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

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.

Mutations in the human lambda5/14.1 gene result in B cell deficiency and agammaglobulinemia

B cell precursors transiently express a pre-B cell receptor complex consisting of a rearranged mu heavy chain, a surrogate light chain composed of lambda5/14.1 and VpreB, and the immunoglobulin (Ig)-associated signal transducing chains, Igalpha and Igbeta. Mutations in the mu heavy chain are associated with a complete failure of B cell development in both humans and mice, whereas mutations in murine lambda5 result in a leaky phenotype with detectable humoral responses. In evaluating patients with agammaglobulinemia and markedly reduced numbers of B cells, we identified a boy with mutations on both alleles of the gene for lambda5/14.1. The maternal allele carried a premature stop codon in the first exon of lambda5/14.1 and the paternal allele demonstrated three basepair substitutions in a 33-basepair sequence in exon 3. The three substitutions correspond to the sequence in the lambda5/14. 1 pseudogene 16.1 and result in an amino acid substitution at an invariant proline. When expressed in COS cells, the allele carrying the pseudogene sequence resulted in defective folding and secretion of mutant lambda5/14.1. These findings indicate that expression of the functional lambda5/14.1 is critical for B cell development in the human.

Molecular cloning and characterization of a highly conserved human 67-kDa laminin receptor pseudogene mapping to Xq21.3

A highly conserved laminin receptor processed pseudogene (LAMRL5) that has been isolated from a fetal brain cDNA library is described. The pseudogene is a complete copy (97.9% identical) of the transcribed laminin receptor (LAMR1) with all the introns precisely removed. The sequence has direct repeats of 18 bp at either end. It has an 885 nucleotide open reading frame from the start methionine codon to the stop codon that contains no deletions, additions or premature stop codons relative to the expressed LAMR1 gene and has the coding potential for a protein of 295 amino acids. Although TATA and CAAT boxes exist in the region 5' to the open reading frame and a polyadenylation signal is present in the 3' region, no evidence could be obtained either by reverse transcriptase-polymerase chain reaction (RT-PCR) or in the expressed sequence tag (EST) database that LAMRL5 is expressed in vivo. If not expressed, it is estimated that this LAMRL5 pseudogene was incorporated into the human genome approximately 3.5-5 million years ago.

Transcriptional Regulatory Elements Within the First Intron of Bruton's Tyrosine Kinase

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

Transcriptional Regulatory Elements Within the First Intron of Bruton's Tyrosine Kinase

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

BTKbase, mutation database for X-linked agammaglobulinemia (XLA)

X-linked agammaglobulinemia (XLA) is an immunodeficiency caused by mutations in the gene coding for Bruton's agammaglobulinemia tyrosine kinase (BTK). A database (BTKbase) of BTK mutations has been compiled and the recent update lists 463 mutation entries from 406 unrelated families showing 303 unique molecular events. In addition to mutations, the database also lists variants or polymorphisms. Each patient is given a unique patient identity number (PIN). Information is included regarding the phenotype including symptoms. Mutations in all the five domains of BTK have been noticed to cause the disease, the most common event being missense mutations. The mutations appear almost uniformly throughout the molecule and frequently affect CpG sites that code for arginine residues. The putative structural implications of all the missense mutations are given in the database. The improved version of the registry having a number of new features is available at http://www. helsinki.fi/science/signal/btkbase.html

Phosphatidylinositol 3-kinase-gamma activates Bruton's tyrosine kinase in concert with Src family kinases

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

Predominant Expression of Murine Bmx Tyrosine Kinase in the Granulo-Monocytic Lineage

In the course of systematic cloning of protein tyrosine kinases (PTKs) expressed in hematopoietic stem and progenitor cells, we have identified the murine homologue of human Bmx. It encodes a protein containing the five domains characteristic of the Tec family of cytoplasmic src-related PTKs: pleckstrin homology (PH), Tec homology (TH), src homology 3 and 2 (SH3 and SH2), and tyrosine kinase (TK). In adults, Bmx expression was found primarily in bone marrow and at a lower level in lung and heart. During fetal development it was also found in the spleen at late stage of gestation and in neonates. Analysis of bone marrow subpopulations showed that Bmx was expressed in the progenitor cell population and maturing hematopoietic cells of the granulo/monocytic lineage where expression increased with maturation and differentiation. At the periphery, a high level of Bmx expression was also found in neutrophils and monocytes/macrophages. Bmx expression was not detected in the primitive hematopoietic stem cell population, and cells of the B-, T-, and erythroid-lineages. It was also not detected in most of the cell lines examined. Our results indicate that Bmx is another member of the Btk/Itk/Tec PTK family, which is predominantly expressed in the granulo-monocytic lineage within the hematopoietic system.

