Molecular dissection of mutations at the heterozygous thymidine kinase locus in mouse lymphoma cells

Key Challenges for In Vitro Testing of Tobacco Products for Regulatory Applications: Recommendations for the In Vitro Mouse Lymphoma Assay

The Institute for In Vitro Sciences (IIVS) is sponsoring a series of workshops to develop recommendations for optimal scientific and technical approaches for conducting in vitro assays to assess potential toxicity within and across traditional tobacco and various tobacco and nicotine next-generation products (NGPs), including Heated Tobacco Products (HTPs) and Electronic Nicotine Delivery Systems (ENDS). This report was developed by a working group composed of attendees of the seventh IIVS workshop, 'Approaches and recommendations for conducting the mouse lymphoma gene mutation assay (MLA) and introduction to in vitro disease models', which was held virtually on 21-23 June 2022. This publication provides a background overview of the MLA, and includes the description of assay conduct and data interpretation, key challenges and recommended best practices for evaluating tobacco and nicotine products, with a focus on the evaluation of NGPs, and a summary of how the assay has been used to evaluate and compare tobacco and nicotine products.

Thymidine Kinase+/− Mammalian Cell Mutagenicity Assays for Assessment of Nanomaterials

The methods outlined here are part of a series of papers designed specifically for genotoxicity assessment of nanomaterials (NM). Common Considerations such as NM characterization, sample preparation and dose selection, relevant to all genotoxicity assays, are found in an accompanying paper. The present paper describes methods for evaluation of mutagenicity in the mammalian (mouse) thymidine kinase (Tk) gene occurring in L5178Y mouse lymphoma (ML) cells and in the designated TK gene in human lymphoblastoid TK6 cells. Mutations change the functional genotype from TK^+/− to TK^−/−, detectable as cells surviving on media selective for the lack of thymidine kinase (TK) function. Unlike cells with TK enzyme function, the TK^−/− cells are unable to integrate the toxic selection agent, allowing these cells to survive as rare mutant colonies. The ML assay has been shown to detect a broad spectrum of genetic damage, including both small scale (point) mutations and chromosomal alterations. This assay is a widely used mammalian cell gene mutation assay for regulatory purposes and is included in the core battery of genotoxicity tests for regulatory decision-making. The TK6 assay is an assay using a human cell line derived similarly via mutagenic manipulations and optimal selection. Details are provided on the materials required, cell culture methods, selection of test chemical concentrations, cytotoxicity, treatment time, mutation expression, cloning, and data calculation and interpretation. The methods describe the microwell plate version of the assays without metabolic activation.

Detection of Loss of Heterozygosity in Tk-Deficient Mutants from L5178Y Tk+/--3.7.2C Mouse Lymphoma Cells

The mouse lymphoma assay (MLA), a forward mutation assay using the Tk^+/--3.7.2C clone of the L5178Y mouse lymphoma cell line and the Thymidine kinase (Tk) gene, has been widely used as an in vitro genetic toxicity assay for more than four decades. The MLA can evaluate the ability of mutagens to induce a wide range of genetic events including both gene mutations and chromosomal mutations and has been recommended as one component of several genotoxicity test batteries. Tk-deficient mutants often exhibit chromosomal abnormalities involving the distal end of chromosome 11 where the Tk gene is located, in mice, and the type of chromosome alteration can be analyzed using a loss of heterozygosity (LOH) approach. LOH has been considered an important event in human tumorigenesis and can result from any of the following several mechanisms: large deletions, mitotic recombination, and chromosome loss. In this chapter, the authors describe the procedures for the detection of LOH in the Tk mutants from the MLA, and apply LOH analysis for understanding the types of genetic damage that is induced by individual chemicals.

Genotoxicity Assessment of Nanomaterials: Recommendations on Best Practices, Assays, and Methods

Nanomaterials (NMs) present unique challenges in safety evaluation. An international working group, the Genetic Toxicology Technical Committee of the International Life Sciences Institute's Health and Environmental Sciences Institute, has addressed issues related to the genotoxicity assessment of NMs. A critical review of published data has been followed by recommendations on methods alterations and best practices for the standard genotoxicity assays: bacterial reverse mutation (Ames); in vitro mammalian assays for mutations, chromosomal aberrations, micronucleus induction, or DNA strand breaks (comet); and in vivo assays for genetic damage (micronucleus, comet and transgenic mutation assays). The analysis found a great diversity of tests and systems used for in vitro assays; many did not meet criteria for a valid test, and/or did not use validated cells and methods in the Organization for Economic Co-operation and Development Test Guidelines, and so these results could not be interpreted. In vivo assays were less common but better performed. It was not possible to develop conclusions on test system agreement, NM activity, or mechanism of action. However, the limited responses observed for most NMs were consistent with indirect genotoxic effects, rather than direct interaction of NMs with DNA. We propose a revised genotoxicity test battery for NMs that includes in vitro mammalian cell mutagenicity and clastogenicity assessments; in vivo assessments would be added only if warranted by information on specific organ exposure or sequestration of NMs. The bacterial assays are generally uninformative for NMs due to limited particle uptake and possible lack of mechanistic relevance, and are thus omitted in our recommended test battery for NM assessment. Recommendations include NM characterization in the test medium, verification of uptake into target cells, and limited assay-specific methods alterations to avoid interference with uptake or endpoint analysis. These recommendations are summarized in a Roadmap guideline for testing.

In Vitro Mouse Lymphoma Cell (L5178Y Tk+/- -3.7.2.C) Forward Mutation Assay

The in vitro mouse lymphoma cell assay (MLA) is one of the most widely practiced assays in genetic toxicology. MLA detects forward mutations at the thymidine kinase (Tk) locus of the L5178Y (Tk^+/- -3.7.2C) cell line derived from a mouse thymic lymphoma. This assay is capable of detecting a wide range of genetic events including point mutations, deletions and multilocus, chromosomal rearrangements, mitotic recombination and nondisjunction. There are two equally accepted versions of the assay, one using soft agar cloning and the second method using liquid media cloning in 96-microwell plates. There are two morphologically distinct types of mutant colonies recovered in the MLA; small and large colony mutants. The induction of small colony mutants is associated with chemicals inducing gross chromosomal aberrations, whereas the induction of large mutant colonies is generally associated with chemicals inducing point mutations. The source and karyotype of the cell line as well as the culture conditions are important variables that could influence the assay performance. The assay when performed according to the standards recommended by the International Workshops on Genotoxicity Testing (IWGT) and the Organization of Economic Cooperation and Development Test Guideline 490 is capable of providing valuable genotoxicity hazard information as part of the overall safety assessment process of various classes of test substances.

