Topoisomerase-II-mediated lesions in nascent DNA: comparison of the effects of epipodophyllotoxin derivatives, VM-26 and VP-16, and 9-anilinoacridine derivatives, m-AMSA and o-AMSA.

This study compares the effects of the epipodophyllotoxin derivatives, VM-26 and VP-16, and the 9-anilinoacridine derivatives, m-AMSA and o-AMSA, on nascent and mature DNA. Two types of lesion which are putatively mediated by topoisomerase II, DNA-protein crosslinks and DNA double-strand breaks, were analyzed in drug-treated nuclei from 3H/14C labelled L1210 cells. Potassium/dodecyl sulfate precipitation assay was used to assess DNA-protein crosslinks in mature and nascent (1 min old) DNA. Both epipodophyllotoxins and m-AMSA showed a strong preference for nascent DNA. DNA double-strand cleavage induced by VM-26 and m-AMSA also showed a preference for nascent DNA as indicated by neutral elution technique. Sedimentation on neutral sucrose gradients revealed that these drugs generated highly degraded fragments (under 30 S) in nascent DNA substantially faster than in mature DNA. Lesions in nascent DNA were diminished substantially by the omission of ATP or the addition of novobiocin. The ability to induce lesions in nascent DNA correlates with cytotoxic potency of the agents studied. The results suggest that replicating DNA may constitute a preferential target for antitopoisomerase II drugs.