In vivo and in vitro stimulation by antitumor drugs of the topoisomerase II-induced cleavage sites in c-myc proto-oncogene.

Stimulation of topoisomerase II cleavage activity by antitumor drugs with or without DNA intercalative ability has been tested in vivo on the c-myc proto-oncogene. Two human tumor cell lines (N417 and HL60 cells) were treated with mAMSA, OH-9-ellipticine, VM26, and BD-40 (an aza-ellipticine analog), and DNA breaks were mapped in the gene locus by Southern blot hybridization with c-myc probes. Most of the major cleavage sites induced in vivo by drugs in the 5' end of c-myc were also observed in vitro using purified topoisomerase II and a c-myc gene DNA insert. Moreover, they closely mapped to some DNAse I hypersensitive sites, the presence of which reflects gene activity. DNA from drug treated cells probed with a human beta 1 globin pseudogene, and the c-mos proto-oncogene did not reveal topoisomerase II cleavage bands, suggesting that topoisomerase II-mediated drug activity may correlate with gene activity.