Triple-negative breast cancer (TNBC) is a poor prognosis disease with no clinically approved targeted therapies. Here, using in vitro and in vivo RNA interference (RNAi) screens in TNBC patient-derived xenografts (PDX), we identify cyclin dependent kinase 19 (CDK19) as a potential therapeutic target. Using in vitro and in vivo TNBC PDX models, we validated the inhibitory effect of CDK19 knockdown on tumor initiation, proliferation and metastases. Despite this, CDK19 knockdown did not affect the growth of non-transformed mammary epithelial cells. Using CD10 and EpCAM as novel tumor initiating cell (TIC) markers, we found the EpCAM(med/high)/(CD10-/low) TIC sub-population to be enriched in CDK19 and a putative cellular target of CDK19 inhibition. Comparative gene expression analysis of CDK19 and CDK8 knockdowns revealed that CDK19 regulates a number of cancer-relevant pathways, uniquely through its own action and others in common with CDK8. Furthermore, although it is known that CDK19 can act at enhancers, our CHIP-Seq studies showed that CDK19 can also epigenetically modulate specific H3K27Ac enhancer signals which correlate with gene expression changes. Finally, to assess the potential therapeutic utility of CDK19, we showed that both CDK19 knockdown and chemical inhibition of CDK19 kinase activity impaired the growth of pre-established PDX tumors in vivo. Current strategies inhibiting transcriptional co-factors and targeting TICs have been limited by toxicity to normal cells. Because of CDK19s limited tissue distribution and the viability of CDK19 knockout mice, CDK19 represents a promising therapeutic target for TNBC.

CDK19 is a Regulator of Triple-Negative Breast Cancer Growth