Giulia Fragola

Somatic cells can revert to a pluripotent state following ectopic overexpression of the four transcription factors (TF): c-Myc, Kl4, Oct4 and Sox2. Upon TF-induced reprogramming, cells undergo genome-wide resetting of both transcriptional and epigenetic states. Among the different epigenetic marks, Histone H3 lysine 27 tri-methylation (H3K27me3) has been proposed to play a major role in this process. Here I provide a functional dissection of H3K27me3 function in TF-induced reprogramming. To this purpose, I derived induced pluripotent stem cells (iPSCs) from mouse embryonic fibroblasts (MEFs) expressing a catalytically dead form of EZH2, the major mammalian H3K27 methyltransferase. Remarkably, loss of EZH2 activity did not affect reprogramming efficiency and was compatible with the derivation of iPSCs globally devoid of H3K27me3. Ezh2-deficient iPSCs were indistinguishable from Ezh2 control iPSCs at both molecular and functional level since they could efficiently silence the somatic transcriptome and differentiate into tissues derived from the three germ layers. Interestingly, genome-wide analysis of H3K27me3 in Ezh2 mutant iPSCs revealed the retention of residual levels of this mark on a highly selected group of Polycomb targets enriched for developmental regulators controlling the expression of lineage specific genes. Retention of H3K27me3 mark was guaranteed by the presence of an alternative PRC2 complex that compensated EZH2 loss by the recruitment of its homologue EZH1. Erasure of residual H3K27me3 from these targets led to a striking impairment in TF-induced reprogramming indicating that PRC2-mediated H3K27 trimethylation is required on a highly selective core of Polycomb targets whose repression enables TF-dependent cell reprogramming.