Influence of DNA repair on mutation spectra in Salmonella

Hazardous gas emissions from drop-in biofuels: mutagenicity, cytotoxicity, and unregulated pollutants

This study investigates cancer-related mutations (TA98 and YG5185 strains/Ames test), cell death (human A549 cell line/MTT assay) and unregulated pollutants (16 PAH, 13 carbonyls) from the gas exhaust emissions from a last-mile delivery vehicle following the WLTC driving cycle, operating with hydrotreated vegetable oil and biodiesel. Both biofuels were used pure and blended 20 % by volume with diesel fuel. Gas phase samples were collected using XAD-2 Amberlite® resin. Total carbonyl emission factors for the different fuels ranged from 9.4 ± 0.4 (HVO100) to 14.8 ± 1.6 mg/km (B20), while PAH emission factors ranged from 1.8 ± 0.5 (B100) to 4.3 ± 0.9 mg/km (HVO20). The ester group in biodiesel demonstrated a significant impact on increasing carbonyl emissions. All fuels were cytotoxic at the highest concentration of exhaust gases, causing more than 30 % cell death in human cell line A549 (HVO100 ≈ HVO20 > B100 ≈ B20 > ULSD). No significant correlation was found between cytotoxicity and most of PAH and carbonyls. A strong correlation between PAH and mutagenicity (Pearson correlation coefficient higher than 0.6 for PAH with 3 or more rings) was observed with strain YG5185, particularly when using the metabolic activator. These results indicate that the exhaust gases from the tested biofuels pose potential health risks, particularly in chronic exposure scenarios.

Lung cancer associated with combustion particles and fine particulate matter (PM2.5) - The roles of polycyclic aromatic hydrocarbons (PAHs) and the aryl hydrocarbon receptor (AhR)

Air pollution is the leading cause of lung cancer after tobacco smoking, contributing to 20% of all lung cancer deaths. Increased risk associated with living near trafficked roads, occupational exposure to diesel exhaust, indoor coal combustion and cigarette smoking, suggest that combustion components in ambient fine particulate matter (PM2.5), such as polycyclic aromatic hydrocarbons (PAHs), may be central drivers of lung cancer. Activation of the aryl hydrocarbon receptor (AhR) induces expression of xenobiotic-metabolizing enzymes (XMEs) and increase PAH metabolism, formation of reactive metabolites, oxidative stress, DNA damage and mutagenesis. Lung cancer tissues from smokers and workers exposed to high combustion PM levels contain mutagenic signatures derived from PAHs. However, recent findings suggest that ambient air PM2.5 exposure primarily induces lung cancer development through tumor promotion of cells harboring naturally acquired oncogenic mutations, thus lacking typical PAH-induced mutations. On this background, we discuss the role of AhR and PAHs in lung cancer development caused by air pollution focusing on the tumor promoting properties including metabolism, immune system, cell proliferation and survival, tumor microenvironment, cell-to-cell communication, tumor growth and metastasis. We suggest that the dichotomy in lung cancer patterns observed between smoking and outdoor air PM2.5 represent the two ends of a dose-response continuum of combustion PM exposure, where tumor promotion in the peripheral lung appears to be the driving factor at the relatively low-dose exposures from ambient air PM2.5, whereas genotoxicity in the central airways becomes increasingly more important at the higher combustion PM levels encountered through smoking and occupational exposure.

Analysis of chemical structures and mutations detected by Salmonella TA98 and TA100

As part of an analysis performed under the auspices of the International Workshop on Genotoxicity Testing (IWGT) in 2017, we and others showed that Salmonella frameshift strain TA98 and base-substitution strain TA100 together + /- S9 detected 93% of the mutagens detected by all the bacterial strains recommended by OECD TG471 (Williams et al., Mutation Res. 848:503081, 2019). We have extended this analysis by identifying the numbers and chemical classes of chemicals detected by these two strains either alone or in combination, including the role of S9. Using the Leadscope 2021 SAR Genetox database containing > 21,900 compounds, our dataset containing 7170 compounds tested in both TA98 and TA100. Together, TA98 and TA100 detected 94% (3733/3981) of the mutagens detected using all the TG471-recommended bacterial strains; 39% were mutagenic in one or both strains. TA100 detected 77% of all of these mutagens and TA98 70%. Considering the overlap of detection by both strains, 12% of these mutagens were detected only by TA98 and 19% only by TA100. In the absence of S9, sensitivity dropped by 31% for TA98 and 29% for TA100. Overall, 32% of the mutagens required S9 for detection by either strain; 9% were detected only without S9. Using the 2021 Leadscope Genetox Expert Alerts, TA100 detected 18 mutagenic alerting chemical classes with better sensitivity than TA98, whereas TA98 detected 10 classes better than TA100. TA100 detected more chemical classes than did TA98, especially hydrazines, azides, various di- and tri-halides, various nitrosamines, epoxides, aziridines, difurans, and half-mustards; TA98 especially detected polycyclic primary amines, various aromatic amines, polycyclic aromatic hydrocarbons, triazines, and dibenzo-furans. Model compounds with these structures induce primarily G to T mutations in TA100 and/or a hotspot GC deletion in TA98. Both TA98 and TA100 + /- S9 are needed for adequate mutagenicity screening with the Salmonella (Ames) assay.

