Mutagenic responses of five independent genetic loci in CHO cells to a variety of mutagens. Development and characteristics of a mutagen screening system based on selection for multiple drug-resistant markers

Differences in β-naphthylamine metabolism and toxicity in Chinese hamster ovary cell lines transfected with human CYP1A2 and NAT2*4, NAT2*5B or NAT2*7B N-acetyltransferase 2 haplotypes

β-naphthylamine (BNA) is an important aromatic amine carcinogen. Current exposures derive primarily from cigarette smoking including e-cigarettes. Occupational and environmental exposure to BNA is associated with urinary bladder cancer which is the fourth most frequent cancer in the United States. N-acetyltransferase 2 (NAT2) is an important metabolizing enzyme for aromatic amines. Previous studies investigated mutagenicity and genotoxicity of BNA in bacteria and in rabbit or rat hepatocytes. However, the effects of human NAT2 genetic polymorphism on N-acetylation and genotoxicity induced by BNA still need to be clarified. We used nucleotide excision repair-deficient Chinese hamster ovary (CHO) cells that were stably transfected with human CYP1A2 and NAT2 alleles: NAT2*4 (reference allele), NAT2*5B (variant slow acetylator allele common in Europe) or NAT2*7B (variant slow acetylator allele common in Asia). BNA N-acetylation was measured both in vitro and in situ via high-performance liquid chromatography (HPLC). Hypoxanthine phosphoribosyl transferase (HPRT) mutations, double-strand DNA breaks, and reactive oxygen species (ROS) were measured as indices of toxicity. NAT2*4 cells showed significantly higher BNA N-acetylation rates followed by NAT2*7B and NAT2*5B. BNA caused concentration-dependent increases in DNA damage and ROS levels. NAT2*7B showed significantly higher levels of HPRT mutants, DNA damage and ROS than NAT2*5B (p < 0.001, p < 0.0001, p < 0.0001 respectively) although both are slow alleles. Our findings suggest that BNA N-acetylation and toxicity are modified by NAT2 polymorphism. Furthermore, they confirm heterogeneity among slow acetylator alleles for BNA metabolism and toxicity supporting differential risk for individuals carrying NAT2*7B allele.

Characterization of benzo[a]pyrene and colchicine based on an in vivo repeat-dosing multi-endpoint genotoxicity quantitative assessment platform

Two prototypical genotoxicants, benzo[a]pyrene (B[a]P) and colchicine (COL), were selected as model compounds to deduce their quantitative genotoxic dose–response relationship at low doses in a multi-endpoint genotoxicity assessment platform. Male Sprague-Dawley rats were treated with B[a]P (2.5–80 mg/kg bw/day) and COL (0.125–2 mg/kg bw/day) daily for 28 days. The parameters included were as follows: comet assay in the peripheral blood and liver, Pig-a gene mutation assay in the peripheral blood, and micronucleus test in the peripheral blood and bone marrow. A significant increase was observed in Pig-a mutant frequency in peripheral blood for B[a]P (started at 40 mg/kg bw/day on Day 14, started at 20 mg/kg bw/day on Day 28), whereas no statistical difference for COL was observed. Micronucleus frequency in reticulocytes of the peripheral blood and bone marrow increased significantly for B[a]P (80 mg/kg bw/day on Day 4, started at 20 mg/kg bw/day on Days 14 and 28 in the blood; started at 20 mg/kg bw/day on Day 28 in the bone marrow) and COL (started at 2 mg/kg bw/day on Day 14, 1 mg/kg bw/day on Day 28 in the blood; started at 1 mg/kg bw/day on Day 28 in the bone marrow). No statistical variation was found in indexes of comet assay at all time points for B[a]P and COL in the peripheral blood and liver. The dose–response relationships of Pig-a and micronucleus test data were analyzed for possible point of departures using three quantitative approaches, i.e., the benchmark dose, breakpoint dose, and no observed genotoxic effect level. The practical thresholds of the genotoxicity of B[a]P and COL estimated in this study were 0.122 and 0.0431 mg/kg bw/day, respectively, and our results also provided distinct genotoxic mode of action of the two chemicals.

Value and limitation of in vitro bioassays to support the application of the threshold of toxicological concern to prioritise unidentified chemicals in food contact materials

Some of the chemicals in materials used for packaging food may leak into the food, resulting in human exposure. These include so-called Non-intentionally Added Substances (NIAS), many of them being unidentified and toxicologically uncharacterized. This raises the question of how to address their safety. An approach consisting of identification and toxicologically testing all of them appears neither feasible nor necessary. Instead, it has been proposed to use the threshold of toxicological concern (TTC) Cramer class III to prioritise unknown NIAS on which further safety investigations should focus. Use of the Cramer class III TTC for this purpose would be appropriate if amongst others sufficient evidence were available that the unknown chemicals were not acetylcholinesterase inhibitors or direct DNA-reactive mutagens. While knowledge of the material and analytical chemistry may efficiently address the first concern, the second could not be addressed in this way. An alternative would be use of a bioassay capable of detecting DNA-reactive mutagens at very low levels. No fully satisfactory bioassay was identified. The Ames test appeared the most suitable since it specifically detects DNA-reactive mutagens and the limit of biological detection of highly potent genotoxic carcinogens is low. It is proposed that for a specific migrate, the evidence for absence of mutagenicity based on the Ames test, together with analytical chemistry and information on packaging manufacture could allow application of the Cramer class III TTC to prioritise unknown NIAS. Recommendations, as well as research proposals, have been developed on sample preparation and bioassay improvement with the ultimate aim of improving limits of biological detection of mutagens. Although research is still necessary, the proposed approach should bring significant benefits over the current practices used for safety evaluation of food contact materials.

