Lesions in DNA

Humans are constantly exposed to background levels of ionizing radiation, which will generate some OH in vivo. This radical may also arise by reaction of metal ions with H2O2 in vivo. Thus, it is not surprising to find that repair systems have evolved to remove at least some of the lesions in DNA that can result from attack of OH (for review, see Breimer, 1991). 

Electron donation to nucleobases is a fundamental process exploited by nature to achieve the efficient repair of UV induced lesions in DNA [27, 28]. Nature developed to this end two enzymes, CPD photolyases and (6-4) photolyases, which both inject electrons into the UV damaged DNA bases [29, 30]. Both enzymes are, in many species, including plants, essential for the repair of the UV-light induced DNA lesions depicted in Scheme 1 [31]. 

Scheme 1 UV-light induced formation of the two major photo lesions in DNA. T=T cyclobutane pyrimidine dimer. (6-4)-photo product (6-4)-lesion, formed after ring opening of an oxetane intermediate, which is the product of a Paterno-Buchi reaction...

The exact nature of the lesion in DNA is unknown, and so is the type of DNA that is attacked. Recent X-ray crystallographic studies, as well as other physicochemical studies, have made it clear that DNA is not simply a polynucleotide, folded as Watson and Crick (106) proposed. There are three main conformational types of DNA they each keep the hydrogen-bonded bases in the center of the helix, but may tilt them by a "propellor twist," may slide them from the center of the helix in the plane of the base pairs, and may vary the amount of rotation from one base pair to the next up the helical axes. 

So we are still left with two models of the stereochemistry of DNA alkylated by a PAH diol epoxide the PAH either lies in a groove of DNA or else tries to intercalate between the bass of DNA. Since it is covalently bonded to a base it must cause considerable distortion if it tries to lie between the bases. However, the stacking observed in the crystalline state seems to argue for partial intercalation. We will need crystal structures of at least one appropriately alkylated polynucleotide before this problem can be resolved. And when this is done it will be just the beginning of the answer to the problem of alkylation of DNA by activated carcinogens. The subsequent question is, what is the lesion in DNA that is important in carcinogenesis, and then what does it cause to happen so that tumor formation is initiated ... 

Perhaps the best-characterized lesion in DNA associated with uv inactivation and mutagenesis is that involving the intrastrand photodimerization of adjacent thymine residues this lesion is almost wholly repaired by photodissociation of the dimers at shorter wavelengths in the photoreactivation process. Production of the chh dimer in this case, promoted by the configuration of adjacent molecules on the same sugar-phosphate strand, must however involve a rotational displacement of 36°, following the reduction of 0.6 A in molecular separation. 

Steenken et al. have concluded that in double-stranded DNA direct hydrogen atom abstraction from 2 -deoxyribose by G(-H) radical is very unlikely due to steric hindrance effects and a small thermodynamic driving force [94]. The EPR studies performed in neutral aqueous solutions at room temperature have shown that, in the absence of specific reactive molecules, the lifetime of the G(-H) radical in double-stranded DNA is as long as -5 s [80]. Therefore, the fates of G(-H) radicals are mostly determined by the presence of other reactive species and radicals. Thus, the G(-H) radical can be a key precursor of diverse guanine lesions in DNA. In the next section we begin from a discussion of the site-selective generation of the G(-H) radical in DNA, and then continue with a discussion of the reaction pathways of this guanine radical. 

An exposure to low-LET radiation that is lethal to 50 percent of dividing cells—i.e., approximately 1.5 Sv (150 rem)—produces substantial DNA damage, hundreds of strand breaks per cell. Nevertheless, roughly 50 percent of the cells survive, indicating that much of the damage is reparable (Cole et oL, 1980). The effectiveness of repair processes is thus an important factor in determining the fate and effects of lesions in DNA. 