An academic centre for gene therapy research with clinical grade manufacturing capability

Huddinge University Hospital is a major teaching hospital affiliated with the Karolinska Institute in Southern Stockholm. For the past few years several groups have been working there in different areas of gene therapy relating to cancer, genetic and infectious diseases. However, a facility to produce clinical grade material under good manufacturing practice was lacking. To this end, Huddinge University Hospital has taken the initiative to open a Gene Therapy Research Center in 1996. This facility, which is unique of its kind in Scandinavia, is located in the Novum Research Park, Huddinge, and is a part of the existing Clinical Research Center. The newly built centre will allow clinicians and researchers to develop and produce vectors (viral and nonviral) for clinical trials and do basic research to understand the mechanisms of diseases. Although the centre will primarily serve the academic institutions it will also extend its facilities to other investigators in this field. The production unit is run in collaboration with the Faculty of Medicine, University of Lund. On-going projects include production of plasmid vectors for prevention of postangioplasty restenosis, DNA vaccine for HIV-1, cationic liposome DNA complexes for cystic fibrosis and retroviral vectors for HIV-1.

Predominant Expression of Murine Bmx Tyrosine Kinase in the Granulo-Monocytic Lineage

In the course of systematic cloning of protein tyrosine kinases (PTKs) expressed in hematopoietic stem and progenitor cells, we have identified the murine homologue of human Bmx. It encodes a protein containing the five domains characteristic of the Tec family of cytoplasmic src-related PTKs: pleckstrin homology (PH), Tec homology (TH), src homology 3 and 2 (SH3 and SH2), and tyrosine kinase (TK). In adults, Bmx expression was found primarily in bone marrow and at a lower level in lung and heart. During fetal development it was also found in the spleen at late stage of gestation and in neonates. Analysis of bone marrow subpopulations showed that Bmx was expressed in the progenitor cell population and maturing hematopoietic cells of the granulo/monocytic lineage where expression increased with maturation and differentiation. At the periphery, a high level of Bmx expression was also found in neutrophils and monocytes/macrophages. Bmx expression was not detected in the primitive hematopoietic stem cell population, and cells of the B-, T-, and erythroid-lineages. It was also not detected in most of the cell lines examined. Our results indicate that Bmx is another member of the Btk/Itk/Tec PTK family, which is predominantly expressed in the granulo-monocytic lineage within the hematopoietic system.

SH3 domain of Bruton's tyrosine kinase can bind to proline-rich peptides of TH domain of the kinase and p120cbl

X-linked agammaglobulinemia (XLA), an inherited disease, is caused by mutations in the Bruton's tyrosine kinase (BTK). The absence of functional BTK leads to failure of B-cell differentiation; this incapacitates antibody production in XLA patients, who suffer from recurrent, sometimes lethal, bacterial infections. BTK plays an important role in B-cell development; it interacts with several proteins in the context of signal transduction. Point mutation in the BTK gene that leads to deletion of C-terminal 14 aa residues of BTK SH3 domain was found in a patient family. To understand the role of BTK, we studied binding of BTK SH3 domain (aa 216-273, 58 residues) and truncated SH3 domain (216-259, 44 residues) with proline-rich peptides; the first peptide constitutes the SH3 domain of BTK, while the latter peptide lacks 14 amino acid residues of the C terminal. Proline-rich peptides selected from TH domain of BTK and p120cbl were studied. It is known that BTK TH domain binds to SH3 domains of various proteins. We found that BTK SH3 domain binds to peptides of BTK TH domain. This suggests that BTK SH3 and TH domains may associate in inter- or intramolecular fashion, which raises the possibility that the kinase may be regulating its own activity by restricting the availability of both its ligand-binding modules. We also found that truncated SH3 domain binds to BTK TH domain peptide less avidly than does normal SH3 domain. Also, we show that the SH3 and truncated SH3 domains bind to peptide of p120cbl, but the latter domain binds weakly. It is likely that the truncated SH3 domain fails to present to the ligand the crucial residues in the correct context, hence the weaker binding. These results delineate the importance of C-terminal in binding of SH3 domains and indicate also that improper folding and the altered binding behavior of mutant BTK SH3 domain likely leads to XLA.

Btk dosage determines sensitivity to B cell antigen receptor cross-linking

Mutations in Btk result in the B cell immunodeficiencies X-linked agammaglobulinemia (XLA) in humans and X-linked immunodeficiency (xid) in mice. Btk is a critical component of signaling pathways regulating B cell development and function. We used a genetic approach to determine whether Btk is also limiting for these processes. One allele of a murine Btk transgene expressed a dosage of Btk (25% of endogenous levels in splenic B cells) sufficient to restore normal numbers of phenotypically mature conventional B cells in xid mice. 2,4,6-trinitrophenyl-Ficoll response, anti-IgM-induced proliferation, B1 cell development, and serum IgM and IgG3 levels remained significantly impaired in these animals. B cells from Btk -/- transgenic mice also responded poorly to anti-IgM, indicating that the xid mutation does not create a dominant negative form of Btk. Response to 2,4,6-trinitrophenyl-Ficoll and B cell receptor cross-linking were increased 3- to 4-fold in xid mice homozygous for the transgene. These results demonstrate that Btk is a limiting component of B cell antigen receptor signaling pathways and suggest that B cell development and response to antigen may require different levels of Btk activity.