Evaluation of the RBC Pig-a assay and the PIGRET assay using benzo[a]pyrene in rats

The red blood cell (RBC) Pig-a assay has the potential to detect the in vivo mutagenicity of chemicals. Recently, use of the Pig-a assay with reticulocytes (the PIGRET assay) reportedly enabled the in vivo mutagenicity of chemicals to be detected earlier than using the RBC Pig-a assay. To evaluate whether the PIGRET assay is useful and effective as a short-term test, compared with the RBC Pig-a assay, we performed both assays using benzo[a]pyrene (BP), which is a well-known mutagen. BP was used to dose 8-week-old male rats orally at 0, 75.0, 150, and 300mg/kg administered as a single administration. Peripheral blood samples were then collected on days 0, 7, 14, and 28 after treatment and were used in both assays. In the treatment groups receiving 150mg/kg of BP or more, both the RBC Pig-a assay and the PIGRET assay detected the in vivo mutagenicity of BP. In the 300mg/kg treatment group, in which a significant increase in the mutant frequency (MF) was observed at all the sampling points using both the RBC Pig-a assay and the PIGRET assay, the reticulocyte (RET) Pig-a MF was higher than the RBC Pig-a MF on days 7 and 14 after treatment; nevertheless, the negative control RET Pig-a MF was comparable to the negative control RBC Pig-a MF. In addition, the RET Pig-a MF began to increase after day 7 and reached a maximum value on day 14 after treatment, whereas the RBC Pig-a MF increased continuously from day 7 until day 28 after treatment. These results indicate that the PIGRET assay has a higher sensitivity than the RBC Pig-a assay and that the PIGRET assay is useful for the earlier detection of the in vivo mutagenicity of chemicals, compared with the RBC Pig-a assay.

Chronic exposure to nanosized, anatase titanium dioxide is not cyto- or genotoxic to Chinese hamster ovary cells

Titanium dioxide nanoparticles (nano-TiO(2) ) are widely used in cosmetics, skin care products, paints, and water treatment processes. Disagreement remains regarding the safety of nano-TiO(2) , and little epidemiological data is available to provide needed resolution. Most studies have examined effects using acute exposure experiments with relatively few studies using a chronic exposure design. We examined cyto- and genotoxicity in CHO-K1 cells following 60 days of continuous exposure to defined levels of nano-TiO(2) (0, 10, 20, or 40 μg/ml). Oxidative stress increased in a concentration-dependent manner in short- (2 days) and long-term cultures, but long-term cultures had lower levels of oxidative stress. The primary reactive oxygen species appeared to be superoxide, and ROS indicators were lowered with the addition of superoxide dismutase (SOD). No cyto- or genotoxic effects were apparent using the XTT, trypan-blue exclusion, and colony-forming assays for viability and the Comet and Hprt gene mutation assays for genotoxicity. Nano-TiO(2) increased the percentage of cells in the G2/M phase of the cell cycle, but this effect did not appear to influence cell viability or cell division. Cellular Ti content was dose-dependent, but chronically exposed cells had lower amounts than acutely exposed cells. CHO cells appear to adapt to chronic exposure to nano-TiO(2) and detoxify excess ROS possibly through upregulation of SOD in addition to reducing particle uptake.

SFTG international collaborative study on in vitro micronucleus test I. General conditions and overall conclusions of the study

This study, coordinated by the SFTG (French branch of European Environmental Mutagen Society), included 38 participants from Europe, Japan and America. Clastogens (bleomycin, urethane), including base and nucleoside analogs (5-fluorouracil and cytosine arabinoside), aneugens and/or polyploidy inducers (colchicine, diethylstilboestrol, griseofulvin and thiabendazole), as well as non-genotoxic compounds (mannitol and clofibrate), were tested. Four cell types were used, i.e. human lymphocytes in the presence of cytochalasin B and CHO, CHL and L5178Y cell lines, in the presence or absence of cytochalasin B, with various treatment-recovery schedules. Mitomycin C was used as a positive control for all cell types. Mannitol and clofibrate were consistently negative in all cell types and with all treatment-recovery conditions. Urethane, known to induce questionable clastogenicity, was not found as positive. Bleomycin and mitomycin C were found positive in all treatment-recovery conditions. The base and nucleoside analogs were less easy to detect, especially 5-fluorouracil due to the interference with cytotoxicity, while cytosine arabinoside was detected in all cell types depending on the treatment-recovery schedule. Aneugens (colchicine, diethylstilboestrol and griseofulvin) were all detected in all cell types. In this study, the optimal detection was ensured when a short treatment followed by a long recovery was associated with a long continuous treatment without recovery. There was no impact of the presence or absence of cytochalasin B on the detection of micronucleated cells on cell lines. Scoring micronucleated cells in both mononucleated and binucleated cells when using cytochalasin B was confirmed to be useful for the detection and the identification of aneugens. In conclusion, these results, together with previously published validation studies, provide a useful contribution to the optimisation of a study protocol for the detection of both clastogens and aneugens in the in vitro micronucleus test.

