Contiguous Erosion of the Inactive X in Human Pluripotency Concludes With Global DNA Hypomethylation

SUMMARY Female human pluripotent stem cells (hPSCs) are prone to undergoing X chromosome erosion (XCE), a progressive loss of key epigenetic features on the inactive X that initiates with repression of XIST , the long non-coding RNA required for X inactivation. As a result, previously silenced genes on the eroding X (Xe) reactivate, some of which are thought to provide selective advantages. To-date, the sporadic and progressive nature of XCE has largely obscured its scale, dynamics, and key transition events. To address this knowledge gap, we performed an integrated analysis of DNA methylation (DNAme), chromatin accessibility, and gene expression across hundreds of hPSC samples. Differential methylation across the Xe enables ordering female hPSCs across a trajectory of XCE from initiation to terminal stages. Our results identify a crucial cis -regulatory element for XIST expression, trace contiguously growing domains of reactivation to a few euchromatic origins on the Xi, and indicate that the late-stage Xe impairs DNAme genome-wide. Surprisingly, from this altered epigenetic landscape emerge select features of naïve pluripotency, suggesting its link to X chromosome dosage may be partially conserved in human embryonic development.