Impact of Heavy Metals and Chemicals on Epigenetic Changes

i1 More than 13 million people die each year as a result of environmental contaminants, and it is estimated that up to 24% of diseases are brought on by avoidable environmental exposures. Different environmental chemicals were discovered in the blood and urine of the US population during a screening sponsored by the US Center for Disease Control and Prevention, revealing the level of human exposure to environmental chemicals. A growing body of research indicates that environmental toxins may affect gene expression in ways that affect the development of diseases. The first stages of gene transcription need dynamic chromatin remodeling, which is accomplished by changing how accessible gene promoters and regulatory regions are. These regulatory procedures involve epigenetic factors such as DNA methylation, histone changes, and microRNAs (miRNAs), which in turn regulate gene expression. Exposure to various environmental toxins has been found to create changes in these epigenetic markers, and some of them have been connected to a variety of diseases, along with their causes, treatments, and preventative measures.


Introduction
Most preventative programs cover the possible environmental toxins' impact on human health as well as how to control them. However, it is also a social phenomenon to be afraid of passing on negative traits to succeeding generations. We should distinguish between transgenerational transmission, in which subsequent offspring generations inherit the phenotypic features, and intergenerational transmission, which only affects the first offspring generation of the exposed individual (Lopez-Casas et al., 2012). The germ line, which contains DNA sequences of both coding and non-coding genes, can be Mendelian transferred to the following generations if a toxicant is able to cause mutations on DNA in individuals impacted. Reprotoxicants are included in this group. However, due to the nature of the affected system, mutations that impact reproduction characteristics are under strong negative selection. Epimutations are mutations that impact phenotypes but do not change the DNA sequence. A notion closely related to epigenetics is the transgenerational inheritance of phenotypes brought on by environmental stress (Miller et al., 1974;Bartel and Chen, 2004).
Due to their very nature, epigenetic alterations can be seen as a way for organisms to adapt to changes in their living environment. As a result, epigenetic modifications in germ cells exhibit the highest rate of environmental modification but also the highest rate of reversibility. After parental exposure, changes in the embryonic gonads' gene expression patterns have been hypothesized to affect the epigenetic marks (Yoder et al., 1997;Reik, et al 2001). DNA methylation, chromatin remodeling via histone modifications, and more recently, ncRNA-mediated epigenetic gene regulation are the three main processes behind epigenetic alterations.

Epigenetic Changes in Human Histone Modifications
Histone proteins play a significant role in the protection and packing of the genetic material in humans. They also provide a method for controlling DNA 1This work is published open access under the Creative Commons Attribution License 4.0, which permits free reuse, remix, redistribution and transformation provided due credit is given transcription, replication, and repair. According to Eden et al. (2003), Gaudet et al. (2003) and Laird (2005), histones are nuclear globular proteins that can be covalently altered to affect chromatin structure and gene expression. These modifications include acetylation (Ac), methylation, phosphorylation, glycosylation, sumoylation, ubiquitination, and adenosine diphosphate (ADP) ribosylation. Yang et al., 2004;Laird, 2005. Ac and methylation of lysine residues at the amino terminus of histone 3 (H3) and H4 are the most frequent histone modifications that have been proven to be altered by environmental pollutants. While histone methylation (Me) occurs as mono-, di-, and trimethyl group states (Deshmukh et al., 2011), it can either increase or decrease gene expression depending on the amino acid position that is modified. Histone Ac, which typically has only one acetyl group added to each amino acid residue, increases gene transcriptional activity (Irizarry et al., 2009;Orta et al., 2010) miRNAs miRNAs are short single-stranded RNAs of approximately 20-24 nucleotides in length that are transcribed from DNA but not translated into proteins. miRNAs negatively regulate expression of target genes at the posttranscriptional level by binding to 3′-untranslated regions of target mRNAs (Andrews et al., 2012). Each mature miRNA is partially complementary to multiple target mRNAs and directs the RNA-induced silencing complex (RISC) to identify the target mRNAs for inactivation (Kafri et al., 1999). miRNAs are initially transcribed as longer primary transcripts (pri-miRNAs) and processed first by the RNase enzyme complex, and then by Dicer, leading to incorporation of a single strand into the RISC. miRNAs guide RISC to interact with mRNAs and determine post-transcriptional repression. miRNAs are involved in the regulation of gene expression through the targeting of mRNAs during cell proliferation, apoptosis, control of stem cell self-renewal, differentiation, metabolism, development and tumour metastasis (Del Mazo et al., 1994) Compared with other mechanisms involved in gene expression, miRNAs act directly before protein synthesis and may be more directly involved in fine-tuning of gene expression or quantitative regulation (Kafri et al., 1999). Moreover, miRNAs also play key roles in modifying chromatin structure and participating in the maintenance of genome stability (Herbst et al., 1971, Williams, 2008. miRNAs can regulate various physiological and pathological processes, such as cell growth, differentiation, proliferation, apoptosis and metabolism (Andrews et al., 2012, Newbold, 2004. More than 100000 miRNAs have been reported in animals, plants and viruses by using computational and experimental methods in miRNA-related public databases. The aberrant expression of miRNAs has been linked to various human diseases, including Alzheimer's disease, cardiac hypertrophy, altered heart repolarization, lymphomas, leukaemias, and cancer at several sites (Martin and Zhang, 2005;Sato et al., 2009).