ATSDR evaluation of health effects of chemicals. IV. Polycyclic aromatic hydrocarbons (PAHs): understanding a complex problem

Polycyclic Aromatic Hydrocarbons (PAHs) are a group of chemicals that are formed during the incomplete burning of coal, oil, gas, wood, garbage, or other organic substances, such as tobacco and charbroiled meat. There are more than 100 PAHs. PAHs generally occur as complex mixtures (for example, as part of products such as soot), not as single compounds. PAHs are found throughout the environment in the air, water, and soil. As part of its mandate, the Agency for Toxic Substances and Disease Registry (ATSDR) prepares toxicological profiles on hazardous chemicals, including PAHs (ATSDR, 1995), found at facilities on the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) National Priorities List (NPL) and which pose the most significant potential threat to human health, as determined by ATSDR and the Environmental Protection Agency (EPA). These profiles include information on health effects of chemicals from different routes and durations of exposure, their potential for exposure, regulations and advisories, and the adequacy of the existing database. Assessing the health effects of PAHs is a major challenge because environmental exposures to these chemicals are usually to complex mixtures of PAHs with other chemicals. The biological consequences of human exposure to mixtures of PAHs depend on the toxicity, carcinogenic and noncarcinogenic, of the individual components of the mixture, the types of interactions among them, and confounding factors that are not thoroughly understood. Also identified are components of exposure and health effects research needed on PAHs that will allow estimation of realistic human health risks posed by exposures to PAHs. The exposure assessment component of research should focus on (1) development of reliable analytical methods for the determination of bioavailable PAHs following ingestion, (2) estimation of bioavailable PAHs from environmental media, particularly the determination of particle-bound PAHs, (3) data on ambient levels of PAHs metabolites in tissues/fluids of control populations, and (4) the need for a critical evaluation of current levels of PAHs found in environmental media including data from hazardous waste sites. The health effects component should focus on obtaining information on (1) the health effects of mixtures of PAHs particularly their noncarcinogenic effects in humans, and (2) their toxicokinetics. This report provides excerpts from the toxicological profile of PAHs (ATSDR, 1995) that contains more detailed information.

Benzidine mechanistic data and risk assessment: species- and organ-specific metabolic activation

The aromatic amine benzidine (BZ) has produced various tumors, including liver tumors, in mice, rats and hamsters. BZ forms DNA adducts in rodent liver, and it is positive in most genotoxicity tests. Only bladder tumors are produced in dogs and in humans who have been occupationally exposed, possibly related to the slow rate of liver detoxification by acetylation, allowing activation of BZ or its metabolites in urine. Despite these differences, risk assessment for humans, based on liver tumors in mice, was approximately predictive of the incidence of bladder tumors observed in industrially exposed humans.

International Commission for Protection Against Environmental Mutagens and Carcinogens. Working paper no. 2. Spontaneous mutations in mammalian cells

Spontaneous or background mutation in mammals plays an important role in both medical and evolutionary contexts. However, establishing mutation frequencies or rates has not always been easy. When the field of mammalian mutagenesis was in its infancy, the word "variant" rather than "mutant" was often used because the genetic nature of the observed phenotypic alterations could not be adequately proven. Nowadays numerous target genes have been identified in which mutant frequencies can be measured, and occasionally even rates can be estimated. Indeed, the genetic basis for 'variants' now often comes from direct DNA sequencing. This review describes the most often used and best understood genetic markers for mutation research and examines their usefulness. In addition, mutational specificity is compared for several loci and the use of DNA-sequence data in determining the origins of spontaneous mutation is also discussed. An important observation is that spontaneous mutation frequencies of similarly sized genes can vary by more than an order of magnitude. Chromosomal location, the nature of the gene product and mutational specificity may offer a partial explanation.

Evaluation of the genotoxic potential of zinc pyrithione in the Salmonella mutagenicity (Ames) assay, CHO/HGPRT gene mutation assay and mouse micronucleus assay

The mutagenic potential of zinc pyrithione (Znpt) was evaluated in vitro in the Salmonella/mammalian microsome plate incorporation mutagenicity (Ames) assay and the CHO/HGPRT gene mutation assay. The clastogenic potential of Znpt was evaluated in vivo using the mouse micronucleus test. Znpt was negative in the Ames test in five tester strains in the presence and absence of rat liver microsomal enzymes when assayed at concentrations ranging between 10 and 333 micrograms per plate and between 0.03 and 33 micrograms per plate, respectively. Znpt also produced negative results in the CHO/HGPRT assay. No significant increases in mutant frequencies were seen in the presence and absence of rat liver microsomal enzymes. In each case, the highest concentrations reduced cellular viability by 83% and 85%, respectively. Znpt also did not induce increased frequencies of micronuclei in mouse bone marrow cells when tested at the maximally tolerated dose (MTD) (44 mg kg-1). These data support the conclusion that Znpt lacks genotoxic activity under the conditions of these tests.

Gene mutation assay of acrolein in the CHO/HGPRT test system

The mutagenic potential of acrolein has been studied with a wide range of in vitro and in vivo genetic toxicity assays. The data often have been conflicting, especially with the Ames assay. This study was undertaken to assess the mutagenic potential of acrolein using the CHO/HGPRT assay, both with and without metabolic activation. This assay system was chosen because it provides eukaryotic DNA as the target and is capable of detecting a range of mutational events. Because of its considerable toxicity, acrolein was tested over a very narrow dose range of 0.2-2 nl ml-1 without exogenous activation and 0.5-8 nl ml-1 with rat S-9 activation. Multiple assays were performed under both conditions. The results indicated that while acrolein was clearly very cytotoxic, it did not induce a significant mutagenic response in the presence or absence of metabolic activation.