Mutagenicity and carcinogenicity of combustion emissions are impacted more by combustor technology than by fuel composition: A brief review

Studies during the past 50 years have characterized the carcinogenicity and mutagenicity of extractable organic material (EOM) of particulate matter (PM) in ambient air and from combustion emissions. We have summarized conclusions from these studies and present data supporting those conclusions for 50 combustion emissions, including carcinogenic potencies on mouse skin (papillomas/mouse/mg EOM), mutagenic potencies (revertants/μg EOM) in the Salmonella (Ames) mutagenicity assay, and mutagenicity emission factors (revertants/kg fuel or revertants/MJthermal ) in Salmonella. Mutagenic potencies of EOM from PM in ambient air and combustion emissions span 1-2 orders of magnitude, respectively. In contrast, the revertants/m3 span >5 orders of magnitude due to variable PM concentrations in ambient air. Carcinogenic potencies of EOM from combustion emissions on mouse skin and EOM-associated human lung cancer risk from those emissions both span ~3 orders of magnitude and are highly associated. The ubiquitous presence of polycyclic aromatic hydrocarbons (PAHs), nitroarenes, and aromatic amines results in mutagenic and carcinogenic potencies of PM that span only 1-3 orders of magnitude; most PM induces primarily G to T mutations. Mutagenicity emission factors of combustion emissions span 3-5 orders of magnitude and correlate with PAH emission factors (r > 0.9). Mutagenicity emission factors were largely a function of how material was burned (highly efficient modern combustors versus open burning) rather than what materials were burned. Combustion systems that minimize kinetic and mass-transfer limitations and promote complete oxidation also minimize the mutagenicity of their emissions. This fundamental engineering principle can inform environmental and public health assessments of combustion emissions.

Mechanistic understanding of cellular responses to genomic stress

Within the past half century we have learned of multiple pathways for repairing damaged DNA, based upon the intrinsic redundancy of information in its complementary double strands. Mechanistic details of these pathways have provided insights into environmental and endogenous threats to genomic stability. Studies on bacterial responses to ultraviolet light led to the discovery of excision repair, as well as the inducible SOS response to DNA damage. Similar responses in eukaryotes promote upregulation of error-prone translesion DNA polymerases. Recent advances in this burgeoning field include duplex DNA sequencing to provide strikingly accurate profiling of mutational signatures, analyses of gene expression patterns in single cells, CRISPR/Cas9 to generate changes at precise genomic positions, novel roles for RNA in gene expression and DNA repair, phase-separated aqueous environments for specialized cellular transactions, and DNA lesions as epigenetic signals for gene expression. The Environmental Mutagenesis and Genomics Society (EMGS), through the broad range of expertise in its membership, stands at the crossroad of basic understanding of mechanisms for genomic maintenance and the field of genetic toxicology, with the need for regulation of exposures to toxic substances. Our future challenges include devising strategies and technologies to identify individuals who are susceptible to specific genomic stresses, along with basic research on the underlying mechanisms of cellular stress responses that promote disease-causing mutations. As the science moves forward it should also be a responsibility for the EMGS to expand its outreach programs for the enlightenment and benefit of all humans and the biosphere. Environ. Mol. Mutagen. 61:25-33, 2020. © 2019 Wiley Periodicals, Inc.

Oxidative stress, DNA damage, and mutagenicity induced by the extractable organic matter of airborne particulates on bacterial models

The biological activity induced by the extractable organic matter (EOM) of size-segregated airborne Particulate Matter (PM) from two urban sites, urban traffic (UT) and urban background (UB), was assessed by using bacterial assays. The Gram-negative Escherichia coli (E. coli) coliform bacterium was used to measure the intracellular formation of Reactive Oxygen Species (ROS) by employing the Nitroblue tetrazolium (NBT) reduction assay and the lipid peroxidation by malondialdehyde (MDA) measurement. To the best of our knowledge, this is the first study using E. coli for assessing the bioactivity of ambient air in term of oxidative mechanism studies. E. coli BL21 cells were further used for DNA damage assessment by employing the reporter (β-galactosidase) gene expression assay. The bacterial strain S. typhimurium TA100 was used to assess the mutagenic potential of PM by employing the well-known mutation assay (Ames test). Four PM size fractions were assessed for bioactivity, specifically the quasi-ultrafine mode (<0.49 μm), the upper accumulation mode (0.49-0.97 μm), the upper fine mode (0.97-3 μm), and the coarse mode (>3.0 μm). The EOM of each PM sample included three organic fractions of successively increased polarity: the non-polar organic fraction (NPOF), the moderately polar organic fraction (MPOF), and the polar organic fraction (POF). The toxicological endpoints induced by each organic fraction were correlated with the concentrations of various organic chemical components determined in previous studies in an attempt to identify the chemical classes involved.

