Transcriptional Targeting Of Oncogene Addiction In Medullary Thyroid Cancer [ChIP-Seq]

Metastatic medullary thyroid cancer (MTC) is a currently incurable disease. FDA approved therapies that target RET, a commonly mutated receptor tyrosine kinase in MTC, and other receptor tyrosine kinases, do not result in complete responses and acquired resistance is universal due to “gatekeeper” mutation in Ret or overactivation of alternative signaling pathways. Based on data from human MTCs and a number of murine models, the CDK/RB cell cycle pathway is a potential alternative target for MTC. The objective of this study was to determine if CDKs represent therapeutic targets for MTC and to define mechanisms of activity. We demonstrate that targeting the CDK/RB pathway with Palbociclib (CDK4/6 inhibitor) is not cytotoxic to MTC cells but that Dinaciclib (CDK1/2/5/9 inhibitor) remarkably reduced cell viability and proliferation at low doses in two MTC cell lines accompanied by loss of CDK9 and RET protein and mRNA levels. In human tumors, CDK9 protein was highly expressed and array CGH demonstrated copy number gain in 11/30 analyzed tumors. RNA sequencing demonstrated that RNA polymerase II-dependent transcription was markedly reduced by Dinaciclib, consistent with transcriptional mode of action. Subsequent studies using the CDK7 inhibitor, THZ1, demonstrated high potency vs. the MTC cell lines and marked loss of RET mRNA and protein. In silico analysis, and CHIP-Sequencing using H3K27Ac antibody confirmed that RET is associated with a super-enhancer in RET-mutated MTC cells. In summary, this study reveals a novel mechanism of RET transcription regulation that represents a potentially translatable finding for new therapeutic approaches for RET-mutated MTC. Examination of BRD4 and H3K27Ac peaks in TT cell DNA.