Overview of mutation assays in transgenic mice for routine testing

Proliferating primary hepatocytes from the pUR288 lacZ plasmid mouse are valuable tools for genotoxicity assessment in vitro

Safety assessments of substances with regard to genotoxicity are generally based on a combination of in vitro and in vivo tests. These tests are performed according to a (tiered) test strategy whereby a positive result in vitro usually triggers further testing in vivo. A low specificity and high frequency of irrelevant positive results associated with most in vitro mammalian cell genotoxicity assays necessitates the design and validation of suitable alternatives. As such, we examined the feasibility of culturing primary hepatocytes from the pUR288 lacZ reporter mouse, and moreover, using established cultures to reliably assess genotoxic activity in vitro. Initial studies characterizing the metabolic capacity of proliferating lacZ primary hepatocytes indicated that these cells retained at least some activities important for xenobiotic metabolism: cytochrome P450 1A1 enzyme activities were markedly increased in the hepatocytes after exposure to benzo[a]pyrene, and also UDP-glucuronosyl transferase and glutathione-S-transferase activities, both Phase II enzymes, were detected. Increasing levels of phosphorylated p53 at residue serine 389 after ultraviolet treatment indicated a properly functioning p53, one of the criteria for an effective new test system. Four genotoxic substances with different mechanisms of genotoxicity, i.e., benzo[a]pyrene, bleomycin, etoposide, and cyclophosphamide, were tested in the lacZ rescue assay. For etoposide and cyclophosphamide, the induction of mutant colonies was rather low. Exposure to benzo[a]pyrene and bleomycin, however, yielded a clear concentration-dependent induction of the lacZ mutant frequency. Based on our preliminary observations, proliferating lacZ primary hepatocytes are a promising new tool for the assessment of genotoxic hazard.

Ionizing radiation-induced mutant frequencies increase transiently in male germ cells of older mice

Spontaneous mutant frequency in the male germline increases with age, thereby increasing the risk of siring offspring with genetic disorders. In the present study we investigated the effect of age on ionizing radiation-induced male germline mutagenesis. lacI transgenic mice were treated with ionizing radiation at 4-, 15- and 26-month-old, and mutant frequencies were determined for pachytene spermatocytes and round spermatids at 15 days or 49 days after ionizing radiation treatment. Cells collected 15 days after treatment were derivatives of irradiated differentiating spermatogenic cells while cells collected 49 days later were derivatives of spermatogonial stem cells. The results showed that (1) spontaneous mutant frequency increased in spermatogenic cells recovered from nonirradiated old mice (26-months-old), particularly in the round spermatids; (2) mutant frequencies were significantly increased in round spermatids obtained from middle-aged mice (15-months-old) and old age mice (26-months-old) at 15 and 49 days after irradiation compared to the sham-treated old mice; and (3) pachytene spermatocytes obtained from 15- or 26-month-old mice displayed a significantly increased mutant frequency at 15 days post irradiation. This study indicates that age modulates the mutagenic response to ionizing radiation in the male germline.

Epigenetic regulation of genetic integrity is reprogrammed during cloning

Cloning by somatic cell nuclear transfer (SCNT) circumvents processes that normally function during gametogenesis to prepare the gamete genomes to support development of new progeny following fertilization. One such process is enhanced maintenance of genetic integrity in germ cells, such that germ cells typically carry fewer spontaneously acquired mutations than somatic cells in the same individual. Thus, embryos produced from somatic cells by SCNT could directly inherit more mutations than naturally conceived embryos. Alternatively, they could inherit epigenetic programming that predisposes more rapid accumulation of de novo mutations during development. We used a transgenic mouse system to test these possibilities by producing cloned midgestation mouse fetuses from three different donor somatic cell types carrying significantly different initial frequencies of spontaneous mutations. We found that on an individual locus basis, mutations acquired spontaneously in a population of donor somatic cells are not likely to be propagated to cloned embryos by SCNT. In addition, we found that the rate of accumulation of spontaneous mutations was similar in fetuses produced by either natural conception or cloning, indicating that cloned fetuses do not acquire mutations more rapidly than naturally conceived fetuses. These results represent the first direct demonstration that the process of cloning by SCNT does not lead to an increase in the frequency of point mutations. These results also demonstrate that epigenetic mechanisms normally contribute to the regulation of genetic integrity in a tissue-specific manner, and that these mechanisms are subject to reprogramming during cloning.

Detailed review of transgenic rodent mutation assays

Induced chromosomal and gene mutations play a role in carcinogenesis and may be involved in the production of birth defects and other disease conditions. While it is widely accepted that in vivo mutation assays are more relevant to the human condition than are in vitro assays, our ability to evaluate mutagenesis in vivo in a broad range of tissues has historically been quite limited. The development of transgenic rodent (TGR) mutation models has given us the ability to detect, quantify, and sequence mutations in a range of somatic and germ cells. This document provides a comprehensive review of the TGR mutation assay literature and assesses the potential use of these assays in a regulatory context. The information is arranged as follows. (1) TGR mutagenicity models and their use for the analysis of gene and chromosomal mutation are fully described. (2) The principles underlying current OECD tests for the assessment of genotoxicity in vitro and in vivo, and also nontransgenic assays available for assessment of gene mutation, are described. (3) All available information pertaining to the conduct of TGR assays and important parameters of assay performance have been tabulated and analyzed. (4) The performance of TGR assays, both in isolation and as part of a battery of in vitro and in vivo short-term genotoxicity tests, in predicting carcinogenicity is described. (5) Recommendations are made regarding the experimental parameters for TGR assays, and the use of TGR assays in a regulatory context.

Induction of gene mutations by 5-(2-chloroethyl)-2'-deoxyuridine (CEDU), an antiviral pyrimidine nucleoside analogue

5-(2-chloroethyl)-2'-deoxyuridine (CEDU) had been developed for the treatment of herpes simplex infections. In the Salmonella reverse mutation test, the compound was found to be mutagenic in strains TA1535 and TA102 at very high concentrations (> or =2500 micro g/plate), both with and without S9-mix. The mutagenic potential of CEDU was further investigated in vivo and in vitro. It did not induce DNA repair in rat hepatocyte primary cultures, and was negative in the micronucleus test in V79 cells and in the comet assay in human leukocytes. In vivo, CEDU was negative in the bone marrow micronucleus test in CD1 mice. The mouse spot test provided a clearly positive result. Treatment of mice on day 9 of pregnancy with 2000 mg/kg resulted in 5.9% of the F1 animals having genetically relevant spots, whereas the corresponding vehicle control group had a spot rate of 1.9%. Since these data clearly identified CEDU as an inducer of gene mutations in vivo, this potential was further investigated in lacZ transgenic Muta Mouse. Six female animals were treated daily on five consecutive days with 2000 mg/kg/day and sacrificed, after a treatment-free sampling time, 14 days later. The data showed a clear increase in the mutant frequency in the bone marrow, the lung and in the spleen. CEDU is an exception in the group of nucleoside analogues, because it was found to be a strong gene mutagen and, in contrast to the other compounds of this group investigated so far, had no considerable clastogenic effects.

In vivo mutational analysis of liver DNA ingpt delta transgenic rats treated with the hepatocarcinogensN-nitrosopyrrolidine, 2-amino-3-methylimidazo[4,5-f]quinoline, and di(2-ethylhexyl)phthalate

In order to cast light on carcinogen-specific molecular mechanisms underlying experimental hepatocarcinogenesis in rats, in vivo mutagenicity and mutation spectra of known genotoxic rat hepatocarcinogens N-nitrosopyrrolidine (NPYR), and 2-amino-3-methylimidazo[4,5-f]quinoline (IQ), as well as the nongenotoxic hepatocarcinogen di(2-ethylhexyl)phthalate (DEHP) and the noncarcinogen acetaminophen (AAP), were investigated in guanine phosphoribosyltransferase (gpt) delta transgenic rats, a recently developed animal model for genotoxicity analysis. After 13-wk treatment, glutathione S-transferase placental form (GST-P)-positive liver cell foci were significantly increased in NPYR-treated and IQ-treated rats. In the DEHP-treated rats, marked hepatomegaly with centrilobular hypertrophy of hepatocytes occurred, although GST-P staining was consistently negative. Positive mutagenicity was detected in IQ- and NPYR-treated rats. Mutant frequencies (MFs) in the liver DNA were 188.0 x 10(-6) and 56.5 x 10(-6), approximately 35-fold and 10-fold higher, respectively, than that of nontreatment control rats (5.5 x 10(-6)). There were no increases in MFs in the DEHP- or AAP-treated rats as compared to the nontreatment control value. IQ induced mainly base substitutions leading to G:C to T:A transversions (56.9%) and deletions of G:C base pairs. In contrast, NPYR primarily caused specific A:T to G:C transitions (49.3%), which are very rare in the other groups. These data provided support for the conclusion that IQ and NPYR hepatocarcinogenesis depends on genotoxic processes and specific DNA adduct formation while DEHP exerts its influence via a nongenotoxic promotional pathway. Our data also indicate that analysis of specific in vivo mutational responses with transgenic animal models can provide crucial information for understanding the molecular mechanisms underlying chemical carcinogenesis.

