Evaluation of the L5178Y mouse lymphoma cell mutagenesis assay: quality-control guidelines and response categories

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.

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.

Mutant spectra of irradiated CHO AL cells determined with multiple markers analyzed by flow cytometry

We have previously developed a sensitive and rapid mammalian cell mutation assay which is based on a Chinese hamster ovary cell line that stably incorporates human chromosome 11 (CHO A(L)) and uses flow cytometry to measure mutations in CD59. We now show that multiparameter flow cytometry may be used to simultaneously analyze irradiated CHO A(L) cells for mutations in five CD genes along chromosome 11 (CD59, CD44, CD90, CD98, CD151) and also a GPI-anchor gene. Using this approach, 19 different mutant clones derived from individual sorted mutant cells were analyzed to determine the mutant spectrum induced by ionizing radiation. All clones analyzed were negative for CD59 expression and PCR confirmed that at least CD59 exon 4 was also absent. As expected, ionizing radiation frequently caused large deletions along chromosome 11. This technology can readily be used to rapidly analyze the mutant yield as well as the spectrum of mutations caused by a variety of genotoxic agents and provide greater insight into the mechanisms of mutagenesis.

Identification of rodent carcinogens and noncarcinogens using genetic toxicity tests: premises, promises, and performance

The basic premises that guide genetic toxicity testing for identifying carcinogens and to support administrative and regulatory decisions are: the Salmonella mutagenicity test is a necessary component of testing schemes; a chromosome aberration test is needed in addition to a gene mutation test; a mammalian cell mutagenicity test is needed in addition to the Salmonella test; in vivo tests are needed to confirm the results of in vitro tests; and test batteries are more predictive than the individual tests of the battery. Results from the Salmonella mutagenicity, in vitro chromosome aberration, mutations in mouse lymphoma cells, rodent bone marrow micronucleus, and rodent carcinogenicity tests, performed by the U.S. National Toxicology Program, were used to evaluate these premises. A positive Salmonella test was most predictive of carcinogenicity. However, the data do not support using the other tests in addition to Salmonella for predicting carcinogenicity. The genetic toxicity tests did not complement each other, and batteries or combinations of the tests were no more predictive of carcinogenicity than Salmonella alone. If a chemical is mutagenic in Salmonella it should be considered a potential rodent carcinogen, unless ancillary information suggests otherwise. Positive responses in the other in vitro or in vivo tests do not increase the probability that the chemical is a carcinogen, and negative responses in the other tests do not diminish the implications of the positive Salmonella response.

An in situ protocol for measuring the expression of chemically-induced mutations in mammalian cells

The generation of expression curves and the evaluation of mutagenic responses of mammalian cells using standard mutagenesis assays can be inaccurate because mutant and wild-type cells are usually mixed during the expression phase. If some mutant progenitors or mutants grow more slowly than the wild-type cells during the expression period, there will be a decrease in the mutant to wild-type ratio with time and the mutant fraction will not accurately represent the number of mutational events that occurred. The mutant fraction may also inaccurately assess the number of mutations if these mutations are expressed over a number of generations during the time before selection. We previously showed that recovery of L5178Y mouse cell mutants is not complete when mutations are allowed to express in suspension because slowly growing mutants and/or mutant progenitors are diluted out during this time (Rudd et al., 1990). In order to more accurately quantitate the mutagenic response of the cells, we developed an in situ procedure which segregates and immobilizes cells during expression. Because of this immobilization, slowly growing mutant progenitors and mutants expressed at different times will have an equal probability of being scored as mutants. Thus, one mutation leads to one mutant colony and the measurement of the mutagenic response of the cells to the chemical accurately reflects the mutational events that occurred. We plated L5178Y tk+/- mouse cells in semisolid medium immediately after treatment. As the cells grew and formed microcolonies, the selective agent TFT was added as an overlay at specified times, permitting only TFTr cells to survive. In this procedure, each mutation was captured as an individual colony; consequently, the measured mutation fraction accurately reflected the mutational events that occurred at the selected locus. In addition, the induced mutant colonies arising in the agar are the result of independent mutational events. We previously described the in situ protocol for L5178Y cells and showed that the spontaneous mutation rate measured was 50-fold greater than when the cells expressed the phenotype in suspension (Rudd et al., 1990). From this we concluded that the slow growth phenotype was expressed before TFT resistance. In the present paper, we evaluate the effect of chemical treatment on the mutation fraction as a function of the time to TFT addition. Using the in situ protocol, we generated expression curves for three nucleotide analogs, 5-azacytidine, TFT and AraC. The numbers of TFTr colonies produced at various times after treatment indicated that chemically-treated cultures had higher mutation fractions than the solvent controls.(ABSTRACT TRUNCATED AT 400 WORDS)