Evaluation of U. S. National Toxicology Program (NTP) mouse lymphoma assay data using International Workshop on Genotoxicity Tests (IWGT) and the Organization for Economic Co-Operation and Development (OECD) criteria

The forward gene mutation mouse lymphoma assay (MLA) is widely used, as part of a regulatory test battery, to identify the genotoxic potential of chemicals. It identifies mutagens capable of inducing a variety of genetic events. During the 1980s and early 1990s, the U.S. National Toxicology Program (NTP) developed a publicly available database (https://tools.niehs.nih.gov/cebs3/ui/) of MLA results. This database is used to define the mutagenic potential of chemicals, to develop structure-activity relationships (SAR), and to draw correlations to animal carcinogenicity findings. New criteria for MLA conduct and data interpretation were subsequently developed by the International Workshop for Genotoxicity Testing (IWGT) and the Organization of Economic Cooperation and Development (OECD). These recommendations are included in a new OECD Test Guideline (TG490). It is essential that early experimental data be re-examined and classified according to the current criteria to build a curated database to better inform chemical-specific evaluations and SAR models. We re-evaluated more than 1900 experiments representing 342 chemicals against the newly defined acceptance criteria for background mutant frequency (MF), cloning efficiency (CE), positive control values (modified for this evaluation due to lack of colony sizing), appropriate dose selection, and data consistency. Only 17% of the evaluated experiments met all acceptance criteria used in this re-evaluation. Results from 211 chemicals were determined to be uninterpretable, 92 were positive, and 39 equivocal. The authors could not classify any responses as negative because colony sizing was not performed for any of these experiments and it is clear, based on many experiment with unacceptably low background and positive control MFs, that mutant colony recovery was often suboptimal. This re-evaluation provides a curated database for the MLA. A similar curation should be done for other widely used genetic toxicology assays, but will be more difficult for certain assays (e.g., in vitro chromosomal aberrations) because important parameters such as level of cytotoxicity were often not evaluated/reported. Environ. Mol. Mutagen. 59:829-841, 2018. © 2018 Wiley Periodicals, Inc.

Quantitative analysis of the relative mutagenicity of five chemical constituents of tobacco smoke in the mouse lymphoma assay

Quantifying health-related biological effects, like genotoxicity, could provide a way of distinguishing between tobacco products. In order to develop tools for using genotoxicty data to quantitatively evaluate the risk of tobacco products, we tested five carcinogens found in cigarette smoke, 4-aminobiphenyl (4-ABP), benzo[a]pyrene (BaP), cadmium (in the form of CdCl2), 2-amino-3,4-dimethyl-3H-imidazo[4,5-f]quinoline (MeIQ) and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), in the mouse lymphoma assay (MLA). The resulting mutagenicity dose responses were analyzed by various quantitative approaches and their strengths and weaknesses for distinguishing responses in the MLA were evaluated. L5178Y/Tk (+/-) 3.7.2C mouse lymphoma cells were treated with four to seven concentrations of each chemical for 4h. Only CdCl2 produced a positive response without metabolic activation (S9); all five chemicals produced dose-dependent increases in cytotoxicity and mutagenicity with S9. The lowest dose exceeding the global evaluation factor, the benchmark dose producing a 10%, 50%, 100% or 200% increase in the background frequency (BMD10, BMD50, BMD100 and BMD200), the no observed genotoxic effect level (NOGEL), the lowest observed genotoxic effect level (LOGEL) and the mutagenic potency expressed as a mutant frequency per micromole of chemical, were calculated for all the positive responses. All the quantitative metrics had similar rank orders for the agents' ability to induce mutation, from the most to least potent as CdCl2(-S9) > BaP(+S9) > CdCl2(+S9) > MeIQ(+S9) > 4-ABP(+S9) > NNK(+S9). However, the metric values for the different chemical responses (i.e. the ratio of the greatest value to the least value) for the different chemicals ranged from 16-fold (BMD10) to 572-fold (mutagenic potency). These results suggest that data from the MLA are capable of discriminating the mutagenicity of various constituents of cigarette smoke, and that quantitative analyses are available that can be useful in distinguishing between the exposure responses.

In vitro investigation of the mutagenic potential of Aloe vera extracts

A 2-year cancer bioassay in rodents with a preparation of Aloe vera whole leaf extract administered in drinking water showed clear evidence of carcinogenic activity. To provide insight into the identity and mechanisms associated with mutagenic components of the Aloe vera extracts, we used the mouse lymphoma assay to evaluate the mutagenicity of the Aloe vera whole leaf extract (WLE) and Aloe vera decolorized whole leaf extract (WLD). The WLD extract was obtained by subjecting WLE to activated carbon-adsorption. HPLC analysis indicated that the decolorization process removed many components from the WLE extract, including anthraquinones. Both WLE and WLD extracts showed cytotoxic and mutagenic effects in mouse lymphoma cells but in different concentration ranges, and WLD induced about 3-fold higher levels of intracellular reactive oxygen species than WLE. Molecular analysis of mutant colonies from cells treated with WLE and WLD revealed that the primary type of damage from both treatments was largely due to chromosome mutations (deletions and/or mitotic recombination). The fact that the samples were mutagenic at different concentrations suggests that while some mutagenic components of WLE were removed by activated carbon filtration, components with pro-oxidant activity and mutagenic activity remained. The results demonstrate the utility of the mouse lymphoma assay as a tool to characterize the mutagenic activity of fractionated complex botanical mixtures to identify bioactive components.

Can the L5178Y Tk+/- mouse lymphoma assay detect epigenetic silencing?

The mouse lymphoma L5178Y Tk(+/-) assay is broadly used in toxicology to assess genotoxicity because of its known sensitivity to genotoxicants that act through a variety of mechanisms, which may include epigenetic DNA methylation. This brief article highlights the studies that have contributed to this conjecture and suggests an addition to the experimental design that could identify if the test substance is a potential epimutagen acting via hypermethylation.

In vitro mouse lymphoma (L5178Y Tk⁺/⁻-3.7.2C) forward mutation assay

The in vitro mouse lymphoma assay (MLA) is one of the most widely practiced assays in genetic toxicology. MLA detects forward mutations at the thymidine kinase (Tk) locus of the L5178Y (Tk (+/-) -3.7.2C) cell line derived from a mouse thymic lymphoma. This assay is capable of detecting a wide range of genetic events including point mutations, deletions (intragenic) and multilocus, chromosomal rearrangements, mitotic recombination, and nondisjunction. There are two equally accepted versions of the assay, one using soft agar cloning and the second method using liquid media cloning in 96-microwell plates. There are two morphologically distinct types of mutant colonies recovered in the MLA: small- and large-colony mutants. The induction of small-colony mutants is associated with chemicals inducing gross chromosomal aberrations whereas the induction of large mutant colonies is generally associated with chemicals inducing point mutations. The source and karyotype of the cell line as well as the culture conditions are important variables that could influence the assay performance. The assay when performed according to the standards recommended by the International Workshops on Genotoxicity Testing is capable of providing valuable genotoxicity hazard information as part of the overall safety assessment process of various classes of test substances.

Silver nanoparticle-induced mutations and oxidative stress in mouse lymphoma cells

Silver nanoparticles (Ag-NPs) have increasingly been used for coatings on various textiles and certain implants, for the treatment of wounds and burns, as a water disinfectant, and in air-freshener sprays. The wide use of Ag-NPs may have potential human health impacts. In this study, the mutagenicity of 5-nm Ag-NPs was evaluated in the mouse lymphoma assay system, and modes of action were assessed using standard alkaline and enzyme-modified Comet assays and gene expression analysis. Treatments of L5178Y/Tk(+/-) mouse lymphoma cells with 5-nm uncoated Ag-NPs resulted in a significant yield of mutants at doses between 3 and 6 μg/mL; the upper range was limited by toxicity. Loss of heterozygosity analysis of the Tk mutants revealed that treatments with uncoated Ag-NPs induced mainly chromosomal alterations spanning less than 34 megabase pairs on chromosome 11. Although no significant induction of DNA damage in Ag-NP-treated mouse lymphoma cells was observed in the standard Comet assay, the Ag-NP treatments induced a dose-responsive increase in oxidative DNA damage in the enzyme-modified Comet assay in which oxidative lesion-specific endonucleases were added. Gene expression analysis using an oxidative stress and antioxidant defense polymerase chain reaction (PCR) array showed that the expressions of 17 of the 59 genes on the arrays were altered in the cells treated with Ag-NPs. These genes are involved in production of reactive oxygen species, oxidative stress response, antioxidants, oxygen transporters, and DNA repair. These results suggest that 5 nm Ag-NPs are mutagenic in mouse lymphoma cells due to induction of oxidative stress by the Ag-NPs.

