Mechanisms of DNA Damage and Repair in Alzheimer Disease

Reddy PV, Perry G, Cooke MS, et al.

Publication Details

Introduction

Reactive oxygen species (ROS) are produced during the respiratory cycle in mitochondria, 1 as well as normal cellular and xenobiotic metabolism. Exposure to various noxious insults can also lead to ROS production. In addition, ROS are also generated through metal-catalyzed reactions. A consequence of ROS production is the modification of cellular biomolecules, such as DNA, protein and lipids.2,3 In addition to mutation, which is commonly considered, oxidative modification of DNA can have other, broad-ranging effects upon the function of the cell, impacting upon telomeres, microsatellite sequences, promoters and sites of methylation.4,5 Perhaps as a consequence, such damage appears to have an important role in the pathogenesis of many diseases,4 including Alzheimer disease (AD). Notably in AD, oxidative stress is regarded as one of the earliest pathological changes6,7 and likely involves metabolic changes,8 and redox-active metals9,10 as well as other factors.11 Oxidative modification of nuclear and mitochondrial DNA are thought to exacerbate AD, and their measurement is often used as a marker of oxidative stress. Indeed, extensive mitochondrial DNA damage was observed when PC12 cells were exposed to amyloid-β (Aβ), showing a direct correlation between oxidative stress and DNA damage.12 Treatment of the exposed cells with endonuclease III or formamidopyrimidine (FaPy) glycosylase revealed significant damage to pyrimidine or purine bases, although in recent studies Aβ was found to sequester ROS, thus acting as an antioxidant equivalent.13

Nucleic acids may also be damaged by reactive nitrogen species (RNS), leading to deamination of the nucleotide bases. The RNS-mediated damage of DNA is associated with upregulation of nitrotyrosine which is also used as a marker for DNA damage by RNS.14 In addition, DNA is also damaged by advanced lipid peroxidation end products (ALEs), for example, trans-4-hydroxy-2-nonenal (HNE), through the formation of DNA-HNE adducts.15

It is essential that damage to DNA does not persist. Repair of damaged DNA is accomplished mostly through nucleotide- or base-excision pathways (NER and BER, respectively). Base excision repair occurs through the mediation of enzymes called glycosylases which function by cleaving the damaged bases, which will be subsequently transported into cerebrospinal fluid (CSF). These enzymes selectively cleave phosphodiester bonds 5' and 3' to damaged bases, producing free hydroxyl and phosphate groups at the respective termini.16 The released oxidized nucleosides such as 8-hydroxy-2'-deoxyguanosine (8-OHdG) are then passed through the circulation into urine and eventually excreted.17 The decreased levels of damaged deoxynucleosides in CSF from cases of AD, as compared to those of control cases, give an indication of the absence or failure of the repair enzymes in AD. The latter effect is reflected also in the accumulation of damaged bases in intact DNA strands. In this review, we will focus upon studies related to the effects of DNA damage in AD and neuronal response to repair the damaged DNA.

Free Radical Formation and Biomolecular Damage

Formation of ROS

Metal (Fe2+ or Cu+) catalyzed reduction of molecular oxygen gives rise to the superoxide anion which can be protonated at pH below 6 to give the hydroperoxyl radical (HOO•), which in turn can be converted to hydrogen peroxide (H2O2) by metal catalyzed reduction followed by protonation. Superoxide dismutase (SOD) can also catalyze the transformation of superoxide radical anion into hydrogen peroxide. Nonenzymatic superoxide dismutation is very rapid and will occur in biological compartments when the concentration of SOD is low. However, this can generate singlet oxygen, which can damage DNA. The superoxide anion and hydrogen peroxide by themselves are not highly oxidizing species, but the metal ion (e.g., Fe2+ or Cu+) catalyzed Fenton reaction of H2O2 results in the formation of the highly toxic hydroxyl (•OH) radical. The generalized reaction sequence for the hydroxyl radical formation is shown below (Fig. 1).

Figure 1. Intracellular formation of superoxide radical anions, hydrogen peroxide and hydroxyl radicals.

Figure 1

Intracellular formation of superoxide radical anions, hydrogen peroxide and hydroxyl radicals.

