Identification of mutations of Bruton's tyrosine kinase gene (BTK) in Brazilian patients with X-linked agammaglobulinemia

Mutations in the Bruton tyrosine kinase (BTK) gene are responsible for X-linked agammaglobulinemia (XLA), which is characterized by recurrent bacterial infections, profound hypogammaglobulinemia, and decreased numbers of mature B cells in the peripheral blood. We evaluated 17 male Brazilian patients from 13 unrelated families who showed markedly reduced numbers of blood B cells and hypogammaglobulinemia. BTK gene analysis detected mutations in 10 of the 13 presumed XLA families. Seven mutations (Q196X, G613D, R28L, 251-273del, Q234X, H364P, and R13X) had been reported previously, whereas the remaining three mutations (M501T, IVS15+1G>C, and IVS14+1G>A) were novel. Mutation IVS15+1G>C occurred in a splice donor site and caused exons 15 and 16 to be skipped, and IVS14+1G>A might cause exon 14 to be skipped. Flow cytometry revealed deficient expression of BTK protein in 10 of the 13 families. This is the first report of the diagnosis of XLA by analysis of mutations of the BTK gene in Brazilian patients.     

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

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. © 1996 Wiley-Liss, Inc.     

Characterization of novel Bruton's tyrosine kinase gene mutations in Central European patients with agammaglobulinemia

X-linked agammaglobulinemia (XLA) is an immunodeficiency disorder caused by mutations in the gene coding for Bruton's tyrosine kinase (BTK). In this study we investigated 10 male patients with XLA-compatible phenotype (agammaglobulinemia and undetectable B cells in peripheral blood) from 9 unrelated Central European families. We identified seven different mutations, six of which were novel. One previously described point mutation caused a premature stop codon (p.C464X), two point mutations resulted in amino acid exchanges (p.W588R; p.G419E), and two point mutations affected splice sites (c.305-1G>A; c.391+1G>A). We further detected one deletion (c.1921_1927del CGTCCCA) and one large duplication. The duplication resulted from Alu element-induced unequal homologous recombination, which was only detectable by extended analysis of cDNA, while direct sequencing of genomic DNA gave a false negative result. Western blot analysis revealed that the patients with the p.W588R and the p.G419E amino acid substitutions, respectively, produced full length BTK, but in clearly diminished amounts. The patient with the 7bp deletion expressed low amounts of protein which might represent truncated BTK. All other genomic alterations resulted in complete loss of BTK protein. In two patients from unrelated families BTK protein expression was normal and no Btk gene mutation was detected. The results of this study further substantiate the importance of using elaborate molecular analysis with different detection techniques to obtain an explicit molecular diagnosis in patients with suspected XLA.     

Genomic organization of the Btk gene and exon scanning for mutations in patients with X-linked agammaglobulinemia

The defective gene responsible for the recessively inheritedimmunodeficiency X-linked agammaglobulinemia (XLA) has beenshown to encode a cytoplasmic protein tyrosine kinase of theSrc family designated Btk (Bruton's tyrosine kinase). To facilitatethe search for germline mutations of the Btk gene, we have characterizedits genomic structure. Eighteen introns were positioned withinthe approximately 37 kb gene. Each of the exon/intron boundarieswere defined and sequenced, and all but two conform to consensussequences. We have utilized the genomic organization of Btkand the intervening sequence data to design an assay for amplifyingeach of the 19 exons from XLA patient DNA for single strandconformation polymorphism (SSCP) analysis. By using this methodwe have identified mutations in 12 of 14 unrelated affectedmales: seven different base substitutions and two small deletions.Two of the mutations described in exon 15 of the kinase domainwere found in more than one patient and may represent a mutationhot spot. Exon scanning has proven to be a valuable method foridentifying the patient mutations in genomic DNA without theuse of cDNA. The mutations are easily confirmed with directsequencing of the amplified exons. This approach will greatlybenefit XLA family studies involving carrier detection and prenataldiagnosis. In addition, the mutations identified may revealresidues involved in the specific protein interactions necessaryin the B-cell developmental pathway, of which Btk is an integralcomponent.     

Bruton's tyrosine kinase mutations in 8 Chinese families with X-linked agammaglobulinemia

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.     

Mutation pattern in the Bruton's tyrosine kinase gene in 26 unrelated patients with X-linked agammaglobulinemia

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. Hum Mutat 9:418–425, 1997. © 1997 Wiley-Liss, Inc.     

Molecular analysis of Bruton’s tyrosine kinase gene in Spain

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. © 2001 Wiley‐Liss, Inc.     

Unusual Mutations in Btk: An Insertion,a Duplication,an Inversion,and Four Large Deletions

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.     

Maternal germinal mosaicism of X‐linked agammaglobulinemia

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. © 2001 Wiley‐Liss. Inc.     

