YEAST TOPOISOMERASE-II MUTANTS RESISTANT TO ANTI-TOPOISOMERASE AGENTS- IDENTIFICATION AND CHARACTERIZATION OF NEW YEAST TOPOISOMERASE-II MUTANTS SELECTED FOR RESISTANCE TO ETOPOSIDE

We describe a system that allows us to easily isolate and characterize mutants in yeast topoisomerase II that are resistant to antitumor age nts that target this enzyme. The system uses yeast strains that are se nsitive to those agents and that carry temperature-sensitive top2 muta tions. The temperature-sensitive mutation allows the isolation of rece ssive drug-resistant mutations. The mutagenized TOP2 gene we have used is under the control of the yeast DED1 promoter; this overexpression of TOP2 is designed to avoid isolating mutants that are drug resistant solely because the mutated topoisomerase II has low enzymatic activit y. We describe three mutants that we isolated using this system. Two o f the three mutants show resistance to etoposide and amsacrine, while the third mutant is partially resistant to etoposide and fluoroquinolo nes but not to amsacrine. DNA sequence changes have been identified in all of these mutant TOP2 genes. The mutant with partial resistance to etoposide and fluoroquinolones has an amino acid change at position 7 38 of TOP2, which is three amino acids from the site homologous to Ser (83) of E. coli gyrA, an amino acid which had previously been shown to be an important target for resistance to quinolones in bacteria. One of the alleles that confers resistance to both etoposide and amsacrine , top2-103, has changes in amino acid 824 and amino acid 1186 of TOP2. Reconstruction of the mutations by oligonucleotide-directed mutagenes is demonstrates that the change at amino acid 824 is responsible for t he drug resistance of this allele.