The use of Chinese hamster ovary cells to quantify specific locus mutation and to determine mutagenicity of chemicals. A report of the gene-tox program.

Abstract The GENE-TOX Group on Specific Gene Mutations in Chinese Hamster Ovary (CHO) Cells has evaluated the use of mutational systems in these cells for identification of mutagenic chemicals from 261 references in the file of the Environmental Mutagen Information Center, Oak Ridge National Laboratory by February, 1979; 68 references were found to be relevant to the stated task. After establishing that the end-point of mutational measurement occurs at a specific locus and the determinations are quantifiable and reproducible, data from 21 references were found to fulfill such requirements. Among them, 14 were concerned with chemically-induced mutations to resistance to a purine analogue, 6-thioguanine, which selects for variants deficient in the enzyme hypoxanthine—guanine phosphoribosyl transferase (HGPRT). This mutational system is referred to as the CHO/HGPRT assay. Studies with other genetic markers offer promise for the development of quantitative specific genemutational assays, but these studies have not advanced thoroughly enough to assess their value. Several lines of genetic, physiological and biochemical evidence support the premise that the CHO/HGPRT system fulfills the criteria for measurement of specific gene mutations using CHO-K1-BH4 subclone and other appropriate CHO subclones. Based largely on published information, this Work Group has suggested a protocol for testing of chemical agents with consideration of the following: cells, media, culture conditions and their quality control, treatment with test compounds with and without an exogenous metabolic activation system, estimation of cytoxicity (cloning efficiency), optimum expression and selection of the mutant phenotype, calculation of mutation frequency, positive and negative controls, vehicles or solvents, spontaneous mutation frequency, dosage selection and number of doses, and collection of raw data. For interpretation of the mutagenesis data, this Work Group recommends various ways of presenting data, numerous criteria for acceptability of data, the need to use appropriate statistical procedures for data evaluation, and a potential applicability of results to hazard evaluation. Evaluation of test performances with 18 chemicals revealed that the correlation between mutagenicity in CHO/HGPRT assay and animal mutagenicity and carcinogenicity is high. Since the number of chemicals tested was small and 17 of the 18 compounds were direct-acting agents, the utility of the system for identification of various classes of potential mutagens and carcinogens cannot be adequately assessed until more chemical classes, especially promutagens, are tested. However, the assay has a sound genetic and biochemical basis for quantifying specific locus mutation reproducibly. The fact that CHO cells are also useful for determination of chemically-induced chromosome aberration and sister-chromatid exchange adds an additional strength to the assay. Future research should address the possible improvement of procedures for phenotypic expression and application for testing gaseous and volatile liquids, as well as such problems as appropriate metabolic activation system(s) and effective statistical procedures common to perhaps all short-term cellular assays. Recent rapid development of mutagen test systems like the CHO/HGPRT assay calls for a need to update and evaluate the data base generated.