Recommended protocols based on a survey of current practice in genotoxicity testing laboratories: II. Mutation in Chinese hamster ovary, V79 Chinese hamster lung and L5178Y mouse lymphoma cells

Laboratory protocols and guidelines have been developed for the performance of point mutation assays using Chinese hamster ovary (CHO) cells, V79 cells, and L5178Y mouse lymphoma cells. Since only minor differences in the treatment of CHO and V79 cells exist, these two assays could be combined in one procedural guideline. A second protocol was developed for the mouse lymphoma assay in order to incorporate concerns and methods specific to that cell type and genetic locus. The protocols were based primarily on current laboratory practices as determined by responses to a detailed questionnaire completed by North-American and European governmental, university and contract laboratories involved with in vitro mutation testing. This report identifies those modifications to previously described methodologies which are being used on a regular basis, provides recommendations, and also serves to clarify confusing or inconsistent practices.

The in vitro transformation of a rat liver epithelial cell line with chromium

The transformation of a rat liver epithelial cell line under a wide range of doses of chromium was determined by anchorage-independent growth and tumor formation in syngeneic animals. Chronic exposure to low concentrations and brief exposure to high concentrations of hexavalent chromium (K2CrO4) transformed the cells, but one dose (1 mM K2CrO4, 2 h) was clearly optimal in this regard. The cytotoxicity, effects on cell cycle, rates of chromium uptake, and mutagenic activity under the different treatment conditions were evaluated. The results showed that cells could adapt to the presence of chromium under certain treatment conditions, but this was not the case for the optimal transforming dose. Cells treated with chromium above the optimal transforming dose showed evidence of a transient G2 arrest, whereas all lower levels of treatment did not. A low level continuous exposure to chromate was mutagenic, whereas high level short exposures, including the optimal transforming dose, were not. An increase in the amount of protein complexed with isolated nucleic acids was detected in cells following treatment with the optimal transforming dose of chromate. The results indicate that the effects of chromium on this in vitro system vary with dose, and the identification of those events relevant to metal carcinogenesis will require consideration of treatment conditions.

Chromosomal aberrations as a contributing factor for tumor promotion in the mouse skin

Tumor promotion in mouse skin can be dissected in two stages: stage I (conversion) and stage II. Whereas for stage II clonal expansion of transformed cells is believed to play a major role, the mechanism(s) underlying conversion is still a matter of debate. Because conversion can be achieved upon treatment with phorbol ester tumor promoters prior to initiation, it is unlikely to represent simply proliferative stimulation of initiated cells (due to epigenetic changes induced). Since tumor promoters exert clastogenic activities and, on the other hand, the clastogen methyl methanesulfonate proved to be convertogenic, the possibility arises that chromosomal changes are involved in conversion. Based on this hypothesis, several findings concerning the action of tumor promoters and the process of tumor promotion in the mouse skin system are discussed and interpreted: the frequency, reversibility, and transient nature of conversion, dependence of tumor promotion on DNA synthesis, induction of DNA breaks by tumor promoters, and the protecting effect of scavengers of free radicals. A model is presented suggesting tumor formation in mouse skin (and other systems) to proceed in discrete, genetically determined steps. Initiation is considered to be due to the induction of point mutations in a dominant-acting oncogene that becomes thereupon activated, whereas the decisive event in the conversion stage of tumor promotion is the induction of numerical and/or structural chromosomal changes with the consequence of loss or inactivation of gene(s) involved in suppression of the tumor phenotype.

A review and analysis of the Chinese hamster ovary/hypoxanthine guanine phosphoribosyl transferase assay to determine the mutagenicity of chemical agents. A report of phase III of the U.S. Environmental Protection Agency Gene-Tox Program

Published literature on the Chinese hamster ovary cell/hypoxanthine guanine phosphoribosyl transferase (CHO/HGPRT) assay from mid-1979 through June 1986 was reviewed and evaluated. Data from the papers considered acceptable include test results on 121 chemicals belonging to 25 chemical classes. A total of 87 chemicals were evaluated positive, 3 negative, and 31 inconclusive. Mutagenicity data on 49 of the 121 chemicals evaluated could also be compared with in vivo animal carcinogenicity data. 40 of the 43 reported animal carcinogens were considered mutagenic. Caprolactam, the only definitive noncarcinogen in the group of 49, was not mutagenic. The CHO/HGPRT assay was concluded to be an appropriate assay system for use in the screening of chemicals for genotoxicity.

Mutagenic potency at the Na+/K+ ATPase locus correlates with cycle-dependent killing of 10T1/2 cells

Perturbation of DNA replication by chemical-DNA adducts produced by exposure to mutagenic/carcinogenic chemicals results in mutagenic or cytotoxic damage in the DNA. Demonstration of a correlation between cell cycle dependency of cytotoxicity and point mutation at the Na+/K+ ATPase gene could suggest that the two consequences of chemical exposure are caused by the same damage in the template DNA and that both are mediated through DNA replication-associated mechanisms. N-methyl-N'-nitro-N-nitrosoguanidine, N-ethyl-N'-nitro-N-nitrosoguanidine, 4-nitroquinoline-1-oxide, and benzo(a)pyrene-trans-7,8-dihydrodiol-9,10-epoxide demonstrated cell cycle-related patterns of cytotoxicity in 10T1/2 cells, with maximal cell killing produced by exposure in early S phase, and were highly efficient mutagens of the Na+/K+ ATPase gene relative to their cytotoxic potential. In contrast, methyl methanesulfonate and N-acetoxy-N-2-fluorenylacetamide were maximally cytotoxic in cell populations exposed in early G1 phase and were weak mutagens of the Na+/K+ ATPase gene at comparable levels of cytotoxicity. These data suggest that mutagenic/carcinogenic chemicals that are effective at producing mutations by misreplication kill cells by a related mechanism that may be associated with the perturbation of DNA replication.

