Secondary literature sources for BTK

The following references were automatically generated.

Sakamoto M, Kanegane H, Fujii H, Tsukada S, Miyawaki T, Shinomiya N
Maternal germinal mosaicism of X-linked agammaglobulinemia.
Am J Med Genet. 2001; 99: 234-7
Display abstract
X-linked agammaglobulinemia (XLA) is an immunodeficiency caused by abnormalities in tyrosine kinase (BTK), and is characterized by a deficiency of peripheral blood B cells. We studied cytoplasmic expression of BTK protein and analyzed the BTK gene (BTK) in peripheral blood mononuclear cells from two siblings with XLA and additional family members. Cytoplasmic expression of BTK protein in monocytes was not detected in either patient with XLA. A single base deletion (C563) in BTK-exon 6, which encodes the TH domain, was identified in both XLA patients. However, normal cytoplasmic expression of BTK protein in monocytes was detected in their mother without any BTK mutation. These results strongly suggest germinal mosaicism in the mother. Copyright 2001 Wiley-Liss. Inc.

Speletas M et al.
Analysis of Btk mutations in patients with X-linked agammaglobulinaemia (XLA) and determination of carrier status in normal female relatives: a nationwide study of Btk deficiency in Greece.
Scand J Immunol. 2001; 54: 321-7
Display abstract
Bruton's tyrosine kinase (Btk) is a nonreceptor tyrosine kinase, critical for B-cell development and function. Mutations that inactivate this kinase were found in families with X-linked agammaglobulinaemia (XLA). In this study the Btk gene was analyzed in 13 registered Greek patients with XLA phenotype originated from 12 unrelated families, in order to provide a definite diagnosis of the XLA. The structure of Btk was analyzed at the cDNA level using the recently developed method, NIRCA (Non-Isotopic-Rnase-Cleavage-Assay). Alterations were detected in all patients and sequencing analysis confirmed the results and defined six novel XLA-associated Btk mutations (three missense mutations: C337G, L346R, L452P; one nonsense mutation: Y392X, and two frameshift alterations: c1211-1212delA, c1306-1307insA). Having defined the genetic alteration in the affected males of these families, the information was used to design polymerase chain reaction (PCR) primers and the Btk segments containing the mutated sequences were amplified from peripheral blood derived genomic DNA of potential female carriers. The PCR products were directly sequenced and carrier status was determined in 12 out of 16 phenotypically normal females analyzed. This protocol can be used once the nature of the Btk mutation has been defined in one of the affected males and provides a convenient, simple and reliable way to determine the carrier status of other female family members. Molecular genetic analysis constitutes a determinative tool for the definitive diagnosis of XLA and may allow accurate carrier and prenatal diagnosis for genetic counselling.

Rodriguez MC, Granados EL, Cerdan AF, Casariego GF
Molecular analysis of Bruton's tyrosine kinase gene in Spain.
Hum Mutat. 2001; 18: 84-84
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Mutations in Bruton's tyrosine kinase (BTK) gene result in X linked agammaglobulinemia (XLA). Using Single Strand Conformation Polymorphism (SSCP) followed by direct sequencing 21 mutations were found in 27 patients with an XLA phenotype from 21 unrelated families. We identified 13 novel and 8 known mutations: seven missense (R288W, R544G, P566S, K430E; K374N, L512P, R544S), 5 nonsense (Q196X, Y361X, L249X, Q612X, Q466X), 2 deletions of one nucleotide (A207fsX216, Q612fsX648), 2 deletion-insertions (V219fsX227, K218fsX228), one insertion of two nucleotides (S572fsX587) and 4 point mutations in donor/acceptor splice sites (g.IVS1+1G>C, g.IVS6+5G>A, g.IVS10+1G>T, g.IVS13-1GG>CT). Carrier detection was performed in 18 mothers. Only in one case the mutation was found to be de novo. Additionally, BTK mutations were not found in four patients without family history, but with XLA-compatible phenotype. Hum Mutat 18:84, 2001. Copyright 2001 Wiley-Liss, Inc.

Futatani T, Watanabe C, Baba Y, Tsukada S, Ochs HD
Bruton's tyrosine kinase is present in normal platelets and its absence identifies patients with X-linked agammaglobulinaemia and carrier females.
Br J Haematol. 2001; 114: 141-9
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X-linked agammaglobulinaemia (XLA) is a primary immunodeficiency caused by mutations in the gene coding for Bruton's tyrosine kinase (Btk) and is characterized by an arrest of B-cell development. We analysed Btk protein expression in platelets using flow cytometry and found that normal platelets express large amounts of Btk. Assessment of affected males from 45 unrelated XLA families revealed that platelets of the majority of the patients (37 out of 45 families) had decreased or absent Btk expression, and that platelets from carrier females of these families had both normal and mutated Btk expression, indicating that megakaryocytes in XLA carriers undergo random X-chromosome inactivation. These observations demonstrate that Btk is not crucial for maturation of megakaryocytes and the production of platelets. No correlation between Btk expression in platelets and clinical phenotype was observed in this study. Flow cytometric evaluation using platelets is a simple and rapid method to test Btk expression. It may be used as a screening test for XLA and for carrier detection, followed, if necessary, by more expensive mutation analyses.

Stewart DM, Tian L, Nelson DL
A case of X-linked agammaglobulinemia diagnosed in adulthood.
Clin Immunol. 2001; 99: 94-9
Display abstract
X-linked agammaglobulinemia (XLA), caused by mutations in Bruton's tyrosine kinase (BTK), typically presents in early childhood. We report here the case of a male diagnosed at age 23 years with hypogammaglobulinemia, originally classified as common variable immunodeficiency (CVID). On further analysis at age 40, flow cytometric analysis of lymphocytes showed only 0.1% B cells and Western blot analysis showed a deficiency of BTK protein in peripheral blood mononuclear cells, indicating the patient has XLA. BTK cDNA and genomic DNA analysis revealed a splice site mutation at the 3' end of intron 13. Multiple abnormally spliced mRNA species were identified, one of which was predicted to produce a protein with a 24-amino-acid insertion between the SH2 and kinase domains. In vitro kinase assay of this product showed weak kinase activity, perhaps resulting in milder than usual disease. XLA can present in adult males, and sporadic cases may be misdiagnosed as CVID. Copyright 2001 Academic Press.

Saito K, Scharenberg AM, Kinet JP
Interaction between the Btk PH domain and phosphatidylinositol-3,4,5-trisphosphate directly regulates Btk.
J Biol Chem. 2001; 276: 16201-6
Display abstract
Bruton's tyrosine kinase (Btk) binds to phosphatidylinositol-3,4,5-trisphosphate (PtdIns-3,4,5-P(3)) through the Btk pleckstrin homology (PH) domain, an interaction thought to be required for Btk membrane translocation during B cell receptor signaling. Here, we report that interaction of PtdIns-3,4,5-P(3) with the PH domain of Btk directly induces Btk enzymatic activation in an in vitro kinase assay. A point mutation that reduces interaction of PtdIns-3,4,5-P(3) with the Btk PH domain blocks in vitro PtdIns-3,4,5-P(3)-dependent Btk activation, whereas the PH domain deletion enhances Btk basal activity but eliminates the PtdIns-3,4,5-P(3)-dependent stimulation. Btk kinase activity and the Btk activation loop phosphorylation site are both required for the PtdIns-3,4,5-P(3)-mediated stimulation of Btk kinase activity. Together, these results suggest that the Btk PH domain is positioned such that it normally suppresses both Btk kinase activity and access to substrates; when interacting with PtdIns-3,4,5-P(3), this suppression is relieved, producing apparent Btk activation. In addition, using Src family kinase inhibitors and Btk catalytically inactive mutants, we demonstrate that in vivo, the activation of Btk is due to both Lyn phosphorylation and PtdIns-3,4,5-P(3)-mediated direct activation. Thus, the Btk-PtdIns-3,4,5-P(3) interaction serves to translocate Btk to the membrane and directly regulate its signaling function.

Jo EK et al.
Characterization of mutations, including a novel regulatory defect in the first intron, in Bruton's tyrosine kinase gene from seven Korean X-linked agammaglobulinemia families.
J Immunol. 2001; 167: 4038-45
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In this report, we describe seven mutations, including a novel single base pair substitution in intron 1, of the Bruton's tyrosine kinase (Btk) gene found in 12 Korean patients with X-linked agammaglobulinemia. Various mutations, including three novel genetic alterations, were discovered using single-strand conformation polymorphism analysis and direct DNA sequencing. The effect of the intron 1 point mutation (intron 1 +5G-->A) was further evaluated using reporter constructs. Using luciferase assay experiments, we showed that the transcriptional activity of the mutant was significantly lower than in normal counterparts, indicating that the intronic mutation was functional. In addition, DNase I footprinting analysis showed that a single protected region spanning the position +3 to +15 bp hybridized with a mutant-specific probe, but not with a wild-type probe. EMSA indicated that a distinct nuclear protein has the ability to bind the mutant oligonucleotides to produce a new DNA-protein complex. We also observed decreased expression of Btk proteins in monocytes of patients having the point mutation in intron 1. Taken together with the functional analysis, our results strongly suggest the existence of a novel cis-acting element, which might be involved in the down-regulation of Btk gene transcription. Precise definition of the regulatory defect in the Btk intron 1 may provide valuable clues toward elucidating the pathogenesis of X-linked agammaglobulinemia.

Pienaar S, Eley B, Beatty DW, Henderson HE
X-linked agammaglobulinaemia and the underlying genetics in two kindreds.
J Paediatr Child Health. 2000; 36: 453-6
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OBJECTIVE: Molecular analysis of the Bruton's tyrosine kinase (Btk) gene in two unrelated families, with a combined total of seven boys, affected by X-linked agammaglobulinaemia (XLA). METHODS: Protein electrophoresis and western blotting were used for the examination of Btk protein synthesis in blood leucocytes. Isolation of the coding sequence of the Btk gene was performed by amplification using the reverse transcription-polymerase chain reaction (RT-PCR) technique. Sequence alterations were screened for by the single-stranded conformation polymorphism (SSCP) method and characterized by standard sequencing protocols. RESULTS: Western blotting revealed Btk protein to be absent in leucocytes of affected males from both families. A novel 3 b.p. deletion in exon 3 of the Btk gene was found to be responsible for the XLA phenotype in the affected proband in one family (kindred I). A diagnostic PCR assay was established to detect this mutation in other affected male siblings and carrier females. For the second family (kindred II), the coding sequence of the Btk gene and the promoter region were found to be normal. CONCLUSIONS: The present study has demonstrated genetic heterogeneity in the Btk gene in South African XLA patients and has identified a novel mutation in this gene in the largest of the affected kindreds. The gene mutation in the second kindred was undetermined and may be indicative of a defect in some other gene associated with Btk function or stability. Western blotting was found to be informative in establishing a deficiency of Btk protein in both probands and is recommended as a frontline procedure in the molecular diagnosis and work-up of XLA.

Satterthwaite AB, Witte ON
The role of Bruton's tyrosine kinase in B-cell development and function: a genetic perspective.
Immunol Rev. 2000; 175: 120-7
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Mutations in Bruton's tyrosine kinase (Btk) result in the B-cell immunodeficiencies X-linked agammaglobulinemia in humans and X-linked immunodeficiency in mice. These diseases are characterized by blocks in B-cell development at multiple stages and impaired function of residual mature B cells. This review focuses on a series of in vivo genetic studies that have begun to define the mechanism by which Btk regulates B-cell development and function. The functional interactions between Btk and other signaling molecules defined by this approach are more complex than initially appreciated from in vitro biochemical and cell culture studies.

Vihinen M, Mattsson PT, Smith CI
Bruton tyrosine kinase (BTK) in X-linked agammaglobulinemia (XLA).
Front Biosci. 2000; 5: 91728-91728
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X-linked agammaglobulinemia (XLA) is a heritable immunodeficiency disorder that is caused by a differentiation block leading to almost complete absence of B lymphocytes and plasma cells. The affected protein is a cytoplasmic protein tyrosine kinase, Bruton's agammaglobulinemia tyrosine kinase (Btk). Btk along with Tec, Itk, Bmx and Txk belong to a distinct family of protein kinases. These proteins contain five regions; PH, TH, SH3, SH2 and kinase domains. Mutations causing XLA may affect any of these domains. About 380 unique mutations have been identified and are collected in a mutation database, BTKbase. Here, we describe the structure, function, and interactions of the affected signaling molecules in atomic detail.

Kanegane H et al.
Detection of Bruton's tyrosine kinase mutations in hypogammaglobulinaemic males registered as common variable immunodeficiency (CVID) in the Japanese Immunodeficiency Registry.
Clin Exp Immunol. 2000; 120: 512-7
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CVID is frequently diagnosed in male and female individuals with hypogammaglobulinaemia of unknown aetiology. To examine the possibility that sporadic male cases with X-linked agammaglobulinaemia (XLA), which is caused by mutations in the Bruton's tyrosine kinase (Btk) gene, might be misregistered as having CVID, we employed a flow cytometric test to identify XLA in hypogammaglobulinaemic males registered as CVID in the Japanese Immunodeficiency Registry. From 30 male cases registered as having CVID between 1992 and 1998, we selected 21 males with low or unreported peripheral B cell counts. Blood samples could be obtained from 11 patients and their mothers. Using flow cytometric analysis, the Btk-deficient status in monocytes was demonstrated in seven out of nine cases with decreased numbers of peripheral B cells. The diagnosis of XLA was confirmed in each of the seven patients by demonstration of Btk gene mutations in the patients or cellular mosaicism in the mother. This study demonstrates misregistration of XLA as CVID.

Yip KL, Chan SY, Ip WK, Lau YL
Bruton's tyrosine kinase mutations in 8 Chinese families with X-linked agammaglobulinemia.
Hum Mutat. 2000; 15: 385-385
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Bruton's tyrosine kinase (BTK) is involved in B-cell development. Mutation of BTK results in X-linked agammaglobulinemia (XLA). BTK is expressed in most haemopoietic lineages except mature T cells and plasma cells. We identified six novel and two known mutations of BTK in 11 Chinese XLA patients from 8 families. Family 1 had a novel point mutation at the start codon (135G-->T) in exon 2. Family 2 had known mutation of single A insertion in a stretch of 7 A residues (341-347insA) recognized as mutation hotspot in exon 3. Family 3 had a novel point mutation in exon 11 (1074A-->G) which led to aberrant splicing. Family 4 had known mutation in exon 19 (2053C-->T) in CpG mutation hotspot. The novel mutation of family 5 was an A deleted in a run of three As (1017-1019delA) in exon 10. In family 6, exons 2 and 3 were lost in BTK mRNA, a novel deletion. Family 7 had a novel substitution in exon 2 (227T-->C) which led to change of a conserved leucine to serine. Family 8 had a novel point mutation at beginning of intron 14 (IVS14+ 6 T-->G) resulting in aberrant splicing. Hum Mutat 15:385, 2000.

