EZH2 directs breast cancer progression through the modulation of epithelial cellular plasticity

Breast cancer remains a leading cause of death among women, encompassing diverse molecular subtypes with distinct clinical outcomes. The HER2+ subtype is particularly aggressive, with many patients eventually developing resistance to HER2-targeted therapies, underscoring the urgent need for alternative treatment strategies. Enhancer of Zeste Homolog 2 (EZH2), the catalytic subunit of Polycomb Repressive Complex 2, represses the expression of genetic programs crucial for differentiation, control of proliferation, and apoptosis through histone H3 lysine 27 trimethylation (H3K27me3). To investigate the role of EZH2 in HER2+ tumor progression, we crossed a genetically engineered mouse model (GEMM) of HER2-driven breast cancer with a conditional Ezh2 knockout strain. The results showed that Ezh2 is essential for accelerating tumor initiation and metastatic dissemination in HER2-driven breast cancer. Combined bulk and single cell RNA sequencing analyses revealed a significant downregulation of basal cell populations in the absence of Ezh2, characterized by the loss of Tp63 expression, and an upregulation of luminal progenitor cell populations, driven by crucial transcription factors such as Esr1, thereby driving luminal lineage commitment. Further, inhibition of EZH2 in vitro resulted in increased expression of ER in HER2+ human breast cancer cell lines and rendered them susceptible to tamoxifen. Altogether, these findings demonstrate that EZH2 dictates breast cancer plasticity through control of cellular identity and provides rationale for combining EZH2 inhibitors with endocrine therapies to improve outcomes in HER2+ breast cancer.