FIGURE 7.

Methylation status of DNMT1 does not predict global 5MeC levels in extraembryonic tissue. Global 5MeC content was measured in a variety of placental and umbilical cord blood cells. Human full term (FT), first trimester, full term pre-eclamptic (PE), and full term marmoset tissue (with DNMT1 promoter methylation) show mean global 5MeC levels of <3% (range 2.64–2.93%). Despite lacking methylation of the equivalent DNMT1 gene promoter, bovine and guinea pig FT placental tissues show a similar mean global 5MeC level (2.77 and 3.1% methylation, respectively), indicating that DNMT1 promoter methylation is not an absolute prerequisite for reduced genomic methylation in extraembryonic tissue. Human cord blood (also lacking DNMT1 promoter methylation) shows a mean global 5MeC content of 3.82%, consistent with previous reports in peripheral blood (5). Human embryonic stem cells showed the highest global 5MeC level of all tissues/cells examined with a mean of 4.33%. Differentiation of these cells into cytotrophoblast stem cells was associated with a reduction in global methylation levels (mean 3.59%) in the absence of concomitant DNMT1 promoter methylation (data not shown). However, the small sample numbers resulted in a lack of significance when tested using a paired Student's t test. The dark bars correspond to samples with DNMT1 promoter methylation, whereas light bars identify samples lacking DNMT1 promoter methylation.