Abstract P6-03-02: Functional genomic screening identifies USP11 as a novel therapeutic target in breast cancer

The estrogen receptor (ER) is the principal driver of growth and differentiation in breast cells and de-regulated receptor function is a key feature of almost 75% of breast cancers. Here, we investigated the role of de-ubiquitinating enzymes (DUBs), which act to remove ubiquitin moieties from proteins, in regulating the transcriptional activity of ER in breast cancer. To identify DUBs involved in the regulation of ER transcriptional activity, we performed an RNAi loss-of-function screen using a library of shRNA vectors targeting all known human DUB genes (108 genes/432 shRNAs in total). We found that suppression of a number of DUBs markedly repressed or enhanced the activity of an estrogen-response-element (ERE) luciferase reporter to estradiol (E2). In particular, suppression of the BRCA2-associated DUB, USP11, was found to down-regulate ERα transcriptional activity (both in the presence and absence of E2), as demonstrated by a pronounced decrease in estrogen-response element (ERE) luciferase reporter activity. Subsequent validation of the screen using multiple individual hairpins and ZR-75-1 stable USP11 knockdown cells confirmed the suppression of ERE-reporter activity and further revealed a notable reduction in expression of the endogenous ER target genes TFF1 and PgR following USP11 knockdown. In vitro phenotypic analysis also revealed a global decrease in cell survival, decreased ERK and AKT phosphorylation and increased sensitivity to DNA-damaging agents in USP11 knockdown cell lines compared to non-targeting controls. In silico analysis of publically available breast cancer gene expression datasets revealed a highly significant association between high USP11 mRNA levels and poor prognosis. We observed a highly significant correlation between high expression of USP11 mRNA in ER positive patients and poor overall survival (OS)(HR 1.51, CI 1.07-2.14, p=0.018) and distant metastasis-free survival (DMFS)(HR 1.35, CI 1.04-1.73, p=0.023). This correlation was also significant in ER positive patients who had received endocrine therapy (OS, HR 3.4, CI 1.2-9.81, p=0.014), DMFS, HR 2.16, CI 1.23-3.8, p=0.0083). These results suggest a role for USP11 in driving cellular growth and identify USP11 as a rationale and novel therapeutic target in breast cancer. Citation Format: Darran P O9Connor, Lisa Dwane, Aisling E O9Connor, Annette M Dirac, Rene Bernards, William M Gallagher. Functional genomic screening identifies USP11 as a novel therapeutic target in breast cancer [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P6-03-02.