Abstract 3754: PPP6C regulation of ERK signaling in melanoma

Abstract

Abstract Melanoma, the deadliest form of skin cancer, is characterized by aberrant hyperactivation of the ERK mitogen-activated protein kinase signaling pathway. Inhibitors of BRAF and MEK1/2, both members of this pathway, have shown clinical efficacy and are used in combination to treat BRAFV600E melanoma, the most common melanoma genomic subtype. However, not all BRAFV600E melanomas respond to these inhibitors, and those that do respond eventually acquire resistance. To better understand mechanisms of drug susceptibility in BRAFV600E melanoma, we performed a loss-of-function screen to identify kinases and phosphatases that modulate sensitivity to two clinical MEK1/2 inhibitors. In this screen, we identified PPP6C, the catalytic subunit of protein phosphatase 6 (PP6), as a factor promoting sensitivity to MEK1/2 inhibition. PPP6C is a potential tumor suppressor mutated in 7-12% of melanomas, and has roles in cell cycle regulation and DNA damage repair. shRNA-mediated knockdown and CRISPR/Cas9-mediated knockout of PPP6C in BRAF mutant melanoma cells both lead to hyperactivation of MEK1/2 and ERK1/2, resulting in decreased sensitivity to pathway inhibitors. This finding was confirmed in a panel of BRAFV600E melanoma cell lines. Through in vitro biochemical and cell-based studies, we have established that PP6 directly dephosphorylates MEK1/2. As with complete loss of PPP6C, cells expressing most recurrent cancer-associated PPP6C mutants similarly have hyperactive ERK signaling. Our studies support a novel negative regulatory role for PPP6C in ERK signaling that contributes to its activity as a tumor suppressor and promotes pathway inhibitor sensitivity in BRAFV600E melanoma. Citation Format: Eunice Cho, Hua Jane Lou, Benjamin E. Turk. PPP6C regulation of ERK signaling in melanoma [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 3754.