Mutational spectra and mutational signatures: Insights into cancer aetiology and mechanisms of DNA damage and repair

Reporter gene assays, in which a single mutation from each experiment can contribute to the assembly of a mutation spectrum for an agent, have provided the basis for understanding the mutational processes induced by mutagenic agents and for providing clues to the origins of mutations in human tumours. More recently exome and whole genome sequencing of human tumours has revealed distinct patterns of mutation that could provide additional clues for the causative origins of cancer. This can be tested by examining the mutational signatures induced in experimental systems by putative cancer-causing agents. Such signatures are now being generated in vitro in a number of different mutagen-exposed cellular systems. Results reveal that mutagens induce characteristic mutation signatures that, in some cases, match signatures found in human tumours. Proof of principle has been established with mutational signatures generated by simulated sunlight and aristolochic acid, which match those signatures found in human melanomas and urothelial cancers, respectively. In an analysis of somatic mutations in cancers for which tobacco smoking confers an elevated risk, it was found that smoking is associated with increased mutation burdens of multiple different mutational signatures, which contribute to different extents in different tissues. One of these signatures, mainly found in tissues directly exposed to tobacco smoke, is attributable to misreplication of DNA damage caused by tobacco carcinogens. Others likely reflect indirect activation of DNA editing by APOBEC cytidine deaminases and of an endogenous clock-like mutational process. The results are consistent with the proposition that smoking increases cancer risk by increasing the somatic mutation load although direct evidence for this mechanism is lacking in some cancer types. Thus, next generation sequencing of exomes or whole genomes is providing new insights into processes underlying the causes of human cancer.

Carcinogenicity of ethylene oxide: key findings and scientific issues

In support of the Integrated Risk Information System (IRIS), the U.S. Environmental Protection Agency (EPA) completed an evaluation of the inhalation carcinogenicity of ethylene oxide (EtO) in December 2016. This article reviews key findings and scientific issues regarding the carcinogenicity of EtO in EPA's Carcinogenicity Assessment. EPA's assessment critically reviewed and characterized epidemiologic, laboratory animal, and mechanistic studies pertaining to the human carcinogenicity of EtO, and addressed some key scientific issues such as the analysis of mechanistic data as part of the cancer hazard evaluation and to inform the quantitative risk assessment. The weight of evidence from the epidemiologic, laboratory animal, and mechanistic studies supports a conclusion that EtO is carcinogenic in humans, with the strongest human evidence linking EtO exposure to lymphoid and breast cancers. Analyses of the mechanistic data establish a key role for genotoxicity and mutagenicity in EtO-induced carcinogenicity and reveal little evidence supporting other mode-of-action hypotheses. In conclusion, EtO was found to be carcinogenic to humans by inhalation, posing a potential human health hazard for lymphoid and breast cancers.

Evaluation of an Air Quality Health Index for Predicting the Mutagenicity of Simulated Atmospheres

No study has evaluated the mutagenicity of atmospheres with a calculated air quality health index (AQHI). Thus, we generated in a UV-light-containing reaction chamber two simulated atmospheres (SAs) with similar AQHIs but different proportions of criteria pollutants and evaluated them for mutagenicity in three Salmonella strains at the air-agar interface. We continuously injected into the chamber gasoline, nitric oxide, and ammonium sulfate, as well as either α-pinene to produce SA-PM, which had a high concentration of particulate matter (PM): 119 ppb ozone (O3), 321 ppb NO2, and 1007 μg/m3 PM2.5; or isoprene to produce SA-O3, which had a high ozone (O3) concentration: 415 ppb O3, 633 ppb NO2, and 55 μg/m3 PM2.5. Neither PM2.5 extracts, NO2, or O3 alone, nor nonphoto-oxidized mixtures were mutagenic or cytotoxic. Both photo-oxidized atmospheres were largely direct-acting base-substitution mutagens with similar mutagenic potencies in TA100 and TA104. The mutagenic potencies [(revertants/h)/(mgC/m3)] of SA-PM (4.3 ± 0.4) and SA-O3 (9.5 ± 1.3) in TA100 were significantly different ( P < 0.0001), but the mutation spectra were not ( P = 0.16), being ∼54% C → T and ∼46% C → A. Thus, the AQHI may have some predictive value for the mutagenicity of the gas phase of air.