Formation of RNS

Similarly, RNS, especially peroxynitrite, can initiate DNA damage. Peroxynitrite is derived from the reaction of nitric oxide (a free radical) with superoxide radical anion. Rapid protonation of peroxynitrite anion in cells gives peroxynitrous acid (ONOOH), which is a good nitrating agent for tyrosine and tryptophan side chains in proteins. Peroxynitrous acid can also react with deoxyribose backbones and cause single- and double-strand breaks. Further, it can cause nitration or deamination of DNA bases. Guanine, for example, upon deamination gives xanthine, and adenine similarly gives hypoxanthine. The mispairing of these newly generated oxidized forms of nucleic acids can result in nuclear base transversion mutations. Similar deaminative damage may result from alkylperoxynitrites, themselves formed by the reaction of nitric oxide with alkylperoxy radicals (Fig. 2).

Figure 2. Formation of RNS, peroxynitrite and alkyl peroxynitrrite.

Figure 2

Formation of RNS, peroxynitrite and alkyl peroxynitrrite.

Advanced Glycation End Products and Advanced Lipid Peroxidation End Products

Adduction of reducing sugars to proteins and evolution of the initial adducts under aerobic conditions leads to advanced glycation end products (AGEs), which contribute to the histopathological and biochemical hallmarks of AD. Among such AGEs, pentosidine (a cross-link between lysine and arginine), pyrraline (a lysine-derived adduct), and carboxymethyllysine (CML, the product of condensation of glyoxal with lysine, which can also form from oxidative cleavage of a glycated lysine) are identified in elevated levels in AD brains.18-23 Similarly, advanced lipid peroxidation end products (ALEs), formed by the reaction of proteins with lipid-peroxidation products, such as 4-hydroxy-trans-2-nonenal (HNE), are elevated in AD (Fig. 3). HNE derived adducts of proteins were shown to be present in AD by immunocytochemical studies, using antibodies specific to various HNE adducts.24-26 The AGEs and ALEs can potentially act as further sources of ROS by their ability to chelate, and in some cases reduce, redox active transition metals. The AGEs also activate the receptor for AGE (RAGE), indirectly contributing to an increased production of ROS.23,27

Figure 3. Structures of typical AGEs, and HNE found in AD.

Figure 3

Structures of typical AGEs, and HNE found in AD.

DNA Damage Involving Lipoxidation-Derived Aldehydes

Lipid peroxidation generates numerous cytotoxic aldehydes. HNE, in particular, has been extensively explored in recent years in the pathogenesis of AD. It can form exocyclic propano- adducts by reaction with nucleoside purine and pyrimidine bases. One such adduct involving the reaction of HNE with 2'-deoxyguanosine has been isolated.28 The reaction presumably proceeds through the initial Michael addition of the free amino group (in the purine ring) to the HNE followed by nucleophilic addition of the N1 (in the purine ring) to the carbonyl group. Subsequent dehydration gives the stable propano-adduct.

Among other products of lipid peroxidation, 4-oxo-2-nonenal (4-ONE) has been well characterized29,30 and shown to be involved in DNA damage.29,31-35 DNA forms exocyclic five-membered ‘etheno-’ adducts with 4-ONE through reactions with purine and pyrimidine rings. Several DNA/4-ONE adducts such as those derived from 2'-deoxyadenosine, 2'-deoxyguanosine, and 2'-deoxycytosine have been characterized by Blair and coworkers using atmospheric pressure ionization/MS/MS techniques33 (Fig. 4). The formation of these etheno-adducts was postulated to involve initial nucleophilic addition of the free amino group of the purine/pyrimidine rings to the carbonyl group of 4-ONE, followed by Michael addition and subsequent dehydration reactions.33 The etheno adducts are formed in high yields when 2'-deoxyguanosine and 2'-deoxyadenosine are treated with 4-ONE.29

Figure 4. HNE and 4-ONE induced DNA damage.

Figure 4

HNE and 4-ONE induced DNA damage.

