Placenta-specific DMRs maintain methylation across gestation.

One possible mechanism leading to the apparent polymorphic placenta-specific DMRs would be the failure to maintain allelic methylation during gestation. For a temporal comparison, we performed methylation profiling on first trimester chorionic villus sampling (CVS) and compared it with corresponding samples at term. This revealed that DNA methylation level at placenta-specific DMRs is highly stable between the two points. In addition, to ensure that the methylation profiles were uniform across the placental plate, we determined the placenta-specific DMR profiles from multiple biopsies from the same term placentas. Biopsies collected from the opposite sides of the cord insertion site also showed high correlations, suggesting that methylation does not vary greatly between sampling sites. Finally, we compared placenta samples from dizygotic twins and triplets. As in the other cases, this revealed that the correlations between samples of the same gestations (sharing the same in utero environment and maternal exposures) were also higher than between unrelated samples. Bisulphite converted DNA from 12 human placenta samples (two pairs of first trimester CVS and samples at term, two pairs of opposite sides biopsies from the same term placentas, one pair of dyzigotic twin term placentas and one trio of triplet placenta samples) were hybridised to the Illumina Infinium HumanMethylationEPIC and HumanMethylation450 BeadChips.