title: p53 prevents the accumulation of double-strand DNA breaks at stalled-replication forks induced by UV in human cells.
abstract: To investigate the mechanism by which UV irradiation causes S-phase-dependent chromosome aberrations and thereby genomic instability, we have developed an assay to study the DNA structure of replication forks (RFs) in UV-irradiated mammalian cells, using pulse-field gel electrophoresis for the DNA analysis. We demonstrate that replication stalling at UV-induced pyrimidine dimers results in the formation of single-strand DNA (ssDNA) regions and incomplete RF structures. In normal and in nucleotide-excision-repair (NER)-defective xeroderma pimentosum (XP) cells, stalling at dimers is rapid and prolonged and recovery depends on dimer repair or bypass. By contrast, XP variant (XPV) cells, defective in replication of a UV-damaged template due to mutation of bypass-polymerase epsilon, fail to arrest at dimers, resulting in a much higher frequency of ssDNA regions in the stalled RFs. We show that the stability of UV-arrested RFs depends directly on functional p53, and indirectly on NER and pol eta. In p53-deficient cells, the stalled sites give rise to double-strand DNA breaks (DSBs), at a frequency inversely correlated with repair capacity of the cell. In normal cells only a fraction of the stalled sites give rise to DSBs, while in XPASV, XPDSV and also XPVSV, all the sites do. XPVSV cells, although repair proficient, accumulate almost double the number of DSBs, suggesting that a high frequency of ssDNA regions in UV-arrested forks cause RF instability. These replication-associated DSBs do not accumulate in p53-proficient human cells. We propose that a major mechanism by which p53 maintains genome stability is the prevention of DSB accumulation at long-lived ssDNA regions in stalled-replication forks.
authors.affiliation: Department of Zoology, University of Cambridge, Cambridge, UK. ss17@cus.cam.ac.uk
authors.forename: Shoshana
authors.initials: S
authors.lastname: Squires
authors.forename: Julia A
authors.initials: JA
authors.lastname: Coates
authors.forename: Michal
authors.initials: M
authors.lastname: Goldberg
authors.forename: Lorraine H
authors.initials: LH
authors.lastname: Toji
authors.forename: Stephen P
authors.initials: SP
authors.lastname: Jackson
authors.forename: Duncan J
authors.initials: DJ
authors.lastname: Clarke
authors.forename: Robert T
authors.initials: RT
authors.lastname: Johnson
mesh_terms: D045744:Cell Line, Tumor; D002470:Cell Survival; D002478:Cells, Cultured; D004247:DNA / Q000096:biosynthesis / Q000235:genetics; D004249:DNA Damage / Q000235:genetics*; D004260:DNA Repair; D004261:DNA Replication / Q000528:radiation effects*; D004307:Dose-Response Relationship, Radiation; D004587:Electrophoresis, Agar Gel; D004706:Endodeoxyribonucleases; D005347:Fibroblasts / Q000528:radiation effects; D005720:Gamma Rays; D015894:Genome, Human; D006801:Humans; D007700:Kinetics; D008954:Models, Biological; D009097:Multienzyme Complexes; D011740:Pyrimidine Dimers; D013997:Time Factors; D016159:Tumor Suppressor Protein p53 / Q000172:deficiency / Q000378:metabolism*; D014466:Ultraviolet Rays / Q000009:adverse effects*
chemical_list: D009097:Multienzyme Complexes; D011740:Pyrimidine Dimers; D016159:Tumor Suppressor Protein p53; C032219:UV endonuclease; D004247:DNA; D004706:Endodeoxyribonucleases