ALTERED DNA TOPOISOMERASE-II IN MULTIDRUG-RESISTANCE

The characteristic feature of multidrug resistance (MDR) associated with drugs that interact with DNA topoisomerase II (topo II) is alterations in topo II activity or amount (at-MDR). We have characterized the at-MDR phenotype in human leukemic CEM cells selected for resistance to the topo II inhibitor, VM-26. Compared to drug-sensitive cells, the key findings are that at-MDR cells exhibit (i) decreased topo II activity; (ii) decreased drug sensitivity, activity and amount of nuclear matrix topo II; (iii) increased ATP requirement of topo II; (iv) a single base mutation in topo II resulting in a change of Arg to Gln at position 449, at the start of the motif B/nucleotide binding site; and (v) decreased topo II phosphorylation, suggesting decreased kinase or increased phosphatase activities. Recent results using single-stranded conformational polymorphism analysis reveals the presence of a mutation in the motif B/nucleotide binding site of the topo IIalpha gene in CEM at-MDR cells and in another leukemic cell line selected for resistance to m-AMSA. Finally, we have observed marked changes in the nuclear distribution of topo II in cells treated with anti-topo II drugs and have also found these changes to be attenuated in drug-resistant cells. We postulate that traditional inhibitors of topo II alter the equilibrium of the strand-passing reaction such that the number of enzyme-DNA covalent complexes increases. We further suggest that when the enzyme is bound to DNA it is protected from proteolysis, thus allowing more topo II molecules to be detected. We propose that MDR associated with alterations in topo II may have clinical consequences, and our current efforts involve exploiting these biochemical and molecular observations in the development of probes that may be useful to identify such drug resistant cells in the tumors of patients.