Mutagenesis to aminopterin resistance in cultured hamster cells

Abstract

Spontaneous hamster cell mutants resistant to aminopterin have been shown previously to contain an elevated folate reductase activity, not corrected upon somatic cell hybridization with low enzyme parent lines. In an attempt to isolate other types of aminopterin resistant mutants we have employed the chemical mutagens, ethylmethane sulfonate and nitrosoguanidine. Mutants with low resistance were produced at an increased frequency, but highly resistant clones could not be produced directly with these agents. Highly resistant lines were obtained by a second exposure to mutagens. Unexpectedly, these induced mutants contained no more folate reductase than wild-type cells. Mechanisms of resistance considered possible from work in other systems but excluded here were decreased sensitivity of the reductase to inhibition by drug, increased thymidylate synthetase activity, and decreased drug uptake. Drug catabolism or the metabolism of folate derivatives within the cell may be altered in these mutants. These results underscore the importance of defining the biochemical alterations in induced mutants, and suggest that chemical mutagens may be particularly useful in the production of novel mutants in animal cell systems.