Amplification of Several Different Genes in Multidrug-Resistant Chinese Hamster Cell Lines

Abstract

In 1984, when we started our experiments on genes involved in multidrug resistance (MDR), we were struck by the genetic instability and the phenotypic diversity of MDR. Several MDR cell lines had been shown to rapidly revert to normal when grown under nonselective conditions, a genetic property associated with unstable forms of gene amplification, i.e., double-minute (DM) chromosomes. Indeed, DM chromosomes and homogeneously staining regions (HSRs), the cytogenetic representations of gene amplification, had been observed in MDR cells (see Chapters 6–8). It seemed likely, therefore, that MDR would be associated with increased expression of one or more genes. Phenotypic diversity appeared not to be due to interaction of a single MDR gene with diverse genetic backgrounds, because a single parental line could give rise to several MDR daughter lines that differed in the detailed spectrum of cross-resistance (see Chapters 1 and 2). This strongly indicated that more than one gene could contribute to MDR. We have tried to make an inventory of these genes in MDR hamster cells by collecting genes that were overexpressed in a resistant line relative to the parental sensitive line. Whereas Roninson and co-workers developed a DNA-DNA renaturation approach to directly isolate amplified genes (see Chapter 3), we made clone banks of DNA complementary to the mRNAs of MDR cells and selected the cDNAs corresponding to mRNAs present at increased levels in the MDR cell line.