title: Ultrastructural Insights into the Biological Significance of Persisting DNA Damage Foci after Low Doses of Ionizing Radiation.
abstract: OBJECTIVE
  Intensity-modulated radiotherapy (IMRT) enables the delivery of high doses to target volume while sparing surrounding nontargeted tissues. IMRT treatment, however, substantially increases the normal tissue volume receiving low-dose irradiation, but the biologic consequences are unclear.
  
  
  METHODS
  Using mouse strains that varied in genetic DNA repair capacity, we investigated the DNA damage response of cortical neurons during daily low-dose irradiation (0.1 Gy). Using light and electron microscopic approaches, we enumerated and characterized DNA damage foci as marker for double-strand breaks (DSBs).
  
  
  RESULTS
  During repeated low-dose irradiation, cortical neurons in brain tissues of all mouse strains had a significant increase of persisting foci with cumulative doses, with the most pronounced accumulation of large-sized foci in repair-deficient mice. Electron microscopic analysis revealed that persisting foci in repair-proficient neurons reflect chromatin alterations in heterochromatin, but not persistently unrepaired DSBs. Repair-deficient SCID neurons, by contrast, showed high numbers of unrepaired DSBs in eu- and heterochromatin, emphasizing the fundamental role of DNA-PKcs in DSB rejoining, independent of chromatin status. In repair-deficient ATM-/-neurons, large persisting damage foci reflect multiple unrepaired DSBs concentrated at the boundary of heterochromatin due to disturbed KAP1 phosphorylation.
  
  
  CONCLUSIONS
  Repeated low-dose irradiation leads to the accumulation of persisting DNA damage foci in cortical neurons and thus may adversely affect brain tissue and increase the risk of carcinogenesis. Multiple unrepaired DSBs account for large-sized foci in repair-deficient neurons, thus quantifying foci alone may underestimate extent and complexity of persistent DNA damage. Clin Cancer Res; 22(21); 5300-11. ©2016 AACR.
authors.affiliation: Department of Radiation Oncology, Saarland University, Homburg, Saarland, Germany.
authors.forename: Yvonne
authors.initials: Y
authors.lastname: Lorat
authors.affiliation: Department of Radiation Oncology, Saarland University, Homburg, Saarland, Germany.
authors.forename: Stefanie
authors.initials: S
authors.lastname: Schanz
authors.affiliation: Department of Radiation Oncology, Saarland University, Homburg, Saarland, Germany. claudia.ruebe@uks.eu.
authors.forename: Claudia E
authors.initials: CE
authors.lastname: Rübe
mesh_terms: D000818:Animals; D053903:DNA Breaks, Double-Stranded / Q000187:drug effects; D004249:DNA Damage / Q000187:drug effects*; D004260:DNA Repair / Q000187:drug effects; D004307:Dose-Response Relationship, Radiation; D051379:Mice; D008810:Mice, Inbred C57BL; D009474:Neurons / Q000187:drug effects; D011836:Radiation Tolerance / Q000187:drug effects; D011839:Radiation, Ionizing; D050397:Radiotherapy, Intensity-Modulated / Q000009:adverse effects*; D000076183:Tripartite Motif-Containing Protein 28 / Q000378:metabolism
chemical_list: D000076183:Tripartite Motif-Containing Protein 28