Gene amplification and chromosome rearrangements: a study of a single cell lineage selected for amplification and deamplification of the UMP synthase gene.

The UMP synthase gene is stably amplified in Chinese hamster lung cells selected for resistance to pyrazofurin (PF) and 6-azauridine (6AUR), inhibitors of the decarboxylase activity of the bifunctional UMP synthase enzyme. The amplified DNA is located intrachromosomally as expanded chromosomal regions (ECRs). Growth of these cells in 5-fluorouracil enables rapid selection of cells that have undergone deamplification and consequently lost resistance to PF + 6AUR. Detailed cytogenetic analyses and fluorescence in situ hybridization on three consecutive amplification-deamplification cycles in descendants of the same cloned cell showed a unique position and structure for the ECR. In the first cycle of amplification, the ECR forms a homogeneously staining region on a small marker chromosome (M3). In the second cycle of amplification, a chromosomal break was noted at the site of the endogenous UMP synthase gene on another derivative chromosome, M2, with amplification resulting in an abnormally banded region on a third marker (M3). The third cycle of amplification produced a cell line with an ECR on the distal portion of M2. This ECR was unstable, showing variations in size as well as translocations and other chromosome rearrangements. Our data, taken in its entirety, suggest a relationship between amplification as an ECR and chromosome rearrangements in Chinese hamster cells.