Epigenetic changes regulating the epithelial-mesenchymal transition in human trophoblast differentiation

The phenotype of term, human placental extravillous trophoblast (EVT) reflects both the first trimester differentiation from villous cytotrophoblast (CTB) and later gestational changes, including the loss of proliferative and invasive capacity. Invasion abnormalities, as observed in preeclampsia and placenta accreta spectrum, are not usually diagnosed until the second or even third trimester of pregnancy. Characterization of the normal processes at term, including arrest of invasion is therefore crucial. In this report gene expression analysis demonstrates definitively the epithelial-mesenchymal transition (EMT) mechanism which underlies differentiation and provides a trophoblast-specific EMT signature. Methylation profiling shows that CTB, already hypomethylated relative to other somatic cells, show a further degree of hypomethylation in their transition to EVT. A small fraction of genes show both gain of methylation and changes in gene expression. Prominent are genes involved in the EMT such as the transcription factor RUNX1, loss of which leads to reduced migratory capacity in JEG3 trophoblast cells. Examination of these EMT genes leads us to suggest that the gains of methylation may assist in maintaining term EVT in a mesenchymal but non-invasive state. To examine differences between term cytotrophoblast (CTB) and extravillous trophoblast (EVT), we isolated these cells in a paired manner from the basal plate of the human placenta and extracted total RNA RNA extracted from 9 paired samples of cytotrophoblast and extravillous trophoblast and 4 sets of villous core cytotrophoblast (vCTB) was used to generate a cDNA library. RNA-sequencing was followed by gene expression analysis.