Transgenic and knockout mice for DNA repair functions in carcinogenesis and mutagenesis

Genetically modified mouse models with defects in DNA repair pathways, especially in nucleotide excision repair (NER) and mismatch repair (MMR), are powerful tools to study processes like carcinogenesis and mutagenesis. The use of mutant mice in these studies has many advantages over using normal wild type mice with respect to costs, number of animals, predictive value towards carcinogenic compounds and the duration of study. Short-term carcinogenicity assays still require considerable number of animals and extensive pathological analyses. Therefore, alternatives demanding less animals and shorter exposure times would be desirable. In this respect, one approach could be the use of transgenic mice harbouring marker genes, that can easily detect mutagenic features of carcinogenic compounds, especially when such models are in a DNA repair deficient background. Here, we review the progress made in the development and use of DNA repair deficient mouse models as replacements for long-term cancer assays and discuss the applicability of enhanced gene mutant frequencies as early indicators of tumourigenesis. Although promising models exist, there is still a need for more universally responding and highly sensitive mouse models, since it is likely that non-genotoxic carcinogens will go undetected in a DNA repair deficient mouse. One attractive candidate mouse model, having a presumptive broad detective range, is the Xpa/p53 mutant mouse model, which will be discussed in more detail.

In vivo transgenic mutation assays

Transgenic rodent gene-mutation models provide relatively quick and statistically reliable assays for gene mutations in the DNA from any tissue. This report summarizes those issues that have been agreed upon at a previous IWGT meeting [Environ. Mol. Mutagen. 35 (2000) 253], and discusses in depth those issues for which no consensus was reached before. It was previously agreed that for regulatory applications, assays should be based upon neutral genes, be generally available in several laboratories, and be readily transferable. For phage-based assays, five to ten animals per group should be analyzed, assuming a spontaneous mutant frequency (MF) of approximately 3x10(-5) mutants/locus and 125,000-300,000 plaque or colony forming units (pfu or cfu) per tissue per animal. A full set of data should be generated for a vehicle control and two dose groups. Concurrent positive control animals are only necessary during validation, but positive control DNA must be included in each plating. Tissues should be processed and analyzed in a blocked design, where samples from negative control, positive control and each treatment group are processed together. The total number of pfus or cfus and the MF for each tissue and animal are reported. Statistical tests should consider the animal as the experimental unit. Nonparametric statistical tests are recommended. A positive result is a statistically significant dose-response and/or statistically significant increase in any dose group compared to concurrent negative controls using an appropriate statistical model. A negative result is a statistically non-significant change, with all mean MFs within two standard deviations of the control. During the current workshop, a general protocol was agreed in which animals are treated daily for 28 consecutive days and tissues sampled 3 days after the final treatment. This recommendation could be modified by reducing or increasing the number of treatments or the length of the treatment period, when scientifically justified. Normally male animals alone are sufficient and normally at least one rapidly proliferating and one slowly proliferating tissue should be sampled. Although, as agreed previously, sequencing data are not normally required, they might provide useful additional information in specific circumstances, mainly to identify and correct for clonal expansion and in some cases to determine a mechanism associated with a positive response.

Characterization of mutant spectra generated by a forward mutational assay for gene A of Phi X174 from ENU-treated transgenic mouse embryonic cell line PX-2

The sensitivity of in vivo transgenic mutation assays benefits from the sequencing of mutations, although the large number of possible mutations hinders high throughput sequencing. A forward mutational assay exists for Phi X174 that requires an altered, functional Phi X174 protein and therefore should have fewer targets (sense, base-pair substitutions) than forward assays that inactivate a protein. We investigated this assay to determine the number of targets and their suitability for detecting a known mutagen, N-ethyl-N-nitrosourea (ENU). We identified 25 target sites and 33 different mutations in Phi X174 gene A after sequencing over 350 spontaneous and ENU-induced mutants, mostly from mouse embryonic cell line PX-2 isolated from mice transgenic for Phi X174 am3, cs70 (line 54). All six types of base-pair substitution were represented among both the spontaneous and ENU-treated mutant spectra. The mutant spectra from cells treated with 200 and 400 microg/ml ENU were both highly different from the spontaneous spectrum (P < 0.000001) but not from each other. The dose trend was significant (P < 0.0001) for a linear regression of mutant frequencies (R(2) = 0.79), with a ninefold increase in mutant frequency at the 400 microg/ml dose. The spontaneous mutant frequency was 1.9 x 10(-5) and the spontaneous spectrum occurred at 11 target base pairs with 15 different mutations. Thirteen mutations at 12 targets were identified only from ENU-treated cells. Seven mutations had highly significant increases with ENU treatment (P < 0.0001) and 15 showed significant increases. The results suggest that the Phi X174 forward assay might be developed into a sensitive, inexpensive in vivo mutagenicity assay.

The role of nucleotide excision repair and loss of p53 in mutagenesis and carcinogenesis

Xpa mice, which have a completely defective nucleotide excision repair (NER) pathway, have a cancer predisposition when exposed to several carcinogens. NER is one of the major DNA repair pathways in the mammalian cell, and is involved in the removal of a wide variety of DNA lesions, such as those induced by UV light, bulky adducts and DNA crosslinks. To study the role of NER in both mutagenesis and carcinogenesis, NER-defective Xpa mice were crossed with transgenic lacZ/pUR288 mutation-indicator mice. Furthermore, the relationship between the tumor suppressor gene p53, NER, induction of mutations and tumor development was studied in Xpa/p53+/-/lacZ triple transgenic mice. Using the genotoxic carcinogens benzo[a]pyrene (B[a]P) and 2-acetylaminofluorene (2-AAF), it is shown that mutations in the inactive (non-transcribed) lacZ reporter gene reliably predict cancer risk. In tissues at risk for the development of tumors, increased mutant frequencies could be found at much earlier stages. A heterozygous loss of p53 appears to act synergistically to a NER defect, both in mutation- as well as tumor-induction. Surprisingly, however, the effect of a heterozygous loss of p53 appeared to be tissue-restricted, being apparent in the bladder but absent in liver and spleen.

Dietary restriction during murine development provides protection against MNU-induced mutations

The developmental stage is the most rapid period for the accumulation of somatic mutations. Epidemiological studies have also suggested a significant role of early life for cancer susceptibility, showing a protective effect of modest dietary restriction early in life. To determine if mutation rate, diet, and cancer risk are related, we have investigated the effect of dietary restriction on somatic mutations early in life. The diet of mouse dams was restricted during pregnancy and lactation by 10% from ad libitum control. F(1) pups (SWRxMutaMouse) were weaned at 3 weeks of age. Pups from dams that were on a restricted diet were kept under dietary restriction (40% until 5 weeks of age and then 20% until sacrifice). Only females from litters of seven or eight were used in this study. A portion of pups from both groups were treated with N-methyl-N-nitrosourea (MNU, 50mg/kg, i.p.) at 5 weeks of age and all mice were sacrificed at 10 weeks of age. The frequency of induced mutations was reduced by about 30% at the three loci studied, lacZ (P=0.028) and cII (P=0.042) and Dlb-1 (P=0.032) in the small intestine in the restricted group. A similar decrease in the lacZ mutant frequency was observed in the bone marrow, but the results did not reach statistical significance (P=0.074). Few differences in the lacZ mutant frequency were observed in the colon and the mammary epithelium, but variability of the mutant frequencies was such that an effect of similar magnitude could not be excluded statistically. Analysis of 47 cII mutants revealed that the majority of MNU-induced mutations were G:C to A:T transition at non-CpG sites, with no difference in the mutation spectrum between the two dietary groups.

