Genetic transformation of murine bone marrow cells to methotrexate resistance

Genetic transformation of murine bone marrow stem cells to methotrexate resistance was achieved using a modified calcium phosphate-DNA coprecipitation procedure. Bone marrow cells were transformed by DNA derived from methotrexate-resistant mouse 3T6 cells. In vivo selection of drug-resistant bone marrow cells resulted from thrice weekly injections of methotrexate (MTX) for a period of 6-8 wk. Following selection, dihydrofolate reductase activity encoded by the donor DNA species was easily detectable in extracts of recipient mouse spleens. In addition, selection of methotrexate-resistant cells was indicated by the persistence of spleen colony-forming units (CFU-S) in drug-treated animals. Also, changes in ratios of mixed syngeneic bone marrow cells derived from CBA and CBA/T6T6 mice resulted from initial treatment of either cell type with 3T6 DNA. These results confirm and extend the observations of Cline and coworkers that normal bone marrow cells can be genetically transformed to methotrexate resistance.