Prenatal Tobacco Smoke Exposure Affects Global and Gene-specific DNA Methylation

Rationale: Prenatal exposure to tobacco smoke increases the risk for diseases later in the child's life that may be mediated through alterations in DNA methylation. Objectives: To demonstrate that differences in DNA methylation patterns occur in children exposed to tobacco smoke and that variation in detoxification genes may alter these associations. Methods: Methylation of DNA repetitive elements, LINE1 and AluYb8, was measured using bisulfite conversion and pyrosequencing in buccal cells of 348 children participating in the Children's Health Study. Gene-specific CpG methylation differences associated with smoke exposure were screened in 272 participants in the Children's Health Study children using an Illumina GoldenGate panel. CpG loci that demonstrated a statistically significant difference in methylation were validated by pyrosequencing. Estimates were standardized across loci using a Z score to enable cross-comparison of results. Measurements and Main Results: DNA methylation patterns were associated with in utero exposure to maternal smoking. Exposed children had significantly lower methylation of AluYb8 (beta, -0.31; P = 0.03). Differences in smoking-related effects on LINE1 methylation were observed in children with the common GSTM1 null genotype. Differential methylation of CpG loci in eight genes was identified through the screen. Two genes, AXL and PTPRO, were validated by pyrosequencing and showed significant increases in methylation of 0.37 (P = 0.005) and 0.34 (P = 0.02) in exposed children. The associations with maternal smoking varied by a common GSTP1 haplotype. Conclusions: Life-long effects of in utero exposures may be mediated through alterations in DNA methylation. Variants in detoxification genes may modulate the effects of in utero exposure through epigenetic mechanisms.