Mutagenicity of actinomycin D in mammalian cells due to clastogenic effects

Actinomycin D was clastogenic and mutagenic in L5178Y/TK +/- -3.7.2C mouse lymphoma cells. The majority of the mutants were small colonies, indicating that actinomycin D acts primarily by a clastogenic mechanism.

A mammalian cell line designed to test the mutagenic activity of anti-herpes nucleosides

The herpes simplex virus (HSV) thymidine kinase (tk) gene was transfected into Chinese hamster ovary (CHO) 51-11 gly- cells to test its effect on the cytotoxic and mutagenic activity of anti-herpetic nucleoside analogues. Insertion of the viral tk was verified by Southern blot analysis, by sensitivity to acyclovir, and by elevated in vitro thymidine kinase (TK) activity. TK activity was increased by superinfection with a tk- virus and inhibited by antibody to viral TK. Acyclovir (ACV) was somewhat more cytotoxic in the 51-D3 cell line that expresses the viral TK than in the 51-11 parent line. Growth in ACV did not increase over background mutations at the hprt locus. FIAC (2'-fluoro-5-iodio-aracytosine) was slightly cytotoxic to the parent 51-11 line and the tk-containing clone 51-D3. FMAU (2'-fluoro-5-methyl-arauracil) had pronounced cytotoxicity in both cell lines: the 50% survival points were 1.0 microM for 51-11 cells and 0.2 microM for 51-D3. The clone 51-D3 was more sensitive than 51-11 to low concentrations of FIAU (2'-fluoro-5-iodo-arauracil), and when treated with FIAU 51-D3 had a mutation frequency to glycine independence 5 times greater than that of 51-11 cells. With both cell lines the mutation frequency at the hprt locus did not increase after growth in the presence of FIAC or FIAU. A 7-fold increase in mutation frequency at the hprt locus was detected after 51-D3 cells were grown with iododeoxyuridine. Trifluorothymidine was more toxic to 51-D3 than to 51-11 cells and increased the mutation frequency 2-fold. Cytosine-beta-D-arabinofuranoside showed no differential cytotoxicity on the two cell lines and did not increase the mutation frequency at the hprt locus.

Plasmid mediated mutagenesis of a cellular gene in transfected eukaryotic cells

NIH3T3 cells are widely used in transformation assays and readily take up transfected DNA. A system has been devised using NIH3T3 cells to measure the mutagenic effect of transfected DNA on recipient cell genes. NIH3T3 cells can be mutated to 6-thioguanine resistance at a frequency which suggests that at least a portion of the cells have only one functional copy of the HGPRT gene. They have a low spontaneous background mutation frequency (approximately 1 X 10(-7)). Transfection of three different plasmids into NIH3T3 cells induced 6-thioguanine resistant mutants at frequencies ranging from 3 to 11 fold above background. The mutant phenotype is stable and reversion frequencies of several mutants are less than or equal to 1 X 10(-7). Southern blot analysis of the HGPRT gene in several mutants showed that 4 of 26 mutants (15.4%) had detectable alterations in the structure of the HGPRT gene. Interestingly 3 of the 4 mutants showing rearrangements were obtained by transfection of the HSV-2 morphological transforming region.

Functional hemizygosity for the MDH2 locus in Chinese hamster ovary cells

Isolation of electrophoretic mobility shift mutants for a large number of enzyme loci in CHO cells has allowed the identification of many genes which are functionally hemizygous. To gain further insight into the nature of hemizygosity in CHO cells and the mechanisms by which it has arisen, we are attempting to determine whether hemizygous gene loci are clustered in a few localized chromosomal regions in CHO or are more generally distributed throughout the genome. Isozyme analysis of a series of CHO electrophoretic mobility shift mutants for MDH2 (malate dehydrogenase 2, EC 1.1.1.37) revealed that this locus is functionally hemizygous in CHO cells, but the locus could not be mapped by conventional approaches because of the similar electrophoretic mobilities of Chinese hamster and mouse MDH2 isozymes. Construction of intraspecific CHO X CHO hybrids using electrophoretic mobility shift mutants with secondary, selectable drug-resistance markers allowed us to determine that MDH2 is not closely linked to any previously mapped hemizygous marker loci in CHO, but is linked to alleles for two dizygous gene loci, PGM3 and APRT, on CHO chromosome Z7. A possible genetic basis for hemizygosity of the MDH2 locus in CHO cells is discussed.

Sodium arsenite potentiates the clastogenicity and mutagenicity of DNA crosslinking agents

To see if sodium arsenite enhances the clastogenicity and the mutagenicity of DNA crosslinking agents, Chinese hamster ovary (CHO) cells and human skin fibroblasts were exposed to cis-diamminedichloroplatinum (II) (cis-Pt(II)) or 8-methoxypsoralen (8-MOP) plus long-wave ultraviolet light (UVA) and then to sodium arsenite. The results indicate that the clastogenicity of cis-Pt(II) and 8-MOP plus UVA are enhanced by the post-treatment with sodium arsenite. Chromatid breaks and exchanges are predominantly increased in doubly treated cells. Furthermore, the mutagenicity of cis-Pt(II) at the hypoxanthine-guanine phosphoribosyl transferase locus is also potentiated by sodium arsenite in CHO cells.

