210 DNA METHYLATION AND HYDROXYMETHYLATION ANALYSIS IN A MODEL OF OOCYTE DIFFERENTIAL DEVELOPMENTAL COMPETENCE IN SHEEP

Abstract

DNA methylation is an important epigenetic mark that plays a role in gene regulation by the addition of a methyl group to CpG islands in the DNA. Despite being relatively stable in somatic cells, DNA methylation is subject to reprogramming during embryo development and gametogenesis. The aim of this work was to evaluate different aspects of DNA methylation in relation to oocyte quality in the ovine species. A model of differential developmental competence consisting in ovine oocytes and in vitro produced (IVP) blastocysts derived from adult (AD) and prepubertal (PR) donors, was used. The methylation was analysed in terms of: expression of a panel of genes involved in DNA methylation [DNA methyltransferases (DNMTs)] and demethylation [ten-eleven translocation dioxygenases (TET)] in oocytes and blastocysts; global methylation and hydroxymethylation by direct immunofluorescence; locus-specific methylation of 2 imprinted genes by pyrosequencing. Gene relative quantification was performed by RNA reverse transcription followed by real-time PCR. Pools of 10 immature (GV) and in vitro-matured (MII) oocytes and (IVP) blastocysts derived from AD and PR donors (4 replicates per class) were analysed. Lower expression of TET1, TET2, and TET3 was observed in PR GV oocytes (ANOVA; P < 0.05), while no significant differences were found for the enzymes involved in methylation (DNMT1, DNMT3A, DNMT3B; ANOVA; P > 0.05). The levels of all the genes studied showed no significant differences in embryos at blastocyst stage (ANOVA; P > 0.05). Methylation and hydroxymethylation immunostaining were performed in GV and MII oocytes using anti-5-methylcytosine mouse mAb and 5-hydroxymethylcytosine rabbit pAB. High levels of DNA methylation were observed in both AD and PR GV and MII oocytes, while hydroxymethylation immunopositivity was scattered evident throughout the gamete chromatin. Pyrosequencing of bisulfite converted DNA was used to determine the methylation status within differentially methylated regions (DMR) of maternally imprinted H19 (CTCF binding site IV; 11 CpG sites) and paternally imprinted IGF2R (17CpG sites within intron 2). No differences were observed between classes of oocytes for each gene (pools of 40 oocytes per replicate, 3 replicates per class; ANOVA; P > 0.05). Our work shows no differences in the expression of the enzymes involved in methylation, in accordance with the results of global and locus specific methylation analysis. Conversely, we observed lower expression of the TET genes in PR GV oocytes (ANOVA; P > 0.05). TET1, TET2, and TET3, whose expression has never been studied in ovine, generate 5-hydroxymethlcytosine (5hmC) by oxidation of 5-methylcytosine (5mC), and are involved in active DNA demethylation during early embryo development. Our observation of lower expression of the TET genes in lower competence PR GV oocytes suggests that epigenetic mechanisms may affect oocyte quality and paves the way to better understand methylation dynamics during sheep pre-implantation development.