NIH3T3 cell lines expressing different polyoma virus oncogenes were found to differ by almost three orders of magnitude in frequencies of methotrexate-resistant (MTXr) cells in single-step selections. Cells expressing the polyoma small T antigen showed the highest frequencies of MTX resistance, whereas cells expressing the polyoma middle T antigen showed the lowest frequencies of MTX resistance. Amplification of the dihydrofolate reductase (dhfr) gene was not a mechanism of MTX resistance in these cell lines. Cells expressing large T antigen also showed a high frequency of MTXr colonies with only two of 20 such colonies showing evidence of marginal (less than threefold) dhfr gene amplification. Cells carrying the cloning vector without an oncogene showed intermediate frequencies of MTX resistance, and in contrast to the oncogene expressing lines, two-thirds of these MTXr colonies showed marked dhfr gene amplification. The results suggest that the plasticity of the genome as measured by frequency or extent of dhfr gene amplification is not increased by expression of either immortalizing or transforming polyoma oncogenes. These oncogenes may, however, influence MTX resistance through epigenetic mechanisms. In addition, there appears to be no relationship between frequency of methotrexate resistance and frequency of dhfr gene amplification. It is clear that mechanisms of resistance other than dhfr gene amplification can be exclusively associated with either very high or very low frequencies of resistance.