The mouse lymphoma assay

The mouse lymphoma TK assay (MLA) is part of an in vitro battery of tests designed to predict risk assessment prior to in vivo testing. The test has the potential to detect mutagenic and clastogenic events at the thymidine kinase (tk) locus of L5178Y mouse lymphoma tk ( +/- ) cells by measuring resistance to the lethal nucleoside analogue triflurothymidine (TFT). Cells may be plated for viability and mutation in semi-solid agar (agar assay) or in 96-well microtitre plates (microwell assay). When added to selective medium containing TFT, wild-type tk ( +/- ) cells die, but TFT cannot be incorporated into the DNA of mutant tk ( -/- ) cells, which survive to form colonies that may be large (indicative of gene mutation) or small (indicative of chromosomal mutation) in nature. Mutant frequency is expressed as the number of mutants per 10(6) viable cells.

The ability of the mouse lymphoma TK assay to detect aneugens

There is some evidence that the mouse lymphoma TK assay (MLA) can detect aneugens, and this is accepted in the current International Conference on Harmonisation guidance for testing pharmaceuticals. However, whether or not it can be used as a reliable screen for aneugenicity has been the subject of debate. Consequently, aneugens with diverse mechanisms of action were tested in the MLA using 24-h exposure. No evidence of increased mutant frequency was seen with noscapine, diazepam or colchicine and increases were seen with taxol, carbendazim, econazole and chloral hydrate only at high levels of toxicity (for all but one taxol concentration survival reduced to ≤10% of control). None of these agents would be unequivocally classified as positive using currently accepted criteria. The largest increases in mutant number were seen with taxol and carbendazim; therefore, trifluorothymidine (TFT)-resistant clones resulting from treatment with them were cultured and analysed for chromosome 11 copy number using fluorescent in situ hybridisation (FISH) and loss of heterozygosity (LOH). High concentrations of these aneugens induced LOH at all loci examined indicating only one chromosome 11 was present but, perhaps surprisingly, all were found to have two copies of chromosome 11 using FISH. This would be consistent with loss of the tk(+) chromosome 11b with concomitant duplication of chromosome 11a, which has been proposed as a likely mechanism for induction of TFT-resistant clones. However, it was also surprising that analysis of centromere size showed that almost all the clones had both small and large centromeres, i.e. suggesting the presence of both chromosomes 11a and 11b. In conclusion, it appears that the TFT-resistant mutants resulting from treatment with toxic concentrations of some aneugens such as taxol and carbendazim have undergone complex genetic changes. However, these data show that the MLA cannot be used as a routine screen to detect aneugens.

Follow-up actions from positive results of in vitro genetic toxicity testing

Appropriate follow-up actions and decisions are needed when evaluating and interpreting clear positive results obtained in the in vitro assays used in the initial genotoxicity screening battery (i.e., the battery of tests generally required by regulatory authorities) to assist in overall risk-based decision making concerning the potential effects of human exposure to the agent under test. Over the past few years, the International Life Sciences Institute (ILSI) Health and Environmental Sciences Institute (HESI) Project Committee on the Relevance and Follow-up of Positive Results in In Vitro Genetic Toxicity (IVGT) Testing developed a decision process flow chart to be applied in case of clear positive results in vitro. It provides for a variety of different possibilities and allows flexibility in choosing follow-up action(s), depending on the results obtained in the initial battery of assays and available information. The intent of the Review Subgroup was not to provide a prescriptive testing strategy, but rather to reinforce the concept of weighing the totality of the evidence. The Review Subgroup of the IVGT committee highlighted the importance of properly analyzing the existing data, and considering potential confounding factors (e.g., possible interactions with the test systems, presence of impurities, irrelevant metabolism), and chemical modes of action when analyzing and interpreting positive results in the in vitro genotoxicity assays and determining appropriate follow-up testing. The Review Subgroup also examined the characteristics, strengths, and limitations of each of the existing in vitro and in vivo genotoxicity assays to determine their usefulness in any follow-up testing.

Mutagenicity of 11 cigarette smoke condensates in two versions of the mouse lymphoma assay

Cigarette smoke condensate (CSC) is genotoxic in nearly all assays in which it has been tested. In this study, we investigated the mutagenicity of 11 CSCs using the microwell and soft-agar versions of the mouse lymphoma assay (MLA). These CSCs were prepared from commercial or experimental cigarettes, 10 of them were produced using International Organisation for Standardisation (ISO) conditions and one CSC was generated using intense Massachusetts Department of Public Health (MDPH) conditions. In the presence of rat liver S9, the L5178Y/Tk(+/-) mouse lymphoma cells were treated with 11 CSCs at different concentrations (25-200 μg/ml) for 4 h. All CSCs resulted in dose-dependent increases of both cytotoxicity and mutagenicity in both versions of the MLA. The mutagenic potencies of the CSCs were calculated as mutant frequency per microgram CSC from the slope of the linear regression of the dose-response curves and showed no correlations with the tar yield of the cigarette or nicotine concentrations of the CSCs. Comparing two CSCs produced from the same commercial cigarettes using two different smoking conditions, the one generated under ISO conditions was more mutagenic than the other generated under intense conditions on a per microgram CSC basis. We also examined the loss of heterozygosity (LOH) at four microsatellite loci spanning the entire chromosome 11 for the mutants induced by 11 CSCs. The most common type of mutation observed was LOH with chromosome damage spanning less than ∼34 Mbp. These results indicate that the MLA identifies different genotoxic potencies among a variety of CSCs and that the results from both versions of the assay are comparable.

Cytotoxicity and mutagenicity of retinol with ultraviolet A irradiation in mouse lymphoma cells

Vitamin A (all-trans-retinol; retinol) is an essential human nutrient and plays an important role in several biological functions. However, under certain circumstances, retinol treatment can cause free radical generation and induce oxidative stress. In this study, we investigated photocytotoxicity and photomutagenicity of retinol using L5178Y/Tk(+/-) mouse lymphoma cells concomitantly exposed to retinol and ultraviolet A (UVA) light. While the cells treated with retinol alone at the doses of 5 or 10microg/ml in the absence of light did not increase the mutant frequency (MF) in the Tk gene, the treatment of the cells with 1-4microg/ml retinol under UVA light (1.38mW/cm(2) for 30min) increased the MF in the Tk gene in a dose-responsive manner. To elucidate the underlying mechanism of action, we also examined the mutational types of the Tk mutants by determining their loss of heterozygosity (LOH) at four microsatellite loci spanning the entire chromosome 11 on which the Tk gene is located. The mutational spectrum for the retinol+UVA treatment was significantly different from those of the control and UVA alone. More than 93% of the mutants from retinol+UVA treatment lost heterozygosity at the Tk1 locus and the major type (58%) of mutations was LOHs extending to D11Mit42, an alternation involving approximately 6cM of the chromosome, whereas the main type of mutations in the control was non-LOH mutations. These results suggest that retinol is mutagenic when exposed to UVA in mouse lymphoma cells through a clastogenic mode-of-action.

