Quantitative Approaches to Assess Key Carcinogenic Events of Genotoxic Carcinogens

Concept for the Evaluation of Carcinogenic Substances in Population-Based Human Biomonitoring

The Human Biomonitoring (HBM) Commission at the German Environment Agency holds the opinion that for environmental carcinogens for which no exposure levels can be assumed and are harmless to health, health-based guidance values corresponding to the classical definition of the HBM-I or HBM-II value cannot be established. Therefore, only reference values have been derived so far for genotoxic carcinogens from exposure data of the general population or subpopulations. The concept presented here opens up the possibility of performing health risk assessments of carcinogenic substances in human biomonitoring, and thus goes decisively beyond the purely descriptive statistical reference value concept. Using the presented method, quantitative dose descriptors of internal exposure can be derived from those of external exposure, provided that sufficient toxicokinetic information is available. Dose descriptors of internal exposure then allow the simple estimate of additional lifetime cancer risks for measured biomarker concentrations or, conversely, of equivalent concentrations for selected risks, such as those considered as tolerable for the general population. HBM data of chronic exposures to genotoxic carcinogens can thus be used to assess the additional lifetime cancer risk referring to the general population and to justify and prioritize risk management measures.

A Review of Stable Isotope Labeling and Mass Spectrometry Methods to Distinguish Exogenous from Endogenous DNA Adducts and Improve Dose-Response Assessments

Cancer remains the second most frequent cause of death in human populations worldwide, which has been reflected in the emphasis placed on management of risk from environmental chemicals considered to be potential human carcinogens. The formation of DNA adducts has been considered as one of the key events of cancer, and persistence and/or failure of repair of these adducts may lead to mutation, thus initiating cancer. Some chemical carcinogens can produce DNA adducts, and DNA adducts have been used as biomarkers of exposure. However, DNA adducts of various types are also produced endogenously in the course of normal metabolism. Since both endogenous physiological processes and exogenous exposure to xenobiotics can cause DNA adducts, the differentiation of the sources of DNA adducts can be highly informative for cancer risk assessment. This review summarizes a highly applicable methodology, termed stable isotope labeling and mass spectrometry (SILMS), that is superior to previous methods, as it not only provides absolute quantitation of DNA adducts but also differentiates the exogenous and endogenous origins of DNA adducts. SILMS uses stable isotope-labeled substances for exposure, followed by DNA adduct measurement with highly sensitive mass spectrometry. Herein, the utilities and advantage of SILMS have been demonstrated by the rich data sets generated over the last two decades in improving the risk assessment of chemicals with DNA adducts being induced by both endogenous and exogenous sources, such as formaldehyde, vinyl acetate, vinyl chloride, and ethylene oxide.

Chitosan and flavonoid glycosides are promising combination partners for enhanced inhibition of heterocyclic amine formation in roast beef

The effects of different kinds of chitosan, oligomer (ChiO) and monomer (Gluco), and the combinations of polymer (Chi) or ChiO with flavonoid aglycones and glycosides against the formation of major HAs were investigated to find out potential combination partners for enhanced suppression of HA formation. Results in roast beef patties showed ChiO and Gluco significantly inhibited PhIP and MeIQx formation by 43-80% and 31-57%, respectively. Of which, ChiO was the most effective. In combinations with flavonoid glycosides (phloridzin, rutin and hesperidzin, respectively), Chi, but not ChiO, generated enhanced inhibitory effects. Further analysis showed Chi and phloridzin combined at a ratio of 1:1 was the most promising, especially in inhibiting PhIP, and the mechanism behind involved: 1) water retention by Chi, and 2) reduction of phenylalanine availability by phloridzin. These findings suggest that appropriate combination of Chi and flavonoid glycosides contributes to significant improvement in the safety of meat products.

The safety evaluation of food flavoring substances: the role of genotoxicity studies

The Flavor and Extract Manufacturers Association (FEMA) Expert Panel relies on the weight of evidence from all available data in the safety evaluation of flavoring substances. This process includes data from genotoxicity studies designed to assess the potential of a chemical agent to react with DNA or otherwise cause changes to DNA, either in vitro or in vivo. The Panel has reviewed a large number of in vitro and in vivo genotoxicity studies during the course of its ongoing safety evaluations of flavorings. The adherence of genotoxicity studies to standardized protocols and guidelines, the biological relevance of the results from those studies, and the human relevance of these studies are all important considerations in assessing whether the results raise specific concerns for genotoxic potential. The Panel evaluates genotoxicity studies not only for evidence of genotoxicity hazard, but also for the probability of risk to the consumer in the context of exposure from their use as flavoring substances. The majority of flavoring substances have given no indication of genotoxic potential in studies evaluated by the FEMA Expert Panel. Examples illustrating the assessment of genotoxicity data for flavoring substances and the consideration of the factors noted above are provided. The weight of evidence approach adopted by the FEMA Expert Panel leads to a rational assessment of risk associated with consumer intake of flavoring substances under the conditions of use.

