Immediate and durable effects of maternal tobacco consumption on placental DNA methylation: a replication and discovery study

An increasing number of epigenome-wide association studies report tobacco smoking-associated DNA methylation levels. However, comprehensive replication studies remain scarce, particularly in placenta, despite their crucial interest in such a large-scale context. Using DNA methylation data from the EPIC array of 341 new placentas (85 smokers, 219 non-smokers, and 37 former smokers) from the EDEN cohort, we used a candidate approach to replicate maternal smoking-associated CpGs and regions previously identified using the 450K array, and an exploratory approach to discover new associations within EPIC-specific CpGs. Smoking-associated changes in DNA methylation in CpGs and regions were classified as either transient or persistent (indicating epigenetic memory), depending on the stability of their association with smoking status. Among candidate loci, 38% of probes and 9% of regions were replicated, providing robust evidence of effects of prenatal smoke exposure on methylation patterns of these loci. <i>LEKR1</i> was the top hit in both the initial and replication studies. Most of the replicated loci were transient CpGs (i.e. current smokers), while persistent CpGs (i.e. former smokers) remained scarce and somewhat inconsistent with previous findings. The additional exploratory analysis identified 733 novel probes and 75 novel regions, including 18% and 30% of transient loci, respectively. Results suggested that most of the effects were related to <i>in utero</i> exposure only, supporting pregnant women's efforts to quit smoking. This replication study also evidences the importance of reproducible work in omic investigations to provide a more in-depth and robust understanding of the effects of environmental exposures on health biomarkers..