A study of drug-induced topoisomerase II-mediated DNA lesions on episomal chromatin.

A well defined extrachromosomal DNA element, referred to as an episome (Ostrowski, M., Richard-Foy, H., Wolford, R., Berard, D., and Hager, G. (1983) Mol. Cell. Biol. 3, 2045-2057), was employed as a target for the topoisomerase II inhibitors amsacrine and teniposide. Both drugs have distinct mechanisms of action in cleaving the episome, as defined by topological forms conversion assays. The concentration ranges required to measure episomal cleavage are similar. The onset of damage induced by amsacrine begins within 1 min and is maintained at that level for at least 1 h. Teniposide induces damage that peaks between 30 and 60 min. The amsacrine-induced damage is only partially reversible, whereas teniposide-induced damage is almost completely reversible. Sites of specific cleavage are quite dissimilar. Multiple cleavage sites are formed in the episomal regulatory regions after amsacrine treatment, whereas a single cleavage in the regulatory region and one outside this region are found after teniposide treatment. Transcriptional activation using dexamethasone does not change the amount or site preference of episomal cleavage induced by either agent. Damage to the episome was quantitatively compared with damage produced in genomic DNA between 500 and 24,000 rad equivalents. The study showed that amsacrine has a significant (33-38-fold) preference for episomal DNA over genomic DNA.