Cu(II)/H2O2-induced DNA damage is enhanced by packaging of DNA as a nucleosome

Copper is a physiologically important, redox-active metal that may be invol ved in endogenous DNA damage and mutagenesis. To understand the factors tha t affect the location and quantity of copper-induced oxidative DNA damage i n cells, we used the 5S rDNA nucleosome as a model to assess the effect of chromatin structure on DNA damage produced by Cu(II)/H2O2. Packaging of DNA into a nucleosome increased the extent of Cu(II)/H2O2-induced strand break s by a factor of 2, while the extent of base lesions sensitive to Fpg and e ndo III glycosylases increased 8-fold. We also observed that Cu(II)/H2O2 ca used slightly more strand breaks than base lesions in isolated 5S rDNA (rat io of base lesions to strand breaks of similar to0.6), while base lesions o utnumbered strand breaks by a factor of 3-4 when the DNA was incorporated i nto a nucleosome. Apart from several sites of enhanced or diminished DNA da mage, there were no major changes in the sequence selectivity of Cu(II)/H2O 2, and there was no apparent footprinting effect associated with nucleosome structure, such as that observed with the Fe(II)-EDTA complex. Possible me chanisms for explaining these observations include (1) an increase in Cu(II ) concentration in the vicinity of nucleosomal DNA caused by binding of Cu to histone proteins or (2) increased reactivity or accessibility of nucleob ases caused by DNA conformational changes associated with nucleosome struct ure. The enhancement of Cu(II)/H2O2-induced DNA damage in nucleosomes stand s in contrast to the protective effect afforded DNA by proteins in chromati n against radiation-induced DNA damage.