title: Telomeric DNA damage by topoisomerase I. A possible mechanism for cell killing by camptothecin.
abstract: Topoisomerase I adjusts torsional stress in the genome by breaking and resealing one strand of the helix through a transient covalent coupling between enzyme and DNA. Camptothecin, a specific topoisomerase I poison, traps this covalent intermediate, thereby damaging the genome. Here we examined the activity of topoisomerase I at telomeric repeats to determine whether telomere structures are targets for DNA damage. We show that topoisomerase I is catalytically active in cleaving the G-rich telomeric strand in vitro in the presence of camptothecin but not in cleaving the C-rich strand. The topoisomerase I cleavage site is 5'-TT (downward arrow) AGGG-3' (cleavage site marked by the downward arrow). We also show that endogenous topoisomerase I can access telomeric DNA in vivo and form camptothecin-dependent covalent complexes. Therefore, each telomeric repeat represents a potential topoisomerase I cleavage site in vivo. Because telomere structures are comprised of a large number of repeats, telomeres in fact represent a high concentration of nested topoisomerase I sites. Therefore, more telomeric DNA damage by camptothecin could occur in cells with longer telomeres when cells possess equivalent levels of topoisomerase I. The evidence presented here suggests that DNA damage at telomeric repeats by topoisomerase I is a prominent feature of cell killing by camptothecin and triggers camptothecin-induced apoptosis.
authors.affiliation: Department of Biology and Molecular Aging Research Center, Yonsei University, 134 Shinchon-dong, Seoul 120-749, Korea.
authors.forename: Mi Ran
authors.initials: MR
authors.lastname: Kang
authors.forename: Mark T
authors.initials: MT
authors.lastname: Muller
authors.forename: In Kwon
authors.initials: IK
authors.lastname: Chung
mesh_terms: D017209:Apoptosis; D001665:Binding Sites; D001681:Biological Assay; D002166:Camptothecin / Q000494:pharmacology*; D003545:Cysteine / Q000737:chemistry; D004247:DNA / Q000187:drug effects*; D004249:DNA Damage; D004264:DNA Topoisomerases, Type I / Q000737:chemistry*; D004791:Enzyme Inhibitors / Q000494:pharmacology; D016678:Genome; D006367:HeLa Cells; D006801:Humans; D008954:Models, Biological; D010957:Plasmids / Q000378:metabolism; D011188:Potassium / Q000737:chemistry; D012594:Scleroderma, Localized / Q000097:blood; D012967:Sodium Dodecyl Sulfate / Q000494:pharmacology; D016615:Telomere
chemical_list: D004791:Enzyme Inhibitors; D012967:Sodium Dodecyl Sulfate; D004247:DNA; D004264:DNA Topoisomerases, Type I; D003545:Cysteine; D011188:Potassium; D002166:Camptothecin