Selective CDK7 inhibition suppresses cell cycle progression and MYC signaling while enhancing apoptosis in therapy-resistant estrogen receptor positive breast cancer
Purpose: Resistance to endocrine therapy (ET) and CDK4/6 inhibitors (CDK4/6i) is a clinical challenge in estrogen receptor (ER) positive (ER+) breast cancer (BC). Cyclin-dependent kinase 7 (CDK7) is a candidate target in endocrine resistant ER+ BC models and selective CDK7 inhibitors (CDK7i) are in clinical development for the treatment of ER+ BC. Nonetheless, the precise mechanisms responsible for the activity of CDK7i in ER+ BC remain elusive. Herein, we sought to unravel these mechanisms. Experimental Design: We conducted multi-omic analyses in ER+ BC models in vitro and in vivo including models with different genetic backgrounds. We also performed genome wide CRISPR knock-out library screens to identify potential therapeutic vulnerabilities in CDK4/6i resistance models. Results: We found that the on-target anti-tumor effects of CDK7 inhibition in ER+ BC are in part p53 dependent, involve cell-cycle inhibition and suppression of c-Myc. Moreover, CDK7 inhibition exhibited cytotoxic effects, distinctive from the cytostatic nature of ETs and CDK4/6i. CDK7 inhibition resulted in suppression of ER phosphorylation at S118, however, long-term CDK7 inhibition resulted in increased ER signaling, supporting the combination of ET with a CDK7i. Lastly, genome wide CRISPR/Cas9 screens identified CDK7 and MYC signaling as putative vulnerabilities in CDK4/6i resistance, and CDK7 inhibition effectively inhibited CDK4/6i resistant models. Conclusions: Taken together, these findings support the clinical investigation of selective CDK7 inhibition combined with ET to overcome treatment resistance in ER+ BC. In addition, our study highlights the potential of increased c-Myc activity and intact p53 as predictors for sensitivity to CDK7 inhibitor-based treatments. Overall design: 1. Comparative gene expression profiling analysis of RNA-seq data for ER-WT MCF7 cells and its ER-mutant derivatives  with and without CDK7 mutation and in two PDX models (WT:1415 and Mut:1526). 2. Comparative gene expression profiling analysis of RNA-seq data for Palbociclib-sensitive and -resistant T47D and MCF7 cells.