Focus Formation of DNA Repair Proteins in Normal and Repair-Deficient Cells Irradiated With high-LET Ions

Radiat Res. 2004 May;161(5):517-27. doi: 10.1667/rr3171.


To investigate the repair of clustered lesions within the DNA/chromatin, the focus formation and persistence of foci of the phosphorylated histone protein H2AX and the repair protein MRE11 were studied in normal cells and in cells lacking DNA-PKcs (M059J) or ATM (GM2052D) after irradiation with high-LET nitrogen ions or low-LET photons. There was a rapid formation of MRE11 and gamma-H2AX foci, and 0.5 h after high-LET irradiation, the number of foci in normal cells correlated well with the number of particle hits per cell nucleus. After 8 h of repair, there were significantly more gamma-H2AX foci than MRE11 foci remaining in the normal cells, independent of radiation quality. The difficulty in repairing clustered breaks was detected as slower rejoining of DSBs (measured by DNA fragmentation analysis), as quantification of the amount of gamma-H2AX over time, and as a larger fraction of repair foci remaining after 24 h in cells irradiated with high- LET ions. These data indicate that clustered lesions are repaired by a pathway involving the same proteins that repair sparsely distributed breaks. Further, for both low- and high- LET radiation, no reduction of the initial number of gamma-H2AX and MRE11 foci was detected in M059J cells up to 21 h after irradiation, which was in accordance with a complete absence of DSB rejoining in these cells. In the GM2052D cells there was also a higher level of foci remaining after 21 h; however, this was not accompanied by unrejoined DSBs, indicating that these foci not only represent DSBs but also may be a sign of persistent problems even when breaks are rejoined.

Publication types

  • Comparative Study
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Ataxia Telangiectasia Mutated Proteins
  • Calcium-Binding Proteins / metabolism*
  • Cell Cycle Proteins
  • Cesium Radioisotopes
  • DNA / radiation effects*
  • DNA Damage
  • DNA Repair / radiation effects*
  • DNA-Binding Proteins / metabolism*
  • Dose-Response Relationship, Radiation
  • Fibroblasts / cytology
  • Fibroblasts / metabolism*
  • Fibroblasts / radiation effects*
  • Gamma Rays
  • Histones / metabolism*
  • Humans
  • Ions
  • Linear Energy Transfer
  • MRE11 Homologue Protein
  • Protein-Serine-Threonine Kinases / metabolism*
  • Radiation Dosage
  • Tumor Suppressor Proteins


  • CIB1 protein, human
  • Calcium-Binding Proteins
  • Cell Cycle Proteins
  • Cesium Radioisotopes
  • DNA-Binding Proteins
  • H2AX protein, human
  • Histones
  • Ions
  • MRE11 protein, human
  • Tumor Suppressor Proteins
  • DNA
  • ATM protein, human
  • Ataxia Telangiectasia Mutated Proteins
  • Protein-Serine-Threonine Kinases
  • MRE11 Homologue Protein