title: Long time persistence of residual 53BP1/γ-H2AX foci in human lymphocytes in relationship to apoptosis, chromatin condensation and biological dosimetry.
abstract: OBJECTIVE
  Novel assay for radiosensitivity is based on measurements of residual DNA repair foci produced by several proteins including phosphorylated H2AX (γ-H2AX), recombinase Rad51 (Rad51) and tumour suppressor p53 binding protein 1 (53BP1), which co-localise with radiation-induced DNA double-strand breaks (DSB). Here, we studied dose-response for residual 53BP1, Rad51, and γ-H2AX foci in relationship to apoptosis and chromatin condensation in human G(0)-lymphocytes.
  
  
  METHODS
  Residual foci, apoptosis and condensation of chromatin were studied following irradiation with γ-rays at doses of 0.5-10 Gy.
  
  
  RESULTS
  No clear dose response for residual Rad51 was seen. Residual 53BP1/γ-H2AX foci remained in human lymphocytes up to four weeks after irradiation. No foci formed during radiation-induced apoptosis. We provide evidence that irreversible apoptotic condensation of chromatin is responsible for arrest of residual foci and preventing de novo focus formation. Similar linear dose dependences up to 2 Gy were observed for the 53BP1/γ-H2AX foci at all studied time points. At higher doses, saturation and decline were caused by preferential elimination of apoptotic lymphocytes with residual foci. While primary 53BP1 and γ-H2AX foci almost completely co-localised, co-localisation of residual foci did not exceed 17%, indicating that 53BP1 and γ-H2AX proteins may remain for different times at the locations of DSB repair.
  
  
  CONCLUSIONS
  Prolonged persistence of residual 53BP1/γ-H2AX foci may be used for biological dosimetry within the dose range up to 2 Gy. While foci are not formed during radiation-induced apoptosis in human lymphocytes, elimination of apoptotic cells with residual foci may affect the dose response.
authors: Department of Genetics, Microbiology and Toxicology, Stockholm University, Sweden.
authors: Eva
authors: E
authors: Marková
authors: Jesper
authors: J
authors: Torudd
authors: Igor
authors: I
authors: Belyaev
mesh_terms: D017209:Apoptosis / Q000528:radiation effects*; D001681:Biological Assay / Q000379:methods; D002478:Cells, Cultured; D002843:Chromatin / Q000378:metabolism* / Q000528:radiation effects; D004307:Dose-Response Relationship, Radiation; D006657:Histones / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D008214:Lymphocytes / Q000502:physiology* / Q000528:radiation effects*; D011829:Radiation Dosage; D011874:Radiometry / Q000379:methods*; D016159:Tumor Suppressor Protein p53 / Q000235:genetics / Q000378:metabolism*
chemical_list: D002843:Chromatin; C468783:H2AX protein, human; D006657:Histones; C495901:TP53 protein, human; D016159:Tumor Suppressor Protein p53