TY - JOUR AU - Goodrich, Jaclyn, M AB - Abstract Maternal prenatal exposures, including bisphenol A (BPA), are associated with offspring’s risk of disease later in life. Alterations in DNA methylation may be a mechanism through which altered prenatal conditions (e.g. maternal exposure to environmental toxicants) elicit this disease risk. In the Michigan Mother and Infant Pairs Cohort, maternal first-trimester urinary BPA, bisphenol F, and bisphenol S concentrations were tested for association with DNA methylation patterns in infant umbilical cord blood leukocytes (N = 69). We used the Illumina Infinium MethylationEPIC BeadChip to quantitatively evaluate DNA methylation across the epigenome; 822 020 probes passed pre-processing and quality checks. Single-site DNA methylation and bisphenol models were adjusted for infant sex, estimated cell-type proportions (determined using cell-type estimation algorithm), and batch as covariates. Thirty-eight CpG sites [false discovery rate (FDR) <0.05] were significantly associated with maternal BPA exposure. Increasing BPA concentrations were associated with lower DNA methylation at 87% of significant sites. BPA exposure associated DNA methylation sites were enriched for 38 pathways significant at FDR <0.05. The pathway or gene-set with the greatest odds of enrichment for differential methylation (FDR <0.05) was type I interferon receptor binding. This study provides a novel understanding of fetal response to maternal bisphenol exposure through epigenetic change. Introduction Bisphenol A (BPA), a chemical commonly used in receipts, plastics, and food packaging, is considered to be a ‘ubiquitous exposure’, principally because of its wide-spread usage and high rate (over 95%) of detection in human urine [1]. Exposure to BPA and two of its commonly used replacement analogues, bisphenol F (BPF), and bisphenol S (BPS), are readily detectable in the US populations [1–4]. BPF and BPS are now increasingly utilized in place of BPA particularly as a result of consumer and scientific-based advocacy efforts. This pressure effectively elicited the US Food and Drug Administration’s ban of BPA in infant-related plastics and products [5]. However, significantly less is known about BPF and BPS, with new evidence suggesting that these replacement chemicals with close structural similarities to BPA may have comparable or increased levels of potency as endocrine disruptors and may also negatively impact the reproductive system [5–7]. In the USA, BPF and BPS were only recently added to the list of chemicals measured in the National Health and Nutrition Examination Survey (NHANES), appearing for the first time from 2013 to 2014 [4]. While the proportion of adults and children with detectable levels of these bisphenols is concerning, the exposure patterns experienced by pregnant and lactating mothers introduce an additional layer of consideration. Specifically, when the potential impact on fetal development and lifetime health trajectory are evaluated. Pregnant women in the USA and internationally are typically exposed to or have biological concentrations of urinary bisphenols at similar levels to non-pregnant women [8–11]. Furthermore, BPA, BPF, and BPS have the potential to cross the placenta at differing rates and with inter-individual variation [12–14]. Environmental research establishes the framework of time around conception, gestation, and birth as one of the most developmentally susceptible times of life. This aligns with the Developmental Origins of Health and Disease hypothesis, which recognizes the connection between maternal exposure during pregnancy and the risks posed to her offspring’s health and later-life disease [15]. An increasing number of studies have investigated the impact of prenatal exposure to bisphenols on phenotypic outcomes in infants and children. Maternal and prenatal exposure to bisphenols in humans is associated with pregnancy duration and birth weight [16–19], increased risk of preeclampsia [20], early childhood behavior [21–23], childhood body mass index (BMI) [24], and peripubertal metabolic homeostasis [25, 26]. Studies in mice have demonstrated that prenatal or early-life exposure to bisphenols is associated with altered brain development and behavior [27] as well as disruptions in metabolic homeostasis [28–31], glucose metabolism [32, 33], neuroendocrine function [34, 35], and immune function [36, 37]. Despite these developments in understanding of the association between prenatal bisphenol exposure and phenotypic outcomes in offspring, less is known of the possible mechanism through which bisphenols elicit these outcomes. Recent work indicates that environmentally induced disease etiology may be mediated by changes in the epigenetic profile [38–40]. For the purposes of this investigation, we define the epigenome as consisting of chemical modifications (e.g. DNA methylation and histone modification) that are mitotically heritable and regulate gene expression but are not the result of a change in the DNA sequence [15]. Currently, very few studies exist that evaluate prenatal bisphenol exposure and its consequent longitudinal impact on the fetal and later-life epigenome [41, 42]. Most studies were completed in mice; with evidence suggesting that prenatal exposure to bisphenols is associated with changes in DNA