Persistent organic pollutants and promoter hypermethylation of the O(6)-methylguanine-DNA methyltransferase gene

Epigenetic processes involved in response to pesticide exposure in human populations: a systematic review and meta-analysis

In recent decades, the use of pesticides in agriculture has increased dramatically. This has resulted in these substances being widely dispersed in the environment, contaminating both exposed workers and communities living near agricultural areas and via contaminated foodstuffs. In addition to acute poisoning, chronic exposure to pesticides can lead to molecular changes that are becoming better understood. Therefore, the aim of this study was to assess, through a systematic review of the literature, what epigenetic alterations are associated with pesticide exposure. We performed a systematic review and meta-analysis including case-control, cohort and cross-sectional observational epidemiological studies to verify the epigenetic changes, such as DNA methylation, histone modification and differential microRNA expression, in humans who had been exposed to any type of pesticide. Articles published between the years 2005 and 2020 were collected. Two different reviewers performed a blind selection of the studies using the Rayyan QCRI software. Post-completion, the data of selected articles were extracted and analyzed. Most of the 28 articles included evaluated global DNA methylation levels, and the most commonly reported epigenetic modification in response to pesticide exposure was global DNA hypomethylation. Meta-analysis revealed a significant negative correlation between Alu methylation levels and β-hexachlorocyclohexane, p,p'-dichlorodiphenyldichloroethane and p,p'-dichlorodiphenylethylene levels. In addition, some specific genes were reported to be hypermethylated in promoter regions, such as CDKN2AIGF2, WRAP53α and CDH1, while CDKN2B and H19 were hypomethylated due to pesticide exposure. The expression of microRNAs was also altered in response to pesticides, as miR-223, miR-518d-3p, miR-597, miR-517b and miR-133b that are associated with many human diseases. Therefore, this study provides evidence that pesticide exposure could lead to epigenetic modifications, possibly altering global and gene-specific methylation levels, epigenome-wide methylation and microRNA differential expression.

Epigenetic alterations induced by genotoxic occupational and environmental human chemical carcinogens: An update of a systematic literature review

Epigenetic alterations, such as changes in DNA methylation, histones/chromatin structure, nucleosome positioning, and expression of non-coding RNAs, are recognized among key characteristics of carcinogens; they may occur independently or concomitantly with genotoxic effects. While data on genotoxicity are collected through standardized guideline tests, data collected on epigenetic effects is far less uniform. In 2016, we conducted a systematic review of published studies of genotoxic carcinogens that reported epigenetic endpoints to better understand the evidence for epigenetic alterations of human carcinogens, and the potential association with genotoxic endpoints. Since then, the number of studies of epigenetic effects of chemicals has nearly doubled. This review stands as an update on epigenetic alterations induced by occupational and environmental human carcinogens that were previously and recently classified as Group 1 by the International Agency for Research on Cancer. We found that the evidence of epigenetic effects remains uneven across agents. Studies of DNA methylation are most abundant, while reports concerning effects on non-coding RNA have increased over the past 5 years. By contrast, mechanistic toxicology studies of histone modifications and chromatin state alterations remain few. We found that most publications of epigenetic effects of carcinogens were studies in exposed humans or human cells. Studies in rodents represent the second most common species used for epigenetic studies in toxicology, in vivo exposures being the most predominant. Future studies should incorporate dose- and time-dependent study designs and also investigate the persistence of effects following cessation of exposure, considering the dynamic nature of most epigenetic alterations.

Dioxin-like compound exposures and DNA methylation in the Anniston Community Health Survey Phase II

The Anniston Community Health Survey (ACHS-I) was initially conducted from 2005 to 2007 to assess polychlorinated biphenyl (PCB) exposures in Anniston, Alabama residents. In 2014, a follow-up study (ACHS-II) was conducted to measure the same PCBs as in ACHS-I and additional compounds e.g., polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), and dioxin-like non-ortho (cPCBs) substituted PCBs. In this epigenome-wide association study (EWAS), we examined the associations between PCDD, PCDF, and PCB exposures and DNA methylation. Whole blood DNA methylation was measured using Illumina EPIC arrays (n=292). We modeled lipid-adjusted toxic equivalencies (TEQs) for: ΣDioxins (sum of 28 PCDDs, PCDFs, cPCBs, and mPCBs), PCDDs, PCDFs, cPCBs, and mPCBs using robust multivariable linear regression adjusting for age, race, sex, smoking, bisulfite conversion batch, and estimated percentages of six blood cell types. Among all exposures we identified 10 genome-wide (Bonferroni p≤6.74E-08) and 116 FDR (p≤5.00E-02) significant associations representing 10 and 113 unique CpGs, respectively. Of the 10 genome-wide associations, seven (70%) occurred in the PCDDs and four (40%) of these associations had an absolute differential methylation ≥1.00%, based on the methylation difference between the highest and lowest exposure quartiles. Most of the associations (six, 60%) represented hypomethylation changes. Of the 10 unique CpGs, eight (80%) were in genes shown to be associated with dioxins and/or PCBs based on data from the 2019 Comparative Toxicogenomics Database. In this study, we have identified a set of CpGs in blood DNA that may be particularly susceptible to dioxin, furan, and dioxin-like PCB exposures.

