title: The amount of DNA damage needed to activate the radiation-induced G2 checkpoint varies between single cells.
abstract: OBJECTIVE
  The radiation-induced G2 checkpoint helps facilitate DNA repair before cell division. However, recent work has revealed that human cells often escape the G2 checkpoint with unrepaired DNA breaks. The purpose was to explore whether G2 checkpoint activation occurs according to a threshold level of DNA damage.
  
  
  METHODS
  G2 checkpoint activation was assayed at 75-90 min and 24-48 h after X-ray irradiation of BJ diploid fibroblasts and U2OS osteosarcoma cells. Multiparameter flow cytometry with pacific blue barcoding, and flow cytometry-based sorting of phospho-H3 positive cells to microscope slides, were used to examine the DNA damage marker γ-H2AX in individual mitotic cells that had escaped the G2 checkpoint.
  
  
  RESULTS
  For all radiation doses and times tested, the number of γ-H2AX foci varied between individual mitotic cells. At 75 min the median levels of γ-H2AX in mitotic cells increased with higher radiation doses. At 24-48 h, following a prolonged G2 checkpoint, cells were more resistant to checkpoint re-activation by a second dose of radiation.
  
  
  CONCLUSIONS
  Our results suggest that different amounts of DNA damage are needed to activate the G2 checkpoint in individual cells. Such single cell variation in checkpoint activation may potentially contribute to radiation-induced genomic instability.
authors: Department of Radiation Biology, Oslo University Hospital, Norway.
authors: Kinga
authors: K
authors: Tkacz-Stachowska
authors: Christin
authors: C
authors: Lund-Andersen
authors: Angeliki
authors: A
authors: Velissarou
authors: June H
authors: JH
authors: Myklebust
authors: Trond
authors: T
authors: Stokke
authors: Randi G
authors: RG
authors: Syljuåsen
mesh_terms: D014408:Biomarkers, Tumor / Q000378:metabolism; D002470:Cell Survival; D004249:DNA Damage; D004260:DNA Repair / Q000235:genetics / Q000502:physiology; D005347:Fibroblasts / Q000528:radiation effects*; D005434:Flow Cytometry; D005455:Fluorescent Antibody Technique; D059565:G2 Phase Cell Cycle Checkpoints / Q000235:genetics*; D005720:Gamma Rays; D042822:Genomic Instability / Q000235:genetics*; D006657:Histones / Q000378:metabolism; D006801:Humans; D008938:Mitosis / Q000235:genetics / Q000528:radiation effects; D012516:Osteosarcoma / Q000235:genetics* / Q000378:metabolism / Q000532:radiotherapy*; D011829:Radiation Dosage; D012016:Reference Values; D014407:Tumor Cells, Cultured / Q000187:drug effects / Q000528:radiation effects
chemical_list: D014408:Biomarkers, Tumor; C468783:H2AX protein, human; D006657:Histones