Drug-gene Interaction Screens Coupled to Tumor Data Analyses Identify the Most Clinically Relevant Cancer Vulnerabilities Driving Sensitivity to PARP Inhibition

PARP inhibitors (PARPi) are currently indicated for the treatment of ovar-ian, breast, pancreatic, and prostate cancers harboring mutations in the tumor suppressor genes BRCA1 or BRCA2. In the case of ovarian and prostate cancers, their classification as homologous recombination re-pair (HRR) deficient (HRD) or mutated also makes PARPi an available treatment option beyond BRCA1 or BRCA2 mutational status. However, identification of the most relevant genetic alterations driving the HRD phe-notype has proven difficult and recent data have shown that other genetic alterations not affecting HRR are also capable of driving PARPi responses. To gain insight into the genetics driving PARPi sensitivity, we performed CRISPR-Cas9 loss-of-function screens in six PARPi-insensitive cell lines and combined the output with published PARPi datasets from eight addi-tional cell lines. Ensuing exploration of the data identified 110 genes whose inactivation is strongly linked to sensitivity to PARPi. Parallel cell line gen-eration of isogenic gene knockouts in ovarian and prostate cancer cell lines identified that inactivation of core HRR factors is required for driving in vitro PARPi responses comparable with the ones observed for BRCA1 or BRCA2 mutations. Moreover, pan-cancer genetic, transcriptomic, and epi-genetic data analyses of these 110 genes highlight the ones most frequently inactivated in tumors, making this study a valuable resource for prospective identification of potential PARPi-responsive patient populations. Impor-tantly, our investigations uncover XRCC3 gene silencing as a potential new prognostic biomarker of PARPi sensitivity in prostate cancer.Significance: This study identifies tumor genetic backgrounds where to expand the use of PARPis beyond mutations in BRCA1 or BRCA2. This is achieved by combining the output of unbiased genome-wide loss-of-function CRISPR-Cas9 genetic screens with bioinformatics analysis of biallelic losses of the identified genes in public tumor datasets, unveiling loss of the DNA repair gene XRCC3 as a potential biomarker of PARPi sensitivity in prostate cancer.