The levels of DNA methylation of sheep cloned embryos in different development stages

DNA methylation is a kind of epigenetic modification. During the early development of cloned embryos, parental genomic DNA undergoes active-and passive demethylation as well as low level of DNA methylation in cloned embryos at morula stage. In this study, sheep skin fibroblasts were used as nuclear donors, and sheep oocytes were used as nuclear recipient, sheep cloned embryos were constructed by the technology of somatic cell nuclear transfer, the development ability of cloned embryos was evaluated. Moreover, the dynamic change in DNA methylation level was also examined by the technology of methyl sensitive amplified polymorphism. The results showed that sheep cloned embryo had the ability to develop to blastocyst stage, and the level of DNA methylation decreased from nonactivated 1-cell stage to 4-cell stage, and then increased onward to blastocyst stage; while the level of DNA hemimethylation decreased from nonactivated 1-cell stage to 2-cell stage, then increased onward to blastocyst stage; moreover, the level of total methylation (including hemi-and methylation) decreased from nonactivated 1-cell stage to 4-cell stage, and then increased onward to blastocyst stage. This study indicated that incomplete DNA methylation could be a reason of low developmental ability and high loss rate of cloned embryos/fetuses in early pregnancy and low efficiency of somatic cell nuclear transfer.