Placental DNA hypomethylation in association with particulate air pollution in early life

BACKGROUND: There is evidence that altered DNA methylation is an important epigenetic mechanism in prenatal programming and that developmental periods are sensitive to environmental stressors. We hypothesized that exposure to fine particles (PM(2.5)) during pregnancy could influence DNA methylation patterns of the placenta. METHODS: In the ENVIRONAGE birth cohort, levels of 5’-methyl-deoxycytidine (5-mdC) and deoxycytidine (dC) were quantified in placental DNA from 240 newborns. Multiple regression models were used to study placental global DNA methylation and in utero exposure to PM(2.5) over various time windows during pregnancy. RESULTS: PM(2.5) exposure during pregnancy averaged (25(th)-75(th) percentile) 17.4 (15.4-19.3) μg/m(3). Placental global DNA methylation was inversely associated with PM(2.5) exposures during whole pregnancy and relatively decreased by 2.19% (95% confidence interval [CI]: -3.65, -0.73%, p = 0.004) for each 5 μg/m(3) increase in exposure to PM(2.5). In a multi-lag model in which all three trimester exposures were fitted as independent variables in the same regression model, only exposure to PM(2.5) during trimester 1 was significantly associated with lower global DNA methylation (-2.13% per 5 μg/m(3) increase, 95% CI: -3.71, -0.54%, p = 0.009). When we analyzed shorter time windows of exposure within trimester 1, we observed a lower placental DNA methylation at birth during all implantation stages but exposure during the implantation range (6-21d) was strongest associated (-1.08% per 5 μg/m(3) increase, 95% CI: -1.80, -0.36%, p = 0.004). CONCLUSIONS: We observed a lower degree of placental global DNA methylation in association with exposure to particulate air pollution in early pregnancy, including the critical stages of implantation. Future studies should elucidate genome-wide and gene-specific methylation patterns in placental tissue that could link particulate exposure during in utero life and early epigenetic modulations.