The gene involved in X-linked agammaglobulinaemia is a member of the src family of protein-tyrosine kinases

Antibody deficiency, growth retardation, spondyloepiphyseal dysplasia and retinal dystrophy: a novel syndrome

The clinical and laboratory combination of recurrent infections due to antibody deficiency, spondyloepiphyseal dysplasia, growth retardation and retinal dystrophy is novel. Four patients with strikingly similar phenotypes from three different families of diverse genetic backgrounds are described, suggesting a similar underlying genotype. Increased awareness of this syndrome will hopefully lead to the description of a larger number of affected individuals, which ultimately might be critical for its genetic characterization.

Human Emt tyrosine kinase is specifically expressed both in mature T-lymphocytes and T-cell associated hematopoietic malignancies

The pattern of expression of the human Emt tyrosine kinase was established in healthy individuals and hematological malignancies by RT-PCR from bone marrow and blood samples, fractionated into T-cells, B-cells, monocytes, granulocytes and thrombocytes. Previously studied mostly in murine samples or established human cell lines, the in vivo correlation was here further clarified. In hematopoietic cells, expression of the EMT gene was associated with T-cell fractions, but Emt was not detected in cord blood CD34+ cells. In fetal tissues, Emt mRNA was strongly expressed in thymus, no expression could be detected in non-hematopoietic tissues. The expression pattern of the 48 malignant bone marrow samples (23 ALL, 1 PLL, 9 AML, 7 CLL and 8 CML cases) paralleled the findings from normal hematopoietic cells: 9/11 T cell associated ALLs, as well as one T-PLL sample, but only 1/12 samples of B-ALL expressed Emt markedly. Only minor signs of Emt expression could be shown in the AML samples, while CML and CLL samples were totally devoid of expression. In addition the Emt protein could be detected by Western blotting from T-lymphocytes and T-cell associated ALL, corresponding to mRNA expression. In conclusion, Emt (Itk) is T-cell associated both in normal and leukemic cells, but is not expressed in cord blood CD34+ cells, suggesting that Emt expression is switched on only later in T-cell development. In addition, an association between Emt and CD2 expression remains even in malignancies.

Phosphoinositide 3-kinase and integrin signalling are involved in activation of Bruton tyrosine kinase in thrombin-stimulated platelets

Bruton tyrosine kinase (Btk) plays a crucial role in the differentiation of B lymphocytes and belongs to the group of Tec kinases, which are characterised by the presence of a pleckstrin homology domain. Here we show that Btk is activated and undergoes tyrosine phosphorylation upon challenge of platelet thrombin receptor, these responses requiring engagement of alphaIIb/beta3 integrin and phosphoinositide 3-kinase activity. These data unravel a novel signalling pathway involving Btk downstream of an adhesive receptor via a complex regulation implicating the products of phosphoinositide 3-kinase, which might act to anchor Btk at the membrane.

Bruton's tyrosine kinase as an inhibitor of the Fas/CD95 death-inducing signaling complex

Bruton's tyrosine kinase (BTK) is a member of the Src-related Tec family of protein tyrosine kinases. Mutations in the btk gene have been linked to severe developmental blocks in human B-cell ontogeny leading to X-linked agammaglobulinemia. Here, we provide unique biochemical and genetic evidence that BTK is an inhibitor of the Fas/APO-1 death-inducing signaling complex in B-lineage lymphoid cells. The Src homology 2, pleckstrin homology (PH), and kinase domains of BTK are all individually important and apparently indispensable, but not sufficient, for its function as a negative regulator of Fas-mediated apoptosis. BTK associates with Fas via its kinase and PH domains and prevents the FAS-FADD interaction, which is essential for the recruitment and activation of FLICE by Fas during the apoptotic signal. Fas-resistant DT-40 lymphoma B-cells rendered BTK-deficient through targeted disruption of the btk gene by homologous recombination knockout underwent apoptosis after Fas ligation, but wild-type DT-40 cells or BTK-deficient DT-40 cells reconstituted with wild-type human btk gene did not. Introduction of an Src homology 2 domain, a PH domain, or a kinase domain mutant human btk gene into BTK-deficient cells did not restore the resistance to Fas-mediated apoptosis. Introduction of wild-type BTK protein by electroporation rendered BTK-deficient DT-40 cells resistant to the apoptotic effects of Fas ligation. BTK-deficient RAMOS-1 human Burkitt's leukemia cells underwent apoptosis after Fas ligation, whereas BTK-positive NALM-6-UM1 human B-cell precursor leukemia cells expressing similar levels of Fas did not. Treatment of the anti-Fas-resistant NALM-6-UM1 cells with the leflunomide metabolite analog alpha-cyano-beta-methyl-beta-hydroxy-N-(2, 5-dibromophenyl)propenamide, a potent inhibitor of BTK, abrogated the BTK-Fas association without affecting the expression levels of BTK or Fas and rendered them sensitive to Fas-mediated apoptosis. The ability of BTK to inhibit the pro-apoptotic effects of Fas ligation prompts the hypothesis that apoptosis of developing B-cell precursors during normal B-cell ontogeny may be reciprocally regulated by Fas and BTK.

Impaired B cell development and proliferation in absence of phosphoinositide 3-kinase p85alpha

Phosphoinositide 3-kinase (PI3K) activation has been implicated in many cellular responses, including fibroblast growth, transformation, survival, and chemotaxis. Although PI3K is activated by several agents that stimulate T and B cells, the role of PI3K in lymphocyte function is not clear. The mouse gene encoding the PI3K adapter subunit p85alpha and its splice variants p55alpha and p50alpha was disrupted. Most p85alpha-p55alpha-p50alpha-/- mice die within days after birth. Lymphocyte development and function was studied with the use of the RAG2-deficient blastocyst complementation system. Chimeric mice had reduced numbers of peripheral mature B cells and decreased serum immunoglobulin. The B cells that developed had diminished proliferative responses to antibody to immunoglobulin M, antibody to CD40, and lipopolysaccharide stimulation and decreased survival after incubation with interleukin-4. In contrast, T cell development and proliferation was normal. This phenotype is similar to defects observed in mice lacking the tyrosine kinase Btk.

