Abstract IA03: PARP inhibitor and platinum resistance in ovarian cancer-genome wide CRISPR screens

Abstract High-grade serous ovarian carcinoma (HGSOC) patients with germline mutations in BRCA1/2 exhibit high sensitivity and improved outcome to platinum-based drugs and PARP inhibitors. Three PARP inhibitors (olaparib, rucaparib, niraparib) have recently gained FDA approval for the treatment of HGSOCs. However, de novo and acquired resistance to these agents is common even in the BRCA mutation carriers and pose a significant, unsolved clinical challenge. Here we identify multiple gene complexes involved in PARPi resistance through a loss-of-function CRISPR screen. High-confidence hits include previously identified 53BP1, PARP1, shieldin complex, and ATMIN/DYNLL1 complex. Only for DYNLL1 does low expression correlate with worse outcome in BRCA1-mutant ovarian cancer patients; incidentally, all these patients had platinum-based therapy. In BRCA1-defective HGSOC patients, DNA end resection is greatly compromised and contributes to the loss of HR and PARP inhibitor sensitivity. DNA end resection is a vital process that initiates homologous recombination (HR)-mediated repair of double-stranded DNA breaks and consequently influences genome stability. Loss of DYNLL1 allows DNA end resection and restores HR in BRCA1-mutant cells, thereby inducing resistance to platinum drugs and PARP inhibitors. In primary ovarian carcinomas low BRCA1 expression correlates with increased chromosomal aberrations, and the junction sequences of somatic structural variants indicate the loss of HR. Concurrent decrease in DYNLL1 expression in BRCA1-low ovarian cancers “rescued” this phenotype with reduced genomic alterations and increased homology at putative lesions. From the mechanistic standpoint, we observed that DYNLL1 limits nucleolytic degradation of DNA ends by interacting with the DNA end resection machinery (MRN complex, BLM helicase, and DNA2) in cells. The impact of DYNLL1 on end resection can be recapitulated in vitro; this is dependent on direct interaction with MRE11. Furthermore, DYNLL1 mutants that do not interact with MRE11 phenocopy the PARPi resistance phenotype of DYNLL1/BRCA1-deficient cells. Therefore, we infer that DYNLL1 is an important antiresection factor that significantly influences genomic stability and response to DNA-damaging chemotherapy in ovarian cancer patients. Citation Format: Dipanjan Chowdhury. PARP inhibitor and platinum resistance in ovarian cancer-genome wide CRISPR screens [abstract]. In: Proceedings of the AACR Special Conference on Advances in Ovarian Cancer Research; 2019 Sep 13-16, 2019; Atlanta, GA. Philadelphia (PA): AACR; Clin Cancer Res 2020;26(13_Suppl):Abstract nr IA03.