Bruton's tyrosine kinase: cell biology, sequence conservation, mutation spectrum, siRNA modifications, and expression profiling

Essential roles for the Tec family kinases Tec and Btk in M-CSF receptor signaling pathways that regulate macrophage survival

Tec family kinases have important roles in lymphocytes; however, little is known about their function in monocytes/macrophages. In this study we report that Tec family kinases are essential for M-CSF (M-CSF)-induced signaling pathways that regulate macrophage survival. Compared with wild-type bone marrow-derived macrophage (BMM) cultures, Tec(-/-)Btk(-/-) BMM cultures displayed increased cell death that correlated with a severe drop in macrophage numbers. In addition, macrophages deficient in either Tec or Btk showed expression and activation of caspase-11. Elucidation of M-CSF receptor (M-CSFR) signaling pathways revealed that the total tyrosine phosphorylation pattern upon M-CSF stimulation was altered in Tec(-/-)Btk(-/-) macrophages despite normal expression and phosphorylation of the M-CSFR. Further, Tec and Btk are required for proper expression of the GM-CSF receptor alpha (GM-CSFRalpha) chain in macrophages but not dendritic cells, implicating Tec family kinases in the lineage-specific regulation of GM-CSFRalpha expression. Taken together, our study shows that Tec and Btk regulate M-CSFR signaling-induced macrophage survival and provides a novel link between Tec family kinases and the regulation of caspase-11 and GM-CSFRalpha expression.

Genotype-phenotype correlation in Bruton's tyrosine kinase deficiency

Bruton's tyrosine kinase (Btk) belongs to the Tec family of nonreceptor protein tyrosine kinases. Mutations in the BTK gene cause X-linked agammaglobulinemia (XLA); a primary immunodeficiency disorder in human. No clear genotype-phenotype correlation has been established in XLA so far. To determine how differently mutations in BTK affect the severity of the disease and if BTK promoter polymorphic variant or intron 1 polymorphic variant in Tec, a cytoplasmic tyrosine kinase that might substitute for Btk, could contribute to the clinical phenotype, we analyzed the clinical and molecular findings in a cohort of XLA patients. Polymorphisms in BTK promoter and TEC intron 1 regions include substitutions of C>T (rs2071219) and T>C (rs2664019), respectively. Btk expression was evaluated by means of western immunoblotting and fluorescence-activated cell sorter analysis. Mutations were categorized as mild or severe and patients were evaluated for the clinical severity of disease. On the basis of the results, severe genotypes do not necessarily lead to severe phenotypes. More over, in a considerable number of patients with mild phenotype we showed a severe mutation with a tendency toward C substitution in the polymorphic site on TEC intron 1.

Adults with X-linked agammaglobulinemia: impact of disease on daily lives, quality of life, educational and socioeconomic status, knowledge of inheritance, and reproductive attitudes

Since many children with X-linked agammaglobulinemia (XLA) can now be expected to reach adulthood, knowledge of the status of adults with XLA would be of importance to the patients, their families, and the physicians caring for these patients. We performed the current study in adults with XLA to examine the impact of XLA on their daily lives and quality of life, their educational and socioeconomic status, their knowledge of the inheritance of their disorder, and their reproductive attitudes. Physicians who had entered adult patients with XLA in a national registry were asked to pass on a survey instrument to their patients. The patients then filled out the survey instrument and returned it directly to the investigators. Adults with XLA were hospitalized more frequently and missed more work and/or school than did the general United States population. However, their quality of life was comparable to that of the general United States population. They achieved a higher level of education and had a higher income than did the general United States population. Their knowledge of the inheritance of their disease was excellent. Sixty percent of them would not exercise any reproductive planning options as a result of their disease. The results of the current study suggest that although the disease impacts the daily lives of adults with XLA, they still become productive members of society and excel in many areas.

Epidemiology, etiology, pathogenesis, and diagnosis of recurrent bacterial meningitis

Recurrent bacterial meningitis is a rare phenomenon and generally poses a considerable diagnostic challenge to the clinician. Ultimately, a structured approach and early diagnosis of any underlying pathology are crucial to prevent further episodes and improve the overall outcome for the affected individual. In this article, we are reviewing the existing literature on this topic over the last two decades, encompassing 363 cases of recurrent bacterial meningitis described in 144 publications. Of these cases, 214 (59%) were related to anatomical problems, 132 (36%) were related to immunodeficiencies, and 17 (5%) were related to parameningeal infections. The review includes a detailed discussion of the underlying pathologies and microbiological aspects as well as recommendations for appropriate diagnostic pathways for investigating this unusual entity.

