Clustered lesions

The amount of information is still insufficient to predict how packaging of DNA in chromatin modifies the final product distribution in DNA. If the stable end products reflect a doubling in reductive damage while oxidative damage remains the same, then one would expect an increase in frequency and complexity of clustered lesions. If that proves true, the histone proteins and attending nucleosomes structure would act as radiation sensitizers. 

Clustered Lesions and Damage Amplification Reactions (Tandem Lesions) 391... 

The main types of damage that can be formed in DNA (base damage, apy-rimidnic/apurinic (AP) site, single-strand break (SSB), double-strand break (DSB), tandem lesions and various clustered lesions) are shown schematically in Fig. 12.1. There are, however, further lesions such as DNA/DNA and DNA/pro-tein cross-links. 

Also, in the treatment with BLM, 150 SSBs and 30 DSBs are formed at the LD37 level of survival (Ward 1988). It has hence be concluded that the cellular repair enzymes can cope very efficiently with these simple lesions, and much more complex lesions (clustered lesions, above the level of a DSB, see below) are required to cause a lethal event. This is supported by the observation that densely ionizing radiations show a higher RBE then low-LET radiation (Goodhead 1994). 

Nearly all modifications that have been detected on the model level (Chap. 10) are also found in free-radical damaged DNA. Obviously the DNA-bound lesions are much more difficult to detect, and there is an ongoing discussion as to the best procedure of their excision (Chap. 13.2 for a review on the excision and repair of base lesions in vivo see Wallace, 2002). Mechanistic details concerning the formation of the base lesions have been discussed in Chapters 10 and 11, and only some additional information will be given below and in the section on clustered lesions where the phenomenon of tandem lesions, two damaged bases that are formed side by side, is dealt with. The yields of damaged bases formed upon y-irradiation in aqueous solution, as has been determined by the GC-MS/SIM technique, are compiled in Table 12.5. 

As is seen from the table, the yield of detectable damaged bases is noticeably lower for the high LET radiation despite its higher RBE. This agrees with the concept of clustered lesions being responsible for lethality (for the repair of clustered lesions see Dianov et al. 2001). 

Table 12.10. Clustered lesions in (Jenner et al. 2001) percent of SSB induced by y and a-partide irradiation. ...

Clustered lesions are also apparent when DNA films are subjected to y-radia-tion (Yokoya et al. 2002). Here, additional DSBs are at least twice that of prompt DSBs. In contrast, enzymatic treatment of DNA films by a-radiation does not raise the number of DSBs, and it has been suggested that in this case the complexity of the clustered damage may be greater than that formed upon y-radia-tion and that the enzymes can no longer adequately work on such sites (Yokoya et al. 2003). 

The chemical repair rates for the survival of V70 cells and two typical lesions, DSBs and SSBs are shown in Table 12.19. There is no difference between cell survival and DSB formation when the large error bars are taken into account. In contrast, the repair rate for SSBs is much slower. This difference may be accounted for on the basis of the clustered lesion model (Sect. 12.5) considering that if one of two precursors of nearby SSBs has been successfully repaired, a DSB can no longer develop. Note that the probability of one or the other of these radicals being chemically repaired is twice that of a that of an SSB free-radical precursor. 

As the LET of the ionizing radiation increases, the RBE increases as do clustered lesions such as DSBs. The presence of O2 has then no longer such a dominating effect on increasing the complexity of DNA lesions and thus the OER drops. This is exemplified for ultrasoft X-rays in Table 12.24. 

Dianov GL, O Neill P, Goodhead DT (2001) Securing genome stability by orchestrating DNA repair removal of radiation-induced clustered lesions in DNA. BioEssays 23 745-749 Dirksen M-L, Blakely WF, Holwitt E, Dizdaroglu M (1988) Effect of DNA conformation on the hydroxyl radical-induced formation of 8,5 -cyclopurine 2 -deoxyribonucleoside residues in DNA. Int J Radiat Biol 54 195-204... 

DNA damage induced by ionizing radiation leads single lesions also to the formation of clustered lesions such as two close-by damaged bases on opposite strands (Chap. 12). For their detection, DNA is treated with an endonuclease that induces a SSB at a damaged site. If there are two closely separated lesions on opposite strands, such treatment induces a DSB which can be detected on a non-denaturing gel (Sutherland et al. 2000). The enzymes that have been used and their targets are compiled in Table 13.1. 

Blaisdell J.O., Wallace S.S., Abortive base-excision repair of radiation-induced clustered lesions in Escherichia coli, Proc. Natl. Acad. Sci. USA, 2001,98,7426-7430. 

Double Strand Breaks DNA-protein XL Clustered lesions... 

Methods for Determining Biomarkers of Exposure and Effect. Analytical methods with satisfactory sensitivity and precision are available to determine the levels of strontium in human tissues and body fluids. Strontium and radiostrontium are found in essentially all food, water, and air, so everyone is exposed to some levels. Recently, Sutherland et al. (2000a, 2000b) developed a molecular biological strategy to identify clustered lesions in DNA resulting from in vitro cellular exposure to gamma radiation. 

Deoxyuridinyl)methyl radical (74) and the respective radical from 5-methyl-2 -deoxycytidine have not been proposed to yield strand breaks. However, 74 and other nucleobase radicals have been impHcated in reactions with adjacent nucleotides, resulting in two contiguously damaged nucleotides (eg 15). Such lesions are typically referred to as tandem lesions.Tandem lesions are a subset of clustered lesions, which are defined as two or more damaged nucleotides within one to two helical turns. Clustered lesions are of particular interest to radiation scientists because of their formation via HO spurs and the difficulties that they pre-sent for DNA repair. In addition, 74 has been invoked as an inter-... 

Saloua KS, Sonia G, Pierre C, Leon S, Darel HJ. The relative contributions of DNA strand breaks, base damage and clustered lesions to the loss of DNA functionality induced by ionizing radiation. Radiat Res. 2014 181 99-110. 

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