Methylation profile of the promoters of Nanog and Oct4 in ICSI human embryos

What is the methylation status of the Nanog and Oct4 promoters in human gametes and ICSI embryos and is abnormal reprogramming of their methylation associated with developmental failure of ICSI embryos? Developmental failure of human ICSI embryos is associated with high methylation of the Oct4 promoter. Nanog and Oct4 genes play critical roles in the establishment and maintenance of pluripotency during normal early embryonic development, and both are negatively regulated through the methylation of their promoters. We analysed the methylation profile of Nanog and Oct4 promoters in 5 control sperm from normally fertile men, 70 metaphase II oocytes, 21 4-cell control ICSI embryos, 7 control blastocysts and 45 ICSI embryos arrested at 2- to 8-cell stage following prolonged culture. Embryos and gametes were donated for research by patients from the Department of Reproductive Medicine at the Hpital Femme Mre Enfant (Bron, France) and the Clinique du Tonkin (Villeurbanne, France) after giving their informed consent. For both promoters, high methylation was observed in sperm cells. Although, in general, the promoters were unmethylated in oocytes, the methylation of some alleles was observed, particularly in oocytes from women with known infertility. Both gene promoters were hypomethylated in control blastocyst ICM (inner cell mass) and in control 28-cells embryos obtained from 6 out of 8 couples. However, they appeared highly methylated in embryos obtained from the other two couples. In most arrested ICSI embryos, the Nanog promoter was unmethylated while the Oct4 promoter was highly methylated. High methylation of the Oct4 promoter was significantly more pronounced in embryos from couples where a male factor was the only known cause of infertility. When the embryos were heterozygous for a G/A single nucleotide polymorphism, both alleles could be methylated, each likely representing a paternally inherited or a maternally inherited copy. The study was done on a limited number of oocytes and embryos and the gametes of the couples were not available. These results provide new insight regarding the roles of epigenetic abnormalities in early developmental failure in humans. No external funding was obtained for this study. There was no competing interest.