4-ONE is more reactive and cytotoxic than HNE, and may be more biologically important. Its adduct with Vitamin C has been recently found in human plasma, verifying its generation in vivo.36 Since 4-ONE also readily modifies proteins,37 4-ONE could interact with proteins responsible for nucleotide excision repair, damaging DNA repair mechanisms.

The effects of HNE on DNA damage and inhibition of DNA repair has been explored in human cells in the case of mutations responsible for cancer,38 although there has been no such evidence of HNE modification of DNA repair enzymes in AD. In the absence of appropriate DNA repair mechanisms, the DNA adducts can induce apoptosis,39 an important event in AD.

HNE Detoxification

HNE can be detoxified by anti-inflammatory agents such as carnosine through Michael adduct formation.40,41 The glutathione transferase superfamily of enzymes catalyzes the nucleophilic attack of glutathione on HNE for detoxification (Fig. 5).42

Figure 5. HNE detoxification by carnosine and glutathione (GSH).

Figure 5

HNE detoxification by carnosine and glutathione (GSH).

Free Radical Formation from Amyloid- β (A β)

Aβ may also serve as a source of ROS, as it has been shown to bind to Cu2+ and Zn2+, inducing its aggregation. However, it has been shown that copper binding to Aβ results in conformational changes that enable SOD-like or Cu/Zn SOD activity.43 Thus Aβ functions to maintain oxidative balance. Further, it can act as an antioxidant by sequestering free radicals.44 For this reason therapeutic intervention aimed at disrupting the relationship between heavy metals and Aβ is in doubt.13

Mechanisms of DNA Damage by ROS/RNS

ROS and RNS can damage DNA by modifying nucleic acid bases. Most of the damage of the nucleic acids arises directly or indirectly from the •OH radical produced from the metal catalyzed Fenton reaction.45 ROS, for example the hydroxyl radical, can modify guanine, forming 8-hydroxyguanine, which energetically base pairs more favorably with adenine rather than with cytosine, the resulting mispairing producing GC to TA transversions.46 Thymine similarly gives rise to 5-hydroxymethyluracil, as a common (but not sole) product upon reaction with ROS. It has a low probability of mispairing with guanine.47 8-Hydroxyadenine, a ROS derived product of adenine, also mispairs with guanine.47

Reaction of RNS (e.g., HNO2 or ONOOH) with nucleic acid bases normally results in oxidative deamination, replacing NH2 groups with OH groups. Thus adenine, cytosine and guanine are transformed into hypoxanthine, uracil, and xanthine, respectively (Fig. 6). Hypoxanthine mispairs with cytosine, and uracil mispairs with adenine. Deamination of nucleic acid bases can therefore result in AT to GC transversions.47 A variety of oxidatively modified nucleic acid bases have been identified and measured by HPLC-MS/MS techniques. However, artifactual DNA damage during isolation and sample preparation makes the latter technique somewhat unreliable, and a number of work-up procedures to minimize such potential alterations have been introduced.48 Immunocytochemical studies, using antibodies specific to known DNA modifications, are useful as they circumvent many of the problems associated with DNA extraction.49

Figure 6. RNS and ROS induced oxidative deamination of nucleic acid bases.

Figure 6

RNS and ROS induced oxidative deamination of nucleic acid bases.

Protection against ROS and RNS

Antioxidants, such as ascorbic acid, vitamin E, glutathione, and β-carotene can intercept superoxide and hydroperoxide radical anions as well as hydroxyl radicals, thereby limiting their toxic effects. In general, combinations of vitamin C and vitamin E are more effective as antioxidants compared to either one alone. In these cases, vitamin E can be regenerated from its oxidatively modified form by reaction with vitamin C, which is relatively more abundant in cells. CuZn superoxide dismutase and Mn superoxide dismutase react with superoxide radical anions and convert them into hydrogen peroxide and molecular oxygen. Downregulation of these enzymes can result in exacerbation of oxidative stress. However, in AD brains these enzymes are abundant, ruling out the possibility that the deficiency of these enzymes results in the pathogenesis of AD.50

