SPONTANEOUS DNA LESIONS POISON HUMAN TOPOISOMERASE II-ALPHA AND STIMULATE CLEAVAGE PROXIMAL TO LEUKEMIC 11Q23 CHROMOSOMAL BREAKPOINTS

Topoisomerase II-targeted drugs, such as etoposide, ''poison'' this en zyme and kill cells by increasing levels of covalent topoisomerase II- cleaved DNA complexes. In spite of the success of this drug in the tre atment of human cancers, a significant proportion of patients treated with etoposide eventually develop secondary leukemias that are charact erized by translocations at chromosome band 11q23. Since similar trans locations are associated with primary leukemias in previously untreate d infants, we questioned whether they could also be triggered by the a ctions of ''endogenous topoisomerase II poisons''. Recent studies, whi ch demonstrated that several forms of spontaneous DNA damage stimulate cleavage mediated by Drosophila topoisomerase II, suggest that DNA le sions may act as these endogenous poisons. Therefore, to determine whe ther the ability to recognize spontaneous DNA damage has been conserve d from this lower eukaryote to mammalian species, the effects of apuri nic sites, apyrimidinic sites, and deaminated cytosine residues on hum an topoisomerase II alpha were assessed. All three lesions were potent poisons of the human enzyme and stimulated cleavage when located with in the four-base overhang generated by enzyme-mediated DNA scission. F urthermore, these lesions increased levels of cleavage at five sites p roximal to 11q23 translocation breakpoints and did so with an efficacy that was comparable to or greater than that of therapeutic concentrat ions of etoposide. Although the physiological relevance of these findi ngs has yet to be established, they suggest a potential role for endog enous topoisomerase II poisons in the initiation of leukemic chromosom al breakpoints.