Topoisomerase II Inhibitors Etoposide: A Genetic System for Determining the Targets of Temperature Dependent Drug Resistance on Amsacrine and A Temperature Sensitive Topoisomerase II Allele Confers

We have developed a simple system for determining the specific con tribution of topoisomerase II targeting to the cytotoxic activity of a drug. We have constructed yeast strains that are permeable to anti-topoisomerase II drugs, carry a DNA repair mutation, rot/52, and also have a temperature sensitive topoisomerase II mutation, top2-l. Strains carrying the top2-l mutation have nearly normal topoisomerase II activity at 25 (' but less than 10% of wild type activity at 36°C.We find that at a semipermissive temperature (30°C),there is sufficient topoisomerase II activity for viability; but since the topoisomerase II activity is greatly reduced, the cells are very resistant to anti-topoisomerase II drugs. Conversely, such cells are hypersensitive to the topoisomerase I inhibitor camptothecin. These results provide strong support for the model that drug stabilized DNA cleavage, rather than a lack of enzyme activity, is responsible for cell killing by eukaryotic anti-topoisomerase II agents. They also show that there is a minimum level of topoisomerase II activity in yeast that is consistent with viability but also allows a high degree of resistance to anti-topoisomerase II agents.