Abstract B098: GPX4 is a broadly shared gene vulnerability among residual tumors
Authors: Hangauer Matthew J. , Viswanathan Vasanthi S. , Ryan Matthew J. , Bole Dhruv , Eaton John K. , Matov Alexandre , Galeas Jacqueline , Dhruv Harshil D. , Berens Michael E. , Schreiber Stuart L. , McCormick Frank , McManus Michael T. 
Published: August 2018

Abstract
The heterogeneity of tumor cells underlies acquired drug resistance to a variety of therapeutic approaches. The advent of next-generation sequencing has facilitated a wave of studies identifying genetic mutations, which may be preexisting or acquired during treatment, that drive drug resistance and tumor relapse. However, it has recently become clear that non-mutational mechanisms of drug resistance, such as cell state switching from an epithelial to mesenchymal state, can also play an important role in the process of acquired drug resistance. Non-mutational drug resistance is relatively poorly understood and represents fertile ground for the discovery of novel therapeutic targets. Drug-tolerant “persister” cells are an experimental model of non-mutational cancer drug resistance in which small fractions (&lt;5%) of cells within cancer cell lines survive cytotoxic drug exposure despite lacking resistance-conferring mutations. These residual surviving persister cells occupy a reversible quiescent state with a unique chromatin landscape. Persister cells regrow and become resensitized to drug, reminiscent of clinical observations of secondary responses from retreatment after a drug holiday. Persister cells also eventually obtain genetic mutations and reenter the cell cycle after weeks or months of continuous drug exposure, modeling the process of acquisition of resistance-conferring genetic mutations in patients during treatment. Here, we report on our efforts to identify a widely shared gene vulnerability in persister cells that transcends tissue lineage, genetic mutation background, and drug treatment regimens. Through a functional genomics approach entailing RNA-seq, pathway analysis, and a focused chemical inhibitor screen, we have identified a gene, glutathione peroxidase 4 (GPX4), that is specifically essential to persister cells. When GPX4 is chemically inhibited or genetically ablated, persister cells across a wide range of tissue lineages undergo ferroptosis–a recently discovered mechanism of non-apoptotic caspase-independent cell death. Ferroptosis occurs when lipid peroxides accumulate in cells, and as the only human enzyme capable of scavenging lipid peroxides, GPX4 plays a key role in preventing ferroptosis. Compared to drug-naïve parental cells or nontransformed normal cells, persister cells are strongly differentially sensitive to GPX4 inhibition and ferroptosis. This sensitivity is the result of a disabled antioxidant program in persister cells marked by a global downregulation of antioxidant genes including Nrf2 targets, and decreased levels of reducing cofactors glutathione and NADPH. As a first step toward raising GPX4 as a promising preclinical drug target in vivo, we also show that targeting GPX4 in residual melanoma xenograft tumors prevents tumor relapse. Therefore, GPX4 is an extremely promising drug target that may be exploited to prevent tumor relapse across a wide spectrum of tumor types and drug treatments.
Citation Format: Matthew J. Hangauer, Vasanthi S. Viswanathan, Matthew J. Ryan, Dhruv Bole, John K. Eaton, Alexandre Matov, Jacqueline Galeas, Harshil D. Dhruv, Michael E. Berens, Stuart L. Schreiber, Frank McCormick, Michael T. McManus. GPX4 is a broadly shared gene vulnerability among residual tumors [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2017 Oct 26-30; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2018;17(1 Suppl):Abstract nr B098.