In vivo genotoxicity of 2-amino-3,8-dimethylimidazo[4, 5-f]quinoxaline in lacI transgenic (Big Blue) mice

2-Amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx), a heterocyclic amine found in cooked meat, is a strong mutagen in the Salmonella/microsome assay and was proven to be a hepatocarcinogen in rodents. We used the lacI transgenic (Big Blue(R)) mouse to investigate MeIQx genotoxicity in vivo. lacI mutant frequencies were examined in liver and colon after single intragastric administration of MeIQx (males) or 12 weeks of feeding in the diet (males and females). Micronucleus induction was monitored in the peripheral blood and cell proliferating activity was monitored by proliferating cell nuclear antigen (PCNA) immunostaining, but only after the intragastric administration. Intragastric treatment with MeIQx (100 mg/kg) did not increase mutant frequency (MF) in liver or colon but it did induce a slight but statistically significant increase in the incidence of micronucleated reticulocytes 48 h after the treatment. No apparent increase in PCNA-positive foci was observed in any of tissues analyzed 14 days after the treatment. Administration of MeIQx (300 ppm) in diet for 12 weeks, however, caused MF increases in liver and colon in male and female mice, with greater increases in the females. An increase was also obvious after 4 weeks, but only in females. The sex difference in MF is consistent with the fact that female mice are more susceptible to MeIQx carcinogenesis. These results demonstrated that in the transgenic mouse mutation assay, long-term feeding of MeIQx was more effective than single gastric exposures in revealing the compound's mutagenicity in the target organs of carcinogenicity and that sex differences in susceptibility can also be observed.

Mutagenesis and carcinogenesis in nucleotide excision repair-deficient XPA knock out mice

Mice with a defect in the xeroderma pigmentosum group A (XPA) gene have a complete deficiency in nucleotide excision repair (NER). As such, these mice mimic the human XP phenotype in that they have a >1000-fold higher risk of developing UV-induced skin cancer. Besides being UV-sensitive, XPA(-/-) mice also develop internal tumors when they are exposed to chemical carcinogens. To investigate the effect of a total NER deficiency on the induction of gene mutations and tumor development, we crossed XPA(-/-) mice with transgenic lacZ/pUR288 mutation-indicator mice. The mice were treated with various agents and chemicals like UV-B, benzo[a]pyrene and 2-aceto-amino-fluorene. Gene mutation induction in several tumor target- and non-target tissues was determined in both the bacterial lacZ reporter gene and in the endogenous Hprt gene. Furthermore, alterations in the p53- and ras genes were determined in UV-induced skin tumors of XPA(-/-) mice. In this work, we review these results and discuss the applicability and reliability of enhanced gene mutant frequencies as early indicators of tumorigenesis.

In vivo transgenic mutation assays

Transgenic rodent gene mutation models provide quick and statistically reliable assays for mutations in the DNA from any tissue. For regulatory applications, assays should be based on neutral genes, be generally available in several laboratories, and be readily transferable. Five or fewer repeated treatments are inadequate to conclude that a compound is negative but more than 90 daily treatments may risk complications. A sampling time of 35 days is suitable for most tissues and chemicals, while shorter sampling times might be appropriate for highly proliferative tissues. For phage-based assays, 5 to 10 animals per group should be analyzed, assuming a spontaneous mutant frequency (MF) of approximately 3 x 10(-5) mutants/locus and 125,000-300,000 plaque or colony forming units (PFU or CFU) per tissue. Data should be generated for two dose groups but three should be treated, at the maximum tolerated dose (MTD), two-thirds the MTD, and one-third the MTD. Concurrent positive control animals are only necessary during validation, but positive control DNA must be included in each plating. Tissues should be processed and analyzed in a block design and the total number of PFUs or CFUs and the MF for each tissue and animal reported. Sequencing data would not normally be required but might provide useful additional information in specific circumstances. Statistical tests used should consider the animal as the experimental unit. Nonparametric statistical tests are recommended. A positive result is a statistically significant dose-response and/or statistically significant increase in any dose group compared to concurrent negative controls using an appropriate statistical model. A negative result is statistically nonsignificant with all mean MF within two standard deviations of the control.

Mutation spectrum of 4-nitroquinoline N-oxide in the lacI transgenic Big Blue Rat2 cell line

This paper describes the spectrum of mutations induced by 4-nitroquinoline N-oxide (4-NQO) in the lacI target gene of the transgenic Big Blue Rat2 cell line. There are only a few report for the mutational spectrum of 4-NQO in a mammalian system although its biological and genetic effects have been well studied. Big Blue Rat2 cells were treated with 0.03125, 0.0625 or 0.125 microg/ml of 4-NQO, the highest concentration giving 85% survival. Our results indicated that the mutant frequency (MF) induced by 4-NQO was dose-dependent with increases from three- to seven-fold. The DNA sequence analysis of lacI mutants from the control and 4-NQO treatment groups revealed an obvious difference in the spectra of mutations. In spontaneous mutants, transition (60%) mutations, especially G:C-->A:T transition (45%), were most frequent. However, the major type of base substitution after treatment of 4-NQO was transversions (68.8%), especially G:C-->T:A (43.8%), while only 25% of mutants were transitions. These results are consistent with those produced by 4-NQO in other systems and the transgenic assay system will be a powerful tool to postulate more accurately the mechanism of chemical carcinogenesis involved.

Procarbazine genotoxicity in the MutaMouse; strong clastogenicity and organ-specific induction of lacZ mutations

Procarbazine, a drug used for cancer chemotherapy, is carcinogenic in rodent bioassays. We analyzed the mutagenicity of procarbazine in various organs and the clastogenicity of the drug in hematopoietic cells of the lacZ transgenic MutaMouse. This was part of the second collaborative study of the Mammalian Mutagenesis Study Group of the Japanese Environmental Mutagen Society on the transgenic mouse mutation assay. At 50 mg kg(-1), procarbazine induced micronuclei in hematopoietic cells, but it did not increase the lacZ mutant frequency (MF) in bone marrow. It was also negative in liver, testis, spleen, kidney, and lung. Five daily administrations of 150 mg kg(-1) yielded highly positive responses in the drug's target organs for carcinogenesis (lung, bone marrow, and spleen). Lower positive responses were detected in kidney, which is a minor target organ. Liver showed only a slight increase in lacZ MF and brain showed no increase. The testis MF more than doubled which suggest that procarbazine is mutagenic to germ cells. Thus, we demonstrated that procarbazine has a strong clastogenic effect in hematopoietic cells and is mutagenic in a variety organs after high dose treatment. The induced MF was especially high in procarbazine's target organs for carcinogenesis, which supports the relevance of the transgenic mouse mutation assay for the assessment of potential genotoxins in vivo.

Induction of lacZ mutation by 7,12-dimethylbenz[a]anthracene in various tissues of transgenic mice

The induction of gene mutations was examined in MutaMouse after an intraperitoneal injection of 7, 8-dimethylbenz[a]anthracene (DMBA) at 20 mg/kg in a collaborative study participated by four laboratories. Although the DMBA dose used was lower than the level that has been reported to induce micronucleated erythrocytes maximally in several mouse strains, a killing effect appeared after day 9 of the post-treatment interval. Mutations in lacZ transgene were detected by the positive selection assay following in vitro packaging of phage lambda from the genomic DNA of the transgenic animals that survived. The mutant induction was evaluated in the bone marrow, liver, skin, colon, kidney, thymus, and testis 7 to 28 days after the treatment. In the bone marrow, the mutant frequency reached a maximum, approximately a 30-fold increase, 14 days after the treatment and the increased frequency persisted at least up to day 28 of the post-treatment. Induction of mutants was detected in the liver, colon, thymus, and skin to lesser extents. Marginal responses were obtained in the kidney and testis. The slight increases in the mutant frequencies in the kidney and testis observed in some laboratories were within laboratory-to-laboratory or animal-to-animal variations. In contrast to the gene mutation induction in the bone marrow, the frequency of micronucleated reticulocytes increased transiently 3 days after the treatment and returned to a control level before day 8 of the post-treatment. It was suggested that DMBA induced gene mutation is fixed in stem cells depending on cell proliferation while DNA damages responsible for chromosome breakage are not transmitted to progeny cells.

