Aberrant DNA methylation in cloned ovine embryos

By using the approach of immunofluorescence staining with an antibody against 5-methylcytosine(5MeC),the present study detected the DNA methylation patterns of cloned ovine embryos.The em-bryos derived from in vitro fertilization were also examined for reference purpose.The results showed that:(1)during the pre-implantation development,cloned embryos displayed a similar demethylation profile to the fertilized embryos;that is,the methylation level decreased to the lowest at 8-cell stage,and then increased again at morulae stage.However,methylation level was obviously higher in cloned embryos than in stage-matched fertilized embryos,especially at 8-cell stage and afterwards;(2)at blastocyst stage,the methylation pattern in cloned embryos was different from that in fertilized em-bryos.In cloned blastocyst,inner cell mass(ICM)exhibited a comparable level to trophectoderm cells(TE),while in in-vitro fertilized blastocyst the methylation level of ICM was lower than that of TE,which is not consistent with that reported by other authors.These results indicate that DNA methylation is abnormally reprogrammed in cloned embryos,implying that aberrant DNA methylation reprogramming may be one of the factors causing cloned embryos developmental failure.