Poly-(ADP-ribose) polymerase (PARP) inhibitors (PARPis) selectively kill BRCA1/2-deficient cells, but their efficacy in BRCA-deficient patients is limited by drug resistance. Here, we used derived cell lines and cells from patients to investigate how to overcome PARPi resistance. We found that the functions of BRCA1 in homologous recombination (HR) and replication fork protection are sequentially bypassed during the acquisition of PARPi resistance. Despite the lack of BRCA1, PARPi-resistant cells regain RAD51 loading to DNA double-stranded breaks (DSBs) and stalled replication forks, enabling two distinct mechanisms of PARPi resistance. Compared with BRCA1-proficient cells, PARPi-resistant BRCA1-deficient cells are increasingly dependent on ATR for survival. ATR inhibitors (ATRis) disrupt BRCA1-independent RAD51 loading to DSBs and stalled forks in PARPi-resistant BRCA1-deficient cells, overcoming both resistance mechanisms. In tumor cells derived from patients, ATRis also overcome the bypass of BRCA1/2 in fork protection. Thus, ATR inhibition is a unique strategy to overcome the PARPi resistance of BRCA-deficient cancers.
Bibliographical noteFunding Information:
We thank S. Cantor, J. Chen, D. Durocher, N. Dyson, L. Ellisen, S. Orsulic, and B. Xia for reagents and discussions. This work was supported by grants from the National Institutes of Health (GM076388 and CA197779 to L.Z., and CA129933 to D.A.H.), National Cancer Institute Federal Share of Program Income (to L.Z.), Jim and Ann Orr MGH Research Scholar Award (to L.Z.), Susan Komen for the Cure (KG09042 to S.M.), and the Wellcome Trust (102696 to C.H.B.). The Flow Cytometry Core is supported by National Institutes of Health instrumentation grant 1S10RR023440-01A1. S.A.Y. was supported by a fellowship from the Department of Defense (W81XWH-13-1-0027). V.C. and R.B. have received support from the Philippe Foundation. M.-M.G. is a fellow of Fonds de recherche du Qu?bec-Sant?. H. D. N. was a Medical Discovery Post-doctoral Fellow. R.B. was a Marsha Rivkin Scholar. L.Z. is the James and Patricia Poitras Endowed Chair in Cancer Research.
- BRCA-deficient cancer
- PARP inhibitor