Genome-wide DNA methylation study in human placenta identifies novel loci associated with maternal smoking during pregnancy

We conducted an epigenome-wide association study (EWAS) of DNA methylation in placenta in relation to maternal tobacco smoking during pregnancy and examined whether smoking-induced changes lead to low birthweight. DNA methylation in placenta was measured using the Illumina HumanMethylation450 BeadChip in 179 participants from the INfancia y Medio Ambiente (INMA) birth cohort. Methylation levels across 431 311 CpGs were tested for differential methylation between smokers and non-smokers in pregnancy. We took forward three top-ranking loci for further validation and replication by bisulfite pyrosequencing using data of 248 additional participants of the INMA cohort. We examined the association of methylation at smoking-associated loci with birthweight by applying a mediation analysis and a two-sample Mendelian randomization approach. Fifty CpGs were differentially methylated in placenta between smokers and non-smokers during pregnancy [false discovery rate (FDR)