Mutant frequencies and loss of heterozygosity induced by N-ethyl-N-nitrosourea in the thymidine kinase gene of L5178Y/TK+-3.7.2C mouse lymphoma cells

N-ethyl-N-nitrosourea (ENU) is a potent monofunctional ethylating agent that has been found to be mutagenic in a wide variety of organisms from viruses to mammalian germ cells. To elucidate the mutagenicity of ENU at the Tk(+/-) locus of mouse lymphoma cells and to confirm the ability of the mouse lymphoma assay (MLA) to detect both point mutations and large DNA alterations, Tk(+/-) L5178Y cells were exposed to different doses of ENU. Treatment of the cells with ENU resulted in a linear dose response with mutant frequencies of up to 16-fold over control. Evaluation of mutant clone size showed that 36% of the 100 mug/ml ENU-induced clones (66% in control) were small colony mutants and 64% (34% in control) were large colony mutants. DNA isolated from mutants in the control culture and the 100 mug/ml ENU treatment group was analyzed for loss of heterozygesity (LOH) using allele-specific PCR. The majority of the small colony mutants, both ENU-treated (97%) and spontaneous (91%), lost the Tk1b allele. The percentage of allele loss in ENU-Induced large colony mutants was distinctly different from that of the control. Twenty-three percent of ENU-induced large colony mutants lost their Tk1b alleles, whereas 73% of the large colony mutants from the control culture lost the allele (P < 0.001). Overall, 50% of the Tk mutants from the 100 μg/ml ENU-treated cultures (86% in control) showed LOH. Our data indicate that ENU is a potent mutagen in mouse lymphoma cells and that 100 μg/ml ENU induces equal numbers of point mutations and chromosomal mutations. This study serves to verify that the MLA detects both point mutations and chromosomal mutations.