Tk+/- mouse model for detecting in vivo mutation in an endogenous, autosomal gene

Tk+/- transgenic mice were created using an embryonic stem cell line in which one allele of the endogenous thymidine kinase (Tk) gene was inactivated by targeted homologous recombination. Breeding Tk+/- parents produced viable Tk-/- knockout (KO) mice. Splenic lymphocytes from KO mice were used in reconstruction experiments for determining the conditions necessary for recovering Tk somatic cell mutants from Tk+/- mice. The cloning efficiency of KO lymphocytes was not affected by the toxic thymidine analogues 5-bromo-2'-deoxyuridine (BrdUrd) or trifluorothymidine (TFT), or by BrdUrd in the presence of lymphocytes from Tk+/- animals; however, it was easier to identify clones resistant to BrdUrd than to TFT when Tk+/- cells were present. Tk+/- mice were treated with vehicle or 100 mg/kg of N-ethyl-N-nitrosourea (ENU), and after 4 months, the frequency of Tk mutant lymphocytes was measured by resistance to BrdUrd. The frequency of Tk mutants was 22+/-5.9x10-6 in control animals and 80+/-31x10-6 in treated mice. In comparison, the frequency of Hprt mutant lymphocytes, as measured by resistance to 6-thioguanine, was 2.0+/-1.2x10-6 in control animals and 84+/-28x10-6 in the ENU-treated mice. Analysis of BrdUrd-resistant lymphocyte clones derived from the ENU-treated animals revealed point mutations in the non-targeted Tk allele. These results indicate that the selection of BrdUrd-resistant lymphocytes from Tk+/- mice may be used for assessing in vivo mutation in an endogenous, autosomal gene.

Spontaneous mutations in the Big Blue transgenic system are primarily mouse derived

The Big Blue transgenic mouse mutation detection system provides a powerful approach for measuring spontaneous and induced mutations in vivo. The observed mutations may contain a fraction of ex vivo or prokaryotic mutational events. Indeed, a modified, selectable form of the Big Blue assay seem to generate artifactual mutants under certain circumstances. Herein we review the evidence that circular mutants (i.e., the plaque circumference is at least 50% blue) collected in the standard Big Blue assay are derived primarily from the mouse. The most direct evidence is the similarity in the types of mutations found in jackpot and nonjackpot mutations. In addition, about half of the spontaneous mutations in the lacI transgene are transitions and transversions at CpG dinucleotides, a mammalian-specific feature. The mutation pattern observed at lacI is consistent with AT mutation pressure operating in a GC rich DNA and approaches that reported for observed germline human factor IX mutations. Furthermore, the spontaneous mutation pattern of circular Big Blue mutants differs significantly from that of an endogenous lacI gene in E. coli. Pinpoint mutants (a dot of blue color peripherally located in a wild type plaque), which a priori were not expected to be mouse-derived, have a mutation pattern consistent with the mutation pattern of an endogenous E. coli lacI gene. Analysis of induced mutagenesis studies reveals mutation frequencies and patterns for the Big Blue circular mutants which are comparable to endogenous genes. In reconstruction experiments, blue plaques derived from a superinfection with wild type and mutant phage produced approximately 50% blue and 50% clear plaques on replating. This phenomenon has not been seen when plaques derived from mouse were replated in the Big Blue assay. Collectively, the evidence strongly supports a murine origin for circular mutants recovered in the standard Big Blue assay. Validation of current assays is an essential step in determining the frequency and pattern of spontaneous murine-specific mutations. Defining this benchmark will be helpful in evaluating the next generation of transgenic mutation detection systems.

Tandem-base mutations occur in mouse liver and adipose tissue preferentially as G:C to T:A transversions and accumulate with age

Tandem-base mutations (TBM) are associated with ultraviolet light and other mutagens. Herein, we report an age- and tissue-specific difference in the frequency of spontaneous TBM in Big Blue transgenic mice. A total of 390 mutants from liver and adipose tissue contained 17 and 4 TBM, respectively, while no TBM were detected in 683 mutants from six other tissues. There was a proportional increase in the frequency of TBM in liver with age (29 days postconception to 25 months of age). Nine TBM (43%) were GG to TT transversions that preferentially occurred at specific sites. The remaining 12 mutants contained at least one transversion mutation each. We speculate that the increase of TBM in liver and adipose tissue with age is due to chronic mutagen exposure, perhaps derived from fat in the diet.

Gene-mutation assays in lambda lacZ transgenic mice: comparison of lacZ with endogenous genes in splenocytes and small intestinal epithelium

Comparison of results derived from transgenic animal gene-mutation assays with those from mutation analyses in endogenous genes is an important step in the validation of the former. We have used lambda lacZ transgenic mice to study alkylation-induced mutagenesis in vivo in (a) lacZ and hprt in spleen cells, and (b) lacZ and Dlb-I in small intestine from lambda lacZ+/0/Dlb-Ia/b mice. Induction of mutations by ethyl- and methylnitrosourea (ENU, MNU) and ethyl methanesulphonate (EMS) was investigated at 7 weeks after a single i.p. dose of each of these chemicals. In the small intestine, treatment with various dosages of ENU (10-150 mg/kg) resulted in a linear dose-response in both lacZ and Dlb-I. MNU (30 mg/kg) was also mutagenic in lacZ and Dlb-I, while EMS (250 mg/kg) did not significantly induce mutations in either gene. In spleen, ENU gave a linear dose-related response in both lacZ and hprt, MNU induced mutation sin both lacZ and hprt, and EMS was only positive for lacZ. No differences in response were observed between single and split-dose treatment with ENU (1 x 50 or 5 x 10 mg/kg with a 1- or 7-day interval), both in spleen and small intestine, except for lacZ in small intestine, where the single high dose gave a significantly higher induction than the split dose with the 7-day interval. The overall results suggest that mutagenic effects of fractionated doses are generally additive. In most cases, the induction factor (ratio treated over controls) for mutations in lacZ was lower than that for hprt and Dlb-I, presumably due to a higher background in lacZ and/or a lower mutability of lacZ. The general concordance between the data for lacZ and the endogenous genes indicates that lambda lacZ transgenic mice are a suitable model to study induction of gene mutations in vivo.

Transgenic assays for mutations and cancer: current status and future perspectives

Transgenic mouse modelling has proved to be a powerful approach to explore the various steps involved in spontaneous and induced carcinogenesis. Some of the multitude of models currently available have the potential to become a substitute for the expensive, long-term rodent bioassay to predict carcinogenicity of environmental compounds. Here, we review the progress in the development and use of transgenic mouse models specifically for the purpose of carcinogenicity and mutagenicity testing.

4-Chloro-o-phenylenediamine: a 26-week oral (in feed) mutagenicity study in Big Blue mice

4-Chloro-o-phenylenediamine (4-C-o-PDA) is a liver carcinogen in mice and was found to be weakly mutagenic in the liver of female Big Blue mice after short term treatment. In the present study the test compound was given subchronically in the diet for 26 weeks at doses of 0, 5000 and 10,000 ppm. The corresponding average test substance intake was 2166 mg kg-1 day-1 (males: 1794 mg kg-1 day-1; females: 2539 mg kg-1 day-1) and 4610 mg kg-1 day-1 (males: 3926 mg kg-1 day-1; females 5925 mg kg-1 day-1) at the low and high dose, respectively. After sacrifice, tissues were flash frozen in liquid nitrogen. The lacI mutant frequency in the liver was determined from three male and three female mice per dose group. The genomically integrated transgene was recovered by packaging into lambda phage using Transpack packaging extract (Stratagene, La Jolla, USA) followed by infection of Escherichia coli strain SCS-8. Blue mutant plaques were scored against a background of clear non-mutant plaques. Food consumption decreased initially at 10,000 ppm, while no treatment related effect on food intake was observed at 5000 ppm. Body weight gain was found to be decreased in all treated animals. Absolute and relative liver weight increased in a dose-related manner, but only the latter effect was statistically significant. A clear dose dependent increase in lacI mutant frequencies was observed in the liver of both sexes. The following mutant frequencies (x10(-5)) were observed: 2.73+/-1.01 (males, untreated), 7.24+/-1.50 (females, untreated), 18.91+/-5.30 (5000 ppm, males), 24.91+/-7.58 (5000 ppm, females), 20.47+/-6.68 (10,000 ppm, males) and 36.17+/-14.98 (10,000 ppm, females). It is therefore concluded that 4-C-o-PDA is a strong mutagen in the liver of mice treated subchronically for 26 weeks.

