AN INVESTIGATION OF CLUSTERED RADIATION-INDUCED LESIONS IN DNA

Recent track structure modelling studies indicate that radiation induc ed damage to DNA consists of a spectrum of different lesions of varyin g complexity. There is considerable evidence to suggest that, in repai r-proficient systems, it is only the small proportion of more complex forms that is responsible for most of the biological effect. The compl ex lesions induced consist initially of clustered radical sites and a knowledge of their special chemistry is important in modelling how the y react to form the more stable products that are processed by the rep air systems. However, much of the current understanding of the chemica l stage of radiation has developed from single-radical systems and the re is a need to translate this to the more complex reactions that are likely to occur at the important multiple radical sites. With low LET radiation, DNA dsb may derive either from single-radical attack that d amages both strands by a transfer mechanism, or from pairs of radical sites induced in close proximity, with one or more radical on each str and. With high LET radiation, modelling studies indicate that there is an increased probability of dsb arising from sites with more than two radical centres, leading to a greater frequency of more complex types of break. The spectrum of these lesions depends on the overall outcom e of consecutive physical and chemical processes. The initial pattern of radical damage is determined by the energy depositions on and aroun d the DNA, according to the type of radiation. This pattern is then mo dified by scavengers that inhibit the formation of radicals on the DNA , and by agents that either chemically repair (e.g. thiols) or fix (e. g. oxygen) a large fraction of these radicals. The reaction kinetics a ssociated with clustered radical sites will differ from those of singl e sites: (1) because of the opportunities for interactions between the radicals themselves; and (2) because certain endpoints, e.g. a dsb, m ay require a combination of the products of two or more radicals. Fast response techniques using pulsed low and high LET irradiation have be en established to measure the reactions of radical sites on pBR322 pla smid DNA with oxygen and thiols with a view to obtaining information a bout cluster size. This paper describes experimental approaches to exp lore the role of the chemical stage of the radiation effect in relatio n to lesion complexity.