Extreme variation in X-linked agammaglobulinemia phenotype in a three-generation family

BACKGROUND

X-linked agammaglobulinemia is typically a severe life-threatening disease characterized by the failure of B-cell differentiation and antibody production, which manifests in infancy and early childhood. Recently, we reported a novel mutation (Cys145-->STOP) in Bruton's tyrosine kinase in a 51-year-old man who was referred for evaluation because of chronic nasal congestion, recurrent sinusitis, sporadic pneumonia, and a family history suggestive of an X-linked immunodeficiency disease. He had not been treated with gammaglobulin.

OBJECTIVE

This study was performed to investigate the clinical and immunologic phenotypes of this patient's other affected male family members.

METHODS

A detailed family history and comprehensive review of medical records was carried out. Genetic mutation analysis of the gene encoding Bruton's tyrosine kinase was carried out in the proband's brother and nephew.

RESULTS

Clinically affected male family members exhibit marked phenotypic variation with manifestations ranging from extremely mild to severe recurrent infections. Immunologic evaluation revealed extreme variation in immunoglobulin levels, B-cell numbers, and functional antibody titers. Genetic analysis documented a novel mutation in the gene encoding Bruton's tyrosine kinase in the proband, his brother, and his nephew.

CONCLUSIONS

Despite their sharing the same genetic abnormality, extreme variation was noted in the immunologic findings and phenotypic expression of affected family members. This family study is extraordinary in that clinically affected male members who did not receive aggressive medical treatment died of the disease in childhood or survived into late adulthood.

Identification of two tyrosine phosphoproteins, pp70 and pp68, which interact with phospholipase Cgamma, Grb2, and Vav after B cell antigen receptor activation

Tyrosine phosphorylation of cellular proteins mediates the assembly and localization of effector proteins through interactions facilitated by modular Src homology 2 (SH2) and phosphotyrosine binding domains. We describe here two tyrosine-phosphorylated proteins with Mr values of 70,000 and 68,000 that interact with Grb2, phospholipase C (PLCgamma1 and PLCgamma2), and Vav after B cell receptor cross-linking. The interaction of pp70 and pp68 with PLC and Vav is mediated by the carboxyl-terminal SH2 domain of PLC and the SH2 domain of Vav. In contrast, the interaction of pp70 and pp68 with Grb2 requires cooperative binding of the SH2 and SH3 domains of Grb2. Western blot analysis demonstrated that neither pp70 nor pp68 represented the recently described linker protein SLP-76, which binds Grb2, PLC, and Vav in T cells after T cell receptor activation. Moreover, SLP-76 protein was not detected in a number of B cell lines or in normal mouse B cells. Hence, we propose that pp70 and pp68 likely represent B cell homologs of SLP-76 which facilitate and coordinate B cell activation.

SOCS-1/JAB/SSI-1 can bind to and suppress Tec protein-tyrosine kinase

Tec is the prototype of a recently emerging subfamily among nonreceptor type protein-tyrosine kinases and is known to become tyrosine-phosphorylated and activated by a wide range of cytokine stimulations in hematopoietic cells. Although Tec was recently shown to be involved in the cytokine-driven activation mechanism of c-fos transcription, it is yet obscure how Tec relays the signals from cell surface receptors to the nucleus. To identify signaling molecules acting downstream of Tec, we have looked for Tec-interacting proteins (TIPs) by using the yeast two-hybrid system. Here we report the identification and characterization of a novel protein, TIP3, which has been simultaneously identified by other groups as SOCS-1, JAB, or SSI-1. TIP3 carries one Src homology 2 domain with a sequence similarity to that of CIS. In 293 cells, TIP3 associates with Tec and suppresses its kinase activity. Interestingly, TIP3 can also down-regulate the activity of Jak2 but not that of Lyn. We propose that SOCS-1/JAB/SSI-1/TIP3 is a novel type of negative regulator to a subset of protein-tyrosine kinases.

A PCR-based non-radioactive X-chromosome inactivation assay for genetic counseling in X-linked primary immunodeficiencies

The Wiskott-Aldrich syndrome (WAS), X-linked severe combined immunodeficiency (SCIDX1), and X-linked agammaglobulinemia (XLA) are severe congenital immunodeficiencies with X-linked inheritance. Although rare, they are all associated with severe infections from early in life, and high morbidity and mortality. Female carriers of these diseases can be identified by a non-random pattern of X-chromosomal inactivation in cell lineages targeted by each gene defect. For patients with WAS, SCIDX1 or XLA, the demonstration of non random X-Chromosome inactivation in their mothers can be used to confirm clinical diagnosis. Furthermore, analysis of X-Chromosome inactivation in at risk females allows preconceptional carrier detection, thus representing an important aid in genetic counseling. For each disease we established a PCR-based, non radioactive assay at the human androgen receptor (HUMARA) locus, that allows analysis of X-Chromosome inactivation in the affected cell types and in tissue specific controls to exclude the issue of skewed X-chromosomal inactivation. In our study, 50 females with a known family history of XLA [19], WAS [18], and SCIDX1 [13],were examined. A carrier status was established in 19 females (7 XLA, 6 WAS, 6 SCIDX1) and excluded in 29 ( 11 XLA, 11 WAS, 7 SCIDX1). Only in 2 cases (4%) the assay was not informative.

