Abstract B051: PARG inhibition leads to the formation of toxic nuclear PARP1 aggregates

Abstract

Abstract Mutations in DNA repair genes, such as BRCA1/2 mutations, occur in 50-70% of breast cancers among others. PARP inhibitors (PARPis) have shown remarkable efficacy against these tumors, however, emerging clinical evidence has revealed that PARPi resistance represents a major concern in these cancers with up to 90% of initial responders eventually developing resistance to the therapy. To overcome the limitations of PARPis, researchers have been working towards developing PARG inhibitors (PARGis) for the treatment of DNA repair-deficient cancers. In response to DNA damage, PARP1 synthesizes poly(ADP-ribose) (PAR) chains at break sites, which are crucial for the proper release of PARP1 and the recruitment of downstream repair proteins to the break site. PAR glycohydrolase (PARG) is the primary enzyme responsible for the resolution of PAR chains following repair. Owing to their central role in PARP-dependent processes, PARG inhibitors are currently being developed for cancer therapy, however, their precise mechanism of action has yet to be determined. Here we report that PARG loss or inhibition leads to cytotoxic effects through the formation of toxic PARP1 aggregates in response to DNA damage. Using live cell imaging assays, we determined that PARP1 aggregates form at sites distant from the original site of damage, causing a mislocalization of DNA repair factors from the damage site. We also find that these aggregates are formed by and composed of long, branched PAR chains, which are likely sequestering ATP and NAD+ reserves in the nucleus. Finally, we show that these aggregates are associated with increased cell death in BRCA-deficient cell lines suggesting that these aggregates are critical for PARGi mechanism of toxicity. Overall, this work elucidates the mechanism of action of PARG inhibitors, which will help identify patient populations in which these drugs could be used. Citation Format: Sateja Paradkar, Annie Cui, Julia Purcell, Sam Friedman, Ryan Jensen, Ranjit Bindra. PARG inhibition leads to the formation of toxic nuclear PARP1 aggregates [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2023 Oct 11-15; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2023;22(12 Suppl):Abstract nr B051.