Differential DNA methylation in umbilical cord blood of infants exposed to mercury and arsenic in utero

Abstract

Mercury and arsenic are known developmental toxicants. Prenatal exposures are associated with adverse childhood health outcomes that could be in part mediated by epigenetic alterations that may also contribute to altered immune profiles. In this study, we examined the association between prenatal mercury exposure on both DNA methylation and white blood cell composition of cord blood, and evaluated the interaction with prenatal arsenic exposure. A total of 138 mother-infant pairs with postpartum maternal toenail mercury, prenatal urinary arsenic concentrations, and newborn cord blood were assessed using the Illumina Infinium Methylation450 array. White blood cell composition was inferred from DNA methylation measurements. A doubling in toenail mercury concentration was associated with a 2.5% decrease (95% CI: 5.0%, 1.0%) in the estimated monocyte proportion. An increase of 3.5% (95% CI: 1.0, 7.0) in B-cell proportion was observed for females only. Among the top 100 CpGs associated with toenail mercury levels (ranked on P-value), there was a significant enrichment of loci located in North shore regions of CpG islands (P = 0.049), and the majority of these loci were hypermethylated (85%). Among the top 100 CpGs for the interaction between arsenic and mercury, there was a greater than expected proportion of loci located in CpG islands (P = 0.045) and in South shore regions (P = 0.009) and all of these loci were hypermethylated. This work supports the hypothesis that mercury may be contributing to epigenetic variability and immune cell proportion changes, and suggests that in utero exposure to mercury and arsenic, even at low levels, may interact to impact the epigenome.

Keywords: DNA methylation; arsenic; epigenetics; illumina 450K; in-utero exposure; mercury; neurodevelopment.

Figure 1.

Differentially methylated loci based on maternal toenail Hg. (A) Volcano plot for the relationship between log₂ toenail Hg on DNA methylation at all 348,569 CpGs analyzed. Red and blue lines indicate −log₁₀(1 × 10⁻⁴) and −log₁₀(0.05) P-values, respectively. Colors: Yellow=CpG island, Black=CpG Shore, Blue=Shelfs and Open sea. Symbols: Circle = Infinium Type II, Square = Infinium Type I (B) Location of the top 100-CpGs on the basis of P-values compared to all CpGs on the methylation array. Among the top 100 CpGs with lowest P-value, there was a significant enrichment in the N. Shore regions of CpG islands (P = 0.049).

Figure 2.

Differentially methylated loci based on the interaction between maternal toenail Hg and urinary As. (A) Volcano plot for the relationship between the multiplicative interaction of log₂ toenail Hg and log₂ urinary As on DNA methylation at all 348,569 CpGs analyzed. Red and blue lines indicate −log₁₀(1 × 10⁻⁴) and −log₁₀(0.05) P-values, respectively. Colors: Yellow = CpG island, Black = CpG Shore, Blue=Shelfs and Open sea. Symbols: Circle = Infinium Type II, Square = Infinium Type I (B) Location of the top 100-CpGs on the basis of P-values compared to all CpGs on the methylation array. Among the top 100 CpGs with lowest P-value, there was a significant enrichment of CpG islands (P = 0.045) and S. Shore regions (P = 0.009).