We have reported that beta-lapachone, a plant alkaloid, greatly increases the lethality of a variety of DNA damaging agents to mammalian dells. The drug was shown to increase the double strand breaks in DNA of the damaged cells, and to decrease the rate of disappearance of single strand breaks. It also-increased the unwinding activity of purified topoisomerase T. We hypothesized from these results that topoisomerase I is involved in repair of damaged DNA. Consistent with this, we showed that camptothecin, a specific inhibitor of topoisomerase I, also enhanced the lethality of DNA damage in cells. Our three specific aims are: (1) To test the hypothesis that topoisomerase I is involved in excision repair of damaged DNA. (2) To determine whether camptothecin or beta-lapachone enhance lethality of antineoplastic DNA damaging agents by altering topoisomerase I activity. (3) To correlate the above lethality with the increased conversion of single stranded DNA damage to double stranded breaks. The experimental approach, described in more detail below, depends upon three principle variations of established systems for studying topoisomerase I, namely (A) To use nicked gapped, and randomly damaged DNA's as substrates for the enzyme in comparison with intact DNA. (B) To determine the effects of the two drugs on both damaged cells and topoisomerase 1 activity in purified systems. (C) To make mutations of topoisomerase I and investigate the above phenomena with them as compared to the wild type enzyme, both in vivo and in purified preparations. The research proposed- in-this grant is intended to fill several gaps in our knowledge. The first concerns the role of topoisomerase I in DNA repair. How are DNA repair processes dependent upon the type of initial lesion? Is topoisomerase I involved in the repair of all kinds of DNA lesions? From a practical point of view, the combination of topoisomerase I inhibitors with DNA damaging agents may lead to enhanced antitumor activity with diminished host toxicity, and thereby improve the therapeutic indices of established antineoplastic agents.
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