methylation in genes regulating hepatic function [43], metabolism [44, 45], neuronal [46] and inflammatory pathways [47], and other regulatory epigenetic machinery [48]. Four human studies have evaluated the epigenetic impact of prenatal exposure to bisphenols [49–52]. From these collective investigations comes significant insight into elements of the association between prenatal BPA exposure and DNA methylation in offspring growth and neurological function in addition to its sexually dimorphic nature. However, these studies are not uniform across their approach in three key elements: (i) the time point and sample type in which bisphenol exposure was measured (e.g. urinalysis during pregnancy or cord blood), (ii) the type of DNA methylation profiling (e.g. in candidate genes or epigenome-wide), and (iii) the timepoint at which DNA methylation was analyzed in offspring. With advances in exposure science and DNA methylation technology, it is critical to evaluate exposure to multiple bisphenols from the first trimester, a time during which the epigenome is highly susceptible to reprogramming; and measure outcomes at birth, utilizing methods that generate data at all genes. This study aimed to test the association between maternal exposure to the bisphenol BPA or its substitute chemicals, BPF and BPS, and cord blood leukocyte DNA methylation at >800 000 loci in a longitudinal pregnancy cohort. This study is of the few to evaluate prenatal exposure to bisphenols during the first trimester and its epigenome-wide association with DNA methylation infant cord blood. Importantly, we are the first to use this method to also investigate the replacement phenols BPF and BPS. Methods Study Population The samples used in this study were derived from the Michigan Mother−Infant Pairs pregnancy cohort (MMIP), which initiated in 2011. Briefly, women providing informed, written consent were enrolled during their first prenatal visit to the University of Michigan Women’s Hospital clinic. At this visit, maternal first-trimester blood and urine were collected. Women also completed a questionnaire that gathered socio-demographic factors, health behaviors, food consumption and personal care product use, among other measures. Exclusion criteria included: age <18 years, prior infertility treatment, pregnancy with multiple fetuses, and pregnancy <8 weeks or >14 weeks gestation. Women were provided study materials between weeks 34 and 38 of gestation for blood and urine collection upon admission into labor. Maternal blood and urine were collected when admitted and umbilical cord blood samples were collected at delivery. At the time of writing, 331 mothers have enrolled in MMIP, and 200 have been followed-up through labor and delivery. For the analysis described here, a subset of MMIP families enrolled between 2011 and 2017 with first-trimester exposure assessment of three urinary bisphenols and DNA methylation analysis via the Infinium EPIC were included (n = 69). The University of Michigan Medical School Institutional Review Board approved all study procedures (HUM00017941). Epigenome-Wide DNA Methylation Analysis of Infant Umbilical Cord Blood Infant cord blood samples (N = 69) were collected into PaxGene Blood DNA tubes (PreAnalytix) with the use of butterfly needles at the time of birth and stored at −80°C until processing. Total DNA was extracted with the PaxGene Blood DNA kit. DNA quality and concentration were assessed via Qubit at the University of Michigan Advanced Genomics Core. DNA was bisulfite converted using the EZ-96 DNA Methylation Kit (Zymo), wherein ∼500 ng of input DNA was used. The kit utilized sodium bisulfite to convert un-methylated cytosines to uracil and ultimately thymine, while methylated cytosines were protected [53]. Following bisulfite treatment, DNA methylation at >850 000 CpG sites was evaluated using the Illumina Infinium MethylationEPIC BeadChip (EPIC) at the University of Michigan Advanced Genomics Core according to standard protocols. Cord blood samples were run on three separate days, and these experimental batches are considered in statistical models. Processing and Quality Control of Infinium MethylationEPIC Data Arrays were assessed for quality of samples and probes using a standard pipeline. Briefly, the pipeline utilized the minfi package [54] (R Project for Statistical Computing) to read in raw data image files. Quality control of samples was assessed by comparing estimated sex (from methylation values on the X and Y chromosomes) with known infant sex, detection P-values of probes, and intensity signals. Probes with poor detection (positions that failed detection in more than 10% of samples N = 1475), cross-reactive probes, and probes that target polymorphic CpG sites in the Illumina HumanMethylation arrays were dropped [55]. The Functional Normalization [56] R package was used to correct for background and perform dye-bias normalization. Using estimateCellCounts, the relative proportion of B cells, CD4, CD8T, granulocytes, monocytes, neutrophils, and nucleated red blood cells (nRBCs) were estimated for each cord blood sample using an established algorithm based on DNA methylation profiles of sorted major cord blood cell types [57]. estimateCellCounts is a cell proportion estimation algorithm that estimates the relative proportions of cell types within