Gaspar HB et al.
Kinase mutant Btk results in atypical X-linked agammaglobulinaemia phenotype.
Clin Exp Immunol. 2000; 120: 346-50
Display abstract
X-linked agammaglobulinaemia (XLA) is a B cell humoral abnormality arising from mutations in the gene encoding Bruton's tyrosine kinase (Btk). The phenotype of XLA can be variable, with some individuals having a less severe immunophenotype, although in most cases this cannot be correlated with the Btk mutation or expression of Btk protein. In this study we describe clinical and immunological heterogeneity within the same pedigree. Analysis of the genetic defect identified a missense mutation in the kinase domain of Btk which, unusually, preserved Btk protein expression but at reduced levels, and also considerably diminished autophosphorylation activity. Structural analysis of the effect of this mutation on the kinase domain suggests that this mutation is not an integral part of the ATP or substrate binding domains but may affect the interaction of the kinase domain with its own kinase domain and other substrates. Together, these data may provide an explanation for the variable XLA phenotype.

Staehelin F, Kuhne T
Identification of a new Bruton's tyrosine kinase (BTK) mutation associated with a mild phenotype in a child with X-linked agammaglobulinemia (XLA).
Clin Lab Haematol. 2000; 22: 123-5
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This study reports a new X-linked agammaglobulinemia (XLA) mutation and its phenotypic features in a 6(1/2)-year-old boy. Different clinically defined subtypes of XLA may exist according to different genetic alterations and to other defect signalling molecules or pathways of B cell maturation.

Curtis SK, Hebert MD, Saha BK
Twin carriers of X-linked agammaglobulinemia (XLA) due to germline mutation in the Btk gene.
Am J Med Genet. 2000; 90: 229-32
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We report on an X-linked agammaglobulinemia (XLA) family in which mothers of two affected cousins were monozygotic twins. We analyzed the Btk gene of several members in three generations of the family by SSCP analysis, DNA sequencing, and RFLP analysis following polymerase chain reaction-amplification of the individual exons. We identified a missense point mutation, G1817C (R562P), in exon 17 of the Btk gene in the affected cousins. The same mutation was also present in both mothers (twin sisters) of the cousins identifying them as carriers. However, the mutation was absent in all other relatives including the grandmother of the cousins (mother of the twin sisters). This strongly suggests that the mutation in the Btk gene had originated in one of the germ lines or in the zygote. This may be the first demonstration of a germ line (or zygotic) mutation in XLA.

Mattsson PT et al.
Six X-linked agammaglobulinemia-causing missense mutations in the Src homology 2 domain of Bruton's tyrosine kinase: phosphotyrosine-binding and circular dichroism analysis.
J Immunol. 2000; 164: 4170-7
Display abstract
Src homology 2 (SH2) domains recognize phosphotyrosine (pY)-containing sequences and thereby mediate their association to ligands. Bruton's tyrosine kinase (Btk) is a cytoplasmic protein tyrosine kinase, in which mutations cause a hereditary immunodeficiency disease, X-linked agammaglobulinemia (XLA). Mutations have been found in all Btk domains, including SH2. We have analyzed the structural and functional effects of six disease-related amino acid substitutions in the SH2 domain: G302E, R307G, Y334S, L358F, Y361C, and H362Q. Also, we present a novel Btk SH2 missense mutation, H362R, leading to classical XLA. Based on circular dichroism analysis, the conformation of five of the XLA mutants studied differs from the native Btk SH2 domain, while mutant R307G is structurally identical. The binding of XLA mutation-containing SH2 domains to pY-Sepharose was reduced, varying between 1 and 13% of that for the native SH2 domain. The solubility of all the mutated proteins was remarkably reduced. SH2 domain mutations were divided into three categories: 1) Functional mutations, which affect residues presumably participating directly in pY binding (R307G); 2) structural mutations that, via conformational change, not only impair pY binding, but severely derange the structure of the SH2 domain and possibly interfere with the overall conformation of the Btk molecule (G302E, Y334S, L358F, and H362Q); and 3) structural-functional mutations, which contain features from both categories above (Y361C).

Orrico A et al.
A mutation in the pleckstrin homology (PH) domain of the FGD1 gene in an Italian family with faciogenital dysplasia (Aarskog-Scott syndrome).
FEBS Lett. 2000; 478: 216-20
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Aarskog-Scott Syndrome (AAS) is an X-linked disorder characterised by short stature and multiple facial, limb and genital abnormalities. A gene, FGD1, altered in a patient with AAS phenotype, has been identified and found to encode a protein with homology to Rho/Rac guanine nucleotide exchange factors (Rho/Rac GEF). However, since this original report on identification of a mutated FGD1 gene in an AAS patient, no additional mutations in the FGD1 gene have been described. We analysed 13 independent patients with clinical diagnosis of AAS. One patient presented a mutation that results in a nucleotide change in exon 10 of the FGD1 gene (G2559>A) substituting a Gln for Arg in position 610. The mutation was found to segregate with the AAS phenotype in affected males and carrier females in the family of this patient. Interestingly, Arg-610 is located within one of the two pleckstrin homology (PH) domains of the FGD1 gene and it corresponds to a highly conserved residue which has been involved in InsP binding in PH domains of other proteins. The same residue is often mutated in the Bruton's tyrosine kinase (Btk) gene in patients with an X-linked agammaglobulinemia. The Arg610Gln mutation represents the first case of a mutation in the PH domain of the FGD1 gene and additional evidence that mutations in PH domains can be associated to human diseases.

Mohamed AJ, Vargas L, Nore BF, Backesjo CM, Christensson B, Smith CI
Nucleocytoplasmic shuttling of Bruton's tyrosine kinase.
J Biol Chem. 2000; 275: 40614-9
Display abstract
Bruton's tyrosine kinase (Btk), a nonreceptor cytoplasmic tyrosine kinase belonging to the Tec family of kinases, has been shown to be critical for B cell proliferation, differentiation, and signaling. Loss-of-function mutations in the Btk gene lead to X-linked agammaglobulinemia (XLA), a primary immunodeficiency in humans, and the less severe condition xid in mice. Although Btk is mainly localized in the cytoplasm under steady state conditions, it translocates to the plasma membrane upon growth factor stimulation and cross-linking of the B cell receptor. Nevertheless, in ectopically as well as endogenously Btk-expressing cells, it can also translocate to the nucleus. Deletion of the pleckstrin homology (PH) domain (DeltaPH1) leads, however, to an even redistribution of Btk within the nucleus and cytoplasm in the majority of transfected cells. In contrast, an SH3-deleted (DeltaSH3) mutant of Btk has been found to be predominantly nuclear. We also demonstrate that the nuclear accumulation of DeltaPH1 is dependent on Src expression. This nucleocytoplasmic shuttling is sensitive to the exportin 1/CRM1-inactivating drug, leptomycin B, indicating that Btk utilizes functional nuclear export signals. In addition, while the DeltaPH1 mutant of Btk was found to be active and tyrosine-phosphorylated in vivo, DeltaSH3 displayed decreased autokinase activity and was not phosphorylated. Our findings indicate that the nucleocytoplasmic shuttling of Btk has implications regarding potential targets inside the nucleus, which may be critical in gene regulation during B cell development and differentiation.

Orlandi P et al.
Identification of nine novel mutations in the Bruton's tyrosine kinase gene in X-linked agammaglobulinaemia patients.
Hum Mutat. 2000; 15: 117-117
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Mutations in the Bruton's tyrosine kinase (BTK ) gene are responsible for X-linked Agammaglobulinemia (XLA), an immunodeficiency caused by a block in B cell differentiation. Non Isotopic RNAse Cleavage Assay (NIRCA), followed by sequencing was used to screen for BTK mutations in 11 Italian XLA patients. Nine novel mutations were identified: 6 missense (Y39S, L512P, L512Q, R544G, S578Y, E589K), one non-sense (Q260X), one frameshift (1599-1602del GCGC) and one in-frame insertion (2037-2038insTTTTAG), that represents the first case of premature stop codon introduction in the BTK coding frame. These data support the high molecular heterogeneity of BTK gene in XLA disease and provide new insight to the diagnosis and to the role of BTK domain in XLA and in B cell signal transduction and development. Hum Mutat 15:117, 2000.

Moschese V et al.
X-chromosome inactivation and mutation pattern in the Bruton's tyrosine kinase gene in patients with X-linked agammaglobulinemia. Italian XLA Collaborative Group.
Mol Med. 2000; 6: 104-13
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BACKGROUND: The diagnosis of X-linked agammaglobulinemia (XLA) is not always clearcut. Not all XLA conform to the classic phenotype and less than 50% of affected boys have a family history of immunodeficiency. Mutations in the gene for Bruton's tyrosine kinase (BTK) are responsible for the majority of agammaglobulinemia cases. However, a certain proportion of patients may have mutations involving other genes, although they show with an XLA phenotype. We performed BTK gene mutation analysis in 37 males with presumed XLA and analyzed the pattern of X-chromosome inactivation (XCI) in 31 mothers to evaluate the relevance of these approaches to diagnosis and genetic counseling. MATERIALS AND METHODS: Twenty affected males with a sporadic occurrence and 17 familial cases belonging to nine families were enrolled within the framework of the Italian Multicenter Clinical Study on XLA. We used non-isotopic RNase cleavage assay (NIRCA), followed by cDNA sequence determination to screen for BTK mutations and X-chromosome inactivation analysis for carrier detection. RESULTS: Using the cDNA-based approach, the identification of BTK gene abnormalities confirmed the clinical diagnosis of XLA in 31 of 37 affected infants. Missense was the most frequent mutational event and the kinase domain was mostly involved. In addition, nine novel mutations were identified. In sporadic cases, BTK gene abnormalities were identified in 9 out of 10 patients whose mothers had a nonrandom pattern of XCI and in 5 out of 6 patients whose mother had a random pattern of XCI. With the exception of one family, all patients with a familial occurrence and born to mothers with a nonrandom pattern of XCI had mutations of the BTK gene. CONCLUSIONS: Our findings indicate that in sporadic cases BTK gene sequencing is the only reliable tool for a definitive diagnosis of XLA and support XCI as the first diagnostic tool in the mothers of affected males in multiple generations. Furthermore, our molecular analysis confirms that 10-20% of BTK-unaltered patients have disorders caused by defects in other genes.

Mano H
Tec family of protein-tyrosine kinases: an overview of their structure and function.
Cytokine Growth Factor Rev. 1999; 10: 267-80
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The Tec family is a recently emerging subfamily of non-receptor protein-tyrosine kinases (PTKs) represented by its first member, Tec. This family is composed of five members, namely Tec, Btk. Itk/Emt/Tsk, Bmx and Txk/Rlk. The most characteristic feature of this family is the presence of a pleckstrin homology (PH) domain in their protein structure. The PH domain is known to bind phosphoinositides; on this basis, Tec family PTKs may act as merge points of phosphotyrosine-mediated and phospholipid-mediated signaling systems. Many Tec family proteins are abundantly expressed in hematopoietic tissues, and are presumed to play important roles in the growth and differentiation processes of blood cells. Supporting this, mutations in the Btk gene cause X chromosome-linked agammaglobulinemia (XLA) in humans and X chromosome-linked immunodeficiency (Xid) in mice, indicating that Btk activity is indispensable for B-cell ontogeny. In addition, Tec family kinases have been shown to be involved in the intracellular signaling mechanisms of cytokine receptors, lymphocyte surface antigens, heterotrimeric G-protein-coupled receptors and integrin molecules. Efforts are being made to identify molecules which interact with Tec kinases to transfer Tec-mediated signals in vivo. Candidates for such second messengers include PLC-gamma2, guanine nucleotide exchange factors for RhoA and TFII-I/BAP-135. This review summarizes current knowledge concerning the input and output factors affecting the Tec kinases.

Ohya K et al.
Molecular cloning of a docking protein, BRDG1, that acts downstream of the Tec tyrosine kinase.
Proc Natl Acad Sci U S A. 1999; 96: 11976-81
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Tec, Btk, Itk, Bmx, and Txk constitute the Tec family of protein tyrosine kinases (PTKs), a family with the distinct feature of containing a pleckstrin homology (PH) domain. Tec acts in signaling pathways triggered by the B cell antigen receptor (BCR), cytokine receptors, integrins, and receptor-type PTKs. Although upstream regulators of Tec family kinases are relatively well characterized, little is known of the downstream effectors of these enzymes. The yeast two-hybrid system has identified several proteins that interact with the kinase domain of Tec, one of which is now revealed to be a previously unknown docking protein termed BRDG1 (BCR downstream signaling 1). BRDG1 contains a proline-rich motif, a PH domain, and multiple tyrosine residues that are potential target sites for Src homology 2 domains. In 293 cells expressing recombinant BRDG1 and various PTKs, Tec and Pyk2, but not Btk, Bmx, Lyn, Syk, or c-Abl, induced marked phosphorylation of BRDG1 on tyrosine residues. BRDG1 was also phosphorylated by Tec directly in vitro. Efficient phosphorylation of BRDG1 by Tec required the PH and SH2 domains as well as the kinase domain of the latter. Furthermore, BRDG1 was shown to participate in a positive feedback loop by increasing the activity of Tec. BRDG1 transcripts are abundant in the human B cell line Ramos, and the endogenous protein underwent tyrosine phosphorylation in response to BCR stimulation. BRDG1 thus appears to function as a docking protein acting downstream of Tec in BCR signaling.

Hashimoto S et al.
Atypical X-linked agammaglobulinemia diagnosed in three adults.
Intern Med. 1999; 38: 722-5
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OBJECT: X-linked agammaglobulinemia (XLA) is one of the most common humoral immunodeficiencies characterized from childhood by the absence of peripheral B lymphocytes, reduced levels of serum immunoglobulins and recurrent and severe bacterial infections. These characteristics are the result of Bruton's tyrosine kinase (Btk) protein deficiency in peripheral B lymphocytes. In addition to typical XLA, several atypical cases have been recognized, who exhibited mild or even no clinical symptoms, although they were definitely deficient in Btk protein (atypical XLA). In these patients peripheral B lymphocytes and serum immunoglobulins (Igs) are detectable though at a lower level than in normal people. To clarify the discrepancies between the Btk gene mutations and the phenotypes more atypical patients should be examined. In this study we evaluated the cytoplasmic Btk protein in peripheral monocytes of some hypogammaglobulinemia adults by means of flowcytometric analysis. MATERIALS AND METHODS: Heparinized venous blood samples were collected from some hypogammaglobulinemia adults. Mononuclear cells were separated from their blood and first reacted with a phycoerythrin-labeled CD14 monoclonal antibody (MoAb) (staining of monocyte membrane). Next, the cells were fixed and permeabilized. And then these permeabilized cells were reacted with an anti-Btk MoAb (staining of cytoplasmic Btk protein) and incubated with a FITC-conjugated goat antimouse IgG1. The double-stained cells were analyzed on a flowcytometer. RESULTS AND CONCLUSION: By means of flowcytometric analysis we diagnosed three hypogammaglobulinemia adults as XLA, who did not show typical clinical progress of XLA. Advancements in diagnostic methods has facilitated a prompt and definite diagnosis of this disease.

