Nuclear structure and the microdistribution of radiation damage in DNA

Evidence for the roles of proteins and metal ions in the microheterogeneity of DNA damage is reviewed. Decondensation of chromatin in hypotonic buffers markedly sensitizes the DNA to radiation, while treatment of nuclei with hypertonic buffers strips the DNA of histones and other nuclear proteins and enhances the radiosensitivity of the DNA with respect to double-strand break (dsb) formation. Addition of the radical scavenger DMSO reduces the yield of strand breaks, but dehistonized chromatin remains ∼2.5 times more sensitive to radiation than does native chromatin at 0.1 M DMSO. DNA-protein crosslink (DPC) formation is relatively unaffected by the removal of the majority of histones from chromatin. Most DPC form at or near the nuclear matrix, and matrix is stabilized and radiosensitized by Cu++. To elucidate the role of Cu++, the induction of dsb and DPC by γ-radiation has been compared with that by hydroxyl radical from Fe++-EDTA, or Cu++ catalysed Fenton reactions. Data comparing the size of DNA fragments produced, the effect of expanding or dehistonizing chromatin, and the effects of radical scavengers suggest that γ-radiation and Fe++-EDTA produce dsb at open chromatin sites, whereas Cu++-generated dsb are similar to radiation-induced DCP in their location at the nuclear matrix. (Author)