Cellular membranes and lipid-binding domains as attractive targets for drug development

Interdisciplinary research focused on biological membranes has revealed them as signaling and trafficking platforms for processes fundamental to life. Biomembranes harbor receptors, ion channels, lipid domains, lipid signals, and scaffolding complexes, which function to maintain cellular growth, metabolism, and homeostasis. Moreover, abnormalities in lipid metabolism attributed to genetic changes among other causes are often associated with diseases such as cancer, arthritis and diabetes. Thus, there is a need to comprehensively understand molecular events occurring within and on membranes as a means of grasping disease etiology and identifying viable targets for drug development. A rapidly expanding field in the last decade has centered on understanding membrane recruitment of peripheral proteins. This class of proteins reversibly interacts with specific lipids in a spatial and temporal fashion in crucial biological processes. Typically, recruitment of peripheral proteins to the different cellular sites is mediated by one or more modular lipid-binding domains through specific lipid recognition. Structural, computational, and experimental studies of these lipid-binding domains have demonstrated how they specifically recognize their cognate lipids and achieve subcellular localization. However, the mechanisms by which these modular domains and their host proteins are recruited to and interact with various cell membranes often vary drastically due to differences in lipid affinity, specificity, penetration as well as protein-protein and intramolecular interactions. As there is still a paucity of predictive data for peripheral protein function, these enzymes are often rigorously studied to characterize their lipid-dependent properties. This review summarizes recent progress in our understanding of how peripheral proteins are recruited to biomembranes and highlights avenues to exploit in drug development targeted at cellular membranes and/or lipid-binding proteins.

XLA-associated neutropenia treatment: a case report and review of the literature

X-linked agammaglobulinemia (XLA) is a primary B-cell deficiency syndrome with an incidence of 5 to 10 cases per million. The current treatment approach includes intravenous immunoglobulin and aggressive antibiotic regimens for infections. Besides recurrent infections, XLA patients may present with other manifestations, such as alopecia, enteropathy, amyloidosis, and neutropenia. Neutropenia, which has been shown in up to 25% of affected patients, might also contribute to the degree of severity of bacterial infections that have been reported in these cases. Here we present our experience with the granulocyte colony-stimulant factor, filgrastim (Neupogen), in the treatment of neutropenia in a 14-month-old child with XLA.

Physical and functional characterization of the genetic locus of IBtk, an inhibitor of Bruton's tyrosine kinase: evidence for three protein isoforms of IBtk

Bruton's tyrosine kinase (Btk) is required for B-cell development. Btk deficiency causes X-linked agammaglobulinemia (XLA) in humans and X-linked immunodeficiency (Xid) in mice. Btk lacks a negative regulatory domain and may rely on cytoplasmic proteins to regulate its activity. Consistently, we identified an inhibitor of Btk, IBtk, which binds to the PH domain of Btk and down-regulates the Btk kinase activity. IBtk is an evolutionary conserved protein encoded by a single genomic sequence at 6q14.1 cytogenetic location, a region of recurrent chromosomal aberrations in lymphoproliferative disorders; however, the physical and functional organization of IBTK is unknown. Here, we report that the human IBTK locus includes three distinct mRNAs arising from complete intron splicing, an additional polyadenylation signal and a second transcription start site that utilizes a specific ATG for protein translation. By northern blot, 5′RACE and 3′RACE we identified three IBTKα, IBTKβ and IBTKγ mRNAs, whose transcription is driven by two distinct promoter regions; the corresponding IBtk proteins were detected in human cells and mouse tissues by specific antibodies. These results provide the first characterization of the human IBTK locus and may assist in understanding the in vivo function of IBtk.

Forward genetic dissection of immunity to infection in the mouse

Forward genetics is an experimental approach in which gene mapping and positional cloning are used to elucidate the molecular mechanisms underlying phenotypic differences between two individuals for a given trait. This strategy has been highly successful for the study of inbred mouse strains that show differences in innate susceptibility to bacterial, parasitic, fungal, and viral infections. Over the past 20 years, these studies have led to the identification of a number of cell populations and critical biochemical pathways and proteins that are essential for the early detection of and response to invading pathogens. Strikingly, the macrophage is the point of convergence for many of these genetic studies. This has led to the identification of diverse pathways involved in extracellular and intracellular pathogen recognition, modification of the properties and content of phagosomes, transcriptional response, and signal transduction for activation of adaptive immune mechanisms. In models of viral infections, elegant genetic studies highlighted the pivotal role of natural killer cells in the detection and destruction of infected cells.

Molecular mechanism of membrane targeting by the GRP1 PH domain

The general receptor for phosphoinositides isoform 1 (GRP1) is recruited to the plasma membrane in response to activation of phosphoinositide 3-kinases and accumulation of phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P(3)]. GRP1's pleckstrin homology (PH) domain recognizes PtdIns(3,4,5)P(3) with high specificity and affinity, however, the precise mechanism of its association with membranes remains unclear. Here, we detail the molecular basis of membrane anchoring by the GRP1 PH domain. Our data reveal a multivalent membrane docking involving PtdIns(3,4,5)P(3) binding, regulated by pH and facilitated by electrostatic interactions with other anionic lipids. The specific recognition of PtdIns(3,4,5)P(3) triggers insertion of the GRP1 PH domain into membranes. An acidic environment enhances PtdIns(3,4,5)P(3) binding and increases membrane penetration as demonstrated by NMR and monolayer surface tension and surface plasmon resonance experiments. The GRP1 PH domain displays a 28 nM affinity for POPC/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine/PtdIns(3,4,5)P(3) vesicles at pH 6.0, but binds 22-fold weaker at pH 8.0. The pH sensitivity is attributed in part to the His355 residue, protonation of which is required for the robust interaction with PtdIns(3,4,5)P(3) and significant membrane penetration, as illustrated by mutagenesis data. The binding affinity of the GRP1 PH domain for PtdIns(3,4,5)P(3)-containing vesicles is further amplified (by approximately 6-fold) by nonspecific electrostatic interactions with phosphatidylserine/phosphatidylinositol. Together, our results provide new insight into the multivalent mechanism of the membrane targeting and regulation of the GRP1 PH domain.

