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

X-linked agammaglobulinaemia (XLA) is a human immunodeficiency caused by failure of pre-B cells in the bone marrow to develop into circulating mature B cells. A novel gene has been isolated which maps to the XLA locus, is expressed in B cells, and shows mutations in families with the disorder. The gene is a member of the src family of proto-oncogenes which encode protein-tyrosine kinases. This is, to our knowledge, the first evidence that mutations in a src-related gene are involved in human genetic disease.

Deficient expression of a B cell cytoplasmic tyrosine kinase in human X-linked agammaglobulinemia

We describe a novel cytoplasmic tyrosine kinase, termed BPK (B cell progenitor kinase), which is expressed in all stages of the B lineage and in myeloid cells. BPK has classic SH1, SH2, and SH3 domains, but lacks myristylation signals and a regulatory phosphorylation site corresponding to tyrosine 527 of c-src. BPK has a long, basic amino-terminal region upstream of the SH3 domain. BPK was evaluated as a candidate for human X-linked agammaglobulinemia (XLA), an inherited immunodeficiency characterized by a severe deficit of B and plasma cells and profound hypogammaglobulinemia. BPK mapped to within 100 kb of a probe defining the polymorphism most closely linked to XLA at DXS178. Reduction in or the absence of BPK mRNA, protein expression, and kinase activity was observed in XLA pre-B and B cell lines. BPK is likely the XLA gene and functions in pathways critical to B cell expansion.

Autosomal dominant immune dysregulation syndrome in humans with CTLA4 mutations

The protein cytotoxic T lymphocyte antigen-4 (CTLA-4) is an essential negative regulator of immune responses, and its loss causes fatal autoimmunity in mice. We studied a large family in which five individuals presented with a complex, autosomal dominant immune dysregulation syndrome characterized by hypogammaglobulinemia, recurrent infections and multiple autoimmune clinical features. We identified a heterozygous nonsense mutation in exon 1 of CTLA4. Screening of 71 unrelated patients with comparable clinical phenotypes identified five additional families (nine individuals) with previously undescribed splice site and missense mutations in CTLA4. Clinical penetrance was incomplete (eight adults of a total of 19 genetically proven CTLA4 mutation carriers were considered unaffected). However, CTLA-4 protein expression was decreased in regulatory T cells (Treg cells) in both patients and carriers with CTLA4 mutations. Whereas Treg cells were generally present at elevated numbers in these individuals, their suppressive function, CTLA-4 ligand binding and transendocytosis of CD80 were impaired. Mutations in CTLA4 were also associated with decreased circulating B cell numbers. Taken together, mutations in CTLA4 resulting in CTLA-4 haploinsufficiency or impaired ligand binding result in disrupted T and B cell homeostasis and a complex immune dysregulation syndrome.

Colocalization of X-linked agammaglobulinemia and X-linked immunodeficiency genes

Mice that bear the X-linked immunodeficiency (xid) mutation have a B lymphocyte-specific defect resulting in an inability to make antibody responses to polysaccharide antigens. A backcross of 1114 progeny revealed the colocalization of xid with Bruton's agammaglobulinemia tyrosine kinase (btk) gene, which is implicated in the human immune deficiency, X-linked agammaglobulinemia. Mice that carry xid have a missense mutation that alters a highly conserved arginine near the amino-terminus of the btk protein, Btk. Because this region of Btk lies outside any obvious kinase domain, the xid mutation may define another aspect of tyrosine kinase function.

