Residual DNA and chromosomal damage in ex vivo irradiated blood lymphocytes correlated with late normal tissue response to breast radiotherapy

Abstract Purpose To test the association of DNA double-strand break (DSB) repair and chromosomal radiosensitivity in ex vivo irradiated blood lymphocytes with late-onset normal tissue responses following breast radiotherapy. Methods Breast cancer patients with minimal (controls) or marked late radiotherapy changes (cases) were retrospectively selected. DSB were quantified by γH2AX/53BP1 immunofluorescence microscopy 0.5 and 24h after exposure of unstimulated blood lymphocytes to 0.5 and 4Gy X-rays, respectively. Chromosomal aberrations were scored in blood lymphocyte metaphases after 6Gy X-rays. Results Despite similar foci levels at 0.5h in cases ( n =7) and controls ( n =7), foci levels 24h after 4Gy irradiation differed significantly between them (foci per cell were 12.8 in cases versus 10.2 in controls, p =0.004). Increased chromosomal radiosensitivity was also observed in cases (aberrations per cell were 5.84 in cases versus 3.79 in controls, p =0.001) with exchange and deletion type aberrations contributing equally to the difference between cases and controls. Residual foci correlated with formation of deletions (Spearman's R= 0.589, p =0.027) but not exchanges ( R =0.367, p =0.197) in blood lymphocytes from the same patients. Conclusions Higher levels of exchange type aberrations observed among radiosensitive breast cancer patients suggest a role for DSB misrepair, in addition to residual damage, as determinants of late normal tissue damage. Correlation of residual foci levels with deletion type aberration yields in the same cohort confirms their mechanistic linkage.