Chromosomal and Extrachromosomal Localization of Amplified Dihydrofolate Reductase Genes in Cultured Mammalian Cells

Previous reports from this (Alt et al. 1976; Schimke et al. 1978, 1979) and other laboratories (Fisher 1961; Hakala et al. 1961; Friedkin et al. 1962; Nakamura and Littlefield 1972; Biedler and Spengler 1976; Flintoff et al. 1976; Bostock et al. 1979) have shown that stepwise selection of cultured mouse and hamster cells for resistance to the 4-amino analog of folic acid, methotrexate (MTX), results in elevated levels of dihydrofolate reductase (DHFR). We have shown that the elevated DHFR enzyme levels result from a corresponding increase in the number of dhfr genes (i.e., gene amplification; Alt et al. 1978; Schimke et al. 1979) and occur in a number of different cell lines of both mouse and hamster origin irrespective of whether the karyotype is grossly aneuploid or is relatively normal. The amplified dhfr genes can exist in either a stable or an unstable state when cells are grown in the...