Mutations in the chemokine receptor gene CXCR4 are associated with WHIM syndrome, a combined immunodeficiency disease

WHIM syndrome is an immunodeficiency disease characterized by neutropenia, hypogammaglobulinemia and extensive human papillomavirus (HPV) infection. Despite the peripheral neutropenia, bone marrow aspirates from affected individuals contain abundant mature myeloid cells, a condition termed myelokathexis. The susceptibility to HPV is disproportionate compared with other immunodeficiency conditions, suggesting that the product of the affected gene may be important in the natural control of this infection. We describe here the localization of the gene associated with WHIM syndrome to a region of roughly 12 cM on chromosome 2q21 and the identification of truncating mutations in the cytoplasmic tail domain of the gene encoding chemokine receptor 4 (CXCR4). Haplotype and mutation analyses in a pedigree transmitting myelokathexis as an apparently autosomal recessive trait support genetic heterogeneity for this aspect of the WHIM syndrome phenotype. Lymphoblastoid cell lines carrying a 19-residue truncation mutation show significantly greater calcium flux relative to control cell lines in response to the CXCR4 ligand, SDF-1, consistent with dysregulated signaling by the mutant receptor. The identification of mutations in CXCR4 in individuals with WHIM syndrome represents the first example of aberrant chemokine receptor function causing human disease and suggests that the receptor may be important in cell-mediated immunity to HPV infection.

An antibody-deficiency syndrome due to mutations in the CD19 gene

BACKGROUND

The CD19 protein forms a complex with CD21, CD81, and CD225 in the membrane of mature B cells. Together with the B-cell antigen receptor, this complex signals the B cell to decrease its threshold for activation by the antigen.

METHODS

We evaluated four patients from two unrelated families who had increased susceptibility to infection, hypogammaglobulinemia, and normal numbers of mature B cells in blood. We found a mutation in the CD19 gene in all four patients. The CD19 gene in the patients and their first-degree relatives was sequenced, and flow-cytometric immunophenotyping of B cells, immunohistochemical staining of lymphoid tissues, and DNA and messenger RNA analysis were performed. B-cell responses on the triggering of the B-cell receptor were investigated by in vitro stimulation; the antibody response after vaccination with rabies vaccine was also studied.

RESULTS

All four patients had homozygous mutations in the CD19 gene. Levels of CD19 were undetectable in one patient and substantially decreased in the other three. Levels of CD21 were decreased, whereas levels of CD81 and CD225 were normal, in all four patients. The composition of the precursor B-cell compartment in bone marrow and the total numbers of B cells in blood were normal. However, the numbers of CD27+ memory B cells and CD5+ B cells were decreased. Secondary follicles in lymphoid tissues were small to normal in size and had a normal cellular composition. The few B cells that showed molecular signs of switching from one immunoglobulin class to another contained V(H)-C(alpha) and V(H)-C(gamma) transcripts with somatic mutations. The response of the patients' B cells to in vitro stimulation through the B-cell receptor was impaired, and in all four patients, the antibody response to rabies vaccination was poor.

CONCLUSIONS

Mutation of the CD19 gene causes a type of hypogammaglobulinemia in which the response of mature B cells to antigenic stimulation is defective.

Deleterious mutations in LRBA are associated with a syndrome of immune deficiency and autoimmunity

Most autosomal genetic causes of childhood-onset hypogammaglobulinemia are currently not well understood. Most affected individuals are simplex cases, but both autosomal-dominant and autosomal-recessive inheritance have been described. We performed genetic linkage analysis in consanguineous families affected by hypogammaglobulinemia. Four consanguineous families with childhood-onset humoral immune deficiency and features of autoimmunity shared genotype evidence for a linkage interval on chromosome 4q. Sequencing of positional candidate genes revealed that in each family, affected individuals had a distinct homozygous mutation in LRBA (lipopolysaccharide responsive beige-like anchor protein). All LRBA mutations segregated with the disease because homozygous individuals showed hypogammaglobulinemia and autoimmunity, whereas heterozygous individuals were healthy. These mutations were absent in healthy controls. Individuals with homozygous LRBA mutations had no LRBA, had disturbed B cell development, defective in vitro B cell activation, plasmablast formation, and immunoglobulin secretion, and had low proliferative responses. We conclude that mutations in LRBA cause an immune deficiency characterized by defects in B cell activation and autophagy and by susceptibility to apoptosis, all of which are associated with a clinical phenotype of hypogammaglobulinemia and autoimmunity.