Recommendations for the evaluation of complex genetic toxicity data sets when assessing carcinogenic risks to humans

Genotoxicity testing plays an important role in the assessment of carcinogenic and heritable risks. In many cases, experts charged with assessing genotoxicity test results need to evaluate widely varying numbers and types of bioassays of differing quality, conducted in a variety of cells and species using a wide range of protocols. The recommendations in this article were initially prepared as general guidelines to assist experts involved in the 2016 Joint Food and Agricultural Organization and World Health Organization Meeting on Pesticide Residues (JMPR) in their evaluation of the human health risks associated with exposure to pesticide residues in the diet. A weight of evidence approach is recommended in which studies are evaluated based on quality, reproducibility and consistency, significance of the genetic alteration, phylogenetic relevance to humans, type (in vivo vs. in vitro), and relevance of the route of administration. Using the recommended approach, the most weight will generally be given to high quality in vivo studies of gene and chromosome mutations (including aberrations) in humans or mammals exposed to the chemical through a physiologically relevant route such as oral or dermal administration or by inhalation. The guidelines are intended to give reviewers flexibility in evaluating all relevant scientific information, and allow them to use their best scientific judgment in reaching conclusions about the significance of the genotoxicity results. The use of these guidelines and the associated weighting considerations should facilitate the evaluation of complex and sometimes contradictory data sets, and provide more consistency in evaluations across risk assessments. Environ. Mol. Mutagen. 58:380-385, 2017. © 2017 Wiley Periodicals, Inc.

Antimutagenic and antioxidant activities of some bioflavours from wine

Monoterpenes limonene and its metabolic derivatives, α-terpineol and 1,8-cineol, commonly found as aroma wine components, were studied for their antimutagenicity by the bacterial reverse mutation assay on different strains. Substances were also tested for their antioxidant activity, i.e. radical scavenger, chelation, reduction, and lipid peroxidation inhibition. Limonene and its metabolites, α-terpineol and 1,8-cineol, resulted able to inhibit the chemically-induced mutagenesis, although with a different specificity. The antimutagenicity of limonene has been generally retained by its metabolites and sometimes increased. In particular, α-terpineol exhibited the strongest inhibition, moreover it showed to be a remarkable ferrous ions chelating agent. Limonene and 1,8-cineol were devoid of antioxidant activity. Present results are a starting point in evaluating the potential of α-terpineol as a chemopreventive agent and suggest potential functional dietary benefits of wine.

Association between mutation spectra and stable and unstable DNA adduct profiles in Salmonella for benzo[a]pyrene and dibenzo[a,l]pyrene

Benzo[a]pyrene (BP) and dibenzo[a,l]pyrene (DBP) are two polycyclic aromatic hydrocarbons (PAHs) that exhibit distinctly different mutagenicity and carcinogenicity profiles. Although some studies show that these PAHs produce unstable DNA adducts, conflicting data and arguments have been presented regarding the relative roles of these unstable adducts versus stable adducts, as well as oxidative damage, in the mutagenesis and tumor-mutation spectra of these PAHs. However, no study has determined the mutation spectra along with the stable and unstable DNA adducts in the same system with both PAHs. Thus, we determined the mutagenic potencies and mutation spectra of BP and DBP in strains TA98, TA100 and TA104 of Salmonella, and we also measured the levels of abasic sites (aldehydic-site assay) and characterized the stable DNA adducts ((32)P-postlabeling/HPLC) induced by these PAHs in TA104. Our results for the mutation spectra and site specificity of stable adducts were consistent with those from other systems, showing that DBP was more mutagenic than BP in TA98 and TA100. The mutation spectra of DBP and BP were significantly different in TA98 and TA104, with 24% of the mutations induced by BP in TA98 being complex frameshifts, whereas DBP produced hardly any of these mutations. In TA104, BP produced primarily GC to TA transversions, whereas DBP produced primarily AT to TA transversions. The majority (96%) of stable adducts induced by BP were at guanine, whereas the majority (80%) induced by DBP were at adenine. Although BP induced abasic sites, DBP did not. Most importantly, the proportion of mutations induced by DBP at adenine and guanine paralleled the proportion of stable DNA adducts induced by DBP at adenine and guanine; however, this was not the case for BP. Our results leave open a possible role for unstable DNA adducts in the mutational specificity of BP but not for DBP.

