title: Impact of homologous recombination on individual cellular radiosensitivity.
abstract: OBJECTIVE
  Individual radiosensitivity as measured with in vitro irradiated lymphocytes using metaphase analysis can predict the risk of normal tissue effects after radiotherapy. This parameter is considered to be primarily determined by the cellular repair capacity of DNA double-strand breaks (DSBs). It is now tested to which extent this capacity also depends on homologous recombination (HR), which is a pathway available when cells are in S/G2 phase.
  
  
  METHODS
  Experiments were performed with CHO K1 cells, in which HR was suppressed via knock-down of RAD51 using RNA interference (RNAi). RAD51 was measured via western and foci formation, cell survival by colony forming, DSBs by gammaH2AX foci formation, and chromosomal damage using PCC, G0 or G2 assay.
  
  
  RESULTS
  In quiescent G1 cells DSB repair is completed 6h after irradiation. But there is still a substantial fraction of non-repaired DSBs. Most of these DSBs are repaired when G1 cells are stimulated into cell cycle. Suppression of HR by down-regulation of RAD51 did not affect this repair. In contrast, repair was inhibited when cells were irradiated in late S/G2. In line with these data down-regulation of HR did affect survival of cells irradiated in late S/G2, but not in G1.
  
  
  CONCLUSIONS
  Individual radiosensitivity as measured for G0/1 cells using metaphase analysis does not depend on homologous recombination.
authors.affiliation: Centre of Oncology, University Medical Center Hamburg-Eppendorf, Germany.
authors.forename: Kerstin
authors.initials: K
authors.lastname: Koch
authors.forename: Agnieszka
authors.initials: A
authors.lastname: Wrona
authors.forename: Ekkehard
authors.initials: E
authors.lastname: Dikomey
authors.forename: Kerstin
authors.initials: K
authors.lastname: Borgmann
mesh_terms: D000818:Animals; D016466:CHO Cells; D002453:Cell Cycle / Q000528:radiation effects; D002460:Cell Line; D002869:Chromosome Aberrations / Q000528:radiation effects; D006224:Cricetinae; D003412:Cricetulus; D053903:DNA Breaks, Double-Stranded / Q000528:radiation effects*; D004260:DNA Repair; D016195:G2 Phase / Q000528:radiation effects; D006657:Histones / Q000378:metabolism; D051135:Rad51 Recombinase / Q000378:metabolism; D011836:Radiation Tolerance / Q000235:genetics*; D011995:Recombination, Genetic
chemical_list: C468783:H2AX protein, human; D006657:Histones; D051135:Rad51 Recombinase