Hemi-methylated CpG sites connect -knockdown-induced and -induced DNA demethylation during somatic cell reprogramming.

The relationship between active DNA demethylation induced by overexpressing and passive DNA demethylation induced by suppressing remains unclear. Here, we found that DNMT1 preferentially methylated, but TET1 preferentially demethylated, hemi-methylated CpG sites. These phenomena resulted in a significant overlap in the targets of these two types of DNA demethylation and the counteractions of and during somatic cell reprogramming. Since the hemi-methylated CpG sites generated during cell proliferation were enriched at core pluripotency loci, DNA demethylation induced by or sh-RNA against () was enriched in these loci, which, in combination with Yamanaka factors, led to the up-regulation of these genes and promoted somatic cell reprogramming. In addition, since induces DNA demethylation by impairing the further methylation of hemi-methylated CpG sites generated during cell proliferation, while induced DNA demethylation by demethylating these hemi-methylated CpG sites, -induced DNA demethylation, compared with sh-Dnmt1-induced DNA demethylation, exhibited a higher ability to open the chromatin structure and up-regulate gene expression. Thus, -induced but not -induced DNA demethylation led to the up-regulation of an additional set of genes that can promote the epithelial-mesenchymal transition and impair reprogramming. When vitamin C was used to further increase the demethylation ability of TET1 during reprogramming, induced a larger up-regulation of these genes and significantly impaired reprogramming. Therefore, the current studies provide additional information regarding DNA demethylation during somatic cell reprogramming.