Mediator Subunit Med4 Enforces Metastatic Dormancy in Breast Cancer.

Long term survival of breast cancer patients is limited due to recurrence from metastatic dormant cancer cells. However, the mechanisms by which these dormant breast cancer cells survive and awaken remain poorly understood. Our unbiased genome-scale genetic screen in mice identified Med4 as a novel cancer-cell intrinsic gatekeeper in metastatic reactivation. MED4 haploinsufficiency is prevalent in metastatic breast cancer patients and correlates with poorer prognosis. Syngeneic xenograft models revealed that Med4 enforces breast cancer dormancy. Contrary to the canonical function of the Mediator complex in activating gene expression, Med4 maintains 3D chromatin compaction and enhancer landscape, by preventing enhancer priming or activation through the suppression of H3K4me1 deposition. Med4 haploinsufficiency disrupts enhancer poise and reprograms the enhancer dynamics to facilitate extracellular matrix (ECM) gene expression and integrin-mediated mechano-transduction, driving metastatic growth. Our findings establish Med4 as a key regulator of cellular dormancy and a potential biomarker for high-risk metastatic relapse.