Src-induced activation of inducible T cell kinase (ITK) requires phosphatidylinositol 3-kinase activity and the Pleckstrin homology domain of inducible T cell kinase

The Tec family of tyrosine kinases are involved in signals emanating from cytokine receptors, antigen receptors, and other lymphoid cell surface receptors. One family member, ITK (inducible T cell kinase), is involved in T cell activation and can be activated by the T cell receptor and the CD28 cell surface receptor. This stimulation of tyrosine phosphorylation and activation of ITK can be mimicked by the Src family kinase Lck. We have explored the mechanism of this requirement for Src family kinases in the activation of ITK. We found that coexpression of ITK and Src results in increased membrane association, tyrosine phosphorylation and activation of ITK, which could be blocked by inhibitors of the lipid kinase phosphatidylinositol 3-kinase (PI 3-kinase) as well as overexpression of the p85 subunit of PI 3-kinase. Removal of the Pleckstrin homology domain (PH) of ITK resulted in a kinase that could no longer be induced to localize to the membrane or be activated by Src. The PH of ITK was also able to bind inositol phosphates phosphorylated at the D3 position. Membrane targeting of ITK without the PH recovered its ability to be activated by Src. These results suggest that ITK can be activated by a combination of Src and PI 3-kinase.

Phosphorylation of two regulatory tyrosine residues in the activation of Bruton's tyrosine kinase via alternative receptors

Mutation of Bruton's tyrosine kinase (Btk) impairs B cell maturation and function and results in a clinical phenotype of X-linked agammaglobulinemia. Activation of Btk correlates with an increase in the phosphorylation of two regulatory Btk tyrosine residues. Y551 (site 1) within the Src homology type 1 (SH1) domain is transphosphorylated by the Src family tyrosine kinases. Y223 (site 2) is an autophosphorylation site within the Btk SH3 domain. Polyclonal, phosphopeptide-specific antibodies were developed to evaluate the phosphorylation of Btk sites 1 and 2. Crosslinking of the B cell antigen receptor (BCR) or the mast cell Fcepsilon receptor, or interleukin 5 receptor stimulation each induced rapid phosphorylation at Btk sites 1 and 2 in a tightly coupled manner. Btk molecules were singly and doubly tyrosine-phosphorylated. Phosphorylated Btk comprised only a small fraction (</=5%) of the total pool of Btk molecules in the BCR-activated B cells. Increased dosage of Lyn in B cells augmented BCR-induced phosphorylation at both sites. Kinetic analysis supports a sequential activation mechanism in which individual Btk molecules undergo serial transphosphorylation (site 1) then autophosphorylation (site 2), followed by successive dephosphorylation of site 1 then site 2. The phosphorylation of conserved tyrosine residues within structurally related Tec family kinases is likely to regulate their activation.

Lck phosphorylates the activation loop tyrosine of the Itk kinase domain and activates Itk kinase activity

The Tec family tyrosine kinase Itk has been implicated in T cell receptor (TCR) signaling, yet its precise role and mechanism of activation remain undefined. To investigate these issues, we examined the biochemical response of Itk to TCR stimulation. We found that Itk is tyrosine-phosphorylated after TCR cross-linking and that this phosphorylation depends on the presence of functional Lck. To determine if this Lck dependence results from direct phosphorylation of Itk by Lck, we generated recombinant Itk and Lck using a baculovirus expression system and used these proteins in subsequent biochemical analyses. We found that Lck phosphorylates Itk upon co-expression in insect cells and, further, that this phosphorylation of Itk results in increased Itk in vitro kinase activity. The major site of Lck phosphorylation on Itk was mapped to the conserved tyrosine (Tyr511) in the activation loop of the Itk kinase domain. Substitution of this tyrosine with phenylalanine abolishes Itk kinase activity in insect cells, indicating that phosphorylation at this site plays a critical role in regulating Itk function.

Antiviral immune responses in Itk-deficient mice

Mice lacking Itk, a T-cell-specific protein tyrosine kinase, have reduced numbers of T cells and reduced responses to allogeneic major histocompatibility molecules. This study analyzed antiviral immune responses in mice deficient for Itk. Primary cytotoxic T-lymphocyte (CTL) responses were analyzed after infection with lymphocytic choriomeningitis virus (LCMV), vaccinia virus (VV), and vesicular stomatitis virus (VSV). Ex vivo CTL activity was consistently reduced by a factor of two to six for the different viruses. CTL responses after restimulation in vitro were similarly reduced unless exogenous cytokines were added. In the presence of interleukin-2 or concanavalin A supernatant, Itk-deficient and control mice responded similarly. Interestingly, while LCMV was completely eliminated by day 8 in both Itk-deficient and control mice, VV cleared from itk-/- mice with delayed kinetics. Antibody responses were evaluated after VSV infection. Both the T-cell-independent neutralizing immunoglobulin M (IgM) and the T-cell-dependent IgG responses were similar in Itk-deficient and control mice. Taken together, the results show that CTL responses are reduced in the absence of Itk whereas antiviral B-cell responses are not affected.