Hashimoto S et al.
Identification of the SH2 domain binding protein of Bruton's tyrosine kinase as BLNK--functional significance of Btk-SH2 domain in B-cell antigen receptor-coupled calcium signaling.
Blood. 1999; 94: 2357-64
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Bruton's tyrosine kinase (Btk) is a critical component in the B-cell antigen receptor (BCR)-coupled signaling pathway. Its deficiency in B cells leads to loss or marked reduction in the BCR-induced calcium signaling. It is known that this BCR-induced calcium signaling depends on the activation of phospholipase Cgamma (PLCgamma), which is mediated by Btk and another tyrosine kinase Syk and that the SH2 and pleckstrin homology (PH) domains of Btk play important roles in this activation process. Although the importance of the PH domain of Btk has been explained by its role in the membrane targeting of Btk, the functional significance of the SH2 domain in the calcium signaling has remained merely a matter of speculation. In this report, we identify that one of the major Btk-SH2 domain-binding proteins in B cells is BLNK (B-cell linker protein) and present evidences that the interaction of BLNK and the SH2 domain of Btk contributes to the complete tyrosine phosphorylation of PLCgamma.

Rohrer J, Minegishi Y, Richter D, Eguiguren J, Conley ME
Unusual mutations in Btk: an insertion, a duplication, an inversion, and four large deletions.
Clin Immunol. 1999; 90: 28-37
Display abstract
Mutations in Bruton's tyrosine kinase (Btk) result in the immunodeficiency X-linked agammaglobulinemia (XLA). In a previous study of 101 patients with presumed XLA, we identified seven patients with large genomic alterations in Btk. The recent completion of 100 kb of contiguous DNA sequence at the Btk locus has allowed us to characterize these mutations in detail and to identify four different types of alterations. These alterations included a 253-bp retroposon insertion at position +5 within intron 9, an inversion of greater than 48 kb that disrupted Btk between exons 4 and 5, a 12.9-kb duplication including Btk exons 2 to 5, and four deletions ranging from 2.8 to 38 kb in size. The duplication and three of the deletions resulted from unequal crossovers of Alu repeats. Further, three of the deletions terminated within a repeat-rich cluster spanning 30 kb of sequence 3' of Btk exon 19, suggesting that this region was more susceptible to unequal crossovers than the rest of the Btk gene. These studies describe the first reports of an insertion, an inversion, and a duplication in Btk and demonstrate the utility of large-scale sequencing in the elucidation of disease-causing mutations.

Rawlings DJ
Bruton's tyrosine kinase controls a sustained calcium signal essential for B lineage development and function.
Clin Immunol. 1999; 91: 243-53
Display abstract
Genetic data support a role for Btk during the B lineage development transitions regulated by signaling through both the pre-B and the B cell antigen receptors. Dysregulated signaling at each of these transitions can result in failure of these cell populations to proliferate and subsequent cell death. Btk-dependent IP3 production is crucial for maintaining the sustained calcium signal in response to BCR engagement and is likely to regulate a subset of transcriptional events essential for B lineage growth or survival. Identification of these Btk-dependent signals will be important in understanding B cell activation, differentiation, and cell death. This information may lead to therapies specifically targeting these events in B cell autoimmunity or malignancy and provide a fuller understanding of the appropriate target populations and potential negative consequences of Btk gene therapy in XLA. Identification of Btk/Tec family kinases in an increasing number of vertebrate and invertebrate cell lineages suggests that the link between Btk and the PLC gamma/IP3/calcium signaling pathways may be broadly conserved.

Copley RR
The gene for X-linked anhidrotic ectodermal dysplasia encodes a TNF-like domain.
J Mol Med. 1999; 77: 361-3

Varnai P, Rother KI, Balla T
Phosphatidylinositol 3-kinase-dependent membrane association of the Bruton's tyrosine kinase pleckstrin homology domain visualized in single living cells.
J Biol Chem. 1999; 274: 10983-9
Display abstract
Phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P3) has been proposed to act as a second messenger to recruit regulatory proteins to the plasma membrane via their pleckstrin homology (PH) domains. The PH domain of Bruton's tyrosine kinase (Btk), which is mutated in the human disease X-linked agammaglobulinemia, has been shown to interact with PI(3,4,5)P3 in vitro. In this study, a fusion protein containing the PH domain of Btk and the enhanced green fluorescent protein (BtkPH-GFP) was constructed and utilized to study the ability of this PH domain to interact with membrane inositol phospholipids inside living cells. The localization of expressed BtkPH-GFP in quiescent NIH 3T3 cells was indistinguishable from that of GFP alone, both being cytosolic as assessed by confocal microscopy. In NIH 3T3 cells coexpressing BtkPH-GFP and the epidermal growth factor receptor, activation of epidermal growth factor or endogenous platelet-derived growth factor receptors caused a rapid (<3 min) translocation of the cytosolic fluorescence to ruffle-like membrane structures. This response was not observed in cells expressing GFP only and was completely inhibited by treatment with the PI 3-kinase inhibitors wortmannin and LY 292004. Membrane-targeted PI 3-kinase also caused membrane localization of BtkPH-GFP that was slowly reversed by wortmannin. When the R28C mutation of the Btk PH domain, which causes X-linked agammaglobulinemia, was introduced into the fluorescent construct, no translocation was observed after stimulation. In contrast, the E41K mutation, which confers transforming activity to native Btk, caused significant membrane localization of BtkPH-GFP with characteristics indicating its possible binding to PI(4,5)P2. This mutant, but not wild-type BtkPH-GFP, interfered with agonist-induced PI(4,5)P2 hydrolysis in COS-7 cells. These results show in intact cells that the PH domain of Btk binds selectively to 3-phosphorylated lipids after activation of PI 3-kinase enzymes and that losing such binding ability or specificity results in gross abnormalities in the function of the enzyme. Therefore, the interaction with PI(3,4,5)P3 is likely to be an important determinant of the physiological regulation of Btk and can be utilized to visualize the dynamics and spatiotemporal organization of changes in this phospholipid in living cells.

Mano H
The Tec family protein-tyrosine kinases: a subset of kinases for a subset of signalings.
Int J Hematol. 1999; 69: 6-12
Display abstract
The Tec family has emerged recently as a subfamily among nonreceptor type protein-tyrosine kinases, consisting of Tec, Btk, Itk/Tsk/Emt, Bmx, and Txk/Rlk. Because many members of this family have been shown to be activated in response to growth and differentiation stimuli in hematopoietic tissues, they are presumed to function in vivo as important signaling mediators. Although that hypothesis was further strengthened by the knowledge that mutations in Btk cause agammaglobulinemia in humans, we have only limited information concerning the molecular interaction through which Tec kinases exert their effects. One characteristic feature of Tec family members is the presence of a pleckstrin homology domain in their protein structure, suggesting a physical and functional interaction with the phospholipid-dependent signaling pathways. Recent data have revealed that Tec kinases regulate phospholipase C isoforms. This review summarizes current knowledge concerning the in vivo roles of the Tec family proteins.

Johannes FJ et al.
Bruton's tyrosine kinase (Btk) associates with protein kinase C mu.
FEBS Lett. 1999; 461: 68-72
Display abstract
Bruton's tyrosine kinase (Btk) is considered an essential signal transducer in B-cells. Mutational defects are associated with a severe immunodeficiency syndrome, X-chromosome linked agammaglobulinemia (XLA). Here we show by coimmunoprecipitation that a member of the protein kinase C (PKC) family, PKCmu, is constitutively associated with Btk. Neither antigen receptor (Ig) crosslinking nor stimulation of B-cells with phorbol ester or H(2)O(2) affected Btk/PKCmu interaction. GST precipitation analysis revealed association of the Btk pleckstrin/Tec homology domain with PKCmu. Transient overexpression of PKCmu deletion mutants as well as expression of selected PKCmu domains in 293T cells revealed that both the kinase domain and the regulatory C1 region are independently capable of binding to the Btk PH-TH domain. These data show the existence of a PKCmu/Btk complex in vivo and identify two PKCmu domains that participate in Btk interaction.

Vihinen M et al.
Mutations of the human BTK gene coding for bruton tyrosine kinase in X-linked agammaglobulinemia.
Hum Mutat. 1999; 13: 280-5
Display abstract
X-linked agammaglobulinemia (XLA) is an immunodeficiency caused by mutations in the gene coding for Bruton agammaglobulinemia tyrosine kinase (BTK). A database (BTKbase) of BTK mutations lists 544 mutation entries from 471 unrelated families showing 341 unique molecular events. In addition to mutations, a number of variants or polymorphisms have been found. Mutations in all the five domains of BTK cause the disease, the single most common event being missense mutations. Most mutations lead to truncation of the enzyme. The mutations appear almost uniformly throughout the molecule. About one-third of point mutations affect CpG sites, which usually code for arginine residues. The putative structural implications of all the missense mutations are provided in the database. BTKbase is available at http://www.uta.fi/imt/bioinfo.

Minegishi Y, Rohrer J, Conley ME
Recent progress in the diagnosis and treatment of patients with defects in early B-cell development.
Curr Opin Pediatr. 1999; 11: 528-32
Display abstract
Mutation detection for X-linked agammaglobulinemia (XLA) has revealed the heterogeneity of the clinical phenotype of patients with defects in Bruton's tyrosine kinase (Btk), the gene that is abnormal in XLA. Over 50% of patients with mutations in Btk have no family history of the disease because their cases are the first manifestation of a new mutation in their family. In 10% to 20% of patients, the serum immunoglobulins are higher than expected or the onset of disease is delayed; however, a marked reduction in B-cell numbers is consistent in all patients. Mutation detection has also shown that not all patients with presumed XLA have mutations in Btk. Mutations in mu heavy chain, and other components of the pre-B cell receptor complex, including lambda 5/14.1, cause a disorder that is clinically identical to XLA. Although new strategies for therapy are not yet available, the groundwork is being laid for cell or gene therapy.

Nonoyama S
Recent advances in the diagnosis of X-linked agammaglobulinemia.
Intern Med. 1999; 38: 687-8

Mohamed AJ, Nore BF, Christensson B, Smith CI
Signalling of Bruton's tyrosine kinase, Btk.
Scand J Immunol. 1999; 49: 113-8
Display abstract
Bruton's tyrosine kinase, which is encoded by the BTK gene, is a cytoplasmic protein tyrosine kinase (PTK) crucial for B-cell development and differentiation. It belongs to the Tec family of PTKs containing several domains that are characteristic of signalling molecules. In humans, mutations that disrupt the function of this gene lead to the classical XLA syndrome (X-linked agammaglobulinaemia), a primary immunodeficiency mainly characterized by lack of mature B cells as well as low levels of immunoglobulins. In contrast, animal models of this disease such as the xid mice display profoundly milder XLA phenotype. BTK phosphorylation and activation in response to engagement of the B-cell receptor (BCR) by antigen is a dynamic process whereby a variety of proteins interact with each other and recruit signalling molecules resulting in a physiological response such as B-cell proliferation and antibody production. The main players, however, that participate in the intracellular downstream cascade have not yet been identified and are therefore under intense scrutiny in several laboratories. This review discusses certain aspects of BTK activation following receptor stimulation by agonists and how this event is translated into the biochemical signals within the cell that eventually lead to nuclear responses.

Yamadori T et al.
Bruton's tyrosine kinase activity is negatively regulated by Sab, the Btk-SH3 domain-binding protein.
Proc Natl Acad Sci U S A. 1999; 96: 6341-6
Display abstract
Bruton's tyrosine kinase (Btk) is a cytoplasmic tyrosine kinase that is crucial for human and murine B cell development, and its deficiency causes human X-linked agammaglobulinemia and murine X-linked immunodeficiency. In this report, we describe the function of the Btk-binding protein Sab (SH3-domain binding protein that preferentially associates with Btk), which we reported previously as a newly identified Src homology 3 domain-binding protein. Sab was shown to inhibit the auto- and transphosphorylation activity of Btk, which prompted us to propose that Sab functions as a transregulator of Btk. Forced overexpression of Sab in B cells led to the reduction of B cell antigen receptor-induced tyrosine phosphorylation of Btk and significantly reduced both early and late B cell antigen receptor-mediated events, including calcium mobilization, inositol 1, 4,5-trisphosphate production, and apoptotic cell death, where the involvement of Btk activity has been demonstrated previously. Together, these results indicate the negative regulatory role of Sab in the B cell cytoplasmic tyrosine kinase pathway.

Merkel AL, Atmosukarto II, Stevens K, Rathjen PD, Booker GW
Splice variants of the mouse Tec gene are differentially expressed in vivo.
Cytogenet Cell Genet. 1999; 84: 132-9
Display abstract
Tec is a cytoplasmic protein tyrosine kinase that participates in the signalling pathways of a broad range of cytokines. Up to five different Tec isoforms have been reported in the literature. We report here the genomic organisation of the mouse Tec gene and the tissue expression pattern of the two predominant transcripts, TecIII and TecIV. The mouse Tec gene consists of 18 exons, spans more than 86 kb, and is 2.6 kb 5' to the gene for Txk, a Tec family member. Comparison of mouse and human Btk, human TXK, and mouse Tec genomic structures shows a high level of conservation of exon/intron boundaries. Compared with TecIV, the TecIII transcript has a 66-bp deletion in the SH3 domain encoding region and is revealed here to arise by alternative splicing of exon 8. We show that both TecIII and TecIV are expressed as early as embryonic day 10.5 in mouse development, as well as in adult and embryonic organs. The ratio of TecIV to TecIII expression is markedly reduced in adult liver and kidney tissues and d16 embryonic limb.

Abo K et al.
A novel single basepair insertion in exon 6 of the Bruton's tyrosine kinase (Btk) gene from a Japanese X-linked agammaglobulinemia patient with growth hormone insufficiency.
Hum Mutat. 1998; 11: 336-336
Display abstract
A novel insertion mutation in exon 6 of the Btk gene was detected in a 17 year-old XLA patient with GH insufficiency. We synthesized cDNA from leukocyte total RNA and amplified every region of the Btk-coding sequence. Sequencing of cDNA fragments revealed a single basepair insertion mutation at codon 157 in exon 6 (CAG-->CAAG) which leads to premature termination at codon 193 in exon 7. To confirm the results, we also performed a PCR-DdeI digestion analysis using leukocyte genomic DNA. The PCR product from the patient's genomic DNA was uncleaved with DdeI, as expected. PCR-DdeI digestion analysis of the family members showed that the mother and elder sister were carriers with the mutation and that the younger sister did not carry the mutation.