In vivo mutagenesis by the hepatocarcinogen quinoline in the lacZ transgenic mouse: evidence for its in vivo genotoxicity

Quinoline is carcinogenic to the liver of rats and mice and mutagenic to bacterial tester strains in the presence of rat liver microsomal enzymes. The unscheduled DNA synthesis (UDS) study suggested that quinoline might be a non-genotoxic carcinogen because of the lack of UDS-inducing capacity. In order to determine whether or not cancer induction is initiated by mutagenic DNA lesions, the present study was undertaken to evaluate the mutagenicity of quinoline in an in vivo mutation assay system using the lac Z transgenic mouse (Muta Mouse). Mutation was only induced in the liver, the target organ of carcinogenesis by quinoline, but not in the other organs examined, i.e. lung, kidney and spleen. Mutant frequency in the liver was 4-fold higher than in the untreated control animals. Dimethylnitrosamine, used as a positive control, induced mutation at a frequency 5-fold higher in the liver and 3-fold higher in the spleen than in their respective control organs. It can be concluded that the genotoxicity of quinoline is responsible for its hepatocarcinogenesis, although UDS was not induced under the conditions previously reported.

Carbon tetrachloride does not induce micronucleus in either mouse bone marrow or peripheral blood

We performed mouse bone marrow and peripheral blood micronucleus tests on carbon tetrachloride (CCl4). In the bone marrow assay, bone marrow cells were collected once after 24 h and twice, with a 24-h interval at a dose of 500, 1000 and 2000 mg/kg. In the peripheral blood assay, blood samples were collected 0, 24, 48 and 72 h after a single intraperitoneal injection at a dose of 1000, 2000 and 3000 mg/kg. As a result, micronucleated polychromatic erythrocytes (MNPCEs) were observed neither in the bone marrow assay nor the peripheral blood assay. We concluded that CCl4 does not induce chromosomal aberrations in the mouse bone marrow cells under these experimental conditions.

Comparative study on organ-specificity of tumorigenicity, mutagenicity and cell proliferative activity induced by dimethylnitrosamine in Big Blue mice

Recently, we have shown that dimethylnitrosamine (DMN) treatments increase lacI mutant frequency in the liver, kidney and lung but not in other organs, and also enhance cell proliferation only in the bronchial epithelia. In the present study, organ specificity of tumorigenicity induced by DMN was compared to those of lacI mutation and cell proliferation in Big Blue mice. Male 8-week-old Big Blue mice were treated with daily i.p. injections of 1 or 10 mg/kg DMN for 5 days, or a single i.p. injection of 5 or 10 mg/kg DMN. Except for the 10 mg/kg x 5 DMN group, all animals survived until 78 weeks after the first treatment of DMN. In the present study, the induction of cell proliferation in the bronchial epithelia was confirmed in a dose-dependent manner. At the termination of 78 weeks, it was histopathologically shown that the DMN-treated mice developed liver cell tumor in three out of seven (43%) of the 5 mg/kg group, renal tubule dysplasia in three out of seven (43%) of the 1 mg/kg x 5 group, and duodenal adenocarcinoma in one of seven (14%) of the 1 mg/kg x 5 group, although no neoplastic or preneoplastic lesions were found in the control mice. Because non-transgenic C57BL/6 mice are resistant to developing spontaneous liver cell and duodenal tumors, it was speculated that even these low doses of DMN could be sufficient to initiate target cells. Our results thus suggest that organ specificity of tumorigenicity by DMN is in favorable agreement with that of lacI mutation but not with possibly temporal cell proliferation induced by DMN.

In vivo mutagenicity of ethylene oxide at the hprt locus in T-lymphocytes of B6C3F1 lacI transgenic mice following inhalation exposure

Ethylene oxide (EO) is a direct-acting alkylating agent with the potential to induce cytogenetic alterations, mutations, and cancer. In the present study, the in vivo mutagenicity of EO at the hypoxanthine guanine phosphoribosyltransferase (hprt) locus of T-lymphocytes was evaluated following inhalation exposure of male B6C3F1 lacI transgenic mice. For this purpose, groups of male Big Blue mice at 6-8 (n = 4/group) and 8-10 (n = 5/group) weeks of age were exposed to 0, 50, 100, or 200 ppm EO for 4 weeks (6 h/day, 5 days/week). At necropsy, T-cells were isolated from thymus and/or spleen and cultured in the presence of concanavalin A, IL-2, and 6-thioguanine [Skopek, T.R., V.E. Walker, J.E. Cochrane et al. (1992) Proc. Natl. Acad. Sci. USA, 89, 7866-7870]. The time course for expression of hprt-negative lymphocytes in thymus was determined in mice necropsied 2 h, 2 weeks, and 8 weeks after exposure to 200 ppm EO. The dose-response for hprt mutant T-cells in thymus and spleen was defined in mice necropsied 2 and 8 weeks post-exposure, respectively. The hprt mutant frequency (Mf) in thymus of exposed mice was increased 2 h after exposure and reached a maximum of 7.5 +/- 0.9 x 10(-6) (average Mf +/- SE) at 2 weeks post-exposure, compared with 2.3 +/- 0.8 x 10(-6) in thymus of control mice. Dose-related increases in hprt Mfs were found in thymus from mice exposed to 100 and 200 ppm EO. In addition, a nonlinear dose-dependent increase in hprt Mfs was observed in splenic T-cells, with greater mutagenic efficiency (mutations per unit dose) found at higher concentrations than at lower concentrations of EO. Average induced Mfs (i.e. induced Mf = treatment Mf - background Mf) in splenic T-cells were 1.6, 4.6, and 11.9 x 10(-6) following exposures to 50, 100, or 200 ppm EO, respectively, while the average control Mf value was 2.2 +/- 0.3 x 10(-6). In aliquots of lymphocytes (both B- and T-cells) isolated from spleen for analysis of lacI mutations in the same animals, only two of three EO-exposed mice at the 200 ppm exposure level demonstrated an elevated lacI Mf and these elevations were apparently due to the in vivo replication of preexisting mutants and not due to the induction of new mutations associated with EO exposure [Sisk, S., L.J. Pluta, K.G. Meyer and L. Recio (1996) Mutation Res., submitted]. These data demonstrate that repeated inhalation exposures to high concentrations of EO produce dose-related increases in mutations at the hprt locus of T-lymphocytes in male lacI transgenic mice of B6C3F1 origin.

Methods for predicting carcinogenic hazards: new opportunities coming from recent developments in molecular oncology and SAR studies

Without epidemiological evidence, and prior to either short-term tests of genotoxicity or long-term tests of carcinogenicity in rodents, an initial level of information about the carcinogenic hazard of a chemical that perhaps has been designed on paper, but never synthesized, can be provided by structure-activity relationship (SAR) studies. Herein, we have reviewed the interesting strategies developed by human experts and/or computerized approaches for the identification of structural alerts that can denote the possible presence of a carcinogenic hazard in a novel molecule. At a higher level of information, immediately below epidemiological evidence, we have discussed carcinogenicity experiments performed in new types of genetically engineered small rodents. If a dominant oncogene is already mutated, or if an allele of a recessive oncogene is inactivated, we have a model animal with (n-1) stages in the process of carcinogenesis. Both genotoxic and receptor-mediated carcinogens can induce cancers in 20-40% of the time required for classical murine strains. We have described the first interesting results obtained using these new artificial animal models for carcinogenicity studies. We have also briefly discussed other types of engineered mice (lac operon transgenic mice) that are especially suitable for detecting mutagenic effects in a broad spectrum of organs and tissues and that can help to establish mechanistic correlations between mutations and cancer frequencies in specific target organs. Finally, we have reviewed two complementary methods that, while obviously also feasible in rodents, are especially suitable for biomonitoring studies. We have illustrated some of the advantages and drawbacks related to the detection of DNA adducts in target and surrogate tissues using the 32P-DNA postlabeling technique, and we have discussed the possibility of biomonitoring mutations in different human target organs using a molecular technique that combines the activity of restriction enzymes with polymerase chain reaction (RFLP/PCR). Prediction of carcinogenic hazard and biomonitoring are very wide-ranging areas of investigation. We have therefore selected five different subfields for which we felt that interesting innovations have been introduced in the last few years. We have made no attempt to systematically cover the entire area: such an endeavor would have produced a book instead of a review article.

