Abstract A10: BRD4 is a new therapeutic target in melanoma

Abstract

Metastatic melanoma remains a mostly incurable disease. Although newly approved targeted therapies are efficacious in a subset of patients, resistance and relapse rapidly ensue. Alternative therapeutic strategies to manipulate epigenetic regulators and disrupt the transcriptional program that maintains tumor cell identity are emerging. Bromodomain and extraterminal domain (BET) family of proteins consists of BRD2, BRD3, BRD4, and testis- specific BRDT, and are epigenome readers known to exert key roles at the interface between chromatin remodeling and transcriptional regulation. We investigated the role of BET proteins in melanoma tumor maintenance and assessed their value as therapeutic targets. Data mining of our previously published gene expression profile of melanoma cell lines and immunostaining of melanoma tissue microarray revealed that BRD4 is significantly upregulated in primary and metastatic melanoma tissues compared to melanocytes and nevi, thus suggesting a potential role for BET family proteins in promoting melanoma tumorigenesis. Treatment with BET inhibitors impaired melanoma cell proliferation and colony formation in vitro. Moreover, tumor growth and metastatic behavior assessed by a xenograft model also revealed impairment of melanoma proliferation in vivo. These effects were mostly recapitulated by individual silencing of BRD4, and not of other BET family members. RNA sequencing of BET inhibitor-treated cells followed by gene ontology analysis showed a striking impact on transcriptional programs controlling cell growth, proliferation, cell-cycle regulation and differentiation. In particular, we found that, rapidly after BET displacement, key cell cycle genes (SKP2, ERK1 and c-MYC) were downregulated concomitantly with the accumulation of CDK inhibitors (p21, p27), followed by melanoma cell cycle arrest. However, single genetic manipulation of these cell cycle genes did not rescue the cytostatic effect of BET inhibition, suggesting that BET inactivation leads to a non-redundant, simultaneous regulation of multiple cell cycle effectors. Interestingly, SKP2 and ERK1 mRNA levels directly correlated with those of BRD4 in a panel of melanoma tissues, suggesting that these two factors may be direct BRD4 targets. Importantly, the effects of the BET inhibitor were not influenced by BRAF or NRAS mutational status, opening the possibility of using these small molecule compounds to treat patients for whom no effective targeted therapy currently exists. Collectively, our results strongly support a critical role for BRD4 in melanoma tumor maintenance, and render it a legitimate and novel target for epigenetic therapy directed against the core transcriptional program of melanoma. Citation Format: Barbara Fontanals-Cirera, Miguel F. Segura, Avital Gaziel-Sovran, Maria V. Guijarro, Doug Hanniford, Pilar Gonzalez-Gomez, Weijia Zhang, Guantao Zhang, Farbod Darvishian, Michael Ohlmeyer, Iman Osman, Ming-Ming Zhou, Eva Hernando. BRD4 is a new therapeutic target in melanoma. [abstract]. In: Proceedings of the AACR Special Conference on Chromatin and Epigenetics in Cancer; Jun 19-22, 2013; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2013;73(13 Suppl):Abstract nr A10.