Membrane recognition by phospholipid-binding domains

Many different globular domains bind to the surfaces of cellular membranes, or to specific phospholipid components in these membranes, and this binding is often tightly regulated. Examples include pleckstrin homology and C2 domains, which are among the largest domain families in the human proteome. Crystal structures, binding studies and analyses of subcellular localization have provided much insight into how members of this diverse group of domains bind to membranes, what features they recognize and how binding is controlled. A full appreciation of these processes is crucial for understanding how protein localization and membrane topography and trafficking are regulated in cells.

Development of an enhanced B-specific lentiviral vector expressing BTK: a tool for gene therapy of XLA

Further development of haematopoietic stem cell (HSC) gene therapy will depend on enhancement of gene transfer safety: ad hoc improvement of vector design relating to each particular disease is thus a crucial issue for HSC gene therapy. We modified a previously described lentiviral vector by adding the Emumar B-specific enhancer to a human CD19 promoter-derived sequence (Mol Ther 2004;10:45-56). We thus significantly improved the level of expression of the green fluorescent protein (GFP) reporter gene while retaining the specificity of expression in B-cell progeny of transduced human CD34+ progenitor cells obtained from cord blood or adult bone marrow. Indeed, GFP was strongly expressed from early medullary pro-B cells to splenic mature B cells whereas transgene expression remained low in transduced immature progenitors as in myeloid and T-lymphoid progeny retrieved from xenografted NOD/SCID/gammac(null) mice. Using this lentiviral vector, we further demonstrated the possibility to express a functional human BTK protein in long-term human CD34+ cell B-lymphoid progeny. This newly designed lentiviral vector fulfils one of the pre-requisites for the development of efficient and safe gene therapy for X-linked agammaglobulinaemia, the most common primary humoral immunodeficiency disorder.

Proteasome-dependent autoregulation of Bruton tyrosine kinase (Btk) promoter via NF-kappaB

Bruton tyrosine kinase (Btk) is critical for B-cell development. Btk regulates a plethora of signaling proteins, among them nuclear factor-[kappa]B (NF-kappaB). Activation of NF-kappaB is a hallmark of B cells, and NF-kappaB signaling is severely compromised in Btk deficiency. We here present strong evidence indicating that NF-kappaB is required for efficient transcription of the Btk gene. First, we found that proteasome blockers and inhibitors of NF-kappaB signaling suppress Btk transcription and intracellular expression. Similar to Btk, proteasome inhibitors also reduced the expression of other members of this family of kinases, Itk, Bmx, and Tec. Second, 2 functional NF-kappaB-binding sites were found in the Btk promoter. Moreover, in live mice, by hydrodynamic transfection, we show that bortezomib (a blocker of proteasomes and NF-kappaB signaling), as well as NF-kappaB binding sequence-oligonucleotide decoys block Btk transcription. We also demonstrate that Btk induces NF-kappaB activity in mice. Collectively, we show that Btk uses a positive autoregulatory feedback mechanism to stimulate transcription from its own promoter via NF-kappaB.

Expression of Fc gamma and complement receptors in monocytes of X-linked agammaglobulinaemia and common variable immunodeficiency patients

Recently we reported that monocyte phagocytosis and chemotaxis are impaired in X-linked agammaglobulinaemia (XLA) and common variable immunodeficiency (CVI) patients. Few data exist on the in vivo expression of receptors for the constant region of immunoglobulin (IgG) (Fc gammaR) and complement receptors (CR) in these patients. The objective of this study was to investigate the expression of Fc gammaR and CR on monocytes from XLA and CVI patients and compare it to that of healthy controls. Whole blood samples were obtained from 10 patients with XLA, 12 with CVI and 18 healthy controls. Monocyte phenotype was determined by flow cytometry with gating on CD14+ cells. Surface expression of Fc gammaRI (CD64), Fc gammaRII (CD32) and Fc gammaRIII (CD16), CR1 (CD35) and CR3 (CD11b and CD18) was measured by determination of the proportion of CD14+ cells positive for each receptor and by receptor density. Compared to controls, a significantly higher percentage of CD16 and CD35+ monocytes from XLA (P = 0.002 and P = 0.007, respectively) were observed. The relative fluorescence intensity (RFI) expression of Fc gammaRII (CD32) and Fc gammaRIII (CD16) were significantly lower on CVI monocytes compared to controls (P = 0.001 and P = 0.035, respectively). XLA patients, who have a reduction of Bruton's tyrosine kinase (Btk), showed normal or increased percentages of monocytes expressing Fc gamma and complement receptors. CVI patients, who have normal expression of Btk, showed reduced expression of CD16 and CD32 on monocytes. Inefficient chemotaxis and phagocytosis, reported previously in XLA patients, could be due to defects of cytoplasmatic transduction mechanisms.

