title: Poly (ADP-ribose) polymerase (PARP) is not involved in base excision repair but PARP inhibition traps a single-strand intermediate.
abstract: Base excision repair (BER) represents the most important repair pathway of endogenous DNA lesions. Initially, a base damage is recognized, excised and a DNA single-strand break (SSB) intermediate forms. The SSB is then ligated, a process that employs proteins also involved in SSB repair, e.g. XRCC1, Ligase III and possibly PARP1. Here, we confirm the role of XRCC1 and PARP in direct SSB repair. Interestingly, we uncover a synthetic lethality between XRCC1 deficiency and PARP inhibition. We also treated cells with alkylating agent dimethyl sulfate (DMS) and monitored the SSB intermediates formed during BER. DMS-induced SSBs were quickly repaired in wild-type cells; while a rapid accumulation of SSBs was observed in cells where post-incision repair was blocked by a PARP inhibitor or by XRCC1 deficiency (EM9 cells). Interestingly, DMS-induced SSBs did not accumulate in PARP1 siRNA depleted cells, demonstrating that PARP1 is not required for efficient completion of BER. Based on these results we suggest no immediate role for PARP1 in BER, but that PARP inhibitors trap PARP on the SSB intermediate formed during BER. Unexpectedly, addition of PARP inhibitor 2 h after DMS treatment still increased SSB levels indicating ongoing repair even at this late time point.
authors.affiliation: Department of Genetics, Microbiology and Toxicology, Stockholm University, S-106 91 Stockholm, Sweden.
authors.forename: Cecilia E
authors.initials: CE
authors.lastname: Ström
authors.forename: Fredrik
authors.initials: F
authors.lastname: Johansson
authors.forename: Mathias
authors.initials: M
authors.lastname: Uhlén
authors.forename: Cristina Al-Khalili
authors.initials: CA
authors.lastname: Szigyarto
authors.forename: Klaus
authors.initials: K
authors.lastname: Erixon
authors.forename: Thomas
authors.initials: T
authors.lastname: Helleday
mesh_terms: D000477:Alkylating Agents / Q000633:toxicity; D000818:Animals; D002460:Cell Line; D053904:DNA Breaks, Single-Stranded; D045647:DNA Glycosylases / Q000502:physiology; D004260:DNA Repair; D004268:DNA-Binding Proteins / Q000502:physiology; D055785:Gene Knockdown Techniques; D006801:Humans; D007700:Kinetics; D000067856:Poly(ADP-ribose) Polymerase Inhibitors; D011065:Poly(ADP-ribose) Polymerases / Q000235:genetics / Q000502:physiology*; D013463:Sulfuric Acid Esters / Q000633:toxicity; D000076105:X-ray Repair Cross Complementing Protein 1
chemical_list: D000477:Alkylating Agents; D004268:DNA-Binding Proteins; D000067856:Poly(ADP-ribose) Polymerase Inhibitors; D013463:Sulfuric Acid Esters; D000076105:X-ray Repair Cross Complementing Protein 1; C000619371:XRCC1 protein, human; D011065:Poly(ADP-ribose) Polymerases; D045647:DNA Glycosylases; C007482:dimethyl sulfate