The efficiency of cloning by somatic cell nuclear transfer (SCNT) has remained low.
In most cloned embryos, epigenetic reprogramming is incomplete, and usually the
genome is hypermethylated. The DNA methylation inhibitor 5-aza-2’-deoxycytidine
(5-aza-dC) could improve the developmental competence of cow, pig, cat and human SCNT
embryos in previous studies. However, the parameters of 5-aza-dC treatment among
species are different, and whether 5-aza-dC could enhance the developmental
competence of porcine cloned embryos has still not been well studied. Therefore, in
this study, we treated porcine fetal fibroblasts (PFF) that then were used as donor
nuclei for nuclear transfer or fibroblast-derived reconstructed embryos with
5-aza-dC, and the concentration- and time-dependent effects of 5-aza-dC on porcine
cloned embryos were investigated by assessing pseudo-pronucleus formation,
developmental potential and pluripotent gene expression of these reconstructed
embryos. Our results showed that 5-aza-dC significantly reduced the DNA methylation
level in PFF (0 nM vs. 10 nM vs. 25 nM
vs. 50 nM, 58.70% vs. 37.37%
vs. 45.43% vs. 39.53%, P<0.05), but did not
improve the blastocyst rate of cloned embryos derived from these cells. Treating
cloned embryos with 25 nM 5-aza-dC for 24 h significantly enhanced the blastocyst
rate compared with that of the untreated group. Furthermore, treating cloned embryos,
but not donor cells, significantly promoted pseudo-pronucleus formation at 4 h post
activation (51% for cloned embryos treated, 34% for donor cells treated and 36% for
control, respectively, P<0.05) and enhanced the expression levels of pluripotent
genes (Oct4, Nanog and Sox2) up to
those of in vitro fertilized embryos during embryo development. In
conclusion, treating cloned embryos, but not donor cells, with 5-aza-dC enhanced the
developmental competence of porcine cloned embryos by promotion of pseudo-pronucleus
formation and improvement of pluripotent gene expression.