Metals
Numerous studies have shown a connection between ambient metals like nickel, cadmium, lead, and especially arsenic and DNA methylation. These results across various metals may be explained by oxidative stress caused by metals. Redox cycling has been shown to catalytically boost the formation of reactive oxygen species (ROS) in metals (Miller et al., 1999;Bartel and Chen, 2004;Lopez-Casas et al., 2012). According to Yoder et al. (1999) and Reik et al. (2001), oxidative DNA damage can prevent methyltransferases from interacting with DNA, which leads to a generalized altered methylation of cytosine residues at CpG sites (Jones and Baylin, 2002).

Cadmium
Cadmium is a known carcinogen with very little mutagenesis potential (Bird, 2002). Induction of ROS and changes in DNA methylation appear to perform a biological role that predominates among the several potential mechanisms of cadmium carcinogenesis that have been proposed. (Chen and others, 1998) According to Takiguchi et al. (2003), cadmium decreases genome methylation while non-competitively blocking DNA methyltransferases. This study raises the possibility that cadmium may interact with the DNA binding domain of the methyltransferase, interfering with the enzyme-DNA interaction . Cadmium can also prevent proto-oncogene DNA methylation, which promotes the production of oncogenes and leads to cell growth (Chen et al., 1998;Eden et al., 2003).

Arsenic
Arsenic is an established carcinogen that lacks carcinogenicity in animal models. Inorganic arsenic is enzymatically methylated for detoxication, using up S-adenosyl-methionine (SAM) in the process. The observation that DNA methyltransferases also require SAM as their methyl donor suggested a role for DNA methylation in arsenic carcinogenesis and other arsenic-related effects . In rat-liver epithelial cell lines treated with chronic low arsenic doses, showed malignant transformation associated with depressed SAM levels, global DNA hypomethylation, and decreased DNA methyltransferase activity . Following these findings, several studies have shown that arsenic is associated with gene-specific hypermethylation (Laird, 2005;Weisenberger et al. 2005), as well as global DNA hypomethylation (Gronniger et al., 2010;Zhu et al., 2012). An unexpected finding was recently reported in vivo, as a global dose-dependent hypermethylation of blood DNA was observed in Bangladeshi adults with chronic arsenic exposure (Deshmukh et al., 2011). This effect was modified by folate, suggesting that arsenic-induced increases in DNA methylation were dependent from methyl availability (Deshmukh et al., 2011). The same group, however, reported that lower blood DNA methylation was a risk factor for arsenic-induced skin lesions in a related Bangladeshi population.
In a human study from India, significant DNA hypermethylation of p53 and p16 promoter regions was observed in blood DNA of subjects exposed to toxic level of arsenic compared to controls (Laird, 2005). In this study, hypermethylation showed a dose-response relationship with arsenic measured in drinking water.
Arsenic toxicity has been recently related to changes in miRNA expression. Marsit et al. showed alterations in miRNA profiles of human lymphoblastoid cells grown under sodium arsenite treatment (Bergman et al., 2015). Interestingly, Arsenic altered expression of specific miRNAs that were involved in one-carbon metabolism (Bergman et al., 2015).