Induced reversion of a Chinese hamster ovary triple auxotroph. Validation of the system with several mutagens

A Chinese hamster ovary triple auxotroph (CHO AUXB1) requires glycine, adenosine, and thymidine (GAT) for growth and survival due to a defect in the structural gene for folylpolyglutamate synthetase (FPGS). This auxotroph and others like it contain less than 3% of the parental amounts of FPGS activity. In order to develop a reverse mutation assay with CHO AUXB1, we determined the optimal conditions for measuring reversion and characterized some of the revertants. We also obtained quantitative mutagenicity data for several direct-acting mutagens for comparison to the parental CHO-S/HGPRT locus. Induced revertants appear in the culture immediately following 20-22 h exposures in +GAT complete medium, indicative of dominant genetic changes. They are maximally expressed after 2 population doublings and can be conveniently selected after 44-48 h of expression growth by plating 1 X 10(6) cells/100-mm dish into -GAT-deficient medium and incubating 12-13 days. Plating reconstruction experiments show that the cloning efficiencies of revertants in -GAT medium are not influenced by the presence of up to 1 X 10(6) CHO AUXB1 cells. Dose-dependent increases above the spontaneous revertant frequency (average = 5 X 10(7)) are induced with cis-Pt(NH3)2Cl2 (14-fold) (but not trans-Pt(NH3)2Cl2), PtCl4(10-fold), Pt(SO4)2 (14-fold), K2CrO4 (8-fold), EMS (10-fold), 4-NQO (53-fold), ICR-191 (60-fold), and ICR-170 (30-fold). All of the revertants that have been isolated are stable to repeated subculturing in -GAT medium; 40 out of 42 that have been analyzed are characterized by an increased 72-h growth incorporation of labeled folate and their extracts contain 5-94% as much FPGS as the original, parental CHO-S line. Spontaneous and induced reversion to the GAT+ phenotype primarily reflects mutations involving the FPGS gene locus. But the re-acquisition by most of the revertants of much less than normal amounts of FPGS activity suggests that they arise from compensatory second-site mutations within this gene. Comparison of the mutagenicity patterns of the foregoing compounds as a function of the applied concentration and the relative percent survival reveals some interesting similarities, as well as differences, between the CHO AUXB1/FPGS and CHO-S/HGPRT loci. In particular, the FPGS locus is rather insensitive to EMS (or other simple alkylating agents). However, it seems to be quite susceptible to reversion by other chemicals that are known to react selectively with guanine bases in DNA. CHO AUXBI is a useful supplemental mammalian assay system for assessing quantitatively the generally weak mutagenic activities of metal compounds.

Linkage of the MBG locus to another functionally hemizygous gene locus (IDH2) on chromosome Z3 in Chinese hamster ovary cells

To gain insight into the nature of hemizygosity in Chinese hamster ovary (CHO) cells and the mechanisms by which it has arisen, we are attempting to map and determine linkage relationships for as many hemizygous loci as possible. In this study, we have shown by segregation analysis of intraspecific somatic cell hybrids that the hemizygous gene locus associated with resistance to methylglyoxalbisguanyl hydrazone (MBG) in CHO cells is linked to the hemizygous IDH2 locus on chromosome Z3. Nine of the ten autosomal hemizygous gene loci that have been mapped to date in CHO cells are clustered on three chromosomes, with five such markers on chromosome 2, two on chromosome 8, and now two on the Z3 chromosome. With the mapping of MBG to the Z3 chromosome, selectable drug resistance markers are now available on eight different CHO chromosomes.

Sodium arsenite enhances the cytotoxicity, clastogenicity, and 6-thioguanine-resistant mutagenicity of ultraviolet light in Chinese hamster ovary cells

Cytotoxicity, chromosome aberrations, and mutations to 6-thioguanine resistance were synergistically increased by incubating the ultraviolet light (UV)-irradiated Chinese hamster ovary (CHO) cells in medium containing sodium arsenite. However, the frequencies of sister-chromatid exchanges and mutations to ouabain resistance induced by UV were not synergistically increased by sodium arsenite. The synergistic effect of sodium arsenite on UV-induced chromosome aberrations varied with cell-harvesting time and decreased with increasing time intervals between UV and sodium arsenite treatments.

Linkage of the MBG locus to another functionally hemizygous gene locus (IDH2) on chromosome Z3 in Chinese hamster ovary cells

To gain insight into the nature of hemizygosity in Chinese hamster ovary (CHO) cells and the mechanisms by which it has arisen, we are attempting to map and determine linkage relationships for as many hemizygous loci as possible. In this study, we have shown by segregation analysis of intraspecific somatic cell hybrids that the hemizygous gene locus associated with resistance to methylglyoxalbisguanyl hydrazone (MBG) in CHO cells is linked to the hemizygous IDH2 locus on chromosome Z3. Nine of the ten autosomal hemizygous gene loci that have been mapped to date in CHO cells are clustered on three chromosomes, with five such markers on chromosome 2, two on chromosome 8, and now two on the Z3 chromosome. With the mapping of MBG to the Z3 chromosome, selectable drug resistance markers are now available on eight different CHO chromosomes.

HGPRT- mutants of V79 cells that revert specifically by base pair substitution and frameshift mutations

In order to determine the mutagenic specificity of mutagenic and carcinogenic agents in mammalian cells, a reversion system capable of distinguishing between frameshift mutations and various kinds of base pair substitutions would be useful. We report here a method for the isolation and characterization of HGPRT- Chinese hamster V79 cell mutants that might form the basis for such a system. Two mutants of different specificity have been partially characterized. DEW-1, isolated following N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) treatment, is revertible by the base pair substitution mutagens MNNG and ethyl methanesulfonate (EMS), but not by frameshift mutagens. DSW-3, isolated following ICR-191 treatment, is specifically reverted by frameshift mutagens, but not by EMS or MNNG. With the further characterization of these and other mutants, it should be feasible to determine not only whether an agent is mutagenic in V79 cells, but also to determine the type(s) of mutation(s) it produces.

