MON-LB80 Human Thyroid Cancer Cells Are Highly Sensitive to CDK7 and 9 Inhibition

Cancer cells display lineage and cancer-specific transcriptional control of expression of oncogenic driver genes, particularly those regulated through super-enhancers. Compounds that target CDK7 and 9, key regulators of RNA polymerase II (RNAPII)-mediated gene transcription, selectively reduce transcription of “superenhanced” oncogenes and are in clinical trials. We previously identified that medullary cancer cell lines are highly sensitive to CDK7 inhibition and that the RET-gene is super-enhanced. In the present study, we sought to determine sensitivities of a panel of follicular cell-derived thyroid cancer cell lines to the CDK7 inhibitor, THZ1 and CDK9 inhibitor, AZD4573. We selected 8 independently confirmed thyroid cancer cell lines (TPC1, FTC133, BCPAP, SW1376; K1, THJ16, C643, and 8505C) from a variety of histological subtypes with different drive mutations and performed WST assays with increasing concentration of THZ1 or AZD4573 to determine IC(50) for each cell line. Western blots were performed in parallel for target validation. IC(50) values ranged from 5-100 nM for both compounds for all cell lines. All cell lines showed inhibition of CDK 7 phosphorylation (Ser 5) of RNAPII with retained CDK7 levels with THZ1 and loss of CDK9 RNAPII phosphorylation (Ser 2) with both compounds. However, treatment with either agent unexpectedly caused a reduction of total RNAPII protein levels. qRT-PCR did not reveal reduced mRNA levels in TPC1 cells with THZ1 treatment while, Bortezomib (proteasome inhibitor) co-treatment with THZ1 rescued RNAPII protein. These results are consistent with THZ1-induced proteasome degradation of RNAPII. BRAF protein levels also decreased in the hemizygous BRAF V600E-mutated cell line (8505C) but not the BRAF WT cell line (TPC1). qRT-PCR of both cell lines treated THZ-1 showed stable BRAF gene expression; further mechanistic studies are ongoing. In summary, human thyroid cancer cell lines are sensitive to inhibition of CDK7 and CDK9 likely through several mechanisms not all directly attributable to RNAPII inhibition.