Plasmid DNA breakage by decay of DNA-associated Auger electron emitters: approaches to analysis of experimental data

Plasmid DNA is a popular substrate for the assay of DNA strand breakage by a variety of agents. The use of the plasmid assay relies on the assumption that individual damaging events occur at random, which allows the application of Poisson statistics. This assumption is not valid in the case of damage arising from decay of DNA-associated Auger electron emitters, since a single decay event can generate a few breaks in the same DNA strand, which is indistinguishable from a single break in the assay. The consequent analytical difficulties are overcome by considering relaxation events rather than single-strand breaks, and linearization events rather than double-strand breaks. A further consideration is that apart from damage at the site of DNA-associated decay, which is the principal interest of the analysis, some DNA damage also arises from the radiation field created by all decay events. These two components of damage are referred to as internal and external breakage, respectively, and they can be separated in the analysis since their contribution depends on the experimental conditions. The DNA-binding ligand Hoechst 33258 labeled with 125I was used in our experiments to study breakage in pBR322 plasmid DNA arising from the decay of this Auger electron emitter. The values obtained for the efficiency (per decay) of plasmid relaxation and linearization by the 125I-labeled ligand were 0.090 +/- 0.035 and 0.82 +/- 0.04, respectively. When dimethylsulfoxide was included as a radical scavenger, the efficiency values for relaxation and linearization were 0.15 +/- 0.02 and 0.65 +/- 0.05, respectively.