PCNA-RECQL5-RPRD1B recruit repair components to stressed replication forks to promote survival of BRCA1-deficient cancer cells.

Given their defect in homologous recombination, BRCA1-deficient cancers use polymerase theta (POLQ) microhomology-mediated end joining (MMEJ) for repair of stressed replication forks. We found that BRCA1-deficient cells repress microRNA (miR) 4485-3p, and reconstituting expression of this miR resulted in the death of BRCA1-deficient cells. In our interrogation of the mechanism of this synthetic lethality, we discovered that miR-4485-3p suppresses expression of RPRD1B protein. Depletion of RPRD1B led to selective BRCA1-deficient cell death, decreased MMEJ, and decreased replication fork repair after replication stress. RPRD1B promotes the recruitment of the MMEJ components 53BP1, PARP1, and POLQ to the stressed replication fork. However, RPRD1B does not have a canonical DNA-binding domain, and our search for the mechanism of its recruitment to stressed replication forks discovered that this is mediated by RECQL5 and ubiquitinated (Ub) PCNA. Thus, our results unveil an essential signaling cascade at the stressed replication fork in BRCA1-deficient cancer cells, in which Ub PCNA recruits RECQL5, which recruits RPRD1B to mediate the assembly of the MMEJ repair apparatus. These data suggest that this repair scaffold may be targeted in the development of a novel synthetic therapy of BRCA1-deficient cancers.