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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 ∼3 × 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. Environ. Mol. Mutagen. 35:253–259, 2000 © 2000 Wiley‐Liss, Inc.

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Transgenic mouse mutation assays, such as MutaMouse (lacZ, CD2F1) and Big Blue (lacI, B6C3F1), afford the opportunity to evaluate the mutagenic potential of chemicals in any target organ in vivo. This paper discusses published data collected from the analysis of the skin, stomach and lung DNA after topical, oral and inhalation exposure, respectively. These data indicate that both MutaMouse and Big Blue should play an important part in the evaluation of genotoxicity in vivo, particularly where the endpoint or target tissue available in the more conventional tests is inappropriate. It is concluded that there is a distinct role for this type of assay in genetic toxicology testing. For substances applied to the skin or dosed orally or by inhalation and which are unlikely to reach either the bone marrow or the liver, then data derived from these assays may be more relevant to an assessment of possible risk to man than the currently used unscheduled DNA synthesis in liver and cytogenetics assays in bone marrow or peripheral blood.

Two complementary assay systems have been adapted for the detection of compounds which may form mutagenic photoproducts during exposure to UV light from an Osram Ultra-Vitalux sunlamp as used in the evaluation of the effectiveness of sun filters. The effects of UVA and of UVB were evaluated. A bacterial plate test using Escherichia coli strain WP2 allowed the bacteria, co-plated with test chemical in soft agar, to be irradiated with various doses of UV light. Mutagenesis was assessed by scoring numbers of tryptophan-independent colonies. The chosen reference compound was 8-methoxypsoralen (8-MOP) which was non-mutagenic alone at the highest dose tested (1000 micrograms/plate). Following simultaneous exposure of bacteria to 8-MOP and doses of UV light which alone had little effect, large numbers of revertants were scored. Numbers of mutants were dependent upon the doses of both 8-MOP and of UV light. The second test system involved the exposure of Chinese hamster ovary cells to UV light in the presence of test chemical to determine the clastogenic effects of photoproducts. Treatment with 8-MOP alone up to 50 micrograms/ml was not clastogenic but concomitant exposure to non-damaging doses of UV light caused large increases in the incidence of chromosome aberrations of all types. Damage was again dependent on the doses of both components. Two additional photoactive compounds, para-aminobenzoic acid and chlorpromazine both show photoclastogenic but not photomutagenic properties. These two complementary assay systems take advantage of using no-effect levels of UV light as a baseline against which photomutagenicity readily can be compared.(ABSTRACT TRUNCATED AT 250 WORDS)