A temperature sensitive topoisomerase II allele confers temperature dependent drug resistance on amsacrine and etoposide: a genetic system for determining the targets of topoisomerase II inhibitors.

We have developed a simple system for determining the specific contribution 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, rad52, and also have a temperature sensitive topoisomerase II mutation, top2-1. Strains carrying the top2-1 mutation have nearly normal topoisomerase II activity at 25 degrees C but less than 10% of wild type activity at 36 degrees C. We find that at a semi-permissive temperature (30 degrees 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.