Cutting edge: A/WySnJ transitional B cells overexpress the chromosome 15 proapoptotic Blk gene and succumb to premature apoptosis.

Better knowledge of peripheral B lymphocyte homeostasis is needed to address the human hypogammaglobulinemia diseases. A defect in the Bcmd gene shortens the B cell life span and causes B cell deficiency in A/WySnJ mice. Previous genetic mapping placed Bcmd near Srebf2 on chromosome 15. Inspection of the human chromosome 22 syntenic region identified the proapoptotic Bik gene as a candidate. Two mapping methods placed the homologous mouse gene, Blk, near Srebf2. The Blk genomic structure was highly homologous to BIK: Sequence analysis ruled out coding region mutations, but Blk transcripts were overly abundant in sorted A/WySnJ T1 B cells. Moreover, enriched transitional B cells showed a cell-autonomous defect leading to excessive apoptosis. Thus, Bcmd may be a direct mutation in Blk, or in a gene involved in Blk regulation, such that excess expression pushes the A/WySnJ transitional B cells past the apoptosis checkpoint to cell death.