Further evidence for the genetic origin of mutations in mammalian somatic cells: the effects of ploidy level and selection stringency on dose-dependent chemical mutagenesis to purine analogue resistance in Chinese hamster ovary cells
Hsie, A.W.; Brimmer, P.A.; Machanoff, R.; Hsie, M.H.
Mutation Research 45(2): 271
1977
ISSN/ISBN: 0027-5107 PMID: 201836 Document Number: 123417
Whether resistance to purine analogues 8-azaguanine (AG) and 6-thioguanine (TG) in mammalian cells is due to gene mutation or to epigenetic changes was investigated by an ethyl methanesulfonate (EMS) dose-dependent induced resistance to these analogues in 2 near-diploid (2N) and 1 tetraploid (4N) Chinese hamster ovary (CHO) cells. EMS produced higher cell killing in 2N than in 4N cells. In the 2N cells, EMS-induced mutations to TG (1.7 .mu.g/ml) resistance increased approximately as a linear function of the dose from 0-400 .mu.g/ml. EMS was ineffective in inducing such mutation in the 4N cells. These observations are consistent with the notion that the induced TG resistance arose as a result of mutation at the gene or chromosome level. In each cell type, both the observed spontaneous and the EMS-induced frequency to purine analogue resistance decreased with increasing concentration of purine analogues. Among the resistant clones a high proportion of those selected at 1.2 and 3.0 .mu.g/ml of AG, a small portion selected at 7.5 .mu.g/ml of AG, and virtually none at 1.7 and 6.0 .mu.g/ml of TG are capable of growth in medium containing aminopterin (10 .mu.M). Under less stringent selective conditions, some resistant variants were apparently selected through mechanisms not yet defined.