Selection for the expression of one form of Chinese hamster dihydrofolate reductase over another during growth in methotrexate.

Two mammalian expression plasmids, each carrying a cDNA encoding a different allele of dihydrofolate reductase (DHFR) present in the Chinese hamster lung cell line DC-3F, were constructed. These simian virus 40 promoter-enhancer-based plasmids, designated pSVA75 and pSVMQ19, are identical except for a G-->A transition at nucleotide 286 of the DHFR coding sequence. Due to this change, the enzyme expressed by pSVA75 contains Asp95, while the enzyme expressed by pSVMQ19 has Asn95 [Melera et al., J. Biol. Chem. (1988) 1978-1990]. Both forms of the enzyme are catalytically equivalent and are produced to similar levels in DC-3F cells [Yu and Melera, Cancer Res. (1993) 6031-6035; H.Y., A.H. and P.W.M., in preparation]. Clonal cell lines expressing one or the other DHFR allele were obtained via transfection of DHFR- Chinese hamster ovary cells, and 74 clones of each type isolated. These were pooled and divided into 40 aliquots, each of which was then subjected to selection by growth in sequentially increasing concentrations of methotrexate (MTX). Analysis of the resulting drug-resistant populations revealed that cells producing Asp95 DHFR dominated with an overall frequency of 3:1, and therefore, under these growth conditions, display a selective advantage over those producing Asn95 DHFR. These data extend previous observations showing that independent selections of the heterozygous parent cell line DC-3F in MTX result in threefold more MTX-resistant lines overexpressing the Asp95-encoding DHFR allele than the Asn95-encoding DHFR allele.(ABSTRACT TRUNCATED AT 250 WORDS)