Impaired expansion of mouse B cell progenitors lacking Btk

Mutations in the gene encoding the protein tyrosine kinase Btk are associated with the human B cell immunodeficiency X-linked agammaglobulinemia (XLA). In the mouse, a point mutation in the Btk pleckstrin homology domain segregates with a milder X-linked immunodeficiency (xid). To assess the importance of Btk function in murine lymphopoiesis, we generated multiple embryonic stem cell clones bearing a targeted disruption of the btk gene and examined their potential to produce lymphocytes in both C57BL/6 and RAG2-/- host chimeric animals. These mice provide a complementary set of in vivo competition assays that formally establish the genetic basis for the xid phenotype. Although the null mutation yields a phenotype quite similar to that of xid, it also compromises expansion of B cell precursors. Our results suggest that the murine and human consequences of Btk deficiency differ only quantitatively, and represent the same disease process.

Bruton's tyrosine kinase is required for activation of IkappaB kinase and nuclear factor kappaB in response to B cell receptor engagement

Mutations in the gene encoding Bruton's tyrosine kinase (btk) cause the B cell deficiency diseases X-linked agammaglobulinemia (XLA) in humans and X-linked immunodeficiency (xid) in mice. In vivo and in vitro studies indicate that the BTK protein is essential for B cell survival, cell cycle progression, and proliferation in response to B cell antigen receptor (BCR) stimulation. BCR stimulation leads to the activation of transcription factor nuclear factor (NF)-kappaB, which in turn regulates genes controlling B cell growth. We now demonstrate that a null mutation in btk known to cause the xid phenotype prevents BCR-induced activation of NF-kappaB. This defect can be rescued by reconstitution with wild-type BTK. This mutation also interferes with BCR-directed activation of IkappaB kinase (IKK), which normally targets the NF-kappaB inhibitor IkappaBalpha for degradation. Taken together, these findings indicate that BTK couples IKK and NF-kappaB to the BCR. Interference with this coupling mechanism may contribute to the B cell deficiencies observed in XLA and xid.

An essential role for BLNK in human B cell development

The signal transduction events that control the progenitor B cell (pro-B cell) to precursor B cell (pre-B cell) transition have not been well delineated. In evaluating patients with absent B cells, a male with a homozygous splice defect in the cytoplasmic adapter protein BLNK (B cell linker protein) was identified. Although this patient had normal numbers of pro-B cells, he had no pre-B cells or mature B cells, indicating that BLNK plays a critical role in orchestrating the pro-B cell to pre-B cell transition. The immune system and overall growth and development were otherwise normal in this patient, suggesting that BLNK function is highly specific.

The pleckstrin homology domain: an intriguing multifunctional protein module

Pleckstrin homology (PH) domains are a family of compact protein modules defined by sequences of roughly 100 amino acids. These domains are common in vertebrate, Drosophila, C. elegans and yeast proteins, suggesting an early origin and fundamental importance to eukaryotic biology. Many enzymes which have important regulatory functions contain PH domains, and mutant forms of several such proteins are implicated in oncogenesis and developmental disorders. Numerous recent studies show that PH domains bind various proteins and inositolphosphates. Here I discuss PH domains in detail and conclude that they form a versatile family of membrane binding and protein localization modules.

Early and prolonged intravenous immunoglobulin replacement therapy in childhood agammaglobulinemia: a retrospective survey of 31 patients

OBJECTIVE

To evaluate the outcome of children who received prolonged intravenous immunoglobulin (IVIg) replacement therapy early in life for X-linked agammaglobulinemia (XLA).

STUDY DESIGN

We performed a retrospective study of the clinical features and outcome of patients with genetic and/or immunologic results consistent with XLA. Patients receiving IVIg replacement therapy within 3 months of the diagnosis and for at least 4 years between 1982 and 1997 were included.

