Loss of XIST Impairs Human Mammary Stem Cell Differentiation and Increases Tumorigenicity Through Enhancer and Mediator Complex Hyperactivation

X-chromosome inactivation (XCI) is triggered by up-regulation of XIST, which coats the chromosome in cis and promotes recruitment of chromatin modifiers that transform the X into a silent heterochromatic domain. Whether XIST plays a role beyond initiation of XCI is unclear. Here, we demonstrate that XIST loss impairs differentiation of human mammary stem cells (MaSC) and, upon oncogenic transformation, promotes emergence of highly metastatic carcinomas. On the Xi, XIST-deficient MaSC display epigenetic erosion and reactivation of genes overlapping Polycomb domains. Among reactivated loci we identify MED14 , a critical backbone of Mediator, and show that MED14 overdosage is sufficient to explain how defective XCI maintenance can stabilize MaSC enhancer landscape and transcriptional program, making differentiation less favorable. We conclude that XIST is a gatekeeper of mammary epithelium homeostasis, thus unveiling a new paradigm in the control of somatic cell identity with potential consequences in our understanding of gender-specific malignancies.