Full methylation of H3K27 by PRC2 is dispensable for initial embryoid body formation but required to maintain differentiated cell identity

Polycomb repressive complex 2 (PRC2) catalyzes methylation of histone H3 on lysine 27 and is required for normal development of complex eukaryotes. The nature of that requirement is not clear. H3K27me3 is associated with repressed genes, however the modification is not sufficient to induce repression, and in some instances is not required. We blocked full methylation of H3K27 with both a small molecule inhibitor, GSK343 and by introducing a point mutation into EZH2, the catalytic subunit of PRC2. Cells with substantively decreased H3K27 methylation differentiate into embryoid bodies, which contrasts with EZH2 null cells. PRC2 targets had varied requirements for H3K27me3, with a subset that maintained normal levels of repression in the absence of methylation. The primary cellular phenotype of blocked H3K27 methylation was an inability of altered cells to maintain a differentiated state when challenged. This phenotype was determined by H3K27 methylation in embryonic stem cells through the first four days of differentiation. Full H3K27 methylation therefore was not necessary for formation of differentiated cell states during embryoid body formation but was required to maintain a stable differentiated state.