Multiparameter analysis of CHO AL mutant populations sorted on CD59 expression after gamma irradiation

The flow cytometry mutation assay is based on detecting mutations in the CD59 gene on human chromosome 11 in CHO A(L) cells with flow cytometry, but the kinetics of mutant expression and the histogram region for mutant selection have not been studied in detail. CHO A(L) cells were analyzed by flow cytometry for CD59 expression at various times after irradiation. The mutant fraction increased to a maximum at day 6 but decreased to near background levels by day 20. Cells were sorted from six different regions on the CD59 histograms after irradiation. The growth rate was similar for cells from all regions, and the surviving fraction was 50% of that for control cells. By 14 days the CD59 expression of cells from regions 2-5 was reduced to that of region 1. Cells were also analyzed for simultaneous expression of CD59, CD44 and CD90 (all on chromosome 11) to roughly characterize the size of the mutations. Triple mutants from the sorted populations were reduced from 41% on day 6 to 8% on day 24. We conclude that the mutant region should be increased to include cells with intermediate CD59 expression; also, the loss of CD59 mutant expression over time could be explained in part by the loss of triple mutants from the population.

Integration of in vivo and in vitro approaches to characterize the toxicity of Antalarmin, a corticotropin-releasing hormone receptor antagonist

Non-clinical studies were conducted to evaluate the toxicity of Antalarmin, a corticotropin-releasing hormone type 1 receptor antagonist being developed for therapy of stress-related pathologies. Antalarmin was not genotoxic in bacterial mutagenesis assays, mammalian cell mutagenesis assays, or in vivo DNA damage assays. In a 14-day range-finding study in rats, Antalarmin doses >or=500 mg/kg/day (3,000 mg/m(2)/day) induced mortality. In a 90-day toxicity study in rats, no gross toxicity was seen at doses of 30, 100, or 300 mg/kg/day (180, 600, or 1,800 mg/m(2)/day, respectively). Antalarmin (300 mg/kg/day) induced mild anemia, increases in serum gamma-glutamyl transferase activity, and microscopic hepatic pathology (bile duct hyperplasia and epithelial necrosis, periportal inflammation). Microscopic renal changes (cortical necrosis, inflammation, hypertrophy, nephropathy) were observed in rats at all Antalarmin doses. In a 14-day range-finding study in dogs, Antalarmin doses >or=50mg/kg/day (1,000 mg/m(2)/day) induced repeated emesis and bone marrow suppression. In a 90-day toxicity study in dogs, Antalarmin (4, 8, or 16 mg/kg/day (80, 160, or 320 mg/m(2)/day, respectively)) induced bone marrow and lymphoid depletion, but no gross toxicity. Comparative in vitro studies using rat, dog, and human neutrophil progenitors demonstrated that canine bone marrow cells are highly sensitive to Antalarmin cytotoxicity, while rat and human bone marrow cells are relatively insensitive. As such, the bone marrow toxicity observed in dogs is considered likely to over-predict Antalarmin toxicity in humans. The hepatic and renal toxicities seen in rats exposed to Antalarmin identify those tissues as the most likely targets for Antalarmin toxicity in humans.

Gene-mutation induction by arsenic compounds in the mouse lymphoma assay

Arsenic compounds are generally considered as poor inducers of gene mutations. To investigate the mutagenicity of several arsenic compounds at the thymidine kinase (Tk) gene, a reporter gene for mutation induction, we used the mouse lymphoma assay (MLA). This test is widely applied and detects a broad spectrum of mutational events, from point mutations to chromosome alterations. The selected arsenic compounds were two inorganic (sodium arsenite and arsenic trioxide) and four organic compounds (monomethylarsonic acid, dimethylarsinic acid, tetraphenylarsenium and arsenobetaine). The results show that sodium arsenite, arsenic trioxide, monomethylarsonic acid and dimethylarsinic acid are mutagenic, showing a clear dose-response pattern. On the other hand, tetraphenylarsenium and arsenobetaine are not mutagenic. Inorganic arsenic compounds are the more potent agents producing significant effects in the micromolar range, while the mutagenic organic arsenic compounds induce similar effects but in the millimolar range.

Genotoxic effects of acrylamide and glycidamide in mouse lymphoma cells

In addition to occupational exposures to acrylamide (AA), concerns about AA health risks for the general population have been recently raised due to the finding of AA in food. In this study, we evaluated the genotoxicity of AA and its metabolite glycidamide (GA) in L5178Y/Tk(+/-) mouse lymphoma cells. The cells were treated with 2-18 mM of AA or 0.125-4 mM of GA for 4 h without metabolic activation. The DNA adducts, mutant frequencies and the types of mutations for the treated cells were examined. Within the dose range tested, GA induced DNA adducts of adenine and guanine [N3-(2-carbamoyl-2-hydroxyethyl)-adenine and N7-(2-carbamoyl-2-hydroxyethyl)-guanine] in a linear dose-dependent manner. The levels of guanine adducts were consistently about 60-fold higher across the dose range than those of adenine. In contrast, no GA-derived DNA adducts were found in the cells treated with any concentrations of AA, consistent with a lack of metabolic conversion of AA to GA. However, the mutant frequency was significantly increased by AA at concentrations of 12 mM and higher. GA was mutagenic starting with the 2mM dose, suggesting that GA is much more mutagenic than AA. The mutant frequencies were increased with increasing concentrations of AA and GA, mainly due to an increase of proportion of small colony mutants. To elucidate the underlying mutagenic mechanism, we examined the loss of heterozygosity (LOH) at four microsatellite loci spanning the entire chromosome 11 for mutants induced by AA or GA. Compared to GA induced mutations, AA induced more mutants whose LOH extended to D11Mit22 and D11Mit74, an alteration of DNA larger than half of the chromosome. Statistical analysis of the mutational spectra revealed a significant difference between the types of mutations induced by AA and GA treatments (P=0.018). These results suggest that although both AA and GA generate mutations through a clastogenic mode of action in mouse lymphoma cells, GA induces mutations via a DNA adduct mechanism whereas AA induces mutations by a mechanism not involving the formation of GA adducts.

Photomutagenicity of anhydroretinol and 5,6-epoxyretinyl palmitate in mouse lymphoma cells

Retinyl palmitate (RP) is frequently used as an ingredient in cosmetics and other retail products. We previously reported that, under UVA light irradiation, RP is facilely decomposed into multiple products, including anhydroretinol (AR) and 5,6-epoxyretinyl palmitate (5,6-epoxy-RP). We also determined that combined treatment of mouse lymphoma cells with RP and UVA irradiation produced a photomutagenic effect. In this study, we evaluated the photomutagenicity of AR and 5,6-epoxy-RP, in L5178Y/Tk+/- mouse lymphoma cells. Treatment of cells with AR or 5,6-epoxy-RP alone at 10 and 25 microg/mL for 4 h did not show a positive mutagenic response. However, because these doses did not induce the required amount of cytotoxicity for mouse lymphoma assay, we are unable to determine whether or not these two compounds are mutagenic. Treatment of cells with 1-25 microg/mL AR or 5,6-epoxy-RP under UVA light (315-400 nm) for 30 min (1.38 mW/cm2) produced a synergistic photomutagenic effect. At 10 microg/mL (37.3 microM) AR with UVA exposure, the mutant frequency (MF) was about 3-fold higher than that for UVA exposure alone, whereas the MF for 25microg/mL (46.3microM) of 5,6-epoxy-RP + UVA was approximately 2-fold higher than that for UVA exposure alone. Compared with previous results for RP + UVA treatment, the potency of the induced phototoxicity and photomutagenicity was AR > RP > 5,6-epoxy-RP. To elucidate the underlying photomutagenic mechanism, we examined the loss of heterozygosity (LOH) at four microsatellite loci spanning the entire chromosome 11 for mutants induced by AR or 5,6-epoxy-RP. Most mutants lost the Tk+ allele, and more than 70% of the chromosome damage extended to 38 cM in chromosome length. AR + UVA induced about twice as many mutants that lost all four microsatellite markers from the chromosome 11 carrying the Tk+ allele as RP + UVA or 5,6-epoxy-RP + UVA. These results suggest that two of RP's photodecomposition products are photomutagenic in mouse lymphoma cells, causing events that affect a large segment of the chromosome.