L5178Y mouse lymphoma cell mutation assay results with 41 compounds

Forty-one chemicals were tested for their abilities to induce trifluorothymidine resistance in L5178Y mouse lymphoma (MOLY) cells. These chemicals were included in the National Toxicology Program's evaluation of four in vitro short-term toxicity assays for predicting carcinogenicity in the rodent bioassay. Of the 41 chemicals examined for this report, 8 were equivocal in the rodent bioassay, and 7 were questionable in- the MOLY assay. If these chemicals are eliminated from an analysis of concordance, the remaining 26 chemicals lead to a concordance of 69% with a sensitivity of 71%. The specificity could not be determined because only two non-carcinogens were detected.

Evaluation of the L5178Y mouse lymphoma cell mutagenesis assay: interlaboratory reproducibility and assessment

The L5178Y mouse lymphoma cell mutagenesis assay is used to detect the mutagenic activity of chemicals in a mammalian cell system. To evaluate this assay we compared the results of assays performed independently on 63 chemicals by laboratories at SRI International and Litton Bionetics, Inc. The two laboratories used similar protocols. The solvent and positive control mutant frequencies and cloning efficiencies obtained by the two laboratories were similar, which justified the use of the same quality-control criteria and analytical procedures for analyzing the results from both laboratories. The rate of concordance between the two laboratories was 92% for tests in the absence of S9 activation and 95% for tests in its presence. The results of the assays agreed for 57 of the 63 chemicals; three chemicals could not be compared because there were questionable calls in at least one of the laboratories; the results disagreed for the three remaining chemicals. The concordance rate for these overall assay evaluations was 95%. The interlaboratory concordance rates were similar to concordance rates for replicate experiments within the laboratories (96% at LBI, 94% at SRI). The mouse lymphoma cell mutagenicity results are concordant with the rodent chronic assay results in 78% of 50 chemicals and with the Salmonella assay results in 79% of 56 chemicals. Fifteen carcinogens were examined for genotoxic effects in mouse lymphoma, Salmonella, Chinese hamster ovary (CHO) chromosomal aberration, and CHO sister chromatid exchange assay. Eight of these were positive in all four assays. Of the seven noncarcinogens that were tested in these four assays, none was negative in all four. The main conclusion to be drawn from this study is that the mouse lymphoma cell forward mutation assay, as performed and evaluated in this study, detects chemical mutagenicity in a manner that is highly consistent with other genetic endpoints as well as rodent carcinogenicity studies. Thus the assay quality control and response criteria established in this study led not only to a high degree of reproducibility but also to an apparently reliable detection of mutagenic activity.

Evaluation of the L5178Y mouse lymphoma cell mutagenesis assay: methods used and chemicals evaluated

A general protocol, modified from the one described by Clive and Spector (Mutat Res 31:17-29, 1975), was followed by two laboratories, Litton Bionetics, Inc., and SRI International, to evaluate 63 coded chemicals from 16 chemical classes for mutagenic activity at the thymidine kinase locus in L5178Y TK+/- 3.7.2C mouse lymphoma cells. The general protocol is discussed. Some procedural variations introduced by both laboratories are described and discussed in terms of their potential effect on the comparative results of the assay. Also included are the chemical structures, molecular weights, and functional classifications of the 63 chemicals. The assay appeared to tolerate the specific procedural variations in each laboratory without changing its reliability.

