Novel BTK Mutation in Patient with Late Diagnosis of X-Linked Agammaglobulinemia.

X-linked agammaglobulinemia (XLA) is a genetic disorder with mutation in Bruton's tyrosine kinase (BTK). Defects in B cell development and immunoglobulin production lead to recurrent infections following loss of maternal IgG at 6 months of age. A 55-year-old male with a longstanding common variable immunodeficiency diagnosis on infusion therapy presented to the clinic with cutaneous T-cell lymphoma, which inspired overall repeat evaluation. Immunoglobulin levels and lymphocyte markers, family history, and genetic testing prompted a true diagnosis of XLA and novel mutation in the BTK gene. Disease-associated mutations have been noted in all five domains of BTK, with missense variants most commonly cited among the 100s of reported genetic alterations. The BTK protein is expressed in hematopoietic lineages and plasma cells, with the exception of T lymphocytes. Disruption in the protein function or absence of BTK halts normal B cell development at the pre-B transitional cell stage and induces premature apoptosis. We present the first reported case of a novel hemizygous BTK c.1492C > G mutation in a patient causing XLA.