Conley ME, Mathias D, Treadaway J, Minegishi Y, Rohrer J
Mutations in btk in patients with presumed X-linked agammaglobulinemia.
Am J Hum Genet. 1998; 62: 1034-43
Display abstract
In 1993, two groups showed that X-linked agammaglobulinemia (XLA) was due to mutations in a tyrosine kinase now called Btk. Most laboratories have been able to detect mutations in Btk in 80%-90% of males with presumed XLA. The remaining patients may have mutations in Btk that are difficult to identify, or they may have defects that are phenotypically similar to XLA but genotypically different. We analyzed 101 families in which affected males were diagnosed as having XLA. Mutations in Btk were identified in 38 of 40 families with more than one affected family member and in 56 of 61 families with sporadic disease. Excluding the patients in whom the marked decrease in B cell numbers characteristic of XLA could not be confirmed by immunofluorescence studies, mutations in Btk were identified in 43 of 46 patients with presumed sporadic XLA. Two of the three remaining patients had defects in other genes required for normal B cell development, and the third patient was unlikely to have XLA, on the basis of results of extensive Btk analysis. Our techniques were unable to identify a mutation in Btk in one male with both a family history and laboratory findings suggestive of XLA. DNA samples from 41 of 49 of the mothers of males with sporadic disease and proven mutations in Btk were positive for the mutation found in their son. In the other 8 families, the mutation appeared to arise in the maternal germ line. In 20 families, haplotype analysis showed that the new mutation originated in the maternal grandfather or great-grandfather. These studies indicate that 90%-95% of males with presumed XLA have mutations in Btk. The other patients are likely to have defects in other genes.

Petroni D, Bartolini E, Chiaramonte R, Ottolenghi S, Comi P
Computer sequence analysis of human highly conserved zinc finger modules.
DNA Seq. 1998; 9: 163-9
Display abstract
We defined a sub-family of zinc finger proteins by computer analyses and comparisons of five new finger domains against protein databases. This subclass of the cysteine-cysteine/histidine-histidine motif shows additional well conserved amino acid patterns and belongs to the human kox and gli-Kruppel gene family, sharing also the same stretches of regulatory zinc finger-containing proteins of mouse and Xenopus. We particularly describe ZF6 cDNA which contains the most interesting sequence, encoding a putative multi-domain regulatory protein.

Nonoyama S et al.
Functional analysis of peripheral blood B cells in patients with X-linked agammaglobulinemia.
J Immunol. 1998; 161: 3925-9
Display abstract
X-linked agammaglobulinemia (XLA) is a primary immunodeficiency disease caused by mutations of Bruton tyrosine kinase (Btk); Btk plays an essential role in the development of mature B cells. However, small numbers of B cells ("leaky B cells") are present in the peripheral blood of most XLA patients. In this study, we analyzed the function of these leaky B cells obtained from XLA patients. Enough numbers of B cells were available for analysis from five of nine XLA patients originally screened. Sequence analysis revealed missense mutations of Btk in four of the five XLA patients. No mutation was found in the coding region of Btk in one patient. Western blotting and/or flow cytometric analysis failed to detect Btk protein in all five patients. B cells isolated from peripheral blood of these XLA patients were CD5-, CD20+, CD19+, and CD21-. If stimulated with anti-CD40 and IL-4, XLA B cells proliferated normally and produced significant amounts of IgE. Anti-CD40 stimulation of XLA B cells resulted in normal expression of CD23. In addition, three of the five XLA patients studied were immunized with bacteriophage phiX174 and produced low but detectable levels of antiphage-specific Ab. Similarly, X-linked immunodeficiency mice, which carry a missense mutation in Btk, produced substantial amounts of antiphage Ab. These results indicate that CD40 signaling is intact in B cells lacking demonstrable Btk, and that leaky B cells in XLA patients can proliferate, undergo isotype switching, and differentiate into specific Ab-producing cells.

Haire RN, Strong SJ, Litman GW
Tec-family non-receptor tyrosine kinase expressed in zebrafish kidney.
Immunogenetics. 1998; 47: 336-7

Futatani T et al.
Deficient expression of Bruton's tyrosine kinase in monocytes from X-linked agammaglobulinemia as evaluated by a flow cytometric analysis and its clinical application to carrier detection.
Blood. 1998; 91: 595-602
Display abstract
The B-cell defect in X-linked agammaglobulinemia (XLA) is caused by mutations in the gene for Bruton's tyrosine kinase (BTK). Using the anti-BTK monoclonal antibody (48-2H), a flow cytometric analysis of intracytoplasmic BTK protein expressed in monocytes was successfully performed. To examine the possible identification of XLA patients and female carriers by this assay, we studied 41 unrelated XLA families with (35) or without (6) known BTK mutations. A flow cytometric assay showed deficient expression of the BTK protein in 40 of 41 patients, complete BTK deficiency in 35, and partial BTK deficiency in 5. One patient exhibited a normal level of BTK expression. All 6 patients with partial BTK deficiency or normal BTK expression had missense BTK mutations. The cellular mosaicism of BTK expression in monocytes from obligate carriers was clearly shown in 35 of 41 families. The results suggested that most BTK mutations in XLA might result in deficient expression of the BTK protein. We conclude that deficient expression of BTK protein can be evaluated by a flow cytometric assay, and the clinical usefulness and limitations in diagnosis of XLA patients and carriers are discussed.

Rohrer J, Conley ME
Transcriptional regulatory elements within the first intron of Bruton's tyrosine kinase.
Blood. 1998; 91: 214-21
Display abstract
Defects in the gene for Bruton's tyrosine kinase (Btk) result in the disorder X-linked agammaglobulinemia (XLA). Whereas XLA is characterized by a profound defect in B-cell development, Btk is expressed in both the B lymphocyte and myeloid cell lineages. We evaluated a patient with XLA who had reduced amounts of Btk transcript but no abnormalities in his coding sequence. A single base-pair substitution in the first intron of Btk was identified in this patient, suggesting that this region may contain regulatory elements. Using reporter constructs we identified two transcriptional control elements in the first 500 bp of intron 1. A strong positive regulator, active in both pre-B cells and B cells, was identified within the first 43 bp of the intron. Gel-shift assays identified two Sp1 binding sites within this element. The patient's mutation results in an altered binding specificity of the proximal Sp1 binding site. A negative regulator, active in pre-B cells only, was located between base pairs 281 and 491 of the intron. These findings indicate that regulation of Btk transcription is complex and may involve several transcriptional regulatory factors at the different stages of B-cell differentiation.

Ishida F, Kobayashi H, Saito H, Futatani T, Miyawaki T, Kiyosawa K
[The oldest case with X-linked agammaglobulinemia in Japan lacking Bruton-type tyrosine kinase protein detected by flow cytometry]
Rinsho Ketsueki. 1998; 39: 44-7
Display abstract
We report a case of sporadic X-linked agammaglobulinemia previously diagnosed as variable immunodeficiency (VID). An 39-year-old male had recurrent episodes of respiratory tract infection since his early childhood. At the age of four, he developed partial paresis of the left limbs after polio immunization. After diagnosis of VID based on marked decrease of serum IgG, IgA and IgM levels and no antibody production against antigenetic stimuli at age 22 years old, he received intravenous immunoglobulin supplementation irregularly. We reexamined him and found marked decrease in B cells in the peripheral blood. In addition, we investigated the expression of Bruton-type tyrosine kinase on monocytes by flow cytometry and confirmed its deficiency. His mother was diagnosed as a carrier of XLA. The patient is probably the oldest case with XLA in Japan.

Smith CI et al.
X-linked agammaglobulinemia: lack of mature B lineage cells caused by mutations in the Btk kinase.
Springer Semin Immunopathol. 1998; 19: 369-81

Holinski-Feder E et al.
Mutation screening of the BTK gene in 56 families with X-linked agammaglobulinemia (XLA): 47 unique mutations without correlation to clinical course.
Pediatrics. 1998; 101: 276-84
Display abstract
OBJECTIVES: To determine the utility of single-stranded conformation polymorphism (SSCP) analysis for mutation screening in the BTK (Bruton's tyrosine kinase) gene, we investigated 56 X-linked agammaglobulinemia (XLA) families. To obtain genotype/ phenotype correlations, predicted protein aberrations were correlated with the clinical course of the disease. PATIENTS: This study included 56 patients with XLA, with or without a positive family history, who were diagnosed on the basis of their clinical features, low peripheral B-cell count, and low immunoglobulin levels. Ten patients with isolated hypogammaglobulinemia and 50 healthy males served as controls. METHODS: SSCP analysis was performed for the entire BTK gene, including the exon-intron boundaries and the promoter region. Structural implications of the missense mutations were investigated by molecular modeling, and the functional consequences of some mutations also were evaluated by in vitro kinase assays and Western blot analysis. RESULTS: We report the largest series of patients with XLA to date. All but 5 of the 56 index patients with XLA screened with SSCP analysis showed BTK gene abnormalities, and in 2 of the 5 SSCP-negative patients, no BTK protein was found by Western blot analysis. There were 51 mutations, including 37 novel ones, distributed across the entire gene. This report contains the first promoter mutation as well as 14 novel missense mutations with the first ones described for the Tec homology domain and the glycine-rich motif in the SH1 domain. Each index patient had a different mutation, except for four mutations, each in two unrelated individuals. This result supports the strong tendency for private mutations in this disease. No mutations were found in the controls. CONCLUSIONS: Our results demonstrate that molecular genetic testing by SSCP analysis provides an accurate tool for the definitive diagnosis of XLA and the discrimination of borderline cases, such as certain hypogammaglobulinemia or common variable immunodeficiency patients with overlapping clinical features. Genotype/ phenotype correlations are not currently possible, making prediction of the clinical course based on molecular genetic data infeasible.

Gaspar HB, Lester T, Levinsky RJ, Kinnon C
Bruton's tyrosine kinase expression and activity in X-linked agammaglobulinaemia (XLA): the use of protein analysis as a diagnostic indicator of XLA.
Clin Exp Immunol. 1998; 111: 334-8
Display abstract
Mutations in the Bruton's tyrosine kinase (BTK) gene result in XLA. Despite the large numbers of BTK mutations reported, no correlation can be made between the clinical phenotype and the gene defects. Analysis of Btk protein expression and activity in individuals with XLA was performed to characterize the relationship between a particular mutation, the resultant Btk protein and the clinical phenotype. In most patients studied, including those with atypical phenotypes, there was complete absence of protein expression and activity. Furthermore, in two undiagnosed individuals with a clinical phenotype suggestive of XLA, lack of protein expression was used to confirm an abnormality in Btk. These results underline the importance of protein analysis prior to speculating on protein structure and function based on the gene mutation. Lack of Btk expression in atypical phenotypes suggests that there is redundancy in B lymphocyte signalling such that alternative signalling molecules, or mechanisms, can compensate for the lack of Btk. We also suggest that analysis of Btk expression can be used as an indicator of XLA. These rapid assays may be used to screen a wider spectrum of individuals with humoral immunodeficiency in order to characterize fully the extent of Btk deficiency.

Vihinen M, Mattsson PT, Smith CIE
BTK, the tyrosine kinase affected in X-linked agammaglobulinemia
Front Biosci. 1997; 2: 2742-2742
Display abstract
X-linked agammaglobulinemia (XLA) is a heritable immunodeficiency disorder that is caused by a differentiation block leading to almost complete absence of B lymphocytes and plasma cells. The affected protein is a cytoplasmic protein tyrosine kinase, Bruton's agammaglobulinemia tyrosine kinase (Btk). Btk along with Tec, Itk and Bmx belong to a distinct family of protein kinases. These proteins contain five regions; PH, TH, SH3, SH2 and kinase domains. Mutations causing XLA may affect any of these domains. About 200 unique mutations have been identified and are collected in a mutation database, BTKbase. Here, we describle, the structure, function, and interactions of the affected signaling molecules in atomic detail.

Li T, Rawlings DJ, Park H, Kato RM, Witte ON, Satterthwaite AB
Constitutive membrane association potentiates activation of Bruton tyrosine kinase.
Oncogene. 1997; 15: 1375-83
Display abstract
Mutations in the nonreceptor tyrosine kinase Btk result in the B cell immunodeficiencies X-linked agammaglobulinemia (XLA) in humans and X-linked immunodeficiency (xid) in mice. Genetic and biochemical evidence implicates Btk as a key component of several B cell signaling pathways. Activation of Btk by a point mutation (E41K) within the PH domain (Btk*) results in fibroblast transformation and is correlated with increased membrane localization of Btk. When wild type Btk is activated by coexpression with Lyn, the tyrosine phosphorylated pool of Btk is highly enriched in the membrane fraction. To determine whether membrane association is sufficient to activate Btk, we targeted Btk to the plasma membrane using a series of fusion proteins including GagBtk, CD16Btk and CD4Btk. Constitutive membrane association greatly enhanced the ability of Btk to transform Rat2 fibroblasts in the presence of high levels of Src activity. All membrane targeted forms of Btk were highly tyrosine phosphorylated. Transformation required membrane localization, Btk kinase activity, transphosphorylation by Src family kinases, and an intact SH2 domain but not the PH or SH3 domains. These data suggest that membrane localization is a critical early step in Btk activation.

Vihinen M et al.
BTKbase, mutation database for X-linked agammaglobulinemia (XLA)
Nucleic Acids Res. 1997; 25: 166-71
Display abstract
X-linked agammaglobulinemia (XLA) is an immunodeficiency caused by mutations in the gene coding for Bruton's agammaglobulinemia tyrosine kinase (BTK). A database (BTKbase) of BTK mutations has been compiled and the recent update lists 368 entries from 318 unrelated families showing 228 unique molecular events. In addition to mutations the database lists also some polymorphisms and site-directed mutations. Each patient is given a unique patient identity number (PIN). Information is provided regarding the phenotype including symptoms. Mutations in all the five domains of BTK have been noticed to cause the disease, the most common event being missense mutations. The mutations appear almost uniformly throughout the molecule and frequently affect CpG sites forming arginine residues. These hot spots have generally pyrimidines 5'and purines 3'to the mutated cytosine. A decreased frequency of missense mutations was found in the TH, SH3 and the upper lobe of the kinase domain. The putative structural implications of all the missense mutations are given in the database showing 228 unique molecular events, including a novel missense mutation causing an R28C substitution as previously seen in the Xid mouse.

Tsukada S
Symposium on gene abnormalities in medical diseases. 1. Immunological diseases: Bruton's agammaglobulinemia.
Intern Med. 1997; 36: 148-50

Saha BK, Curtis SK, Vogler LB, Vihinen M
Molecular and structural characterization of five novel mutations in the Bruton's tyrosine kinase gene from patients with X-linked agammaglobulinemia.
Mol Med. 1997; 3: 477-85
Display abstract
BACKGROUND: The Btk (Bruton's tyrosine kinase) gene has been shown to be mutated in the human immunodeficiency disease, XLA (X-linked agammaglobulinemia). Btk is a member of the Tec family of cytosolic protein tyrosine kinases with distinct functional domains PH, TH, SH3, SH2, and kinase. Mutations have been observed in each of the Btk subdomains in XLA. We have analyzed the Btk gene in six XLA patients from five unrelated families. MATERIALS AND METHODS: DNA was prepared from the patients peripheral blood. The Btk exons including the junctional sequences were analyzed by single-strand conformation polymorphism (SSCP) followed by direct nucleotide sequencing after PCR-amplification. For structural analysis, the missense mutations were introduced into three-dimensional models of the PH and kinase domains of Btk and the outcome was predicted based on the knowledge of the protein function. RESULTS: Five novel mutations and two novel polymorphisms, all of which resulted from single-base alterations, were identified. Three of the five mutations were in the PH domain and two were in the kinase domain of Btk. Three of these mutations were of the missense type, two of which altered the same codon in the PH domain; the third one was located in the kinase domain. The fourth mutation was a point deletion in the PH domain causing a frameshift followed by premature termination. The fifth mutation was a splice donor-site mutation within the kinase domain which could result in an exon skipping. In four of the five instances, mothers of the patients were shown to be obligate carriers. In one instance, a sibling sister was identified as a heterozygote establishing her as a carrier. CONCLUSIONS: Functional consequences of the mutations causing frameshifts and altered splicing can be inferred directly. Functional consequences of the missense mutations were interpreted by 3-dimensional structural modeling of Btk domains. It is proposed that the two PH domain mutations will interfere with membrane localization while the kinase domain mutation will interfere with the enzymatic function of Btk. This study provides further insight into the role of Btk in XLA.

