Identification of novel non-pathogenic mutation in SH3 domain of Btk in an XLA patient.

A first report of an XLA patient with a polymorphism in Btk SH3 domain has been identified after sequencing of the entire gene. SH3 domain variants might not be detected due to well characterized mutations outside the domain.

A leaky mutation in CD3D differentially affects αß and γδ T cells and leads to a Tαß-Tγδ+B+NK+ human SCID.

T cells recognize antigens via their cell surface TCR and are classified as either αß or γδ depending on the variable chains in their TCR, α and ß or γ and δ, respectively. Both αß and γδ TCRs also contain several invariant chains, including CD3δ, which support surface TCR expression and transduce the TCR signal. Mutations in variable chains would be expected to affect a single T cell lineage, while mutations in the invariant chains would affect all T cells. Consistent with this, all CD3δ-deficient patients described to date showed a complete block in T cell development. However, CD3δ-KO mice have an αß T cell-specific defect. Here, we report 2 unrelated cases of SCID with a selective block in αß but not in γδ T cell development, associated with a new splicing mutation in the CD3D gene. The patients' T cells showed reduced CD3D transcripts, CD3δ proteins, surface TCR, and early TCR signaling. Their lymph nodes showed severe T cell depletion, recent thymus emigrants in peripheral blood were strongly decreased, and the scant αß T cells were oligoclonal. T cell-dependent B cell functions were also impaired, despite the presence of normal B cell numbers. Strikingly, despite the specific loss of αß T cells, surface TCR expression was more reduced in γδ than in αß T cells. Analysis of individuals with this CD3D mutation thus demonstrates the contrasting CD3δ requirements for αß versus γδ T cell development and TCR expression in humans and highlights the diagnostic and clinical relevance of studying both TCR isotypes when a T cell defect is suspected.

Bruton's tyrosine kinase is not essential for LPS-induced activation of human monocytes.

BACKGROUND: X-linked agammaglobulinemia (XLA) is characterized by impaired B-cell differentiation caused by mutations in the Bruton's tyrosine kinase (Btk) gene. The natural disease model, the X-linked immunodeficiency mouse, shows a less severe phenotype, indicating a different requirement of Btk in human and mouse B cells. Btk is also expressed in the myeloid line and participates in LPS signaling. Deficient oxidative burst and myeloid differentiation have been reported in the X-linked immunodeficiency mouse, but the precise mechanism and relevance of Btk activity in human monocytes is poorly understood. OBJECTIVE: The apparent absence in XLA of clinical manifestations of myeloid deficiency prompted us to explore the relevance of complete Btk absence in human myeloid cells. METHODS: Seven patients with XLA with BTK mutations conditioning a null protein expression were included in the study. Monocyte LPS-induced mitogen-activated protein kinase activation, TNF-alpha and IL-6 production in monocytes, and oxidative burst in monocytes and granulocytes were analyzed by means of flow cytometry. RESULTS: We show that in response to LPS, Btk-null monocytes from patients with XLA induce early mitogen-activated protein kinase activation and intracellular TNF-alpha and IL-6 production with the same intensity as cells from age- and sex-matched control subjects. In addition, the oxidative burst in response to LPS and other stimulants was completely normal in Btk-null monocytes and neutrophils. CONCLUSION: Our results indicate that Btk is not essential for early LPS signaling in human monocytes and that different Btk dependency might exist between human and mouse myeloid cells. CLINICAL IMPLICATIONS: These findings provide a better understanding of XLA, and they show the differences between human XLA and murine Xid models.

A genotype-phenotype correlation study in a group of 54 patients with X-linked agammaglobulinemia.

BACKGROUND: X-linked (Bruton's) agammaglobulinemia (XLA) is a rare immunodeficiency caused by a block in B-cell development caused by mutations in the Bruton's tyrosine kinase (BTK) gene. Many aspects of XLA and BTK function remain unresolved; atypical presentations have been reported, and no clear genotype-phenotype correlation has been established. OBJECTIVES: We sought to contribute to the understanding of XLA through the phenotypic and biochemical characterization of a large group of Spanish patients with agammaglobulinemia. We also sought to classify the mutations according to their severity to analyze a genotype-phenotype correlation. METHODS: Clinical and analytic data were collected from the clinical records. We studied the BTK gene, protein expression, and function, and the findings were correlated with the phenotypic information. RESULTS: Fifty-four patients were given diagnoses of XLA. We identified 38 different mutations in BTK, 26 not described in other patients, and several uncommon clinical phenotypes or analytic characteristics were found. The statistical analysis shows that less severe mutations or minimal detection of protein by means of flow cytometry are associated with decreased severity in clinical and analytic data, demonstrating a clear relation between the type of mutation and the disease expressivity. However, some exemptions to this rule were noted. CONCLUSIONS: XLA is a variable disease. Globally, a genotype-phenotype correlation is observed, but individual discrepancies between the severity of the mutation and the clinical and analytic phenotype suggest that other loci or ambient factors significantly influence the disease presentation and evolution.