A conserved function of EZH2-PRC2 in repression of primitive endoderm fate in mouse and human pluripotent cells

In early blastocyst, inner cell mass (ICM) undergoes a second wave of lineage commitment to generate epiblast (Epi) and primitive endoderm (PrE), restricting the developmental potential of their progeny. However, the epigenetic mechanism underlying this lineage segregation remains unclear. Here, we reveal that enhancer of zeste homolog 2 (EZH2) inhibition with EPZ-6438 transdifferentiates murine embryonic stem cells (ESCs) into primitive endoderm stem cells (PrESCs). Genetic ablation of Ezh2 in ESCs reduces the trimethylation of lysine 27 on histone 3 (H3K27me3) deposition at the promoters of PrE-specific genes, derepressing their expression and facilitating the PrESC conversion. Reconstitution with wild-type EZH2, but not catalytically inactive or EED-binding-deficient mutants in EZH2-deficient ESCs, blocks this transition. Strikingly, EZH2 inhibition reduces epiblast cell numbers in blastocysts, with 45.9% of embryos exhibiting ICMs composed of no SOX2-positive cells. Furthermore, EPZ-6438 treatment in human naive ESCs upregulates hypoblast-associated genes. In sum, these results reveal an evolutionarily conserved role for EZH2-polycomb repressive complex 2 (PRC2) in safeguarding pluripotency from primitive endoderm cell fate.