hnRNPL–CstF64 complex: coordinating CSR and LSR in IgH locus recombination dynamics through eRNA and NHEJ regulation

Class switch recombination (CSR) and locus suicide recombination (LSR) are critical processes involved in the immune system's ability to diversify antibody responses. Both are initiated by activation-induced cytidine deaminase, which induces DNA double-strand breaks (DSBs) at specific regions within the immunoglobulin heavy chain (IgH) locus. In CSR, DSBs occur at the switch (S) regions, allowing B cells to replace the IgM heavy chain constant region (CH) with other isotypes, thereby enhancing immune adaptability. This process is regulated by both cis and trans mechanisms, including the IgH super-enhancer 3' regulatory region (3'RR) and the production of enhancer RNAs (eRNAs). A recent study highlighted the role of MED12 in CSR through enhancer activation and the transcription of eRNA. Now, we show that heterogeneous ribonucleoprotein L (hnRNPL) acts as an additional regulator of CSR and LSR by forming an eRNA-associated complex with CstF64, a polyadenylation factor. This complex facilitates RNA polymerase II elongation and eRNA transcription at the 3'RR. Moreover, the hnRNPL/CstF64 complex promotes NHEJ-mediated DNA repair at both S and 3'RR regions, facilitating 53BP1 and Ku80 recruitment, thereby impacting the efficiency of CSR and LSR. This discovery highlights the intricate, multimodal regulation of these processes, linking eRNA transcription to DNA repair in the process of antibody diversification.