Polychlorinated biphenyl exposure and DNA methylation in the Anniston Community Health Survey

Anniston, Alabama was home to a major polychlorinated biphenyl (PCB) production facility from 1929 until 1971. The Anniston Community Health Survey I and II (ACHS-I 2005-2007, ACHS-II 2013-2014) were conducted to explore the effects of PCB exposures. In this report we examined associations between PCB exposure and DNA methylation in whole blood using EPIC arrays (ACHS-I, n = 518; ACHS-II, n = 299). For both cohorts, 35 PCBs were measured in serum. We modelled methylation versus PCB wet-weight concentrations for: the sum of 35 PCBs, mono-ortho substituted PCBs, di-ortho substituted PCBs, tri/tetra-ortho substituted PCBs, oestrogenic PCBs, and antiestrogenic PCBs. Using robust multivariable linear regression, we adjusted for age, race, sex, smoking, total lipids, and six blood cell-type percentages. We carried out a two-stage analysis; discovery in ACHS-I followed by replication in ACHS-II. In ACHS-I, we identified 28 associations (17 unique CpGs) at p ≤ 6.70E-08 and 369 associations (286 unique CpGs) at FDR p ≤ 5.00E-02. A large proportion of the genes have been observed to interact with PCBs or dioxins in model studies. Among the 28 genome-wide significant CpG/PCB associations, 14 displayed replicated directional effects in ACHS-II; however, only one in ACHS-II was statistically significant at p ≤ 1.70E-04. While we identified many novel CpGs significantly associated with PCB exposures in ACHS-I, the differential methylation was modest and the effect was attenuated seven years later in ACHS-II, suggesting a lack of persistence of the associations between PCB exposures and altered DNA methylation in blood cells.

BDE-47 Decreases Progesterone Levels in BeWo Cells by Interfering with Mitochondrial Functions and Genes Related to Cholesterol Transport

Polybrominated diphenyl ethers (PBDEs) have been reported to exert reproductive endocrine toxicity, but the mechanisms for this process remain unclear. Currently available studies have concentrated on the enzymatic reactions during steroidogenesis, but the results are not consistent. In this study, we explored the effects of 2,2',4,4'-tertrabromodiphenyl ether (BDE-47) on progesterone biosynthesis and the potential mechanisms in human placental choriocarcinoma cells. The results showed that BDE-47 decreased progesterone production in a dose-dependent manner but had no effect on key enzymes (Cyp11a1 and 3β-HSD). BDE-47 exposure depolarized the mitochondrial membrane potential and downregulated adenosine triphosphate levels. The gene expression levels of Mfn2, Tspo, Atad3, Vdac1, Fis1, and Drp1, which are involved in mitochondrial dynamics and cholesterol transport, were disturbed. The demethylation of some CpG loci of mitochondrial biomarkers (Drp1, Opa1, Vdac2, and Atad3) was induced in the 1 μM BDE-47 exposure group, but no methylation change was observed with 50 μM treatment. Our findings unveiled that the reduction of progesterone synthesis induced by BDE-47 might be associated with cholesterol transportation, mitochondrial dynamics, and mitochondrial functions. These findings provide substantial data on the reproductive endocrine toxicity of PBDEs.

Environmental Influences on the Epigenome: Exposure- Associated DNA Methylation in Human Populations

DNA methylation is the most well studied of the epigenetic regulators in relation to environmental exposures. To date, numerous studies have detailed the manner by which DNA methylation is influenced by the environment, resulting in altered global and gene-specific DNA methylation. These studies have focused on prenatal, early-life, and adult exposure scenarios. The present review summarizes currently available literature that demonstrates a relationship between DNA methylation and environmental exposures. It includes studies on aflatoxin B₁, air pollution, arsenic, bisphenol A, cadmium, chromium, lead, mercury, polycyclic aromatic hydrocarbons, persistent organic pollutants, tobacco smoke, and nutritional factors. It also addresses gaps in the literature and future directions for research. These gaps include studies of mixtures, sexual dimorphisms with respect to environmentally associated methylation changes, tissue specificity, and temporal stability of the methylation marks.

DNA methylation: a mechanism linking environmental chemical exposures to risk of autism spectrum disorders?

There is now compelling evidence that gene by environment interactions are important in the etiology of autism spectrum disorders (ASDs). However, the mechanisms by which environmental factors interact with genetic susceptibilities to confer individual risk for ASD remain a significant knowledge gap in the field. The epigenome, and in particular DNA methylation, is a critical gene expression regulatory mechanism in normal and pathogenic brain development. DNA methylation can be influenced by environmental factors such as diet, hormones, stress, drugs, or exposure to environmental chemicals, suggesting that environmental factors may contribute to adverse neurodevelopmental outcomes of relevance to ASD via effects on DNA methylation in the developing brain. In this review, we describe epidemiological and experimental evidence implicating altered DNA methylation as a potential mechanism by which environmental chemicals confer risk for ASD, using polychlorinated biphenyls (PCBs), lead, and bisphenol A (BPA) as examples. Understanding how environmental chemical exposures influence DNA methylation and how these epigenetic changes modulate the risk and/or severity of ASD will not only provide mechanistic insight regarding gene-environment interactions of relevance to ASD but may also suggest potential intervention strategies for these and potentially other neurodevelopmental disorders.