De Weers M et al.
Expression of Bruton's tyrosine kinase in B lymphoblastoid cell lines from X-linked agammaglobulinaemia patients.
Clin Exp Immunol. 1997; 107: 235-40
Display abstract
X-linked agammaglobulinaemia (XLA) is an immunodeficiency caused by mutations in Bruton's tyrosine kinase (Btk) and is characterized by an almost complete arrest of B cell development. We analysed expression of Btk in B lymphoblastoid cell lines (BLCL) derived from four unrelated XLA patients. In one patient, with a 3 x 5 kb genomic deletion encompassing the first (untranslated) exon, mRNA levels and in vitro kinase activities were very low. The patient manifested a mild phenotype with a delayed onset of the disease. Another mutation, in which the intron 3 donor splice site is lost, was also associated with very low mRNA levels and an absence of detectable Btk protein. Patients with this mutation showed extensive heterogeneity of the immunological phenotype. In the BLCL of a third patient, with an Arg288 substitution in the SH2 domain, the mutation did not appear to affect the expression level, nor to abrogate in vitro phosphorylation activity. In the BLCL of the fourth patient, with an Arg28 mutation in the PH domain, tyrosine kinase activity in BTK precipitates appeared to be decreased compared with control BLCL.

Li Z, Wahl MI, Eguinoa A, Stephens LR, Hawkins PT, Witte ON
Phosphatidylinositol 3-kinase-gamma activates Bruton's tyrosine kinase in concert with Src family kinases.
Proc Natl Acad Sci U S A. 1997; 94: 13820-5
Display abstract
Bruton's tyrosine kinase (Btk) is essential for normal B lymphocyte development and function. The activity of Btk is partially regulated by transphosphorylation within its kinase domain by Src family kinases at residue Tyr-551 and subsequent autophosphorylation at Tyr-223. Activation correlates with Btk association with cellular membranes. Based on specific loss of function mutations, the Btk pleckstrin homology (PH) domain plays an essential role in this activation process. The Btk PH domain can bind in vitro to several lipid end products of the phosphatidylinositol 3-kinase (PI 3-kinase) family including phosphatidylinositol 3,4,5-trisphosphate. Activation of Btk as monitored by elevation of phosphotyrosine content and a cellular transformation response was dramatically enhanced by coexpressing a weakly activated allele of Src (E378G) and the two subunits of PI 3-kinase-gamma. This activation correlates with new sites of phosphorylation on Btk identified by two-dimensional phosphopeptide mapping. Activation of Btk was dependent on the catalytic activity of all three enzymes and an intact Btk PH domain and Src transphosphorylation site. These combined data define Btk as a downstream target of PI 3-kinase-gamma and Src family kinases.

Brooimans RA et al.
Identification of novel Bruton's tyrosine kinase mutations in 10 unrelated subjects with X linked agammaglobulinaemia.
J Med Genet. 1997; 34: 484-8
Display abstract
Mutations of the Bruton's tyrosine kinase (Btk) gene cause X linked agammaglobulinaemia (XLA). This inherited immunodeficiency disease causes an arrest in B cell differentiation of pre-B cells to mature B cells. In this study we report the characterisation of mutations in the Btk gene in 10 unrelated XLA families. The screening approach we used was based on reverse transcriptase PCR and direct cycle sequencing of the amplified products followed by analysis of the observed mutations at the level of genomic DNA. The single strand confirmation polymorphism (SSCP) technique was used for assessment of the carriers in some of these families. Various mutations throughout the coding gene were observed, including missense and nonsense mutations, a deletion, and several splicing defects. None of the mutations except one has been previously described. There were three point mutations resulting in a single amino acid substitution. One of these missense mutations was observed in a conserved region of the PH domain, the other two were found in the src homology domain 2 that is involved in phosphotyrosyl peptide binding. Two mutations were single base pair substitutions resulting in premature stop codons. In four patients abnormal Btk transcripts were found that were the result of aberrant splicing. One small deletion was observed causing a frameshift and a secondary premature termination signal. Characterisation of the mutations responsible for XLA allowed us to diagnose the disease conclusively and identify the phenotypically normal female carriers.

Haire RN, Strong SJ, Litman GW
Identification and characterization of a homologue of Bruton's tyrosine kinase, a Tec kinase involved in B-cell development, in a modern representative of a phylogenetically ancient vertebrate.
Immunogenetics. 1997; 46: 349-51

Vorechovsky I et al.
Mutation pattern in the Bruton's tyrosine kinase gene in 26 unrelated patients with X-linked agammaglobulinemia.
Hum Mutat. 1997; 9: 418-25
Display abstract
Mutation pattern was characterized in the Bruton's tyrosine kinase gene (BTK) in 26 patients with X-linked agammaglobulinemia, the first described immunoglobulin deficiency, and was related to BTK expression. A total of 24 different mutations were identified. Most BTK mutations were found to result in premature termination of the translation product. Mutations were detected in most BTK exons with a predominance of frameshift and nonsense mutations in the 5' end of the gene and missense mutations in its 3' part, corresponding to the catalytic domain of the enzyme. Nonsense and frameshift mutations were associated with diminished levels of BTK mRNA expression, except for a frameshift mutation in exon 17 and two nonsense mutations in exon 2, indicating that these cases are not confined to penultimate exons. One amino acid substitution (R28H) was found in the pleckstrin homology domain's residue, which is mutated in mice bearing the X-linked immunodeficiency phenotype; another substitution (R307G) was identified in the src homology domain 2. All remaining amino acid substitutions were found in the catalytic domain of Btk.

Haire RN et al.
Unusual patterns of exon skipping in Bruton tyrosine kinase are associated with mutations involving the intron 17 3' splice site.
Am J Hum Genet. 1997; 60: 798-807
Display abstract
Seven individuals with the diagnosis of X-linked agammaglobulinemia were analyzed for mutations in Bruton tyrosine kinase (Btk) gene at both the cDNA transcript and genomic DNA levels. In addition, maternal carrier status was determined in six of the seven families by examining X chromosome-inactivation patterns for B cells in comparison with other types of blood cells. Three categories of mutations were identified: (1) three patients have missense mutations in either the pleckstrin or SH2 domains of Btk; (2) three patients exhibit mutations at or near intron/exon splice sites, two of which represent inherited mutations within the kinase domain; and (3) one patient has inherited a 2.5-kb deletion with the loss of a DNA segment encoding three exons of the kinase domain. Variation in the lengths of Btk transcripts was evident in two patients with splice-site mutations and in the patient with the DNA deletion. Sequences of the different cDNA transcripts from the patients with 3' splice-site mutations reveal complex patterns of exon skipping involving from one to four exons of the kinase domain. These findings implicate 3' splice sites of the penultimate exon in the recognition or processing of upstream exons.

Wahl MI, Fluckiger AC, Kato RM, Park H, Witte ON, Rawlings DJ
Phosphorylation of two regulatory tyrosine residues in the activation of Bruton's tyrosine kinase via alternative receptors.
Proc Natl Acad Sci U S A. 1997; 94: 11526-33
Display abstract
Mutation of Bruton's tyrosine kinase (Btk) impairs B cell maturation and function and results in a clinical phenotype of X-linked agammaglobulinemia. Activation of Btk correlates with an increase in the phosphorylation of two regulatory Btk tyrosine residues. Y551 (site 1) within the Src homology type 1 (SH1) domain is transphosphorylated by the Src family tyrosine kinases. Y223 (site 2) is an autophosphorylation site within the Btk SH3 domain. Polyclonal, phosphopeptide-specific antibodies were developed to evaluate the phosphorylation of Btk sites 1 and 2. Crosslinking of the B cell antigen receptor (BCR) or the mast cell Fcepsilon receptor, or interleukin 5 receptor stimulation each induced rapid phosphorylation at Btk sites 1 and 2 in a tightly coupled manner. Btk molecules were singly and doubly tyrosine-phosphorylated. Phosphorylated Btk comprised only a small fraction (

Brorson K et al.
xid affects events leading to B cell cycle entry.
J Immunol. 1997; 159: 135-43
Display abstract
X-linked agammaglobulinemia patients and X-linked immunodeficient (xid) mice possess mutations in the Bruton's tyrosine kinase (Btk kinase) gene and display defects in B cell development and activation by sIg cross-linking. Btk is an early activation kinase in sIg-cross-linked B cells. xid does not ablate Btk protein kinase activity, and immediate signal transduction events, such as tyrosine phosphorylation, occur in sIg-activated xid B cells. These cells do not subsequently progress into cell division and have a high rate of apoptosis, which has been shown to correlate with an absence of sIg-mediated induction of the bcl-xL protein. To establish the point where Btk activity is critical for progression beyond immediate signaling, we examined early and late events in sIg-cross-linked xid B cells. Induction of proto-oncogenes and nuclear factors occurred normally in xid cells. However, induction of cyclins and increased GAPDH mRNA was not observed in xid cells. Degradation of the cyclin inhibitor p27Kip1 occurred normally in xid cells. After 24 h of culture with anti-mu, the remaining live, nonapoptotic xid cells were enlarged, viable, and primed for subsequent stimulation by LPS. Our data suggest that the Btk kinase is not essential for several G1 events and that the failure of sIg-activated xid B cells to enter cell cycle correlates with a defect of cyclin induction. Moreover, these data suggest that Btk is important not only for immediate events following B cell activation and control of apoptosis but also for subsequent events leading to cyclin activation.

Kornfeld SJ, Haire RN, Litman GW
Primary immunodeficiency disease presenting in midlife.
JAMA. 1996; 276: 1953-4

Kornfeld SJ et al.
A novel mutation (Cys145-->Stop) in Bruton's tyrosine kinase is associated with newly diagnosed X-linked agammaglobulinemia in a 51-year-old male.
Mol Med. 1996; 2: 619-23
Display abstract
BACKGROUND: X-linked agammaglobulinemia (XLA) is a severe, life-threatening disease characterized by failure of B cell differentiation and antibody production and is associated with mutations in Bruton's tyrosine kinase (Btk). The proband in this study is a 51-year-old male presenting with chronic nasal congestion, recurrent sinusitis, sporadic pneumonia, and pronounced B cell deficiency. A family history suggestive of an X-linked immunodeficiency disease was noted. MATERIALS AND METHODS: cDNA was synthesized from mRNA prepared from peripheral blood mononuclear leukocytes. Btk cDNA amplified by polymerase chain reaction (PCR) was subjected to both manual and automated DNA sequencing. A DNA sequence corresponding to exons 6 and 7 of Btk was amplified from genomic DNA. Western blot analysis employed both polyclonal and monoclonal antibodies to Btk and reaction patterns were obtained both by chemiluminescence and an in vitro kinase assay. RESULTS: A mutation (Cys145-->Stop) was identified in Btk cDNA and was confirmed in amplified exon 6 of genomic DNA from both the proband and an affected nephew. Neither Btk nor a truncated peptide was detected in Western blot analyses of peripheral blood mononuclear cell lysates. CONCLUSIONS: The C145A mutation reported here is novel. This family study is extraordinary in that affected male members who did not undergo aggressive medical management either succumbed to complications in early life or survived into later life. The proband is the oldest de novo diagnosed patient with XLA reported to date.

Jones A et al.
X linked agammaglobulinaemia with a 'leaky' phenotype.
Arch Dis Child. 1996; 74: 548-9
Display abstract
Typical X linked agammaglobulinaemia (XLA) is characterised by absence of immunoglobulin production and lack of mature B cells. The gene responsible for XLA has recently been identified, and codes for a B cell tyrosine kinase, BTK. A family affected by a B cell immunodeficiency, which is less severe than classical XLA, is described but they had a pedigree suggestive of X linked inheritance. Demonstration of a mutation in the BTK gene confirms that this is a mild form of XLA.

Schuster V, Seidenspinner S, Kreth HW
Detection of a novel mutation in the SRC homology domain 2 (SH2) of Bruton's tyrosine kinase and direct female carrier evaluation in a family with X-linked agammaglobulinemia.
Am J Med Genet. 1996; 63: 318-22
Display abstract
X-linked agammaglobulinemia (XLA) is an inherited immunodeficiency disease with a block in differentiation from pre-B to B cells resulting in a selective defect in the humoral immune response. Affected males have very low concentrations of serum immunoglobulins leading predominantly to recurrent bacterial infections beginning at age 6 to 18 months. The gene responsible for XLA was identified recently to encode a cytoplasmatic tyrosine kinase (Bruton's tyrosine kinase, BTK). We have analyzed the BTK gene in a large family in which two brothers presented with the severe phenotype of XLA. Genomic DNA of affected boys and from healthy relatives was amplified by PCR with primers specific for the putative promoter region and for all 19 exons, including flanking intron boundaries, and subsequently screened for mutations using single strand conformation polymorphism (SSCP) analysis. Altered single strand band patterns were found using primers specific for exon 10, 15, and 18. Direct cycle-sequencing of these BTK segments detected two known polymorphisms in intron 14 and in exon 18. Sequencing of exon 10 from two boys with XLA demonstrated a novel point mutation in the SH2 domain of BTK. Direct identification of healthy female carriers in three generations was performed by amplification mutagenesis using PCR with a modified first primer. This method can easily be applied also to prenatal diagnosis.

Ollila J, Lappalainen I, Vihinen M
Sequence specificity in CpG mutation hotspots.
FEBS Lett. 1996; 396: 119-22
Display abstract
CpG dinucleotides are efficiently methylated in vertebrate genomes except in the CpG islands having a high C+G content. Methylated CpGs are the single most mutated dinucleotide. Sequences surrounding disease causing CpG mutation sites were analyzed from locus-specific mutation databases. Both tetra- and heptanucleotide analyses indicated clear overall sequence preference for having pyrimidines 5' and purines 3' to the mutated 5-methylcytosine. The most mutated tetranucleotides are TCGA and TCGG, the former being also a frequent restriction and modification site. The results will help in elucidating the still controversial mutation mechanism of CpG doublets.

