title: Homologous recombination mediates cellular resistance and fraction size sensitivity to radiation therapy.
abstract: OBJECTIVE
  Cellular sensitivity to radiotherapy total dose and fraction size is strongly influenced by DNA double strand break (DSB) repair. Here, we investigate response to radiotherapy fraction size using CHO cell lines deficient in specific DNA repair pathways in response to radiation induced DNA double strand breaks (DSB).
  
  
  METHODS
  We irradiated CHO cell lines, AA8 (WT), irs1SF (XRCC3-), V3-3 (DNA-PKcs-) and EM9 (XRCC1-) with 16 Gy in 1 Gy daily fractions over 3 weeks or 16 Gy in 4 Gy daily fractions over 4 days, and studied clonogenic survival, DNA DSB repair kinetics (RAD51 and 53BP1 foci staining) and cell cycle profiles (flow cytometry).
  
  
  RESULTS
  In response to fractionated radiotherapy, wild-type and DNA repair defective cells accumulated in late S/G2 phase. In cells proficient in homologous recombination (HR), accumulation in S/G2 resulted in reduced sensitivity to fraction size and increased cellular resistance (clonogenic survival). Sensitivity to fraction size was also lost in NHEJ-defective V3-3 cells, which likely rely on functional HR. By contrast, HR-defective irs1SF cells, with functional NHEJ, remained equally sensitive to fractionation throughout the 3-week treatment.
  
  
  CONCLUSIONS
  The high fidelity of HR, which is independent of induced DNA damage level, is postulated to explain the low fractionation sensitivity and cellular resistance of cells in S/G2 phase. In conclusion, our results suggest that HR mediates resistance to fractionated radiotherapy, an observation that may help future efforts to improve radiotherapy outcome.
authors.affiliation: Gray Institute for Radiation Oncology & Biology, University of Oxford, UK. navita.somaiah@gmail.com
authors.forename: Navita
authors.initials: N
authors.lastname: Somaiah
authors.forename: John
authors.initials: J
authors.lastname: Yarnold
authors.forename: Anne
authors.initials: A
authors.lastname: Lagerqvist
authors.forename: Kai
authors.initials: K
authors.lastname: Rothkamm
authors.forename: Thomas
authors.initials: T
authors.lastname: Helleday
mesh_terms: D000818:Animals; D016466:CHO Cells; D006224:Cricetinae; D003412:Cricetulus; D053903:DNA Breaks, Double-Stranded; D059766:DNA End-Joining Repair; D019583:Dose Fractionation, Radiation; D016195:G2 Phase; D059765:Homologous Recombination; D011836:Radiation Tolerance; D016196:S Phase
keywords: Cell cycle; DNA double-strand break repair; Fraction size sensitivity; Fractionated radiotherapy; Homologous recombination