APURINIC SITES ARE POSITION-SPECIFIC TOPOISOMERASE-II POISONS

Many anticancer drugs ''poison'' topoisomerase II by enhancing its dou ble-stranded DNA cleavage activity. To determine whether DNA lesions a ct as endogenous topoisomerase II poisons, we characterized the effect s of position-specific apurinic sites on enzyme activity. Lesions loca ted within the 4-base overhang generated by enzyme-mediated DNA scissi on stimulated cleavage similar to 10-18-fold without altering the spec ificity of topoisomerase II. DNA breaks were double-stranded in nature , protein-linked, and readily reversible. In contrast, apurinic sites located immediately outside the cleavage overhang were inhibitory. Thu s, apurinic sites, which are the most commonly formed lesion in DNA, a re position-specific topoisomerase II poisons. A model is proposed tha t encompasses the actions of endogenous and exogenous topoisomerase II poisons and provides a pre-existing pathway for the cellular actions of topoisomerase II-targeted anticancer drugs.