Yel L et al.
Mutations in the mu heavy-chain gene in patients with agammaglobulinemia.
N Engl J Med. 1996; 335: 1486-93
Display abstract
BACKGROUND: Most patients with congenital hypogammaglobulinemia and absent B cells are males with X-linked agammaglobulinemia, which is caused by mutations in the gene for Bruton's tyrosine kinase (Btk); however, there are females with a similar disorder who do not have mutations in this gene. We studied two families with autosomal recessive defects in B-cell development and patients with presumed X-linked agammaglobulinemia who did not have mutations in Btk. METHODS: A series of candidate genes that encode proteins involved in B-cell signal-transduction pathways were analyzed by linkage studies and mutation screening. RESULTS: Four different mutations were identified in the mu heavy-chain gene on chromosome 14. In one family, there was a homozygous 75-to-100-kb deletion that included D-region genes, J-region genes, and the mu constant-region gene. In a second family, there was a homozygous base-pair substitution in the alternative splice site of the mu heavy-chain gene. This mutation would inhibit production of the membrane form of the mu chain and produce an amino acid substitution in the secreted form. In addition, a patient previously thought to have X-linked agammaglobulinemia was found to have an amino acid substitution on one chromosome at an invariant cysteine that is required for the intrachain disulfide bond and, on the other chromosome, a large deletion that included the immunoglobulin locus. CONCLUSIONS: Defects in the mu heavy-chain gene are a cause of agammaglobulinemia in humans. This implies that an intact membrane-bound mu chain is essential for B-cell development.

Chen YJ, Lin SC, Tzeng SR, Patel HV, Lyu PC, Cheng JW
Stability and folding of the SH3 domain of Bruton's tyrosine kinase.
Proteins. 1996; 26: 465-71
Display abstract
Bruton's tyrosine kinase (BTK) plays an important role in B cell development. Deletion of C-terminal 14 amino acids of the SH3 domain of BTK results in X-linked agammaglobulinemia (XLA), an inherited disease. We report here on the stability and folding of SH3 domain of BTK. Peptides corresponding to residues 216-273 (58 residues) and 216-259 (44 residues) of BTK SH3 domain were synthesized by solid phase methods; the first peptide constitutes the entire SH3 domain of BTK while the latter peptide lacks 14 amino acid residues of the C-terminal. The 58 amino acid peptide forms mainly a beta-barrel type folding unit. Although small and lacking disulfide bonds, this peptide is extremely stable to thermal denaturation. Based on circular dichroism measurements, its melting temperature was found to be high, 82 degrees C at pH 6.0. However, the Gibbs free energy (delta GH2O) of the intrinsic stability and thermodynamic spontaneity of unfolding were found to be low, 2.6 kcal/mol by Gdn.HCl denaturation experiments, as compared to 12 kcal/mol obtained for larger single domain proteins, indicating poor stability of SH3 domain. Addition of 500 mM of Na2SO4 increased the free energy change delta GH2O to 4.0 kcal/mol, suggesting an ionic strength effect. The truncated peptide fails to fold correctly and adopts random coil conformation in contrast to 58 amino acid beta-barrel peptide, which exhibits high thermal stability but normal or low stability at ambient temperature. These results, to our knowledge the first to delineate the importance of C-terminal in structural integrity of SH3 domains, indicate also that improper folding and/or poor stability of mutant SH3 domain in BTK likely causes XLA.

Bykowsky MJ et al.
Discordant phenotype in siblings with X-linked agammaglobulinemia.
Am J Hum Genet. 1996; 58: 477-83
Display abstract
X-linked agammaglobulinemia (XLA) is a congenital humoral immunodeficiency caused by a defect in a B-cell-specific signaling molecule, Btk. There has been little concordance of phenotype with genotype in this disorder, and defects in Btk cause immunodeficiencies that range from mild impairment to complete inability to produce antibodies. The factors modifying the phenotype of XLA are not understood. The current study is the first description of two male siblings with identical T134 --> C mutations in the translation initiation ATG of Btk who have different clinical phenotypes. The proband lacks immunoglobulins and B cells and has recurrent infections, while the elder, affected brother has normal levels of IgG and IgM and very few infections. Both have undetectable levels of Btk kinase activity in circulating mononuclear cells. Complete sequencing of Btk gene transcripts in both brothers revealed no additional mutations to account for the discordant phenotypes. This description provides unequivocal evidence that the phenotype of XLA is influenced by factors additional to the Btk gene.

Hashimoto S et al.
Identification of Bruton's tyrosine kinase (Btk) gene mutations and characterization of the derived proteins in 35 X-linked agammaglobulinemia families: a nationwide study of Btk deficiency in Japan.
Blood. 1996; 88: 561-73
Display abstract
Deficiencies of Bruton's tyrosine kinase (Btk) have been implicated in the pathogenesis of human X-linked agammaglobulinemia (XLA). The distinctive phenotype observed in B-cell deficiency indicates the crucial role of Btk in B-cell development. This report describes a nationwide study of Btk deficiency in Japan, covering 51 XLA patients (35 independent families). Along with the identification of mutations, the resulting protein products were characterized by an in vitro kinase assay and a Western blot analysis. Thirty-one of the families were found to have mutations in the coding region of Btk. Although mutations were not found in the cDNA of 4 families, the Btk transcripts of these patients were greatly reduced. The identification of several novel missense mutations, in combination with the result of other studies, clarified the presence of two (missense) mutation hot spots, one in the SH1 and the other in the PH domain. The absence of kinase activity seen in 32 of the families underscored the importance of Btk protein analysis as a diagnostic indicator of XLA. The protein analysis also clarified the different effects of missense mutations on kinase activity and protein stability.

Vihinen M
BTKbase: XLA-mutation registry.
Immunol Today. 1996; 17: 502-6

Kobayashi S et al.
Mutations of the Btk gene in 12 unrelated families with X-linked agammaglobulinemia in Japan.
Hum Genet. 1996; 97: 424-30
Display abstract
Alterations of the Bruton's tyrosine kinase (Btk) gene are responsible for X-linked agammaglobulinemia (XLA). Although mutations in various regions were reported mainly in the Caucasian population, correlation between the locations of mutation and the clinical phenotypes remains unclear. We report 12 abnormalities of the Btk gene found in 12 unrelated families out of 14 XLA families in Japan and their clinical features. We utilized Southern blotting and single-strand conformation polymorphism (SSCP) analysis. Gene rearrangement in the kinase domain was identified in two patients by Southern blotting. Seven point mutations, two small deletions, and one small insertion were detected by SSCP and sequencing. The SSCP analysis also provided information about the carriers in these families. We found some clinical heterogeneity in the affected family members with the same gene mutation. Moreover, there is considerable inconsistency between the locations of gene aberrations and the immunological phenotypes. Some patients with a nonsense mutation, which may result in the lack of kinase domain, have detectable B cells and immunoglobulins. These identified alterations will provide valuable clues to the Btk protein function and the pathogenesis of XLA.

Farrar JE, Rohrer J, Conley ME
Neutropenia in X-linked agammaglobulinemia.
Clin Immunol Immunopathol. 1996; 81: 271-6
Display abstract
X-linked agammaglobulinemia (XLA) is usually considered a disorder of B cell development; however, the gene that is defective in XLA encodes a cytoplasmic tyrosine kinase called Btk, that is expressed throughout myeloid as well as B cell differentiation. A review of medical records of patients in whom mutations in Btk had been identified indicated that 13 of 50 patients (26%) had experienced episodes of profound neutropenia. In 12 of the 13 patients, neutropenia was part of the acute illness that precipitated an evaluation for immunodeficiency. These boys were more likely to be less than a year of age at the time of diagnosis and they were less likely to have a family history of immunodeficiency. Neutropenia was associated with staphylococcal or pseudomonas sepsis in 6 of the patients. The duration of neutropenia was variable but was often more than 1 week. Neutropenia was not seen in any patient with XLA receiving intravenous gammaglobulin. Although neutropenia was not associated with any specific mutation in Btk, most of the alterations in this gene in the patients with XLA and neutropenia resulted in the absence of Btk protein or in amino acid substitutions in sites thought to be critical to Btk function. Btk may not be required for neutrophil production under normal circumstances; however, it may play a role in the response to stress.

Ochs HD, Smith CI
X-linked agammaglobulinemia. A clinical and molecular analysis.
Medicine (Baltimore). 1996; 75: 287-99
Display abstract
X-linked agammaglobulinemia (XLA), characterized by a profound deficiency of B lymphocytes due to an arrest in B lymphocyte development, is caused by mutations in the gene encoding Btk (Bruton tyrosine kinase). The BTK gene has been cloned and the genomic organization determined. BTK codes for 19 exons and is expressed in all hematopoietic cell lineages but is selectively down-regulated in T lymphocytes and plasma cells. The different Btk domains include PH, TH, SH3, SH2, and the kinase (SH1) domains. Btk, a cytoplasmic protein tyrosine kinase, is involved in cell signaling, although the precise pathway remains elusive. Mutation analysis has been performed in 236 families representing 282 patients. Mutations are scattered throughout the gene and consist of missense, nonsense, and splice site mutations as well as deletions and insertions. The major consequence of nonfunctional Btk appears to be a delay or block of the development of pro-B cells to pre-B cells and then to mature lymphocytes. Because IgG is actively transported across the placenta, affected newborns have normal levels of serum IgG at birth followed by gradually decreasing IgG levels and development of hypogammaglobulinemia and increased susceptibility to infections. Bacterial infections are the most common clinical manifestation. Resistance to viral infection is intact, except for an unusual susceptibility to infections with enteroviruses that may result in vaccine-related paralytic poliomyelitis or a dermatomyositis-meningoencephalitis syndrome. The diagnosis of XLA is based on the presence of lymphoid hypoplasia, markedly reduced serum levels of all 3 major classes of immunoglobulins, failure to make antibody to antigenic stimulation, and almost complete absence of B lymphocytes in the peripheral blood. Carrier detection and prenatal diagnosis are possible. The prophylactic infusion of high-dose intravenous immunoglobulin (IVIG) and the use of antibiotics have markedly improved the long-term prognosis of patients with XLA.

Mattsson PT, Vihinen M, Smith CI
X-linked agammaglobulinemia (XLA): a genetic tyrosine kinase (Btk) disease.
Bioessays. 1996; 18: 825-34
Display abstract
X-linked agammaglobulinemia is a heritable immunodeficiency disease caused by a differentiation abnormality, resulting in the virtual absence of B lymphocytes and plasma cells. The affected gene encodes a cytoplasmic protein tyrosine kinase, Bruton's agammaglobulinemia tyrosine kinase, designated Btk. Btk and the other family members, Tec, ltk and Bmx, contain five regions, four of which are common structural and functional modules that are found in other signaling proteins. Mutations affect all domains of the gene, but amino acid substitutions seem to be confined to certain regions. More than 150 unique mutations have been identified and are collected in a mutation database, BTKbase. Here we discuss the three-dimensional structural implications of such mutations and their putative functional role. Of special interest are mutations affecting the pleckstrin homology domain, as Btk is the only disease-associated protein so far reported to carry mutations in this particular module.

Maniar HS, Vihinen M, Webster AD, Nilsson L, Smith CI
Structural basis for X-linked agammaglobulinemia (XLA): mutations at interacting Btk residues R562, W563, and A582.
Clin Immunol Immunopathol. 1995; 76: 198202-198202
Display abstract
It has been suggested that tryptophan 563 is sandwiched between residues R562 and A582 in Bruton's agammaglobulinemia tyrosine kinase (Btk). Mutations of the surrounding residues have been shown to cause X-linked agammaglobulinemia. Substitutions R562P and A582V were noticed to have impaired kinase activity. However, based on Western blot analysis, the mutant proteins were expressed at normal levels. Molecular modeling of the kinase domain has previously indicated that these residues presumably govern the position of the W563 side chain, which is thought to interact with the catalytic loop. W563 is inside the molecule and too far away from the catalytic center to interact directly with the substrate or cofactors. To prove these model-based conclusions, a conservative substitution with phenylalanine for W563 was made, and the resultant mutant lacked kinase activity. These results confirm our previous assumption that the side chain of W563, invariant in protein tyrosine kinases, is crucial for Btk kinase activity. Mutations in the surrounding residues seem to inactivate Btk by affecting the location of W563.

Hagemann TL, Rosen FS, Kwan SP
Characterization of germline mutations of the gene encoding Bruton's tyrosine kinase in families with X-linked agammaglobulinemia.
Hum Mutat. 1995; 5: 296-302
Display abstract
Bruton's tyrosine kinase (Btk) has been identified as the protein responsible for the primary immunodeficiency X-linked agammaglobulinemia (XLA) and has been described as a new member of Src-related cytoplasmic protein tyrosine kinases. We have recently characterized the structure of the entire gene encoding Btk and developed a polymerase chain reaction (PCR)-based assay to detect germline mutations within it. In this report we describe six mutations, five of which are novel, of the Btk gene in patients with XLA and demonstrate the inheritance pattern of the defect within the families of the affected individuals. The mutations found include two nonsense and two missense mutations, a single base deletion at an intron acceptor splice site, and a 16-bp insertion. A single strand conformation polymorphism was also found in the 5' end of intron 8 with the same assay. This technique has provided a powerful tool for direct analysis of the Btk gene for the diagnosis of XLA and carrier detection. The identification of new mutations may eventually reveal the role of Btk in the signaling pathways involved in B-cell development.

Rawlings DJ, Witte ON
The Btk subfamily of cytoplasmic tyrosine kinases: structure, regulation and function.
Semin Immunol. 1995; 7: 237-46
Display abstract
The genetic defect associated with two closely related primary immunodeficiencies was recently identified as a deficiency of function of a new cytoplasmic tyrosine kinase, Bruton's tyrosine kinase (Btk). Btk and related genes expressed primarily in hematopoietic cells (Itk, Tec, Drsrc28C and Txk) comprise a new subfamily of cytoplasmic tyrosine kinases. These proteins share significant structural and sequence homology including an amino-terminal pleckstrin homology (PH) domain not present in other cytoplasmic tyrosine kinase subfamilies. This domain plays an essential role in regulation and function of the Btk subfamily proteins. Genetic evidence supports a critical role for Btk in B-lineage development. Additional studies demonstrate activation of these proteins in multiple hematopoietic signaling pathways including the B cell antigen receptor, several cytokine receptors, and a potential novel role in heterotrimeric G protein associated receptor signaling.

Ohashi Y, Tsuchiya S, Konno T
A new point mutation involving a highly conserved leucine in the Btk SH2 domain in a family with X linked agammaglobulinaemia.
J Med Genet. 1995; 32: 77-8

Vorechovsky I, Luo L, de Saint Basile G, Hammarstrom L, Webster AD, Smith CI
Improved oligonucleotide primer set for molecular diagnosis of X-linked agammaglobulinaemia: predominance of amino acid substitutions in the catalytic domain of Bruton's tyrosine kinase.
Hum Mol Genet. 1995; 4: 2403-5

Gaspar HB et al.
Mutation analysis in Bruton's tyrosine kinase, the X-linked agammaglobulinaemia gene, including identification of an insertional hotspot.
Hum Mol Genet. 1995; 4: 755-7

Vihinen M et al.
BTKbase: a database of XLA-causing mutations. International Study Group.
Immunol Today. 1995; 16: 460-5

Jin H et al.
Identification of Btk mutations in 20 unrelated patients with X-linked agammaglobulinaemia (XLA).
Hum Mol Genet. 1995; 4: 693-700
Display abstract
X-linked agammaglobulinaemia (XLA) is an inherited immunodeficiency resulting from mutations in the gene for a cytoplasmic protein tyrosine kinase (Btk). We have utilised reverse-transcription-based PCR in combination with the chemical cleavage and mismatch technique (CCM) to screen for Btk mutations in 42 unrelated patients having classical XLA or 'leaky' XLA-like phenotypes. A variety of mutations, including point mutations, large deletions and splicing defects were detected using this strategy. In total, 20 mutations were found in these patients. All the mutations were different with the exception of three unrelated patients who all showed the same Arg-->His amino acid substitution (R641H) at a highly-conserved residue in the kinase domain. We have also used structural modelling of the Btk kinase domain to predict how two different amino acid substitution mutations at highly-conserved residues are likely to affect the Btk kinase activity.