3'-azido-3'-deoxythymidine induces deletions in L5178Y mouse lymphoma cells

3'-Azido-3'-deoxythymidine (AZT), a nucleoside analogue used for the treatment of acquired immunodeficiency syndrome (AIDS), induced a significant dose-related increase in the thymidine kinase (Tk) mutant frequency (MF) in L5178Y/Tk(+/-) 3.7.2C mouse lymphoma cells. Treatment with 1 mg/ml (3,742 muM) AZT for 24 hr resulted in a MF of 407 x 10(-6) compared to a control MF of 84 x 10(-6). The MFs of the large and small colony mutants resulting from AZT exposure were 142 x 10(-6) and 265 x 10(-6), respectively. One hundred and fifty mutants from the 1 mg/ml (3,742 muM) AZT-treated culture and sixty-nine mutants from independent untreated cultures were isolated and analyzed. LOH analysis using a heteromorphic microsatellite locus located in the Tk gene was performed to determine the presence or absence of the Tk(+) allele. Eight other microsatellite markers spanning the entire mouse chromosome 11 also were examined for heterozygosity to determine the extent of LOH. In addition, Tk gene dosage analysis was conducted using Real-Time PCR in those mutants showing LOH at the Tk locus. The presence of only one Tk allele based on Real-Time PCR indicated that the mutant resulted from deletion while the presence of two alleles was consistent with a recombination event. More mutants from the AZT-treated culture showed Tk LOH than did independent mutants from the untreated cultures (91% vs. 64%) and the induced mutants also showed distinct chromosome 11 LOH patterns. The mutation spectrum of mutants from AZT-treated cells was also significantly different from that of spontaneous mutants. More deletions and fewer intragenic mutations were observed in the mutants from the AZT-treated culture than independent mutants from the untreated control. Our data indicate that AZT primarily induced LOH mutations in L5178Y mouse lymphoma cells and a large number of LOH mutations resulted from deletions.

The mouse lymphoma thymidine kinase assay for the assessment and comparison of the mutagenic activity of cigarette mainstream smoke particulate phase

The mouse lymphoma thymidine kinase assay (MLA) has been optimized to quantitatively determine the in vitro mutagenicity of cigarette mainstream smoke particulate phase. To test whether the MLA is able to discriminate between different cigarette types, specially constructed cigarettes each containing a single tobacco type - Bright, Burley, or Oriental - were investigated. The mutagenic activity of the Burley cigarette was statistically significantly lower, up to approximately 40%, than that of the Bright and Oriental cigarettes. To determine the impact of two different sets of smoking conditions, American-blend cigarettes were smoked under US Federal Trade Commission/International Organisation for Standardisation conditions and under Massachusetts Department of Public Health (MDPH) conditions. Conventional cigarettes - eight from the US commercial market plus the Reference Cigarettes 1R4F and 2R4F - and an electrically heated cigarette smoking system (EHCSS) prototype were tested. There were no statistically significant differences between the two sets of smoking conditions on a per mg total particulate matter basis, although there was a consistent trend towards slightly lower mutagenic activity under MDPH conditions. The mutagenic activity of the EHCSS prototype was distinctly lower than that of the conventional cigarettes under both sets of smoking conditions. These results show that the MLA can be used to assess and compare the mutagenic activity of cigarette mainstream smoke particulate phase in the comprehensive toxicological assessment of cigarette smoke.

Microarray analysis distinguishes differential gene expression patterns from large and small colony Thymidine kinase mutants of L5178Y mouse lymphoma cells

Background

The Thymidine kinase (Tk) mutants generated from the widely used L5178Y mouse lymphoma assay fall into two categories, small colony and large colony. Cells from the large colonies grow at a normal rate while cells from the small colonies grow slower than normal. The relative proportion of large and small colonies after mutagen treatment is associated with a mutagen's ability to induce point mutations and/or chromosomal mutations. The molecular distinction between large and small colony mutants, however, is not clear.

Results

To gain insights into the underlying mechanisms responsible for the mutant colony phenotype, microarray gene expression analysis was carried out on 4 small and 4 large colony Tk mutant samples. NCTR-fabricated long-oligonucleotide microarrays of 20,000 mouse genes were used in a two-color reference design experiment. The data were analyzed within ArrayTrack software that was developed at the NCTR. Principal component analysis and hierarchical clustering of the gene expression profiles showed that the samples were clearly separated into two groups based on their colony size phenotypes. The Welch T-test was used for determining significant changes in gene expression between the large and small colony groups and 90 genes whose expression was significantly altered were identified (p < 0.01; fold change > 1.5). Using Ingenuity Pathways Analysis (IPA), 50 out of the 90 significant genes were found in the IPA database and mapped to four networks associated with cell growth. Eleven percent of the 90 significant genes were located on chromosome 11 where the Tk gene resides while only 5.6% of the genes on the microarrays mapped to chromosome 11. All of the chromosome 11 significant genes were expressed at a higher level in the small colony mutants compared to the large colony mutants. Also, most of the significant genes located on chromosome 11 were disproportionally concentrated on the distal end of chromosome 11 where the Tk mutations occurred.

Conclusion

The results indicate that microarray analysis can define cellular phenotypes and identify genes that are related to the colony size phenotypes. The findings suggest that genes in the DNA segment altered by the Tk mutations were significantly up-regulated in the small colony mutants, but not in the large colony mutants, leading to differential expression of a set of growth regulation genes that are related to cell apoptosis and other cellular functions related to the restriction of cell growth.

A compilation of two decades of mutagenicity test results with the Ames Salmonella typhimurium and L5178Y mouse lymphoma cell mutation assays

As previously reported [Cameron, T. P., Rogers-Back, A. M., Lawlor, T. E., Harbell, J. W., Seifried, H. E., and Dunkel, V. C. (1991) Gentoxicity of multifunctional acrylates in the Salmonella/mammalian-microsome assay and mouse lymphoma TK+/- assay. Environ. Mol. Mutagen. 17, 264-271], the National Cancer Institute (NCI) shares the responsibility of selecting the most significant chemicals for carcinogenicity testing by the National Toxicology Program (NTP) and has used data from Salmonella and mouse lymphoma mutagenicity assays to aid in the selection and prioritization of chemicals to be further evaluated in chronic 2 year rodent studies. In addition, a number of antineoplastic and anti-AIDS drugs in preclinical evaluation were tested for the NCI's Division of Cancer Treatment Toxicology Branch. In the NCI/NTP chemical selection process, it is no longer necessary to test chemicals prior to sending them to the NTP so the NCI program has ceased performing mutagenicity tests. Some of the testing data has been made available in summary form in the Chemical Carcinogenisis Research Information System (CCRIS), which is searchable on the NLM TOXNET system. The limitations in using this source are that only summary results are available and many negative test results are not included. A summary table that presents the results for each compound is provided in the Appendix with raw data provided in the Supporting Information. The Appendix table contains the compound name, CAS number, and a summary of the data from the Ames test and the mouse lymphoma assay.