Genetic toxicology in the 21st century: reflections and future directions

A symposium at the 40th anniversary of the Environmental Mutagen Society, held from October 24-28, 2009 in St. Louis, MO, surveyed the current status and future directions of genetic toxicology. This article summarizes the presentations and provides a perspective on the future. An abbreviated history is presented, highlighting the current standard battery of genotoxicity assays and persistent challenges. Application of computational toxicology to safety testing within a regulatory setting is discussed as a means for reducing the need for animal testing and human clinical trials, and current approaches and applications of in silico genotoxicity screening approaches across the pharmaceutical industry were surveyed and are reported here. The expanded use of toxicogenomics to illuminate mechanisms and bridge genotoxicity and carcinogenicity, and new public efforts to use high-throughput screening technologies to address lack of toxicity evaluation for the backlog of thousands of industrial chemicals in the environment are detailed. The Tox21 project involves coordinated efforts of four U.S. Government regulatory/research entities to use new and innovative assays to characterize key steps in toxicity pathways, including genotoxic and nongenotoxic mechanisms for carcinogenesis. Progress to date, highlighting preliminary test results from the National Toxicology Program is summarized. Finally, an overview is presented of ToxCast™, a related research program of the U.S. Environmental Protection Agency, using a broad array of high throughput and high content technologies for toxicity profiling of environmental chemicals, and computational toxicology modeling. Progress and challenges, including the pressing need to incorporate metabolic activation capability, are summarized.

Assessment of heritable genetic effects using new genetic tools and sentinels in an era of personalized medicine

The challenge of estimating human health effects from damage to the germ line may be met in the genomic era. Understanding the genetic, as opposed to postconception developmental basis of birth defects is critical to their use in monitoring heritable genetic damage. The causes of common birth defects are analyzed here: mendelian genetic, multigenic, developmental, inherited, or combinational. Only a small fraction of these (noninherited, mendelian genetic) are likely to be informative relative to germ cell mutagenesis, and these won't be discernible against the general background of birth defects. Targeted genetic testing as part of personalized medicine could be integrated into a strategy for assessing germ cell alterations in populations. Thus, "sentinel mutations," as originally proposed by Mulvihill and Ceizel, need not be restricted to X-linked or dominant mutations or conditions visible at birth. Several new sentinels related to personalized medicine are proposed, based on health impact (likelihood of monitoring), frequency, and genetic target suitability (responsiveness to diverse mutational mechanisms). Candidates could include CYP genes (related to metabolism of xenobiotics) important in optimizing drug doses and avoiding adverse reactions. High frequency LDLR mutations (related to familial high cholesterol) predict myocardial infarction in approximately50% of individuals. The more common recessive genetic diseases (cystic fibrosis, phenylketonuria, and others) monitored in newborn screening programs could be informative given parental analysis. New opportunities for genetic analyses need to be coupled with epidemiological studies on environmental exposures. These could focus on adverse outcomes related to tobacco, the mostubiquitous and potent environmental mutagen.

Assessing the cytotoxic and mutagenic effects of secondary metabolites produced by several fungal biological control agents with the Ames assay and the VITOTOX(®) test

The potential genotoxic effects of several pure secondary metabolites produced by fungi used as biological control agents (BCAs) were studied with the Ames Salmonella/microsome mutagenicity assay and the Vitotox test, with and without metabolic activation. A complete set of Salmonella tester strains was used to avoid false negative results. To detect possible mutagenic and/or cytotoxic effects of fungal secondary metabolites due to synergistic action, crude extracts and fungal cell extracts of the BCAs were also examined. Although the sensitivity of the methods varied depending on the metabolite used, clearly no genotoxicity was observed in all cases. The results of the two assays are discussed in the light of being used in a complementary fashion for a convincing risk-assessment evaluation of fungal BCAs and their secondary metabolites.

Umbuzeiro G, DeMarini DM. The Salmonella mutagenicity assay: the stethoscope of genetic toxicology for the 21st century

OBJECTIVES

According to the 2007 National Research Council report Toxicology for the Twenty-First Century, modern methods (e.g., "omics," in vitro assays, high-throughput testing, computational methods) will lead to the emergence of a new approach to toxicology. The Salmonella mammalian microsome mutagenicity assay has been central to the field of genetic toxicology since the 1970s. Here we document the paradigm shifts engendered by the assay, the validation and applications of the assay, and how the assay is a model for future in vitro toxicology assays.

DATA SOURCES

We searched PubMed, Scopus, and Web of Knowledge using key words relevant to the Salmonella assay and additional genotoxicity assays.

DATA EXTRACTION

We merged the citations, removing duplicates, and categorized the papers by year and topic.

DATA SYNTHESIS

The Salmonella assay led to two paradigm shifts: that some carcinogens were mutagens and that some environmental samples (e.g., air, water, soil, food, combustion emissions) were mutagenic. Although there are > 10,000 publications on the Salmonella assay, covering tens of thousands of agents, data on even more agents probably exist in unpublished form, largely as proprietary studies by industry. The Salmonella assay is a model for the development of 21st century in vitro toxicology assays in terms of the establishment of standard procedures, ability to test various agents, transferability across laboratories, validation and testing, and structure-activity analysis.