Oeltjen JC et al.
Sixty-nine kilobases of contiguous human genomic sequence containing the alpha-galactosidase A and Bruton's tyrosine kinase loci.
Mamm Genome. 1995; 6: 334-8
Display abstract
Several disease loci have been mapped to the Xq21.3-Xq22 region of the human X Chromosome (Chr) including X-linked agammaglobulinemia (XLA), Fabry disease, Alport syndrome, and Pelizaeus Merzbacher disease. Upon cloning of the XLA gene, Bruton's tyrosine kinase (btk), both Fabry disease and XLA were mapped within the same 50- to 70-kb interval. In order to investigate the genomic organization of the region surrounding btk and the Fabry disease gene, alpha-galactosidase A (gla), we constructed a 6-cosmid contig spanning the region from 5' of gla to 3' of btk. Two of these cosmids spanning most of the coding sequence and the upstream region of btk and gla, U237D10 and U230D1, were sequenced by a random shotgun strategy combined with automated sequencing, resulting in 69 kb of contiguous genomic sequence. Sequencing of U237D10 showed btk to be comprised of 19 exons spanning over 35 kb. Sequencing of U230D1 showed that the 3' end of gla is 9 kb from the 5' end of btk and also demonstrated the presence of two additional genes in the region immediately 5' to btk. The surprisingly high gene density is similar to that seen previously only in the human major histocompatibility locus.

Li T et al.
Activation of Bruton's tyrosine kinase (BTK) by a point mutation in its pleckstrin homology (PH) domain.
Immunity. 1995; 2: 451-60
Display abstract
Bruton's tyrosine kinase (BTK) is a nonreceptor tyrosine kinase critical for B cell development and function. Mutations in BTK result in X-linked agammaglobulinemia (XLA) in humans and X-linked immunodeficiency (xid) in mice. Using a random mutagenesis scheme, we isolated a gain-of-function mutant called BTK* whose expression drives growth of NIH 3T3 cells in soft agar. BTK* results from a single point mutation in the pleckstrin homology (PH) domain, where a Glu is replaced by Lys at residue 41. BTK* shows an increase in phosphorylation on tyrosine residues and an increase in membrane targeting. Transforming activity requires kinase activity, a putative autophosphorylation site, and a functional PH domain. Mutation of the SH2 or SH3 domains did not affect the activity of BTK*. Expression of BTK* could also relieve IL-5 dependence of a B lineage cell line. These results show that transformation activation and regulation of BTK are critically dependent on the PH domain.

Conley ME, Rohrer J
The spectrum of mutations in Btk that cause X-linked agammaglobulinemia.
Clin Immunol Immunopathol. 1995; 76: 1927-1927

Stewart DM, Notarangelo LD, Kurman CC, Staudt LM, Nelson DL
Molecular genetic analysis of X-linked hypogammaglobulinemia and isolated growth hormone deficiency.
J Immunol. 1995; 155: 2770-4
Display abstract
In 1980 the clinical syndrome of X-linked hypogammaglobulinemia and isolated growth hormone deficiency (XLA/GHD) was described. XLA/GHD patients have reduced serum levels of Ig and normal cell-mediated immunity, and thus resemble patients with Bruton's X-linked agammaglobulinemia (XLA). However, XLA/GHD patients also have isolated GHD. Mutations and deletions in the Bruton's tyrosine kinase gene (BTK) are responsible for Bruton's XLA. We investigated BTK gene expression in an XLA/GHD patient from the family originally described by Northern analysis, cDNA sequencing, and Western analysis of protein production using mAb to BTK. BTK mRNA was normal in size and abundance, and the mRNA sequence was normal over the coding region, except for a single silent mutation. BTK protein was present in normal amounts in PBMC of this patient. Thus, at the molecular level, XLA/GHD is a different disease entity from Bruton's XLA. These results suggest that undescribed genes critical for B cell development and growth hormone production exist on the X chromosome.

Stewart DM, Kurman CC, Nelson DL
Production of monoclonal antibodies to Bruton's tyrosine kinase.
Hybridoma. 1995; 14: 243-6
Display abstract
Bruton's X-linked agammaglobulinemia is caused by mutations in a cytoplasmic protein tyrosine kinase termed Bruton's tyrosine kinase (BTK). The protein is expressed in all members of the B cell lineage and is critical for B cell development. The protein consists of several modules, including a pleckstrin homology domain and the Src homology domains SH1, SH2, and SH3. We report here the production of monoclonal antibodies against the pleckstrin homology domain of human BTK. The antibody was produced by immunizing mice with a FLAG-BTK fusion protein. Hybridoma supernatants were screened by ELISA using a GST-BTK fusion protein as the antigen. Selected monoclonal antibodies recognize denatured BTK on Western blots of peripheral blood mononuclear cell lysates. Mouse BTK protein is also detected. These antibodies should be useful in assessing patients with immune deficiency, as well as in studying normal B cell development.

Vorechovsky I et al.
DNA-based mutation analysis of Bruton's tyrosine kinase gene in patients with X-linked agammaglobulinaemia.
Hum Mol Genet. 1995; 4: 51-8
Display abstract
The identification of the BTK (Bruton's tyrosine kinase) gene defective in human immunoglobulin deficiency X-linked agammaglobulinaemia (XLA) and characterisation of BTK exon-intron boundaries has now allowed the analysis of mutations and polymorphisms at the level of genomic DNA. Using Southern blot analysis and the polymerase chain reaction single strand conformation polymorphism (PCR-SSCP) assay, amplifying all 19 exons and the putative promoter region with a single annealling temperature, mutations have been identified in 19 out of 24 unrelated patients diagnosed as having XLA. Apart from a large deletion involving exon 19, nine missense (F25S, R288W, 1370M, M509V, R525P, N526K, R562W, A582V and G594R), two nonsense (E277X and R525X), five frameshift and two splice site mutations have been found affecting most coding exons and all major enzyme domains. No mutations or polymorphisms were detected in the putative promoter region. A single nucleotide deletion located in the last exon, resulting in a truncation of the eight C-terminal residues of Btk and a typical XLA phenotype, indicates structural and/or functional importance of Btk helix I in the catalytic domain. Although allelic heterogeneity at the BTK locus may partly explain clinical variability in families with XLA, compensatory and redundant mechanisms involved in B-cell development must play a role in the phenotypic diversity of the disease.

Buckley RH
Assessing inheritance of agammaglobulinemia.
N Engl J Med. 1994; 330: 1526-8

de Weers M, Mensink RG, Kraakman ME, Schuurman RK, Hendriks RW
Mutation analysis of the Bruton's tyrosine kinase gene in X-linked agammaglobulinemia: identification of a mutation which affects the same codon as is altered in immunodeficient xid mice.
Hum Mol Genet. 1994; 3: 161-6
Display abstract
X-linked agammaglobulinemia (XLA) is an inherited immunodeficiency disease in man, reflecting an arrest in differentiation of pre-B cells to mature B cell stages. The gene defective in XLA has been identified as a cytoplasmic protein tyrosine kinase, named btk (Bruton's tyrosine kinase). Here we report the characterization of mutations in the btk gene of five unrelated XLA families. Amplified products were generated from cDNA, cloned and sequenced. Three single point mutations and two small insertions were identified. One of the point mutations and the two insertions created stop codons that would lead to truncated btk proteins. In one XLA patient we found a single basepair substitution that altered the highly conserved Arg288 within the SH2 domain and would therefore abrogate interactions with substrate phosphotyrosines. In another XLA patient a single basepair substitution was observed that altered the conserved Arg28 residue in the N-terminal unique region of unknown function. This residue is also mutated in the xid mouse, which has a different, less severe, B cell deficiency. We conclude that a similar mutation in the btk gene leads in man to an almost complete arrest at an early stage of B cell differentiation, but in the mouse to only limited B cell abnormalities.

Bradley LA et al.
Mutation detection in the X-linked agammaglobulinemia gene, BTK, using single strand conformation polymorphism analysis.
Hum Mol Genet. 1994; 3: 79-83
Display abstract
The gene defective in X-linked agammaglobulinemia (XLA) has recently been isolated and identified as btk, a non-receptor protein tyrosine kinase. We have utilized the technique of single strand conformation polymorphism (SSCP) analysis for the btk gene to identify mutations in XLA patients. The btk gene in affected boys from 10 families was analysed and mutations were identified in eight cases; seven of these were point mutations and one was a small insertion. The mutations were found throughout the gene coding region. Six of the patients have classical XLA and two have less severe forms of the disease. We have also identified a polymorphism at nucleotide position 2031. This technique will allow us to provide more accurate diagnoses of the disease and to determine the nature of the functional defects in the btk gene in these families.

Tsukada S, Rawlings DJ, Witte ON
Role of Bruton's tyrosine kinase in immunodeficiency.
Curr Opin Immunol. 1994; 6: 623-30
Display abstract
The genetic defect associated with human X-linked agammaglobulinemia and murine X-linked immunodeficiency was recently shown to result from lack of function of a new cytoplasmic tyrosine kinase, called Bruton's tyrosine kinase (Btk). The phenotypes associated with these immunodeficiencies indicate that Btk plays a critical role in B-lymphocyte development. The distinctive protein structure of Btk and preliminary functional studies suggest that Btk may act in a novel manner in a variety of signaling pathways.

Zhu Q, Zhang M, Winkelstein J, Chen SH, Ochs HD
Unique mutations of Bruton's tyrosine kinase in fourteen unrelated X-linked agammaglobulinemia families.
Hum Mol Genet. 1994; 3: 1899-900

Conley ME, Fitch-Hilgenberg ME, Cleveland JL, Parolini O, Rohrer J
Screening of genomic DNA to identify mutations in the gene for Bruton's tyrosine kinase.
Hum Mol Genet. 1994; 3: 1751-6
Display abstract
Mutations in the gene for Bruton's tyrosine kinase (Btk) are responsible for X-linked agammaglobulinemia (XLA). Thus far, mutations in this gene have been identified based on alterations in Southern or Northern blot analysis or cDNA sequence. To permit detection of mutations in genomic DNA, we designed PCR primers to flank each of the 19 exons of Btk with splice sites. Two overlapping PCR products were employed for exons longer than 230 base pairs. Single strand conformation polymorphism (SSCP) analysis was used to screen PCR products from 30 unrelated families presumed to carry a Btk mutation. It was possible to amplify DNA in every reaction from every patient, indicating that large deletions in Btk are uncommon. Twenty three different mutations were found in 25 unrelated families, including one family in whom DNA was available from a carrier but not an affected patient. Seven mutations were single base pair substitutions resulting in premature stop codons scattered throughout the gene. Small insertions or deletions causing frameshifts and secondary premature stop codons constituted an additional seven mutations. One patient had a point mutation in the start codon and one patient had a mutation in a splice donor site. Point mutations resulting in amino acid substitutions were seen in nine patients. Northern blot analysis of RNA from three patients with premature stop codons showed an absence of Btk transcript whereas four patients with amino acid substitutions had normal amounts of transcript of normal size. These studies document the considerable variability in the Btk mutations causing XLA and they demonstrate an approach that will be useful for carrier detection as well as mutation identification.

Hagemann TL, Chen Y, Rosen FS, Kwan SP
Genomic organization of the Btk gene and exon scanning for mutations in patients with X-linked agammaglobulinemia.
Hum Mol Genet. 1994; 3: 1743-9
Display abstract
The defective gene responsible for the recessively inherited immunodeficiency X-linked agammaglobulinemia (XLA) has been shown to encode a cytoplasmic protein tyrosine kinase of the Src family designated Btk (Bruton's tyrosine kinase). To facilitate the search for germline mutations of the Btk gene, we have characterized its genomic structure. Eighteen introns were positioned within the approximately 37 kb gene. Each of the exon/intron boundaries were defined and sequenced, and all but two conform to consensus sequences. We have utilized the genomic organization of Btk and the intervening sequence data to design an assay for amplifying each of the 19 exons from XLA patient DNA for single strand conformation polymorphism (SSCP) analysis. By using this method we have identified mutations in 12 of 14 unrelated affected males: seven different base substitutions and two small deletions. Two of the mutations described in exon 15 of the kinase domain were found in more than one patient and may represent a mutation hot spot. Exon scanning has proven to be a valuable method for identifying the patient mutations in genomic DNA without the use of cDNA. The mutations are easily confirmed with direct sequencing of the amplified exons. This approach will greatly benefit XLA family studies involving carrier detection and prenatal diagnosis. In addition, the mutations identified may reveal residues involved in the specific protein interactions necessary in the B-cell developmental pathway, of which Btk is an integral component.

Rohrer J, Parolini O, Belmont JW, Conley ME, Parolino O [corrected to Parolini O]
The genomic structure of human BTK, the defective gene in X-linked agammaglobulinemia.
Immunogenetics. 1994; 40: 319-24
Display abstract
It has recently been demonstrated that mutations in the gene for Bruton's tyrosine kinase (BTK) are responsible for X-linked agammaglobulinemia. Southern blot analysis and sequencing of cDNA were used to document deletions, insertions, and single base pair substitutions. To facilitate analysis of BTK regulation and to permit the development of assays that could be used to screen genomic DNA for mutations in BTK, we determined the genomic organization of this gene. Subcloning of a cosmid and a yeast artificial chromosome showed that BTK is divided into 19 exons spanning 37 kilobases of genomic DNA. Analysis of the region 5' to the first untranslated exon revealed no consensus TATAA or CAAT boxes; however, three retinoic acid binding sites were identified in this region. Comparison of the structure of BTK with that of other nonreceptor tyrosine kinases, including SRC, FES, and CSK, demonstrated a lack of conservation of exon borders. Information obtained in this study will contribute to our understanding of the evolution of nonreceptor tyrosine kinases. It will also be useful in diagnostic studies, including carrier detection, and in studies directed towards gene therapy or gene replacement.