Xid-like immunodeficiency in mice with disruption of the p85alpha subunit of phosphoinositide 3-kinase

Mice with a targeted gene disruption of p85alpha, a regulatory subunit of phosphoinositide 3-kinase, had impaired B cell development at the pro-B cell stage, reduced numbers of mature B cells and peritoneal CD5+ Ly-1 B cells, reduced B cell proliferative responses, and no T cell-independent antibody production. These phenotypes are nearly identical to those of Btk-/- or xid (X-linked immunodeficiency) mice. These results provide evidence that p85alpha is functionally linked to the Btk pathway in antigen receptor-mediated signal transduction and is pivotal in B cell development and functions.

Comparative genomics and host resistance against infectious diseases

The large size and complexity of the human genome have limited the identification and functional characterization of components of the innate immune system that play a critical role in front-line defense against invading microorganisms. However, advances in genome analysis (including the development of comprehensive sets of informative genetic markers, improved physical mapping methods, and novel techniques for transcript identification) have reduced the obstacles to discovery of novel host resistance genes. Study of the genomic organization and content of widely divergent vertebrate species has shown a remarkable degree of evolutionary conservation and enables meaningful cross-species comparison and analysis of newly discovered genes. Application of comparative genomics to host resistance will rapidly expand our understanding of human immune defense by facilitating the translation of knowledge acquired through the study of model organisms. We review the rationale and resources for comparative genomic analysis and describe three examples of host resistance genes successfully identified by this approach.

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.

Follicular Exclusion and Rapid Elimination of Hen Egg Lysozyme Autoantigen-Binding B Cells Are Dependent on Competitor B Cells, But Not on T Cells

In mice with a diverse B cell repertoire, hen egg lysozyme (HEL) autoantigen-binding B cells are excluded from follicles and eliminated in 3 days. To explore the roles of competitor B cells and of T cells in this mechanism of self-tolerance, HEL-specific B cells were transferred into mice containing HEL and deficient in endogenous B cells (μMT), T cells (TCR−/−), or B and T cells (RAG1−/−). Previous studies suggested a dual requirement for B cell receptor (BCR) engagement and competition in HEL autoantigen-binding B cell elimination, but interpretation of these experiments has been confounded by the possible failure to independently regulate autoantigen concentration and competitor B cell frequency. In experiments in this study, we have fixed one variable, HEL concentration, while varying the second, the presence or absence of other B cells. By this approach, we find that follicular exclusion and rapid elimination of autoreactive B cells require BCR engagement plus competition with other B cells, rather than BCR engagement alone. We also find, by transfers into T cell-deficient mice, that T cells are not required for this peripheral tolerance mechanism. Unexpectedly, in mice lacking both T cells and competitor B cells (RAG1−/−), transferred HEL-binding cells survive less well than in mice just lacking competitor B cells. These results suggest T cells can enhance autoreactive B cell survival. Enhanced survival of autoreactive B cells, due to the presence of T cells and the lack of competitor B cells, might contribute to the elevated frequency of autoimmunity in B cell-deficient individuals.

MPK38 expression is upregulated in immature T cells activated by concanavalin A

We have previously identified a cDNA clone from a murine teratocarcinoma PCC4 cDNA library, which we have termed MPK38 for murine protein kinase 38. MPK38 is a new member of the SNF1 serine/threonine kinase family. To understand the role of the MPK38 in thymocytes, we have investigated the level of MPK38 expression by Northern blot analysis. Interestingly, incubation of thymocytes at 37 degrees C resulted in the loss of the MPK38 transcript, however the transcript could be reinduced by treatment with Con A or PHA, but not with PMA and growth factors such as IL-2, IL-7, TGF-beta, TNF-alpha, and EGF. In addition, stimulation of mature T cells from the spleen with Con A failed to reinduce the MPK38 transcript, indicating a developmental regulation of MPK38 expression, whereas non-T cell populations significantly reinduced the transcript. These results suggest that MPK38 may play a functional role as one of the signal regulators in early T cell activation and, potentially, certain lineages of hematopoietic cell activation induced by Con A.

Defective degranulation and calcium mobilization of bone-marrow derived mast cells from Xid and Btk-deficient mice

Regulation of adhesion and degranulation of mast cells plays an important role in allergy and inflammation. We investigated a possible role of Bruton's tyrosine kinase (Btk) in the regulation of adhesion and degranulation by using bone marrow-derived mast cells from X-linked immunodeficiency (Xid) and Btk-deficient mice. Cross-linking of the high affinity IgE receptor (Fc epsilonRI) and steel factor (SLF) induced indistinguishable adhesive responses of mast cells to fibronectin in kinetics, and these adhesive responses were comparable among wild type, Xid, and Btk-deficient mast cells. Cross-linking of Fc epsilonRI, but not SLF triggered degranulation of bone marrow-derived mast cells. However, Fc epsilonRI-induced degranulation was impaired in Xid and Btk-deficient mast cells. Calcium influx induced by Fc epsilonRI cross-linking and SLF were also reduced in Xid and Btk-deficient mast cells. Degranulation and calcium influx were reduced more severely in Btk-deficient than in Xid mast cells. Consistently, cross-linking Fc epsilonRI and SLF augmented Btk kinase activities transiently. Inositol triphosphate (IP3) production was also severely reduced in Btk-deficient mast cells, indicating Btk play a critical role of Fc epsilonRI-induced IP3 production. The differential sensitivity of wortmannin on calcium influx in wild type and Xid mast cells suggested that the activation of phosphatidylinositol 3 kinase (PI 3-kinase) was required in calcium influx. Furthermore, abnormal secretory granules with translucent contents and variable in size were observed both in Xid and Btk-deficient mast cells. Our study demonstrated a critical role of Btk in regulating intracellular calcium and granule exocytosis.

