Back to topSummary
Environmental and occupational carcinogens such as arsenic and cadmium are implicated in both epigenetic changes and carcinogenesis, but the mechanisms underlying this connection remain unclear. Recent studies elucidate the changes induced by these chemicals and provide potential explanations for how they may lead to cancer, including disruption in expression of homeobox genes, DNA repair genes, and epigenetic regulatory enzymes.
Back to topIntroduction
Epigenetic changes such as differential DNA methylation and altered histone modifications are implicated in many cancers, and a number of toxicants are linked to these changes. For example, the environmental contaminant arsenic is known to cause repression of tumor suppressor genes via methylation and to drive global DNA hypomethylation, a phenomenon frequently associated with cancer. DNA methylation changes also accompany exposure to carcinogenic industrial chemicals such as 1,3-butadiene and drugs that cause cancer in animal models, including phenobarbital (1).
Investigation into the mechanisms underlying carcinogenesis induced by poisons like arsenic and cadmium has led to increased focus on these epigenetic changes. Recent publications offer insight into which genes are targeted as well as the consequences of altered methylation and the relationship between changes observed during in vitro exposure and the methylation patterns in human tumor samples. As understanding of the epigenetic impact of environmental carcinogens increases, so will the potential for identifying epigenetic biomarkers for toxicant exposure that could predict the risk for developing disease.
Back to topToxicant exposure and long-range epigenetic silencing (LRES)
Agglomerative DNA methylation, in which many differentially methylated regions (DMRs) are concentrated within a certain region of a chromosome, has been observed in human breast cancer (3) and can be part of long-range epigenetic silencing (LRES), which is implicated in cancer progression and associated with changes in histone modifications (4). While the role of LRES in cancers continues to gain supporting evidence, which environmental factors trigger this kind of epigenetic change, if any, has been unclear. Recent work by Severson et al. links toxicant exposure with agglomerative DNA methylation, suggesting that this may be one avenue by which these chemicals drive carcinogenesis (5).
Severson's team compared methylation patterns in cancer cell lines and bladder tumor biopsy cells with those of immortalized, non-tumorigenic prostate or ureter cells driven to malignant transformation by arsenite or cadmium. They found that biopsy cells and cancer cell lines shared similar methylation patterns, and toxicant-induced transformation caused immortalized cells to shift toward greater similarity to these cancerous methylation profiles. Hundreds of DMRs, mostly hypermethylated, were present after toxicant transformation.
Transformation by toxicants caused striking agglomerative methylation in the PCDH, HOXC, and HOXD gene clusters. Tumor samples and cancer cell lines also showed hypermethylation at these regions; therefore, as noted by the authors, these may represent common pathways undergoing hypermethylation during the transformation process. Indeed, agglomerative hypermethylation occurs at a HOX gene cluster (HOXA) and protocadherin family clusters in breast cancer (3). Toxicant-induced agglomerative methylation may also be related to histone modifications, as the PCDH, HOXC, and HOXD clusters all showed overlap between the methylation regions and regions enriched for H3K9me3 stem cell domains.
Back to topDNA methylation and expression changes in DNA repair genes after cadmium exposure
Cadmium chloride exposure causes malignant transformation of human bronchial epithelial cells, and a recent study has suggested a potential carcinogenic mechanism involving deficiencies in DNA repair. Zhou et al. observed the methylation changes in six DNA repair genes as well as DNA methyltransferases across epithelial cells representing different stages of the carcinogenic process; these cells were either not exposed to cadmium, exposed for different passage lengths, or isolated from xenograft cadmium-induced tumors.
Over the course of cadmium-induced transformation, global DNA methylation and DNMT activity increased, along with DNMT1 and DNMT3a expression. Moreover, four key DNA repair genes, hMSH2, ERCC1, XRCC1, and hOGG1, were hypermethylated at the promoter and suppressed at the mRNA and protein levels in cadmium-transformed cells compared to controls. Tying these findings together, DNA demethylation treatment partially restored mRNA and protein expression of the DNA repair genes, brought down the elevated levels of DNMTs, and suppressed growth of the cells isolated from tumors and those passaged 35 times with cadmium chloride. Hypermethylation and repression of DNA repair genes, therefore, appears to be an early signature for cadmium-induced cancer and may also constitute part of the mechanism by which the toxicant induces tumorigenesis (6).