Identification of four novel mutations of the XLRS1 gene in Japanese patients with X-linked juvenile retinoschisis. Mutation in brief no. 234. Online

The XLRS1 gene (HUGO-approved symbol, RS1) has been found to cause X-linked recessive retinoschisis (RS) which is characterized by splitting of the superficial layer of the retina. Recent mutation analysis of this gene revealed 82 different mutations in 214 patients with RS. We have now identified 10 mutations of the XLRS1 gene in 11 unrelated Japanese males with RS. Mutations found in these patients were; 1) a 20-kb deletion in exon 1 region; 2) mutations in the initiation sequence (M1V); 3) mutations in the splice donor site (IVS1 + 1 g-->a); 4) two nonsense mutations (Q88X, W163X); and 5) five missense mutations (E72K, Y89C, R182C, G109E, P203L). Four (M1V, Q88X, G109E, and W163X) of the 10 mutations were novel. The R182C mutation was identified in 2 unrelated patients. The 3 mutations found between exons 1 and 3 cause premature translation termination in the XLRS1 protein. The rest of the 7 mutations were clustered between exons 4 and 6. This region of the protein is homologous to the proteins implicated in cell-cell adhesion.     

Identification of nine novel mutations in the Bruton's tyrosine kinase gene in X-linked agammaglobulinaemia patients

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.     

Bruton's tyrosine kinase is not essential for LPS-induced activation of human monocytes

BACKGROUND: X-linked agammaglobulinemia (XLA) is characterized by impaired B-cell differentiation caused by mutations in the Bruton's tyrosine kinase (Btk) gene. The natural disease model, the X-linked immunodeficiency mouse, shows a less severe phenotype, indicating a different requirement of Btk in human and mouse B cells. Btk is also expressed in the myeloid line and participates in LPS signaling. Deficient oxidative burst and myeloid differentiation have been reported in the X-linked immunodeficiency mouse, but the precise mechanism and relevance of Btk activity in human monocytes is poorly understood. OBJECTIVE: The apparent absence in XLA of clinical manifestations of myeloid deficiency prompted us to explore the relevance of complete Btk absence in human myeloid cells. METHODS: Seven patients with XLA with BTK mutations conditioning a null protein expression were included in the study. Monocyte LPS-induced mitogen-activated protein kinase activation, TNF-alpha and IL-6 production in monocytes, and oxidative burst in monocytes and granulocytes were analyzed by means of flow cytometry. RESULTS: We show that in response to LPS, Btk-null monocytes from patients with XLA induce early mitogen-activated protein kinase activation and intracellular TNF-alpha and IL-6 production with the same intensity as cells from age- and sex-matched control subjects. In addition, the oxidative burst in response to LPS and other stimulants was completely normal in Btk-null monocytes and neutrophils. CONCLUSION: Our results indicate that Btk is not essential for early LPS signaling in human monocytes and that different Btk dependency might exist between human and mouse myeloid cells. CLINICAL IMPLICATIONS: These findings provide a better understanding of XLA, and they show the differences between human XLA and murine Xid models.     

Twin carriers of X-linked agammaglobulinemia (XLA) due to germline mutation in the Btk gene

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.     

BTK: 22 novel and 25 recurrent mutations in European patients with X-linked agammaglobulinemia

X linked agammaglobulinemia (XLA) is an immunodeficiency disease caused by mutations in the gene coding for Bruton's agammaglobulinemia tyrosine kinase (BTK), that is involved in signal transduction pathways regulating survival, activation, proliferation, and differentiation of B lineage lymphoid cells. XLA is a primary immunodeficiency disorder characterized by lack of mature, circulating B lymphocytes, and recurrent infections. Using Single Strand Conformation Polymorphism (SSCP) followed by direct sequencing we investigated 57 patients with XLA phenotype, with or without a positive family history, from 52 unrelated families enrolled in the Italian XLA Multicenter Clinical Study. We have identified 25 recurrent mutations, 22 novel mutations including one large deletion comprising the coding sequence from exon 11 to 18. Among the mutations identified, three were detected in different unrelated families, whereas all the others were private mutations.     

Molecular characterization of phenylketonuria in South Brazil

Phenylketonuria (PKU) is an autosomal recessive disorder due to phenylalanine hydroxylase (PAH) deficiency. The PAH gene, located at 12q22-q24.1, includes about 90kb and contains 13 exons. To date, more than 420 different alterations have been identified in the PAH gene. To determine the nature and frequency of PAH mutations in PKU patients from South Brazil, mutation analysis was performed on genomic DNA from 23 unrelated PKU patients. The 13 exons and flanking regions of the PAH gene were amplified by PCR and the amplicons were analyzed by single strand conformation polymorphism (SSCP). Amplicons that showed abnormal migration patterns were analyzed by restriction endonuclease digestion and/or sequencing. Twenty-two previously reported mutations were identified including R261X, R408W, IVS2nt5g-->c, R261Q, and V388M. Polymorphisms were observed in 48.8% of the PKU patients, the most frequent being IVS2nt19t-->c, V245V, and IVS12nt-35c-->t. In addition, two novel sequence variants were identified: 1378g-->t in the 3(')-untranslated region in exon 13 which may be disease-causing and an intron 12 polymorphism, IVS12nt-15t-->c. The mutation spectrum in the patients from Southern Brazil differed from that observed in patients from other Latin American countries and further defined the molecular heterogeneity of this disease.     