Primary immunodeficiency disorders

The primary immunodeficiency diseases are a relatively rare group of congenital disorders that are linked by the expression of an excessive number, duration, or severity of infections. The clinical features of most of the primary immunodeficiency diseases have been well described by astute physicians over several decades and have provided important clues to our basic understanding of human immunology. In contrast, the genetic basis and potential life-saving therapies for many of these disorders have been established only over the past few years. These recent advances have resulted in the prognosis of many of these disorders being largely dependent on their rapid recognition and treatment. Increased awareness of the differentiating epidemiologic, clinical, laboratory and genetic features of these diseases hold the promise of both furthering our understanding of basic human immunology and providing improved care for this challenging group of patients.

Cellular and molecular basis of immunodeficiencies: their consequences for the development and induction of the immune response

This review lists primary immunodeficiencies which essentially involve mutations in genes coding for functionally important molecules, membrane antigens (e.g., MHC), chains of lymphokine receptors, protein kinases of the signal cascade, transcription factors, and important regulators of cellular metabolism. Mutations and subsequent immunodeficiencies occur as early as during embryogenesis (lymphopoiesis-I) as well as during induction of the immune response by antigen (ligand) binding to cell receptors, TCR and BCR (immunopoiesis-II). Immunodeficiencies are classified according the developmental stages in which they occur most markedly, even in clinical terms. Some early mutations are immediately lethal, some express themselves by blocking embryonic lymphopoiesis, while other mutations do not become demonstrable until after cell stimulation by antigens (see the Tables).

Resting lymphocyte kinase (Rlk/Txk) phosphorylates the YVKM motif and regulates PI 3-kinase binding to T-cell antigen CTLA-4

CTLA-4 and CD28 are differentially expressed on T-cells. They bind to a common ligand B71/2 (CD80/86), however with different avidities. Unlike CD28 which augments the T-cell response, CTLA-4 operates predominately as a negative regulator of T-cell proliferation. The mechanism by which CTLA-4 can generate these intracellular signals is unclear. Little is known regarding the identity of the protein-tyrosine kinase(s) responsible for CTLA-4 phosphorylation and thus creating conditions for the reported binding to PI 3-kinase and the protein tyrosine phosphatase SHP-2. In this study, we demonstrate that Rlk (resting lymphocyte kinase) is capable of phosphorylating CTLA-4 at the YVKM motif. Consistent with this finding, Rlk is capable of providing conditions for the binding of the SH2 domains of PI 3-kinase to the receptor. CTLA-4 is therefore the first known substrate for Rlk suggesting the possibility that this kinase may participate in CTLA-4 function.

Functional Analysis of Peripheral Blood B Cells in Patients with X-Linked Agammaglobulinemia

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 φX174 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.

Different protein tyrosine kinases are required for B cell antigen receptor-mediated activation of extracellular signal-regulated kinase, c-Jun NH2-terminal kinase 1, and p38 mitogen-activated protein kinase

B cell antigen receptor (BCR) cross-linking activates three distinct families of nonreceptor protein tyrosine kinases (PTKs): src-family kinases, Syk, and Btk; these PTKs are responsible for initiating downstream events. BCR cross-linking in the chicken DT40 B cell line also activates three distinct mitogen-activated protein kinases (MAPKs): extracellular signal-regulated kinase (ERK)2, c-jun NH2-terminal kinase (JNK)1, and p38 MAPK. To dissect the functional roles of these PTKs in MAPK signaling, activation of MAPKs was examined in various PTK-deficient DT40 cells. BCR-mediated activation of ERK2, although maintained in Lyn-deficient cells, was abolished in Syk-deficient cells and partially inhibited in Btk-deficient cells, indicating that BCR-mediated ERK2 activation requires Syk and that sustained ERK2 activation requires Btk. BCR-mediated JNK1 activation was maintained in Lyn-deficient cells but abolished in both Syk- and Btk-deficient cells, suggesting that JNK1 is activated via a Syk- and Btk-dependent pathway. Consistent with this, BCR-mediated JNK1 activation was dependent on intracellular calcium and phorbol myristate acetate-sensitive protein kinase Cs. In contrast, BCR-mediated p38 MAPK activation was detected in all three PTK-deficient cells, suggesting that no single PTK is essential. However, BCR-mediated p38 MAPK activation was abolished in Lyn/Syk double deficient cells, demonstrating that either Lyn or Syk alone may be sufficient to activate p38 MAPK. Our data show that BCR-mediated MAPK activation is regulated at the level of the PTKs.

Severe B cell deficiency and disrupted splenic architecture in transgenic mice expressing the E41K mutated form of Bruton's tyrosine kinase

To identify B-cell signaling pathways activated by Bruton's tyrosine kinase (Btk) in vivo, we generated transgenic mice in which Btk expression is driven by the MHC class II Ea gene locus control region. Btk overexpression did not have significant adverse effects on B cell function, and essentially corrected the X-linked immunodeficiency (xid) phenotype in Btk- mice. In contrast, expression of a constitutively activated form of Btk carrying the E41K gain-of-function mutation resulted in a B cell defect that was more severe than xid. The mice showed a marked reduction of the B cell compartment in spleen, lymph nodes, peripheral blood and peritoneal cavity. The levels in the serum of most immunoglobulin subclasses decreased with age, and B cell responses to both T cell-independent type II and T cell-dependent antigens were essentially absent. Expression of the E41K Btk mutant enhanced blast formation of splenic B cells in vitro in response to anti-IgM stimulation. Furthermore, the mice manifested a disorganization of B cell areas and marginal zones in the spleen. Our findings demonstrate that expression of constitutively activated Btk blocks the development of follicular recirculating B cells.