Integrin-dependent tyrosine phoshorylation and cytoskeletal translocation of Tec in thrombin-activated platelets

Using a specific polyclonal anti-Tec antibody, we have shown that Tec is expressed in human platelets. In addition, Tec was found to undergo tyrosine phosphorylation during platelet activation. The phosphorylation increased after 1 min and remained stable after 3 min of thrombin treatment. The tetrapeptide RGDS inhibited more than 90% of thrombin-induced tyrosine phosphorylation of Tec and blocked its translocation to the cytoskeleton. These results suggest that Tec participates in platelet signaling downstream of integrin activation.

Identification and characterization of the human homologue of SH3BP2, an SH3 binding domain protein within a common region of deletion at 4p16.3 involved in bladder cancer

In a search for candidate tumor suppressor genes within a 30-kb common region of deletion previously identified in bladder cancer cell lines, we isolated a 2.4-kb cDNA clone comprising 13 exons that spanned approximately 16 kb of genomic DNA. Mutation analysis was carried out by single-strand conformation polymorphism analysis on DNA from 12 bladder carcinoma cell lines and 26 bladder tumors with LOH on chromosome 4p. Direct sequencing of the transcript in 4 bladder carcinoma cell lines with deletions in this region was also carried out. Two polymorphisms in exons 2 and 5 were identified, but no tumor-specific mutations were found. Sequence analysis identified a high degree of homology with the mouse sh3bp2 gene, which is abl-binding, suggesting that this gene is the human homologue. The predicted amino acid sequence of the putative gene product contains a Src homology 2 domain, a Src homology 3 binding domain, and a pleckstrin homology domain, suggesting a possible role in signal transduction. No evidence was found to indicate that SH3BP2 is the tumor suppressor gene at 4p16.3 involved in bladder cancer. However, this study has identified an interesting human gene that is a potential negative regulator of the abl oncogene.

The X-linked immunodeficiency defect in the mouse is corrected by expression of human Bruton's tyrosine kinase from a yeast artificial chromosome transgene

Mutations in the gene for Bruton's tyrosine kinase result in the B cell differentiation defects X-linked agammaglobulinemia in man and X-linked immunodeficiency in mice. Here we describe the generation of two yeast artificial chromosome (YAC)-transgenic mouse strains in which high-level expression of human Btk is provided by endogenous regulatory cis-acting elements that are present on a 340-kb transgene, Yc340-hBtk. The expression pattern of the transgenic human Btk was found to parallel that of the endogenous murine gene. When the Yc340-hBtk-transgenic mice were mated onto a Btk-deficient background, the xid B cell defects were fully corrected: conventional and CD5+ B-1 B cells were present in normal numbers, serum IgM and IgG3 levels as well as responses to T cell-independent type II antigens were in the normal ranges. In vivo competition experiments in Btk+/- female mice demonstrated that in the conventional B cell population the Yc340-hBtk transgene could fully compensate the absence of expression of endogenous murine Btk. We conclude that in the YAC-transgenic mice Btk is appropriately expressed in the context of native regulatory sequences.

The spectrum of primary immunodeficiency disorders in Australia

BACKGROUND

Primary immunodeficiency disorders (PIDs) are uncommon conditions that require specialized immunologic services for diagnosis and management. It is difficult to estimate the prevalence of these disorders from routinely collected health statistics.

OBJECTIVE

We attempted to describe the prevalence of PID in Australia and the requirements for specific therapies, such as intravenous immunoglobulin, ascertained from a national register of PID.

METHODS

A national longitudinal cross-sectional survey of patients with PID under the care of clinical immunologists was established by the Australasian Society of Allergy and Clinical Immunology in 1990. Details of diagnosis and therapy were provided for patients with major PIDs including symptomatic IgA, IgG subclass, and complement deficiencies. Subjects with asymptomatic IgA deficiency were not included. The clinical features of the first 500 cases enrolled in the register were analyzed.

RESULTS

The most frequent type of PID was predominant antibody deficiency (71%). Common variable immunodeficiency, usually first seen as an antibody deficiency, was the single most common disorder with an estimated prevalence of 0.77/100,000 in the general population. Other types of PID were infrequent, and of these, severe combined immunodeficiency accounted for 5.2% of cases. The estimated prevalence of all forms of chronic granulomatous disease was 0.08/100,000. The national prevalence of all PID cases ascertained from the register was 2.1/100,000, with variation between the larger states ranging from 1.18 to 4.57/100,000. Half (247) of the patients were receiving intravenous immunoglobulin therapy with a median duration of care of 5 to 9 years for the different antibody deficiencies. There was also variation in the patterns of intravenous immunoglobulin use across the country. No new forms of PID were encountered.