Markers of DNA Damage in AD

Interaction of the hydroxyl radical with purine and pyrimidine bases can give rise to multiple products51 (Fig. 7). Adenine and guanine can react with hydroxyl radicals at C4, C5, or C8 positions in the pyrimidine ring. Thymine can undergo the addition of hydroxyl radicals at C4 or C5 positions to give the corresponding radicals, which upon reaction with O2 give the peroxide products. Reductive cleavage of the latter peroxides can give cis- and trans-thymine glycols. Thymine can also suffer hydrogen abstraction from the methyl group to give the corresponding free radical, which upon reaction with O2 followed by reductive cleavage gives the corresponding alcohol. Cytosine similarly can give several products including cytosine glycol and 5,6-dihydroxycytosine. Thus although over 20 different oxidation products of DNA have been characterized, most of the investigators have focused attention on 8-hydroxyguanine, or its deoxynucleoside equivalent, 8-OHdG, a major oxidation product of guanine. Although the mechanism of the ring-opening of the imidazolinone ring to give the formamido derivative (Fapy-guanine) has not been established, it can be presumed to involve electron transfer followed by protonation at the carbonyl carbon and oxygen atoms (Fig. 8). 8-OHdG and Fapy-guanine were shown to be the major products of DNA oxidation by radiation-generated ROS and RNS.52 Using gas chromatography/mass spectrometry techniques, Halliwell and coworkers identified the oxidized products of all four DNA bases: 8-hydroxyadenine, 8-hydroxyguanine, thymine glycol, Fapy-guanine, 5-hydroxyuracil, and Fapy-adenine in parietal, temporal, occipital, and frontal lobe, superior temporal gyrus, and hippocampus of AD brains,53 implying a role for such DNA damage in the pathogenesis of AD.

Figure 7. Structures of illustrative ROS generated nucleic acid bases.

Figure 7

Structures of illustrative ROS generated nucleic acid bases.

Figure 8. Oxidative stress of DNA leading to the formation of 8-OHdG, and its ring-opened formanido pyrimidine derivative.

Figure 8

Oxidative stress of DNA leading to the formation of 8-OHdG, and its ring-opened formanido pyrimidine derivative.

Mitochondrial DNA (mtDNA) is more prone to oxidative damage as compared to that of nuclear DNA (nDNA), as the latter is well protected by histones and DNA-binding proteins. The mtDNA is also exposed to increased concentrations of ROS. Elevated levels of 8-OHdG, 8-hydroxyadenine, and 5-hydroxyuracil, a product of cytosine oxidation, were observed from the DNA of parietal, temporal and frontal lobes of AD brains as compared with age-matched controls,54 although there was no significant correlation between the oxidized bases and neurofibrillary tangles or senile plaques. The levels of 8-OHdG were significantly elevated in intact DNA isolated from ventricular CSF of AD subjects, whereas the levels of the free nucleotide, formed due to the action of excision repair, were significantly lower in CSF.55 These observations show that DNA damage is not only greater in AD, but also DNA repair mechanisms may be impaired in AD.

8-OHdG is present in significant amounts (about three-fold increase as compared to control cases) in the mtDNA and nDNA of AD brains.56 Damage to mtDNA was significantly higher than that of nDNA. Lovell and coworkers have found statistically significant decreases in 8-OHdG glycosylase activity in the nuclear fraction of AD hippocampal and parahippocampal gyri (HPG), superior and middle temporal gyri (SMTG), and inferior parietal lobule (IPL). Depletion of helicase activity was also observed in the nuclear fraction in the IPL of AD.57 Thus it is likely that the decreased repair of DNA damage could be one hallmark of the pathogenesis of AD. Furthermore, using the Comet assay with oxidative lesion-specific DNA repair endonucleases (endonuclease III for oxidized pyrimidines, Fapy-glycosylase for oxidized purines) it was determined that AD is associated with elevated levels of 8-OHdG and other oxidized nucleic acids.58

8-Hydroxyguanosine (8OHG)

In situ approaches involving immunochemical techniques are better suited for accurate localization of the 8-OHdG and 8OHG within cells and tissues. Smith and coworkers investigated the formation of these products in AD using immunoreactivity with monoclonal antibodies 1F7 and 15A3 specific to 8-OHdG and 8OHG.59 From these studies it was found that 8-OHdG and 8OHG were prominent in the cytoplasm, and to a lesser extent in the nucleolus and nuclear envelope in neurons within the hippocampus, subiculum, and entorhinal cortex as well as frontal, temporal, and occipital neocortex in AD. Control cases were only faintly immunolabeled in similar structures. Damage to RNA is likely to be more extensive than to DNA, as there are more non-base-paired regions in RNA as compared to DNA, there are no protective histones, and the potential for repair has received only limited study.4 Furthermore, it is plausible that the metals bound to RNA are major sites of redox activity, which result in the formation of hydroxyl radicals.

