RESUMO
STUDY QUESTION: 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? SUMMARY ANSWER: Developmental failure of human ICSI embryos is associated with high methylation of the Oct4 promoter. WHAT IS KNOWN ALREADY: 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. STUDY DESIGN, SIZE AND DURATION: 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. PARTICIPANTS, MATERIALS, SETTING AND METHODS: Embryos and gametes were donated for research by patients from the Department of Reproductive Medicine at the Hôpital Femme Mère Enfant (Bron, France) and the Clinique du Tonkin (Villeurbanne, France) after giving their informed consent. MAIN RESULTS: 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 2-8-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. LIMITATIONS AND REASONS FOR CAUTION: The study was done on a limited number of oocytes and embryos and the gametes of the couples were not available. WIDER IMPLICATIONS OF THE FINDINGS: These results provide new insight regarding the roles of epigenetic abnormalities in early developmental failure in humans. STUDY FUNDING/COMPETING INTEREST(S): No external funding was obtained for this study. There was no competing interest.