Back to topGene expression changes in epigenetic modification enzymes
Because methylation changes are so prevalent following toxicant exposure, changes in the expression of methylation enzymes or other epigenetic regulatory enzymes might be part of the underlying mechanism for toxicant-induced carcinogenesis. As detailed above, DNMT expression increases in response to cadmium chloride. Another study by Treas et al., focusing on prostate epithelial cells, charted expression changes in DNA methylation enzymes and histone modification enzymes after exposure to arsenic, estrogen, or both (7). Previous work has implicated both arsenic and estrogen in prostate cancer. Arsenic can drive malignant transformation and development of androgen independence in prostate epithelial cells, and has also been linked with prostate cancer incidence (8-9). Elevated estrogen, meanwhile, has been shown to initiate carcinogenesis from human prostate epithelia in an in vivo chimera model (10).
The study showed that both normal (100 times below the US Environmental Protection Agency's standards for arsenic, and approximately male physiological concentration for estrogen) and elevated levels of arsenic and estrogen synergistically suppressed several methylation-related genes, including DNMT1, MeCP2, and MBD4, and enhanced DNMT3a expression. Single exposure caused alterations as well, but the changes were stronger when both were administered. Accordingly, DNA hypo- and hypermethylation was observed in various genomic regions upon dual exposure.
Histone modifications and associated genes also showed changes when cells were treated with arsenic, estrogen, or both. Acetylation was impacted by arsenic, as well as arsenic plus estrogen; these treatments led to greater H3 acetylation and diminished expression of HDACs. Histone methylation responded to estrogen and estrogen plus arsenic treatment; H3K4me3 levels increased. However, an unrelated histone methyltransferase, HMT1, showed diminished expression with all treatments. These results suggest that histone modification is a significant path by which arsenic and estrogen affect prostate cell behavior, as the team also saw enhanced growth and ERβ expression after co-treatment (7).
Back to topConclusions
The epigenetic effects induced by environmental contaminants are implicated by these studies and others to have a significant role in tumorigenesis and cancer progression, even at low doses. Early effects, such as those induced by cadmium on DNA repair genes, could potentially be used as biomarkers for exposure and risk of developing cancer. Continuing to monitor the methylation state of genes in response to toxicants, as well as its effects via gene expression studies, will be important in deciphering the contributions of these chemicals to disease.
Back to topReferences
- Pogrinby, I.P. and Rusyn, I. (2013) Environmental toxicants, epigenetics, and cancer. Adv. Exp. Med. Biol. 754, 215.
- Martinez-Zamudio, R. and Ha, H.C. (2011) Environmental epigenetics in metal exposure. Epigenetics 6, 820.
- Novak, P., Jensen, T., Oshiro, M.M., Watts, G.S., Kim, C.J., and Futscher, B.W. (2008) Agglomerative epigenetic aberrations are a common event in human breast cancer. Cancer Res. 68, 8616.
- Coolen, M.W. et al. (2010) Consolidation of the cancer genome into domains of repressive chromatin by long-range epigenetic silencing (LRES) reduces transcriptional plasticity. Nat. Cell Biol. 12, 235.
- Severson, P.L., Tokar, E.J., Vrba, L., Waalkes, M.P., and Futscher, B.W. (2012) Agglomerates of aberrant DNA methylation are associated with toxicant-induced malignant transformation. Epigenetics 7, 1238.
- Zhou, Z., Lei, Y., and Wang, C. (2012) Analysis of aberrant methylation in DNA repair genes during malignant transformation of human bronchial epithelial cells induced by cadmium. Toxicol. Sci. 125, 412.
- Treas, J.N., Tyagi, T., and Singh, K.P. (2012) Effects of chronic exposure to arsenic and estrogen on epigenetic regulatory genes expression and epigenetic code in human prostate epithelial cells. PLoS One.
- Benbrahim-Tallaa, L., Webber, M.M., and Waalkes, M.P. (2005) Acquisition of androgen independence by human prostate epithelial cells during arsenic-induced malignant transformation. Environ. Health Perspect. 113, 1134.
- Benbrahim-Tallaa, L. and Waalkes, M.P. (2008) Inorganic arsenic and human prostate cancer. Environ. Health Perspect. 116, 158.
- Hu, W., Shi, G., Hu, D., Nelles, J.L, and Prins, G.S. (2012) Actions of estrogens and endocrine disrupting chemicals on human prostate stem/progenitor cells and prostate cancer risk. 354, 63.