Repair of DNA Damage

Upon uv irradiation pyrimidine bases in adjacent nucleotides of a DNA strand become covalently cross-linked. The cross-linking occurs mostly between two thymines to form a thymine dimer (Fig. 16-19). The thymine dimer causes a structural distortion within the DNA chain and represents a physical impediment to replication and transcription. 

Excision repair in mammalian cells is considered to occur by a similar mechanism. Inherited defects in this repair pathway can lead to the disease xeroderma pigmentosum, in which the skin is very sensitive to sunlight and results in a high incidence of skin cancer. 

Question Is excision repair the only means of correcting uv-induced damage to DNA  

Dimers can also be repaired directly by enzymatic photoreactivation. The photoreactivating enzyme binds to the DNA in the region of the dimer to form a complex that absorbs visible light and catalyzes cleavage of the covalent linkage between the components of the dimer. Photoreactivation occurs in both bacterial and mammalian cells. 

The ability of human cell (HeLa) extracts to repair the exocydic 4-hydroxy-2-nonenal-dG adducts has been investigated using plasmid DNAs randomly modified by 4-hydroxy-2-nonenal [123]. Incubation of modified plasmids with HeLa nuclear extracts in the presence of [32P]dCTP led to the incorporation of 32P indicative of repair synthesis. 32P-Postlabeling of the modified plasmid revealed that approximately 50-60% of the 4-hydroxy-2-nonenal-dG adducts initially present in the plasmid were removed on incubation with the extracts. This analysis also indicated that two of the four isomeric 4-hydroxy-2-nonenal-dG adducts were preferentially removed. The findings that MjdG, PdG, and the 4-hydroxy-2-none-nal-dG adducts are all repaired by bacterial and mammalian NER suggests that other exocyclic dG adducts are repaired by this pathway. 

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Breimer, L.H. (1991). Repair of DNA damage induced by reactive oxygen species. Free Rad. Res. Commun. 14, 159-171. 

In the case of sulfur mustard, analysis of low molecular weight urinary metabolites suffers from the same drawback as in the case of anticholinesterases, i.e., these products are rapidly excreted and provide therefore limited retrospectivity. Similarly, the in vivo lifetime of DNA adducts of sulfur mustard are less than those of protein adducts due to repair of DNA damage. 

Henle and Linn recently reviewed the formation, prevention, and repair of DNA damage by tron/hydrogen peroxide (so-called Fenton oxidants). Berlett and Stadt-... 

The mechanisms of interaction between fluorouracil and radiation are not clearly understood. Different hypotheses have been postulated to explain the synergistic or potentiated effect of 5-FU with radiation including redistribution of cells to a more radiosensitive cell cycle phase, deranged pyrimidine pools with reduced capacity for repair of DNA damage, and activation of apoptosis. The effect of 5-FU on radiation damage also appears to vary in different cell lines, thus complicating the extrapolation of laboratory results into clinical practice. 

A singular example of how covalent modification can be used to achieve transcription control is found in the regulation of the adaptive repair of DNA damage in proca-... 

A more in-depth characterization of the subimits brought surprising results. Some of the polypeptides of TFIIH could be identified as proteins shown in other experiments to participate in repair of DNA damage, as well as in the regulation of the cell cycle (CDK7/Cyclin H, see above). TFIIH itself, or other individual components of TFIIH, thus participate in the following fundamental processes in the cell ... 

Cleaver, J.E. (1977). Methods for the study of excision repair of DNA damaged by physical and chemical mutagens, page 19 in Handbook on Mutagenicity Tkst Procedures, KiLBEY, B.J., LEGATOR, M., NICHOLS, W., AND Ramel, C., Eds. (Elsevier, Amsterdam). 

Vitamins and minerals, whose main dietary sources are other than fruits and vegetables, are also likely to play a significant role in the prevention and repair of DNA damage, and thus are important to the maintenance of long-term health. Vitamin B12 is found in animal products, and deficiencies of B12 cause a functional folate deficiency, accumulation of the amino acid homocysteine (a risk factor for heart disease),46 and chromosome breaks. B12 supplementation above the RDA was necessary to minimize chromosome breakage.47 Strict vegetarians are at increased risk for developing vitamin B12 deficiency. 

Figure 6.46 Representation of two mechanisms of repair of DNA damage such as ultraviolet light-induced dimerization. The upper line represents cut and patch repair, the lower sister-strand exchange. Thick lines represent newly synthesized DNA. Source From Ref. 12.

Although the example of photoreactivation is the simplest, the earliest demonstration of the repair of DNA damage in vivo was by Boyce and Howard-Flanders and by Setlow and Carrier, working with E. coli. Independently, they showed that UV-induced thymine dimers were removed from DNA during incubation over about 20 min in the dark after induction. This process did not occur in her- mutants (uvrA-, uvrB- or uvrC-). At the same time it was shown that for every dimer removed, about 40 bases were lost from the DNA and later, Pettijohn and Hanawalt (1964) showed that the process was accompanied by a comparable amount of conservative DNA synthesis (i.e. incorporation of radioactive nucleotides into both parental strands of DNA). 

With thiols, protection against free-radical attack and repair of DNA damage (Sect. 12.12) is not always easy to disentangle in in vitro experiments and even more so in cellular systems (e.g., Murray et al. 1990). This has to be kept in mind, when some aspects of thiol protections are discussed here. 

Several amphibian species were examined for photolyase activity. This enzyme is responsible for the repair of DNA damage caused by UV-B radiation. A more than... 

Cleaver, J.E. Methods for studying excision repair of DNA damaged by physical and chemical mutagens, pp. 19-48. In B.J. Kilbey, M. Legator, W. Nichols, and C. Ramel, Eds. Handbook of Mutagenicity Test Procedures. Amsterdam Elsevier/North Holland, 1977. 

Cleaver, J.E. Methods for studying repair of DNA damaged by physical and chemical carcinogens. 

What enzymes are common to the processes of replication of DNA and excision repair of DNA damage in E. colil... 

Honie, E. S., and Linn, S- (1997). Formation, prevention, and repair of DNA damage by iron/hydrogen peroxide. J, Bruf. CJrem. 272,19095-19098. 

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