DELETION WITHIN THE SRC HOMOLOGY DOMAIN-3 OF BRUTON TYROSINE KINASE RESULTING IN X-LINKED AGAMMAGLOBULINEMIA (XLA)

The gene responsible for X-linked agammaglobulinemia (XLA) has been re cently identified to code for a cytoplasmic tyrosine kinase (Bruton's agammaglobulinemia tyrosine kinase, BTK), required for normal B cell d evelopment. BTK, like many other cytoplasmic tyrosine kinases, contain s Src homology domains (SH2 and SH3), and catalytic kinase domain. SH3 domains are important for the targeting of signaling molecules to spe cific subcellular locations. We have identified a family with XLA whos e affected members have a point mutation (g --> a) at the 5' splice si te of intron 8, resulting in the skipping of coding exon 8 and loss of 21 amino acids forming the COOH-terminal portion of the BTK SH3 domai n. The study of three generations within this kinship, using restricti on fragment length polymorphism and DNA analysis, allowed identificati on of the mutant X chromosome responsible for XLA and the carrier stat us in this family. BTK mRNA was present in normal amounts in Epstein-B arr virus-induced B lymphoblastoid cell lines established from affecte d family members. Although the SH3 deletion did not alter BTK protein stability and kinase activity of the truncated BTK protein was normal, the affected patients nevertheless have a severe B cell defect charac teristic for XLA. The mutant protein was modeled using the normal BTK SH3 domain. The deletion results in loss of two COOH-terminal beta str ands containing several residues critical for the formation of the put ative SH3 ligand-binding pocket. We predict that, as a result, one or more crucial SH3 binding proteins fail to interact with BTK, interrupt ing the cytoplasmic signal transduction process required for B cell di fferentiation.