Abstract 1374: Identification of a transcription factor driving BET dependency in SCLC

Abstract

Abstract The regulation, and subsequent recognition, of covalent chromatin modifications by an array of cellular proteins are key determinants of gene expression. For instance, the binding of bromodomain and extra terminal domain (BET) proteins to acetylated histone tails has been shown to regulate c-MYC transcription in some cell types. Consequently, pharmacological inhibition of BET proteins leads to an anti-proliferative activity in cancers dependent on c-MYC activity such as multiple myeloma and acute myelogenous leukemia. Since c-MYC has been implicated as a driver of cell proliferation in many cancer types, we investigated if BET inhibitors may have anti-proliferative activity in solid tumor cancer cell lines as well. Strikingly, in a panel of over 100 mixed histology lung cancer cell lines we identified selective growth inhibition of lines derived from small cell lung cancer (SCLC) histology. To better understand the mechanism of this dependency we performed 12-point dose response microarray analyses using the BET inhibitor, JQ1, in order to identify genes that are regulated by BET proteins in SCLC. In contrast to what has been observed in myeloma and leukemia, BET inhibition is not always accompanied by inhibition of c-MYC expression in the SCLC cell lines. From this analysis a lineage-specific transcription factor was found to be highly regulated in a dose-dependent manner. Previous studies have reported this transcription factor to be essential in maintaining the differentiated neuroendocrine phenotype characteristic of human SCLC. It has also been shown that loss of this transcription factor results in growth inhibition in SCLC tumor models. We have confirmed the growth dependency of SCLC to BET via dysregulation of this transcription factor using RNAi and a small molecule inhibitor. Furthermore, chromatin immunoprecipitation followed by deep-sequencing confirmed binding of BRD4, a BET family member, and RNA polymerase II, a marker of active gene transcription, to the enhancer region of this target gene in two sensitive SCLC cell lines. This binding is disrupted in the presence of BET compound treatment. SCLC is a disease with high unmet medical need, and the results of our studies have identified BET inhibition as a potential novel therapeutic opportunity for this subtype of lung cancer. Our results have also provided a mechanistic basis for the sensitivity of SCLC to BET inhibition, and a rationale for the clinical development of BET inhibitors beyond hematologic cancers. Citation Format: Susan Wee, Tai Wong, BMS. Identification of a transcription factor driving BET dependency in SCLC. [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 1374. doi:10.1158/1538-7445.AM2014-1374