Topoisomerase I poisoning results in PARP-mediated replication fork reversal

Topoisomerase I (Top1) releases torsional stress during DNA replication and transcription and is inhibited by camptothecin and camptothecin-derived cancer chemotherapeutics. Top1 inhibitor cytotoxicity is frequently linked to double-strand break (DSB) formation as a result of Top1 being trapped on a nicked DNA intermediate in replicating cells. Here we use yeast, mammalian cell lines and Xenopus laevis egg extracts to show that Top1 poisons rapidly induce replication-fork slowing and reversal, which can be uncoupled from DSB formation at sublethal inhibitor doses. Poly(ADP-ribose) polymerase activity, but not single-stranded break repair in general, is required for effective fork reversal and limits DSB formation. These data identify fork reversal as a means to prevent chromosome breakage upon exogenous replication stress and implicate proteins involved in fork reversal or restart as factors modulating the cytotoxicity of replication stress-inducing chemotherapeutics. © 2012 Nature America, Inc. All rights reserved.
M.L. and A.R.C. are supported by a Swiss National Science Foundation grant (PP00A-114922). A.R.C. was supported by a European Molecular Biology Organization (EMBO) short-term fellowship. V.C. and Y.H. are supported by Cancer Research UK, the Lister Institute of Preventive Medicine, a European Research Council (ERC-206281) startup grant and the EMBO Young Investigator Program (YIP).