ABSTRACT
Ionizing radiation induces chemically stable molecular changes in cellular DNA (DNA damage). The biological effects of ionizing radiation, such as cell killing, induction of mutations, chromosome aberrations, transformation, and the adaptive responses of irradiated cells, are thought to arise from the formation of such DNA damage. However, it has been gradually recognized that living cells can effectively repair DNA damage through several enzymatic pathways. DNA double-strand breaks (DSBs), for instance, are thought to be one of the main types of DNA damage that induce the cell-killing effect. Most DSBs (80%–90%) initially produced in a cell can be rejoined even in cell extracts (de Lara et al., 1995), but the boundary line that divides repairable and unrepairable DNA damage has not yet been clariŸed. A long-standing and important question remains as to which kinds of DNA damage slip through the defenses of the enzymatic repair system in a living cell, ultimately causing deleterious radiation effects. It has been suggested that the susceptibility of DNA damage to repair strongly depends on the track structure of the radiation that induced the damage, represented by a linear energy transfer (LET) value. LET is deŸned as the ratio of dE to dl (dE/dl), where dE is the mean energy and dl is the distance traversed by the charged particle (ICRU, 1970). Monte Carlo track simulation studies have predicted that radiation with higher LET has a greater tendency to produce clusters of isolated DNA lesions by dense ionization/excitation within a distance of a few nanometers than radiation with lower LET (Nikjoo et al., 1994, 1999). These clustered sites of damage can be categorized into two groups: DSB and non-DSB types of damage. DSB damage consists of two or more single-strand breaks (SSBs) produced within about six base pairs’ separation in DNA (Hanai et al., 1998). Non-DSB damage, on the other hand, comprising two or more lesions, including nucleobase lesions, abasic sites (AP sites), and SSBs formed within about 10 base pairs’ separation by a single radiation track, has been proposed to be more biologically relevant (Ward, 1988; Goodhead, 1994a). It is becoming clear that clustered sites of DNA damage are less readily repaired than isolated nucleobase lesions, SSBs, or AP sites and, therefore, may induce serious genetic changes in cells. In order to understand the initial processes of induction of DNA damage, including the site of clustering of isolated lesions, and the susceptibility of this damage to repair, spectroscopic techniques combined with biochemical assays have been used. In Chapter 19, the molecular mechanisms of radiation damage to DNA are extensively presented in terms of their physicochemical aspects. In this chapter, recent studies aiming at clarifying the nature of DNA damage in terms of its biological reparability, particularly by base-excision repair (BER) proteins, are highlighted. New approaches using spectroscopic techniques combined with a brilliant synchrotron radiation source to reveal the role of the photoelectric effect on DNA induction are also introduced.
DNA damage is thought to be induced by both direct energy deposition on DNA (direct effect) (seereviewbyBernhardandClose,2003)andreactionswithdiffusiblewaterradicals(indirect effect)(seereviewbyO’NeillandFielden,1993).Mostmechanisticstudiestodate(vonSonntag, 1987;O’Neill,2001)havefocusedontheindirecteffectsusingdilute,aqueoussolutionscontainingDNA.Theresultsindicatedthatthehydroxylradical(OH•) is the main water radiolysis species thatinducesSSBsandDSBsinDNA,whereashydratedelectrons,Hatoms,andhydroxylradicals induceDNAnucleobaselesions.Experimental(deLaraet al.,1995)andtheoretical(Nikjooet al., 2002)studieshaveindicatedthatinlivingcellsorunderhighlyscavengingconditionssimilarto thoseinvolvedinOH• scavenging in the cell, ∼40% of the lesions induced in DNA by low-LET radiationcouldbeascribedtodirecteffects,increasingto∼70%forhigh-LETα-particles.Inthe pioneeringworkbyKrischet al.(1991),theyieldsofSSBsandDSBsbybothdirectandindirect effectsweredeterminedusingSV40DNA-irradiatedγ-raysunderconditionsofgreatlyvaryingradicalscavengerconcentration.Theyalsoconcludedthatinhighscavengingconditions,DSBsfrom
indirect effects are produced predominantly by local clusters of OH• from single energy deposition events.Theexperimentallyobtainedratiosbetweendirectandindirecteffectsexaminedbyvarious biological end points have been summarized by Becker and Sevilla (1993).
