Polycomb gene expression and histone H3 lysine 27 trimethylation changes during bovine preimplantation development

Abstract

Trimethylation of histone H3 at lysine 27 (H3K27me3) is established by polycomb group genes and is associated with stable and heritable gene silencing. The aim of this study was to characterize the expression of polycomb genes and the dynamics of H3K27me3 during bovine oocyte maturation and preimplantation development. Oocytes andin vitro-produced embryos were collected at different stages of development. Polycomb gene expression was analyzed by real-time quantitative RT-PCR and immunofluorescence. Global H3K27me3 levels were determined by semiquantitative immunofluorescence. Transcripts forEZH2,EED, andSUZ12were detected at all stages analyzed, withEZH2levels being the highest of the three at early stages of development. By the time the embryo reached the blastocyst stage, the level ofPcGgene mRNA levels significantly increased. Immunofluorescence staining indicated nuclear expression ofEZH2at all stages while nuclear localizedEEDandSUZ12were only evident at the morula and blastocyst stages. Semiquantitative analysis of H3K27me3 levels showed that nuclear fluorescence intensity was the highest in immature oocytes, which steadily decreased after fertilization to reach a nadir at the eight-cell stage, and then increased at the blastocyst stage. These results suggest that the absence of polycomb repressive complex 2 proteins localized to the nucleus of early embryos could be responsible for the gradual decrease in H3K27me3 during early preimplantation development.