Exogenous DNA damage arises from natural exposure to sunlight and cosmic rays, two physical sources that can alter the chemical composition of DNA. In addition, human activities generate a lot a chemicals and radiations that can interfere with biological processes and inevitably generate DNA damage.
31 1.2.1. Physical damage of DNA
A) Ionizing radiation
Ionizing radiation (IR) are electromagnetic waves of high-energysuch as X-rays and g-rays that can ionize a molecule by removing an electron to produce ions and free radicals.
Cosmic rays are the main natural source of ionizing radiations. In addition, a natural background radiation is generated by radioactive isotopes such as 14C and 40K that are naturally absorbed and deposited within organs (Thorne, 2003).
In the early forties, the ability of X-rays to induce breaks into chromosomes was already observed (Giles, 1940). IR can induce DNA double-strand breaks formation which are the most lethal among all DNA lesions (Bernhard et al., 2007). The effect of ionizing radiation on DNA can be direct, when the DNA molecule with its solvation layer absorb the energy of the radiation, or indirect, if the effect on DNA is due to molecules by-products of IR on other molecules (Bernhard et al., 2007). When the energy of IR is absorbed by the sugar residues of DNA, it generates deoxyribose radicals and destabilizes the DNA structure inducing breaks.
Moreover, the water molecules surrounding the DNA in the solvation layer react with the radiations and undergo radiolysis reaction producing hydroxyl radicals (·OH) that contribute to DSBs formation by an electrophilic attack on DNA strands (Bernhard et al., 2007).
In addition, exposure ionizing radiation can also result in ROS production through water radiolysis generating oxidative damage of DNA and the accumulation of 8-oxoG and thymine glycol (Breimer and Lindahl, 1985; Gajewski et al., 1990).
B) DNA damage by UV light
The ultra-violet (UV: 10-400 nm) is one of the main three types of light that constitute the spectrum of sunlight. The two others are visible (400-700 nm) and infra-red lights (700-1000 nm). UV spectrum is subdivided by wavelength into UV-A (315 to 400 nm), UV-B (280 to 315 nm), and UV-C (100 to 280 nm). As wavelength is inversely proportional to the photon energy (E=hC/l, where E: energy, and l: wavelength), UV-C are the most energetic and thus
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the most dangerous. Fortunately, a large proportion of UV-C light is absorbed by the stratospheric ozone layer, while UV-A and UV-B easily penetrate the atmosphere of Earth.
Ultra-violet light creates photochemical crosslinks between adjacent pyrimidines (Lober and Kittler, 1977) generating two major photoproducts, the Cyclobutane Pyrimidine Dimer (CPD) and Pyrimidine-Pyrimidone [6-4] photoproduct ([6-4]PP). Upon UV-light, two thymines form a CPD which is the most frequent UV-photoproduct.This dimer does is not capable of pairing very well with the opposite adenines, because one of the hydrogen bonds is lost at the 5’T (Park et al., 2002). [6-4]PP can rearrange to another photoproduct known as 6-4 Dewar isomer when exposed to UV-A (Rastogi et al., 2010). The [6-4]PP disrupts more the DNA structure than CPD (Kim and Choi, 1995) and is even around 10 times more mutagenic than a cis-syn thymine-thymine CPD (Kamiya et al., 1998). However, CPD is the major cause of UV-induced mutagenesis and tumorigenesis in vivo (Jans et al., 2005) due to its high frequency if compared with [6-4]PP. Both CPD and [6-4]PP form bulky helix-distorting lesions that halt the progression of replicative polymerases Pola and Pold in vitro (Johnson et al., 1999; Masutani et al., 1999). Translesion polymerases (TLS polymerases) can efficiently bypass these lesions, in a process known as translesion synthesis (see paragraph 4.1). Then, they are cleaved by enzymatic excision, in a DNA repair process known as Nucleotide Excision Repair (NER) (see paragraph 2.2.1) (Hanawalt et al., 2003). In addition, UV can introduce mutations. C®T transition are the most frequent in skin cancer and constitute a signature of UV (Brash, 1997), although the mechanism behind is not well understood (Ikehata and Ono, 2011).
Ultra-violet light can also activate chemicals generating reactive aromatic compounds that bind to pyrimidine and form inter-strand cross-links (Hearst et al., 1984). One example is psoralens which are vegetal organic compounds that can induce DNA damage in a UV-dependent manner trough intercalating between DNA strands (Averbeck, 1989).
1.2.2. Chemical damage of DNA
In addition to physical agents, different natural or industry-derived chemical compounds, usually electrophilic, can cause DNA damage trough reacting with the nucleophilic nitrogen atoms in nitrogenous bases at physiological conditions (Lindahl, 1993).
Microorganisms and marine algae produce a natural alkylating agent, called the methyl
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chloride, which can methylate DNA (Crutzen and Andreae, 1990). Methylations very often occur on nitrogen N3 of adenine and the nitrogen N7 of guanine (Lindahl, 1993).
Cis-diamminedichloroplatinum (cisplatin) is a synthetized alkylating-like agent that bridges two adjacent guanine residues (cisPt-1,2-GpG) and forms 1,2-intra-strand cross-link, it is used for its antitumor genotoxic properties (Roberts and Pascoe, 1972). Other antitumor DNA damaging compounds are the Methyl MethaneSulfonate (MMS) alkylating agent and the cross-linking agent MitoMycin C (MMC) (Kim and D'Andrea, 2012).
Car exhausts, cigarette smoke, and burned meat contain a highly carcinogenic polycyclic aromatic hydrocarbon, called Benzo[a]pyrene (Schoket, 1999), which is metabolized in the cell to a very reactive epoxide known as BPDE. This epoxide react with the exocyclic nitrogen N2 of guanine generating bulky DNA lesions called N2-BPDE-guanine (Phillips, 1983). The benzo[a]pyrene induces G®T transversion mutations in vivo (Denissenko et al., 1996).
Aromatic amines are another class of chemical carcinogens (Ames et al., 1973). For example, the N-2-Acetyl-2-AminoFluorene (AAF) is metabolized and produces a highly reactive electrophilic metabolite that can bind covalently the carbon C8 of guanine forming a G-AAF (Kriek et al., 1967) that distorts the DNA double-helix structure (Fuchs, 1975), blocking the replication machinery and inducing frameshift mutations (Belguise-Valladier et al., 1994; Thomas et al., 1994).