Abstract 3538: Targeting the E3 ubiquitin ligase RNF20 in ovarian cancer

Abstract

Abstract E3 ubiquitin ligases are responsible for the final stage of the enzymatic ubiquitination cascade, where they transfer ubiquitin from an E2 conjugating enzyme to a lysine on the recognized substrate. The most common family of E3 ligases are RING (Really Interesting New Gene) domain proteins. Using immunohistochemistry, we have shown that the ring finger protein RNF20 is strongly expressed in 87% (379 of 424) of high-grade serous ovarian cancer (HGSOC) (1). In contrast, other tumors, including breast, metastatic prostate and colorectal cancer, have been reported to show a decrease in RNF20 transcript levels. The Cancer Genome Atlas has reported < 1% (3 of 316) HGSOCs with RNF20 missense variants of unknown pathogenicity. Given the high level of RNF20 expression in HGSOC, it is unlikely that RNF20 transcript levels are reduced in these tumors. RNF20 functioning in a heterodimer with RNF40 is the main E3 ligase complex responsible for monoubiquitination of histone H2B at lysine 120 (H2Bub1) (2). This active histone mark leads to an open chromatin configuration favouring DNA access by transcriptional complexes and DNA repair proteins. The RNF20/RNF40 complex also monoubiquitinates and stabilizes the Eg5 motor protein that functions in spindle assembly during mitosis, preventing spindle assembly defects, cell cycle arrest and apoptosis. RNF20 has also been implicated in polyubiquitination of the tumor suppressor Ebp1, promoting its degradation via the 26S proteasome in malignant cells. We have down-regulated RNF20 in ovarian cancer cell lines to investigate the potential of targeting this E3 ligase to overcome resistance to platinum therapies that is a major problem for the clinical management of women with ovarian cancer. Cell lines studied included the HGSOC lines; COV318, OVCAR-3, OVCAR-4, CaOV3, HEY and OVSAHO, as well as the endometrioid line A2780. Cells were studied as monolayers and spheroid cultures. We have shown that down-regulation of RNF20 leads to lower levels of H2Bub1, consistent with a more closed chromatin configuration. Clonogenic cell survival assays following the treatment of ovarian cancer cell lines with cisplatin showed an impaired ability of cells in which RNF20 had been down-regulated to form colonies. This work provides preliminary evidence that developing strategies to therapeutically down-regulate RNF20 may increase the efficacy of standard-of-care chemotherapy for women with ovarian cancer. (1) Dickson KA et al., Hum Mol Genet (2016) (2) Cole AJ et al., Endocr Relat Cancer (2015) Citation Format: Alexander J. Cole, Kristie-Ann Dickson, Roderick Clifton-Bligh, Deborah J. Marsh. Targeting the E3 ubiquitin ligase RNF20 in ovarian cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3538.