Isolation of genetic suppressor elements, inducing resistance to topoisomerase II-interactive cytotoxic drugs, from human topoisomerase II cDNA

Many cytotoxic anticancer drugs act at topoisomerase II (topo II) by stabilizing cleavable complexes with DNA formed by this enzyme. Several cell lines, selected for resistance to topo II-interactive drugs, show decreased expression or activity of topo II, suggesting that such a decrease may be responsible for drug resistance. In the present study, etoposide resistance was used as the selection strategy to isolate genetic suppressor elements (GSEs) from a retroviral library expressing random fragments of human topo II (a form) cDNA. Twelve GSEs were isolated, encoding either peptides corresponding to short segments of the topo IIα molecule (2.4-6.5% of the protein) or 163- to 220-bp-long antisense RNA sequences. Expression of a GSE encoding antisense RNA led to decreased cellular expression of the topo IIα protein. Both types of GSE induced resistance to several topo II poisons but not to drugs that do not act at topo II. These results provide direct evidence that inhibition of topo II results in resistance to topo II-interactive drugs, indicate structural domains of topo II capable of independent functional interactions, and demonstrate that expression selection of random fragments constitutes an efficient approach to the generation of GSEs in mammalian cells.