Induction of temperature-sensitive 6-thioguanine-resistant mutants as a measure of base-pair substitution mutations in Chinese hamster ovary cells

A method which allows quantification of the frequency of temperature-sensitive (ts) 6-thioguanine-resistant mutants of Chinese hamster ovary cells is described. These mutants, as well as non-ts type of mutant, contain altered hypoxanthine-guanine phosphoribosyl transferase (HGPRT) enzyme activity. The frequency of these altered enzyme mutants allows estimation of the fraction of total mutagenic events in the hgprt gene which results from base-pair substitution and thus provides a measure of the type of lesions induced by mutagenic agents. With an alkylating agent, ethyl methanesulfonate, 16-22% of the total induced mutants show these altered protein phenotypes, while none were found with the putative frameshift mutagen, ICR-191.

Possible epigenetic mechanisms of tumor progression: induction of high-frequency heritable but phenotypically unstable changes in the tumorigenic and metastatic properties of tumor cell populations by 5-azacytidine treatment

Treatment of a variety of highly tumorigenic mouse lines in vitro with chemical mutagens, such as ethyl methane sulfonate (EMS) or N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), can result in extraordinarily high frequencies (sometimes in excess of 90%) of strongly immunogenic clones unable to grow progressively in normal syngeneic hosts. These clones will, however, grow in immunosuppressed hosts and gradually regain tumorigenic ability in normal mice if maintained in long-term (several months-1 year) culture, i.e., they are often phenotypically unstable. These features--phenotypic drift and high frequency--make it unlikely that point mutations are the underlying mechanism involved in the generation of the variants. Results presented here demonstrate that these observations can be reproduced on the same tumor lines using 5-azacytidine--an analogue of cytidine which can be incorporated into DNA causing subsequent extensive hypomethylation of cytosine residues in the absence of any significant mutagenic effects. Furthermore, 5-azacytidine treatment of a nonmetastatic mouse mammary tumor led to the emergence of a small number of heritable but unstable tumor clones capable of spontaneous metastatic spread. Because it is known that DNA hypomethylation can lead to transcriptional activation of normally silent genes, that altered methylation patterns can be somatically replicated with a high but not perfect fidelity, and that mutagens can cause DNA hypomethylation, we propose that DNA hypomethylation followed by de novo methylation represents a plausible mechanism to account not only for the induction of the nontumorigenic variants but for a number of aspects of tumor progression and tumor heterogeneity, as well. In particular, we refer to heritable phenotypic alterations in tumor cell populations which occur at very high frequency but which are not necessarily stable over very long periods of time.

Quantitative mutagenesis at the adenosine kinase locus in Chinese hamster ovary cells. Development and characteristics of the selection system

Stable mutants exhibiting high degree of resistance (100-1000-fold) to various nucleoside analogs viz, toyocamycin, tubercidin, 6-methyl mercaptopurine riboside (6-MeMPR) and pyrazofurin, are obtained at similar frequency (congruent to 1 X 10(-4] in CHO cells. The mutants resistant to any of the above analogs exhibit similar degree of cross-resistance to the other three nucleoside analogs, and all of the mutants examined contained no measurable activity of the purine salvage pathway enzyme adenosine kinase (AK) which converts these analogs to their phosphorylated derivatives. These results indicate that very similar mutants are selected using any of these analogs. The recovery of AK- mutants in CHO cells is not affected by cell density (up to at least 5 X 10(5) cells per 100-mm diameter dish) and after treatment with mutagen(s) maximum mutagenic effect is observed after 7-8 days, which then remains unchanged for the next several days. Treatment of CHO cells with a number of mutagenic agents e.g. ethyl methanesulfonate, ICR170, ultraviolet light, and benzo[a]pyrene, led to a nearly linear concentration-dependent increase in the frequency of the AK- mutants in cultures. The mutagenic response of the AK locus to these agents compared favorably with that of the HGPRT locus (6-thioguanine resistance) within the same experiments. These results show that the selection system for AK- mutants provides an additional valuable genetic marker for quantitative mutagenesis studies in CHO cells.

Utilization of Chinese hamster cells in vitro and in vivo in genetic toxicology: a multiphasic approach

An approach often used to study genetic risk is comparative mutagenesis in different organisms. We have begun the development and validation of a multiphasic genetic toxicity system using the Chinese hamster and its somatic cells to measure mutational events in the same (or similar) gene, the same chromosome derived from the same animal (Fig. 1). This system will eliminate much of the uncertainty generated when different mutational events in such evolutionally divergent organisms as bacteria, insects, and mammals are compared. Using CHO cells we have defined an assay, CHO/HGPRT, to determine mutation at the hgprt locus. Coupled with S9 metabolic activation system, the CHO/HGPRT assay can quantify gene mutation and cytotoxicity induced by various classes of chemicals, physical agents, and the combination thereof. The quantitative nature of this assay permits elucidation of the structure-activity relationship for a given class of direct-acting agents. By incorporating the cytogenetics of CHO cells into this assay we can simultaneously measure induced chromosome aberrations and SCE. Using the stable CHO/human hybrid cell line AL-J1 measurement of chromosome deletion and/loss can be also performed. In order to further expand the usefulness of this genetic toxicity system to the molecular and whole animal levels we have begun development and validation of two additional systems. To study the molecular events which may result in mutation we are developing a CHOpSVgpt system. A Chinese hamster system with treatment in vivo is being developed to study mutation at the hgprt locus, chromosome aberration, and SCE in spleen cells in vitro. The use of this multiphasic genetic toxicity system at the cellular, molecular, and animal levels may soon provide reliable and rapid identification of suspected environmental mutagens.

