Poly (ADP-ribose) polymerase (PARP) inhibitors represent a promising new class of agents that have demonstrated efficacy in treating various cancers, particularly those that carry BRCA1/2 mutations. The cancer associated BRCA1/2 mutations disrupt DNA double strand break (DSB) repair by homologous recombination (HR). PARP inhibitors (PARPis) have been applied to trigger synthetic lethality in BRCA1/2-mutated cancer cells by promoting the accumulation of toxic DSBs. Unfortunately, resistance to PARPis is common and can occur through multiple mechanisms, including the restoration of HR and/or the stabilization of replication forks. To gain a better understanding of the mechanisms underlying PARPi resistance, we conducted an unbiased CRISPR-pooled genome-wide library screen to identify new genes whose deficiency confers resistance to the PARPi olaparib. Our study revealed that ZNF251, a zinc finger protein, is a novel gene whose haploinsufficiency confers PARPi resistance in multiple breast and ovarian cancer lines harboring BRCA1 mutations. Mechanistically, we discovered that ZNF251 haploinsufficiency leads to constitutive stimulation of RAD51-mediated HR and DNA-PKcs -mediated non-homologous end joining (NHEJ) repair of DSBs as well as DNA-PKcs-mediated fork protection in BRCA1-mutated cancer cells. Moreover, we demonstrated that RAD51 and DNA-PKcs inhibitors reversed PARPi resistance of ZNF251 haploinsufficient cancer cells harboring BRCA1 mutations. Our findings provide important insights into the mechanisms underlying PARPi resistance and highlight the unexpected role of RAD51 and DNA-PKcs in this phenomenon.

Haploinsufficiency ofZNF251causes DNA-PKcs-dependent resistance to PARP inhibitors inBRCA1-mutated cancer cells