Evaluation of genetic alterations in cancer-related genes in lung and brain tumors from B6C3F1 mice exposed to 1,3-butadiene or chloroprene

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.

Molecular fingerprints of environmental carcinogens in human cancer

Identification of specific molecular changes (fingerprints) is important to identify cancer etiology. Exploitable biomarkers are related to DNA, epigenetics, and proteins. DNA adducts are the turning point between environmental exposures and biological damage. DNA mutational fingerprints are induced by carcinogens in tumor suppressor and oncogenes. In an epigenetic domain, methylation changes occurs in specific genes for arsenic, benzene, chromium, and cigarette smoke. Alteration of specific microRNA has been reported for environmental carcinogens. Benzo(a)pyrene, cadmium, coal, and wood dust hits specific heat-shock proteins and metalloproteases. The multiple analysis of these biomarkers provides information on the carcinogenic mechanisms activated by exposure to environmental carcinogens.

1,3-Butadiene: II. Genotoxicity profile

1,3-Butadiene’s (BD’s) major electrophilic metabolites 1,2-epoxy-3-butene (EB), 1,2-dihydroxy-3,4-epoxybutane (EBD), and 1,2,3,4-diepoxybutane (DEB) are responsible for both its mutagenicity and carcinogenicity. EB, EBD, and DEB are DNA reactive, forming a variety of adducts. All three metabolites are genotoxic in vitro and in vivo, with relative mutagenic potencies of DEB >> EB > EBD. DEB also effectively produces gene deletions and chromosome aberrations. BD’s greater mutagenicity and carcinogenicity in mice over rats as well as its failure to induce chromosome-level mutations in vivo in rats appear to be due to greater production of DEB in mice. Concentrations of EB and DEB in vivo in humans are even lower than in rats. Although most studies of BD-exposed humans have failed to find increases in gene mutations, one group has reported positive findings. Reasons for these discordant results are examined. BD-related chromosome aberrations have never been demonstrated in humans except for the possible production of micronuclei in lymphocytes of workers exposed to extremely high levels of BD in the workplace. The relative potencies of the BD metabolites, their relative abundance in the different species, and the kinds of mutations they can induce are major considerations in BD’s overall genotoxicity profile.

1,3-Butadiene: III. Assessing carcinogenic modes of action

1,3-Butadiene (BD) is a multisite carcinogen in laboratory rodents following lifetime exposure, with greater potency in the mouse than the rat, and is associated with an increase in leukemia mortality in highly exposed workers. Species differences in the formation of reactive metabolites underlie observed species differences in sensitivity to the carcinogenic effects of BD. The modes of action (MOAs) for human leukemia and rodent tumors are both likely related to mutagenic potencies of one or more of these metabolites. However, differences in the nature of genotoxic lesions associated with human leukemia and rodent tumors, along with their implications for risk assessment, require that they be discussed separately. The MOAs for BD are assessed in this review using the modified Hill criteria and human relevance framework. Key events in MOAs for human and rodent cancers are identified, along with important species differences and sources of nonlinearity for each event that can affect extrapolations made from high- to low-dose exposures. Because occupational exposures to BD have also included co-exposures to styrene and dimethyldithiocarbamide (DMDTC), potential interactions with BD carcinogenicity are also discussed. The MOAs for BD carcinogenesis will be used to guide key decisions made in the quantitative cancer dose-response assessment.