Characterization of Bruton's tyrosine kinase mutations in Mexican patients with X-linked agammaglobulinemia

X-linked agammaglobulinemia (XLA) is a humoral primary immunodeficiency in which affected patients have very low levels of peripheral B cells and a profound deficiency of all immunoglobulin isotypes. Mutations in the gene encoding for Bruton's tyrosine kinase (Btk) are responsible for most of the agammaglobulinemia. In this work, 14 Btk mutations responsible of causing XLA are described; eight of which are novel and six are mutations previously reported. Seven of the mutations were due to deletions and insertions of exons and introns, respectively, which suggest splicing defects. The others were missense mutations, five of which affect arginine residues and have been described, and two new which affect leucine and glutamine residues (L111P and E605G). Most of these mutations were located at the kinase domain of Btk and, less frequently, they were found in PH and SH2 domains. Protein expression was also affected since most of the patients did not express or express very low Btk.     

Identification of the bruton tyrosine kinase (BTK) gene mutations in 20 Australian families with X-linked agammaglobulinemia (XLA)

X-linked agammaglobulinemia (XLA) is an immunodeficiency caused by mutations in the Bruton tyrosine kinase (BTK) gene. Twenty Australian patients with an XLA phenotype, from 15 unrelated families, were found to have 14 mutations. Five of the mutations were previously described c.83G>A (p.R28H), c.862C>T (p.R288W), c.904G>A (p.R302G), c.1535T>C (p.L512P), c.700C>T (p.Q234X), while nine novel mutations were identified: four missense c.82C>A (p.R28S), c.494G>A (p.C165Y), c.464G>A (p.C155Y), c.1750G>A (p.G584E), one deletion c.142_144delAGAAGA (p.R48_G50del), and four splice site mutations c.241-2A>G, c.839+4A>G, c.1350-2A>G, c.1566+1G>A. Carrier analysis was performed in 10 mothers and 11 female relatives. The results of this study further support the notion that molecular genetic testing represents an important tool for definitive and early diagnosis of XLA and may allow accurate carrier status and prenatal diagnosis.     

中国X连锁无丙种球蛋白血症40例基因型表型相关性分析

目的通过中国X连锁无丙种球蛋白血症（XLA）患儿临床表现、免疫功能评价、Bruton＇s酪氨酸激酶（BTK）的表达及BTK基因突变分析,分析基因型和表型间可能存在的关系。方法选取拟诊为XLA患儿,使用抗BTK单克隆抗体通过流式细胞技术分析单核细胞BTK蛋白表达。采用RT-PCR获得患儿cDNA,使用8对不同引物分2步扩增BTKcDNA,PCR产物测序。突变结果通过对DNA外显子相应部位扩增、测序证实。并对确诊XLA患儿的母亲及家族中部分亲属进行BTK蛋白表达和BTK基因分析。结果①40/50例原发性低丙种球蛋白血症患儿经BTK基因突变分析确诊为XLA,以错义突变（16例,40.0%）和无义突变（13例,32.5%）为主。②突变类型为错义突变的患儿平均起病年龄为（1.4±1.1）岁,其他突变类型患儿为（1.4±0.7）岁,差异无统计学意义（P=0.45）。错义突变的发生率随年龄的增长呈上升趋势,无义突变的发生率呈下降趋势。③34/40例（85.0%）B细胞〈0.1%;4例（10.0%）B细胞在1%～2%,其中错义突变2例,无义突变1例,剪接突变1例;2例（5.0%）B细胞为2%,均为错义突变。④血清IgG〈3g·L-1患儿BTK基因突变类型以错义突变和无义突变为主。⑤错义突变患儿BTK蛋白表达水平与其他突变类型无显著差异。⑥6/21例（28.6%）2031C/T多态性患儿伴有严重的关节炎,3/19例（15.8%）无多态性患儿有关节炎表现。⑦28/32例（87.5%）XLA患儿母亲为BTK基因杂合型。结论错义突变可能与确诊年龄较大有关,且某些位点的错义突变可能与较高的外周血B细胞数量和血清IgG水平及正常的BTK蛋白表达水平有关。BTK基因多态性（2031C/T）可能增加关节炎的风险。