The oxidized nucleosides in AD are associated predominantly with RNA, as immunoreactivity towards 8OHG was diminished greatly by pre-incubation with RNase but only slightly by DNase. From these observations it can be inferred that mitochondria may be a major source of ROS that cause oxidative damage to DNA and RNA in AD.6 A recent paper has suggested that mitochondrial ROS damage mitochondrial DNA, but not nuclear DNA.60 It was concluded that the ROS derived from the mitochondrial respiratory chain are detoxified by mitochondrial SOD and other agents such as cytosolic catalase, and thus are not able to travel the distance from the mitochondria to the nucleus.

Quantitatively, neuronal 8OHG is greatest early in AD and is reduced with disease progression. Interestingly, nitrotyrosine, a marker of RNS mediated oxidative stress, is also elevated in the early stages of AD and is reduced with disease progression, indicating the significance of RNS in RNA damage in AD (vide infra). Smith and coworkers observed that the increase in Aβ deposition is associated with decreased oxidative damage.7 Furthermore, neurons with neurofibrillary tangles show a 40% to 56% decrease in 8OHG levels compared with neurons free of neurofibrillary tangles, demonstrating that oxidative stress-induced RNA and DNA damage is an early event in AD, that decreases with disease progression.7 A marked accumulation of 8OHG and nitrotyrosine, was also observed in the cytoplasm of cerebral neurons in Down's syndrome (DS), with the levels of nucleic acid and protein oxidation paralleling each other.6 Thus in AD, increased levels of oxidative damage to DNA occur prior to the onset of Aβ deposition.

Mitochondrial versus Nuclear DNA Damage

Impaired mitochondrial function may result in accelerated DNA damage. The aging human brain accumulates substantial oxidative damage to mitochondrial DNA, which may be due to impaired mitochondrial function resulting in the increase of ROS, or by reducing ATP required for DNA repair.61 The levels of 8-OHdG in DNA isolated from three regions of cerebral cortex and cerebellum increase progressively with normal aging in both nDNA and mtDNA.62 The rate of increase of 8-OHdG, however, was 10-15 times higher in mtDNA than nDNA.62 A significant reduction of mitochondrial fluidity was observed in AD along with increased levels of 8-OHdG in mtDNA. The alteration in membrane fluidity is primarily a result of lipid peroxidation. HNE and malondialdehyde (MDA), two widely studied lipid peroxidation products, were isolated in mitochondria, and more importantly, HNE was shown to modify the membrane fluidity by direct interaction with membrane phospholipids.63 Thus, there is a direct correlation between oxidative stress and DNA damage, implicating oxidative stress in the pathogenesis of AD.62

The ROS and RNS generated as a result of impaired mitochondrial function, combined with reduced ATP levels in these mitochondria, can contribute to damage of vulnerable genes in the aging human brain.61 The 8-OHdG may thus serve as a marker for oxidative stress in the aging brain. However, it is not clear whether it would have a significant effect on total cellular energy metabolism as there are thousands of mitochondria, it is unlikely that DNA damage may occur in the same gene in all of them.64 However, when mutations occur in mtDNA, the corresponding mutant proteins may increase the inefficiency of the mitochondrial respiratory chain, resulting in the excessive production of ROS that cause further mtDNA and nDNA damage.