CONCLUSION

This study highlights the requirement for the continuing provision of immunoglobulin as replacement therapy for these patients. In addition, the register documents a cohort of patients with PID whose long-term response to current therapy can be evaluated prospectively.

Missense mutations affecting a conserved cysteine pair in the TH domain of Btk

Tec family protein tyrosine kinases have in their N-terminus two domains. The PH domain is followed by Tec homology (TH) domain, which consists of two motifs. The first pattern, Btk motif, is also present in some Ras GAP molecules. C-terminal half of the TH domain, a proline-rich region, has been shown to bind to SH3 domains. Mutations in Bruton's tyrosine kinase (Btk) belonging to the Tec family cause X-linked agammaglobulinemia (XLA) due to developmental arrest of B cells. Here we present the first missense mutations in the TH domain. The substitutions affect a conserved pair of cysteines, residues 154 and 155, involved in Zn2+ binding and thereby the mutations alter protein folding and stability.

Role of Btk in B cell development and signaling

Bruton's tyrosine kinase (Btk), the target of inactivating mutations in X-linked immunodeficiency diseases of mice and humans, is essential for normal B cell responsiveness. Recent studies have outlined a mechanism for the activation of Btk by B cell receptor engagement and have identified proximal and distal targets of Btk action.

B cell-deficient muMT mice as an experimental model for Mycoplasma infections in X-linked agammaglobulinemia

B cell-deficient muMT mice were investigated as an experimental model for human X-linked agammaglobulinemia (XLA). Mice were intranasally infected with Mycoplasma pulmonis and in 16 out of 17 muMT mice, dissemination of the bacteria from the airways was observed. More than 50% of these mice developed arthritis and/or changes in periarticular tissues. Mycoplasmal infection in muMT mice thus resembles the disease seen in XLA patients implying the usefulness of the model.

Characterization of the pleckstrin homology domain of Btk as an inositol polyphosphate and phosphoinositide binding domain

We previously reported that the pleckstrin homology (PH) domain of Bruton's tyrosine kinase (Btk) binds Ins(1,3,4,5)P4 and that missense mutations in this domain which cause either human X-linked agammaglobulinemia (XLA) or murine X-linked immunodeficiency (Xid) also dramatically reduce the Ins(1,3,4,5)P4 binding activity. In this paper, we describe the inositol phosphate binding specificity of the Btk PH domain and different inositol polyphosphate binding properties among the PH domains of Tec family kinases. Our results suggest that certain inositol phosphates and/or phosphoinositides are physiological ligands of some Tec family kinases and that Tec family members are differently regulated by inositol molecules.

A complete YAC contig and cosmid interval map covering the entirety of human Xq21.33 to Xq22.3 from DXS3 to DXS287

We have produced a physical map that covers the entirety of Xq21.33 to Xq22.3, from DXS3 to DXS287, approximately 15-17 Mb of the proximal long arm of the human X chromosome. This region has already been shown to contain a number of genes involved in genetic disorders, some of which have yet to be cloned. The physical map consists of a contig of 420 yeast artificial chromosome (YAC) clones ordered with respect to 142 DNA markers, approximately one probe every 110 kb. Forty-three YACs from across the contig have been used to isolate 2019 cosmids that have been mapped into 87 intervals, and 667 of these clones are positive for at least 1 single-copy marker. These YACs and cosmids have been used to confirm data from other published contigs that map to the region. The physical map described here constitutes the first step toward a complete transcriptional map of the region.

A non-XLA primary deficiency causes the earliest known defect of B cell differentiation in humans: a comparison with an XLA case

We report a detailed comparison of B cell defects in two patients, one XLA and one non-XLA. Both had severe agammaglobulinemia with a total absence of CD19+ cells in the periphery. In the non-XLA case, CD19 expression was also highly impaired in the bone marrow, resulting in the absence of both B and preB compartments. Early proB cells were present since CD34+CD10+ and some CD19+CD10+ mostly CD34+ were identified, although diminished. By contrast, in the XLA patient the CD34+CD19+ proB cells were increased whereas the CD34-CD19+ preB cell population was low. Semi-quantitative RT-PCR analysis performed on mononuclear bone marrow cells from the non-XLA patient indicated that lambda-like, VpreB, Rag-1, Rag-2 and TdT transcripts expressed during proB cell stages were found at normal levels whereas E2A, CD10, Syk, Pax-5, CD19, Ig alpha, Ig beta, VH-C mu and V kappa-C kappa transcripts characteristic of later stages were severely depressed. By contrast in the XLA patient most of these transcripts were observed in normal amounts. The phenotype of the non-XLA patient resembles that of Pax-5 or Ig beta knock-out mice, but since the coding sequence of both cDNAs were shown to be normal, the blockage might rather result from an altered regulation of one of these genes or from defect of other genes. All these data indicate that the non-XLA patient suffers from a new genetic defect that results in an arrest of differentiation within the proB cell compartment, before the onset of Ig gene rearrangements. From all agammaglobulinemias reported so far, including XLA cases and those resulting from C mu gene defects, the non-XLA patient exhibits the earliest blockage in the B cell differentiation pathway.

