Effect of ploidy on spontaneous mutation rate to asparagine non-requirement in cultured cells

Thymidine resistance in Chinese hamster V79 cells in vitro

Thymidine resistance in V79 Chinese hamster cells has been investigated. Phenotypically stable variant resistant lines occurred at a high frequency, and the mutation rate (2.67 x 10(-3) per cell per generation) to 400 micrograms/ml thymidine resistance as measured by the standard Luria--Delbrück fluctuation analysis was extremely high. Populations of cells maintained for extended periods in F-10 medium spontaneously increased in resistance, possibly as a result of selective pressures due to the thymidine present in F-10 medium since this change was not observed in Dulbecco's medium. The degree of resistance for a given variant was correlated with the amount of thymidine employed in its selection. Metabolic cooperation, resulting in the suppression of the resistant phenotype, was demonstrated in artificial mixtures of sensitive and resistant clonal lines. Clones isolated in high levels of thymidine possessed lowered uptake of [3H]thymidine and the depression in uptake was related to the level of resistance of the particular clone. Although thymidine kinase specific activity levels were slightly depressed in variant cell lines, growth rate and uridine uptake were unaffected. We conclude that thymidine resistance is due to a genetically controlled depression of external thymidine uptake.

The relationship between induced mutation frequency and chromosome dosage in established mouse fibroblast lines

The frequencies of induced mutation to ouabain (OUA) and 6-thioguanine (6TG) resistance were compared in two established mouse fibroblast lines with different doses of the active X chromosome. The activity states of the X chromosomes were determined by DNA replication studies. Mutation frequency to 6TG R, an X-linked recessive phenotype, was inversely related to dosage whereas OUA R, a codominant phenotpye, occurred with equal frequency in both lines. 6TG R clones isolated from the line containing a majority of cells with two active X chromosomes were monosomic for the active X chromosome. Enzyme activity studies of these cytologically monosomic lines yieled results that were consistent with the presence of a single active X chromosome. The findings strongly support the hypothesis that mutation frequency is dependent on gene dosage.

Prospects for evaluating genetic damage in mammalian cells in culture

Starting from some general considerations on cultured mammalian cells as a biological material for the detection of genetic changes, information is given on cell lines and genetic markers that have been the subject of extensive research. The experimental variables of mutation assays are then considered, with special reference to those involved in the system resistant to 8-azaguanine. Work now in progressin the field of environmental mutagenesis is mentioned at the end of the article.

The fluctuation test as a more sensitive system for determining induced mutation in L5178Y mouse lymphoma cells

Luria-Delbrück fluctuation tests for the determination of the spontaneous mutation rate to ouabain resistance incultured L5178Y mouse lymphoma cells gave values in the range 0.44 to 1.03 X 10(-7) mutants per cell per generation. Addition of very low, non-toxic, levels of EMS (0.1 mM) and MMS (0.012 mM) 30-48 h before plating in selective medium gave a highly significant increase in the number of ouabain-resistant mutants. Methods for the calculation of spontaneous and induced mutation rates are discussed and a method for the computation of induced mutation rates is described. It is suggested that the modified fluctuation test is a simpler and considerably more sensitive assay for mutagens than the conventional experimental design. Some of its limitations are discussed.

Mutagenesis studies on cultured mammalian cells. The sensitivity of the asparagine-requiring phenotype to several chemical agents

The effect of several chemical agents on the mutation frequency from asparagine dependence to asparagine independence has been studied in Jensen sarcoma cells. It was found that ethylmethanesulfonate brought about a dramatic exponential increase, while nitrosoguanidine was not lighly effective as a mutagen, causing only a modest increase in mutation frequency, and quinacrine HCl was ineffective. The results presented here are compared with those obtained in other systems and with our previous work on the effects of UV on mutation induction in the asparagine system. They suggest that the basis of the asparagine requirement of mammalian cell lines resides in a specific genetic alteration in nuclear DNA which is corrected by the mutagenic action of the agents tested here.

On the relationship between spontaneous mutation rates in vivo and in vitro

Recent estimates of spontaneous mutation rates in man, in which previous sources of bias are corrected, indicate that the average is about 3 x 10(-7) per locus per generation, a much lower figure than is generally accepted. Assuming 100 to 1000 cell divisions between each gametic union, this information predicts that cellular mutation rats should be in the order of 10(-9) per locus per generation. Since none of the mutation rates measured in cultured cells are this low (average for seven characters equals 7 x 10(-7)), the size of mutation rates in cultured cells cannot be used to substantiate the claim of epigenetic inheritance. Furthermore, this information suggests that in multicellular organisms the germinal tissue is sequestered from mutagenic insult or subjected to selection against mutational damage so as to keep the genetic load of a species at a tolerable level. Alternatively, cell culture environments may present an extremely abnormal situation to somatic cells, thus elevating the mutation rate.