title: Formation and repair of clustered damaged DNA sites in high LET irradiated cells.
abstract: OBJECTIVE
  Radiation with high linear energy transfer (LET) produces clustering of DNA double-strand breaks (DSB) as well as non-DSB lesions. Heat-labile sites (HLS) are non-DSB lesions in irradiated cells that may convert into DSB at elevated temperature during preparation of naked DNA for electrophoretic assays and here we studied the initial formation and repair of these clustered damaged sites after irradiation with high LET ions.
  
  
  METHODS
  Induction and repair of DSB were studied in normal human skin fibroblast (GM5758) after irradiation with accelerated carbon and nitrogen ions at an LET of 125 eV/nm. DNA fragmentation was analyzed by pulsed-field gel electrophoresis (PFGE) and by varying the lysis condition we could differentiate between prompt DSB and heat-released DSB.
  
  
  RESULTS
  Before repair (t = 0 h), the 125 eV/nm ions produced a significant fraction of heat-released DSB, which appeared clustered on DNA fragments with sizes of 1 Mbp or less. These heat-released DSB increased the total number of DSB by 30-40%. This increase is similar to what has been found in low-LET irradiated cells, suggesting that the relative biological effectiveness (RBE) for DSB induction will not be largely affected by the lysis temperature. After 1-2 hours repair, a large fraction of DSB was still unrejoined but there was essentially no heat-released DSB present.
  
  
  CONCLUSIONS
  These results suggest that high LET radiation, as low LET gamma radiation, induces a significant fraction of heat-labile sites which can be converted into DSB, and these heat-released DSB may affect both induction yields and estimates of repair.
authors.affiliation: a Department of Immunology , Genetics and Pathology, Rudbeck Laboratory , Sweden.
authors.forename: Ann-Sofie
authors.initials: AS
authors.lastname: Gustafsson
authors.affiliation: b The Svedberg Laboratory, Uppsala University , Sweden.
authors.forename: Torbjörn
authors.initials: T
authors.lastname: Hartman
authors.affiliation: a Department of Immunology , Genetics and Pathology, Rudbeck Laboratory , Sweden.
authors.forename: Bo
authors.initials: B
authors.lastname: Stenerlöw
mesh_terms: D002244:Carbon / Q000009:adverse effects; D053903:DNA Breaks, Double-Stranded / Q000528:radiation effects; D004249:DNA Damage; D053938:DNA Fragmentation / Q000528:radiation effects; D004260:DNA Repair / Q000528:radiation effects*; D005347:Fibroblasts / Q000378:metabolism / Q000528:radiation effects; D006801:Humans; D018499:Linear Energy Transfer; D009584:Nitrogen / Q000009:adverse effects; D012867:Skin / Q000166:cytology
chemical_list: D002244:Carbon; D009584:Nitrogen
keywords: Clustered damage; DNA DSB repair; DNA damage; DNA repair; High LET; double-strand breaks