**1. Introduction**

48 Selected Topics in DNA Repair

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cell in vivo. *Carcinogenesis*. Vol. 24, pp. 1811–1817

59–67

42

induced DNA double-strand breaks by nickel and arsenite. *Radiation* 

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a novel molecular mechanism in carcinogenesis. *Toxicology Letters*. Vol.162, pp. 29-

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in relation to cell proliferation and apoptosis in human keratinocytes and leukemia

Oxidative DNA damage has been thought to contribute to the general decline in cellular functions that are associated with a variety of diseases including Alzheimer disease, amyotrophic lateral sclerosis (ALS), Parkinson's disease, atherosclerosis, ischemia/reperfusion neuronal injuries, degenerative disease of the human temporomandibular-joint, cataract formation, macular degeneration, degenerative retinal damage, rheumatoid arthritis , multiple sclerosis , muscular dystrophy, diabetes mellitus, human cancers as well as the aging process itself. Oxidative stress occurs when the production of the reactive oxygen species (ROS) exceeds natural antioxidant defence mechanisms. There are several sources that form the ROS. Most of ROS come from the endogenous sources as by-products of normal and essential metabolic reactions, such as energy generation from mitochondria or the detoxification reactions involving the liver cytochrome *P-450* enzyme system. There are also exogenous ROS sources including exposure to cigarette smoke, environmental pollutants such as emission from automobiles and industries, consumption of alcohol in excess, asbestos, exposure to ionizing radiation, and bacterial, fungal or viral infections. ROS cause damage to biomolecules such as lipid, proteins and DNA by attaching. ROS may directly attack DNA, either the sugar, phosphate or purine and pyrimidine bases. On the other hand, oxidative damage may be indirect by rising of intracellular Ca+2 ions. Free radical-mediated reactions can cause structural alterations in DNA (e.g., nicking, base-pair mutations, rearrangement, deletions insertions and sequence amplification). Degradation of the bases will produce numerous products, including 8-OH-Gua, hydroxymethylurea, urea, thymine glycol; thymine and adenine ring opened and saturated products. Most oxidized bases in DNA are repaired by base excision repair (BER). BER consists of four main steps. The first step involves the removal of the oxidised base by a specific DNA glycosylase, yielding an apurinic/apyrimidinic (AP) site. In the second step, an AP endonuclease removes the deoxyribose phosphate group from the AP site generating a single nucleotide gap. A DNA polymerase, thought to be predominantly DNA polymerase b, fills this gap. Finally, a DNA ligase, probably DNA ligase III, seals the stand break and completes the repair process.

This chapter mainly deals with: (i) formation of ROS in physiological and pathological conditions, (ii ) ROS-mediated DNA damage, leading to cellular pathology and ultimately to cell death (iii) Oxidative DNA damage repair systems, (iv) The molecular mechanism of ROS-mediated diseases such as cancer, cardiovascular disease, neurodegenerative diseases, inflammatory disease, ischemia-reperfusion injury and aging.

Effect of Oxidative Stress on DNA Repairing Genes 51

The **•OH** is an extremely reactive oxidant [Halliwell & Gutteridge, 1999, Khanna & Shiloh, 2009]. It is also a short-lived molecule with an estimated half-life of nanoseconds at 37◦C, traveling only a few Ångstroms. Despite its short life span, •OH is capable of inducing considerable damage to nuclear and mitochondrial DNA. This radical alone can cause over 100 types DNA modifications [Khanna & Shiloh, 2009, Michalik et al., 1995]. In addition, •OH can lead to lipid peroxidation and oxidation of amino acids, sugars, and metals. The •OH is a major product of irradiation due to radiation-induced dissociation of water

Although **H2O2** itself is not a radical, it is included in ROS due to producing highly reactive free radical, •OH. H2O2 is one of the most stable ROS and acts as a messenger in cellular signaling pathways [Khanna & Shiloh, 2009, Kamata & Hirata, 1999]. There are some enzymes that can produce H2O2 directly or indirectly, including SOD, monoamine oxidase (MAO), diamine and polyamine oxidase, and glycolate oxidase. Under normal conditions, H2O2 is not toxic up to a cellular concentration of about 10−8 M [Imlay et al., 1988] H2O2 molecules are freely dissolved in aqueous solution and can easily penetrate biological membranes. Their deleterious chemical effects can be divided into the categories of direct activity, originating from their oxidizing properties, and indirect activity in which they serve as a source for more deleterious species, such as OH. or HClO. In the presence of transition metals such as Fe2+ or Cu+, H2O2 it can be converted to highly reactive •OH, either by Fenton or Harber– Weiss reactions [Yamasaki & Piette, 1991, Halliwell & Gutteridge, 1999, Khanna & Shiloh, 2009]. H2O2 is detoxified by a set of enzymes that includes the

**The nitric oxide (NO), or nitrogen monoxide**, which is a radical (NO• ), is produced by the oxidation of one of the terminal guanido nitrogen atoms of L-arginine. In this reaction, Larginine is converted to NO and L-citrulline by nitric oxide synthase (NOS) which has three isoforms: neuronal NOS, endothelial NOS (eNOS), and inducible NOS (iNOS). NO is quite stable and benign for a free radical, with a lifetime of several seconds. Under normal conditions, NO has many physiological functions such as a neuronal messenger and modulator of smooth muscle contraction. NO can interact with •O2− to form the peroxynitrite anion (ONOO−) that induces a cascade of events that can eventually lead to cell death [Radi et al., 1991a]. This molecule accounts for much of the NO toxicity. The

from its ability to directly nitrate and hydroxylate the aromatic rings of amino acid residues [Schafer & Buettner, 2001] and to react with sulfahydryls [Beckman et al., 1992], lipids [Radi, 1991b], proteins [Moreno & Pryor, 1992] and DNA [King et al., 1992]. Under physiological conditions, ONOOH can react with other components present in high concentrations, such as H2O2 or CO2, to form an adduct that might be responsible for many of the deleterious effects seen in biological sites. Peroxynitrite anion can also affect cellular energy status by inactivating key mitochondrial enzymes [Radi et. Al, 1994], and it may trigger calcium

ROS generated in response to both endogenous and exogenous stimuli can be divided into Endogenous ROS and exogenous ROS. (Figure 1) [Ziech et al., 2010, Fukai & Nakamura,

•. Its toxicity is derived

selenium-dependent glutathione peroxidase (GPx) and catalase.

reactivity of ONOO− is roughly the same as that of •OH and N O2

release from the mitochondria [Packer & Murphy, 1994].

**3. Classification of reactive oxygen species** 

2008, Klaunig & Kamendulis, 2004, Galaris et al., 2008].

molecules.

If the mechanisms of oxidative DNA damage and DNA repairing system are well understood, the diseases resulting from oxidative DNA damage or inefficient DNA repairing could be treated in future and a better understanding of these mechanisms would also allow biomarkers of DNA damage to become potentially useful clinical tools.
