Methylation of CpG island of p14(ARK), p15(INK4b) and p16(INK4a) genes in coke oven workers

DNA Methylation as a Molecular Mechanism of Carcinogenesis in World Trade Center Dust Exposure: Insights from a Structured Literature Review

The collapse of the World Trade Center (WTC) buildings in New York City generated a large plume of dust and smoke. WTC dust contained human carcinogens including metals, asbestos, polycyclic aromatic hydrocarbons (PAHs), persistent organic pollutants (POPs, including polychlorinated biphenyls (PCBs) and dioxins), and benzene. Excess levels of many of these carcinogens have been detected in biological samples of WTC-exposed persons, for whom cancer risk is elevated. As confirmed in this structured literature review (n studies = 80), all carcinogens present in the settled WTC dust (metals, asbestos, benzene, PAHs, POPs) have previously been shown to be associated with DNA methylation dysregulation of key cancer-related genes and pathways. DNA methylation is, therefore, a likely molecular mechanism through which WTC exposures may influence the process of carcinogenesis.

Dynamic changes of DNA methylation induced by benzo(a)pyrene in cancer

Benzo(a)pyrene (BaP), the earliest and most significant carcinogen among polycyclic aromatic hydrocarbons (PAHs), has been found in foods, tobacco smoke, and automobiles exhaust, etc. Exposure to BaP induced DNA damage directly, or oxidative stress-related damage, resulting in cell apoptosis and carcinogenesis in human respiratory system, digestive system, reproductive system, etc. Moreover, BaP triggered genome-wide epigenetic alterations by methylation, which might cause disturbances in regulation of gene expression, and thereby induced cancer. It has been proved that BaP reduced genome-wide DNA methylation, and activated proto-oncogene by hypomethylation in the promoter region, but silenced tumor suppressor genes by promoter hypermethylation, resulting in cancer initiation and progression. Here we summarized the changes in DNA methylation in BaP exposure, and revealed the methylation of DNA plays a role in cancer development.

Toxicomethylomics revisited: A state-of-the-science review about DNA methylation modifications in blood cells from workers exposed to toxic agents

Introduction

Epigenetic marks have been proposed as early changes, at the subcellular level, in disease development. To find more specific biomarkers of effect in occupational exposures to toxicants, DNA methylation studies in peripheral blood cells have been performed. The goal of this review is to summarize and contrast findings about DNA methylation in blood cells from workers exposed to toxicants.

Methods

A literature search was performed using PubMed and Web of Science. After first screening, we discarded all studies performed in vitro and in experimental animals, as well as those performed in other cell types other than peripheral blood cells. Results: 116 original research papers met the established criteria, published from 2007 to 2022. The most frequent investigated exposures/labor group were for benzene (18.9%) polycyclic aromatic hydrocarbons (15.5%), particulate matter (10.3%), lead (8.6%), pesticides (7.7%), radiation (4.3%), volatile organic compound mixtures (4.3%), welding fumes (3.4%) chromium (2.5%), toluene (2.5%), firefighters (2.5%), coal (1.7%), hairdressers (1.7%), nanoparticles (1.7%), vinyl chloride (1.7%), and others. Few longitudinal studies have been performed, as well as few of them have explored mitochondrial DNA methylation. Methylation platforms have evolved from analysis in repetitive elements (global methylation), gene-specific promoter methylation, to epigenome-wide studies. The most reported observations were global hypomethylation as well as promoter hypermethylation in exposed groups compared to controls, while methylation at DNA repair/oncogenes genes were the most studied; studies from genome-wide studies detect differentially methylated regions, which could be either hypo or hypermethylated.

Discussion

Some evidence from longitudinal studies suggest that modifications observed in cross-sectional designs may be transitory; then, we cannot say that DNA methylation changes are predictive of disease development due to those exposures.

Conclusion

Due to the heterogeneity in the genes studied, and scarcity of longitudinal studies, we are far away from considering DNA methylation changes as biomarkers of effect in occupational exposures, and nor can we establish a clear functional or pathological correlate for those epigenetic modifications associated with the studied exposures.

Hypermethylation in Gene Promoters Are Induced by Chronic Exposure to Benzene, Toluene, Ethylbenzene and Xylenes

BACKGROUND AND OBJECTIVE

Gas station attendants are occupationally exposed to benzene, toluene, ethylbenzene and xylene (BTEX) compounds and thus more susceptible to the biological effects of this mixture present in gasoline, especially due to the carcinogenicity of benzene. Furthermore, the harmful effects of BTEX exposure may be potentiated by genetic and epigenetic inactivation of critical genes. The objective was to evaluate such gene-BTEX interactions accessing the promoter methylation status of p14ARF, p16INK4A and GSTP1 in peripheral blood leukocyte samples.

MATERIALS AND METHODS

The 59 exposed and 68 unexposed participants from Rio de Janeiro, Brazil, were included. The promoter methylation status was accessed by methylation-specific PCR (MSP) and GSTP1 Ile105Val polymorphism was investigated by PCR-restriction fragment length polymorphism (PCR-RFLP) technique.