Contiguous X-chromosome deletion syndrome encompassing the BTK, TIMM8A, TAF7L, and DRP2 genes

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

Tec kinases, actin, and cell adhesion

The Tec family non-receptor tyrosine kinases have been recognized for their roles in the regulation of phospholipase C-gamma and Ca(2+) mobilization downstream from antigen receptors on lymphocytes. Recent data, however, show that the Tec family kinase interleukin-2-inducible T-cell kinase (Itk) also participates in pathways regulating the actin cytoskeleton and 'inside-out' signaling to integrins downstream from the T-cell antigen receptor. Data suggest that Itk may function in a kinase-independent fashion to regulate proper recruitment of the Vav1 guanine nucleotide exchange factor. By enhancing actin cytoskeleton reorganization, recruitment of signaling molecules to the immune synapse, and integrin clustering in response to both antigen and chemokine receptors, the Tec kinases serve as modulators or amplifiers that can increase the duration of T-cell signaling and regulate T-cell functional responses.

Defective Toll-like receptor 9-mediated cytokine production in B cells from Bruton's tyrosine kinase-deficient mice

Bruton's tyrosine kinase (Btk), a member of the Tec family of tyrosine kinases, plays an important role in the differentiation and activation of B cells. Mutations affecting Btk cause immunodeficiency in both humans and mice. In this study we set out to investigate the potential role of Btk in Toll-like receptor 9 (TLR9) activation and the production of pro-inflammatory cytokines such as interleukin (IL)-6, tumour necrosis factor (TNF)-alpha and IL-12p40. Our data show that Btk-deficient B cells respond more efficiently to CpG-DNA stimulation, producing significantly higher levels of pro-inflammatory cytokines but lower levels of the inhibitory cytokine IL-10. The quantitative reverse transcription-polymerase chain reaction (RT-PCR) analysis presented in this work shows that mRNA production of one of the important new members of the IL-12 family, IL-27, was significantly increased in Btk-deficient B cells after CpG-DNA stimulation. In this study, we demonstrate significant differences in CpG responsiveness between transitional 1 (T1) and T2 B cells for survival and maturation. Furthermore, TLR9 expression, measured both as protein and as mRNA, was increased in Btk-defective cells, especially after TLR9 stimulation. Collectively, these data provide evidence in support of the theory that Btk regulates both TLR9 activation and expression in mouse splenic B cells.

Chemical proteomic profiles of the BCR-ABL inhibitors imatinib, nilotinib, and dasatinib reveal novel kinase and nonkinase targets

The BCR-ABL tyrosine kinase inhibitor imatinib represents the current frontline therapy in chronic myeloid leukemia. Because many patients develop imatinib resistance, 2 second-generation drugs, nilotinib and dasatinib, displaying increased potency against BCR-ABL were developed. To predict potential side effects and novel medical uses, we generated comprehensive drug-protein interaction profiles by chemical proteomics for all 3 drugs. Our studies yielded 4 major findings: (1) The interaction profiles of the 3 drugs displayed strong differences and only a small overlap covering the ABL kinases. (2) Dasatinib bound in excess of 30 Tyr and Ser/Thr kinases, including major regulators of the immune system, suggesting that dasatinib might have a particular impact on immune function. (3) Despite the high specificity of nilotinib, the receptor tyrosine kinase DDR1 was identified and validated as an additional major target. (4) The oxidoreductase NQO2 was bound and inhibited by imatinib and nilotinib at physiologically relevant drug concentrations, representing the first nonkinase target of these drugs.

The X-files of inflammation: cellular mosaicism of X-linked polymorphic genes and the female advantage in the host response to injury and infection

Females as compared with males display better general health status, longevity, and improved clinical course after injury and infection. It is generally believed that the female advantage is associated with the effects of sex hormones. This review argues that the sex benefit of females during the host response is associated with polymorphism of X-linked genes and cellular mosaicism for X-linked parental alleles. Cells from females carry both parental X chromosomes (maternal, Xm; or paternal, Xp), whereas males carry only one (Xm). Because of dosage compensation and random X inactivation, half of the cells from females express either Xm or Xp. Therefore, females are cellular mosaics for their X-linked polymorphic genes. This cellular mosaicism in females represents a more adaptive and balanced cellular machinery that is advantageous during the innate immune response. Several genes encoding key metabolic and regulatory proteins reside on the X chromosome, including members of the apoptotic cascade, hormone homeostasis, glucose metabolic enzymes, superoxide-producing machinery, and the toll-like receptor/nuclear factor kappaB/c-Jun N-terminal kinase signaling pathway. Polymorphic forms of these X-linked proteins are likely to manifest in phenotypic differences in the mosaic cell populations in females and may contribute to sex-related differences in the host response to injury and infection. The unique inheritance pattern of X-linked polymorphisms and their potential confounding effects in clinical trials are also discussed; furthermore, we present potential biomarkers for studying mosaic cell populations of innate immunity.

