Abstract
Three classes of TFT r variants of L5178Y/TK +/− - 3.7.2C mouse lymphoma cells can be identified — large colony (λ), small colony (σ), and tiny colony (τ). The σ and λ mutants are detectable in the routine mutagenesis assay using soft agar cloning. The τ mutants are extremely slow growing and are quantitated only in suspension cloning in microwells. Variants of all three classes have been analyzed in the process of evaluating the usefulness of the thymidine kinase locus in L5178Y/TK +/− mouse lymphoma cells for detecting induced mutational damage. 150 of 152 variants from mutagen treated cultures and 163 of 168 spontaneous mutants were TFT r when rechallenged approximately 1 week after isolation (3 weeks after induction). All of the 41 mutants assayed for enzyme activity were TK-deficient. The σ and τ phenotypes were found to correlate with slow cellular growth rates (doubling time > 12 h), rather than from effects of the TFT selection or mutagen toxicity. Cytogenetic analysis of σ mutants approximately 3 weeks after induction shows an association between the σ phenotype and readily observable (at the 230–300 band level) chromosomal abnormalities (primarily translocations involving that chromosome 11 carrying the functional TK gene) in 30 of 51 induced mutants studied. Using an early clonal analysis of mutants (approximately 2 weeks after induction) 28 of 30 σ mutants showed chromosome 11 rearrangements. All λ mutants studied (17 of 17 evaluated 3 weeks after induction and 8 of 8 evaluated 2 weeks after induction) showed normal karyotypes (at the 230–300 band resolution level), including the chromosome 11s. These observations support the hypothesis that σ (and likely τ) mutants represent chromosomal mutations and λ mutants represent less extensive mutations affecting the TK locus. The inclusion of σ mutants in the total induced mutant frequency, as well as distinguishing them as a separate subpopulation of TK-deficient mutants, is, therefore, essential in obtaining maximum utility of the information provided by the L5178Y/TK +/− mouse lymphoma assay.
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