DIFFERENT TOPOISOMERASE-II ANTITUMOR DRUGS DIRECT SIMILAR SPECIFIC LONG-RANGE FRAGMENTATION OF AN AMPLIFIED C-MYC GENE LOCUS IN LIVING CELLS AND IN HIGH-SALT EXTRACTED NUCLEI

We have analyzed the long-range distribution of topoisomerase II-media ted cleavages induced in an amplified human c-MYC gene locus in the pr esence of several antitumor agents. The long-range cleavage patterns w ere found to be nonrandom and similar for all antitumor drugs tested. Cleavages occurred within several kilobase-long areas (approximate to 5 kb) highly accessible to topoisomerase II and separated by extended regions (approximate to 70-100 kb) of less accessibility, possibly ref lecting the mode of DNA organization into loops along the chromosome. Within the cleavage areas, the patterns of cleavage sites showed a cer tain dependence on the type of drug used for entrapment of topoisomera se II-DNA complexes. Importantly, distribution of cleavage areas in na tive chromatin and histone-depleted nuclei was very similar, if not id entical, suggesting that the primary target of antitumor agents in viv o is topoisomerase LI associated with the high-salt-insoluble nuclear matrix. These data show that matrix-attached DNA is preferentially dam aged by topoisomerase II-targeting agents, which may be an important c ellular event contributing to drug-induced cell death.