Global DNA hypomethylation has no impact on lung function or serum inflammatory and fibrosis cytokines in asbestos-exposed population

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.

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.

Assessment of the Carcinogenicity of Carbon Nanotubes in the Respiratory System

In 2014, the International Agency for Research on Cancer (IARC) classified the first type of carbon nanotubes (CNTs) as possibly carcinogenic to humans, while in the case of other CNTs, it was not possible to ascertain their toxicity due to lack of evidence. Moreover, the physicochemical heterogeneity of this group of substances hamper any generalization on their toxicity. Here, we review the recent relevant toxicity studies produced after the IARC meeting in 2014 on an homogeneous group of CNTs, highlighting the molecular alterations that are relevant for the onset of mesothelioma. Methods: The literature was searched on PubMed and Web of Science for the period 2015-2020, using different combinations keywords. Only data on normal cells of the respiratory system after exposure to fully characterized CNTs for their physico-chemical characteristics were included. Recent studies indicate that CNTs induce a sustained inflammatory response, oxidative stress, fibrosis and histological alterations. The development of mesothelial hyperplasia, mesothelioma, and lungs tumors have been also described in vivo. The data support a strong inflammatory potential of CNTs, similar to that of asbestos, and provide evidence that CNTs exposure led to molecular alterations known to have a key role in mesothelioma onset. These evidences call for an urgent improvement of studies on exposed human populations and adequate systems for monitoring the health of workers exposed to this putative carcinogen.

Carbon Nanotube- and Asbestos-Induced DNA and RNA Methylation Changes in Bronchial Epithelial Cells

Carbon nanotubes (CNTs) are nanoscale tube-shaped carbon materials used in many industrial areas. Their fiber shape has caused concerns about their toxicity given their structural similarity with asbestos. The aim here was to elucidate the effect of CNTs and asbestos exposure on global DNA and RNA methylation and the methylation of genes associated with cell cycle, inflammation, and DNA damage processes in human lung cells. Human bronchial epithelial cells (16HBE14o-) were exposed for 24 h to 25 and 100 μg/mL CNTs (single-walled CNTs [SWCNTs] and multiwalled CNTs [MWCNTs]) and 2.5 μg/mL asbestos (chrysotile, amosite, and crocidolite). Global DNA and RNA (hydroxy)methylation to cytosines was measured by a validated liquid chromatography tandem-mass spectrometry method. Global RNA methylation to adenines was measured by a colorimetric ELISA-like assay. Gene-specific DNA methylation status at certain cytosine-phosphate-guanine (CpG) sites of cyclin-dependent kinase inhibitor 1A ( CDKN1A), serine/threonine kinase ( ATM), and TNF receptor-associated factor 2 ( TRAF2) were analyzed by using bisulfite pyrosequencing technology. Only MWCNT-exposed cells showed significant global DNA hypomethylation of cytosine and global RNA hypomethylation of adenosine. SWCNT, MWCNT, and amosite exposure decreased DNA methylation of CDKN1A. ATM methylation was affected by chrysotile, SWCNT, and MWCNT. However, SWCNT exposure led to DNA hypermethylation of TRAF2. These findings contribute to further understanding of the effect of CNTs on different carcinogenic pathways.

Increase of global DNA methylation patterns in beauty salon workers exposed to low levels of formaldehyde

Formaldehyde (FA) is a carcinogenic aldehyde illegally added to creams as a hair straightening agent for the Brazilian blowout (BB). This study aimed to investigate the possible effects of occupational exposure to FA on global DNA methylation in salon workers with different exposure levels. FA exposure was monitored using environmental and biological measurements. The study included 49 salon workers divided by FA levels in the workplace into group A (FA < 0.01 ppm; n = 8), group B (0.03 ppm < FA < 0.06 ppm; n = 15), and group C (0.08 ppm < FA < 0.24 ppm; n = 26). The global DNA methylation levels were 3.12%, 4.55%, and 4.29% for groups A, B, and C, respectively, with statistically higher values for groups B and C compared to group A (p = 0.002). A correlation was found between FA in passive samplers and global DNA methylation (r_s = 0.307, p = 0.032). Additionally, when only taking into account the hairdressers that performed the BB on clients instead of the whole group, a stronger correlation was observed between FA in personal passive samplers and global DNA methylation (r_s = 0.764, p = 0.006). For the first time, an increase in DNA methylation was observed in subjects occupationally exposed to FA. In conclusion, our results indicated that even low levels of FA exposure could cause a disturbance in DNA methylation, leading to epigenetic changes, which is associated with cancer development. These data suggest a possible contribution of FA to cancer development through occupational exposure.