Bruton's tyrosine kinase (BTK) as a dual-function regulator of apoptosis

Multiple counterregulatory mechanisms have been identified in B-cell precursors that operate to regulate cell survival and growth, thereby ensuring the orderly development and differentiation of B-cells. Inappropriate apoptosis may underlie the pathogenesis of immunodeficiencies, as well as pathogenesis and drug/radiation resistance of human leukemias and lymphomas, which makes control of apoptosis an important potential target for therapeutic interventions. Therefore, identification of the molecular regulators of apoptosis is an area of intense investigation. Bruton's tyrosine kinase (BTK) is the first tyrosine kinase to be identified as a dual-function regulator of apoptosis, which promotes radiation-induced apoptosis but inhibits Fas-activated apoptosis in B-cells. BTK functions in a pro-apoptotic manner when B-cells are exposed to reactive oxygen intermediates, at least in part, by down-regulating the anti-apoptotic activity of STAT-3 transcription factor. In contrast, BTK associates with the death receptor Fas and impairs its interaction with Fas-associated protein with death domain (FADD), which is essential for the recruitment and activation of FLICE by Fas during the apoptotic signal, thereby preventing the assembly of a pro-apoptotic death inducing signaling complex (DISC) after Fas-ligation. The identification of BTK as a dual-function regulator of apoptosis will significantly increase our understanding of both the biological processes involved in programmed cell death and the diseases associated with dysregulation of apoptosis. New agents with BTK-modulatory activity may have clinical potential in the treatment of B-cell malignancies (in particular acute lymphoblastic leukemia, the most common form of childhood cancer), as well as B-cell immunodeficiencies.

Independent and opposing roles for Btk and lyn in B and myeloid signaling pathways

Transphosphorylation by Src family kinases is required for the activation of Bruton's tyrosine kinase (Btk). Differences in the phenotypes of Btk-/- and lyn-/- mice suggest that these kinases may also have independent or opposing functions. B cell development and function were examined in Btk-/-lyn-/- mice to better understand the functional interaction of Btk and Lyn in vivo. The antigen-independent phase of B lymphopoiesis was normal in Btk-/-lyn-/- mice. However, Btk-/-lyn-/- animals had a more severe immunodeficiency than Btk-/- mice. B cell numbers and response to T cell-dependent antigens were reduced. Btk and Lyn therefore play independent or partially redundant roles in the maintenance and function of peripheral B cells. Autoimmunity, hypersensitivity to B cell receptor (BCR) cross-linking, and splenomegaly caused by myeloerythroid hyperplasia were alleviated by Btk deficiency in lyn-/- mice. A transgene expressing Btk at approximately 25% of endogenous levels (Btklo) was crossed onto Btk-/- and Btk-/-lyn-/- backgrounds to demonstrate that Btk is limiting for BCR signaling in the presence but not in the absence of Lyn. These observations indicate that the net outcome of Lyn function in vivo is to inhibit Btk-dependent pathways in B and myeloid cells, and that Btklo mice are a useful sensitized system to identify regulatory components of Btk signaling pathways.

Absence of Bruton's tyrosine kinase (Btk) mutations in patients with acute myeloid leukaemia

Bruton's tyrosine kinase (Btk) is a non-receptor protein tyrosine kinase (PTK) that is expressed in all haemopoietic lineages except mature T cells and plasma cells. Despite the broad range of expression. mutations that inactivate this molecule affect primarily the development of the B-cell lineage. As a PTK, Btk could potentially be involved directly or indirectly in the processes that relate to the malignant transformation of all the cell lineages where this molecule is expressed. Previous studies have failed to demonstrate mutations in patients with B-cell origin acute lymphoblastic leukaemia (ALL). We have utilized a recently developed method that enables the rapid and convenient detection of mutations at the cDNA level, namely, the non-isotopic RNase cleavage assay (NIRCA) to analyse Btk sequences from 27 patients with different types of acute myeloid leukaemia (AML). The only alteration that we observed was a polymorphism at position 2031. This polymorphism has already been seen in previous studies. Furthermore, using the same methodology, we identified the Btk mutations in six XLA (X-linked agammaglobulinaemia) patients. Our results, although they do not exclude the involvement of Btk mutations in the development or progression of some type of AML, nevertheless suggest that such mutations do not constitute a major co-factor in the development of myeloid malignancies.

The association of nonsense codons with exon skipping

Some genes that contain premature nonsense codons express alternatively-spliced mRNA that has skipped the exon containing the nonsense codon. This paradoxical association of translation signals (nonsense codons) and RNA splicing has inspired numerous explanations. The first is based on the fact that premature nonsense codons often reduce mRNA abundance. The reduction in abundance of full-length mRNA then allows more efficient amplification during PCR of normal, minor, exon-deleted products. This mechanism has been demonstrated to explain an extensive correlation between nonsense codons and exon-skipping for the hamster Hprt gene. The second explanation is that the mutation producing an in-frame nonsense codon has an effect on exon definition. This has been demonstrated for the Mup and hamster Hprt gene by virtue of the fact that missense mutations at the same sites also are associated with the same exon-deleted mRNA. The third general explanation is that a hypothetical process takes place in the nucleus that recognizes nonsense codons, termed 'nuclear scanning', which then has an effect on mRNA splicing. Definitive evidence for nuclear scanning is lacking. My analysis of both nonsense and missense mutations associated with exon skipping in a large number of genes revealed that both types of mutations frequently introduce a T into a purine-rich DNA sequence and are often within 30 base pairs of the nearest exon boundary. This is intriguing given that purine-rich splicing enhancers are known to be inhibited by the introduction of a T. Almost all mutations associated with exon skipping occur in purine-rich or A/C-rich sequences, also characteristics of splicing enhancers. I conclude that most cases of exon skipping associated with premature termination codons may be adequately explained either by a structural effect on exon definition or by nonquantitative methods to measure mRNA, rather than an effect on a putative nuclear scanning mechanism.

