| Summary: | Summary: A dramatic difference in global DNA methylation between male and female cells characterizes mouse embryonic stem cells (ESCs), unlike somatic cells. We analyzed DNA methylation changes during reprogramming of male and female somatic cells and in resulting induced pluripotent stem cells (iPSCs). At an intermediate reprogramming stage, somatic and pluripotency enhancers are targeted for partial methylation and demethylation. Demethylation within pluripotency enhancers often occurs at ESC binding sites of pluripotency transcription factors. Late in reprogramming, global hypomethylation is induced in a female-specific manner. Genome-wide hypomethylation in female cells affects many genomic landmarks, including enhancers and imprint control regions, and accompanies the reactivation of the inactive X chromosome. The loss of one of the two X chromosomes in propagating female iPSCs is associated with genome-wide methylation gain. Collectively, our findings highlight the dynamic regulation of DNA methylation at enhancers during reprogramming and reveal that X chromosome dosage dictates global DNA methylation levels in iPSCs. : Somatic cells can be reprogrammed to iPSCs, inducing reactivation of the inactive X chromosome. Using genome-scale DNA methylation analyses, Plath, Pasque, and colleagues show that iPSCs adopt sex-specific differences in global DNA methylation that correlate with the presence of two active X chromosomes. Upon culture, female iPSCs lose one of the two X chromosomes and adopt male-like DNA methylation. Keywords: induced pluripotency, iPSC, DNA methylation, epigenetics, embryonic stem cells, ESC, pluripotency, reprogramming, X chromosome inactivation
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