Abstract 678: The Ets factor ETV5 regulates E2F target genes and mediates drug resistance
Authors: Taylor-Harding Barbie , Agadjanian Hasmik , Aspuria Paul-Joseph , Cheon Dong-Joo , Mizuno Takako , Greenberg Danielle , Allen Jenieke R. , Orsulic Sandra , Walsh Christine , Karlan Beth Y. , Wiedemeyer W. Ruprecht 
Published: May 2015

Abstract
E2F transcription factors are downstream mediators of several oncogenic drivers, including CDK/cyclin complexes, MYC, and mitogen-activated kinase (MAPK) signaling. E2F transcribe genes involved in G1-S phase transition and couple cell cycle progression with DNA repair. Prominent E2F target genes include CDK1 (CDC2), cyclin E1 (CCNE1) and BRCA1. Cyclin E1 is part of a positive feedback loop: its transcription is induced by activator E2F. Cyclin E1 protein then binds to CDK2, and the complex inactivates the RB protein, a potent inhibitor of activator E2F. The CCNE1 gene is frequently amplified in high-grade serous ovarian cancer (HGSOC, 20%) and other cancers. In the TCGA dataset on HGSOC, E2F target genes, including BRCA1, are co-expressed with CCNE1, and CCNE1-amplified HGSOC overexpress BRCA1. Moreover, a recent study suggests that CCNE1-amplified HGSOC are critically dependent on BRCA1. Loss of BRCA1 function results in synthetic lethality specifically in this subset. This finding provides an explanation for the mutually exclusive relationship between CCNE1 amplification and BRCA1 mutation.
Based on the functional link between cyclin E1 and BRCA1, we devised a targeting strategy for cyclin E1-dependent ovarian cancers using pharmacological CDK2 inhibitors. Mutations in BRCA1 and BRCA2 sensitize cancer cells to platinum-based chemotherapy. We hypothesized that CDK inhibitors (CDKi) would induce loss of BRCA function and chemosensitize cyclin E1-dependent HGSOC. Indeed, we found that the CDK1/2 inhibitor, Dinaciclib, sensitized cyclin E1-transformed p53-null murine ovarian surface epithelial (MOSE) cells to cisplatin, whereas no sensitization was observed in HRASV12-driven MOSE cells. Similarly, progression of human CCNE1-amplified OVCAR3 xenografts was blocked by combined treatment with Dinaciclib and cisplatin but not by either single agent. Mechanistically, Dinaciclib treatment resulted in transcriptional downregulation of both BRCA1 and cyclin E1. Comparison of CDKi-sensitive and CDKi-resistant ovarian cancer cell lines revealed that the Ets family transcription factor, ETV5, was highly expressed in resistant lines. ETV5 is a downstream mediator of MAPK signaling and known oncogene in prostate cancer.
Using inducible shRNA, we show that genetic depletion of ETV5 in RAS-driven ovarian cancer cells reduces tumorigenicity, accompanied by transcriptional downregulation of cyclin E1 and BRCA1. Thus, ETV5 regulates a subset of E2F target genes. These findings suggest that Ets family transcription factors can compensate for loss of E2F function induced by targeted agents, such as CDKi. Functional compensation on the level of receptor tyrosine kinases has been described as a mechanism of drug resistance. Our study demonstrates similar compensatory mechanisms among transcription factors that mediate oncogenic signaling.
Citation Format: Barbie Taylor-Harding, Hasmik Agadjanian, Paul-Joseph Aspuria, Dong-Joo Cheon, Takako Mizuno, Danielle Greenberg, Jenieke R. Allen, Sandra Orsulic, Christine Walsh, Beth Y. Karlan, W. Ruprecht Wiedemeyer. The Ets factor ETV5 regulates E2F target genes and mediates drug resistance. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 678. doi:10.1158/1538-7445.AM2014-678