The BRCA1- RAD51 Axis Regulates SCAI/REV3 Dependent Replication Fork Maintenance
Replication stress is a key contributor to genomic instability and cancers. BRCA1 has well established roles in protecting stalled forks against degradation. Here we show that BRCA1 has a fork protection-independent role via regulation of RAD51 in a manner that confers dependency on SCAI and REV3. SCAI loss leads to elevated DNA breaks, genomic instability and cell death upon DNA damage. We show that the increased DNA breaks occur in S phase, consistent with fork breakage. BRCA1 drives SLX4-SLX1-ERCC1 mediated DNA break formation in the absence of SCAI and REV3, which are required to maintain integrity of stalled forks for efficient restart. Domain analysis revealed that the increased break signaling seen in the absence of SCAI does not require binding to resection factors like CtIP. Surprisingly, loss of fork reversal factors leads to additive increases in damage signaling and elevated fork shortening in the absence of SCAI/REV3 in a manner that depends on RAD51 activity. We propose that Protexin may be required to replicate through and maintain stalled replication forks at fragile genomic regions. Failure to do so leads to increased DNA breakage and genomic instability.