Identification of novel Bruton's tyrosine kinase mutations in 10 unrelated subjects with X linked agammaglobulinaemia

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

Interactions between protein kinase C and pleckstrin homology domains. Inhibition by phosphatidylinositol 4,5-bisphosphate and phorbol 12-myristate 13-acetate

Pleckstrin homology (PH) domains comprised of loosely conserved sequences of approximately 100 amino acid residues are a functional protein motif found in many signal-transducing and cytoskeletal proteins. We recently demonstrated that the PH domains of Tec family protein-tyrosine kinases Btk and Emt (equal to Itk and Tsk) interact with protein kinase C (PKC) and that PKC down-regulates Btk by phosphorylation. In this study we have characterized the PKC-BtkPH domain interaction in detail. Using pure PKC preparations, it was shown that the Btk PH domain interacts with PKC with high affinity (KD = 39 nM). Unlike other tested phospholipids, phosphatidylinositol 4,5-bisphosphate, which binds to several PH domains, competed with PKC for binding to the PH domain apparently because their binding sites on the amino-terminal portion of the PH domains overlap. The minimal PKC-binding sequence within the Btk PH domain was found to correspond roughly to the second and third beta-sheets of the PH domains of known tertiary structures. On the other hand, the C1 regulatory region of PKCepsilon containing the pseudosubstrate and zinc finger-like sequences was found to be sufficient for strong binding to the Btk PH domain. Phorbol 12-myristate 13-acetate (PMA), a potent activator of PKC that interacts with the C1 region of PKC, inhibited the PKC-PH domain interaction, whereas the bioinactive PMA (4-alpha-PMA) was ineffective. The zeta isoform of PKC, which has a single zinc finger-like motif instead of the two tandem zinc finger-like sequences present in conventional and novel PKC isoforms, does not bind PMA. Thus, as expected, PH domain binding with PKCzeta was not interfered with by PMA. Further, inhibitors that are known to attack the catalytic domains of serine/threonine kinases did not affect this PKC-PH domain interaction. In contrast, the presence of physiological concentrations of Ca2+ induced less than a 2-fold increase in PKC-PH domain binding. These results indicate that PKC binding to PH domains involve the beta2-beta3 region of the Btk PH domain and the C1 region of PKC, and agents that interact with either of these regions (i.e. phosphatidylinositol 4,5-bisphosphate binding to the PH domain and PMA binding to the C1 region of PKC) might act to regulate PKC-PH domain binding.

Bruton's tyrosine kinase regulates apoptosis and JNK/SAPK kinase activity

Mast cells derived from Bruton's tyrosine kinase (Btk)-defective xid or btk null mice showed greater expansion in culture containing interleukin-3 (IL-3) than those from wild-type (wt) mice. Although the proliferative response to IL-3 was not significantly different between the wt and xid mast cells, xid and btk null mast cells died by apoptosis more slowly than their wt counterparts upon IL-3 deprivation. Consistent with these findings, the apoptosis-linked c-Jun N-terminal kinase/stress-activated protein kinase (JNK) activity was compromised in these btk-mutated cells upon Fc(epsilon)RI crosslinking or upon stimulation with IL-3 or with stem cell factor. p38 activity was less severely, but significantly, affected by btk mutation, whereas extracellular signal-regulated kinases were not affected by the same mutation. Btk-mediated regulation of apoptosis and JNK activity was confirmed by reconstitution of btk null mutant mast cells with the wt btk cDNA. Furthermore, growth factor withdrawal induced the activation and sustained activity of JNK in wt mast cells, while JNK activity was consistently lower in btk-mutated mast cells. These results support the notion that Btk regulates apoptosis through the JNK activation.

Large-scale comparative sequence analysis of the human and murine Bruton's tyrosine kinase loci reveals conserved regulatory domains

Large-scale genomic DNA sequencing of orthologous and paralogous loci in different species should contribute to a basic understanding of the evolution of both the protein-coding regions and noncoding regulatory elements. We compared 93 kb of human sequence to 89 kb of mouse sequence in the Bruton's tyrosine kinase (BTK) region. In addition to showing the conservation of both position and orientation of the five functionally unrelated genes in the region (BTK, alpha-D-galactosidase A, L44L, FTP-3, and FCI-12), the comparison revealed conservation of clusters of noncoding sequence flanking the first exon of each gene. Furthermore, in the sequence comparison at the BTK locus, the conservation of clusters of noncoding sequence extends throughout the locus; the noncoding sequence is more highly conserved in the BTK locus in comparison to the flanking loci. This suggests a correlation with the complex developmental regulation of expression of btk. To determine whether a highly conserved 3.5-kb segment flanking the first exon of BTK contains transcriptional regulatory signals, we tested various portions of the segment for promoter and expression activity in several appropriate cell lines. The results demonstrate the contribution of the conserved region flanking the first exon to the cell lineage-specific expression pattern of btk. These data show the usefulness of large scale sequence comparisons to focus investigation on regions of noncoding sequence that play essential roles in complex gene regulation.

