Abstract LB-125: Selective inhibition of CDK7 targets MYCN-driven transcriptional amplification in neuroblastoma

Abstract

Abstract Oncogenic MYC family transcription factors act as universal amplifiers of the existing gene expression program in many cancer cells, thus reducing rate-limiting constraints on growth and proliferation. Amplification of the MYCN gene defines approximately 50% of high risk neuroblastomas (NB), and is associated with aggressive disease and a poor clinical outcome. Here we exploit MYCN-driven global transcriptional amplification to specifically target MYCN-deregulated NB cells by inhibiting CDK7, a cyclin-dependent kinase with major roles in transcriptional initiation (as part of the TFIIH complex) and elongation (by activating CDK9/P-TEFb). For this purpose, we chose CDK7-IN-1, a newly developed, highly selective, first-in-class covalent inhibitor of CDK7, and then determined the effects of CDK7 inhibition on MYCN expression and global transcriptional activity. NB cells expressing high levels of MYCN were 10 times more sensitive to CDK7 inhibition than normal cells or NB cells not driven by amplified MYCN. CDK7-IN-1 was more active than its reversible (non-covalent) analogue and two pan-CDK inhibitors, roscovitine and flavopiridol. Cytotoxicity in treated MYCN-amplified NB cells resulted from G2 arrest and apoptosis. We observed a dose-dependent decrease in serine 2, 5 and 7 phosphorylation of RNA Pol II C-terminal domain only in MYCN-amplified NB cells, indicating that CDK7-IN-1 selectively inhibits RNA Pol II-mediated transcriptional initiation and elongation. Growth inhibition was accompanied by downregulation of MYCN and MYCN-associated transcriptional programs. CDK7-IN-1 significantly slowed tumor growth in a xenograft model of MYCN-amplified NB (median growth, 56.8% vs. 100% for vehicle-treated mice, P <0.05; n=6 per group) with tumors showing decreased MYCN expression. Mice remained free of toxicity over 4 weeks of CDK7-IN-1 treatment, suggesting that a therapeutic window may exist for NB cells with high MYCN expression. In conclusion, we show for the first time that selective suppression of MYCN expression and MYCN-associated transcriptional activity can be achieved through CDK7 inhibition, with associated antitumor effects in high-risk NB. Thus, CDK7 inhibition warrants further attention as a potential therapeutic strategy for MYCN-deregulated NB and perhaps other MYC-driven cancers. Citation Format: Edmond Chipumuro, Eugenio Marco, Tinghu Zhang, Camilla Christensen, Nicholas Kwiatkowski, Bandana Sharma, Clark Hatheway, Abigail Altabef, Brian J. Abraham, Kwok-Kin Wong, Guo-Cheng Yuan, Richard A. Young, Nathanael S. Gray, Rani E. George. Selective inhibition of CDK7 targets MYCN-driven transcriptional amplification in neuroblastoma. [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 LB-125. doi:10.1158/1538-7445.AM2014-LB-125