Nickel
The mechanisms underlying nickel health-related effects, including carcinogenicity and cardiorespiratory disease, are still largely unknown. It has been proposed that nickel may replace magnesium in DNA interactions, enhance chromatin condensation, and trigger de novo DNA methylation (Orta et al., 2010). In Chinese hamster G12 cells transfected with E. coli gtp gene, Lee et al. (1995) demonstrated nickel-induced hypermethylation leading to the inactivation of the expression of the transfected gene (Orta et al., 2010). Several studies have shown that nickel affects histone modifications. Exposure to soluble NiCl2 has been shown to reduce histone acetylation, increase demethylation of H3K9, and increase monoubiquitination of H2A and H2B in-vitro (Irizarry et al., 2009). Broday et al. (2000) studied nickel effects, at nontoxic levels, on yeast and mammalian cells and found a decrease in histone H4 acetylation, affecting only lysine 12 in mammalian cells and all of the four H4 lysines in yeasts (Andrews et al., 2012).
Nickel ion exposure has been shown to increase global H3K9 mono-and dimethylation, both of which have been associated with increased DNA methylation and long-term gene silencing. Nickel ions also interfere with the removal of histone methylation in vivo and directly decrease the activity of a Fe(II)-2-oxoglutarate-dependent histone H2K9 demethylase in nuclear extract in vitro (Kafri et al.,199). In human lung cells exposed to soluble nickel compounds, three major changes in histone modifications have been observed: (i) Loss of acetylation of H2A, H2B, H3 and H4; (ii) Increased H3K9 dimethylation; (iii) Increased ubiquitinylation of H2A and H2B (Irizarry et al., 2009). It has been proposed that the binding of Ni 2+ is able to promote a secondary structure with organized side-chain orientation on the N-terminal tail of histone H4. Acetylation of lysine 12 and 16 in yeast exposed to nickel was more robustly affected than lysine 5 and 8. Nickel binding to histidine (Bergman et al., 2015) in histone H4 may be accountable for this effect, acting as an anchoring binding site for metal ions (Kafri et al., 1999). Herbst et al. (1971), Williams (2008) investigated p16 methylation using a methylation-specific PCR method in lung cancer cases associated with chromate exposure and non-chromate lung cancers Deshmukhet al., 2011). A variety of genetic changes in lung cancers from chromate-exposed subjects is known, but the epigenetic effects of chromium are still poorly understood. Kondo et al showed that chromate exposure influenced p16 hypermethylation measured in lung cancer tissues, compared to tissues from non-chromate lung cancer (Herbst et al., 1971;Williams, 2008). Chromium has been shown to reduce in-vitro H3 phosphorilation and trimethylation, as well as various acetylation marks in H3 and H4 (Newbold, 2004).

Methylmercury
Methylmercury is a possible neurotoxin and environmental pollutant that may be found in high concentrations in seafood. When mice are exposed to methylmercury during pregnancy, their behaviors are permanently altered. Brain-derived neurotrophic factor (BDNF) gene expression is epigenetically suppressed in the hippocampus after developmental exposure to low amounts of methylmercury, which predisposes mice to depression (Seto et al., 2009).

Trichloroethylene (TCE), dichloroacetic acid (DCA), and trichloroacetic acid (TCA)
Trichloroethylene (TCE), dichloroacetic acid (DCA), and trichloroacetic acid (TCA) are environmental contaminants that are peroxisome proliferators and carcinogenic in mouse liver. Decreased methylation in the promoter regions of the c-jun and c-myc genes and increased levels of their mRNAs and proteins were found in livers of mice exposed to TCE, DCA, and TCA. Methionine supplement prevented both the decreased methylation and the increased levels of the mRNAs and proteins of the two proto-oncogenes (Edwards and Myers, 200Reik, et al 2001). This work supported the hypothesis that these carcinogens may act by depleting the availability of SAM, whereas methionine would prevent DNA hypomethylation by maintaining adequate SAM levels (Edwards and Myers, 2007;Reik, et al., 2001).

