title: Cell cycle and growth response of CHO cells to X-irradiation: threshold-free repair at low doses.
abstract: OBJECTIVE
  To test the hypothesis of a threshold for induced repair of DNA damage (IR) and, secondarily, of hyperradiosensitivity (HRS) to low-dose X-irradiation.
  
  
  METHODS
  Exponentially growing Chinese hamster ovary cells (CHO) were X-irradiated with doses from 0.2 to 8 Gy. Survival data were established by conventional colony-forming assay and flow-cytometric population counting. The early cell cycle response to radiation was studied based on DNA-profiles and bromodeoxyuridine pulse-labeling experiments.
  
  
  RESULTS
  Colony-forming data were consistent with HRS. However, these data were of low statistic significance. Population counting provided highly reproducible survival curves that were in perfect accord with the linear-quadratic (LQ) model. The dominant cell cycle reaction was a dose-dependent delay of G2 M and late S-phase.
  
  
  CONCLUSIONS
  There was no evidence for a threshold of IR and for low-dose HRS in X-irradiated CHO cells. It is suggested that DNA damage repair activity is constitutively expressed during S-phase and is additionally induced in a dose-dependent and threshold-free manner in late S-phase and G2. The resulting survival is precisely described by the LQ model.
authors.affiliation: Department of Radio-oncology, University Hospital Ulm, Robert-Koch-Str. 6, D-89081 Ulm, Germany. detlef.bartkowiak@medizin.uni-ulm.de
authors.forename: D
authors.initials: D
authors.lastname: Bartkowiak
authors.forename: S
authors.initials: S
authors.lastname: Högner
authors.forename: W
authors.initials: W
authors.lastname: Nothdurft
authors.forename: E M
authors.initials: EM
authors.lastname: Röttinger
mesh_terms: D000818:Animals; D016466:CHO Cells / Q000166:cytology / Q000502:physiology / Q000528:radiation effects; D002453:Cell Cycle / Q000528:radiation effects*; D002455:Cell Division / Q000528:radiation effects; D002470:Cell Survival / Q000528:radiation effects; D003114:Colony-Forming Units Assay; D006224:Cricetinae; D004247:DNA / Q000528:radiation effects; D004249:DNA Damage; D004260:DNA Repair / Q000502:physiology*; D004307:Dose-Response Relationship, Radiation; D016014:Linear Models; D008954:Models, Biological; D011836:Radiation Tolerance
chemical_list: D004247:DNA