CONCLUSIONS

Similar to a stethoscope as a first-line, inexpensive tool in medicine, the Salmonella assay can serve a similar, indispensable role in the foreseeable future of 21st century toxicology.

Antimutagenic activity of a secoisopimarane diterpenoid from Salvia cinnabarina M. Martens et Galeotti in the bacterial reverse mutation assay

The effects of 3,4-secoisopimar-4(18),7,15-trien-3-oic acid, a diterpenoid isolated from Salvia cinnabarina, were evaluated in the Ames test on Salmonella typhimurium TA98 and TA100 and on Escherichia coli WP2uvrA, in presence and in absence of the metabolic activation system. The secoisopimarane diterpenoid not only showed to be devoid of mutagenic activity, but significantly inhibited the effect of some known mutagens, in all strains tested. The reduction of the number of chemically-induced revertant colonies reached the value of 92.2% against 2-aminoanthracene, 59.6% against 2-nitrofluorene, 50.9% against sodium azide and 39.9% against methyl methane sulfonate. It is hypothesized that the secoisopimarane diterpenoid acts by aspecific mechanisms, by alterating the cell permeability thus blocking the mutagen adsorption across the bacterial membrane, or by chemical or enzymatic inhibition of the mutagens.

Induction of abasic sites by the drinking-water mutagen MX in Salmonella TA100

Mutagen X (MX) is a chlorinated furanone that accounts for more of the mutagenic activity of drinking water than any other disinfection by-product. It is one of the most potent base-substitution mutagens in the Salmonella (Ames) mutagenicity assay, producing primarily GC to TA mutations in TA100. MX does not produce stable DNA adducts in cellular or acellular DNA. However, theoretical calculations predict that it might induce abasic sites, which it does in supercoiled plasmid DNA but not in rodents. To investigate the ability of MX to induce abasic sites in cellular DNA, we used an aldehydic site assay to detect abasic sites in DNA from Salmonella TA100 cells treated for 1.5 h with MX. At 0, 2.3, and 4.6 microM, MX induced mutant frequencies (revertants/10(6) survivors) and percent survivals of 2 (100%), 14.9 (111%), and 59.3 (45%), respectively. The frequencies of abasic sites (sites/10(5) nucleotides) for the control and two concentrations were 5.9, 6.2, and 9.7, respectively, with the frequency at the highest concentration being significant (P<0.001). These results provide some evidence for the ability of MX to induce abasic sites in cellular DNA. However, the lack of a dose response makes it unclear whether this DNA damage underlies the mutagenic activity of MX.

Enhanced mutagenesis of Salmonella tester strains due to deletion of genes other than uvrB

The standard Salmonella mutagenicity (Ames) tester strains are missing 15-119 genes due to the extended Delta(gal-bio-uvrB) mutations that render the strains excision-repair deficient (DeltauvrB). We constructed strains of Salmonella that are homologous to tester strains TA98 and TA100 except that in place of the uvrB deletion, they contain single-gene defects in either uvrB, moaA, moeA, or both uvrB and moeA. We then tested the following mutagens in these strains: 2-acetylaminofluorene, Glu-P-1, 4-aminobiphenyl, benzo[a]pyrene, MX, 1-nitropyrene, 6-hydroxylaminopurine (HAP), and 2-amino-6-hydroxylaminopurine (AHAP). We confirmed in Salmonella a previous finding in Escherichia coli that the enhanced mutagenicity of the purine analogues HAP and AHAP is not due to the deletion of the uvrB gene but due to the deletion of moeA and/or moaA, which are involved in molybdenum cofactor biosynthesis. The spontaneous mutant frequency and induced mutagenic potency of mutagens due to the extended DeltauvrB mutation are due largely to the deletion of uvrB and to some extent of moeA/moaA at the frameshift hisD3052 allele of TA98 but involve other genes in addition to uvrB and moeA/moaA at the base-substitution hisG46 allele of TA100. The extended DeltauvrB mutation does not prevent the detection of mutagens that would have been detected in a strain containing a single uvrB defect. Because of the deletion of moeA/moaA, the extended uvrB deletion generally enhanced spontaneous and induced mutagenicity, especially at the base-substitution allele. This enhanced sensitivity may underlay the severe health effects in humans who have mutations in molybdenum cofactor biosynthesis genes.