Bruton's tyrosine kinase together with PI 3-kinase are part of Toll-like receptor 2 multiprotein complex and mediate LTA induced Toll-like receptor 2 responses in macrophages

Lipoteichoic acid (LTA) of Gram-positive bacteria initiates innate immune responses via Toll-like receptor-2 (TLR2), resulting in the activation of intracellular signaling and production of inflammatory cytokines in macrophages. Although Bruton's tyrosine kinase (Btk) is biologically important molecule implicated in immune regulation and recently in TLR signaling its importance for LTA-TLR2 mediated responses has not been evaluated. In this study, we detected Btk in the LTA signaling complex with TLR2 and PI 3-kinase (PI3K). The constitutive interaction of these proteins was mediated via PI3K Src homology (SH3) -domain. Both Btk and PI3K were activated by LTA stimulation and the LTA induced cytokine expression was differentially modulated by these kinases. LTA induced the activation of nuclear factor kappaB (NFkappaB), however, only Btk inhibition affected the LTA induced Ser536 phosphorylation and DNA-binding of NFkappaB. In conclusion, our results demonstrate that Btk and PI3K occupy important roles in TLR2-induced activation of macrophages, resulting in selective regulation of cytokines.

Activation mechanism and steady state kinetics of Bruton's tyrosine kinase

Bruton's tyrosine kinase (BTK) is a member of the Tec non-receptor tyrosine kinase family that is involved in regulating B cell proliferation. To better understand the enzymatic mechanism of the Tec family of kinases, the kinetics of BTK substrate phosphorylation were characterized using a radioactive enzyme assay. We first examined whether autophosphorylation regulates BTK activity. Western blotting with a phosphospecific antibody revealed that BTK rapidly autophosphorylates at Tyr(551) within its activation loop in vitro. Examination of a Y551F BTK mutant indicated that phosphorylation of Tyr(551) causes a 10-fold increase in BTK activity. We then proceeded to characterize the steady state kinetic mechanism of BTK. Varying the concentrations of ATP and S1 peptide (biotin-Aca-AAAEEIY-GEI-NH2) revealed that BTK employs a ternary complex mechanism with KmATP = 84 +/- 20 microM and KmS1 = 37 +/- 8 microM. Inhibition studies were also performed to examine the order of substrate binding. The inhibitors ADP and staurosporine were both found to be competitive with ATP and non-competitive with S1, indicating binding of ATP and S1 to BTK is either random or ordered with ATP binding first. Negative cooperativity was also found between the S1 and ATP binding sites. Unlike ATP site inhibitors, substrate analog inhibitors did not inhibit BTK at concentrations less than 1 mm, suggesting that BTK may employ a "substrate clamping" type of kinetic mechanism whereby the substrate Kd is weaker than Km. This investigation of BTK provides the first detailed kinetic characterization of a Tec family kinase.

Immunodeficiency mutation databases (IDbases)

Primary immunodeficiencies (IDs) are a heterogenic group of inherited disorders of the immune system. Immunodeficiency patients have increased susceptibility to recurrent and persistent, even life-threatening infections. Mutations in a large number of genes can cause defects in different cellular functions and lead to impaired immune response. To date, approximately 150 IDs and more than 100 affected genes have been identified. ID-related genes are distributed throughout the genome, and diseases can be inherited in an X-linked, an autosomal recessive, or an autosomal dominant way. We have collected ID mutation data into locus-specific patient-related mutation databases, IDbases (http://bioinf.uta.fi/IDbases). Mutations are described at DNA, mRNA, and protein levels with links to reference sequences and reference articles. The mutation data has been collated into entries along with some clinical information. IDbases offer an easy way, e.g., to find recently identified mutations, to reveal genotype-phenotype correlations, and to discover a specific mutation or to examine the most common mutations in a single immunodeficiency related gene. At the moment we have databases for 107 ID genes with 4,140 public patient entries. An exhaustive statistical analysis of mutation data from the IDbases was made. Missense and nonsense mutations are the most common mutation types, and the most common single substitution is a nonsense mutation from tryptophan to a stop codon. Arginine is the most mutated as well as the most abundant mutant amino acid.

Clinical, immunological and molecular characteristics of 37 Iranian patients with X-linked agammaglobulinemia

BACKGROUND

X-linked agammaglobulinemia (XLA) is a hereditary immunodeficiency characterized by an early onset of recurrent bacterial infections, a profound deficiency of all immunoglobulin isotypes and a markedly reduced number of peripheral B lymphocytes. Eighty-five percent of the patients with this phenotype have mutations in Bruton's tyrosine kinase (BTK) gene.

METHODS

To provide an informative outlook of clinical and immunological manifestations of XLA in Iran, 37 Iranian male patients with an age range of 1-34 years, followed over a period of 25 years, were studied. Twenty-four of the 37 patients were screened for BTK gene mutation using PCR-SSCP followed by direct sequencing. BTK protein expression assay was done by flow cytometry in 9 families.

RESULTS

All patients first presented with infectious diseases, the most common of which were respiratory tract infections. Eighteen different mutations were identified, 13 of which were novel: IVS1+5G>C, 1896G>A, 349delA, 1618C>T, 1783T>C, 2084A>G, 1346delT, 1351delGAG, 587A>G, IVS14-1G>A, IVS3+2T>C, 1482G>A, 1975C>A.

CONCLUSION

The fact that we found a great number of novel mutations in a relatively limited number of patients underlines the heterogeneity of BTK mutations in the Iranian population. The large number of new mutations indicates that extended studies in this region would be rewarding.

