Production of Clustered DNA Damage by 125I Decay

We previously found a large difference in DNA double strand break (dsb) induction when randomly incorporating I or nonrandom association of I with DNA at so-called `estrogen response elements’ (ERE) in a Chinese hamster ovary cell line transfected with human estrogen receptor (1). A relative biological effectiveness (RBE) value for DNA dsb induction (RBEdsb) is calculated here for accumulated I-iododeoxyuridine (IUdR) decays or I-estrogen decays relative to g radiation from a Cs source. RBEdsb values were 2.9 and 24 for IUdR and I-estrogen, respectively (1). RBE values for cell survival (RBEs) were 3.1 and 3.9 for IUdR and I-estrogen, respectively (see Table 1). Several lines of evidence support the notion of clustered DNA damage (or locally multiply damaged DNA or LMDs (2) produced by I decay (1, 3). First, the production of clustered DNA damage or LMDs is indicated by a drop in the ® rst fraction of elution pro® les (1, 3), suggesting that I decay produces complex lesions composed of small fragments of DNA which elute through the ® lter in the ® rst fraction. Secondly, other workers have observed that RBEdsb decreases relative to RBEs with increasing LET of radiation used (4). The reduction in RBEdsb suggests that dsb assays underestimate DNA dsb induction by not measuring all breaks in complex lesions such as LMDs or clustered lesions. For IUdR, RBEdsb is marginally less than RBEs. Repair of complex lesions may complicate this relationship. Thirdly, numerous physical models of Auger electron emission from 1 decay predict energy deposition patterns consistent with production of clustered DNA damage. The pattern of clustered DNA damage produced by IUdR decay likely consists of short fragments of DNA on the order of 10 ± 100s of base pairs. Martin & Haseltine (5) showed the highest frequencies of strand breaks in an oligonucleotide at about 10 base pairs upstream and downstream of the DNA-incorporated IUdR and Panyutin & Neumann (6) found that approximately 1 DNA dsb is formed opposite to the site of I incorporation and within 10 base pairs of the site of incorporation when they used a I-labeled triplex forming oligonucleotide, which speci® cally binds to a particular nucleotide sequence in the major groove of double stranded DNA in vitro. Brenner & Ward (7) calculated that 2 ± 6 ionizations are required within 1 ± 4 nm volume of DNA for the formation of a DNA dsb. I decay produces a cascade of 22 ± 25 low-energy electrons per decay that could very well produce the requisite 2 ± 6 ionizations within a 1 ± 4 nm volume of DNA. Folding of the mammalian cell DNA into more compact chromatin structures, such as nucleosomes or solenoids, may place adjacent segments of DNA in close proximity to the incorporated radioactivity, resulting in strand breakage further than 10 bp from the site of IUdR incorporation (about 100s of base pairs away since solenoids consist of about 1 kbp DNA). In contrast to the data discussed above using IUdR decay accumulation in cellular DNA, evidence suggests that the clustered DNA damage produced by accumulation of 1-estrogen decays in cellular DNA may comprise both the small fragments (10 ± 100s bp) described above and large domain-sized fragments. In these studies, RBEdsb from I-estrogen decay accumulation studies are much higher than RBEs (RBEdsb was 24 and RBEs was approximately 4) indicating that not only clustered DNA damage is produced but also a greater amount of additional DNA damage is induced by I-estrogen than by IUdR. The physical decay characteristics producing a high density of ionizations in DNA Table 1