a given sample based on DNA methylation signatures of each cell type. These preprocessing steps resulted in 822 020 retained probes from N = 69 cord blood samples that passed all quality control measures. Finally, M-values, defined as the log2 ratio of intensities of methylated probe versus unmethylated probes, were generated for each sample at these CpG sites and were used in downstream statistical analyses unless otherwise noted. Maternal Bisphenol Measurement Bisphenols (BPA, BPF, and BPS) were measured in spot urine samples collected from mothers during their first-trimester visit (between 8 and 14 weeks) for this subset of MMIP participants (n = 69). Samples were collected into polypropylene urine collection containers, aliquoted into glass vials, and frozen at −80°C until analysis. Total urinary BPA, BPF, and BPS were measured at NSF International (Ann Arbor, MI) using isotope dilution-liquid chromatography-tandem mass spectrometry (ID-LC-MS/MS), as reported previously [58]. Specific gravity was measured using a handheld digital refractometer (Atago Co., Ltd., Tokyo, Japan) at the time of sample analysis. Urinary bisphenol values below the limit of detection (LOD, 0.2 ng/ml) were replaced with LOD/√2 (0.141 ng/ml). Statistical Analysis All statistical analyses were performed in R version 3.6.0 (Platform: x86_64-apple-darwin15.6.0 (64-bit) & Running under: macOS Mojave 10.14.6). We first performed univariate analyses on all exposure biomarkers and potential covariates of interest. We then assessed relationships between exposures and covariates to identify potential confounders via chi-square tests, t-tests, and Spearman correlations. First-trimester urinary BPA was modeled as a continuous variable, and BPF and BPS were modeled as categorical (above or below the LOD) (Supplementary Tables S1−S3). Singular value decomposition (SVD) analysis was performed with the ChAMP package [59]. The correlation between principal components of the methylation data with biological and technical covariates was determined using linear regression (continuous variables) or Kruskal−Wallis (categorical variables). We did not identify potential confounders (i.e. covariates associated with both BPA and DNA methylation) to include in the model. However, due to their significant (P < 1 × 10−5) association with the DNA methylation data in the SVD analysis, infant sex, B cells, nRBCs, and sample-plate (batch) were selected as covariates to adjust for in final models. We also performed a sensitivity analysis on the sites significantly associated with BPA exposure to determine the effect of including gestational age and birth weight. Single-Site Association Analysis Linear regression was used to identify differentially methylated CpG sites (using M-values) by each maternal urinary bisphenol exposure, adjusting for covariates described above (infant sex, B cells, nRBCs, sample plate). An empirical Bayes method in the limma [60] R package was then used to shrink probe-wise variances toward a pooled estimate and calculate a moderated t-statistic. M-values were selected for statistical analysis given their advantages which include meeting the assumption of homoscedasticity and superior performance in detection rate and true positive rate, especially for highly methylated and unmethylated sites [61, 62]. P-value correction by the Benjamini−Hochberg false discovery rate (FDR) method was used [63], and a 5% FDR (i.e. q < 0.05) were considered significant. Sensitivity analyses were performed. One maternal urinary BPA sample was identified as a statistical outlier [±2 standard deviations (SD) from the mean]. The outlier was removed, and the single-site analyses were rerun. The direction and significance of the sites identified as significant in the initial model were compared with the results from the model without the outlier. Additional analyses included examining scatterplots of the relationship between BPA and methylation at each significant site. In order to test whether the bisphenol exposures may be influencing the same genes, we calculated the Pearson correlation between the effect estimates of all CpG sites from models for each bisphenol. Lastly, we compared results of previously published epigenome-wide studies focused on BPA exposure with our results [49, 50]. Pearson correlation was run between the effect estimates for sites reported by Miura et al. [49] as significant at P ≤ 0.0001 for all infants and the corresponding results in our BPA model. Results from Junge et al. and Alavian-Ghavanini et al. were compared with our results for replication of the direction of the effect of BPA. Differentially Methylated Regions We utilized dmrcate [64] to test for differentially methylated regions (DMRs) by maternal first-trimester urinary phenols exposure. A DMR had to consist of at least two consecutive probes. Probes that were two nucleotides or closer to a single-nucleotide polymorphism that had minor allele frequency >0.05 were filtered out first. The model was adjusted for cell type (Bcell and nRBC), infant sex, and batch. GenomicRanges [65] was used to graph an annotated representation of the DMRs. GenomicRanges requires the use of beta values (e.g. proportion of DNA methylation at CpG sites), and data are displayed as averaged across quartiles of BPA. Quartile cutoffs are as follows: Q1 [