RESULTS

Thirty-one patients began receiving IVIg replacement therapy at a median age of 24 months and were followed up for a median time of 123 months. IVIg was given at doses >0.25 g/kg every 3 weeks, and mean individual residual IgG levels ranged from 500 to 1140 mg/dL (median, 700 mg/dL). During IVIg replacement, the incidence of bacterial infections requiring hospitalization fell from 0.40 to 0.06 per patient per year (P <. 001). However, viral or unidentified infections still developed, including enteroviral meningoencephalitis (n = 3) causing death in one patient, exudative enteropathy (n = 3), and aseptic arthritis (n = 1). At last follow-up, 30 patients were alive at a median age of 144 months (range, 58 to 253 months). Among 23 patients who were evaluated by respiratory function tests and computed tomography, 3 had an obstructive syndrome, 6 had bronchiectasis, and 20 had chronic sinusitis.

CONCLUSION

Early IVIg replacement therapy achieving residual IgG levels >500 mg/dL is effective in preventing severe acute bacterial infections and pulmonary insufficiency. More intensive therapy may be required to fully prevent the onset of bronchiectasis, chronic sinusitis, and nonbacterial infections, particularly enteroviral infections, in all cases.

Clinical, immunological, and molecular analysis in a large cohort of patients with X-linked agammaglobulinemia: an Italian multicenter study

A questionnaire-based retrospective clinical and immunological survey was conducted in 73 males with a definite diagnosis of X-linked agammaglobulinemia based on BTK sequence analysis. Forty-four were sporadic and 29 familial cases. At December 2000, the patients' ages ranged from 2 to 33 years; mean age at diagnosis and mean duration of follow-up were 3.5 and 10 years respectively. After the mid-1980s all but 2 were on intravenous immunoglobulin (IVIG) substitution therapy, with residual IgG >500 mg/dl in 94% of the patients at the time of enrollment. Respiratory infections were the most frequent manifestation both prior to diagnosis and over follow-up. Chronic lung disease (CLD) was present in 24 patients, in 15 already at diagnosis and in 9 more by 2000. The cumulative risk to present at diagnosis with CLD increased from 0.17 to 0.40 and 0.78 when the diagnosis was made at the ages of 5, 10, and 15 years respectively. For the 9 patients who developed CLD during follow-up, the duration of follow-up, rather than age at diagnosis; previous administration of intramuscular immunoglobulin; and residual IgG levels had a significant effect on the development of CLD. Chronic sinusitis was present in 35 patients (48%), in 15 already at diagnosis and in 20 by 2000. Sistemic infections such as sepsis and meningitis/meningoencephalitis decreased over follow-up, probably due to optimal protection provided by high circulating IgG levels reached with IVIG.

Human ICOS deficiency abrogates the germinal center reaction and provides a monogenic model for common variable immunodeficiency

The homozygous deletion of the inducible costimulator (ICOS), an activation-induced member of the CD28 family on T cells, causes an antibody deficiency syndrome in affected humans. The identification of a total of 9 ICOS-deficient patients revealed that this monogenic disease comprises the full clinical phenotype described for common variable immunodeficiency (CVID), including recurrent bacterial infections, adult as well as childhood onset, splenomegaly, autoimmune phenomena (autoimmune neutropenia), intestinal lymphoid hyperplasia, and malignancy (carcinoma of the vulva). All patients exhibited a profound hypogammaglobulinemia and a disturbed B-cell homeostasis. The severe reduction of class-switched memory B cells resulted from poor germinal center formation in the absence of ICOS. The additional decrease of naive B cells was associated with a partial inhibition of the early B-cell development at the pre-B-I stage. T-cell homeostasis seemed not to be affected, but low IL-10 production by ICOS-deficient T cells may contribute to the disturbed germinal center reaction. Human ICOS deficiency is indistinguishable from CVID and thus serves as a monogenic model for this complex syndrome.

