Pooled genomic screens identify anti-apoptotic genes as targetable mediators of chemotherapy resistance in ovarian cancer

High-grade serous ovarian cancer (HGSOC) is often sensitive to initial treatment with platinum and taxanecombination chemotherapy, but most patients relapse with chemotherapy-resistant disease. To systematically identify genes modulating chemotherapyresponse, we performed pooled functional genomicscreens in HGSOC cell lines treated with cisplatin, paclitaxel, or cisplatin plus paclitaxel. Genes in the intrinsic pathway of apoptosis were among the top candidate resistance genes in both gain-of-function and loss-of-function screens. In an open reading frame overexpression screen, followed by a mini-pool secondary screen, anti-apoptotic genes including BCL2L1 ( BCL-XL) and BCL2L2(BCL-W) were associated with chemotherapyresistance. In a CRISPR- Cas9knockout screen, loss of BCL2L1 decreased cell survival whereas loss of proapoptotic genes promoted resistance. To dissect the role of individual anti-apoptotic proteins in HGSOC chemotherapyresponse, we evaluated overexpression or inhibition of BCL-2, BCL-XL, BCL-W, and MCL1in HGSOC cell lines. Overexpression of anti-apoptotic proteins decreased apoptosis and modestly increased cell viability upon cisplatin or paclitaxeltreatment. Conversely, specific inhibitors of BCL-XL, MCL1, or BCL-XL/BCL-2, but not BCL-2 alone, enhanced cell death when combined with cisplatin or paclitaxel. Anti-apoptotic protein inhibitors also sensitized HGSOC cells to the poly( ADP-ribose) polymeraseinhibitor olaparib. These unbiased screens highlight anti-apoptotic proteins as mediators of chemotherapyresistance in HGSOC, and support inhibition of BCL-XLand MCL1, alone or combined with chemotherapyor targeted agents, in treatment of primary and recurrent HGSOC. Implications: Anti-apoptotic proteins modulate drug resistance in ovarian cancer, and inhibitors of BCL-XLor MCL1promote cell death in combination with chemotherapy.