Abstract LB043: Pharmacological inhibition of ubiquitination sensitizes to DNA damaging agents by preventing homologous recombination repair

Abstract Therapies inducing DNA damage have been the backbone of cancer treatment for decades, but they often cause significant adverse effects reducing quality of life. Recently, inhibition of poly-ADP-ribose polymerase (PARP) has been shown to target tumor cells with deficient DNA homologous recombination (HR) double-strand break repair by converting single-strand DNA breaks into toxic double-strand breaks that cannot be resolved. This approach has been advantageous as healthy cells with functional HR repair are less effective, resulting in a more tolerable treatment. However, most cancers will not respond to PARP inhibition. Here, using a genome-wide CRISPR knockout functional genomics screen we identified that depletion of ubiquitination enzymes induced sensitivity to PARP inhibition in HR proficient ovarian cancer cells, as well as KRAS-mutant colorectal cancer cells which are generally resistant to PARP and other DNA damaging agents. Validation studies across multiple cancer lineages using cell line and organoid models confirmed that either silencing or pharmacological inhibition of ubiquitin-activating enzyme E1 (UBA1), which is required for ubiquitination, sensitized to PARP inhibition and other DNA damaging therapies. In vivo testing of combination PARP and UBA1 inhibition was well tolerated, and significantly extended survival of patient-derived xenografts compared to either therapy alone. Mechanistically, we found that UBA1 inhibition not only blocked HR repair to sensitize to PARP inhibition, but it also induced global increases in PARylation, which was subsequently inhibited by PARP inhibition. Taken together, these results demonstrate that inhibition of UBA1 sensitizes to PARP via multiple pathways, and the combination is well tolerated making it an ideal strategy to expand the therapeutic benefit of PARP inhibition. Citation Format: Sharad Awasthi, S. Stephen Yi, Daniel J. McGrail, Nidhi Sahni. Pharmacological inhibition of ubiquitination sensitizes to DNA damaging agents by preventing homologous recombination repair [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 2 (Clinical Trials and Late-Breaking Research); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(8_Suppl):Abstract nr LB043.