Stimulation of B and T cells activates expression of transcription and differentiation factors

During B and T cell differentiation and proliferation many genes are induced or repressed while certain genes are constitutively expressed. To investigate processes related to B and T cell activation, the gene expression of stimulated and nonstimulated Ramos and Jurkat cells was studied using cDNA microarray technology. Simultaneous analysis of close to 600 genes indicated highest increase in the expression of certain transcription, differentiation and proliferation factors. Many of these genes have not previously been shown to funcion in the stimulated lymphocytes. Also genes encoding proteins involved in DNA replication, binding, transcription and translation were induced. Large part of the activated genes were under very stringent regulation being expressed only after stimulation. The mechanism and function of the expressed genes during lymphocyte differentiation and in disorders is discussed.

T cell activation deficiency associated with an aberrant pattern of protein tyrosine phosphorylation after CD3 perturbation in Down's syndrome

Children affected by Down's syndrome (DS) have an increased susceptibility to viral or bacterial infections and leukemia, associated with several abnormalities of the immune system. We investigated whether the T cell defect was qualitative in nature and associated with abnormalities of the early events occurring during cell activation. The proliferative response of lymphocytes from DS individuals after CD3 cross-linking was clearly depressed, as already reported. In contrast, phorbol ester and ionomycin were able to induce cell cycle progression in DS, suggesting a defect in the early stages of the signal transduction through a T cell receptor/CD3 (TCR/CD3) complex upstream of protein kinase C activation. The functional impairment in DS was not related either to a decrease of circulating mature-type CD3+ cells, which express high levels of surface of CD3 molecules, or to a decrease of the CD4+ subpopulation. The analysis of phosphotyrosine-containing proteins after the cross-linking of CD3 molecules in DS lymphocytes revealed a partial signaling, characterized by increased phosphorylation of proteins of 42-44 kD, comparable to that observed in control subjects, but not of proteins of 70 and 21 kD. Moreover, although the "anti-anergic" gamma element of IL-2, IL-4, IL-7, and IL-15 receptors was normally tyrosine-phosphorylated during cell activation, the CD3 zeta-associated protein kinase (ZAP-70) was not. Our results indicate that in DS there is a T cell activation defect, characterized by partial signal transduction through a TCR/CD3 complex, and associated with a selective failure of ZAP-70 tyrosine phosphorylation.

Genetic basis of abnormal B cell development

A susceptibility gene in the MHC class III region may underlie the defective B-cell differentiation in familial IgA deficiency and common variable immunodeficiency. Mutations in Bruton's tyrosine kinase, immunoglobulin heavy chain and lambda 5/14.1 surrogate light chain loci disrupt B-cell development to cause profound antibody deficiency. Mutational, biochemical and transgenic studies offer insight into the function of these and other 'antibody deficiency genes'.

Involvement of Ras in Bruton's tyrosine kinase-mediated JNK activation

Defects in Bruton's tyrosine kinase (Btk) result in B cell immunodeficiencies in humans and mice. Recent studies showed that Btk is required for maximal activation of JNK, a family of stress-activated protein kinases, induced by several extracellular stimuli including interleukin (IL)-3. On the other hand, IL-3-induced JNK activation is dependent on Ras. In the present study we have investigated whether Ras is involved in Btk-mediated JNK activation in BaF3 mouse pro-B cells. Overexpression of wild-type Btk protein in these cells enhanced JNK activation upon IL-3 stimulation, whereas expression of kinase-dead Btk partially suppressed JNK activation. Induced expression of the dominant negative Ras(N17) in the cells overexpressing wild-type Btk suppressed JNK activation. Importantly, overexpression of Btk enhanced the level of the GTP-bound, active form of Ras in response to IL-3 stimulation. Btk overexpression also increased the Shc-Grb2 association induced by IL-3 stimulation. Expression of either N17Ras or V12Ras did not impose any effects on Btk kinase activity. These data collectively indicate that Ras plays a role of an intermediary signaling protein in Btk-mediated JNK activation induced by the IL-3 signaling pathway.

Grb10/GrbIR as an in vivo substrate of Tec tyrosine kinase

BACKGROUND

Tec is a member of the recently emerging subfamily among nonreceptor protein-tyrosine kinases (PTKs). Although many members of this family have been shown to be involved in a wide range of cytokine-mediated signalling systems, the molecular mechanism by which they exert in vivo effects remains obscure. To gain insights into the downstream pathways of Tec, we here looked for Tec-interacting proteins (TIPs) by using the yeast two-hybrid screening.

RESULTS

One of TIPs turned out to be Grb10/GrbIR, which carries one pleckstrin homology domain and one Src homology 2 domain. Grb10/GrbIR was known to bind receptor PTKs in a ligand-dependent fashion, but not to be phosphorylated on tyrosine residues. In a transient expression system in human kidney 293 cells, however, Grb10/GrbIR becomes profoundly tyrosine-phosphorylated by Tec, but not by Syk, Jak2 or insulin receptor. We also reveal that expression of Grb10/GrbIR suppresses the cytokine-driven and Tec-driven activation of the c-fos promoter.

CONCLUSION

Our results indicate a novel role of Grb10/GrbIR as an effector molecule to a subset of nonreceptor PTKs.

Absence of expression of the Wiskott-Aldrich syndrome protein in peripheral blood cells of Wiskott-Aldrich syndrome patients

Wiskott-Aldrich syndrome (WAS) is an X-linked primary immunodeficiency that is usually associated with thrombocytopenia and eczema. The very variable phenotype of WAS results from defects in the WAS protein (WASP), the function of which is not well understood. In many cases causative mutations have now been identified in the WAS gene. Attempts have been made to correlate the nature of the mutations with the severity of the disease. In this study we investigated mutations in 13 patients with WAS and analyzed the expression of WASP in patient blood samples by immunoblot analysis. We found that despite extensive variation in the nature of the mutations in patients with severe WAS symptoms, none express the protein. However, in 1 patient with a mild clinical phenotype WASP expression was detected. Such an analysis could be used as an initial screening procedure for the diagnosis of WAS prior to genotypic analysis.

