Point mutations at the thymidine kinase locus in L5178Y mouse lymphoma cells: I. Application to genetic toxicological testing

Abstract

We have systematically evaluated the mouse lymphoma TK +/- → TK -/- mutagenesis assay to determine if this somatic-cell test system would be a useful addition to the routine screening battery already used in our laboratory for the detection of chemical mutagens. During these investigations we observe that, with certain modifications of the basic assay, mutagenicity data could be obtained in as little as 9 days once the relative cytotoxic properties of the test substance were known. By improving the culturing conditions, we were able to reduce the serum requirements by as much as 50–75% without appreciably altering either cell viability or the recovery of chemically-induced mutants. Phenotypic stability of test-derived trifluorothymidine resistant (TFT R) mutants was confirmed by demonstrating cross-resistance to bromodeoxyuridine and concomitant sensitivity to methotrexate (THMG) in TFT R cells grown for 20 generations under non-selective conditions. While reduced growth rates resulting from temporary cell-division delay in treated cells is probably not a contributing factor to the observed mutation frequencies, only TFT R colonies which formed large distinct colonies in the presence of trifluorothymidine were clearly phenotypically stable mutants when spontaneous mutants were isolated and verified. When a non-mutagen, a weak mutagen, and a well-established mutagen were compared at equitoxic doses under these modified conditions, clear quantitative differences were seen in the respective mutation frequencies induced by these 3 types of agents. With these technical modifications, we feel this assay is both reliable and amenable to the screening of diverse chemical compounds for point-mutational activity in a mammalian cell.