CDK7 inhibition suppresses Hippo-YAP signaling to reverse CDK4/6 inhibitor resistance in hormone receptor-positive breast cancer

Hormone receptor-positive (HR+) breast cancer is the most prevalent breast cancer subtype, and cyclin-dependent kinase 4/6 inhibitors (CDK4/6i) are the mainstay of treatment for metastatic disease. Amplification or overexpression of CDK6 is an established driver of CDK4/6i resistance, for which no targeted strategy exists. Through a kinase-inhibitor screen for compounds that suppress CDK6, we identified PIK-75 as an agent that downregulates CDK6 and reverses CDK4/6i resistance in HR+ breast cancer. We found that PIK-75 acts as an inhibitor of CDK7, abolishing phosphorylation of serine 5 on the RNA polymerase II (Pol II) carboxy-terminal domain. The structurally distinct CDK7 inhibitor THZ1 and shRNA-mediated CDK7 depletion both phenocopied the suppression of yes-associated protein (YAP) and CDK6, establishing the effect as CDK7-dependent. Chromatin immunoprecipitation demonstrated that CDK7 inhibition reduced Pol II-serine 5 occupancy at the YAP1 promoter, downregulating YAP1 transcription and YAP-TEAD output. Genome-wide, PIK-75 induced promoter-proximal Pol II pausing that was more pronounced at oncogenic-pathway genes, and transcriptomic and proteomic profiling confirmed a bona fide CDK7-inhibition signature, accompanied by widespread intron retention, rather than a phosphoinositide 3-kinase-dominated response. In CDK6-overexpressing xenografts, PIK-75 combined with abemaciclib reversed resistance and suppressed YAP and CDK6 in vivo. These findings define a transcriptional axis linking CDK7 and Pol II-serine 5 phosphorylation to Hippo-YAP signaling and CDK6, and establish CDK7 inhibition as a rational strategy for CDK4/6i-resistant HR+ breast cancer, supporting evaluation of clinical-stage CDK7 inhibitors in combination with continued CDK4/6 inhibition.