Abstract 7131: Base excision repair regulation of replication stress induced poly(ADP-ribose) checkpoints

Abstract PARP-inhibitors (PARPi) have been developed to treat homology-directed repair deficient cancers such as BRCA1/2-defective breast and ovarian tumors. However, acquired resistance to PARPi is now common, suggesting the need to identify new protein targets. To that end, a missed opportunity is found in the PARP1/PARP2 activation mechanism in response to DNA damage and replication stress and the resulting protein complexes and enzymes that depend on and coordinate PARP-mediated DNA repair processes. We have found that PARP1/PARP2 activation, whether from acute DNA damage, from replication stress or both triggers a temporal and coordinated interplay of base excision repair and single-strand break repair (BER/SSBR) proteins and enzymes. In turn, the recruitment of these BER and SSBR proteins suppresses PARP1/PARP2 activation, thereby functioning as a network of PARP1/PARP2 regulators, modulating the PARP-mediated response to replication stress. To that end, we find that loss of expression of these key BER/SSBR proteins releases a break on replication-stress induced PARP1/PARP2 activation, triggering un-restrained PARP1/PARP2 activation and elevated levels of replication-dependent poly(ADP-ribose) (PAR). We next confirmed that the loss of these downstream BER enzymes render cancer cells responsive to PARPi’s when the BRCA1/2 pathways are restored. Further, we show that BER expression regulates the cellular response to inhibitors of the PAR degrading enzyme PARG, suggesting that XRCC1-dependent BER proteins regulate replication-stress dependent PAR accumulation. The increase in PAR due to PARG inhibition, that is further regulated by BER, drives a strong intra-S phase response, indicated by activation of ATR and CHK1 signaling. As the checkpoint governed by ATR and CHK1 functions as a protective mechanism, we show that PARG inhibition is synthetically lethal with ATR and CHK1 inhibition and this may be explained by our demonstration of PARGi-mediated trapping of essential BER proteins at the site of damage and PAR accumulation. Together, these studies reveal a significant role for BER/SSBR proteins in regulating replication stress induced PARylation and the resulting intra-S phase checkpoint. Citation Format: Md Ibrahim, Md Maruf Khan, Rasha Q. Al-Rahaleh, Jennifer Clark, Faisal Hayat, Qingming Fang, Christopher A. Koczor, Marie E. Migaud, Robert W. Sobol. Base excision repair regulation of replication stress induced poly(ADP-ribose) checkpoints [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 7131.