Evaluation of the L5178Y mouse lymphoma cell mutagenesis assay: intralaboratory results for sixty-three coded chemicals tested at Litton Bionetics, Inc

The reliability of the L5178Y TK+/- forward mutation assay as a rapid screen for genotoxicity was evaluated by testing 63 coded chemicals. Replicate treatments were used, and at least two independent experiments were performed for each test condition. The test conditions consisted of no exogenous activation, activation by Aroclor 1254-induced Fischer 344 rat liver S9 homogenate, and in some cases activation by noninduced Fischer 344 rat liver S9. The results were organized into tables that show the mutant colony counts, mutant frequency, and toxicity for each test chemical treatment, positive control treatment, and solvent negative control cultures. The repeat experiments were highly consistent and yielded contradictory evaluations for only a few of the chemicals studied. Fifty-one of the chemicals (81%) were evaluated as mutagenic under one or both of the test conditions. A range in minimum effective concentrations of almost 10(6)-fold (0.008 to 5,000 micrograms/ml) was observed among the mutagenic chemicals. Nine chemicals (14%) were considered to be nonmutagenic. Three chemicals (progesterone, p-rosaniline HCl, and 1,1,1-trichloroethane) gave responses that were not easily evaluated under any test condition: evidence for mutagenesis was obtained in some experiments but not for all repeat studies. Under nonactivation conditions, specifically, the mutagenic activities of 4,4'-bis(dimethylamino)benzophenone, progesterone, and p-rosaniline HCl remained uncertain. With S9 activation, uncertain evidence for mutagenesis was obtained for 2-naphthylamine, progesterone, and 1,1,1-trichloroethane. In some cases, changes in the treatment conditions could lead to different evaluations of the mutagenic activity, and these possibilities are discussed in the descriptive evaluations of each chemical. Comparisons of the observed responses with published results were possible for 29 of the compounds and yielded highly confirmatory evaluations.

Evaluation of the L5178Y mouse lymphoma cell mutagenesis assay: intralaboratory results for sixty-three coded chemicals tested at SRI International

SRI used the L5178Y mouse lymphoma cell forward mutation assay to determine the mutagenic activity of 63 coded chemicals from 16 chemical classes. Replicate experiments were performed to assess the reproducibility of the assay within the laboratory. The evaluations (positive or negative) of the first two repeat experiments with the chemicals were the same for 116 (87%) of 134 tests. Evaluational differences between the first two experiments were fewer in the presence of induced S9 (6 tests) than in the absence of S9 (12 tests). The most commonly observed variability was the magnitude of positive mutagenic responses; this may be attributed to factors such as compound solubilities, S9 activation conditions, and differential recovery of mutant cells. Some consistency was observed in the responses of compounds of various chemical classes. Generally, antibiotics (ABO) and the azo dyes, azoxy and hydrazo compounds, diazoalkanes, nitriles and azides (AZO), were mutagenic with S9; alkyl, acyl, and aryl halides, halogenated ethers, and halohydrins (HAL) were more strongly mutagenic with than without S9; and monofunctional polycyclic aromatic hydrocarbons and fluorenones (PAH) were mutagenic only with S9. Amine-1-oxides (AMO), alkyl and aryl epoxides (EPO), and nitroalkanes, nitroaromatics, nitroquinolines, nitrofurans, and nitroimidazoles (NIT) were mutagenic with and without S9; amides, sulfonamides, aromatic amines, aliphatic amines, hydroxylamines, and benzidine and its derivatives (AMI) were mutagenic without S9; and methyl carbamate (the only monofunctional carbamate) and thioureas (CBM) induced a negative response under both conditions.