Involvement of the splicing factor SART1 in the BRCA1-dependent homologous recombination repair of DNA double-strand breaks

Although previous studies have reported that pre-mRNA splicing factors (SFs) are involved in the repair of DNA double-strand breaks (DSBs) via homologous recombination (HR), their exact role in promoting HR remains poorly understood. Here, we showed that SART1, an SF upregulated in several types of cancer, promotes DSB end resection, which is an essential step in HR, without affecting the expression of HR factors. In the promotion of resection, SART1 is epistatic with BRCA1, an HR factor known to promote resection. Mechanistically, SART1 cooperates with BRCA1 to counteract the resection blockade posed by 53BP1 and RIF1. Moreover, SART1 is required for recruitment of the HR-promoting portion of BRCA1 to DSBs. Laser microirradiation experiments revealed that SART1 is recruited to DSBs in a BRCA1-dependent manner, and that the arginine/serine (RS)-rich domain is required for SART1 recruitment to DSBs. We also obtained evidence that SART1 is involved in transcription-associated HR together with BRCA1. Furthermore, chromosome analysis demonstrated that SART1 and BRCA1 epistatically suppressed genomic alterations caused by DSB misrepair in the G2 phase. Collectively, these results indicate that SART1 and BRCA1 cooperatively facilitate resection of DSBs arising in transcriptionally active genomic regions, thereby promoting faithful HR repair of such DSBs and suppressing genome instability.