Zhu Q et al.
Deletion within the Src homology domain 3 of Bruton's tyrosine kinase resulting in X-linked agammaglobulinemia (XLA).
J Exp Med. 1994; 180: 461-70
Display abstract
The gene responsible for X-linked agammaglobulinemia (XLA) has been recently identified to code for a cytoplasmic tyrosine kinase (Bruton's agammaglobulinemia tyrosine kinase, BTK), required for normal B cell development. BTK, like many other cytoplasmic tyrosine kinases, contains Src homology domains (SH2 and SH3), and catalytic kinase domain. SH3 domains are important for the targeting of signaling molecules to specific subcellular locations. We have identified a family with XLA whose affected members have a point mutation (g-->a) at the 5' splice site of intron 8, resulting in the skipping of coding exon 8 and loss of 21 amino acids forming the COOH-terminal portion of the BTK SH3 domain. The study of three generations within this kinship, using restriction fragment length polymorphism and DNA analysis, allowed identification of the mutant X chromosome responsible for XLA and the carrier status in this family. BTK mRNA was present in normal amounts in Epstein-Barr virus-induced B lymphoblastoid cell lines established from affected family members. Although the SH3 deletion did not alter BTK protein stability and kinase activity of the truncated BTK protein was normal, the affected patients nevertheless have a severe B cell defect characteristic for XLA. The mutant protein was modeled using the normal BTK SH3 domain. The deletion results in loss of two COOH-terminal beta strands containing several residues critical for the formation of the putative SH3 ligand-binding pocket. We predict that, as a result, one or more crucial SH3 binding proteins fail to interact with BTK, interrupting the cytoplasmic signal transduction process required for B cell differentiation.

Sideras P et al.
Genomic organization of mouse and human Bruton's agammaglobulinemia tyrosine kinase (Btk) loci.
J Immunol. 1994; 153: 5607-17
Display abstract
Btk is a cytoplasmic protein tyrosine kinase (PTK) that has been directly implicated in the pathogenesis of X-linked agammaglobulinaemia (XLA) in humans and X-linked immunodeficiency (Xid) in mice. We have isolated phage and cosmid clones that allowed us to deduce the genomic structure of mouse and human Btk loci. The mouse and human genes are contained within genomic regions that span approximately 43.5 kb and 37.5 kb, respectively. Both loci contain 18 coding exons ranging between 55 and 560 bp in size with introns ranging in size from 164 bp to approximately 9 kb. The 5'-untranslated regions are encoded by single exons located approximately 9 kb upstream of the first coding exon. Exon 18 encodes for the last 23 carboxyl-terminal amino acids and the entire 3'-untranslated region. The location of intron/exon boundaries in the catalytic domains of the mouse and human Btk loci differs from that found in other described sub-families of intracellular PTKs, namely that of Src, Fes/Fer, Csk, and Abl/Arg. This observation is consistent with the classification of Btk together with the recently characterized kinases, Tec and Itk, into a separate sub-family of cytoplasmic PTKs. Putative transcription initiation sites in the mouse and human Btk loci have been determined by using the rapid amplification of cDNA ends assay. Similar to many other PTK specific genes, the putative Btk promoters lack obvious TATAA and CAAAT motifs. Putative initiator elements and potential binding sites for Ets (PEA-3), zeste, and PuF transcription factors are located within the 300 bp which are located upstream of the major transcription start site in both species. These sequences can mediate promoter activity when placed upstream of a promotorless chloramphenicol acetyl transferase reporter gene in an orientation-dependent manner. The present analysis will significantly facilitate the mutational analyses of patients with XLA and the further characterization of the function and regulation of the Btk molecule.

Ohta Y et al.
Genomic organization and structure of Bruton agammaglobulinemia tyrosine kinase: localization of mutations associated with varied clinical presentations and course in X chromosome-linked agammaglobulinemia.
Proc Natl Acad Sci U S A. 1994; 91: 9062-6
Display abstract
X chromosome-linked agammaglobulinemia is a life-threatening disease that involves a failure in normal development of B lymphocytes and is associated with missense mutations in BTK, a gene encoding a cytoplasmic tyrosine kinase (Bruton agammaglobulinemia tyrosine kinase, EC 2.7.1.112), a member of the Tec family of protein-tyrosine kinases. The genomic organization has been determined by using conventional restriction fragment mapping, extended DNA sequencing, and PCR fragment-sizing approaches. The DNA sequences of the 18 coding exons composing BTK and their flanking-region sequences are reported; an additional exon(s) encodes a 5' untranslated segment. Single-base-pair substitutions and 4-nt deletions resulted in amino acid replacement, premature termination, frameshift, and exon deletion in a group of X chromosome-linked agammaglobulinemia patients exhibiting different clinical presentations and courses. The nature of the mutations is interpreted in terms of the genomic organization of the BTK gene and the disease course in individual patients. Several examples are found in which the same mutation occurs in unrelated patients, and one of these mutations occurs at the same codon that is substituted in the murine form of BTK, resulting in X chromosome-linked immunodeficiency disease. Considerable variation in presentation and disease course in X chromosome-linked agammaglobulinemia appears associated with the nature and position of different missense mutations.

Katz FE et al.
Expression of the X-linked agammaglobulinemia gene, btk in B-cell acute lymphoblastic leukemia.
Leukemia. 1994; 8: 574-7
Display abstract
The gene which causes X-linked agammaglobulinemia, btk, has recently been identified as a cytoplasmic tyrosine kinase expressed almost exclusively in B cells, and at all stages of B-cell differentiation. To assess the possibility of involvement of this gene in childhood B-cell malignancies, cells from 23 pediatric patients with B-cell acute lymphoblastic leukemia were examined for expression and alteration of the Btk protein and also for mutations in the btk gene. Btk proteins, similar in both molecular weight and quantity to those seen in unaffected individuals, were detected in whole cell lysates from the blasts of 12/12 patients indicating that no abnormal protein was present. cDNAs from the leukemic blasts of all 23 patients were screened with specific primers covering the coding region of the btk cDNA for mutations using single strand conformation polymorphism (SSCP) analysis. No mutations were found but a nucleotide polymorphism was identified in 4/23 patients at the 3' end of btk. Although the sample size in this study was relatively small, these data suggest that btk does not appear to play a critical role in childhood B-cell leukemias.

Tamagnone L et al.
BMX, a novel nonreceptor tyrosine kinase gene of the BTK/ITK/TEC/TXK family located in chromosome Xp22.2.
Oncogene. 1994; 9: 3683-8
Display abstract
The Bmx sequence was identified and cloned during our search for novel tyrosine kinase genes expressed in human bone marrow cells. Bmx cDNA comprises a long open reading frame of 675 amino acids, containing one SH3, one SH2 and one tyrosine kinase domain, which are about 70% identical with Btk, Itk and Tec and somewhat less with Txk tyrosine kinase sequences. The amino terminal sequences of these four tyrosine kinases are about 40% identical and each contains a so-called pleckstrin homology domain. The 2.7 kb Bmx mRNA was expressed in endothelial cells and several human tissues by Northern blotting and an 80 kD Bmx polypeptide was detected in human endothelial cells. Immunoprecipitates of COS cells transfected with a Bmx expression vector and NIH3T3 cells expressing a Bmx retrovirus contained a tyrosyl phosphorylated Bmx polypeptide of similar molecular weight. The BMX gene was located in chromosomal band Xp22.2 between the DXS197 and DXS207 loci. Interestingly, chromosome X also contains the closest relative of BMX, the BTK gene, implicated in X-linked agammaglobulinemia. The BMX gene thus encodes a novel nonreceptor tyrosine kinase, which may play a role in the growth and differentiation of hematopoietic cells.

Tsukada S, Witte ON
X-linked agammaglobulinemia and Bruton's tyrosine kinase.
Adv Exp Med Biol. 1994; 365: 233-8

Conley ME, Parolini O, Rohrer J, Campana D
X-linked agammaglobulinemia: new approaches to old questions based on the identification of the defective gene.
Immunol Rev. 1994; 138: 5-21
Display abstract
The identification of a cytoplasmic tyrosine kinase, Btk, as the defective protein in human XLA and xid in the mouse, supports the hypothesis that both disorders are due to defects in B-cell activation or differentiation. Phenotypic analysis of B-lineage cells and studies on X-chromosome inactivation patterns in both mice and human patients suggest that mutations in Bth do not affect entry of stem cells into the B-lineage pathway but they do inhibit progression at multiple steps along that pathway. Although the exact function of Btk in signal transduction is not yet known, it is probable that studies which correlate specific mutations in different patients with alterations in Btk function will provide clues about critical sites in the molecule. Diagnosis and genetic counseling for families at risk of carrying the gene for XLA will be improved almost immediately by the identification of the responsible gene. Improvements in therapy may come more slowly. The possibility of curative gene therapy is attractive; however, there are several features of Btk that suggest that this will be a challenging undertaking. Overexpression or expression in inappropriate cell lineages may carry unacceptable risks. Mutant proteins may interfere with the function of wild-type proteins provided by gene therapy. However, it is likely that a better understanding of Btk function and regulation will benefit not only patients with XLA but also other patients with defects in B-cell function.

Smith CI et al.
Expression of Bruton's agammaglobulinemia tyrosine kinase gene, BTK, is selectively down-regulated in T lymphocytes and plasma cells.
J Immunol. 1994; 152: 557-65
Display abstract
The gene mutated in the human disease, X-linked agammaglobulinemia (XLA), is related to the Src gene family of cytoplasmic protein-tyrosine kinases and is designated Btk (Bruton's agammaglobulinemia tyrosine kinase; formerly Atk/Bpk; the human gene is denoted BTK, using capital letters according to the kinase nomenclature). We have recently reported that this gene is expressed in B lymphocytes and that the specific mRNA was undetectable in T cells using Northern blotting. Further analyses of different sources of B and T lymphocytes confirmed this pattern. However, BTK transcripts were undetectable in four plasmacytoma lines. Moreover, as virtually normal amounts of BTK transcripts were found in PBMC from two patients carrying a point mutation in BTK, despite low B cell numbers, we anticipated that the gene would also be expressed in cells of other lineages. The erythroleukemia cell line K-562, the promyelocytic line HL-60 and the histiocytic lymphoma line U-937 were found to have BTK mRNA levels comparable to B cells. BTK mRNA was also detected in monocytes from healthy donors as well as in the human immature basophilic cell line KU812, in the human mast cell leukemia cell line HMC-1 and in the CD34 expressing myeloblast KG-1. A similar expression pattern was obtained when BTK protein was analyzed by immunoprecipitation and Western blotting. Using a polymerase chain reaction-based analysis, a small amount (less than 1% of the level in B cells) of BTK mRNA was identified in T lymphocytes. Our findings are compatible with a general expression of the BTK gene in hematopoietic cells, except in T lymphocytes and plasma cells, in which the transcript level is selectively down-regulated.

Kornfeld SJ, Good RA, Litman GW
Atypical X-linked agammaglobulinemia.
N Engl J Med. 1994; 331: 949-51

Saffran DC et al.
Brief report: a point mutation in the SH2 domain of Bruton's tyrosine kinase in atypical X-linked agammaglobulinemia.
N Engl J Med. 1994; 330: 1488-91

Duriez B, Duquesnoy P, Dastot F, Bougneres P, Amselem S, Goossens M
An exon-skipping mutation in the btk gene of a patient with X-linked agammaglobulinemia and isolated growth hormone deficiency.
FEBS Lett. 1994; 346: 165-70
Display abstract
X-linked agammaglobulinemia (XLA) is an inherited immunodeficiency disease associated with a block in differentiation from pre-B to B cells. The XLA gene encodes a 659 amino acids cytoplasmic protein tyrosine kinase named btk (Bruton's tyrosine kinase). The few btk gene alterations so far reported in XLA patients are heterogenous and distributed in all domains of the btk protein. They appear to be responsible for a range of B cell immunodeficiency disorders of variable severity. Rare families in which XLA is inherited together with isolated growth hormone deficiency (IGHD) have been reported. Genetic analysis has shown that this disease association maps to the same region of the X chromosome as XLA, but whether the two phenotypes are caused by a common or different developmental or biochemical mechanism is unknown. We have analyzed the btk gene of a patient with XLA and IGHD. RT-PCR analysis of btk transcripts, sequencing data obtained from cDNA and genomic DNA and in vitro splicing assays showed that an intronic point mutation (1882 + 5G-->A) is responsible for skipping of an exon located in the tyrosine kinase domain. This exon-skipping event results in a frameshift leading to a premature stop codon 14 amino acids downstream, and in the loss of the last 61 residues of the carboxy-terminal end of the protein. Although we studied a sporadic case, the results suggest that an alteration of the btk gene might cause this unusual phenotype.

Vetrie D et al.
The gene involved in X-linked agammaglobulinaemia is a member of the src family of protein-tyrosine kinases.
Nature. 1993; 361: 226-33
Display abstract
X-linked agammaglobulinaemia (XLA) is a human immunodeficiency caused by failure of pre-B cells in the bone marrow to develop into circulating mature B cells. A novel gene has been isolated which maps to the XLA locus, is expressed in B cells, and shows mutations in families with the disorder. The gene is a member of the src family of proto-oncogenes which encode protein-tyrosine kinases. This is, to our knowledge, the first evidence that mutations in a src-related gene are involved in human genetic disease.

Kinnon C, Hinshelwood S, Levinsky RJ, Lovering RC
X-linked agammaglobulinemia--gene cloning and future prospects.
Immunol Today. 1993; 14: 554-8
Display abstract
The btk gene has recently been identified as the causative gene in X-linked agammaglobulinemia (XLA). This has opened up many new possibilities for the treatment of this B-cell immunodeficiency. Christine Kinnon and colleagues review the high degree of sequence of homology of btk to the non-receptor tyrosine kinases and speculate on putative roles for this gene in B-cell development.

Thomas JD, Sideras P, Smith CI, Vorechovsky I, Chapman V, Paul WE
Colocalization of X-linked agammaglobulinemia and X-linked immunodeficiency genes.
Science. 1993; 261: 355-8
Display abstract
Mice that bear the X-linked immunodeficiency (xid) mutation have a B lymphocyte-specific defect resulting in an inability to make antibody responses to polysaccharide antigens. A backcross of 1114 progeny revealed the colocalization of xid with Bruton's agammaglobulinemia tyrosine kinase (btk) gene, which is implicated in the human immune deficiency, X-linked agammaglobulinemia. Mice that carry xid have a missense mutation that alters a highly conserved arginine near the amino-terminus of the btk protein, Btk. Because this region of Btk lies outside any obvious kinase domain, the xid mutation may define another aspect of tyrosine kinase function.

Tsukada S et al.
Deficient expression of a B cell cytoplasmic tyrosine kinase in human X-linked agammaglobulinemia.
Cell. 1993; 72: 279-90
Display abstract
We describe a novel cytoplasmic tyrosine kinase, termed BPK (B cell progenitor kinase), which is expressed in all stages of the B lineage and in myeloid cells. BPK has classic SH1, SH2, and SH3 domains, but lacks myristylation signals and a regulatory phosphorylation site corresponding to tyrosine 527 of c-src. BPK has a long, basic amino-terminal region upstream of the SH3 domain. BPK was evaluated as a candidate for human X-linked agammaglobulinemia (XLA), an inherited immunodeficiency characterized by a severe deficit of B and plasma cells and profound hypogammaglobulinemia. BPK mapped to within 100 kb of a probe defining the polymorphism most closely linked to XLA at DXS178. Reduction in or the absence of BPK mRNA, protein expression, and kinase activity was observed in XLA pre-B and B cell lines. BPK is likely the XLA gene and functions in pathways critical to B cell expansion.