Carcinogens induce reversion of the mouse pink-eyed unstable mutation

Deletions and other genome rearrangements are associated with carcinogenesis and inheritable diseases. The pink-eyed unstable (pun) mutation in the mouse is caused by duplication of a 70-kb internal fragment of the p gene. Spontaneous reversion events in homozygous pun/pun mice occur through deletion of a duplicated sequence. Reversion events in premelanocytes in the mouse embryo detected as black spots on the gray fur of the offspring were inducible by the carcinogen x-rays, ethyl methanesulfonate, methyl methanesulfonate, ethyl nitrosourea, benzo[a]pyrene, trichloroethylene, benzene, and sodium arsenate. The latter three carcinogens are not detectable with several in vitro or in vivo mutagenesis assays. We studied the molecular mechanism of the carcinogen-induced reversion events by cDNA analysis using reverse transcriptase-PCR method and identified the induced reversion events as deletions. DNA deletion assays may be sensitive indicators for carcinogen exposure.

Recent advances in the construction of bacterial genotoxicity assays

Bacterial mutagenicity assays have been widely used in genotoxicology research for two decades. We discuss the development of such assays, especially the Ames test, with particular attention to strain engineering. Genes encoding enzymes of mutagen bioactivation, including N-acetyltransferase, nitroreductase, and cytochrome P450, have been introduced into tester strains. The processing of DNA damage by the bacterial strains has also been modified in several ways, so as to enhance mutagenesis. These efforts have greatly increased the sensitivity of mutation assays and have illuminated the molecular mechanisms of mutagenesis. We also discuss the relationship between bacterial assays and in vivo mutation assays which use transgenic rodents.

Sources of variability in data from a positive selection lacZ transgenic mouse mutation assay: an interlaboratory study

Experimental features of a positive selection transgenic mouse mutation assay based on a lambda lacZ transgene are considered in detail, with emphasis on results using germ cells as the target tissue. Sources of variability in the experimental protocol that can affect the statistical nature of the observations are examined, with the goal of identifying sources of excess variation in the observed mutant frequencies. The sources include plate-to-plate (within packages), package-to-package (within animals), and animal-to-animal variability. Data from five laboratories are evaluated in detail. Results suggest only scattered patterns of excess variability below the animal-to-animal level, but, generally, significant excess variability at the animal-to-animal level. Using source of variability analyses to guide the choice of statistical methods, control-vs-treatment comparisons are performed for assessing the male germ cell mutagenicity of ethylnitrosourea (ENU), isopropyl methanesulfonate (iPMS), and methyl methanesulfonate (MMS). Results on male germ cell mutagenesis of ethyl methanesulfonate (EMS) and methylnitrosourea (MNU) are also reported.

Germ-cell mutagenesis in lambda lacZ transgenic mice treated with ethylating and methylating agents: comparison with specific-locus test

Germ-cell mutagenesis has been studied in male lambda lacZ transgenic mice in such a way that the data can be compared with literature data for germ-cell mutagenesis obtained with the specific-locus test (SLT). Mutagenesis induced by ethylnitrosourea (ENU), ethylmethanesulphonate (EMS), methylnitrosourea (MNU) and methylmethanesulphonate (MMS), has been studied in mature spermatozoa isolated from the epididymis and vas deferens. In order to investigate mutagenesis in different phases of spermatogenesis, animals were sacrificed at various time points after treatment. ENU at 150 mg/kg body weight significantly induced mutations in stem cells (analysis at 100 days post-treatment), but not in post-stem cells (7 days post-treatment). EMS (250 mg/kg) and MMS (60 mg/kg) induced mutations only in post-stem cells (7 days), but not in stem cells (100 days). MNU (70 mg/kg) resulted in an increase of mutations in both post-stem cells (14 and 37 days) and stem cells (100 days), although the latter, due to a limited number of data, was not statistically significant. All these data are in accordance with published SLT data. These results indicate that lambda lacZ transgenic mice are a suitable model to study gene mutations in different phases of spermatogenesis.

Evaluation of the transgenic Lambda/LacI mouse model as a short-term predictor of heritable risk

Transgenic male C57BL/6 lambda/lacI mice were used to assess the mutagenic response in seminiferous tubules and epididymal spermatozoa 3 days after exposure to ethylnitrosourea (ENU), iso-propyl methanesulfonate (iPMS) and methyl methanesulfonate (MMS). No significant mutagenic response was observed in epididymal spermatozoa for all three compounds, as expected 3 days after treatment. However, ENU and iPMS treated samples demonstrated significant mutagenic inductions relative to controls in seminiferous tubules while MMS treated samples did not. The failure of MMS to induce a mutagenic response in lambda/lacI transgenic mice is likely due to a combination of the low dose used, the short expression time after exposure and the reduced sensitivity to large deletion events in transgenic lambda/lacI shuttle vectors. In addition, ex vivo mutations were measured in control samples and iPMS treated samples, where 33% of mutants from control samples and 35% of mutants from iPMS treated samples were mosaic.

Evaluation of lacI mutation in germ cells and micronuclei in peripheral blood after treatment of male lacI transgenic mice with ethylnitrosourea, isopropylmethane sulfonate or methylmethane sulfonate

Male C57B1/6 lacI transgenic mice were used to evaluate germ cell mutagenesis in vivo as part of a collaborative study. Groups of 10 mice were administered single intraperitoneal doses of ethylnitrosourea (ENU; 150 mg/kg), isopropyl methanesulfonate (IPMS; 200 mg/kg), methyl methanesulfonate (MMS; 40 mg/kg) or vehicle. Epididymal spermatozoa and testes were recovered 3 days later and DNA isolated subsequently from epididymal spermatozoa and seminiferous tubules were analyzed for lacI mutations. The mutant frequency in seminiferous tubules (average +/- SEM) increased significantly compared with untreated controls (7.2 +/- 0.7 x 10(-5) following treatment with ENU (11.7 +/- 0.8 x 10(-5), p = 0.003) or with IPMS (9.6 +/- 0.5 x 10(-5), p = 0.018) but not following treatment with MMS (8.1 +/- 0.8 x 10(-5), p = 0.213). Group mutant frequencies were not determined for epididymal spermatozoa from MMS- or IPMS-treated mice because of poor DNA recoveries. As another indicator of the genotoxicity of these alkylating agents, the frequencies of micronuclei were determined in the peripheral blood 48 h after carcinogen administration in the same transgenic mice. The micronuclei frequencies were elevated significantly (p < 0.05) by each treatment (IPMS: 1.0%; MMS: 0.94%) compared to vehicle controls (0.3%). In a separate experiment, 40 mg/kg ENU was previously found to increase the frequency of micronuclei in peripheral blood of lacI transgenic mice 48 h after treatment (3.2%; Gibson et al., 1995). These results demonstrate that the lacI transgenic mouse male germ cells are sensitive to some, but not all, mutagens under the conditions used in this experiment. Investigation of other experimental designs would offer additional perspective on the usefulness of this transgenic model for routine mutagenicity testing in germ cells.

