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.     

Novel insertions of Bruton tyrosine kinase in patients with X-linked agammaglobulinemia

Mutations in the gene encoding Bruton tyrosine kinase (BTK) result in X-linked agammaglobulinemia (XLA), an immunodeficiency of antibody defect. By using base excision sequence scanning method (BESS) followed by direct sequencing we found in seven unrelated families with a classical XLA phenotype various mutations including six novel mutations (g.64512_64513insC, c.108_109insG, c.1700_1701insACTACAG, g.51375_51376GC>TG, g.63991_63992insGGTAGAAAAAA, c.1956_1957insCA) and a previously known silent polymorphism (c.2031C>T). Except for two mutations, the alterations affect the kinase domain. There was exceptionally high proportion of insertions in the cohort. Frameshift insertion was found altogether in five patients, three of which are on introns, one in upstream region, and one in exon 18 leading to frameshift mutation and truncation of the protein. In the intron 4 there is a substitution of two bases. Carrier detection was performed in four families. In one case the mutation was found to be de novo.     

Bruton tyrosine kinase gene mutations in Turkish patients with presumed X‐linked agammaglobulinemia

X‐linked agammglobulinemia (XLA) is a ptototypical humoral immunodeficiency caused by mutations in the gene coding for Bruton tyrosine kinase (BTK). The genetic defect in XLA impairs early B cell development resulting in marked reduction of mature B cells in the blood. Studies from different countries have demonstrated that approximately 90% of males with presumed XLA bear mutations in BTK. In this study, we report for the first time the occurrence of BTK mutations in Turkey. We performed mutational analysis of the BTK gene in 16 Turkish male patients from 13 separate families with presumed XLA based on abnormally low peripheral blood B‐cell numbers (lt; 1%), hypogammaglobulinemia, and recurrent bacterial infections. We found that in nine of the 13 families (69%) a Btk mutation caused XLA. Two of the mutations were previously described, but seven novel mutations were identified: two missense (Y39C, G584R), one nonsense (Q343X), and 4 deletions (1800‐1821del, 1843‐1847del, 1288‐1292del, 291del) resulting in frameshift and premature stop codon. By contrast, no mutations in the BTK gene were identified in the other 4 families. A consanguinity in three of these families raises the possibility that mutations in other autosomal genes which affect early B cell development may contribute to their phenotype resembling XLA. Hum Mutat 18:356, 2001. © 2001 Wiley‐Liss, Inc.     

Delayed diagnosis in X-linked agammaglobulinemia and its relationship to the occurrence of mutations in BTK non-kinase domains

Background: X-linked agammaglobulinemia (XLA) is characterized by the absence of immunoglobulin and B cells. Patients suffer from recurrent bacterial infections from early childhood, and require lifelong immunoglobulin replacement therapy. Mutations in BTK (Bruton’s Tyrosine Kinase) are associated with this phenotype. Some patients that present XLA do not show typical clinical symptoms, resulting in delayed diagnosis due to the lack of a severe phenotype. This study presents a report of five XLA patients from four different families and attempts to determine a relationship between delayed diagnosis and the occurrence of BTK mutations.

Methods: Samples from patients with antibody deficiency were analyzed to determine BTK expression, immunophenotyping and mutation analysis. Clinical and laboratory data was analyzed and presented for each patient.

Results: Most patients presented here showed atypical clinical and laboratory data for XLA, including normal IgM, IgG, or IgA levels. Most patients expressed detectable BTK protein. Sequencing of BTK showed that these patients harbored missense mutations in the pleckstrin homology and Src-homology-2 domains. When it was compared to public databases, BTK sequencing exhibited a new change, along with three other previously reported changes.

Conclusions: Delayed diagnosis and atypical manifestations in XLA might be related to mutation type and BTK expression.     

Characterization of germline mutations of the gene encoding Bruton's tyrosine kinase in families with X-linked agammaglobulinemia

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 Srcrelated 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. © 1995 Wiley-Liss, Inc.     

Characterization of All Possible Single‐Nucleotide Change Caused Amino Acid Substitutions in the Kinase Domain of Bruton Tyrosine Kinase

Knowledge about features distinguishing deleterious and neutral variations is crucial for interpretation of novel variants. Bruton tyrosine kinase (BTK) contains the highest number of unique disease‐causing variations among the human protein kinases, still it is just 10% of all the possible single‐nucleotide substitution‐caused amino acid variations (SNAVs). In the BTK kinase domain (BTK‐KD) can appear altogether 1,495 SNAVs. We investigated them all with bioinformatic and protein structure analysis methods. Most disease‐causing variations affect conserved and buried residues disturbing protein stability. Minority of exposed residues is conserved, but strongly tied to pathogenicity. Sixty‐seven percent of variations are predicted to be harmful. In 39% of the residues, all the variants are likely harmful, whereas in 10% of sites, all the substitutions are tolerated. Results indicate the importance of the entire kinase domain, involvement in numerous interactions, and intricate functional regulation by conformational change. These results can be extended to other protein kinases and organisms.     

