Bcas2 recruits DHX15 and SRSF7 to form a protein complex for the splicing of CSR-related mRNA at the 5′ ss and 3′ ss, generating two mature mRNA isoforms, which are ultimately translated into CSR-related proteins. These CSR-related proteins bind to DNA associated with antibodies, affecting the DSB repair of the relevant DNA after AID induction, thereby influencing the transcription of antibody isotypes and regulating the production of IgG1, IgG2b, IgG3, and IgE. Finally, through the RNA and protein level analysis of blood samples from patients with type I Hyper IgM syndrome, it is speculated that Bcas2 deficiency should affect alternative splicing, thereby regulating antibody CSR in the patient’s blood.
Abstract:
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 Bcas2fl/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.
Author list:
Yu Chen†, Siyuan Sun†, Chenxu Lu†, Yixuan Li†, Bing Fang†, Xiangfeng Tang, Xuepeng Li, Weiru Yu, Yumei Lei, Longjie Sun, Ming Zhang, Jiazeng Sun, Ping Liu, Yongting Luo, Xingwang Zhao, Jing Zhan, Libing Liu, Rong Liu, Jiaqiang Huang, Ziwei Yi, Yifei Yu, Weihan Xiao, Zheng Ding, Lei Li, Dan Su, Fazheng Ren*, Changchang Cao*, Ran Wang*, Wenbiao Shi*, Juan Chen*
How to cite:
Y. Chen, S. Sun, C. Lu, Y. Li, B. Fang, X. Tang, X. Li, W. Yu, Y. Lei, L. Sun, M. Zhang, J. Sun, P. Liu, Y. Luo, X. Zhao, J. Zhan, L. Liu, R. Liu, J. Huang, Z. Yi, Y. Yu, W. Xiao, Z. Ding, L. Li, D. Su, F. Ren, C. Cao, R. Wang, W. Shi, J. Chen, Exploration 2025, 70015.
https://doi.org/10.1002/EXP.70015