Characterization of 5-methylcytosine and 5-hydroxymethylcytosine in human placenta cell types across gestation

The placenta is an important organ in pregnancy, however, very little is understood about placental development at a molecular level. This includes the role of epigenetic mechanisms and how they change throughout gestation. DNA methylation studies in this organ are complicated by the different cell types that make up the placenta, each with their own unique transcriptome and epigenome. Placental dysfunction is often associated with pregnancy complications such as preeclampsia (PE). Aberrant DNA methylation in the placenta has been identified in pregnancy complications. We used immunohistochemistry (IHC) and immunofluorescence (IF) to localize 5-methylcytosine (5-mC) and 5-hydroxymethylcytosine (5-hmC) in placenta tissue from first and second trimester as well as uncomplicated term and PE samples. IHC analysis of whole placental tissues showed 5-mC increased across gestation. When cytotrophoblasts (CTB) and syncytiotrophoblasts (STB) were isolated and assessed using IF, both 5-mC and 5-hmC increased in term CTBs compared to first/second-trimester samples. Staining intensity of 5-hmC was higher in first/second trimester STBs compared to CTBs (P = 0.0011). Finally, IHC staining of term tissue from PE and uncomplicated pregnancies revealed higher 5-mC staining intensity in placentas from PE pregnancies (P = 0.028). Our study has shown increased 5-mC and 5-hmC staining intensities across gestation and differed between two trophoblast populations. Differences in DNA methylation profiles between placental cell types may be indicative of different functions and requires further study to elucidate what changes accompany placental pathologies.