MUTATIONS IN THE GYRB DOMAIN OF EUKARYOTIC TOPOISOMERASE-II CAN LEAD TO PARTIALLY DOMINANT RESISTANCE TO ETOPOSIDE AND AMSACRINE

Anti-topoisomerase II agents represent a major class of anticancer the rapeutic agents. Resistance to this class of agents can be mediated by several possible mechanisms. One mechanism may involve mutations in t he structural gene(s) for topoisomerases, altering the drug sensitivit y of the enzymes. Several mutations have been described in mammalian c ell lines that were selected for resistance to topoisomerase II-target ing drugs such as Adriamycin, etoposide, or amsacrine. The difficulty of performing genetic analysis in mammalian cell lines has complicated the determination of whether the observed mutations are responsible f or drug resistance. We have reconstructed, in the yeast topoisomerase II gene, the arginine to glutamine mutation at position 450 of human t opoisomerase II alpha that was originally identified by Bugg et al. [P roc. Natl. Acad. Sci. USA 88:7654-7658 (1991)]. Mutation of Lys(439), the equivalent amino acid in the yeast protein, to either glutamine or glutamic acid confers resistance to etoposide and amsacrine. Interest ingly, in diploid yeast cells the heterozygous mutation can still conf er partial drug resistance, compared with a diploid strain that is hom ozygous for wild-type topoisomerase II, Because mutations in the topoi somerase II gene that can confer dominant resistance to anti-topoisome rase II agents are relatively rare, mutations in the gyrB region may b e important in the development of clinical drug resistance.