Relevance of biallelic versus monoallelic TNFRSF13B mutations in distinguishing disease-causing from risk-increasing TNFRSF13B variants in antibody deficiency syndromes

TNFRSF13B encodes transmembrane activator and calcium modulator and cyclophilin ligand interactor (TACI), a B cell- specific tumor necrosis factor (TNF) receptor superfamily member. Both biallelic and monoallelic TNFRSF13B mutations were identified in patients with common variable immunodeficiency disorders. The genetic complexity and variable clinical presentation of TACI deficiency prompted us to evaluate the genetic, immunologic, and clinical condition in 50 individuals with TNFRSF13B alterations, following screening of 564 unrelated patients with hypogammaglobulinemia. We identified 13 new sequence variants. The most frequent TNFRSF13B variants (C104R and A181E; n=39; 6.9%) were also present in a heterozygous state in 2% of 675 controls. All patients with biallelic mutations had hypogammaglobulinemia and nearly all showed impaired binding to a proliferation-inducing ligand (APRIL). However, the majority (n=41; 82%) of the pa-tients carried monoallelic changes in TNFRSF13B. Presence of a heterozygous mutation was associated with antibody deficiency (P< .001, relative risk 3.6). Heterozygosity for the most common mutation, C104R, was associated with disease (P< .001, relative risk 4.2). Furthermore, heterozygosity for C104R was associated with low numbers of IgD(-)CD27(+) B cells (P= .019), benign lymphoproliferation (P< .001), and autoimmune complications (P= .001). These associations indicate that C104R heterozygosity increases the risk for common variable immunodeficiency disorders and influences clinical presentation.

Mutations in the mu heavy-chain gene in patients with agammaglobulinemia

BACKGROUND

Most patients with congenital hypogammaglobulinemia and absent B cells are males with X-linked agammaglobulinemia, which is caused by mutations in the gene for Bruton's tyrosine kinase (Btk); however, there are females with a similar disorder who do not have mutations in this gene. We studied two families with autosomal recessive defects in B-cell development and patients with presumed X-linked agammaglobulinemia who did not have mutations in Btk.

METHODS

A series of candidate genes that encode proteins involved in B-cell signal-transduction pathways were analyzed by linkage studies and mutation screening.

RESULTS

Four different mutations were identified in the mu heavy-chain gene on chromosome 14. In one family, there was a homozygous 75-to-100-kb deletion that included D-region genes, J-region genes, and the mu constant-region gene. In a second family, there was a homozygous base-pair substitution in the alternative splice site of the mu heavy-chain gene. This mutation would inhibit production of the membrane form of the mu chain and produce an amino acid substitution in the secreted form. In addition, a patient previously thought to have X-linked agammaglobulinemia was found to have an amino acid substitution on one chromosome at an invariant cysteine that is required for the intrachain disulfide bond and, on the other chromosome, a large deletion that included the immunoglobulin locus.

CONCLUSIONS

Defects in the mu heavy-chain gene are a cause of agammaglobulinemia in humans. This implies that an intact membrane-bound mu chain is essential for B-cell development.

Structure of the PH domain and Btk motif from Bruton's tyrosine kinase: molecular explanations for X-linked agammaglobulinaemia

Bruton's tyrosine kinase (Btk) is an enzyme which is involved in maturation of B cells. It is a target for mutations causing X-linked agammaglobulinaemia (XLA) in man. We have determined the structure of the N-terminal part of Btk by X-ray crystallography at 1.6 A resolution. This part of the kinase contains a pleckstrin homology (PH) domain and a Btk motif. The structure of the PH domain is similar to those published previously: a seven-stranded bent beta-sheet with a C-terminal alpha-helix. Individual point mutations within the Btk PH domain which cause XLA can be classified as either structural or functional in the light of the three-dimensional structure and biochemical data. All functional mutations cluster into the positively charged end of the molecule around the predicted binding site for phosphatidylinositol lipids. It is likely that these mutations inactivate the Btk pathway in cell signalling by reducing its affinity for inositol phosphates, which causes a failure in translocation of the kinase to the cell membrane. A small number of signalling proteins contain a Btk motif that always follows a PH domain in the sequence. This small module has a novel fold which is held together by a zinc ion bound by three conserved cysteines and a histidine. The Btk motif packs against the second half of the beta-sheet of the PH domain, forming a close contact with it. Our structure opens up new ways to study the role of the PH domain and Btk motif in the cellular function of Btk and the molecular basis of its dysfunction in XLA patients.

