Placental DNA Methylation of miR-548 and WWTR1 Genes Influence Insulin Sensitivity during Pregnancy

Insulin sensitivity decrease in pregnancy is a physiologic adaptation to derive nutrients for the growing fetus. The placenta has a major role in this phenomenon, but exact mechanisms remain unknown. A better understanding of the placental role in maternal insulin sensitivity regulation could help us reveal pathophysiologic pathways of gestational diabetes. We tested associations between insulin sensitivity and DNA methylation (DNAm) across the genome in 439 placenta samples from our prospective cohort Gen3G using Illumina EPIC arrays. Women were recruited at 1^st trimester of pregnancy and performed a 2-hour 75g oral glucose tolerance test (OGTT) at 24-28 weeks: we measured glucose and insulin levels at each OGTT point to calculate Matsuda insulin sensitivity index. We applied standard quality control procedures for DNAm arrays analyses and adjusted for technical batch effects. We tested 720,077 individual CpGs using robust linear regression on M-values (adjusted for maternal age, BMI, smoking, parity, gestational age at birth, and child sex) and used Bonferroni to account for multiple testing. We found that maternal insulin sensitivity was associated with DNAm in placenta at 318 CpGs across the genome (P<6.94 x10^-8). Among these, we found differentially methylated CpGs at loci well known for their role in diabetes (TCF7L2, SPRY2), brown adipose tissue regulation (PRDM16), and near the Prolactin Receptor. Mendelian Randomization (MR) analyses supported that placental DNAm causally influence maternal insulin sensitivity at 28 CpGs (P<0.05). Among these, our two strongest MR associations (FDR adjusted P<0.05) pointed at CpGs near miR-548 (P=5.6x10^-5) and WWTR1 (P=9.8x10^-5), a gene implicated in PPARalpha regulation and previously associated with preeclampsia. Our findings highlight the role of placenta epigenetic regulation in insulin sensitivity during pregnancy and reveal novel potential biologic targets implicated in gestational metabolic disorders.