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.     

Mutations of the human BTK gene coding for bruton tyrosine kinase in X-linked agammaglobulinemia

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.     

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.     

Acute primary purulent pericarditis in an adult patient with unknown X-linked agammaglobulinemia

X-linked agammaglobulinemia (XLA) is a rare form of inherited immunodeficiency due to an impairment in B-lymphocyte differentiation and maturation. In the majority of cases XLA is diagnosed in childhood, particularly among males affected by recurrent infections and with a family history of immunodeficiency. Infections of respiratory tract, gastrointestinal apparatus, eyes, nose and ears are frequent in XLA patients; on the contrary, infections of myocardium, cardiac valves and pericardium are rarely described in XLA.A 34-year-old man with unknown XLA was hospitalized because of syncope, due to pericardial tamponade, caused by acute primary purulent pericarditis. Immediate pericardiocentesis was effective in improving hemodynamics, and empiric antibiotic therapy was successful in controlling the infection.Purulent pericarditis is a rare disease with high mortality rate: it is usually caused by hematogenous bacterial propagation, direct infection of pericardial space by chest wounds or thoracic surgery, or extension of infection from adjacent tissues. However, this patient had no recent local or systemic infections. Because of unusual clinical picture during hospitalization he underwent further clinical and laboratory evaluations, that showed low immunoglobulin levels. After exclusion of acquired immunodeficiency, genetic tests were performed: they detected deletion of exons 8-9-10 of Bruton Tyrosine Kinase gene on X chromosome, leading to the diagnosis of XLA.Acute purulent primary pericarditis may also occur in adult XLA patients as first clinical manifestation. According to this case report, a primary immunodeficiency syndrome should be considered in patients with atypical cardiac infections and no predisposing conditions, regardless of age.     

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 mutations in the Bruton's tyrosine kinase gene,including a novel genomic rearrangements resulting in large deletion,in Korean X-linked agammaglobulinemia patients

Mutations in the Bruton's tyrosine kinase (BTK) gene are responsible for X-linked agammaglobulinemia (XLA). We identified BTK mutations in six patients with presumed XLA from unrelated Korean families. Four out of six mutations were novel: two missense mutations (P565T, C154Y), a point mutation in a splicing donor site (IVS11+1G>A), and a large deletion (a 6.1-kb deletion including BTK exons 11–18). The large deletion, identified by long-distance PCR, revealed Alu-Alu mediated recombination extended from an Alu sequence in intron 10 to another Alu sequence in intron 18, spanning a distance of 6.1 kb. The two known mutations consisted of one missense (G462D) mutation, and a point mutation in a splicing acceptor site (IVS7−9A>G). This study suggests that large genomic rearrangements involving Alu repeats are few but an important component of the spectrum of BTK mutations.     

Contiguous X-chromosome Deletion Syndrome Encompassing the BTK, TIMM8A, TAF7L, and DRP2 Genes

X-linked agammaglobulinemia (XLA) is characterized by low levels of B-lymphocytes with early-onset, recurrent, microbial infections occasionally causing neurological symptoms. We observed an atypical clinical course of XLA, complicated since early childhood with neurological impairment, progressive sensorineural deafness, and dystonia in six boys of four unrelated families. The neurologic symptoms suggested the diagnosis of Mohr–Tranebjaerg syndrome, caused by mutations in the TIMM8A gene, previously known as DDP1, and located centromerically of BTK. Deafness dystonia peptide (DDP1) participates in neurological development and is a part of the mitochondrial protein import pathway. Mutation analysis of the BTK gene revealed gross deletions of different lengths in all patients, in one case extending approximately 196 kb, including the genes TIMM8A, TAF7L, and DRP2. The most prominent clinical findings of this contiguous deletion syndrome are the combination of immunodeficiency and sensorineural deafness, which were present in all affected boys. The severity of symptoms, however, did not correlate with the extent of the deletion.     

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。     

Bruton's tyrosine kinase expression and activity in X-linked agammaglobulinaemia (XLA): the use of protein analysis as a diagnostic indicator of XLA

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.     

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.     

Kinase mutant Btk results in atypical X-linked agammaglobulinaemia phenotype

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.