MOLECULAR ANALYSIS OF CHEMICALLY-INDUCED MUTATIONS IN MAMMALIAN-CELL ASSAYS

Two mammalian cell mutation assays, the HPRT/V79 assay and the TK/mous e lymphoma assay, were compared For their ability to respond to the ge notoxic chemicals ethyl methanesulfonate (EMS) and mitomycin C (MMC). Whereas EMS induced a high mutant frequency at both loci, MMC produced few mutants at the hprt locus, but induced a large number of mutants at the tk locus. Southern blotting analysis showed that this differenc e was due to the type of genetic damage induced by the two chemicals. Intragenic changes ranging from point mutations to loss of the entire gene were recovered as viable mutants at both the hprt and tk loci. Th us, EMS which causes mainly intragenic mutations induced similar mutan t frequencies at both loci. The large multilocus deletions induced by MMC, in which the damage was assumed in many cases to extend into a ge ne essential for growth since most TK mutants were slow-growing, could not be recovered at the hprt locus. Whereas both loci will detect int ergenic mutations, mutants carrying large-scale damage are recoverable only at the heterozygous tk locus. At the hemizygous hprt locus no ho mologous chromosome exists to provide the function of essential genes if these are lost along with hprt in multilocus deletions. Most human cancers develop as a highly complex process involving both gene and mu ltilocus mutations in oncogenes and tumour suppressor genes. Thus the TK/mouse lymphoma assay isa more appropriate in vitro test for the det ection of chemicals capable of causing the types of DNA lesions import ant in human cancer.