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Kufe DW, Pollock RE, Weichselbaum RR, et al., editors. Holland-Frei Cancer Medicine. 6th edition. Hamilton (ON): BC Decker; 2003.

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Holland-Frei Cancer Medicine. 6th edition.

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DNA Damage and Repair

, PhD and , MD.

The chemical structure of DNA can be altered by a carcinogen in several ways: the formation of bulky aromatic-type adducts, alkylation (generally small adducts), oxidation, dimerization, and deamination. In addition, double- and single-strand breaks can occur. Chemical carcinogens can cause epigenetic changes, such as altering the DNA methylation status that leads to the silencing of specific gene expression.69 A complex pattern of carcinogen-DNA adducts likely results from exposure to tobacco smoke, because of the mixture of different chemical carcinogens present.70

Benzo[a]pyrene-7,8-diol 9,10-epoxide reacts with the exocyclic (N2) amino group of deoxyguanosine and resides within the minor groove of the double helix; it is typical of polycyclic aromatic hydrocarbons (see Figure 17-3). This adduct appears to be the most common, persistent adduct of benzo[a]pyrene in mammalian systems, but others are possible. Some metabolites bind covalently with deoxyadenosine, and proapurinic adducts form through one electron oxidation (see Figure 17-4). This type of adduct is thought to induce ras gene mutations, which are common in tobacco-smoking-related lung cancers.56,57,71,72 Aromatic amine adducts are more complex, because they have both acetylated and nonacetylated metabolic intermediates, and they form covalent bonds at the C8, N2, and sometimes O6 positions of deoxyguanosine as well as deoxyadenosine. The major adducts, however, are C8-deoxyguanosine adducts, which reside predominantly in the major groove of the DNA double helix (see Figure 17-4).59

Aflatoxin B1 and G1 activation occurs through hydroxylation of the olefinic 8,9-position and adducts are formed at the N7-position of deoxyguanosine. They are relatively unstable and have a half-life of approximately 50 h at neutral pH; depurination products have been detected in rat and human urine.73 The aflatoxin B1-N7-deoxyguanosine adduct also can undergo ring opening to yield two pyrimidine adducts; alternately, aflatoxin B1-8,9-dihydrodiol could result. This latter possibility could restore the DNA molecular structure if hydrolysis of the original adduct occurs, but a potentially promutagenic lesion would result if the 8,9-dihydrodiol results from degradation of open-ring adduct forms.74

DNA alkylation can occur at many sites either following the metabolic activation of certain N-nitrosamines, or directly by the action of the N-alkylureas (N-methyl-N-nitrosourea) or the N-nitrosoguanidines. The protonated alkyl-functional groups that become available to form lesions in DNA generally attack the following nucleophilic centers: adenine (N1, N3, and N7), cytosine (N3), guanine (N2, O6, and N7), and thymine (O2, N3, and O4). Some of these lesions are known to be repaired (O6-methyldeoxyguanosine), while others are not (N7-methyldeoxyguanosine).67,68 Furthermore, O6-methyldeoxyguanosine is a promutagenic lesion, whereas N7-methyldeoxyguanosine is not.

Oxyradical damage can form thymine glycol or 8-hydroxydeoxyguanosine adducts. Exposure to organic peroxides (catechol, hydroquinone, and 4-nitroquinoline-N-oxide) leads to oxyradical damage; however, oxyradicals and hydrogen peroxide can be generated in lipid peroxidation and the catalytic cycling of some enzymes.75 Tobacco smoke is also a source of oxyradicals, and by inducing an inflammatory response, tobacco smoke can contribute to diseases such as asthma and chronic heart disease, as well as cancer.76,77 Also, certain drugs and plasticizers can stimulate cells to produce peroxisomes.78 In addition, increased oxyradical formation is mediated through protein kinase C when inflammatory cells are exposed to tumor promoters like phorbol esters.79

Another potentially mutagenic cause of DNA damage is the deamination of DNA-methylated cytosine residues. 5-Methylcytosine comprises approximately 3% of deoxynucleotides. In this case, deamination at a CpG dinucleotide gives rise to a TpG mismatch. Repair of this lesion most often restores the CpG; however, repair may cause a mutation (TpA).80 Deamination of cytosine also can generate a C to T transition if uracil glycosylation and G-T mismatch repair are inefficient. Oxyradicals can enhance the deamination rate, so the activity of inducible nitric oxide synthase and production of high concentrations of nitric oxide could contribute to DNA damage by this mechanism.81

Maintenance of genome integrity requires abrogation of DNA damage, and diminished DNA-repair capacity is associated with carcinogenesis, birth defects, premature aging, and foreshortened life-span. DNA-repair enzymes act at DNA-damage sites caused by chemical carcinogens, and six major mechanisms are known: direct DNA repair, nucleotide excision repair, base excision repair, nonhomologous end joining (double-strand break repair), mismatch repair, and homologous recombination (postreplication repair).44,82

In the presence of nonlethal DNA damage, cell-cycle progression is postponed for repair mechanisms. This highly coordinated process involves multiple genes. A DNA-damage recognition sensor triggers a signal transduction cascade and downstream factors direct G1 and G2 arrest in concert with the proteins operationally responsible for the repair process. Although there are at least six discrete repair mechanisms, within five of them there are numerous multiprotein complexes comprising all the machinery necessary to accomplish the step-by-step repair function.

