Abstract
Placental DNA methylation varies across gestation and is associated with obstetrical complications. Cell-free DNA (cfDNA) from maternal plasma could provide a noninvasive approach to study placental DNA methylation in ongoing pregnancies. However, research on maternal cfDNA methylation is limited and technologically challenging. Therefore, we aimed to investigate DNA methylation in maternal cfDNA and placental tissues across gestation using the innovative methylation DNA sequencing (MeD-seq) technology. Secondly, we explored the origins of methylation differences in maternal cfDNA across gestation, and aimed to identify gestational age-associated placental DNA methylation markers directly in cfDNA. We longitudinally collected maternal cfDNA in all three trimesters and at birth (n = 10), alongside placental tissues from first trimester, second trimester, and term pregnancies (all n = 10), and used previously collected maternal blood buffy coat samples (n = 20). Different placental cell types, including syncytiotrophoblasts/cytotrophoblasts (SCTs/CTBs) (n = 10), extravillous trophoblasts (n = 7), and syncytial knotting (n = 3), and maternal cell types including spiral arteries (n = 3) and endometrial epithelium (n = 3), were isolated using laser capture microdissection. Differentially methylated regions (DMRs) identified in cfDNA from pregnant compared to non-pregnant women (n = 6) ranged from 798 to 2163 in first and third trimesters, respectively. Gradual DNA methylation changes were observed across gestation in cfDNA, placental tissues, and trophoblasts. We showed an increase in DMRs in cfDNA, that overlap with DNA methylation in placental tissues and especially trophoblasts, and in DNA methylation of placenta-specific markers across gestation, reflecting an increased placental-originated cfDNA fraction. Among 110 DMRs between first trimester and term placental tissues, those related to NXPH4, EPS8L2, AMOTL1, and IRX2 had the strongest association with gestational age in cfDNA, for which comparable associations were found in SCTs/CTBs. These DMRs were all hypomethylated in maternal buffy coat samples. This study indicates the feasibility of identifying gestational age-dependent placental DNA methylation marks in maternal cfDNA and can serve as a reference for future studies.