UVB-induced mutagenesis in hairless lambda lacZ-transgenic mice

UVB-induced mutagenesis was studied in hairless 40.6 transgenic mice (MutaMouse), which contain the lambda gt10lacZ shuttle vector as a target for mutagenesis. Mice were exposed at the dorsal side to either single doses of 200, 500, 800, or 1000 J/m2 UVB or to two successive irradiations of either 200 and 800 J/m2 UVB, with intervals of 1, 3, or 5 days, or to 800 and 200 J/m2 UVB with a 5-day interval. At 23 days after the last exposure, lacZ mutant frequencies (MF) were determined in the epidermis. The lacZ MF increased linearly with increasing dose of UVB. The mutagenic effect of two successive irradiations appeared to be additive. The UV-induced mutation spectrum was dominated by G:C --> A:T transitions at dipyrimidine sites. DNA-sequence analysis of spontaneously mutated phages showed a diverse spectrum consisting of insertions, deletions and G:C --> A:T transitions at CpG sites. The results indicate that the hairless lambda lacZ-transgenic mouse is a suitable in vivo model for studying UVB-induced mutations.

The response of germ cells to ethylene oxide, propylene oxide, propylene imine and methyl methanesulfonate is a matter of cell stage-related DNA repair

We describe the consequences of a defect for nucleotide excision repair (NER) in oocytes for alkylation-induced mutagenesis in different germ-cell stages of Drosophila males. Mutant frequencies induced in NER+ condition (cross NER+female female x NER+male) were compared with those fixed in a NER- background (cross NER-female female x NER+male), using the X-linked recessive lethal assay (SLRL) for the measurement of forward mutations in 700 loci. In successive male germ-cell stages exposed to a low dose of 2.4 mM x h methyl methanesulfonate, efficient repair of premutational damage in spermatogonia and by the maternal repair system after fertilization was observed. Ethylene oxide (EO) and propylene oxide (PO) did not induce high mutant frequencies in postmeiotic germ cells when mutagenized males were mated with NER+ females: a 32-fold increase in dose from 750 ppm x h to 24,000 ppm x h EO (approximately LD50) led to no more than a 3-fold enhancement in mutant frequency. However, up to a 17-fold increase in mutant frequencies were obtained with NER- females. In matings with NER+ females, PO was about 10 times less mutagenic than EO. Suppression of the maternal NER system caused a hypermutability, which, on the average, was 2.4-fold lower than for EO. This indicates that the 2-hydroxyethyl adduct generated by EO is more efficiently repaired than the 2-hydroxypropyl adduct caused by PO. The low SLRL frequencies (0.2-0.9%) estimated for propylene imine (PI) in NER+ genotypes showed no relation to dose in the range from 1,500 to 48,000 ppm x h. In the absence of NER, mutant frequencies were increased up to 29-fold, and a dose-dependent increase in mutations was observed for PI over the entire dose range. This study shows mutation induction by EO in postmeiotic Drosophila germ cells at exposure doses that are 800-fold below those applied previously in the mouse specific-locus test on spermatogonia [with negative response; Russell et al. (1984): Mutat Res 129:381-388] and 11-fold below the EO dose for which increased dominant-lethal responses and heritable translocations were documented in mice spermatozoa and spermatids [Generoso et al. (1990): Environ Mol Mutagen 16:126-131].

Relevance of Animal Studies in Regulatory Toxicology: Current Approaches and Future Opportunities

With rapidly increasing knowledge of toxicological processes, the scientific value and relevance of toxicity studies for risk assessment must be re-evaluated. In this paper, it is proposed that the rigid risk evaluation currently required should be replaced by a more flexible, case-by-case approach, in order to increase the relevance of each animal test conducted. The development of new types of toxicity studies and their application in risk evaluation are also described.

Are lifespan rodent carcinogenicity studies defensible for pharmaceutical agents?

For most pharmaceuticals, the assessment of carcinogenic risk to humans can be made from information available from a) genotoxicity studies in vivo, b) 3-6 month toxicology studies in two or more animal species and c) clinical investigations in Phase I and II studies aimed at assessing the existence of risk factors (genotoxicity, immune suppression, hormonal activity and chronic irritation/inflammation) associated with cancer in humans caused by pharmaceuticals. The rodent carcinogenicity bioassay is redundant for compounds with such properties. In considering the utility of a bioassay, one must recognize that the outcome of the bioassay has been shown to be predictable for about half of a random selection of chemicals in the US National Toxicology Program (NTP). The predictability of bioassay outcomes for many pharmaceuticals should be even better, given the availability of extensive knowledge on genotoxic potential in vivo and of pharmacological mechanisms, this adding to the redundancy of the bioassay. Furthermore, the value of the bioassay is itself questionable. The inconsistencies in tumor responses between rodent species and strains, the simultaneous tumor increase and decreases within a study and the susceptibility to tumorigenicity from non-genotoxic chemicals by mechanisms now shown to be of no relevance to humans, together make the use of rodents highly misleading as predictors of human cancer risk. For pharmaceuticals with a novel or poorly-understood pharmacodynamic mechanism, useful information on long-term adverse effects that might presage a carcinogenic hazard to humans may be obtained from a 12 month study, usually in rats, conducted at clinically relevant dose levels.

A strategy for the application of transgenic rodent mutagenesis assays

The past several years have seen an enormous increase in the development and use of transgenic animal models to measure mutations in specific inserted reporter genes. These systems provide gene mutation data in vivo in a wide range of relevant tissues. Numerous laboratories are now using these systems with consistent results. This paper describes the unique niche that transgenic mutagenesis systems can fill in product development and registration strategies. In addition to tissue-specific mechanistic studies, transgenic assays are available to follow up mutagenic effects demonstrated in Salmonella, Escherichia coli, mouse lymphoma (L5178Y) cells, or other in vitro systems.

Organ variation in the mutagenicity of ethylnitrosourea in Muta mouse: results of the collaborative study on the transgenic mutation assay by JEMS/MMS. The Collaborative Study Group for the Transgenic Mouse Mutation Assay Mammalian Mutagenesis Study Group of the Environmental Mutagen Society of Japan

A collaborative study of the transgenic mouse mutation assay was performed by a subgroup of the Mammalian Mutagenesis Study Group (MMS), which is a suborganization of the Environmental Mutagen Society of Japan (JEMS). Twenty-six laboratories participated in this collaboration, and ethylnitrosourea (ENU) mutagenesis was studied in eight organs of lacZ transgenic mice (Muta Mouse)-liver, spleen, bone marrow, brain, lung, kidney, urinary bladder, and heart. Mice were treated by a single intraperitoneal (ip) injection of 150 mg/kg ENU, and the lacZ mutant frequency (MF) was analyzed by positive selection after 3- and 14-day expression times. The MF in the control group was similar for all organs, approximately 40-60 x 10(-6). ENU increased MF in all organs except the brain with the highest values (more than 10 times the control value) observed in bone marrow on days 3 and 14, and in spleen on day 14. The MF in urinary bladder increased over 400 x 10(-6) and MFs in liver and lung were more than 150 x 10(-6) on day 14, although no increases were apparent in these organs on day 3. Approximately a doubling of control values was observed in kidney and heart but these were less than 100 x 10(-6). These results demonstrated that ENU induces organ specific mutagenesis with specific expression periods.

Organ variation in the mutagenicity of dimethylnitrosamine in Big Blue mice

Organ specificity in the lacI mutant frequency (MF) induced by dimethylnitrosamine (DMN) was analyzed in lung, liver, kidney, bone marrow, urinary bladder, and testis of Big Blue mice. Cell proliferative activity was also analyzed in some of these tissues by immunohistochemical staining of proliferating cell nuclear antigen (PCNA). Clastogenicity of DMN was concomitantly analyzed by the peripheral blood micronucleus assay with the same animals used for the lacI mutation assay. Five daily intraperitoneal (i.p.) treatments with DMN (1 mg/kg) increased MF in liver (6.2 x control), kidney (2.4 x control), and lung (2.1 x control). These are known target organs for DMN carcinogenesis. No MF increase was observed in nontarget organs studied, i.e., bone marrow, bladder, and testis. Single ip treatment with DMN also increased lacI MF in liver but the increases were smaller than in a 5-daily-treatment regimen. This result suggests that multiple dosing is more effective in the transgenic mutation assay. The enhancement of cell proliferation observed was in bronchial epithelia 7 days after treatment. No micronucleus induction in peripheral blood was observed 24 hours after 2 and 3 daily ip treatments with 1 mg/kg DMN. An increase in the incidence of micronucleated reticulocytes in peripheral blood was observed 48 hours after single ip treatment with 5 or 10 mg/kg DMN. The present study demonstrated organ-specific induction of gene mutations by DMN which suggests a relevance of this assay for the prediction of organ-specific carcinogenesis.

