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Oxidatively Induced DNA Damage and Its Repair in Cancer

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Oxidatively Induced DNA Damage and Its Repair in Cancer Oxidatively Induced DNA Damage and Its Repair in Cancer Miral Dizdaroglu Biomolecular Measurement Division, National Institute of Standards and Technology, 100 Bureau Drive, MS 8311, Gaithersburg, MD 20899, USA Corresponding author. Tel.: +1-301-975-2581; fax: +1-301-975-8505 E-mail address: [email protected] . 1 ABSTRACT Oxidatively induced DNA damage is caused in living organisms by endogenous and exogenous reactive species. DNA lesions resulting from this type of damage are mutagenic and cytotoxic and, if not repaired, can cause genetic instability that may lead to disease processes including carcinogenesis. Living organisms possess DNA repair mechanisms that include a variety of pathways to repair multiple DNA lesions. Mutations and polymorphisms also occur in DNA repair genes adversely affecting DNA repair systems. Cancer tissues overexpress DNA repair proteins and thus develop greater DNA repair capacity than normal tissues. Increased DNA repair in tumors that removes DNA lesions before they become toxic is a major mechanism for development of resistance to therapy, affecting patient survival. Accumulated evidence suggests that DNA repair capacity may be a predictive biomarker for patient response to therapy. Thus, knowledge of DNA protein expressions in normal and cancerous tissues may help predict and guide development of treatments and yield the best therapeutic response. DNA repair proteins constitute targets for inhibitors to overcome the resistance of tumors to therapy. Inhibitors of DNA repair for combination therapy or as single agents for monotherapy may help selectively kill tumors, potentially leading to personalized therapy. Numerous inhibitors have been developed and are being tested in clinical trials. The efficacy of some inhibitors in therapy has been demonstrated in patients. More developments of inhibitors of DNA repair proteins are globally underway to help eradicate cancer. 2 Keywords: Cancer therapy DNA damage DNA repair DNA glycosylases Inhibitors 3 Contents 1. Introduction 2. Mechanistic aspects of oxidatively induced DNA damage 2.1. Purines 2.2. Pyrimidines 2.3. Sugar moiety 2.4. Tandem lesions 2.4.1. 8,5'-Cyclopurine-2'-deoxynucleosides 2.4.2. Base-base tandem lesions 2.4.3. DNA-protein cross-links 2.4.4. Clustered lesions 3. Cellular repair of oxidatively induced DNA lesions 3.1. Base excision repair 3.1.1. Substrate specificities of prokaryotic DNA glycosylases 3.1.2. Substrate specificities of eukaryotic DNA glycosylases 3.1.3. Repair of sugar lesions 4. Genetic effects of oxidatively induced DNA lesions 4.1. Purine-derived lesions 4.2. Pyrimidine-derived lesions 4.3. 8,5'-Cyclopurine-2'-deoxynucleosides 4.4. Sugar lesions 5. Oxidatively induced DNA damage and cancer 5.1. Role of DNA glycosylases of BER in carcinogenesis 5.1.1. OGG1 5.1.2. NEIL proteins 4 5.1.3. NTH1 5.2. Other BER proteins 5.2.1. APE1 5.2.2. Pol β 5.3. DNA lesions and DNA repair proteins as biomarkers 5.3.1. DNA lesions as biomarkers 5.3.2. BER proteins as biomarkers 5.3.3. BER proteins as therapy targets 6. Conclusions References 5 ● ●─ ─ Abbreviations: RS, reactive species; OH, hydroxyl radical; O2 , superoxide radical; eaq , hydrated electron; k, reaction rate constant; 8-OH-Gua, 8-hydroxyguanine; FapyGua, 2,6- diamino-4-hydroxy-5-formamidopyrimidine; 8-OH-Ade, 8-hydroxyadenine; FapyAde, 4,6- diamino-5-formamidopyrimidine; Sp, spiroiminohydantoin; Gh, 5-guanidinohydantoin; 5- OHMe-Ura, 5-(hydroxymethyl)uracil; 5-OH-Cyt, 5-hydroxycytosine; 5-OH-Ura, 5- hydroxyuracil; cdA, 8,5'-cyclo-2'-deoxyadenosine; cdG, 8,5'-cyclo-2'-deoxyguanosine; Fo, formamido residue; Thy-Tyr cross-link, 3-[1,3-dihydro-2,4-dioxopyrimidin-5-yl)-methyl]-L- tyrosine; BER, base excision repair; NER, nucleotide excision repair; MMR, mismatch repair; AP site, apyrimidinic/apurinic site; Me-FapyGua, 2,6-diamino-4-hydroxy-N7-methyl- 5-formamidopyrimidine; APE1, apurinic/apyrimidinic endonuclease 1; dRP, 2'-deoxyribose phosphate; Pol β; DNA polymerase β; TS, thymidylate synthetase. 6 1. Introduction Reactive species (RS) including free radicals derived from either oxygen or nitrogen are generated in aerobic organisms by cellular metabolism and by exogenous sources such as ionizing radiations, UV radiation, redox cycling drugs, carcinogenic compounds, environmental toxins, etc. [1]. Antioxidant defense mechanisms exist in living organisms to encounter the production and effects of RS. If the prooxidant-antioxidant balance is disturbed in favor of the former, a state of oxidative stress can occur, leading to oxidative damage to biomolecules including DNA, proteins and lipids [1,2]. Consequences of oxidative stress can be manyfold depending on its severity and the cell type [1]. Among others, these may include increased genetic instability, proliferation, reduction of antioxidants, cell death, apoptosis and angiogenesis [1]. Oxidative stress can also drive the onset of inflammation, which produces RS and is a hallmark of cancer, predisposing individuals to different types of cancers [3-5]. The acute inflammatory response recruits neutrophils that can damage DNA [4,6]. Reactive species are involved in carcinogenesis by damaging DNA and by modulating certain cellular pathways[1,4]. These species can be radicals or non-radicals. Among the oxygen-derived radicals, the hydroxyl radical (●OH) is the most reactive one and reacts with biological molecules such as DNA constituents at or near diffusion-controlled rates [7]. Other radicals ●─ ● ● such as superoxide radical (O2 ), hydroperoxyl radical (HO2 ), peroxyl radicals (RO2 ), ● 1 + alkoxyl radicals (RO ) and singlet oxygen (O2 Σg ) possess very low or intermediate reactivity. Non-radical H2O2 is not reactive, unless its reaction with transition metal ions converts it into ●OH [1]. Nitric oxide (NO●) is also a free radical and possesses low ●─ reactivity; however, its reaction with O2 is diffusion-controlled, yielding peroxynitrite (ONOO─) [8]. Peroxynitrite is a fairly unreactive non-radical. On the other hand, its protonated form peroxynitrous acid (ONOOH) can undergo homolytic fission to yield ●OH ● and NO2 , although this reaction may not be favored [1]. Ionizing radiations also generate 7 ● ● ─ OH and, in addition, H atom (H ) (also a free radical) and hydrated electron (eaq ) from cellular water [9]. Reactions of these endogenously and exogenously generated species with the DNA bases and sugar moiety result in the formation of a multitude of modifications (reviewed in [9,10]). This type of damage, which is called oxidatively induced DNA damage, can be repaired in living cells by a variety of repair mechanisms [11]. Oxidatively induced DNA modifications that escape repair before replication may lead to mutagenesis, which is well known to be a fundamental part of the molecular basis of all cancers [11-13]. Mutations occur throughout the genome, including in genes that maintain genetic stability, leading to genetic instability, which is a hallmark of cancer [3,14,15]. Genetic instability may affect many types of enzymes in various pathways including DNA repair [11]. In healthy cells, there is a balance between DNA damage and DNA repair. In cancer cells, however, this balance may be disturbed in favor of DNA damage, overwhelming DNA repair capacity of cells and thus resulting in mutations at high frequency. Increase in DNA repair capacity may also occur in cancer cells, causing therapy resistance [16-19]. There is mounting evidence that oxidatively induced DNA damage by endogenous and exogenous sources may be a significant source of mutations and genomic instability, and thus an important contributor to carcinogenesis [11,20-22]. 2. Mechanistic aspects of oxidatively induced DNA damage 2.1. Purines Mechanistic aspects of oxidatively induced DNA damage has extensively been reviewed in the past and just recently [9,10,23]. Thus, only a brief summary of this field will be given here. Of the RS, ●OH is the most damaging species to DNA and other biological molecules. Its reactions by addition to the double bonds of purines and pyrimidines in DNA are diffusion-controlled with second-order reaction rate constants (k) of 4‒9 x 109 dm3 mol‒1 s‒1 8 [7,9]. Abstraction of H● from the five C-atoms of the sugar moiety and from the methyl group of thymine also occurs, albeit by slower rates with k ≈ 2 x 109 M‒1 s‒1 [7,9]. Ionizing ─ radiation-generated eaq also adds to the double bonds of DNA bases at diffusion-controlled rates (k = 0.9‒1.7 x 109 dm3 mol‒1 s‒1); however, the addition reactions of H● are slower, but 8 3 ‒1 ‒1 ─ ● still occur at appreciable rates (k = 1‒5 x 10 dm mol s ) [23-27]. Reactions of eaq and H with the sugar moiety of DNA are negligible. Hydroxyl radical preferentially adds to the sites of double bonds of purines and pyrimidines with the highest electron density because of its electrophilic nature. Addition to guanine generates C4-OH‒, C5-OH‒ and C8-OH‒adduct ● ● radicals [10,23,28]. In addition, an H abstraction by OH from the NH2 group at C2 of guanine has been claimed to occur at a rate of ≈ 65%, practically eliminating the addition of ●OH to C4 [29,30]. However, theoretical and experimental studies unequivocally showed that this reaction is energetically not favored and does not occur to an appreciable extent [9,10,31- 36]. Addition reactions of ●OH with adenine mainly produces C4-OH‒ and C8-OH‒adduct radicals, although the C5-OH‒adduct radical is also formed, but to a much lesser extent [23,37,38]. The addition of ●OH to the C2 of adenine also occurs to an extent of 2%. The C4- OH‒ and C5-OH‒adduct radicals of guanine and adenine dehydrate to yield Gua(‒H)● and Ade(‒H)● radicals, respectively, that may be reduced to reconstitute Gua and Ade [23,39]. Gua(‒H)● protonates to give rise to guanine radical cation (Gua●+), which can be converted into the C8-OH‒adduct radical upon hydration (HO‒ addition) [40-42].
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