The RNA Binding Protein Bcas2 is Required for Antibody Class Switch in Activated-B Cells

In children, hyper-IgM syndrome type 1 (HIGM1) is a type of severe antibody disorder, the pathogenesis of which remains unclear. The antibody diversity is partially determined by the alternative splicing (AS) in the germline, which is mainly regulated by RNA-binding proteins, including Breast cancer amplified sequence 2 (Bcas2). However, the effect of Bcas2 on AS and antibody production in activated B cells, the main immune cell type in the germline, remains unknown. To fill this gap, we created a conditional knockout (cKO, B cell-specific AID-Cre Bcas2 ^fl/fl) mouse model and performed integrated mechanistic analysis on alternative splicing (AS) and CSR in B cells through the RNA-sequencing approach, cross-linking immunoprecipitation and sequencing (CLIP-seq) analysis, and interactome proteomics. The results demonstrate that Bcas2-cKO significantly decreased CSR in activated B cells without inhibiting the B cell development. Mechanistically, Bcas2 interacts with SRSF7 at a conservative circular domain, forming a complex to regulate the AS of genes involved in the post-switch transcription, thereby causing broad-spectrum changes in antibody production. Importantly, we identified GAAGAA as the binding motif of Bcas2 to RNAs and revealed its essential role in the regulation of Bcas2-dependent AS and CSR. In addition, we detected a mutation of at the 3'UTR of Bcas2 gene in children with HIGM1 and observed similar patterns of AS events and CSR in the patient that were discovered in the Bcas2-cKO B cells. Combined, our study elucidates the mechanism by which Bcas2-mediated AS affects CSR, offering potential insights into the clinical implications of Bcas2 in HIGM1.