The roles of preB cell receptor in early B cell development and its signal transduction

The preB cell receptor is expressed for a short period after mu heavy chain is produced, that is, at the large preB cell stage in B cell development. The severe impairment of B cell differentiation observed in mice deficient for the preB cell receptor clearly demonstrated the importance of the preB cell receptor in B cell development. Analyses of bone marrow precursor B cells in normal and B cell-deficient mutant mice indicated the preB cell receptor transduced signals to drive cell cycle and to induce allelic exclusion. The proliferation of the preB cell receptor-expressing cells leads to the selective expansion of cells which have succeeded in the productive rearrangement of mu heavy chain gene. This process builds up a preB cell pool large enough to generate sufficient numbers of mature B cells. The preB cell receptor appears to induce allelic exclusion by shutting off the expression of recombinase activation gene (RAG). In order to analyse the signal transduction pathway downstream of the preB cell receptor, we have developed a new system in which cross-linking of Ig beta expressed on bone marrow proB cells mimics the signalling through the preB cell receptor to induce differentiation from proB to small preB cells.

Correction of the X-linked immunodeficiency phenotype by transgenic expression of human Bruton tyrosine kinase under the control of the class II major histocompatibility complex Ea locus control region

Bruton tyrosine kinase (Btk) is essential for the development of pre-B cells to mature B cell stages. Btk-deficient mice manifest an X-linked immunodeficiency (xid) defect characterized by a reduction of peripheral IgMlow IgDhigh B cells, a lack of peritoneal CD5+ B cells, low serum levels of IgM and IgG3, and impaired responses to T cell independent type II (TI-II) antigens. We have generated transgenic mice in which expression of the human Btk gene is driven by the murine class II major histocompatibility complex Ea gene locus control region, which provides gene expression from the pre-B cell stage onwards. When these transgenic mice were mated onto a Btk- background, correction of the xid B cell defects was observed: B cells differentiated to mature IgMlowIgDhigh stages, peritoneal CD5+ B cells were present, and serum Ig levels and in vivo responses to TI-II antigens were in the normal ranges. A comparable rescue by transgenic Btk expression was also observed in heterozygous Btk+/- female mice in those B-lineage cells that were Btk-deficient as a result of X chromosome inactivation. These findings indicate that the Btk- phenotype in the mouse can be corrected by expression of human Btk from the pre-B cell stage onwards.

BAP-135, a target for Bruton's tyrosine kinase in response to B cell receptor engagement

Bruton's tyrosine kinase (Btk) is essential for B cell activation, but downstream targets of Btk have not been defined. We now describe a protein, BAP-135, that is associated with Btk in B cells and is a substrate for phosphorylation by Btk. BAP-135, which exhibits no detectable homology to known proteins, contains six occurrences of a hitherto undescribed amino acid repeat and two motifs, similar to the Src autophosphorylation site, that represent potential targets for tyrosine phosphorylation. The pleckstrin homology domain of Btk comprises the principal site of BAP-135 binding. Btk-dependent phosphorylation of BAP-135 is abolished by mutations that impair activation of Btk by Src-related kinases. Btk and BAP-135 exist in a complex before B cell antigen receptor (BCR) engagement; in response to BCR crosslinking, BAP-135 is transiently phosphorylated on tyrosine. Taken together, these observations suggest that BAP-135 may reside downstream of Btk in a signaling pathway originating at the BCR.

Regulatory intramolecular association in a tyrosine kinase of the Tec family

The T-cell-specific tyrosine kinase Itk is a member of the Tec family of non-receptor tyrosine kinases, and is required for signalling through the T-cell antigen receptor (TCR). The role of Itk in TCR signalling and the manner in which Itk activity is regulated are not well understood. Substrate binding and enzymatic activity of the structurally related Src kinases are regulated by an intramolecular interaction between the Src-homology-2 (SH2) domain and a phosphotyrosine. Although Itk also contains SH3, SH2 and tyrosine kinase domains, it lacks the corresponding regulatory phosphorylation site, and therefore must be regulated by an alternative mechanism. The proline-rich sequence adjacent to the SH3 domain of Tec family kinases contains an SH3 ligand, potentially allowing a different intramolecular interaction. By using multidimensional nuclear magnetic resonance we have determined the structure of a fragment of Itk, confirming that these domains interact intramolecularly. Formation of this intramolecular SH3-ligand complex prevents the Itk SH3 domain and proline-rich region from interacting with their respective protein ligands, Sam68 and Grb-2. We believe that this structure represents the first example of an intramolecular interaction between an SH3 domain and a proline-rich ligand, and has implications for the regulation of Tec family kinases.

Naturally occurring primary deficiencies of the immune system

Naturally occurring genetic disorders of the immune system provide many models for the study of its development and function. In a way, their analysis complements the information provided by the generation of genetic defects in mice created using homologous recombination techniques. In this review, the recent findings made in three areas are focused upon deficiencies in T cell differentiation and in T lymphocyte activation, and on the control process of peripheral immune response.

BTKbase, mutation database for X-linked agammaglobulinemia (XLA)

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