Gastrointestinal pathology in patients with common variable immunodeficiency and X-linked agammaglobulinemia

Review of the medical records of 43 patients with common variable immunodeficiency (CVID) and 23 patients with X-linked agammaglobulinemia (XLAG) revealed a high incidence of chronic gastrointestinal complaints, most commonly diarrhea. Thirty-eight biopsies, four small-bowel resection specimens, and one autopsy from 10 patients with CVID and one patient with XLAG showed a wide range of abnormalities. A pattern resembling acute graft-versus-host disease, with apoptotic bodies and lymphocytes in crypts, was seen in the stomach (four patients), small bowel (three patients), and colon (three patients). Small-bowel specimens from three CVID patients with malabsorption showed mild to severe villous atrophy. Three CVID patients had Giardia in biopsies. Two cases of small bowel lymphoma associated with nodular lymphoid hyperplasia were identified in CVID patients. One patient's small bowel contained foamy histiocytes in the lamina propria, resembling Whipple's disease or chronic granulomatous disease, with numerous apoptotic bodies in crypts. Ultrastructurally, the histiocytes contained cellular debris. The patient with XLAG had recurrent fissuring necrosis of small bowel resembling Crohn's disease; a patient with CVID had colitis with features similar to ulcerative colitis. Poorly formed granulomas were seen in the stomach (one CVID patient) and the colon (two CVID patients). Lymphocyte populations were dominated by T cells; B cells were scarce except in lymphoid follicles in CVID patients with nodular lymphoid hyperplasia. Patients with CVID and XLAG manifest a spectrum of abnormalities in the gastrointestinal tract, with patterns superficially resembling graft-versus-host disease, inflammatory bowel disease, and Whipple's disease, but often lacking some of the diagnostic features of the diseases. Many of the CVID patients with chronic gastrointestinal complaints (62%) also had evidence of autoimmune phenomena, suggesting that in some patients the inflammatory process in the gastrointestinal tract has an autoimmune component.

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.

An immunodeficiency disease with RAG mutations and granulomas

We describe three unrelated girls who had an immunodeficiency disease with granulomas in the skin, mucous membranes, and internal organs. All three girls had severe complications after viral infections, including B-cell lymphoma associated with Epstein-Barr virus (EBV). Other findings were hypogammaglobulinemia, a diminished number of T and B cells, and sparse thymic tissue on ultrasonography. Molecular analysis revealed that the patients were compound heterozygotes for mutations in recombination activating gene 1 or 2 (RAG1 or RAG2). In each case, both parents were heterozygous carriers of a RAG mutation. The mutations were associated with reduced function of RAG in vitro (3 to 30% of normal activity). The parents and one sibling in the three families were healthy.

Detection of aneuploidy and chromosomal mosaicism in human embryos during preimplantation sex determination by fluorescent in situ hybridisation, (FISH)

Five couples at risk of producing offspring with X-linked recessive disease underwent in vitro fertilisation with a view to preimplantation determination of embryo sex and selective transfer of females. On day three postinsemination, one or two blastomeres were removed by embryo biopsy, and used for dual fluorescent in situ hybridisation with X and Y chromosome-specific DNA probes. In two cases, two female embryos were transferred and one pregnancy, (sex confirmed), is ongoing at 19 weeks. All eight embryos from one couple were of such poor quality that diagnosis was possible in one only. In the remaining two cases no embryos were transferred due to the detection of an abnormal number of X chromosome signals. Investigation of the biopsied embryos that were not transferred revealed evidence of mitotic non-disjunction in one and of complete X monosomy in a second. A surviving fetus with this latter constitution would have developed Turner syndrome and would also have been at high risk of X-linked disease. The use of fluorescent in situ hybridisation rather than the polymerase chain reaction allowed the detection of abnormal copy numbers of X chromosomes thus preventing the transfer of potentially abnormal zygotes.