Induction and expression of mutations at multiple drug-resistance marker loci in Chinese hamster ovary cells

We have observed quantitative and qualitative differences in the mutability and mutagen-specificity of various drug-resistance marker loci in Chinese hamster ovary (CHO) cells, which suggest that mammalian gene loci may differ in their relative mutability by a given mutagenic agent. We have used the CHO-AT3-2 multiple-marker mutagenesis assay system to examine the dose-dependent induction and kinetics of expression of mutations at four well-characterized, drug-resistance marker loci, after treatment with chemical agents which produce various types of DNA damage. The CHO-AT3-2 subline allows simultaneous quantitation and direct comparison of induced mutation frequencies at the hgprt, oua (Na+/K+ ATPase), aprt, and tk loci. The agents tested in this study included ethyl methanesulfonate, methyl methanesulfonate, mitomycin C, ICR-191, benzo[a]pyrene, and dimethylnitrosamine. The expression kinetics and optimal expression times for each drug-resistance marker were determined in dose-response experiments in which cells from mutagen-treated populations were plated at 1-2-day intervals over a period of 10 days following mutagenesis. Comparison of induced mutation frequencies for each drug-resistance marker after mutagen treatments yielding equivalent cell survivals (equitoxic doses resulting in relative cell survivals of 0.37) revealed locus-specific differences in the relative mutagenicities of the agents tested. These results indicate that the apparent mutagenicity of a particular agent at a single genetic locus may not necessarily be an accurate indicator of that agent's mutagenic potential for the genome as a whole.

Comparison of the mutagenic responses of 12 anticancer drugs at the hypoxanthine-guanine phosphoribosyl transferase and adenosine kinase loci in Chinese hamster ovary cells

5he mutagenic responses of 12 anticancer drugs--lomustine, dacarbazine, mitomycin C, chlorambucil, cis-diamminedichloroplatinum (II), busulfan, ellipticine, daunomycin, adriamycin, VM-26, VP16-213, and bleomycin--in Chinese hamster ovary (CHO) cells at two independent genetic loci which affect purine salvage pathway enzymes, hypoxanthine-guanine phosphoribosyl transferase (HGPRT) and adenosine kinase (AK) have been compared within the same experiments. The cellular mutants which lack either HGPRT (HGPRT- mutants) or AK (AK- mutants) activity can be directly selected in CHO cells using 6-thioguanine and 6-methyl mercaptopurineriboside (6-MeMPR) respectively, and optimal conditions for their selection have been described. Results of our studies show that all of the above drugs caused a concentration-dependent increase in the frequencies of both 6-thioguanine- and 6-MeMPR-resistant mutants. However, for a number of these drugs, namely adriamycin, daunomycin, bleomycin, and dacarbazine, which are only weakly mutagenic at the HGPRT locus, a relatively strong and clear mutagenic response was observed at the AK locus. In contrast, VM-26 showed a more pronounced effect at the HGPRT locus. For the remaining compounds, the mutagenic responses of the two loci (as indicated by the slopes of the dose-response curves) were comparable. The observed increase in the frequency of AK- mutants after treatment with various drugs, many of which predominantly induce frameshift and chromosome-breakage type of genetic lesions, suggests that like the HGPRT locus the AK locus is also capable of detecting all the different types of genetic lesions. The favorable response of the AK locus as compared to that of HGPRT locus in these studies indicates that the selection system of AK- mutants should provide a very useful genetic marker for mutagenesis studies in CHO cells.

Human fibroblast strains showing increased sensitivity or resistance to ethidium bromide

The sensitivity of a number of human fibroblast cell strains towards the DNA intercalating and mutagenic agent ethidium bromide has been examined. Among the cell strains investigated, 3 were of fetal origin, 6 from clinically normal adult persons, 2 from Lesch-Nyhan individuals and 1 each from persons with 3 genetic disorders, xeroderma pigmentosum, Fanconi anemia and Bloom syndrome, which are known to predispose to cancer. Results of our studies show that cells derived from 2 otherwise normal individuals exhibits a marked sensitivity towards ethidium bromide as compared to the rest of the group. At the same time all 3 cell strains of fetal origin were found to be highly resistant to killing by this agent. These results are discussed in relation to the toxic/mutagenic effects of ethidium bromide.

Mutagenic response to ultraviolet light and X-rays at five independent genetic loci in Chinese hamster ovary cells

The mutagenic response to ultraviolet light and X-irradiation at a number of well characterized independent genetic loci, viz those conferring resistance to 6-thioguanine, ouabain, emetine, methylglyoxal (bis) guanylhydrazone (MGBG), and 5, 6-dichlororibofuranosyl benzimidazole (DRB), has been determined. Upon exposure of cells to UV light in the range of 70 to 180 ergs/mm2, the frequency of mutants at all of the above genetic loci increased in a linear dose-dependent manner. However, some genetic-locus-specific differences in response to UV light were observed in these studies. In contrast to UV light, exposure of cells to X-rays caused a linear dose-dependent increase in the frequency of mutants at only the thioguanine-resistant (Thgr) locus, which affects the purine salvage pathway enzyme hypoxanthine-guanine phosphoribosyltransferase, that is not essential for growth under these conditions. The other genetic loci that affect essential cellular functions (and hence should detect only specific base substitution types of mutations), however, showed no increase in the frequencies of mutants upon X-irradiation of cells. These results are in accordance with the nature of the genetic lesions which are caused by these agents and the properties of these genetic markers.