Stepwise activation of the ATR signaling pathway upon increasing replication stress impacts fragile site integrity

PLoS Genet. 2013;9(7):e1003643. doi: 10.1371/journal.pgen.1003643. Epub 2013 Jul 18.


Breaks at common fragile sites (CFS) are a recognized source of genome instability in pre-neoplastic lesions, but how such checkpoint-proficient cells escape surveillance and continue cycling is unknown. Here we show, in lymphocytes and fibroblasts, that moderate replication stresses like those inducing breaks at CFSs trigger chromatin loading of sensors and mediators of the ATR pathway but fail to activate Chk1 or p53. Consistently, we found that cells depleted of ATR, but not of Chk1, accumulate single-stranded DNA upon Mre11-dependent resection of collapsed forks. Partial activation of the pathway under moderate stress thus takes steps against fork disassembly but tolerates S-phase progression and mitotic onset. We show that fork protection by ATR is crucial to CFS integrity, specifically in the cell type where a given site displays paucity in backup replication origins. Tolerance to mitotic entry with under-replicated CFSs therefore results in chromosome breaks, providing a pool of cells committed to further instability.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Ataxia Telangiectasia Mutated Proteins / genetics
  • Ataxia Telangiectasia Mutated Proteins / metabolism
  • Cell Cycle Proteins / genetics
  • Cell Cycle Proteins / metabolism
  • Cell Line
  • Checkpoint Kinase 1
  • Chromatin / genetics*
  • Chromosome Fragile Sites / genetics*
  • DNA Replication / genetics
  • Fibroblasts / cytology
  • Fibroblasts / metabolism
  • Genomic Instability / genetics*
  • Humans
  • Lymphocytes / cytology
  • Lymphocytes / metabolism
  • Mitosis / genetics
  • Protein Kinases / genetics
  • Replication Origin / genetics
  • Signal Transduction
  • Tumor Suppressor Protein p53 / genetics


  • Cell Cycle Proteins
  • Chromatin
  • TP53 protein, human
  • Tumor Suppressor Protein p53
  • Protein Kinases
  • ATR protein, human
  • Ataxia Telangiectasia Mutated Proteins
  • CHEK1 protein, human
  • Checkpoint Kinase 1

Grant support

MD's team was supported by INCa (Institut National du Cancer) (2009-1-PLBIO-10-IC-1), the Agence Nationale de la Recherche (ANR-09-GENO-000/repinsCFS), and by the Association pour la Recherche sur le Cancer (ARC Subvention Libre n° SL220100601348; Equipements mi-lourds n° 8514). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.