T cell receptor-initiated calcium release is uncoupled from capacitative calcium entry in Itk-deficient T cells

Itk, a Tec family tyrosine kinase, plays an important but as yet undefined role in T cell receptor (TCR) signaling. Here we show that T cells from Itk-deficient mice have a TCR-proximal signaling defect, resulting in defective interleukin 2 secretion. Upon TCR stimulation, Itk-/- T cells release normal amounts of calcium from intracellular stores, but fail to open plasma membrane calcium channels. Since thapsigargin-induced store depletion triggers normal calcium entry in Itk-/- T cells, an impaired biochemical link between store depletion and channel opening is unlikely to be responsible for this defect. Biochemical studies indicate that TCR-induced inositol 1,4,5 tris-phosphate (IP3) generation and phospholipase C gamma1 tyrosine phosphorylation are substantially reduced in Itk-/- T cells. In contrast, TCR-zeta and ZAP-70 are phosphorylated normally, suggesting that Itk functions downstream of, or in parallel to, ZAP-70 to facilitate TCR-induced IP3 production. These findings support a model in which quantitative differences in cytosolic IP3 trigger distinct responses, and in which only high concentrations of IP3 trigger the influx of extracellular calcium.

Electromagnetic field-induced stimulation of Bruton's tyrosine kinase

Here we present evidence that exposure of DT40 lymphoma B-cells to low energy electromagnetic fields (EMF) results in activation of phospholipase C-gamma 2 (PLC-gamma2), leading to increased inositol phospholipid turnover. PLC-gamma2 activation in EMF-stimulated cells is mediated by stimulation of the Bruton's tyrosine kinase (BTK), a member of the Src-related TEC family of protein tyrosine kinases, which acts downstream of LYN kinase and upstream of PLC-gamma2. B-cells rendered BTK-deficient by targeted disruption of the btk gene did not show enhanced PLC-gamma2 activation in response to EMF exposure. Introduction of the wild-type (but not a kinase domain mutant) human btk gene into BTK-deficient B-cells restored their EMF responsiveness. Thus, BTK exerts a pivotal and mandatory function in initiation of EMF-induced signaling cascades in B-cells.

Mutations in btk in patients with presumed X-linked agammaglobulinemia

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.

Bruton's tyrosine kinase-mediated interleukin-2 gene activation in mast cells. Dependence on the c-Jun N-terminal kinase activation pathway

Cross-linking of the high affinity IgE receptor (FcepsilonRI) on mast cells induces secretion of cytokines, including interleukin (IL)-2, through transcriptional activation of cytokine genes. Previously, defects in the gene coding for Bruton's tyrosine kinase (Btk) were shown to result in defective cytokine production in mast cells, and thereby mice carrying btk mutations exhibited diminished anaphylactic reactions in response to IgE and antigen. In this study, we provide evidence that the transcription factors involved in the IL-2 gene expression in T cells are also required for maximal activation of the IL-2 gene in FcepsilonRI-stimulated mast cells. Among them, AP-1 (Jun/Fos) and NF-AT were identified as candidate transcription factors that are regulated by Btk. Consistent with our previous data indicating that Btk regulates stress-activated protein kinases, c-Jun N-terminal kinase (JNK), c-Jun and other JNK-regulatable transcription factors are activated by FcepsilonRI cross-linking in a Btk-dependent manner. Further, FcepsilonRI-induced IL-2 gene activation is dependent on c-Jun and a component, SEK1, of its upstream activation pathway. Collectively, these data demonstrate that Btk regulates the transcription of the IL-2 gene through the JNK-regulatable transcription factors in FcepsilonRI-stimulated mast cells.

The Tec29 tyrosine kinase is required during Drosophila embryogenesis and interacts with Src64 in ring canal development

Tec29 encodes the only known Drosophila member of the Tec tyrosine kinases. By identifying the first mutations in Tec29 (formerly Src29A), we show that it is essential for head involution during embryogenesis and for ring canal development during oogenesis. Tec29 mutant egg chambers are defective in transfer of cytoplasm from the accessory nurse cells through the ring canals into the oocyte. Growth of the mutant ring canals is arrested, and they lack the strong phosphotyrosine localization seen in wild-type ring canals. Mutants lacking the Drosophila Src homolog Src64 show the same phenotype, and we show that Src64 is required for the localization of Tec29 to the ring canals. This interaction is similar to that between vertebrate Src and Tec kinases and suggests that Tec29 is an effector of Src64 that modifies ring canal components required for growth.

The Croonian Lecture 1997. The phosphorylation of proteins on tyrosine: its role in cell growth and disease

The reversible phosphorylation of tyrosines in proteins plays a key role in regulating many different processes in eukaryotic organisms, such as growth control, cell cycle control, differentiation cell shape and movement, gene transcription, synaptic transmission, and insulin action. Phosphorylation of proteins is brought about by enzymes called protein-tyrosine kinases that add phosphate to specific tyrosines in target proteins; phosphate is removed from phosphorylated tyrosines by enzymes called protein-tyrosine phosphatases. Phosphorylated tyrosines are recognized by specialized binding domains on other proteins, and such interactions are used to initiate intracellular signaling pathways. Currently, more than 95 protein-tyrosine kinases and more than 55 protein-tyrosine phosphatase genes are known in Homo sapiens. Aberrant tyrosine phosphorylation is a hallmark of many types of cancer and other human diseases. Drugs are being developed that antagonize the responsible protein-tyrosine kinases and phosphatases in order to combat these diseases.