Assessment of the genotoxic potential of the antipsychotic sigma receptor ligand E-5842

The genotoxic potential of E-5842, a sigma ligand compound being developed as an antipsychotic drug, was evaluated by means of an extensive battery of in vitro and in vivo assays. Negative results were obtained in an Ames test (up to 5000 μg/plate), a mouse lymphoma assay (up to 535.1 μg/ml (-S9) and 891.8 μg/ml (+S9)), an in vivo rat hepatocyte micronucleus assay (up to 100 mg/kg/day on 2 days), and a two-dose mouse micronucleus assay (up to 40 mg/kg/day on 2 days). In a single-dose mouse bone-marrow micronucleus assay (up to 400 mg/kg; 24, 48 and 72 h sampling) a slight and non-statistically significant increase in the frequency of micronucleated polychromatic erythrocytes (MNPCE) was observed 48 h after administration of a 200 mg/kg dose, in the absence of bone-marrow toxicity. This minor increase in MNPCE frequency was considered of questionable biological relevance, because it was observed under conditions of marked animal toxicity including mortality. In addition, it occurred in association with a strong hypothermic effect produced by administration of E-5842. A clear increase in the frequency of structural chromosomal aberrations was observed in human lymphocytes at concentrations ≥350.6 and 1685.4 μg/ml in the presence and absence of S9, respectively. Mitotic accumulation was observed at those concentrations at which clastogenic effects were observed, a condition that may have masked toxicity. Concentrations lacking clastogenic effects in this chromosome aberration assay (300.7 and 173.2 μg/ml in the presence and absence of S9, respectively) were well in excess of maximum human plasma concentrations attained in clinical studies at the maximum tolerated dose (19.1 ng/ml). A weight-of-evidence analysis, taking into consideration the results obtained in the different in vitro and in vivo assays and the conditions of clinical use, suggest that E-5842 would not pose a genotoxic risk under clinical conditions.

Multicolor spectral karyotyping of the L5178Y Tk+/- -3.7.2C mouse lymphoma cell line

The L5178Y/Tk+/- -3.7.2C mouse lymphoma cell line is characterized, at the cytogenetic level, by a karyotype involving both numerical and complex structural aberrations. While the karyotype is remarkably normal for a transformed cell line that has been in culture for almost half a century, there are a number of chromosomal alterations that because of their complexity cannot be fully characterized by routine or even high-resolution G-banding studies. Multicolor spectral karyotyping (SKY) was performed on the cell line in anticipation of identifying the previously unresolved chromosome aberrations and confirming interpretations previously identified by banding studies. New chromosome aberrations detected by SKY include numerical aberrations of chromosome 15, duplications of regions of chromosomes 4, 5, 12, and 18, and deletion of chromosome 14. Complex unbalanced translocations involved segments of chromosomes 6, 14, and 15. In total, the SKY technique was able to provide new refined designations on segments of eight different chromosome pairs (4, 5, 6, 9, 12, 14, 15, 18) and identified all three previously unidentified marker chromosomes. This analysis provides an updated standard reference for the karyotype of the L5178Y/Tk+/- -3.7.2C cell line used in the in vitro mouse lymphoma mutation assay.

Use of genetic toxicology information for risk assessment

Genetic toxicology data are used worldwide in regulatory decision-making. On the 25th anniversary of Environmental and Molecular Mutagenesis, we think it is important to provide a brief overview of the currently available genetic toxicity tests and to outline a framework for conducting weight-of-the-evidence (WOE) evaluations that optimize the utility of genetic toxicology information for risk assessment. There are two major types of regulatory decisions made by agencies such as the Environmental Protection Agency (EPA) and the Food and Drug Administration (FDA): (1) the approval and registration of pesticides, pharmaceuticals, medical devices, and medical-use products, and (2) the setting of standards for acceptable exposure levels in air, water, and food. Genetic toxicology data are utilized for both of these regulatory decisions. The current default assumption for regulatory decisions is that chemicals that are shown to be genotoxic in standard tests are, in fact, capable of causing mutations in humans (in somatic and/or germ cells) and that they contribute to adverse health outcomes via a "genotoxic/mutagenic" mode of action (MOA). The new EPA Guidelines for Carcinogen Risk Assessment [Guidelines for Carcinogen Risk Assessment, USEPA, 2005, EPA Publication No. EPA/630/P-03/001F] emphasize the use of MOA information in risk assessment and provide a framework to help identify a possible mutagenic and/or nonmutagenic MOA for potential adverse effects. An analysis of the available genetic toxicity data is now, more than ever, a key component to consider in the derivation of an MOA for characterizing observed adverse health outcomes such as cancer. We provide our perspective and a two-step strategy for evaluating genotoxicity data for optimal use in regulatory decision-making. The strategy includes integration of all available information and provides, first, for a WOE analysis as to whether a chemical is a mutagen, and second, whether an adverse health outcome is mediated via a mutagenic MOA.

Photomutagenicity of retinyl palmitate by ultraviolet a irradiation in mouse lymphoma cells

Retinyl palmitate (RP), a storage form of vitamin A, is frequently used as a cosmetic ingredient, with more than 700 RP-containing cosmetic products on the U.S. market in 2004. There are concerns for the possible genotoxicity and carcinogenicity of RP when it is exposed to sunlight. To evaluate the photomutagenicity of RP in cells when exposed to ultraviolet A (UVA) light, L5178Y/Tk+/- mouse lymphoma cells were treated with different doses of RP alone/or in the presence of UVA light. Treatment of the cells with RP alone at the dose range of 25-100 microg/ml did not increase mutant frequencies (MFs) over the negative control, whereas treatment of cells with 1-25 microg/ml RP under UVA light (82.8 mJ/cm2/min for 30 min) produced a dose-dependent mutation induction. The mean induced MF (392 x 10(-6)) for treatment with 25 microg/ml RP under UVA exposure was about threefold higher than that for UVA alone (122 x 10(-6)), a synergistic effect. To elucidate the underlying mechanism of action, we examined the mutants for loss of heterozygosity (LOH) at four microsatellite loci spanning the entire chromosome 11, on which the Tk gene is located. The mutational spectrum for the RP + UVA treatment was significantly different from the negative control, but not significantly different from UVA exposure alone. Ninety four percent of the mutants from RP + UVA treatment lost the Tk+ allele, and 91% of the deleted sequences extended more than 6 cM in chromosome length, indicating clastogenic events affecting a large segment of the chromosome. These results suggest that RP is photomutagenic in combination with UVA exposure in mouse lymphoma cells, with a clastogenic mode-of-action.

Genotoxicity screening: the slow march to the future

Genetic toxicology testing in drug discovery and development is slowly moving into the age of high-throughput screening (HTS). This has been helped by the development of new tools, as well as validation studies and data analysis to support their use in hit-to-lead or lead optimisation decisions. This review provides an overview of the current genetic toxicology methodologies and a few HTS methodologies. Comparisons are made between the predictivity of carcinogenesis that can be achieved in screening strategies as well as by the battery of regulatory tests. The importance of false-positive and false-negative calls at different stages in development is considered. There is a good prospect that in genetic toxicology, as in other areas of ADME-Tox, HTS will reduce the growing costs of carrying compounds with undesirable characteristics too far along the drug development process.

