Amplified dihydrofolate reductase genes in unstably methotrexate-resistant cells are associated with double minute chromosomes.

Abstract

Selection of mammalian cells in progressively increasing concentrations of methotrexate results in selective amplification of DNA sequences coding for dihydrofolate reductase (tetrahydrofolate dehydrogenase, 5,6,7,8-tetrahydrofolate:NADP+ oxidoreductase, EC 1.5.1.3). In some cell variants the amplified genes are stable with growth in the absence of methotrexate, whereas in other variants the amplified genes are lost from the population. It was previously reported that in a stably amplified variant of Chinese hamster ovary cells, the genes are localized to a single chromosome. In mouse S-180 (sarcoma) and L5178Y [lymphoma] cell lines, unstably amplified dihydrofolate reductase DNA sequences are associated with small, paired chromosomal elements denoted double minute chromosomes, whereas in stably amplified cells of the same origin, the genes are associated with large chromosomes.