Histone Modification during Mouse Embryo Development

Epigenetic mechanisms such as DNA methylation and histone modifications shape the embryonic epigenome during development. Early embryos are marked by H3 trimethylation on lysine 27 (H3K27me3), a repressive epigenetic marker. It is facilitated by the Polycomb group repressive complex 2 (PRC2) subunit of (EZH1/2), an enzyme that epigenetically regulates chromatin structure and gene expression by engaging DNA methyltransferases with H3K27me3-mediated gene silencing. As a result, EZH2 is crucial for stem cell renewal, maintenance, and differentiation into several cell lineages.

During the preimplantation phase of the embryo, the totipotent zygote undergoes continual cleavage divisions, resulting in the blastocyst. The blastomeres forming the trophectoderm (TE), epithelium that gives rise to placental cells, and the inner cell mass (ICM) that will form the embryo proper are separated at the blastocyst stage. The ICM divides into the epiblast (EPI) and primitive endoderm (PE) before embryo implantation (PrE). This cell lineage specification is assumed to be regulated by transcription factors and epigenetic mechanisms, which govern gene expression in place and time without impacting the genetic code.

In this study, two small molecule inhibitors of H3K27me3 methylase (EZH2), GSK-126 and EPZ-6438, were used to lower H3K27me3 levels and activity in preimplantation mouse embryos in vitro. Mouse embryos were collected at the E0.5 stage and cultured in the presence of GSK-126, EPZ-6438, or DMSO until the E4.5 stage. After that, immunocytochemistry and qPCR were used to evaluate the effect of EZH2 inhibition on expression. The findings were analysed to see if EZH2-mediated gene repression may impact cell fate and proliferation in preimplantation mouse embryos.