Involvement of Bruton's tyrosine kinase in FcepsilonRI-dependent mast cell degranulation and cytokine production

We investigated the role of Bruton's tyrosine kinase (Btk) in FcepsilonRI-dependent activation of mouse mast cells, using xid and btk null mutant mice. Unlike B cell development, mast cell development is apparently normal in these btk mutant mice. However, mast cells derived from these mice exhibited significant abnormalities in FcepsilonRI-dependent function. xid mice primed with anti-dinitrophenyl monoclonal IgE antibody exhibited mildly diminished early-phase and severely blunted late-phase anaphylactic reactions in response to antigen challenge in vivo. Consistent with this finding, cultured mast cells derived from the bone marrow cells of xid or btk null mice exhibited mild impairments in degranulation, and more profound defects in the production of several cytokines, upon FcepsilonRI cross-linking. Moreover, the transcriptional activities of these cytokine genes were severely reduced in FcepsilonRI-stimulated btk mutant mast cells. The specificity of these effects of btk mutations was confirmed by the improvement in the ability of btk mutant mast cells to degranulate and to secrete cytokines after the retroviral transfer of wild-type btk cDNA, but not of vector or kinase-dead btk cDNA. Retroviral transfer of Emt (= Itk/Tsk), Btk's closest relative, also partially improved the ability of btk mutant mast cells to secrete mediators. Taken together, these results demonstrate an important role for Btk in the full expression of FcepsilonRI signal transduction in mast cells.

Identification and characterization of a novel SH3-domain binding protein, Sab, which preferentially associates with Bruton's tyrosine kinase (BtK)

Protein interaction cloning method was used to identify a novel molecule, Sab, which binds to the SH3 domain of Bruton's tyrosine kinase (Btk), the deficient cytoplasmic tyrosine kinase in human X-linked agammaglobulinemia and murine X-linked immunodeficiency. Immunoprecipitation using the anti-Sab antibody identified the protein product of the gene as a 70 kDa molecule. While Sab does not have a proline-rich sequence, it was shown to bind to Btk through the commonly conserved structure among SH3 domains. Remarkably, Sab exhibited a high preference for binding to Btk rather than to other cytoplasmic tyrosine kinases, which suggests a unique role of Sab in the Btk signal transduction pathway.

Transgene expression of bcl-xL permits anti-immunoglobulin (Ig)-induced proliferation in xid B cells

Mutations in the tyrosine kinase, Btk, result in a mild immunodeficiency in mice (xid). While B lymphocytes from xid mice do not proliferate to anti-immunoglobulin (Ig), we show here induction of the complete complement of cell cycle regulatory molecules, though the level of induction is about half that detected in normal B cells. Cell cycle analysis reveals that anti-Ig stimulated xid B cells enter S phase, but fail to complete the cell cycle, exhibiting a high rate of apoptosis. This correlated with a decreased ability to induce the anti-apoptosis regulatory protein, Bcl-xL. Ectopic expression of Bcl-xL in xid B cells permitted anti-Ig induced cell cycle progression demonstrating dual requirements for induction of anti-apoptotic proteins plus cell cycle regulatory proteins during antigen receptor mediated proliferation. Furthermore, our results link one of the immunodeficient traits caused by mutant Btk with the failure to properly regulate Bcl-xL.

Btk/Tec kinases regulate sustained increases in intracellular Ca2+ following B-cell receptor activation

Bruton's tyrosine kinase (Btk) is essential for B-lineage development and represents an emerging family of non-receptor tyrosine kinases implicated in signal transduction events initiated by a range of cell surface receptors. Increased dosage of Btk in normal B cells resulted in a striking enhancement of extracellular calcium influx following B-cell antigen receptor (BCR) cross-linking. Ectopic expression of Btk, or related Btk/Tec family kinases, restored deficient extracellular Ca2+ influx in a series of novel Btk-deficient human B-cell lines. Btk and phospholipase Cgamma (PLCgamma) co-expression resulted in tyrosine phosphorylation of PLCgamma and required the same Btk domains as those for Btk-dependent calcium influx. Receptor-dependent Btk activation led to enhanced peak inositol trisphosphate (IP3) generation and depletion of thapsigargin (Tg)-sensitive intracellular calcium stores. These results suggest that Btk maintains increased intracellular calcium levels by controlling a Tg-sensitive, IP3-gated calcium store(s) that regulates store-operated calcium entry. Overexpression of dominant-negative Syk dramatically reduced the initial phase calcium response, demonstrating that Btk/Tec and Syk family kinases may exert distinct effects on calcium signaling. Finally, co-cross-linking of the BCR and the inhibitory receptor, FcgammaRIIb1, completely abrogated Btk-dependent IP3 production and calcium store depletion. Together, these data demonstrate that Btk functions at a critical crossroads in the events controlling calcium signaling by regulating peak IP3 levels and calcium store depletion.

Etk/Bmx, a tyrosine kinase with a pleckstrin-homology domain, is an effector of phosphatidylinositol 3'-kinase and is involved in interleukin 6-induced neuroendocrine differentiation of prostate cancer cells

Etk/Bmx is the newest member of Btk tyrosine kinase family that contains a pleckstrin homology domain, an src homology 3 domain, an src homology 2 domain, and a catalytic domain. Unlike other members of the Btk family kinases, which are mostly hemopoietic cell-specific, Etk/Bmx is preferentially expressed in epithelial and endothelial cells. We first identified this kinase in prostate cancer [Robinson, D., He, F., Pretlow, T. & Kung, H. J. (1996) Proc. Natl. Acad. Sci. USA 93, 5958-5962). Here we report that Etk is engaged in phosphatidylinositol 3-kinase (PI3-kinase) pathway and plays a pivotal role in interleukin 6 (IL-6) signaling in a prostate cancer cell line, LNCaP. Our evidence that PI3-kinase is involved in Etk activation includes: (i) Wortmannin, a specific inhibitor of PI3-kinase, abolished the activation of Etk by IL-6; (ii) a constitutively active p110 subunit of PI3-kinase was able to activate Etk in the absence of IL-6; and (iii) a dominant negative p85 subunit of PI3-kinase mutant blocked the activation of Etk by IL-6. Interestingly, IL-6 treatment of LNCaP induced a remarkable neuroendocrine-like differentiation phenotype, with neurite extension and enhanced expression of neuronal markers. This phenotype could be abrogated by the overexpression of a dominant-negative Etk, indicating Etk is required for this differentiation process.

