Selective amplification of polymorphic dihydrofolate reductase gene loci in Chinese hamster lung cells.

Abstract

A series of antifolate-resistant Chinese hamster lung sublines that overproduce either a MW 20,000 or a MW 21,000 class of dihydrofolate reductase (DHFR; tetrahydrofolate dehydrogenase; 5,6,7,8-tetrahydrofolate:NADP+ oxidoreductase, EC 1.5.1.3), known to contain amplified DHFR genes, was analyzed by DNA and RNA transfer techniques. The MW 20,000 and MW 21,000 DHFR are encoded by at least 2 polymorphic DHFR genes, both of which are expressed in drug-sensitive parental cells. In drug-resistant sublines only 1 of the 2 DHFR gene types is amplified, thus accounting for the overproduction of one or the other MW class of DHFR. In addition to the known differences between the DHFR whose overproduction they direct, these allelic genes differ in restriction endonuclease profiles and in the relative abundances of their multiple mRNA transcripts.