Impaired active DNA demethylation in zygotes generated by round spermatid injection

STUDY QUESTION: Is the poor development of embryos generated from round spermatid injection (ROSI) in humans and animals associated with abnormal active DNA demethylation? SUMMARY ANSWER: A significant proportion of ROSI-derived embryos failed to undergo active DNA demethylation. WHAT IS KNOWN ALREADY: Active DNA demethylation is initiated by the conversion of 5-methylcytosine (5mC) to 5-hydroxycytosine (5hmC) by the Tet3 enzyme. Active demethylation proceeds in a more pronounced manner in the male pronucleus than in the female one. PARTICIPANTS/MATERIALS, SETTING, METHODS: Mouse zygotes generated by ICSI or ROSI were analyzed for active DNA methylation by quantification of 5mC and 5hmC using specific antibodies. Some ROSI-derived embryos were subjected to time-lapse imaging for DNA methylation levels and were transferred into recipient pseudo-pregnant female mice. MAIN RESULTS AND THE ROLE OF CHANCE: In ICSI-derived embryos, the male: female pronucleus (M/F) ratio of 5mC immunostaining intensity was decreased while that of 5hmC was increased. However, a significant proportion of ROSI-derived embryos showed unchanged M/F ratios for 5mC and 5hmC even at the late zygotic period, indicating that they failed to undergo asymmetric active DNA demethylation. Consistent with this, some ROSI-derived embryos did not show preferential localization of Tet3 to the male pronucleus. ROSI-derived embryos were classified into 'demethylated' or 'non-demethylated' groups by time-lapse imaging and transferred into recipient female mice separately. More normal-sized fetuses were retrieved from the 'demethylated' group than 'non-demethylated' group at Day 11.5 of pregnancy. LIMITATIONS, REASONS FOR CAUTION: A causal relationship between impaired active DNA demethylation and the poor developmental ability of ROSI-derived embryos remains to be determined. WIDER IMPLICATIONS OF THE FINDINGS: We identified two types of ROSI-derived embryos in terms of the degree of active DNA demethylation. Induction of normal DNA demethylation at the zygotic stage might help in the technical improvement of ROSI.