New approaches to assessing the effects of mutagenic agents on the integrity of the human genome

Heritable genetic alterations, although individually rare, have a substantial collective health impact. Approximately 20% of these are new mutations of unknown cause. Assessment of the effect of exposures to DNA damaging agents, i.e. mutagenic chemicals and radiations, on the integrity of the human genome and on the occurrence of genetic disease remains a daunting challenge. Recent insights may explain why previous examination of human exposures to ionizing radiation, as in Hiroshima and Nagasaki, failed to reveal heritable genetic effects. New opportunities to assess the heritable genetic damaging effects of environmental mutagens are afforded by: (1) integration of knowledge on the molecular nature of genetic disorders and the molecular effects of mutagens; (2) the development of more practical assays for germline mutagenesis; (3) the likely use of population-based genetic screening in personalized medicine.

A model for targeted substitution mutagenesis during SOS replication of double-stranded DNA containing cis-syn cyclobutane thymine dimers

A model for ultraviolet mutagenesis is described that is based on the formation of rare tautomeric bases in pyrimidine dimers. It is shown that during SOS synthesis the modified DNA-polymerase inserts canonical bases opposite the dimers; the inserted bases are capable of forming hydrogen bonds with bases in the template DNA. SOS-replication of double-stranded DNA having thymine dimers, with one or both bases in a rare tautomeric conformation, results in targeted transitions, transversions, or one-nucleotide gaps. Structural analysis indicates that one type of dimer containing a single tautomeric base (TT*(1), with the "*" indicating a rare tautomeric base and the subscript referring to the particular conformation) can cause A:T --> G:C transition or homologous A:T --> T:A transversion, while another dimer (TT*(2)) can cause a one-nucleotide gap. The dimers containing T*(4) result in A:T --> C:G transversion, while TT*(5) dimers can cause A:T --> C:G transversion or homologous A:T --> T:A transversion. If both bases in the dimer are in a rare tautomeric form, then tandem mutations or double-nucleotide gaps can be formed. The dimers containing the rare tautomeric forms T*'(1) , T*'(2), T*'(3), T*'(4), and T*'(5) may not result in mutations. The question of whether dimers containing T*'(4) and T*'(5) result in mutations requires further investigation.

Mutation spectra of smoky coal combustion emissions in Salmonella reflect the TP53 and KRAS mutations in lung tumors from smoky coal-exposed individuals

Nonsmoking women in Xuan Wei County, Yunnan Province, China who use smoky coal for cooking and heating in poorly ventilated homes have the highest lung cancer mortality rate in China, and their lung cancer is linked epidemiologically to their use of smoky coal. The emissions contain 81% organic matter, of which 43% is polycyclic aromatic hydrocarbons (PAHs). Exposure assessment and molecular analysis of the lung tumors from nonsmoking women who use smoky coal strongly indicate that PAHs in the emissions are a primary cause of the elevated lung cancer in this population. Here we have determined the mutation spectra of an extract of smoky coal emissions in Salmonella TA98 and TA100; the extract was not mutagenic in TA104. The extract was 8.7 x more mutagenic in TA100 with S9 than without (8.7 rev/microg versus 1.0 rev/microg) and was >3 x more mutagenic in TA100 than in TA98--consistent with a prominent role for PAHs in the mutagenicity of the extract because PAHs are generally more mutagenic in the base-substitution strain TA100 than in the frameshift strain TA98. The extract induced only a hotspot mutation in TA98; another combustion emission, cigarette smoke condensate (CSC), also induces this single class of mutation. In TA100, the mutation spectra of the extract were not significantly different in the presence or absence of S9 and were primarily (78-86%) GC --> TA transversions. This mutation is induced to a similar extent by CSC (78%) and the PAH benzo[a]pyrene (B[a]P) (77%). The frequency of GC --> TA transversions induced in Salmonella by the extract (78-86%) is similar to the frequency of this mutation in the TP53 (76%) and KRAS (86%) genes of lung tumors from nonsmoking women exposed to smoky coal emissions. The mutation spectra of the extract reflect the presence of PAHs in the mixture and support a role for PAHs in the induction of the mutations and tumors due to exposure to smoky coal emissions.

Using base-specific Salmonella tester strains to characterize the types of mutation induced by benzidine and benzidine congeners after reductive metabolism

