Double-strand breaks DSB

The outcome of rapid radiation chemical processes in mammalian cells is to cause a variety of longer-Hved physical alterations in the DNA. Of these, double-strand breaks (DSBs) appear to be most frequently involved in cell killing if not correctly repaired. In general, thiols protect against DSB induction in proportion to their effect on cell killing (7), although there are exceptions (8). 

Figure 1 Action spectra for the induction of single strand break (ssb, open circle) and double strand break (dsb, closed circle) in dry plasmid DNA. Ratio of the cross section of ssb against dsb is also shown in lower panel. Solid line is reconstructed absorption cross section of the plasmid. (From Refs. 12, 13, 22, and 25.)...

Figure 5 Individual examples of simulated sites of damage induced by 3.2 MeV alpha particles in DNA. In each example, the outer and inner rows represent the sugar-phosphate moieties and the pairs of bases, respectively, with single base pair resolution (dots). An x or H represent energy deposition or reaction of hydroxyl radical leading to induction of a single strand break or base damage. A indicates hit sites that did not lead to strand breaks (SB) or base damage (BD). Nomenclature no strand break (No SB) single strand break (SSB), (SSB ), (2SSB) double strand break (DSB), (DSB+), (DSB + ).

It is currently believed that double-stranded break (DSB) repair enzymes may act as baits to attract invading T-DNA molecules to the sites of integration. This DSB repair machinery may be transported via histone modifications as part of a general process of intranuclear protein traffic that also allows transcriptional factors to reach their target promoters (Lacroix et al., 2006b). 

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. 

Although most of the focus on solute effects on macromolecular systems has involved proteins, there is evidence that some of the patterns of solute accumulation reflect the dangers that high salt concentrations pose for the covalent structure of DNA. Using cultured mammalian kidney cells Kiiltz and Chakravarty (2001) showed that hyperosmotic stress could cause double strand breaks (dsb) in DNA. Hyperosmolality due to elevated [NaCl] in the culture medium caused the most dsb. Potassium chloride and mannitol led to less damage and, interestingly, no damage to DNA was found in cells exposed to elevated levels of urea. 

Figure 19-4. Open and solid symbols are the measured quantum yields (events per incident electron) for the induction of single strand breaks (SSB) (a) and double strand breaks (DSB) (b) in DNA films by 4-100 eV electron impact. The solid curves through the data are guides to the eye. The dotted curves symbolize general electron energy dependence of the cross sections for various nonresonant damage mechanisms, such as ionization cross sections, normalized here to the measured strand break yields at lOOeV...

Figure 21-4. Quantum yield of DNA single strand breaks (SSBs) and double-strand breaks (DSBs) vs incident electron energy. The inset shows the dependence of the percentage of circular DNA (i.e., SSBs) on irradiation time for 0.6 eV electrons (Figure 1 of ref. [18]. Reprinted Figure with permission. Copyright 2004 by the American Physical Society.)...

Most studies of DNA damage centre on strand breaks, partly because they are relatively easy to study. Also, there is a large (but controversial) body of opinion that implicates double strand breaks (DSB) as the major lesion leading to cell death. It seems that single strand breakage (SSB) and base damage that does not lead to strand breakage are less important. 

As describe above DNA strand breaks can be induced by LEEs. Recent work has shown LEEs, with energy as low as 1 to 5 eV [19], are effective in this regard. The decay of localized transient anion states (resonances) within DNA is the principai mechanism leading to SSB and double strand breaks (DSB) by electrons with energies below 15 eV [19]. Theoretical calculations support the fragility of the DNA backbone to LEEs and suggest... 

Figure 2 DNA damage induced by ionizing radiation. A) DNA damage and repair. All the constitutive elements of DNA (sugar-phosphate backbone and bases) are possibly modified by ionizing radiation. Single strand breaks (SSB), oxidized bases and abasic site are processed by base excision repair (BER), double strand breaks (DSB) by homologous recombination and non homologous end joining (HR and NHEJ) and DNA-protein crosslinks by nucleotide excision repair (NER). B) Quantitative measurement of radiation-induced and spontaneous DNA damage.

DNA double strand breaks (DSB) can be measured by the same techniques as the SSB, by changing the alkahne conditions to neutral pH and thereby avoiding the denaturation of the DNA helix. The more DSB, the faster the DNA will migrate whereas SSB wiU not be detected under neutral conditions. When measured before and after a defined incubation period, the repair capacity of DSB can be estimated. 

Rassool FV (2003) DNA double strand breaks (DSB) and non-homologous end joining (NHEJ) pathways in human leukemia. Cancer Lett 193 1-9... 

Further studies have implicated BRCAl in the cellular response to DNA double-stranded breaks (DSBs), a potentially lethal form of DNA damage. Cells defective in BRCAl possess numerous cytological and biological features that have been known for years to be correlated with perturbation in the maintenance of chromosome stability. This includes aneuploidy, centrosome amplification, spontaneous chromosome breakage, aberrant recombination events, sensitivity to ionizing radiation, and impaired cell cycle checkpoints. In addition, a variety of experiments have demonstrated roles for BRCAl in enforcing the GfM cell cycle transition, homologous recombination between sister chromatids, as well as the restart of stalled replication in S phase. 

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