Origin of multidrug resistance in cells with and without multidrug resistance genes: Chromosome reassortments catalyzed by aneuploidy

Cancer cells and aneuploid cell lines can acquire resistance against multip le unrelated chemotherapeutic drugs that are over 3,000-fold those of norma l levels and display spontaneous resistances up to 20-fold of normal levels . Two different mechanisms were proposed for this phenotype: (i) classical mutation of drug metabolizing genes or (ii) chromosome reassortments, catal yzed by cancer- and cell line-specific aneuploidy, which generate, via new gene dosage combinations, a plethora of cancer phenotypes, including drug r esistance. To distinguish between these mechanisms, we have asked whether t hree mouse cell lines can become drug resistant, from which two or three ge nes have been deleted, and on which multidrug resistance is thought to depe nd: Mdr1a, Mdr1b, and Mrp1. Because all three lines could acquire multidrug resistance and were aneuploid, whereas diploid mouse cells could not, we c onclude that aneuploid cells become drug resistant via specific chromosome assortments, independent of putative resistance genes. We have asked furthe r whether aneuploid drug-resistant Chinese hamster cells revert spontaneous ly to drug sensitivity in the absence of cytotoxic drugs at the high rates that are typical of chromosome reassortments catalyzed by aneuploidy or at the very low or zero rates (i.e., deletion) of gene mutation. We found that four drug-resistant hamster cell lines reverted to drug sensitivity at rat es of about 2-3% per generation, whereas two closely related lines remained resistant under our conditions. Thus, the karyotypic instability generated by aneuploidy emerges as the common source of the various levels of drug r esistance of cancer cells: minor spontaneous resistances reflect accidental chromosome assortments, the high selected resistances reflect complex spec ific assortments, and multidrug resistance reflects new combinations of uns elected genes located on the same chromosomes as selected genes.