Although benzidine (Bz), 4-aminobiphenyl (ABP), 3,3'-dichlorobenzidine HCl (DCBz), 3,3'-dimethylbenzidine (DMBz), 3,3'-dimethoxybenzidine (DMOBz) and the benzidine congener-based dye trypan blue (TB) produce primarily frameshift mutations in Salmonella typhimurium, the base-substitution strain TA100 also responds to these compounds when S9 is present. Performing DNA sequence analysis, other investigators have shown that ABP induces frameshift, base-pair and complex mutations. Also, it was found that an uninduced hamster liver S9 preparation with glucose-6-phosphate dehydrogenase, FMN, NADH and four times glucose 6-phosphate gave a stronger mutagenic response than the conventional plate incorporation with rat S9 activation mixture for all the compounds tested. Using the base-specific tester strains of S. typhimurium (TA7001-TA7006) with the above reductive metabolic activation system, we surveyed these compounds for the ability to produce specific base-pair substitutions after reductive metabolism. Bz was weakly mutagenic in TA7005 (0.04 revertants/microg). ABP was mutagenic in TA7002 (1.4 revertants/microg), TA7004 (0.6 revertants/microg), TA7005 (2.98 revertants/microg) and TA7006 (0.4 revertants/microg). DCBz was weakly mutagenic in TA7004 (0.01 revertants/microg). It was concluded that benzidine induced some CG->AT transversions in addition to frameshift mutations. ABP induced TA->AT, CG->AT, and CG->GC transversions as well as GC->AT transitions. DCBz induced only GC->AT transitions. Because DMBz, DMOBz and TB were not mutagenic in this base-substitution mutagen detection system, their mutagenic activity was attributed strictly to frameshift mechanisms.

The DeltauvrB mutations in the Ames strains of Salmonella span 15 to 119 genes

The DeltauvrB mutations present in strains of Salmonella enterica Typhimurium used commonly in the Salmonella (Ames) mutagenicity assay were isolated independently for at least five different his mutants. These deletions all involved the galactose operon, biotin operon, nucleotide-excision-repair uvrB gene, and chlorate-resistance genes. Beyond this, the size of the deletions and the number and type of genes deleted have remained unknown for nearly 30 years. Here, we have used genomic hybridization to a Typhimurium microarray to characterize these five DeltauvrB deletions. The number of genes (and amount of DNA) deleted due to the DeltauvrB mutations are 15 (16kb) each in TA97 and TA104, 47 (50kb) in TA100, 87 (96kb) in TA1537, and 119 (125kb) in TA98, accounting for 0.3, 0.3, 1.0, 1.9, and 2.6% of the genome, respectively. In addition, TA97 and TA104 contain an identical three-gene deletion elsewhere in their genomes, and, most remarkably, TA104 contains a 282-gene amplification, representing 7% of the genome. Missing genes include mfdA and mdaA, encoding a multi-drug translocase and a major nitroreductase, respectively, both absent in TA98; dps, encoding a DNA-binding protein absent in TA1537 and TA98; and dinG, encoding a lexA-regulated repair enzyme, absent in three DeltauvrB lineages. Genes involved in molybdenum cofactor biosynthesis and a number of ORFs of unknown functions are missing in all DeltauvrB strains investigated. Studies in DeltauvrB strains of Escherichia coli have found that the enhanced mutagenesis of some base analogues was due to the deletion of genes involved in molybdenum cofactor biosynthesis rather than to deletion of uvrB. These discoveries do not diminish the value of the data generated in the Ames strains. However, absence of genes other than uvrB may account for the enhanced mutagenicity of some compounds in DeltauvrB Ames strains. In general, microarrays will be useful for characterizing the extent and nature of deletion and amplification mutations.

Mutation spectra in Salmonella of analogues of MX: implications of chemical structure for mutational mechanisms

We determined the mutation spectra in Salmonella of four chlorinated butenoic acid analogues (BA-1 through BA-4) of the drinking water mutagen 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX) and compared the results with those generated previously by us for MX and a related compound, MCF. We then considered relationships between the properties of mutagenic potency and mutational specificity for these six chlorinated butenoic acid analogues. In TA98, the three most potent mutagens, BA-3, BA-4, MX, and the organic extract, all induced large percentages of complex frameshifts (33-67%), which distinguish these agents from any other class of compound studied previously. In TA100, which has only GC sites for mutation recovery, >71% of the mutations induced by all of the agents were GC-->TA transversions. The availability of both GC and TA sites for mutation in TA104 resulted in greater distinctions in mutational specificity than in TA100. MX targeted GC sites almost exclusively (98%); the structurally similar BA-4 and BA-2 produced mutations at similar frequencies at both GC and AT sites; and the structurally similar BA-3 and BA-1 induced most mutations at AT sites (69%). Thus, large variations in structural properties influencing relative mutagenic potency appeared to be distinct from the more localized similar structural features influencing mutagenic specificity in TA104. Among a set of physicochemical properties examined for the six butenoic acids, a significant correlation was found between pK(a) and mutagenic potency in TA100, even when the unionized fraction of the activity dose was considered. In addition, a correlation in CLOGP for BA-1 to BA-4 suggested a role for bioavailability in determining mutagenic potency. These results illustrate the potential value of structural analyses for exploring the relationship between chemical structure and mutational mechanisms. To our knowledge, this is the first study in which such analyses have been applied to structural analogues for which both mutagenic potency and mutation spectra date were available.