RESULTS

Both p14ARF and p16INK4A were significantly hypermethylated in exposed subjects compared to unexposed (p = 0.004 and p<0.001, respectively). Additionally, p16INK4A hypermethylation in the exposed group was correlated with chromosomal abnormalities (CAs) (p = 0.018), thus highlighting the influence of the gene-environment interactions on genome instability. Noteworthy, p16INK4A methylation was significantly associated with miscarriage among female attendants (p = 0.047), in which those who reported miscarriage exhibited hypermethylation in at least 2 of the 3 genes analyzed. The GSTP1 heterozygote genotype, which could affect the metabolism of benzene detoxification, was found in both groups but was more frequent in those occupationally exposed. No significant association was observed between GSTP1 genotypes and methylation status.

CONCLUSION

Together, these findings indicate that gas station attendants with the aforementioned epigenetic and genetic profiles may be at greater risk of occupational BTEX exposure-induced genome instability, which could require concerted efforts to establish more preventive actions and constant biomonitoring in gas station attendants.

Overview of biological mechanisms of human carcinogens

This review summarizes the carcinogenic mechanisms for 109 Group 1 human carcinogens identified as causes of human cancer through Volume 106 of the IARC Monographs. The International Agency for Research on Cancer (IARC) evaluates human, experimental and mechanistic evidence on agents suspected of inducing cancer in humans, using a well-established weight of evidence approach. The monographs provide detailed mechanistic information about all carcinogens. Carcinogens with closely similar mechanisms of action (e.g. agents emitting alpha particles) were combined into groups for the review. A narrative synopsis of the mechanistic profiles for the 86 carcinogens or carcinogen groups is presented, based primarily on information in the IARC monographs, supplemented with a non-systematic review. Most carcinogens included a genotoxic mechanism.

FLT1 hypermethylation is involved in polycyclic aromatic hydrocarbons-induced cell transformation

Coke oven emissions (COEs) are common particle pollutants in occupational environment and the major constituents of COEs are polycyclic aromatic hydrocarbons (PAHs). Previously, we identified aberrant methylation of the fms related tyrosine kinase 1 (FLT1) gene over the course of benzo(a)pyrene (BaP)-induced cell transformation via genome-wide methylation array. To quantify FLT1 methylation, we established a bisulfite pyrosequencing assay and examined the FLT1 hypermethylation in several human cancers. The results revealed that 70.0% (21/30 pairs) of lung cancers harbored hypermethylated FLT1 and concomitant suppression of gene expression compared to the adjacent tissues. This implies that FLT1 hypermethylation might play a role in malignant cell transformation. In addition, FLT1 hypermethylation and gene suppression appeared in primary human lymphocytes in a dose-response manner following COEs treatment. To explore whether FLT1 methylation is correlated with COEs exposure and DNA damage, we recruited 144 male subjects who had been exposed to high levels of COEs and 84 male control subjects. Notably, the FLT1 methylation in peripheral blood lymphocytes (PBLCs) of the COEs-exposed group (19.8 ± 3.2%) was enhanced by 17.9% compared to that of the control group (16.8 ± 2.8%) (P < 0.001). The FLT1 methylation status was positively correlated with urinary 1-hydroxypyrene (1-OHP) levels, an internal exposure marker of PAHs (β = 0.029, 95% CI = 0.010-0.048, P = 0.003) and positively correlated with DNA damage (β_OTM = 0.024, 95% CI = 0.007-0.040, P = 0.005; β_{Tail DNA} = 0.035, 95% CI = 0.0017-0.054, P < 0.001) indicated by comet assay. Taken together, these findings indicate that FLT1 might be a tumor suppressor, and its hypermethylation might contribute to PAHs-induced carcinogenicity.

Aberrant DNA Methylation of Two Tumor Suppressor Genes, p14ARF and p15INK4b, after Chronic Occupational Exposure to Low Level of Benzene

BACKGROUND

Exposure to benzene would be associated with many diseases including leukemia. Epigenetic alterations seem to be among the main mechanisms involved.

OBJECTIVE

To determine if chronic occupational exposure to low level of benzene would be associated with DNA methylation.

METHODS

Global DNA methylation and promoter-specific methylation of the two tumor suppressor genes, p14^ARF and p15^INK4b, were assessed employing methylation-specific PCR using the DNA extracted from 40 petrochemical workers exposed to ambient benzene levels of <1 ppm, and 31 office workers not exposed to benzene or its derivatives.

RESULTS

While an increase in global DNA methylation of 5% in p14^ARF (p=0.501) and 28% in p15^INK4b (p=0.02) genes was observed in the exposed group, no hypermethylation in either of the studied genes was observed in the unexposed group. No significant association was found between the frequency of aberrant methylation and either of age, work experience, and smoking habit in the exposed group.

CONCLUSION

Chronic occupational exposure to lower than the permissible exposure limit of benzene may still result in DNA methylation of tumor suppressor genes that may ultimately lead to development of cancer.

