title: Cytogenetic low-dose hyperradiosensitivity is observed in human peripheral blood lymphocytes.
abstract: OBJECTIVE
  The shape of the ionizing radiation response curve at very low doses has been the subject of considerable debate. Linear-no-threshold (LNT) models are widely used to estimate risks associated with low-dose exposures. However, the low-dose hyperradiosensitivity (HRS) phenomenon, in which cells are especially sensitive at low doses but then show increased radioresistance at higher doses, provides evidence of nonlinearity in the low-dose region. HRS is more prominent in the G2 phase of the cell cycle than in the G0/G1 or S phases. Here we provide the first cytogenetic mechanistic evidence of low-dose HRS in human peripheral blood lymphocytes using structural chromosomal aberrations.
  
  
  METHODS
  Human peripheral blood lymphocytes from 2 normal healthy female donors were acutely exposed to cobalt 60 γ rays in either G0 or G2 using closely spaced doses ranging from 0 to 1.5 Gy. Structural chromosomal aberrations were enumerated, and the slopes of the regression lines at low doses (0-0.4 Gy) were compared with doses of 0.5 Gy and above.
  
  
  RESULTS
  HRS was clearly evident in both donors for cells irradiated in G2. No HRS was observed in cells irradiated in G0. The radiation effect per unit dose was 2.5- to 3.5-fold higher for doses ≤0.4 Gy than for doses >0.5 Gy.
  
  
  CONCLUSIONS
  These data provide the first cytogenetic evidence for the existence of HRS in human cells irradiated in G2 and suggest that LNT models may not always be optimal for making radiation risk assessments at low doses.
authors.affiliation: Department of Biological Sciences, Wayne State University, Detroit, Michigan.
authors.forename: Isheeta
authors.initials: I
authors.lastname: Seth
authors.affiliation: Department of Radiation Oncology, Wayne State University, Detroit, Michigan.
authors.forename: Michael C
authors.initials: MC
authors.lastname: Joiner
authors.affiliation: Department of Biological Sciences, Wayne State University, Detroit, Michigan. Electronic address: jtucker@biology.biosci.wayne.edu.
authors.forename: James D
authors.initials: JD
authors.lastname: Tucker
mesh_terms: D000328:Adult; D002869:Chromosome Aberrations; D003037:Cobalt Radioisotopes; D020732:Cytogenetic Analysis; D004307:Dose-Response Relationship, Radiation; D005260:Female; D016195:G2 Phase / Q000235:genetics / Q000528:radiation effects*; D005720:Gamma Rays; D006801:Humans; D008214:Lymphocytes / Q000528:radiation effects*; D011836:Radiation Tolerance / Q000235:genetics*; D016192:Resting Phase, Cell Cycle / Q000528:radiation effects
chemical_list: D003037:Cobalt Radioisotopes