The impact of first trimester phthalate and phenol exposure on IGF2/H19 genomic imprinting and birth outcomes

Abstract

Genomic imprinting leads to parent-of-origin specific gene expression and is determined by epigenetic modification of genes. The paternally expressed gene insulin-like growth-factor 2 (IGF2) is located about ~100kb from the maternally expressed non-coding gene H19 on human chromosome 11, and both genes play major roles in embryonic and placental growth. Given adverse gestational environments can influence DNA methylation patterns in extra-embryonic tissues, we hypothesized that prenatal exposure to endocrine disrupting chemicals (EDCs) alters H19 and IGF2 methylation in placenta. Our study was restricted to a total of 196 women co-enrolled in the Predictors of Preeclampsia Study and the Harvard Epigenetic Birth Cohort. First trimester urine concentrations of 8 phenols and 11 phthalate metabolites were measured and used to characterize EDC exposure profiles. We assessed methylation of differentially methylated regions (DMRs) by pyrosequencing of H19, IGF2DMR0, and IGF2DMR2 and correlated values with phenol and phthalate metabolites. We also assessed overall expression and allele-specific expression of H19 and IGF2. We found several significant associations between DNA methylation and additive biomarker measurements. A significant decrease in H19 methylation was associated with high levels of the sum (Σ) of phthalate metabolites and metabolites of low molecular weight (LMW) phthalates. Σphthalate and LMW phthalate concentrations were inversely associated with IGF2DMR0 methylation values. Variation in methylation was not associated with changes in allele-specific expression. However increased deviation of allele-specific expression of H19 was associated with Σdi(2-ethylhexyl) phthalate metabolites and high molecular weight phthalates. Neither methylation nor expression of these imprinted regions had a significant impact on birth length or birth weight. Overall, our study provides new insight into an epigenetic mechanism that occurs following EDC exposure.

Keywords: Allele-specific expression; Endocrine disruptors; Epidemiology; Imprinting; Methylation.

Figure 1. Summary of Spearman Correlation Coefficients Values for Predicted (A) Phthalates and (B) Phenols Concentrations

We calculated the spearman correlation coefficient between the actual and predicted concentrations of phthalate metabolites (A) and phenols (B) For phthalates, we calculated these values between each individual metabolite and the following groups of metabolites: across all other compounds, grouped by molecular weight (high vs low), DEHP metabolites, the compounds that are the most predictive for the individual phthalate, and the most predictive compounds overall. For phenols, we calculated these values between each individual phenol and the following groups of phenols: across all other compounds, grouped by paraben status, the compounds that are the most predictive for the individual phenol, and the most predictive compounds overall. Increased red shading refers to increased values for spearman correlation coefficient.

Figure 2. Comparison of H19, IGF2DMR2, and IGF2DMR0 DNA Methylation and Additive Maternal Phthalate Urine Concentrations

Shown are the correlations between A) H19, B) IGF2DMR0, and C) IGF2DMR2 placental DNA methylation with additive phthalate biomarker measurements. Phthalates were categorized into Σphthalates, DEHP metabolites, low molecular weight compounds, and high molecular weight compounds. A Wald test was performed to identify if sex had a significant interaction with methylation. If it did not, the overall association between EDC and methylation was reported independently, and if it was, the association was reported separately for males and females. (*p<0.05)

Figure 3. Comparison of H19, IGF2DMR2, and IGF2DMR0 DNA Methylation of the Placenta and Additive Maternal Phenol Urine Concentrations

Shown are the correlations between A) H19, B) IGF2DMR0, and C) IGF2DMR2 placental DNA methylation with additive phenol biomarker measurements. Phenols were categorized into Σphenols, additive parabens, and additive non-parabens. A Wald test was performed to identify if sex had a significant interaction with methylation. If it did not, the overall association between EDC and methylation was reported independently, and if it was, the association was reported separately for males and females. (*p<0.05)