Identification of Bruton tyrosine kinase mutations in 12 Chinese patients with X-linked agammaglobulinaemia by long PCR-direct sequencing

X-linked agammaglobulinaemia (XLA) is an immunodeficiency caused by Bruton tyrosine kinase (BTK) gene mutations. The disease is characterized by recurrent bacterial infections and profound hypogammaglobulinemia with marked reduction or lack of mature B-cells in the peripheral blood. Molecular characterization of BTK gene provides an opportunity for definitive diagnosis of XLA patients, especially for those with atypical phenotype resulting in a milder or late-onset form of the disease. The diagnosis allows accurate carrier detection with subsequent genetic counselling and prenatal diagnosis. In this study, long polymerase chain reaction (PCR)-direct sequencing analysis of the BTK gene in 12 unrelated Chinese XLA patients had been performed. Eight recurrent mutations and four novel mutations were identified. This is the first report of Chinese cases from three different East Asia regions together, including Hong Kong, Singapore and mainland China. Future clinical and genetic information from the undiagnosed Chinese XLA patients may provide insight into the genotype-phenotype correlations of BTK gene.

X-Linked Agammaglobulinemia Diagnosed in Adulthood: A Case Report

X-linked agammaglobulinemia (XLA) is a humoral immunodeficiency caused by mutations in Bruton's tyrosine kinase (BTK). Patients typically become symptomatic during infancy or early childhood and develop recurrent bacterial infections. We report a Japanese case of XLA diagnosed in a patient who was 27 years of age and who had no history of severe infection. The patient's serum immunoglobulin (Ig) G, IgA, and IgM levels were 132,7, and 17 mg/dL, respectively. The percentage of positive cells for CD19 and CD20 was 0.03% and 0.02%, respectively. The patient's brother and sister had no abnormalities. Flow cytometric analysis showed a partially reduced expression of BTK protein in the patient's peripheral monocytes. Sequencing of the BTK. gene revealed a missense mutation (230C>T,T33I). Given this data, this patient was diagnosed as having rare, late onset XLA with a missense mutation in the BTK gene.

Distinct gene expression signature in Btk-defective T1 B-cells

Bruton's tyrosine kinase (Btk) is a cytoplasmic tyrosine kinase important for B-lymphocyte maturation. Mutations in Btk give rise to the primary immunodeficiency disease X-linked agammaglobulinemia (XLA) in man and X-linked immunodeficiency (Xid) in mice. Recent studies have subdivided the mouse immature, or transitional, B-cells into two distinct subsets according to their respective surface markers. Transitional type 1 (T1) and transitional type 2 (T2) cells are also located in distinct anatomic locations. Based on a limited number of markers it has previously been reported that the earliest phenotypic sign of Btk deficiency is manifested at the T2 stage in mice. Here, we report on distinct genome-wide transcriptomic signature differences found in T1 B-lymphocytes from Btk-defective compared to normal mice and demonstrate that Btk deficiency is visible already at this stage.

Skewed Th1 responses caused by excessive expression of Txk, a member of the Tec family of tyrosine kinases, in patients with Behcet's disease

Behcet's disease (BD) is characterized by recurrent attacks of uveitis, oral aphtha, genital ulcers and skin lesions. The etiology and pathogenesis of BD are largely unknown. It has been reported that excessive Th1 cell function is involved in the pathogenesis of BD. Previously, we found that Txk, a member of the Tec family of tyrosine kinases, acts as a Th1 cell-specific transcription factor that is involved in the effector function of Th1 cells. Thus, we studied Th1 cytokine production and Txk expression of T-lymphocytes in patients with BD. Peripheral blood lymphocytes produced excessive Th1-associated cytokines including interferon-gamma (IFN-gamma) and interleukin (IL)-12 in patients with BD. Circulating CD3+ and purified CD4+ T cells expressed excessive Txk protein. The extent of IFN-gamma production by the lymphocytes correlated with the expression of Txk protein in the immunoblotting analysis. The majority of cells infiltrating into the skin lesions of patients with BD expressed IFN-gamma. IL-12 and IL-18 were found in the mononuclear cell aggregates in the skin and intestinal lesions of those with BD. Lymphocytes accumulating in the skin and intestinal lesions expressed higher levels of Txk as compared with other Th2-associated diseases. IFN-gamma, IL-18 and IL-12 detected in skin lesions may induce preferential development of Th1 cells in patients with BD. Collectively, Th1 cells expressing Txk and Th1-associated cytokines may play a critical role in the development of skin and intestinal lesions in patients with BD. This review may serve as a reminder of the importance of excessive Th1 cell function in the pathogenesis of BD and may contribute to the discovery of new molecular targets for the development of a specific therapeutic strategy for BD.

