38 EFFECTS OF 5-AZACYTIDINE ON DNA HYPERMETHYLATION STATUS OF CLONED MINIATURE PIG EMBRYO

During somatic cell nuclear transfer, somatic cells should undergo epigenetic reprogramming in order to attain successful totipotency. Although there are many reasons of low efficiency of cloning, the aberrant DNA methylation status in donor cells is thought to reduce efficiency and increase abnormalities in cloned embryos. The methylation level of cloned embryos is higher than that of in vivo-produced or in vitro-fertilized embryos before occurring de novo methylation. This hyper DNA methylation status has been considered as a reason for the abnormal development of cloned embryos and the related decrease in term number. The aim of this research is to validate the DNA methylation pattern in cloned porcine embryos and to confirm whether reduction of hyper DNA methylation levels results in an improvement in cloning efficiency. First, immunostaining was used to evaluate the stage-specific changing pattern of DNA methylation of cloned embryos. In this result, we demonstrated that the 1-cell stage embryos and blastocyst had the highest level of methylation compared to 2- or 4-cell stage embryos. Second, this methylation level was higher in SCNT-derived embryos compared to parthenogenic embryos, suggesting that the methylation level was associated with the nucleus of donor cells. Third, cloned embryos were treated with various concentration of 5-azacytidine, which is a demethylating agent, to find adequate concentration. In results, 500 nm of 5-azacytidin group from 0 to 24 h after activation produced significantly more blastocysts compared to control group (control; 5.4 ± 2.3, 500 nm/0 24-h treatment; 12.7 ± 2.1, P < 0.05) and also showed low DNA methylation level of inner cell mass different from that of control group. In conclusion, our results suggest that cloned porcine embryos show typical demethylation. But, they are generally on a hyper level of DNA methylation. This high DNA methylation level is associated with somatic cells and affects on blastocyst development. We also suggest that 5-azacytidine could improve blastocyst development rate of cloned porcine embryos by aiding weak and abnormal demethylation process.