Superovulation alters global DNA methylation in early mouse embryo development.

Assisted reproductive technologies are known to alter the developmental environment of gametes and early embryos during the most dynamic period of establishing the epigenome. This may result in the introduction of errors during active DNA methylation reprogramming. Controlled ovarian hyperstimulation, or superovulation, is a ubiquitously used intervention which has been demonstrated to alter the methylation of certain imprinted genes. The objective of this study was to investigate whether ovarian hyperstimulation results in genome-wide DNA methylation changes in mouse early embryos. Ovarian hyperstimulation was induced by treating mice with either low doses (5 IU) or high doses (10 IU) of PMSG and hCG. Natural mating (NM) control mice received no treatment. Zygotes and 8-cell embryos were collected from each group and DNA methylomes were generated by whole-genome bisulfite sequencing. In the NM group, mean CpG methylation levels slightly decreased from zygote to 8-cell stage, whereas a large decrease in mean CpG methylation level was observed in both superovulated groups. A separate analysis of the mean CpG methylation levels within each developmental stage confirmed that significant genome-wide erasure of CpG methylation from the zygote to 8-cell stage only occurred in the superovulation groups. Our results suggest that superovulation alters the genome-wide DNA methylation erasure process in mouse early pre-implantation embryos. It is not clear whether these changes are transient or persistent. Further studies are ongoing to investigate the impact of ovarian hyperstimulation on DNA methylation re-establishment in later stages of embryo development.