TRIM36 hypermethylation is involved in polycyclic aromatic hydrocarbons-induced cell transformation

Long term exposure to polycyclic aromatic hydrocarbons (PAHs) is associated with the increasing risk of lung cancer. To identify differentially hypermethylated genes associated with PAHs-induced carcinogenicity, we performed genome-wide DNA methylation analysis in 20 μM benzo(a)pyrene (BaP)-transformed human bronchial epithelial (HBE) cells at different stages of cell transformation. Several methylated genes (CNGA4, FLT1, GAREM1, SFMBT2, TRIM36) were differentially hypermethylated and their mRNA was suppressed in cells at both pre-transformed and transformed stages. Similar results were observed in HBE cells transformed by 20 μg/mL coke oven emissions (COEs) mixture collected from a coking manufacturing facility. In particular, hypermethylation of TRIM36 and suppression of TRIM36 expression were gradually enhanced over the time of COEs treatment. We developed bisulfite pyrosequencing assay and assessed TRIM36 methylation quantitatively. We found that hypermethylation of TRIM36 and reduced gene expression was prevalent in several types of human cancers. TRIM36 hypermethylation appeared in 90.0% (23/30) of Non-Small Cell Lung Cancer (NSCLCs) tissues compared to their paired adjacent tissues with an average increase of 1.32 fold. Furthermore, an increased methylation rate (5.90% v.s 7.38%) and reduced levels of TRIM36 mRNA were found in peripheral lymphocytes (PBLCs) of 151 COEs-exposed workers. In all subjects, TRIM36 hypermethylation was positively correlated with the level of urinary 1-hydroxypyrene (P < 0.001), an internal exposure marker of PAHs, and the DNA damage (P = 0.013). These findings suggest that aberrant hypermethylation of TRIM36 might be involved in the acquisition of malignant phenotype and could be served as a biomarker for risk assessment of PAHs exposure.

Effects of coke oven emissions and benzo[a]pyrene on blood pressure and electrocardiogram in coke oven workers

Objective:

To evaluate the effects of occupational exposures to coke oven emissions (COEs) and benzo[a]pyrene (B[a]P) on the prevalence of hypertension and abnormal electrocardiogram (ECG) in coke oven workers.

Methods:

We included 880 coke oven workers and 710 oxygen employees in the exposed and control groups, respectively. Blood pressure (BP), ECG, blood lipid levels, and glucose levels of all subjects were measured. COE and B[a]P concentrations at the bottom, side, and top of the oven and control plants were estimated by weighing and high-performance liquid chromatography.

Results:

The COE concentration at the top and side was higher than that at the bottom (P < 0.05). The levels of B[a]P at the top and side significantly exceeded the limit value. Abnormal BP, ECG, the detection ratio of hypertension and left ventricular high voltage were significantly greater in the exposed group than in the control group (P < 0.05). The logistic regression analysis results revealed that age and B[a]P exposure were risk factors for hypertension in coke oven workers (P < 0.05) and both were risk factors for abnormal ECG (P < 0.05). Moreover, B[a]P exposure, age, and gender were risk factors for impaired fasting glucose in coke oven workers (P < 0.05).

Conclusions:

B[a]P and COE exposures are risk factors for hypertension and abnormal ECG in coke oven workers.

Epigenetic alterations induced by genotoxic occupational and environmental human chemical carcinogens: A systematic literature review

Accumulating evidence suggests that epigenetic alterations play an important role in chemically-induced carcinogenesis. Although the epigenome and genome may be equally important in carcinogenicity, the genotoxicity of chemical agents and exposure-related transcriptomic responses have been more thoroughly studied and characterized. To better understand the evidence for epigenetic alterations of human carcinogens, and the potential association with genotoxic endpoints, we conducted a systematic review of published studies of genotoxic carcinogens that reported epigenetic endpoints. Specifically, we searched for publications reporting epigenetic effects for the 28 agents and occupations included in Monograph Volume 100F of the International Agency for the Research on Cancer (IARC) that were classified as "carcinogenic to humans" (Group 1) with strong evidence of genotoxic mechanisms of carcinogenesis. We identified a total of 158 studies that evaluated epigenetic alterations for 12 of these 28 carcinogenic agents and occupations (1,3-butadiene, 4-aminobiphenyl, aflatoxins, benzene, benzidine, benzo[a]pyrene, coke production, formaldehyde, occupational exposure as a painter, sulfur mustard, and vinyl chloride). Aberrant DNA methylation was most commonly studied, followed by altered expression of non-coding RNAs and histone changes (totaling 85, 59 and 25 studies, respectively). For 3 carcinogens (aflatoxins, benzene and benzo[a]pyrene), 10 or more studies reported epigenetic effects. However, epigenetic studies were sparse for the remaining 9 carcinogens; for 4 agents, only 1 or 2 published reports were identified. While further research is needed to better identify carcinogenesis-associated epigenetic perturbations for many potential carcinogens, published reports on specific epigenetic endpoints can be systematically identified and increasingly incorporated in cancer hazard assessments.

CpG site-specific RASSF1a hypermethylation is associated with occupational PAH exposure and genomic instability

Previous studies have shown an etiologic link between exposure to PAHs and lung cancer development.