A review of the molecular mechanisms of chemically induced neoplasia in rat and mouse models in National Toxicology Program bioassays and their relevance to human cancer

Tumor response in the B6C3F1 mouse, F344 rat, and other animal models following exposure to various compounds provides evidence that people exposed to these or similar compounds may be at risk for developing cancer. Although tumors in rodents and humans are often morphologically similar, underlying mechanisms of tumorigenesis are often unknown and may be different between the species. Therefore, the relevance of an animal tumor response to human health would be better determined if the molecular pathogenesis were understood. The underlying molecular mechanisms leading to carcinogenesis are complex and involve multiple genetic and epigenetic events and other factors. To address the molecular pathogenesis of environmental carcinogens, the authors examine rodent tumors (e.g., lung, colon, mammary gland, skin, brain, mesothelioma) for alterations in cancer genes and epigenetic events that are associated with human cancer. National Toxicology Program (NTP) studies have identified several genetic alterations in chemically induced rodent neoplasms that are important in human cancer. Identification of such alterations in rodent models of chemical carcinogenesis caused by exposure to environmental contaminants, occupational chemicals, and other compounds lends further support that they are of potential human health risk. These studies also emphasize the importance of molecular evaluation of chemically induced rodent tumors for providing greater public health significance for NTP evaluated compounds.

Biomarkers in toxicology and risk assessment: informing critical dose-response relationships

Tremendous advances have been made in the study of biomarkers related to carcinogenesis during the past 20 years. This perspective will briefly review improvements in methodology and instrumentation that have increased our abilities to measure the formation, repair, and consequences of DNA adducts. These biomarkers of exposure, along with surrogates such as protein adducts, have greatly improved our understanding of species differences in metabolism and effects of chemical stability and DNA repair on tissue differences in molecular dose. During this same time frame, improvements in assays for biomarkers of effect have provided better data and an improved understanding of the dose responses for both gene and chromosomal mutations. A framework analysis approach was used to examine the mode of action of genotoxic chemicals and the default assumption that cancer can be expected to be linear at very low doses. This analysis showed that biomarkers of exposure are usually linear at low doses, with the exception being when identical adducts are formed endogenously. Whereas biomarkers of exposure extrapolate down to zero, biomarkers of effect can only be interpolated back to the spontaneous or background number of mutations. The likely explanation for this major difference is that at high exposures, the biology that results in mutagenesis is driven by DNA damage resulting from the chemical exposure. In contrast, at very low exposures, the biology that results in mutagenesis is driven by endogenous DNA damage. The shapes of the dose-response curves for biomarkers of exposure and effect can be very different, with biomarkers of effect better informing quantitative estimates of risk for cancer, a disease that results from multiple mutations. It is also clear, however, that low dose data on mutagenesis are needed for many more chemicals.

International Symposium on the Evaluation of Butadiene and Chloroprene Health Risks

These proceedings represent nearly all the platform and poster presentations given during the International Symposium on Evaluation of Butadiene and Chloroprene Health Risks, held in Charleston, South Carolina, USA, on September 20-22, 2005. The Symposium was attended by 78 participants representing private industry (37), academia (21), government (11), not-for-profit organizations (5), and consulting (4). The program followed the format of previous symposia on butadiene, chloroprene, and isoprene in London UK (2000) and butadiene and isoprene in Blaine, Washington USA (1995). This format enabled the exchange of significant new scientific results and discussion of future research needs. Isoprene was not evaluated during the 2005 Symposium because of lack of new data. For background information, the reader is referred to the proceedings of the London 2000 meeting for a thorough historical perspective and overview of scientific and regulatory issues concerning butadiene, chloroprene, and isoprene [Chem.-Biol. Interact. (2001) 135-136:1-7]. The Symposium consisted of seven sessions: (1) Introduction and Opening Remarks, (2) Butadiene/styrene-butadiene rubber (SBR)--Process Overview, Exposure and Health Effects/Human Studies; (3) Chloroprene--Process Overview, Exposure and Health Effects/Human Studies; (4) Mode of Action/Key Events; (5) Risk Assessment; (6) Poster Presentations; and (7) Panel Discussion and Future Directions. The Symposium concluded with a discussion by all participants of issues that arose throughout the course of the Symposium. The Proceedings of the Symposium published in this Special Issue are organized according to the Sessions outlined above. The purpose of this foreword is to summarize the presentations and their key findings and recommend future research directions for each chemical.