Dynamic CpG methylation of the KCNQ1OT1 gene during maturation of human oocytes

Background: Imprinted genes, many of which are involved in development, are marked during gametogenesis to allow their parent-of-origin specific expression, and DNA methylation at CpG islands is part of this epigenetic mark. Maternal imprint is apposed on oocyte during growth and maturation. Factors interfering with normal oocyte differentiation such as gonadotrophin stimulation and in vitro maturation (IVM) may possibly alter imprint resetting. Methods: We examined the methylation of the KCNQ1OT1 differentially methylated region (KvDMR1) in human oocytes at different stages of their development: germinal vesicle (GV), metaphase I (MI) or metaphase II (MII). Results: About 60% of alleles were fully methylated in GV oocytes and that full imprint is acquired in most MII oocytes. Similarly to in vivo, de novo methylation of DNA occurred in vitro during oocyte maturation. Following in vitro culture for 28 h, GV and MI oocytes are significantly more methylated when they are obtained from natural cycles than from patients undergoing gonadotrophin stimulation. Conclusion: This observation suggests that hyperstimulation likely recruits young follicles that are unable to acquire imprint at KvDMR1 during the course of the maturing process.