A new transgenic mouse mutagenesis test system using Spi- and 6-thioguanine selections

A new transgenic mouse mutagenesis test system has been developed for the efficient detection of point mutations and deletion mutations in vivo. The mice carry lambda EG10 DNA as a transgene. When the rescued phages are infected into Escherichia coli YG6020-expressing Cre recombinase, the phage DNA is converted into plasmid pYG142 carrying the chloramphenicol-resistance gene and the gpt gene of E. coli. The gpt mutants can be positively detected as colonies arising on plates containing chloramphenicol and 6-thioguanine. The EG10 DNA carries a chi site along with the red and gam genes so that the wild-type phages display Spi- (sensitive to P2 interference) phenotype. Mutant phages lacking both red and gam genes can be positively detected as plaques that grow in P2 lysogens of E. coli. These mutant phages are called lambda Spi-. The spontaneous gpt mutation frequencies of five independent transgenic lines were 1.7 to 3.3 x 10(-5) in bone marrow. When the mice were treated with ethylnitrosourea (single i.p. treatments with 150 mg/kg body weight; killed 7 days after the treatments), mutation frequencies were increased four- to sevenfold over the background in bone marrow. The average rescue efficiencies were more than 200,000 chloramphenicol-resistant colonies per 7.5 micrograms bone marrow DNA per packaging reaction. In contrast to gpt mutation frequencies, spontaneous Spi- mutation frequencies were 1.4 x 10(-6) and 1.1 x 10(-6) in bone marrow and sperm, respectively. No spontaneous Spi- mutants have been detected so far in spleen, although 930,000 phages rescued from untreated mice were screened. In gamma-ray-treated animals, however, induction of Spi- mutations was clearly observed in spleen, at frequencies of 1.4 x 10(-5) (5 Gy), 1.2 x 10(-5) (10 Gy), and 2.0 x 10(-5) (5O Gy). These results suggest that the new transgenic mouse "gpt delta" could be useful for the efficient detection of point mutations and deletion mutations in vivo.

Database and software for the analysis of mutations at the lacI gene in both transgenic rodents and bacteria

The use of transgenic rodents for the study of genetic toxicology has increased dramatically in the past several years. A great deal of the recent work has employed the lacI locus in transgenic mice. In addition to the transgenic data, a substantial amount of information exists regarding mutation of the lacI gene in bacteria. In an effort to centralize the information regarding mutations in the lacI gene in both rodents and bacteria, we have created a computerized database that contains information about DNA sequence alterations on about 500 mutations in transgenic rodents and 8,000 mutations in bacteria. We have also produced a software package for the analysis of the lacI database. Routines have been developed for the analysis of single base substitutions, including programs to (i) determine if two mutational spectra are different; (ii) determine if mutations show a DNA strand bias; (iii) determine the frequency of transitions and transversions; (iv) display the number and kind of mutations observed at each base in the coding region; (v) perform nearest neighbor analysis; and (vi) display mutable amino acids in the lacI protein. The software runs only on IBM-compatible machines running Microsoft Windows. The software and lacI database are freely available via the internet (http:/(/)sunsite.unc.edu/dnam/mainpage.++ +html).

Initial consideration for use of transgenic mutation assays in a regulatory submission

As an initial consideration for use of TM assays in a regulatory submission, we focus here especially on the following questions: (1) What conditions would trigger the performance of a TM assay? (2) What data set is sufficient to support a negative result? and (3) What increase in mutation frequency is considered to be a positive result? In the first question, we discuss outcomes from traditional tests such as RM, CA, GM, and MN assays. In the second question, we propose a provisional data set including several items, i.e., experimental size [number of animals per group, number of plaques per animal (tissue)], tissue selection, route of administration, top dose level, number of dosings (single vs. multiple), sampling time, number of samples, dose levels, and positive and negative controls. Some statistical and biological considerations are needed for the third question.

Temporal and molecular characteristics of lacZ mutations in somatic tissues of transgenic mice

In order to help establish criteria for optimizing protocols for in vivo mutation studies, lacZ transgenic mice (Muta mouse) were treated with five consecutive daily doses of ethylnitrosourea (50 mg/kg), sampled at times up to 55 days after treatment, and mutant frequencies and DNA sequences determined for liver and bone marrow. In the bone marrow, the mutant frequency rose very rapidly in the first 5 days after treatment to 34 times the control frequency. Subsequently, there was a brood peak where the mutant frequency did not vary significantly, although it did appear to begin to decline after 45 days. In contrast, in the liver, the peak mutant frequency (11 times the control frequency) was not achieved until 35 days, after which there appeared to be a slow decline up to 55 days, which was not statistically significant. Once the maximum mutant frequency was reached, the mutation spectra in the two tissues were indistinguishable. In contrast to the G:C-->A:T transitions in 5'-CpG sites characteristic of untreated mice, A:T-->T:A transversions and A:T-->G:C transitions were prominent in both liver and bone marrow of ENU-treated mice, suggesting the involvement of unrepaired O2- and O4-ethylthymine adducts. In addition, G:C-->T:A transversions were induced in liver. This study demonstrates the possibility that although tissues may have different mutation fixation times, a single mutation fixation time equal to the longest time may be appropriate for in vivo mutation studies, provided that the mutation frequency does not decline appreciably after the peak is reached. This study also illustrates the necessity of ensuring that mutation characteristics are determined after optimal fixation has occurred.

Evaluation of the in vivo genotoxic potential of three carcinogenic aromatic amines using the Big Blue transgenic mouse mutation assay

Three genotoxic mouse carcinogens, 4-chloro-o-phenylenediamine (4-C-o-PDA), 2-nitro-p-phenylenediamine (2-N-p-PDA), and 2,4-diaminotoluene (2,4-DAT), were tested in the Big Blue transgenic mouse mutation assay. Each experiment consisted of a vehicle control group with ten Big Blue C57BL/6 mice, five of either sex, and an equally sized group treated with a high dose of the test chemical. In addition, four animals were treated with the vehicle and six animals with the test compound for the measurement of bromodeoxyuridine (BrdU) incorporation to determine cellular proliferation. Prior to the mutagenicity experiments, the maximally tolerated dose of each compound was determined using nontransgenic C57BL/6 mice. Based on these results the doses used in the main study were 200 mg/kg/day for 4-C-o-PDA, 150 mg/kg/ day for 2-N-p-PDA, and 80 mg/kg/day for 2,4-DAT. Animals were treated for 10 days over a 2 week period and were killed 10 days after the ast treatment. In an additional experiment with 2,4-DAT, animals were killed 28 days after treatment. Since all three chemicals are liver carcinogens in the mouse, the DNA of the liver was analyzed using the standard procedures for the Big Blue assay. Hepatocyte proliferation was assessed by immunohistochemical detection of proliferating cell nuclear antigen (PCNA) and, in some studies, by measuring BrdU incorporation. 4-C-o-PDA and 2-N-p-PDA did not induce an increase in PCNA expression when measured 10 days after the last treatment. There was no increase in BrdU incorporation immediately after treatment with 4-C-o-PDA or with 2,4-DAT. However, 10 days after the last treatment with 2,4-DAT, a strong mitogenic effect was found with both techniques, i.e., in the PCNA and BrdU assays. 4-C-o-PDA, a liver carcinogen in both genders of mice, induced a small, statistically significant increase of the mutant frequencies in females. No increase was found in males. 2-N-p-PDA, which has been reported to induce liver tumors only in females, was found positive in males and was clearly negative in females. 2,4-DAT, a liver carcinogen in female mice, was positive in females and negative in males when the animals were killed 10 days after the last treatment. After an expression time of 28 days, 2,4-DAT induced a statistically significant increase in both sexes. The effect in females was marginally stronger than after 10 days' expression time and almost identical to the effect observed in males under these test conditions. In conclusion, the experiments showed that the Big Blue assay detects the genotoxicity of the three carcinogenic monocyclic aromatic amines tested. However, it seems that the sex specificity of the carcinogenic effects of these compounds is not reflected by the mutagenicity data in Big Blue mice.