Rationale and Roadmap for Developing Panels of Hotspot Cancer Driver Gene Mutations as Biomarkers of Cancer Risk

Cancer driver mutations (CDMs) are necessary and causal for carcinogenesis and have advantages as reporters of carcinogenic risk. However, little progress has been made toward developing measurements of CDMs as biomarkers for use in cancer risk assessment. Impediments for using a CDM-based metric to inform cancer risk include the complexity and stochastic nature of carcinogenesis, technical difficulty in quantifying low-frequency CDMs, and lack of established relationships between cancer driver mutant fractions and tumor incidence. Through literature review and database analyses, this review identifies the most promising targets to investigate as biomarkers of cancer risk. Mutational hotspots were discerned within the 20 most mutated genes across the 10 deadliest cancers. Forty genes were identified that encompass 108 mutational hotspot codons overrepresented in the COSMIC database; 424 different mutations within these hotspot codons account for approximately 63,000 tumors and their prevalence across tumor types is described. The review summarizes literature on the prevalence of CDMs in normal tissues and suggests such mutations are direct and indirect substrates for chemical carcinogenesis, which occurs in a spatially stochastic manner. Evidence that hotspot CDMs (hCDMs) frequently occur as tumor subpopulations is presented, indicating COSMIC data may underestimate mutation prevalence. Analyses of online databases show that genes containing hCDMs are enriched in functions related to intercellular communication. In its totality, the review provides a roadmap for the development of tissue-specific, CDM-based biomarkers of carcinogenic potential, comprised of batteries of hCDMs and can be measured by error-correct next-generation sequencing. Environ. Mol. Mutagen. 61:152-175, 2020. Published 2019. This article is a U.S. Government work and is in the public domain in the USA. Environmental and Molecular Mutagenesis published by Wiley Periodicals, Inc. on behalf of Environmental Mutagen Society.

Genotoxic and Epigenotoxic Alterations in the Lung and Liver of Mice Induced by Acrylamide: A 28 Day Drinking Water Study

Acrylamide has been classified as a "Group 2A carcinogen" (probably carcinogenic to humans) by the International Agency for Research on Cancer. The carcinogenicity of acrylamide is attributed to its well-recognized genotoxicity. In the present study, we investigated the effect of acrylamide on epigenetic alterations in mice. Female B6C3F1 mice received acrylamide in drinking water for 28 days, at doses previously used in a 2 year cancer bioassay (0, 0.0875, 0.175, 0.35, and 0.70 mM), and the genotoxic and epigenetic effects were investigated in lungs, a target organ for acrylamide carcinogenicity, and livers, a nontarget organ. Acrylamide exposure resulted in a dose-dependent formation of N7-(2-carbamoyl-2-hydroxyethyl)guanine and N3-(2-carbamoyl-2-hydroxyethyl)adenine in liver and lung DNA. In contrast, the profiles of global epigenetic alterations differed between the two tissues. In the lungs, acrylamide exposure resulted in a decrease of histone H4 lysine 20 trimethylation (H4K20me3), a common epigenetic feature of human cancer, while in the livers, there was increased acetylation of histone H3 lysine 27 (H3K27ac), a gene transcription activating mark. Treatment with 0.70 mM acrylamide also resulted in substantial alterations in the DNA methylation and whole transcriptome in the lungs and livers; however, there were substantial differences in the trends of DNA methylation and gene expression changes between the two tissues. Analysis of differentially expressed genes showed a marked up-regulation of genes and activation of the gene transcription regulation pathway in livers, but not lungs. This corresponded to increased histone H3K27ac and DNA hypomethylation in livers, in contrast to hypermethylation and transcription silencing in lungs. Our results demonstrate that acrylamide induced global epigenetic alterations independent of its genotoxic effects, suggesting that epigenetic events may determine the organ-specific carcinogenicity of acrylamide. Additionally this study provides strong support for the importance of epigenetic alterations, in addition to genotoxic events, in the mechanism of carcinogenesis induced by genotoxic chemical carcinogens.

Perspectives on trace chemical safety and chemophobia: risk communication and risk management

Safety issues regarding consumer products contaminated with trace amounts of chemicals are of great concern to consumers, with the degree of concern occasionally escalating to the psychological syndrome, chemophobia (i.e., the fear of chemicals). Hazardous substances frequently implicated in safety concerns include heavy metals (arsenic, mercury, cadmium, and lead), volatile organic compounds (VOC) such as benzene and o-toluidine, pesticides, carcinogens, radioactive substances, and endocrine disrupting chemicals (EDC) such as bisphenol A and phthalates. To improve communication of risk to society, members of academia, government, consumer organizations, and industry participated in this workshop to discuss and exchange perspectives on trace chemical safety. From the perspective of academia, integrated risk assessments need to be implemented to encompass various exposure sources and routes. The identification and investigation of new exposure-related biomarkers are also recommended to verify direct causal relationships between specific chemical exposure and effects on human health. As for regulation, governments need to establish and maintain acceptable limits for trace chemicals in products. In addition, harmonized efforts need to be undertaken among government agencies to share regulatory limits and effectively control trace chemicals in consumer products. Manufacturers need to faithfully abide by Good Manufacturing Practice (GMP) guidelines, monitor sources of contamination, and minimize these for consumer safety. To effectively resolve safety issues arising from trace chemicals exposure, collaborative efforts are needed involving academia, government, consumer organizations, and industry. Further, scientific evidence-based risk assessment is a critical approach to effectively manage trace chemical safety issues.