title: Deoxyguanosine enhances the cytotoxicity of the topoisomerase I inhibitor camptothecin by reducing the repair of double-strand breaks induced in replicating DNA.
abstract: Deoxyguanosine (dG) enhances the S phase cytotoxicity of camptothecin (CPT), a topoisomerase I (topo I) inhibitor, but by contrast does not affect the toxicity of VM26, a topoisomerase II inhibitor. The 80% survival of S phase human fibroblasts after a 60 min exposure to 0.2 microM CPT is reduced by half in the presence of 25 microM dG. G1 cells are resistant to CPT toxicity, though the levels of the single-strand DNA breaks induced by the drug are similar in G1 and S phase cells. Higher concentrations of dG retard the recovery of RNA and DNA synthesis and inhibit recovery from the S-G2 cycle block after CPT removal. At 100 microM dG the number of CPT-induced protein-linked single-strand DNA breaks is almost doubled, suggestive of a direct effect of dG on the cellular activity of topo I. In the presence or absence of dG, single-strand breaks disappear within minutes of the removal of CPT. We found that the inhibition of topo I by CPT induces the formation of double as well as single-strand breaks in the chromosomal DNA. Previously we have shown, using a pulse-field gel electrophoresis technique, that the double-strand breaks (DSBs) are generated predominantly at sites of replication and not in the bulk DNA. A number of these DSBs are long-lived. The present study shows that dG affects the repair of these DSBs in a dose-dependent manner, and that a higher proportion of the initial lesions induced in nascent DNA remain 24 h after removal of CPT. We suggest that the long-lived double-strand breaks, formed in replicating DNA at the time of CPT exposure, are the lethal drug-induced lesions, which explains both the selective cytotoxicity of CPT towards S phase cells and the enhancement of CPT cytotoxicity by dG.
authors.affiliation: Department of Zoology, University of Cambridge, UK.
authors.forename: S
authors.initials: S
authors.lastname: Squires
authors.forename: A J
authors.initials: AJ
authors.lastname: Ryan
authors.forename: H L
authors.initials: HL
authors.lastname: Strutt
authors.forename: P J
authors.initials: PJ
authors.lastname: Smith
authors.forename: R T
authors.initials: RT
authors.lastname: Johnson
mesh_terms: D002166:Camptothecin / Q000494:pharmacology*; D002453:Cell Cycle / Q000187:drug effects; D002460:Cell Line; D002470:Cell Survival / Q000187:drug effects; D004260:DNA Repair / Q000187:drug effects*; D004261:DNA Replication / Q000187:drug effects*; D003849:Deoxyguanosine / Q000494:pharmacology* / Q000633:toxicity; D004347:Drug Interactions; D006801:Humans; D012313:RNA / Q000378:metabolism; D013713:Teniposide / Q000494:pharmacology; D059004:Topoisomerase I Inhibitors
chemical_list: D059004:Topoisomerase I Inhibitors; D012313:RNA; D013713:Teniposide; D003849:Deoxyguanosine; D002166:Camptothecin