Neuronal DNA Damage by RNS

Smith and coworkers,65 along with Su and coworkers,14 have found evidence of peroxynitrite-mediated nitration of tyrosine residues of proteins to give 3-nitrotyrosine (NT)-derived proteins (vide supra). Using double labeling experiments with NT and PHF/tau antibodies, these workers have found that the most intense NT-positive neurons usually contained neurofibrillary tangles. However, many neurons lacking neurofibrillary tangles are also intensely stained for NT. NT was undetectable in the cerebral cortex of age-matched control brains. Thus NT upregulation may precede neurofibrillary tangle formation. The peroxynitrous acid and other RNS such as nitrous acid (HONO) can deaminate guanine residues of DNA. Su and coworkers14 showed that the terminal deoxynucleotidyl transferase (TdT)-labeled neurons are elevated in AD, and they have strong NT immunoreactivity, suggesting that the neurons with DNA damage in the absence of tangle formation may degenerate by tangle-independent mechanisms.

The neuronal form of nitric oxide synthase (nNOS) is elevated in reactive astrocytes in the hippocampus and entorhinal cortex in AD.66 Although NO shows protective antioxidant effects, at high concentrations it is known to mediate DNA damage.65 As a result of the upregulation of NO, increased cell death, related to DNA damage, was observed in the hippocampus and entorhinal cortex in AD.66

Diet Restriction and DNA Damage

DNA damage is significantly increased in senescent cells, e.g., in post-mitotic tissues of rodents of various ages.8,67 A significant increase in 8-OHdG was observed in nDNA with increasing age in all tissues and strains of rodents studied, a result of increased sensitivity of these tissues to oxidative stress with age. Conversely, the age-dependent increase in 8-OHdG was reduced in the mtDNA of diet-restricted mice and rats.67 It has been shown that ROS are reduced in diet-restricted rodents, where the rate of aging (e.g., life span extension) is also reduced.68 Thus a major source of oxidative DNA damage is through ROS, which may be reduced through the use of antioxidants or metal chelators.

DNA Repair and AD

DNA damaged cells can undergo apoptosis through a common p53-dependent mechanism.69 DNA damage can also be repaired by a combination of enzymes. If DNA damage is not repaired, it may be bypassed by specialized DNA polymerases.70 The repair of DNA typically involves several steps. Excision enzymes such as DNA glycosylase remove the damaged bases by the cleavage of the base-sugar bonds. An endonuclease nicks the DNA strand at this site and removes it. Then, DNA synthase fills the missing link in the strand, and a DNA ligase joins this new DNA strand with the existing undamaged strand.47 In addition to base excision repair, nucleotide excision repair may also be involved. The nucleotide excision repair involves removal of damaged nucleotides as part of large (30 nucleotide units) fragments.71

The accumulation of the damaged DNA bases in cells may result in the loss of normal cellular function which may be the basis of the causative factors of AD and other age related diseases. The oxidized bases can also be paired with abnormal bases during replication of DNA, leading, for example, to GG and AT combinations. In addition, pyrimidines next to the 8-OHdG residue are also misread72 during DNA transcription.

Nucleotide/Base Excision Repair

Nucleotide excision repair requires the TFIIH transcription-repair complex having helicase activity. These DNA helicases unwind DNA for repair as well as for replication. Base excision repair, nucleotide excision repair, and mismatch repair have been identified and characterized in eukaryotes.71 A variety of other protein complexes are also involved in DNA repair as outlined below.

There are a variety of base-specific glycosylases that cleave specific damaged bases.73 For example, human HeLa cell extracts contain two forms of 8-hydroxyguanine glycosylase (hOGG) repair enzymes. hOGG-1 cleaves 8-hydroxyguanine paired with cytosine and thymine, whereas hOGG-2 cleaves 8-hydroxyguanine paired with adenine. Expression of DNA excision repair cross complementing proteins p80 and p89 was observed in AD brains.74 These are known to repair different types of DNA damage.