Mutagenic effects of 4-hydroxynonenal triacetate, a chemically protected form of the lipid peroxidation product 4-hydroxynonenal, as assayed in L5178Y/Tk+/- mouse lymphoma cells

The lipid peroxidation product 4-hydroxynon-2-enal (4-HNE) is cytotoxic and genotoxic at superphysiological concentrations. To characterize the mechanism of action of 4-HNE, we assessed genotoxic damage by 4-HNE and by 4-HNE triacetate [4-HNE(Ac)(3)] using the mouse lymphoma assay that measures the mutant frequency in the Tk gene. As a strong electrophile, 4-HNE reacts readily with nucleophilic centers on cellular components. When added extracellularly, it may react preferentially with proteins in culture medium or on the cell surface and not reach deeper cellular targets such as nuclear DNA. Therefore, 4-HNE(Ac)(3), a protected form of 4-HNE that is metabolically converted to 4-HNE in cells (Neely MD, Amarnath V, Weitlauf C, and Montine TJ, Chem Res Toxicol 15:40-47, 2002), was assayed in addition to 4-HNE. When added in serum-containing medium, 4-HNE was not mutagenic in the mouse lymphoma assay up to 38 muM (cytotoxicity = 13%). In contrast, exposure to 4-HNE(Ac)(3), which mimics intracellular formation of 4-HNE, resulted in dose-dependent induction of mutations. At 17 muM 4-HNE(Ac)(3) (cytotoxicity = 33%), the mutant frequency was 719 x 10(-6) (>7-fold higher than the spontaneous mutant frequency). Loss of heterozygosity analysis in the Tk mutants revealed that the majority of mutations induced by 4-HNE(Ac)(3) resulted from clastogenic events affecting a large segment of the chromosome. The results indicate that, in the presence of serum that approximates physiological conditions, 4-HNE generated intracellularly but not extracellularly is a strong mutagen via a clastogenic action at concentrations that may occur during oxidative stress.

Bromate induces loss of heterozygosity in the thymidine kinase gene of L5178Y/Tk(+/-)-3.7.2C mouse lymphoma cells

Potassium bromate (KBrO(3)) induces DNA damage and tumors in mice and rats, but is a relatively weak mutagen in microbial assays and the in vitro mammalian Hprt assay. Concern that there may be a human health risk associated with bromate, a disinfectant by-product of ozonation, has accompanied the increasing use of ozonation as an alternative to chlorination for treatment of drinking water. In this study, we have evaluated the mutagenicity of KBrO(3) and sodium bromate (NaBrO(3)) in the Tk gene of mouse lymphoma cells. In contrast to the weak mutagenic activity seen in the previous studies, bromate induced a mutant frequency of over 100 x 10(-6) at 0.6mM with minimal cytotoxicity (70-80% survival) and over 1300 x 10(-6) at 3mM ( approximately 10% survival). The increase in the Tk mutant frequency was primarily due to the induction of small colony of Tk mutants. Loss of heterozygosity (LOH) analysis of 384 mutants from control and 2.7 mM KBrO(3)-treated cells showed that almost all (99%) bromate-induced mutants resulted from LOH, whereas in the control cultures 77% of the Tk mutants were LOH. Our results suggest that bromate is a potent mutagen in the Tk gene of mouse lymphoma cells, and the mechanism of action primarily involves LOH. The ability of the mouse lymphoma assay to detect a wider array of mutational events than the microbial or V79 Hprt assays may account for the potent mutagenic response.

Methylated trivalent arsenicals as candidate ultimate genotoxic forms of arsenic: induction of chromosomal mutations but not gene mutations

Arsenic is a prevalent human carcinogen whose mutagenicity has not been characterized fully. Exposure to either form of inorganic arsenic, As(III) or As(V), can result in the formation of at least four organic metabolites: monomethylarsonic acid, monomethylarsonous acid (MMA(III)), dimethylarsinic acid, and dimethylarsinous acid (DMA(III)). The methylated trivalent species, as well as some of the other species, have not been evaluated previously for the induction of chromosome aberrations, sister chromatid exchanges (SCE), or toxicity in cultured human peripheral blood lymphocytes; for mutagenicity in L5178Y/Tk(+/-) mouse lymphoma cells or in the Salmonella reversion assay; or for prophage-induction in Escherichia coli. Here we evaluated the arsenicals in these assays and found that MMA(III) and DMA(III) were the most potent clastogens of the six arsenicals in human lymphocytes and the most potent mutagens of the six arsenicals at the Tk(+/-) locus in mouse lymphoma cells. The dimethylated arsenicals were also spindle poisons, suggesting that they may be ultimate forms of arsenic that induce aneuploidy. Although the arsenicals were potent clastogens, none were potent SCE inducers, similar to clastogens that act via reactive oxygen species. None of the six arsenicals were gene mutagens in Salmonella TA98, TA100, or TA104; and neither MMA(III) nor DMA(III) induced prophage. Our results show that both methylated As(V) compounds were less cytotoxic and genotoxic than As(V), whereas both methylated As(III) compounds were more cytotoxic and genotoxic than As(III). Our data support the view that MMA(III) and DMA(III) are candidate ultimate genotoxic forms of arsenic and that they are clastogens and not gene mutagens. We suggest that the clastogenicity of the other arsenicals is due to their metabolism by cells to MMA(III) or DMA(III).

Mutant frequency and mutational spectra in the Tk and Hprt genes of N-ethyl-N-nitrosourea-treated mouse lymphoma cellsdagger

The mouse lymphoma assay (MLA) utilizing the Tk gene is widely used to identify chemical mutagens. The autosomal location of the Tk gene allows for the detection of a wide range of mutational events, from point mutations to chromosome alterations. However, chemically induced point mutation spectra in the Tk gene of mouse lymphoma cells have not been characterized. In this study, we determined and compared the mutagenicity and mutational spectra of N-ethyl-N-nitrosourea (ENU) in the Tk and Hprt genes of mouse lymphoma cells. Treatment of L5178Y mouse lymphoma cells with 100 microg/ml ENU induced a Tk mutant frequency of 756 x 10(-6) and an Hprt mutant frequency of 311 x 10(-6). Sequence analysis of Tk and Hprt mutant cDNAs showed a similar overall mutation pattern in the two genes with base-pair substitutions accounting for 83% of non-loss of heterozygosity mutations in the Tk gene and 75% of all mutations in the Hprt gene. The most common point mutation induced by ENU was G:C --> A:T transition (36 and 28% of independent mutations detected in the Tk and Hprt genes, respectively). The mutation spectra induced by ENU in both the Tk and Hprt genes were different from the respective patterns produced in mutants from untreated cells. About 9% of Tk and 7% of Hprt mutations from control cells were in-frame deletions, whereas no such mutations were found among the ENU-induced Tk and Hprt mutations. Our results indicate that ENU produces a chemical-specific point mutational profile in the Tk gene of mouse lymphoma cells that is remarkably similar to that found in the X-linked Hprt gene. This study provides evidence that the MLA can be used not only to detect point mutagens but also for analysis of mutational spectra.

Liver carcinogen aflatoxin B1 as an inducer of mitotic recombination in a human cell line

The mycotoxin aflatoxin B1 (AFB1) is one of the most potent rodent and human liver carcinogens. Upon cytochrome P450-specific metabolism, it induces mutations as well as mitotic recombination events in in vitro systems. We have found that in the lower eukaryote yeast, the recombinagenic activity of AFB1 surpasses its mutagenic activity, and we speculated on possible consequences in terms of the mechanism of liver carcinogenesis. In this study we investigated whether the recombinagenic activity of AFB1 also would be identified in human cells. To address this question, we followed the fate of a heterozygous thymidine kinase (tk) allele in the human lymphoblastoid cell line TK6 upon exposure to AFB1. Individual mutants that had lost tk activity were subjected to loss of heterozygosity analysis of the tk locus and its flanking markers. Fluorescence in situ hybridization analysis on chromosome 17 also was performed. In parallel, a similar analysis was performed on TK6 cells exposed to the alkylating agent N-nitrosomethylurea, a well-known classic point mutagen. Our analysis showed a difference in the molecular mechanism leading to inactivation of the tk allele upon exposure to these two mutagens. In AFB1-exposed cells the fraction of recombination-derived mutants predominated, whereas in N-nitrosomethylurea-exposed cells the fraction of point mutants was higher. Thus, the recombinagenic activity of AFB1 previously identified in a lower eukaryote also was found in the human cell line TK6. Our data support the hypothesis that mitotic recombination represents a central mechanism of action in AFB1-induced liver carcinogenesis.

