*Co-senior authors

Primary high-grade serous ovarian cancer (HGSOC) is often sensitive to platinum and taxane combination chemotherapy, but most patients relapse with chemotherapy-resistant disease. Although alterations in DNA repair function, gene expression, apoptosis, and other pathways have been described that can mediate chemotherapy resistance in HGSOC, the full landscape of HGSOC drug resistance mechanisms and the optimal strategies to eliminate resistant disease have not been fully elucidated. We performed systematic, unbiased near-genome-scale pooled overexpression and CRISPR/Cas9 knockout screens in two BRCA2-mutant HGSOC cell lines to identify genes promoting survival following cisplatin, paclitaxel, or cisplatin/paclitaxel treatment. Anti-apoptotic genes including BCL2L1 (BCL-XL), and BCL2L2 (BCL-W) were among the top hits mediating chemotherapy resistance in the overexpression screen. In the CRISPR/Cas9 screen, loss of pro-apoptotic genes (caspases, APAF1) conferred resistance, and knockout of BCL2L1 sensitized to platinum. A secondary overexpression screen of ~400 genes confirmed anti-apoptotic proteins BCL-XL, BCL-W and BCL-2 as top resistance genes, and validated numerous other candidates. Of note, anti-apoptotic genes BCL2L1 and MCL1 are focally amplified and overexpressed in patients with primary HGSOC. In HGSOC cell lines, overexpression of BCL-XL or BCL-W, and to a lesser extent BCL-2 or MCL1, conferred platinum and taxane resistance and decreased chemotherapy-induced apoptosis in HGSOC cell lines. We systematically tested small molecule inhibitors of BCL-2, BCL-XL, MCL1, or BCL2/BCL-XL as single agents or combined with chemotherapy in HGSOC cell lines. Inhibiting BCL-XL, MCL1, or BCL2/BCL-XL, but not BCL-2, significantly increased cell death when combined with cisplatin or paclitaxel. BCL-XL, MCL1, or BCL2/BCL-XL inhibitors also synergized with olaparib, a poly- ADP-ribose inhibitor. Concomitant overexpression of BCL-XL, BCL-W, or MCL1 abrogated the sensitizing effect of the anti-apoptotic protein inhibitors, depending upon the specific inhibitor. Taken together, unbiased near-genome-scale overexpression screens and patient genomic data highlight the role of the intrinsic pathway of apoptosis in HGSOC chemotherapy resistance. Our studies validate that anti-apoptotic proteins mediate resistance to several clinically relevant drugs in HGSOC, and support that BCL-XL and MCL1 may be therapeutic targets in HGSOC, particularly in combination with DNA-damaging agents.

Citation Format: Elizabeth H. Stover, Maria B. Baco, Ofir Cohen, Yvonne Li, Elizabeth Christie, Mukta Bagul, Amy Goodale, Yenarae Lee, Sasha Pantel, Matthew Rees, Guo Wei, Adam Presser, Ioannis Zervantonakis, Patrick Bhola, Jeremy Ryan, Jennifer Guerriero, Felice Liang, Andrew Cherniack, Federica Piccioni, Ursula A. Matulonis, David D. L. Bowtell, Anthony Letai, Kris Sarosiek, Levi Garraway, Cory M. Johannessen, Matthew Meyerson. POOLED GENOMIC SCREENS IDENTIFY ANTI-APOPTOTIC GENES AS MEDIATORS OF CHEMOTHERAPY RESISTANCE IN OVARIAN CANCER [abstract]. In: Proceedings of the 12th Biennial Ovarian Cancer Research Symposium; Sep 13-15, 2018; Seattle, WA. Philadelphia (PA): AACR; Clin Cancer Res 2019;25(22 Suppl):Abstract nr AP14.