title: Preferential potentiation of topoisomerase I poison cytotoxicity by PARP inhibition in S phase.
abstract: BACKGROUND
  Topoisomerase I (Topo I) poisons (e.g., camptothecin (CPT)), used to treat cancer, cause DNA breaks that are most cytotoxic during S phase. PARP-1 promotes DNA repair and PARP inhibitors (PARPi) sensitise cells to Topo I poisons. We aimed to determine whether chemosensitisation is also S phase specific using rucaparib, a potent PARPi in advanced clinical evaluation.
  
  
  METHODS
  The impact of rucaparib, on CPT-induced cytotoxicity was measured in human colon cancer (LoVo) and leukaemic (K562) cells in asynchronous and cell cycle phase-separated cultures. Topoisomerase I and PARP levels and activity and the effect of rucaparib on DNA single-strand breaks (SSBs), double-strand breaks (DSBs) and collapsed replication fork induction and repair were determined in cell cycle phase-separated cells.
  
  
  RESULTS
  The cytotoxicity of CPT was greatest during S phase, partially attributable to high Topo I activity, and rucaparib preferentially sensitised S-phase cells. Rucaparib increased CPT-induced DNA SSBs in all phases of the cell cycle, and increased DSB and γH2AX foci in S and G2, with γH2AX foci being highest in S-phase cells. Repair of SSBs and DSBs was most rapid during S then G2 phases and was substantially hindered by rucaparib.
  
  
  CONCLUSIONS
  Rucaparib preferentially sensitises S-phase cells by increasing the frequency of collapsed replication forks.
authors.affiliation: Newcastle University, Northern Institute for Cancer Research, Paul O'Gorman Building, Medical School, Newcastle upon Tyne NE2 4HH, UK.
authors.forename: P
authors.initials: P
authors.lastname: Znojek
authors.affiliation: Newcastle University, Northern Institute for Cancer Research, Paul O'Gorman Building, Medical School, Newcastle upon Tyne NE2 4HH, UK.
authors.forename: E
authors.initials: E
authors.lastname: Willmore
authors.affiliation: Newcastle University, Northern Institute for Cancer Research, Paul O'Gorman Building, Medical School, Newcastle upon Tyne NE2 4HH, UK.
authors.forename: N J
authors.initials: NJ
authors.lastname: Curtin
mesh_terms: D002166:Camptothecin / Q000494:pharmacology*; D002470:Cell Survival / Q000187:drug effects; D053903:DNA Breaks, Double-Stranded; D004260:DNA Repair / Q000187:drug effects; D004264:DNA Topoisomerases, Type I / Q000378:metabolism; D004354:Drug Screening Assays, Antitumor; D004357:Drug Synergism; D006801:Humans; D007211:Indoles / Q000494:pharmacology*; D020014:K562 Cells; D000071137:Poly (ADP-Ribose) Polymerase-1; D000067856:Poly(ADP-ribose) Polymerase Inhibitors; D011065:Poly(ADP-ribose) Polymerases / Q000378:metabolism; D059807:S Phase Cell Cycle Checkpoints / Q000187:drug effects*; D059004:Topoisomerase I Inhibitors / Q000494:pharmacology*
chemical_list: D007211:Indoles; D000067856:Poly(ADP-ribose) Polymerase Inhibitors; D059004:Topoisomerase I Inhibitors; C531549:rucaparib; C499320:PARP1 protein, human; D000071137:Poly (ADP-Ribose) Polymerase-1; D011065:Poly(ADP-ribose) Polymerases; D004264:DNA Topoisomerases, Type I; C528909:TOP1 protein, human; D002166:Camptothecin