Comparative induction of gene mutations and chromosome damage by 1-methoxy-1,3,5-cycloheptatriene (MCHT), 2. Results using L5178Y mouse lymphoma cells to detect both gene and chromosome damage; validation with ionizing radiation, methyl methanesulphonate, ethyl methanesulphonate and benzo[a]pyrene

A variation of the mouse lymphoma (L5178Y TK+/(-)-3.7.2c) assay has been developed using a microtiter cloning technique instead of the standard agar method. The cell line has been used to detect both gene mutations (at the Na+/K+ ATPase and thymidine kinase loci) and chromosome damage (micronucleus induction) in the same experiment. The system was validated using gamma-irradiation (a known clastogen), 2 direct-acting mutagens, ethyl and methyl methanesulphonate and an indirect-acting mutagen, benzoa]pyrene. Using the assay, 1-methoxy-1,3,5-cycloheptatriene was shown to be a clastogenic mutagen in the presence of S9, since a clear dose-dependent increase in micronuclei was observed, mainly small colony thymidine kinase mutants were observed, and no ouabain-resistant mutants were induced, a profile very similar to gamma-irradiation. The results suggest that metabolic activation potential explains the results in the accompanying paper (Asquith et al., 1990). The implications for mutagenicity testing are discussed.