2.3.1. DNA Methylation

The best studied epigenetic mechanism in the placenta is DNA methylation, the covalent addition of a methyl group to a cytosine, usually in the context of cytosine-phospho-guanine (CpG) dinucleotides. Several reviews have been dedicated to the role of this mechanism in placental development [10,12,61]. Also, several high-throughput analyses have been performed to analyze the methylation epigenetics of the developing placenta (Table 2, Supplementary Table S1 for the details).

**Table 2.** Summary of DNA methylation studies in developing placenta using genome-wide approaches.


### Di fferentiation of Stem Cells

Contrary to mice, a Trophoblast Stem Cell (TSC) population has not ye<sup>t</sup> been clearly identified in humans, thus limiting our capacity to study the role of DNA methylation in the early stages of trophoblast di fferentiation. A recent study has addressed this question using a side-population trophoblasts, a candidate human TSC [55], isolated from first trimester placenta. The comparison of the methylomes of this side-population trophoblasts and the methylomes of vCTs and EVTs all isolated from the same first trimester placenta, showed that each population had a distinctive methylome [56]. In comparison to mature vCTs, side-population trophoblasts, showed di fferential methylation of genes and miRNAs involved in cell cycle regulation, di fferentiation and regulation of pluripotency. In addition, the comparison of the methylomes and transcriptomes of vCTs and EVTs revealed the methylation of genes involved in epithelial-mesenchymal transition (EMT) and metastatic cancer pathways, which could be involved in the acquisition of the invasive capacities of the EVTs. However, this study, as many others, failed to establish a systematic correlation between hypermethylation of the genes and downregulated expression. Therefore, the authors conclude that although CpG methylation is involved in the trophoblasts di fferentiation, it cannot be the only regulatory process.