Benzene
In a recent investigation, it was assessed whether DNA methylation changes are induced by low-benzene exposure in peripheral blood DNA of gasoline station attendants and traffic police officers. High-level exposure to benzene has been associated with increased risk of acute myelogenous leukemia (AML) (Sterner and Berger, 2000) which is characterized by aberrant global hypomethylation and gene-specific hypermethylation/hypomethylation. In our study, airborne benzene exposure was associated with a significant reduction in global methylation measured in LINE-1 and Alu. Airborne benzene was also associated with hypermethylation in p15 and hypomethylation of the MAGE-1 cancer-antigen gene (Cress and Seto, 2000). This findings show that low-level benzene exposure may induce altered DNA methylation reproducing the aberrant epigenetic patterns found in malignant cells.

Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX)
RDX is a common environmental pollutant resulting from military and civil activities that has been associated with neurotoxicity, immunotoxicity and increased risk of cancer (Wang et al., 2008). Zhang and Pan evaluated the role of RDX in modifying miRNA expression in mouse liver and brain, as measured by miRNA expression microarrays (Klose and Zhang, 2007;Reik et al., 2001) several miRNAs were found to be differentially expressed in exposed mice, with specific miRNA expression profiles in gene pathways related to cancer, toxicant-metabolizing enzymes, and neurotoxicity (Klose and Zhang, 2007;Reik, et al., 2001).

Potential Roles of Environmental Epigenetic Effects in Determining Trans-Generational Risks and Fetal Origins of Diseases
In animal studies, several chemicals including alloxan (Provost, 2010), cyclophosphamide (Shenet al., 2010), orthoaminoasotoluol (Fabbriet al. 2009), benzopyrene (Garzon and Croce, 2008), diethylstilbestrol (DES) (Olive et al., 2010) and vinclozolin (Marcucci et al., 2009) have been reported to induce transgenerational phenotypic effects. Transgenerational transmission of chemically-induced epigenetic changes have been suggested as a potential mechanisms for these effects. Marcucci et al. (2009) showed that gestational exposure of female rats to the endocrine disruptor vinclozolin at the time of gonadal sex determination caused a variety of abnormalities in the offspring that were then transmitted down the male line for at least three generations. The high incidence of the defects (approximately 90% of all males in all generations) and the absence of abnormalities when passed down the female line suggested gametic epigenetic inheritance. In this study, altered DNA methylation in two candidate genes was seen in sperm from vinclozolinexposed males, and these abnormal methylation patterns were inherited. These results indicate that exposure of germ cells, possibly at a specific developmental stage, is necessary to produce heritable epigenetic changes. In addition, epigenetic mechanisms may underlie the effects of in utero and early life exposures on adult health, as in-utero/early-life exposures to epigenetically-active chemicals may produce health effects later in life even independently of environmental risk factors in adults (Motyckova and Stone, 2010). As reported in the sections above, most of the studies on epigenetic effects of environmental chemicals have shown changes in DNA methylation, histone modifications or microRNA in somatic cells of adult individuals. Whether epigenetic changes observed in somatic cells are correlated with germline epigenetic changes is uncertain. Environmentally-induced epigenetic somatic alterations may be sufficient to cause anomalies in biological functions, but these changes are not heritable perse and may not be associated to any transgenerational risk (Motyckova and Stone, 2010).

Concluding remarks and the way forward
Changes in epigenetic factors have been shown to be induced by exposure to various environmental pollutants, and some of them were linked with different diseases. The following recommendations are important: 1 Individuals or persons working with heavy metals should always wear masks or other safety equipment to prevent direct contamination 2 Local fish advisories should be established in every community to ensure that the fishes and other sea food being consumed are safe for human consumption. 3 People living in homes built before 1978 are advised to hire an expert, for heavy metals testing and detection. 5 Drinking contaminated water of any sort should be avoided. 6 Smoking of cigarettes should be avoided and other dietary sources of heavy metal should also be avoided. 7 Selenium contains compounds that serve as antioxidants and plays an important role in preventing and attenuation of toxic effect in humans