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.

Tec and Jak2 Kinases Cooperate to Mediate Cytokine-Driven Activation of c-fos Transcription

Although transcriptional activation of the c-fosproto-oncogene plays an intrinsic role in the mechanism of blood cell growth, it is still obscure how protein-tyrosine kinases (PTKs) regulate the cytokine-driven c-fos activation pathway. We present here that Tec PTK is tyrosine-phosphorylated and activated by granulocyte-macrophage colony-stimulating factor (GM-CSF) stimulation in a human GM-CSF–dependent cell line. Moreover, we could show that introduction of Tec into mouse BA/F3-hGMRαβ cells can profoundly activate the c-fos promoter in response to GM-CSF or to interleukin-3 (IL-3). In contrast, introduction of a kinase-deleted Tec could suppress cytokine-driven c-fos activation, indicating that Tec is directly involved in the regulation of c-fos transcription. Interestingly, strong activation by Tec of the c-fos promoter was blocked by the co-expression of dominant negative Jak2. The molecular interaction between Tec and Jak2 was then investigated both in mammalian and insect cell systems, revealing that they can not only bind to each other, but either of the two can phosphorylate the other. Thus, Tec and Jak2 can “cross-talk” in a complexed way to mediate cytokine-driven c-fos activation.

Tec and Jak2 Kinases Cooperate to Mediate Cytokine-Driven Activation of c-fos Transcription

Although transcriptional activation of the c-fosproto-oncogene plays an intrinsic role in the mechanism of blood cell growth, it is still obscure how protein-tyrosine kinases (PTKs) regulate the cytokine-driven c-fos activation pathway. We present here that Tec PTK is tyrosine-phosphorylated and activated by granulocyte-macrophage colony-stimulating factor (GM-CSF) stimulation in a human GM-CSF–dependent cell line. Moreover, we could show that introduction of Tec into mouse BA/F3-hGMRαβ cells can profoundly activate the c-fos promoter in response to GM-CSF or to interleukin-3 (IL-3). In contrast, introduction of a kinase-deleted Tec could suppress cytokine-driven c-fos activation, indicating that Tec is directly involved in the regulation of c-fos transcription. Interestingly, strong activation by Tec of the c-fos promoter was blocked by the co-expression of dominant negative Jak2. The molecular interaction between Tec and Jak2 was then investigated both in mammalian and insect cell systems, revealing that they can not only bind to each other, but either of the two can phosphorylate the other. Thus, Tec and Jak2 can “cross-talk” in a complexed way to mediate cytokine-driven c-fos activation.

Interleukin 5 and B cell differentiation

Interleukin-5 (IL-5) stimulates proliferation and differentiation of B cells and eosinophils. IL-5 receptor (IL-5R) comprises alpha and (beta)c chains. IL-5 specifically binds to IL-5Ralpha and induces the recruitment of (beta)c to IL-5Ralpha. JAK2 and JAK1 tyrosine kinases are constitutively associated with hIL-5Ralpha and (beta)c, respectively and activated upon IL-5 stimulation. IL-5 induces tyrosine phosphorylations of cellular proteins including (beta)c and STAT5 and activates Btk. X-linked immunodeficient mice have B-cell-specific defects due to missense mutation of the btk gene. The cytoplasmic proline-rich regions of both IL-5Ralpha and (beta)c are essential for the IL-5 signalling. IL-5 appears to play a critical role in hypereosinophilic syndromes and atopic diseases. The treatment of animals with anti-IL-5 mAb can decrease the enhanced bronchial responsiveness induced by allergen sensitization. Clinical studies provide a strong impetus for investigating the means of modulating IL-5 effects.

Expression and Activation of the Nonreceptor Tyrosine Kinase Tec in Human B Cells

The tyrosine kinase Tec belongs to a new group of structurally related nonreceptor tyrosine kinases that also includes Btk and Itk. Previous studies have suggested that these kinases have lineage-specific roles, with Tec being involved mainly in the regulation of cytokine-mediated myeloid cell growth and differentiation. In this study, we investigated expression and activation of Tec in human B-lymphoid cell lines representing different stages of B-cell maturation, including pro-B (RS4;11, 380, REH), pre-B (NALM6), and mature B (Ramos, and one Epstein-Barr virus [EBV]-transformed lymphoblastoid line) cells. Like Btk, Tec protein was expressed in all B-cell lines tested. Tec was also highly expressed in two EBV-transformed lymphoblastoid lines derived from patients with X-linked agammaglobulinemia (XLA) lacking Btk expression, as well as in tonsillar lymphoid cells. In surface immunoglobulin-positive B cells (Ramos), ligation of the B-cell antigen receptor (BCR) with anti-IgM antibodies caused marked tyrosine phosphorylation of Tec and increased Tec tyrosine kinase activity. Likewise, cross-linking of CD19 with a monoclonal antibody in BCR-negative pro-B (RS4;11, 380) and pre-B (NALM6) cells induced Tec tyrosine phosphorylation and increased Tec autophosphorylation, as well as Btk activation. Tyrosine phosphorylation of Tec, but not of Btk, was detectable in RS4;11 cells after CD38 ligation, suggesting that these kinases are regulated differently. We conclude that Tec is expressed and can be stimulated throughout human B-cell differentiation, implying that this tyrosine kinase plays a role in B-cell development and activation.

Mutation screening of the BTK gene in 56 families with X-linked agammaglobulinemia (XLA): 47 unique mutations without correlation to clinical course

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.