Regulation of Bruton tyrosine kinase by the peptidylprolyl isomerase Pin1

Bruton tyrosine kinase (Btk) is expressed in B-lymphocytes. Mutations in Btk cause X-linked agammaglobulinemia in humans. However, the mechanism of activation and signaling of this enzyme has not been fully investigated. We have here shown that the peptidylprolyl cis/trans isomerase (PPIase) Pin1 is a negative regulator of Btk, controlling its expression level by reducing its half-life, whereas the catalytic activity of Btk was unaffected. The negative regulatory effect of Pin1 was observed both in cell lines and in Pin(-/-) mice and was found to be dependent on a functionally intact Btk. This may constitute a feedback loop for the regulation of Btk. The target region in Btk was localized to the pleckstrin homology domain suggesting that interphase phosphorylation of serine 115 (Ser-115) in Btk is required, whereas mitosis phosphorylation of serine 21 (Ser-21) is critical. Accordingly, Pin 1 was shown to associate with Btk through binding to Ser-21 and -115, respectively, both of which lie in a classical Pin1-binding pocket. Using a phosphomitotic antibody, it was found that Btk harbors a bona fide MPM2 epitope corresponding to a phosphorylated serine or threonine residue followed by a proline. Our results indicate that the peptidylprolyl isomerase Pin1 interacts with Btk in a cell cycle-dependent manner, regulating the Btk expression level.

B-cell targeting in rheumatoid arthritis and other autoimmune diseases

B-cell-targeted therapy for autoimmune disease emerged from theoretical proposition to practical reality between 1997 and 1998, with the availability of the B-cell-depleting monoclonal antibody rituximab. Since then, a score of autoantibody-associated disorders have been treated, with most convincing evidence of efficacy seen in subjects with rheumatoid arthritis. Several classes of B-cell-targeted agent are now under investigation. From the outset, a major goal of B-cell targeting has been the re-establishment of some form of immunological tolerance. In some subjects, the observed improvement of disease for years following therapy fuels hope that this goal might ultimately be achievable.

A novel immunodeficiency associated with hypomorphic RAG1 mutations and CMV infection

Amorphic mutations in the recombination activating genes RAG1 and RAG2 have been reported to cause T- B- SCID, whereas hypomorphic mutations led to the expansion of a few autoimmune T cell clones responsible for the Omenn syndrome phenotype. We report here a novel clinical and immunological phenotype associated with recessive RAG1 hypomorphic mutations in 4 patients from 4 different families. The immunological phenotype consists of the oligoclonal expansion of TCR gammadelta T cells combined with TCR alphabeta T cell lymphopenia. The clinical phenotype consists of severe, disseminated CMV infection and autoimmune blood cell manifestations. Repertoire studies suggest that CMV infection, in the setting of this particular T cell immunodeficiency, may have driven the TCR gammadelta T cell clonal expansion. This observation extends the range of clinical and immunological phenotypes associated with RAG mutations, emphasizing the role of the genetic background and microbial environment in determining disease phenotype.

Involvement of SLP-65 and Btk in tumor suppression and malignant transformation of pre-B cells

Signals from the precursor-B cell receptor (pre-BCR) are essential for selection and clonal expansion of pre-B cells that have performed productive immunoglobulin heavy chain V(D)J recombination. In the mouse, the downstream signaling molecules SLP-65 and Btk cooperate to limit proliferation and induce differentiation of pre-B cells, thereby acting as tumor suppressors to prevent pre-B cell leukemia. In contrast, recent observations in human BCR-ABL1(+) pre-B lymphoblastic leukemia cells demonstrate that Btk is constitutively phosphorylated and activated by the BCR-ABL1 fusion protein. As a result, activated Btk transmits survival signals that are essential for the transforming activity of oncogenic Abl tyrosine kinase.

The health status and quality of life of adults with X-linked agammaglobulinemia

Forty-one adults (mean age 33) with a definitive diagnosis of X-linked agammaglobulinemia (XLA) completed a questionnaire concerning current and past medical problems and quality of life. Thirty-six of the 41 were working full time or were full time students; 18 had not missed any work or school due to infection in the previous year. Their quality of life was equivalent to that of the general US male population. Thirteen of the 41 reported that they had chronic lung disease, and 33 indicated that they had one or more episodes of sinusitis in the preceding year. Arthritis, diarrhea and skin infections were common but not debilitating. The 41 study subjects were more likely to have a prior family history of XLA, and they were more likely to have milder mutations in Btk, the gene responsible for XLA. These results indicate that most adults with XLA are moderately healthy and lead productive lives.

BTKbase: the mutation database for X-linked agammaglobulinemia

X-linked agammaglobulinemia (XLA) is a hereditary immunodeficiency caused by mutations in the gene encoding Bruton tyrosine kinase (BTK). XLA patients have a decreased number of mature B cells and a lack of all immunoglobulin isotypes, resulting in susceptibility to severe bacterial infections. XLA-causing mutations are collected in a mutation database (BTKbase), which is available at http://bioinf.uta.fi/BTKbase. For each patient the following information is given (when available): the identification of the entry, a plain English description of the mutation followed by a reference, formal characterization of the mutation, and the various parameters from the patient. BTKbase is implemented with the MUTbase program suite, which provides an easy, interactive, and quality controlled submission of information to mutation databases. BTKbase version 8 lists mutation entries of 1,111 patients from 973 unrelated families showing 602 unique molecular events. The localization of the mutations on the gene and protein for BTK can be analyzed by clicking sequences on the web pages. The distribution of the mutations in the five structural domains is approximately proportional to the length of the domains, except for the Tec homology (TH) domain. The most frequently affected sites are CpG dinucleotides. The majority of the missense mutations are structural-disturbing Bruton tyrosine kinase (Btk) folding or decreasing stability. Many of the mutations affect functionally significant, conserved residues. The structural consequences of the mutations in all the domains have been studied based on crystallographic and nuclear magnetic resonance (NMR) structures as well as computer-aided molecular modeling.