Generically, DNA repair requires damage recognition, damage removal or excision, resynthesis or patch synthesis, and ligation. Recent advances have led to the cloning of more than 130 human genes involved in five of these DNA-repair pathways. A list of these genes and their specific functions was published elsewhere.83 These genes are responsible for the fidelity of DNA repair, and when they are defective the mutation rate increases. This is the mutator phenotype.31 Mutations in at least 30 DNA-repair-associated genes have been linked to increased cancer susceptibility or premature aging (Table 17-2).83 Moreover, it remains to be seen if common polymorphisms in some of these genes are associated with increased susceptibility in a gene-environment interaction scenario.40 However, recent evidence suggests that tobacco-smoking-related lung cancer is associated with a polymorphism in the nucleotide excision repair gene, ERCC2.84

Table 17-2. Examples of Disease Susceptibility and Disease Syndromes Associated with Mutations in DNA-Repair Genes.

Table 17-2

Examples of Disease Susceptibility and Disease Syndromes Associated with Mutations in DNA-Repair Genes.

Direct DNA repair is effected by DNA-alkyltransferases. These enzymes catalyze translocation of the alkyl moiety from an alkylated base (eg, O6-methyldeoxyguanosine) to a cysteine residue at their active site in the absence of DNA strand scission. Thus, one molecule of the enzyme is capable of repairing one DNA alkyl lesion. The inactivation of this mechanism by promoter hypermethylation is associated with K-ras G to A mutations in colon cancer.85

In DNA nucleotide excision repair, lesion recognition, preincision, incision, gap-filling, and ligation are required, and the so-called excinuclease complex comprises 16 or more different proteins. Large distortions caused by bulky DNA adducts (eg, BPDE-dG and 4ABP-dC) are recognized (XPA) and removed by endonucleases (XPF, XPG, FEN). A patch is then constructed (polΔ, polε) and the free ends are ligated.

Base excision repair also removes a DNA segment containing an adduct, however, small adducts (eg, 3-methyladenine) are generally the target so that there is overlap with direct repair. The adduct is removed by a glycosylase (hOgg1, UDG), an apurinic endonuclease (HAP1) degrades a few bases on the damaged strand, and a patch is synthesized (polβ) and ligated (DNA ligases: I, II, IIIα, IIIβ, and IV).

DNA mismatches occasionally occur, because excision repair processes incorporate unmodified or conventional, but noncomplementary, Watson-Crick bases opposite each other in the DNA helix. Transition mispairs (G-T or A-C) are repaired by the mismatch repair process more efficiently than transversion mispairs (G-G, A-A, G-A, C-C, C-T, and T-T). The mechanism for correcting mispairings is similar to that for nucleotide excision repair and resynthesis described earlier, but it generally involves the excision of large pieces of the DNA containing mispairings. Because the mismatch recognition protein is required to bind simultaneously the mismatch and an unmethylated adenine in a GATC recognition sequence, it removes the whole intervening DNA sequence. The parental template strand is then used by the polymerase to fill the gap.

Double-strand DNA breaks can occur from exposure to ionizing radiation and oxidation. Consequences of double-strand DNA breaks are the inhibition of replication and transcription, and loss of heterozygosity. Double-strand DNA break repair occurs through homologous recombination, where the joining of the free ends is mediated by a DNA-protein kinase in a process that also protects the ends from nucleolytic attack. The free ends of the DNA then undergo ligation by DNA ligase IV. Genes known to code for DNA-repair enzymes that participate in this process include XRCC4, XRCC5, XRCC6, XRCC7, HRAD51B, HRAD52, RPA, and ATM.82

Postreplication repair is a damage-tolerance mechanism and it occurs in response to DNA replication on a damaged template. The DNA polymerase stops at the replication fork when DNA damage is detected on the parental strand. Alternately, the polymerase proceeds past the lesion, leaving a gap in the newly synthesized strand. The gap is filled in one of two ways: either by recombination of the homologous parent strand with the daughter strand in a process that is mediated by a helical nucleoprotein (RAD51); or when a single nucleotide gap remains, mammalian DNA polymerases insert an adenine residue. Consequently, this mechanism may lead to recombinational events as well as base-mispairing.

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By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.

Copyright © 2003, BC Decker Inc.
Bookshelf ID: NBK13516

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