Aberrant Epigenetic Reprogramming In The First Cell Cycle Of Bovine Somatic Cell Nuclear Transfer Embryos

Zygotic epigenetic reprogramming is the major initial event in embryo development to acquire a totipotent potential. However, the patterns of epigenetic modifications in bovine zygote were not well clarified, especially in the first cell cycle of bovine somatic cell nuclear transfer (SCNT) embryos. This study was conducted to examine the patterns of DNA methylation (5-methylcytosine [5mc] and 5-hydroxymethylcytosine [5hmc]) and histone H3 lysine 9 methylation (H3K9m2 and H3K9m3) in the first cell cycle of bovine in vitro fertilization (IVF) and SCNT embryos. In bovine zygotic development, the 5mc in the paternal pronucleus (pPN) undergoes partial demethylation from PN1 to PN3, and remethylation from PN4 to PN5, while 5hmc exhibits absolutely different patterns. The 5mc in SCNT embryos underwent much more dramatic demethylation and much earlier de novo methylation compared with their IVF counterparts, while 5hmc stayed stable from PN1 to PN4, and significantly increased at PN5, which made significantly higher level of 5mc and 5hmc at the end of the first cell cycle in SCNT embryos. Different H3K9m2 and H3K9m3 patterns were also observed between IVF and SCNT embryos. H3K9m2 and H3K9m3 asymmetrically distributed in parental genomes in IVF zygote, highly present in the maternal pronucleus, whereas faintly stained in the pPN. H3K9m2 and H3K9m3 in the somatic cell genome were gradually demethylated from PN1-PN4, and significantly increased at the end of the first cell cycle. TET3 dioxygenase was highly present in the first cell cycle of embryos compared with TET1 and TET2. Our results showed that SCNT embryos underwent aberrant epigenetic reprogramming in the first cell cycle; much more dramatic demethylation and significant higher remethylation were observed compared with IVF counterparts.