Selection of wild-type revertants from methotrexate-resistant cells containing an altered dihydrofolate reductase.

A selection system for wild-type revertants from methotrexate-resistant Chinese hamster ovary cells is described. In the absence of exogenous thymidine, cells use the folate metabolic pathway to generate thymidine 5'-monophosphate from deoxyuridine 5'-monophosphate. Thus, in the presence of methotrexate, the incorporation of labeled deoxyuridine into phenotypic wild-type cells is inhibited whereas resistant cells that are cycling incorporate sufficient radioactivity to be killed. Using several suicide cycles, wild-type revertants have been isolated from methotrexate-resistant cells containing a structurally altered dihydrofolate reductase. These revertants possess a wild-type sensitivity to the cytotoxicity of the drug and contain a reductase with similar properties as wild-type enzyme.