Dihydrofolate Reductase Gene Amplification, Altered Dihydrofolate Reductase, and Methotrexate Resistance in Cultured 3T6 Cells Associated with Unstable Amplification of an Altered Dihydrofolate Reductase Gene

Abstract

Resistance of human neoplasms to various cancer chemotherapeutic agents has been a frustrating problem. Extensive studies in many laboratories have investigated the development of resistance to the 4-amino analog of folate, methotrexate (MTX) and have defined three general mechanisms for such resistance, including reduction in MTX transport,1 alteration in affinity of MTX for the target enzyme of MTX inhibition, dihydrofolate reductase (DHFR)2–5 and increased levels of DHFR. 7–13 Our laboratory has shown that the elevated DHFR enzyme levels in various MTX-resistant cell lines result from a corresponding increase in the number of DHFR genes, i.e., gene amplification6,14 irrespective of whether the karyotype is grossly aneuploid or is relatively stable, and irrespective of whether the MTX-resistance is phenotypically stable or unstable.