New insights into the cell biology of the marginal zone of the spleen

In the marginal zone of the spleen the bloodstream passes through an open system of reticular cells and fibers in which various myeloid and lymphoid cells are located. Macrophages in this region are well equipped to recognize pathogens and filter the blood by virtue of unique combinations of pattern recognition receptors. They interact with a specific set of B cells that can be found only in the marginal zone and that are able to react rapidly to bacterial antigens in particular. This combination of strategically located cells is an important factor in our defense against blood-borne pathogens. New data on the development of the marginal zone itself and the marginal zone B cells are reviewed and discussed in light of the function of the spleen in host defense.

Mutational analysis of the SH2-kinase linker region of Bruton's tyrosine kinase defines alternative modes of regulation for cytoplasmic tyrosine kinase families

Bruton's tyrosine kinase (Btk) plays critical roles in B cell development and activation. Mutations of Btk cause X-linked agammaglobulinemia (XLA) in humans and X-linked immunodeficiency in mice. An Src homology domain 2-kinase linker region exists in all Src, Abl, ZAP70/Syk and Btk/Tec non-receptor tyrosine kinase families. Missense mutations in the Btk linker region can cause XLA, supporting an essential role for this protein segment. We investigated the regulatory role of the linker region in Btk function by mutational analysis. XLA-causing mutations L369F and R372G abolished Btk-mediated calcium response without affecting Btk protein stability and kinase activity significantly. Although mutation of a well-conserved tryptophan (W260A) in the linker region of the Src family kinase Hck has been shown to cause a hyperactive kinase, an analogous mutation in Btk (W395A) dramatically decreased Btk kinase activity. Tyrosine phosphorylation in the linker region was previously shown to regulate the function of Abl and ZAP70/Syk kinases. Even though tyrosine phosphorylation was detected on tyrosine 375 in the Btk linker region, no significant alteration was observed in Btk-signaling activity and biological function when this tyrosine was mutated in DT-40 cells or in Y375F knock-in mice. Our data and previous studies suggest that each cytoplasmic tyrosine kinase family has evolved a unique strategy to utilize the linker region to regulate the function of the enzyme.

Variable phenotypic expression of mutations in genes of the immune system

Discovery of mutated genes that cause various types of primary immunodeficiencies has significantly advanced our understanding of the pathogenesis of these diseases and of the functions of normal gene products. However, it is becoming abundantly clear that the phenotypic presentation of mutations in a given gene can be quite different, depending upon the location and type of mutation but also probably upon other genetic factors and environmental influences. In this issue of the JCI, de Villartay et al. describe a third phenotype for mutations in recombination activating gene 1 (RAG1), in addition to the already known phenotypes of SCID and Omenn syndrome (see the related article beginning on page 3291).

Differential expression and molecular characterisation of Lmo7, Myo1e, Sash1, and Mcoln2 genes in Btk-defective B-cells

PURPOSE

Bruton's tyrosine kinase is crucial for B-lymphocyte development. By the use of gene expression profiling, we have identified four expressed sequence tags among 38 potential Btk target genes, which have now been characterised.

METHODS

Bioinformatics tools including data mining of additional unpublished gene expression profiles, sequence verification of PCR products and qualitative RT-PCR were used. Stimulations targeting the B-cell receptor and the protein kinase C were used to activate whole B-cell splenocytes.

RESULTS

Target genes were characterised as Lim domain only 7 (Lmo7); Myosin1e (Myo1e); SAM and SH3 domain containing 1 (Sash1); and Mucolipin2 (Mcoln2). Expression was found in cell lines of different origin and developmental stages as well as in whole B-cell splenocytes and Transitional type 1 (T1) splenic B-cells from wild type and Btk-defective mice, respectively. By the use of semi-quantitative RT-PCR we found Sash1 not to be expressed in the investigated haematopoietic cell lines, while transcripts were found in whole splenic B-cells from both wild type and Btk-defective mice, whereas Lmo7, Myo1e, and Mcoln2 were expressed in both B-cell lines and primary B-lymphocytes. Except for Lmo7, the transcript level was similarly affected by stimulation in control and Btk-defective cells.

RNA interference in biology and disease

RNA interference (RNAi) is a conserved biologic response to double-stranded RNA that results in the sequence-specific silencing of target gene expression. Over the past 5 years, an intensive research effort has facilitated the rapid movement of RNAi from a relatively obscure biologic phenomenon to a valuable tool used to silence target gene expression and perform large-scale functional genomic screens. In fact, recent studies reported in this journal and others have demonstrated success using RNAi to address the role of oncogene expression in leukemia cell lines and to validate the therapeutic potential of RNAi for treating these blood disorders. In order to advance these applications and gain an appreciation for the future of RNAi both in basic research and in the treatment of diseases caused by aberrant gene expression, it is important to have an understanding of the process of RNAi and its limitations.