It was found that the concentrations of 8-OHdG in CSF of AD are 100-fold higher than those of healthy individuals. In addition, the ratio of 8-OHdG to 8-hydroxyguanine is approximately 8-fold higher in CSF than in urine, suggesting that the nucleotide excision repair is a major DNA repair mechanism in removal of oxidatively damaged DNA in brain cells.75

Bcl-2 in DNA Repair

Bcl-2, a 26-kDa integral membrane protein, has an antioxidant effect and is increased in neurons with DNA damage in AD, as are other members of the Bcl family.76,77 Expression of Bcl-2 in PC12 cells inhibit nitric oxide donor (sodium nitroprusside)- and peroxynitrite-induced cell death.78 It was shown that H2O2, nitric oxide and peroxynitrite induced oxidative stress results in DNA damage both in mtDNA and nDNA, even in the presence of Bcl-2. However, recovery from DNA damage was accelerated in cells expressing Bcl-2, implicating that neuronal up-regulation of Bcl-2 may facilitate DNA repair after oxidative stress.78 A growth arrest DNA damage-inducible protein, GADD45, is expressed in AD neurons and is associated with expression of Bcl-2.79 GADD45 may aid in DNA repair. GADD45 was found to bind to PCNA, a normal component of Cdk complexes and a protein involved in DNA replication and repair, and stimulated DNA excision repair in vitro.18

Repair of double strand breaks requires DNA-dependent protein kinase, composed of DNA-PKcs and Ku. It was shown that Ku DNA binding activity was reduced in extracts of postmortem AD midfrontal cortex, and the decreased Ku DNA binding is correlated with reduced protein levels of Ku subunits (DNA-PKcs) and poly(ADP-ribose) polymerase-1.80 Immunohistochemical analysis also suggested that DNA-PK protein levels reflected the number of neurons and regulation of cellular expression.

Repair of 8-OHdG

8-Oxoguanine DNA glycosylase (hOGG1-2a) is one of the excision repair enzymes that repairs 8-OHdG. Using an antibody specific to the mitochondrial form of hOGG1-2a, it was found that hOGG1-2a is expressed mainly in the neuronal cytoplasm in both AD and control cases in regionally different manners.81 Immunoreactivity to hOGG1-2a is associated with neurofibrillary tangles, dystrophic neurites and reactive astrocytes in AD. The relatively low levels of expression of hOGG1-2a in AD indicates that oxidative DNA damage in mitochondria may be involved as the pathogenic factor.81 Another repair enzyme for oxidative DNA damage, purine-nucleoside triphosphatase (hMTH1) was also observed in neurons of AD. In vitro studies showed that hOGG1-2a immunoreactivities in reactive astrocytes and oligodendrocytes were more intense than those to hMTH1.82 By adding H2O2 to the cultured astrocyte cells, rapid induction of hMTH1 was observed, whereas the levels of hOGG1-2a were mildly increased, showing that hMTH1 is an inducible enzyme under oxidative stress, and hOGG1-2a is rather constitutively expressed and also up-regulated in the chronic stage of the disease.82

Mre11 DNA Repair Complex

The Mre11 protein complex consisting of Rad50, Mre11 and Nbs1 is essential for cellular responses to DNA damage, such as initiating cell cycle checkpoints and repairing damaged DNA. It was shown that the Mre11 complex proteins are present in neurons of adult human cortex and cerebellum and were substantially reduced in the neurons of AD cortex. The accumulated DNA damage in AD neurons may be, in part, as a result of the reduced levels of Mre11 protein complexes.83

Poly(ADP-Ribose) Polymerase (PARP)

Poly(ADP-ribose) polymerase (PARP) is one of the DNA repair enzymes that is activated as a result of oxidative stress induced single strand or double strand breaks of DNA. PARP catalyzes the cleavage of NAD+ into adenosine 5'-diphosphoribose (ADP-ribose) and nicotinamide. It also catalyzes the covalent attachment of ADP-ribose polymers to nuclear proteins such as histones. PARP and poly(ADP-ribose)-immunolabelled neurons were detected in a much higher proportion in AD than in controls.84 Overactivation of PARP causes massive NAD+ depletion resulting in cell death due to energy depletion.

Conclusions

As highlighted above, the role of oxidative stress in the pathogenesis of AD is a burgeoning field. However, while much is known, much remains unknown as to the impact of therapeutic intervention in patients with disease. Translation of basic scientific findings into efficacious treatment strategies remains to be determined.

Acknowledgements

Work in the authors' laboratories is supported by the NIH (NS38648 and AG14249) and the Alzheimer's Association (IIRG-03-6263 and IIRG-04-1272).

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