Spindle poisons induce allelic loss in mouse lymphoma cells through mitotic non-disjunction

Aneuploidy is an important contributor to reproductive failure and tumor development. It arises spontaneously or as a result of exposure to aneugenic agents through non-disjunction. Two spindle poisons, colchicine (COL) and vinblastine (VBL) are mutagenic in the mouse lymphoma assay (MLA), a gene mutation assay that targets the heterozygous thymidine kinase (tk) gene on chromosome 11 in mouse lymphoma L5178Y tk+/- 3.7.2c cells. To investigate the mechanisms of spindle poison mutagenesis, we analyzed the COL- and VBL-induced TK mutants at the molecular and cytogenetic level. Loss of heterozygosity (LOH) analysis employing a microsatellite region within the tk locus revealed that almost all mutants had lost the functional tk allele. To determine the extent of the LOH, we further examined LOH mutants for heterozygosity at nine microsatellite loci spanning the entire chromosome 11. Interestingly, every microsatellite marker showed LOH in all COL- and VBL-induced LOH mutants, suggesting that these mutants were generated by loss of the whole chromosome 11 through mitotic non-disjunction. Chromosome painting analysis supported this hypothesis; there were no mutants showing structural changes such as deletions or translocations involving chromosome 11. In contrast, spontaneous TK mutants followed from point mutations, deletions and recombinational events as well as whole chromosome loss. Our present study indicates that spindle poisons induce mutations through mitotic non-disjunction without structural DNA changes and supports a possible mechanism in which a recessive mutation mediated by aneuploidy may develop tumors.

The mouse lymphoma assay

In this paper, the current status of the protocol for the Mouse Lymphoma Assay is discussed. A brief history describes the events leading to current protocol recommendations. Areas for further development such as cytotoxicity, 24-h treatments, acceptability criteria and statistical analysis are also considered. Recent guidelines are reviewed, and consensus issues from the Mouse Lymphoma workgroup assembled as part of the International Workshop on Genotoxicity Test Procedures (IWGTP) are included. There are two versions of the assay - soft agar and microwell - and both will be discussed. For assay procedures, the emphasis will be on a typical microwell protocol but an attempt will be made to highlight protocol variations between laboratories and between the microwell and agar versions of the assay.

Mutagenicity of trichloroethylene and its metabolites: implications for the risk assessment of trichloroethylene

This article addresses the evidence that trichloroethylene (TCE) or its metabolites might mediate tumor formation via a mutagenic mode of action. We review and draw conclusions from the published mutagenicity and genotoxicity information for TCE and its metabolites, chloral hydrate (CH), dichloroacetic acid (DCA), trichloroacetic acid (TCA), trichloroethanol, S-(1, 2-dichlorovinyl)-l-cysteine (DCVC), and S-(1, 2-dichlorovinyl) glutathione (DCVG). The new U.S. Environmental Protection Agency proposed Cancer Risk Assessment Guidelines provide for an assessment of the key events involved in the development of specific tumors. Consistent with this thinking, we provide a new and general strategy for interpreting genotoxicity data that goes beyond a simple determination that the chemical is or is not genotoxic. For TCE, we conclude that the weight of the evidence argues that chemically induced mutation is unlikely to be a key event in the induction of human tumors that might be caused by TCE itself (as the parent compound) and its metabolites, CH, DCA, and TCA. This conclusion derives primarily from the fact that these chemicals require very high doses to be genotoxic. There is not enough information to draw any conclusions for trichloroethanol and the two trichloroethylene conjugates, DCVC and DCVG. There is some evidence that DCVC is a more potent mutagen than CH, DCA, or TCA. Unfortunately, definitive conclusions as to whether TCE will induce tumors in humans via a mutagenic mode of action cannot be drawn from the available information. More research, including the development and use of new techniques, is required before it is possible to make a definitive assessment as to whether chemically induced mutation is a key event in any human tumors resulting from exposure to TCE.

Modeling the mouse lymphoma forward mutational assay: the Gene-Tox program database

An SAR model of the induction of mutations at the tk(+/-) locus of L5178Y mouse lymphoma cells (MLA, for mouse lymphoma assay) was derived based upon a re-evaluation of experimental results reported by a Gene-Tox (GT) working group [A.D. Mitchell, A.E. Auletta, D. Clive, P.E. Kirby, M.M. Moore, B.C. Myhr, The L5178Y/tk(+/-) mouse lymphoma specific gene and chromosomal mutation assay. A phase III report of the U.S. Environmental Protection Agency Gene-Tox Program, Mutation Res. 394 (1997) 177-303.]. The predictive performance of the GT MLA SAR model was similar to that of a Salmonella mutagenicity model containing the same number of chemicals. However, the structural determinants (biophores) derived from the GT MLA SAR model include both electrophilic as well as non-electrophilic moieties, suggesting that the induction of mutations in the MLA may occur by both direct interaction with DNA and by non-DNA-related mechanisms. This was confirmed by the observation that the set of biophores associated with MLA overlapped significantly with those associated with phenomena related to loss of heterozygosity, chromosomal rearrangements and aneuploidy. The MLA SAR model derived from the GT data evaluation was significantly more predictive than an SAR model previously derived from MLA data reported by the US National Toxicology Program [B. Henry, S.G. Grant, G. Klopman, H.S. Rosenkranz, Induction of forward mutations at the thymidine kinase locus of mouse lymphoma cells: evidence for electrophilic and non-electrophilic mechanisms, Mutation Res. 397 (1998) 331-335.]. Moreover, the latter model appeared to be more complex than the former, suggesting that the GT induction data was both simpler mechanistically and more homogeneous than that of the NTP.

Mouse lymphoma thymidine kinase locus gene mutation assay: International Workshop on Genotoxicity Test Procedures Workgroup Report

The Mouse Lymphoma Assay (MLA) Workgroup addressed and reached consensus on a number of issues. Discussion focused on five areas: (1) acceptable assay versions; (2) cytotoxicity measure; (3) 24-hr treatment; (4) microwell colony counting and sizing; and (5) data acceptability/statistical analysis. Although the International Conference on Harmonisation (ICH) indicated a preference for the microwell over the soft agar method, all of the workgroup members agreed that both versions of the MLA are equally acceptable. The workgroup agreed that it is desirable for both assay versions to use the same measure of cytotoxicity to define the acceptable and required concentration range. Currently, laboratories using the microwell version use the relative survival (RS) determined by cloning immediately after the treatment. Laboratories using the soft agar method do not obtain an RS but use the relative total growth (RTG), a combination of the relative suspension growth (RSG) during the expression period and the relative cloning efficiency determined at the time of mutant selection. The workgroup agreed to investigate the RSG, the RS, and the RTG and to develop further guidance. In the interim, the workgroup reached consensus that the RTG be used as the standard measure of cytotoxicity. The ICH recommended a 24-hr treatment in the absence of S9 when negative results are obtained with short (3-4 hr) treatments. The workgroup agreed to retain this requirement but acknowledged that more data are needed prior to making final recommendations concerning the need for and the specific protocol for the 24-hr treatment. Environ. Mol. Mutagen. 35:185-190, 2000 Published 2000 Wiley-Liss, Inc.