The relationship between active DNA demethylation induced by overexpressing Tet1 and passive DNA demethylation induced by suppressing Dnmt1 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 Dnmt1 and Tet1 during somatic cell reprogramming. Since the hemi-methylated CpG sites generated during cell proliferation were enriched at core pluripotency loci, DNA demethylation induced by Tet1 or sh-RNA against Dnmt1 (sh-Dnmt1) 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 sh-Dnmt1 induces DNA demethylation by impairing the further methylation of hemi-methylated CpG sites generated during cell proliferation, while Tet1 induced DNA demethylation by demethylating these hemi-methylated CpG sites, Tet1-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, Tet1-induced but not sh-Dnmt1-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, Tet1 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.

尚不清楚过表达Tet1诱导的主动DNA脱甲基与抑制Dnmt1诱导的被动DNA脱甲基之间的关系。在这里，我们发现DNMT1优先甲基化，而TET1优先去甲基化，半甲基化CpG位点。这些现象导致在这两种类型的DNA脱甲基化的靶标中存在明显的重叠，并且在体细胞重编程过程中Dnmt1和Tet1的反作用。由于细胞增殖过程中产生的半甲基化CpG位点在核心多能性位点富集，因此Tet1或sh-RNA对Dnmt1（sh-Dnmt1）诱导的DNA去甲基化）中富集了这些基因座，再与Yamanaka因子结合，导致这些基因的上调并促进了体细胞的重编程。此外，由于sh-Dnmt1通过破坏细胞增殖过程中产生的半甲基化CpG位点的进一步甲基化而诱导DNA脱甲基，而Tet1通过将这些半甲基化CpG位点脱甲基来诱导DNA脱甲基，因此与sh-Dnmt1相比，Tet1诱导的DNA脱甲基化。 Dnmt1诱导的DNA去甲基化，表现出更高的打开染色质结构和上调基因表达的能力。因此，Tet1诱导但不是sh-Dnmt1-诱导的DNA去甲基化导致上调可促进上皮-间充质转化并损害重编程的另一组基因。当在重新编程过程中使用维生素C进一步提高TET1的脱甲基能力时，Tet1诱导这些基因更大的上调并严重损害了重新编程。因此，当前的研究提供了有关体细胞重编程过程中DNA脱甲基的其他信息。