Scheme 2.2 Examples of reactions catalyzed by and RNA by the protein AlkB [54] (R = sugar al

Delahoussaye YM, Hay MP, Pruijn FB, Denny WA, Brown JM (2003) Improved potency of the hypoxic cytotoxin tirapazamine by DNA targeting. Biochem Pharmacol 65 1807-1815 Demple B, Linn S (1982) 5,6-Saturated thymine lesions in DNA production by UV light or hydrogen peroxide. Nucleic Acids Res 10 3781-3789... 

Dianov GL, O Neill P, Goodhead DT (2001) Securing genome stability by orchestrating DNA repair removal of radiation-induced clustered lesions in DNA. BioEssays 23 745-749 Dirksen M-L, Blakely WF, Holwitt E, Dizdaroglu M (1988) Effect of DNA conformation on the hydroxyl radical-induced formation of 8,5 -cyclopurine 2 -deoxyribonucleoside residues in DNA. Int J Radiat Biol 54 195-204... 

Martin-Bertram H, Rumpf E, Winkler C (1983) Evidence for radiation-induced bulky lesions in DNA. Radiat Environ Biophys 21 305-307... 

Although the basic mechanism of SCE formation is obscure, the SCE test shows that a chemical attacks chromosomes, affects their replication, or both. Most chemicals that induce SCEs produce a spectrum of lesions in DNA itself. 

Most chemicals do not break chromosomes directly instead, they produce lesions in DNA that lead to the formation of chromatid aberrations when lymphocytes cure later stimulated to enter the S phase of the cell cycle. Because DNA lesions can be eliminated in noncycling peripheral lymphocytes before cells are stimulated in culture, the use of chromosomal aberrations in lymphocytes to monitor small exposures to most chemicals is as yet problematic.1+99... 

Thymidine cyclobutane dimers are important photoproducts formed by short-wave UV irradiation (2 = 290-320 nm) of DNA, by [2 + 2] cycloaddition between two adjacent thymine nucleobases in the same oligonucleotide strand (Scheme 4.5.1) [1]. They lead to profound biological effects in vivo, including mutation, cancer, and cell death [2] (Box 21). In a wide range of organisms the repair of these lesions in DNA is accomplished by enzymes (the photolyases), which regenerate undamaged thymidines by means of a photoinduced electron-transfer process [3]. 

Leonard GA, Thomson J, Watson WP, Brown T. High-resolution structure of a mutagenic lesion in DNA. Proc. Natl. Acad. Sci. 39. U.S.A. 1990 87 9573-9576. 

A series of adenosine derivatives have been reported which deal with damaged or adducted analogues. 8-Oxo-7,8-dihydroadenosine has been incorporated into DNA and its templating properties with three reverse transcriptases examined. It was shown that as well as directing the incorporation of TTP, dGTP was incorporated." " Another oxidative lesion of adenosine is formamidino-adeno-sine (Fapy-dA, 71). (71) and its C-nucleoside have been incorporated into DNA and its interaction with base excision repair enzymes assessed." " (5 5)-8, 5 -cyclo-dA is another oxidative lesion, and an assay to determine levels of this lesion in DNA by nuclease digestion has been reported." ... 

Chemicals that cause toxicity by genetic alterations of the somatic cells are called genotoxic carcinogens. Covalent interactions with DNA do not always lead to cell death. The vast majority of lesions in DNA are repaired but in some cases the repair is incorrect or... 

Wilson DM 3rd, Barsky D (2001) The major human abasic endonuclease formation, consequences and repair of abasic lesions in DNA. Mutat Res 485(4) 283-307... 

Box H.C., Dawidzik J.B., Budzinski E.E., Free radical-induced double lesions in DNA, Free Radic. Biol. Med., 2001,31,856-868. 

Methods for Determining Biomarkers of Exposure and Effect. Analytical methods with satisfactory sensitivity and precision are available to determine the levels of strontium in human tissues and body fluids. Strontium and radiostrontium are found in essentially all food, water, and air, so everyone is exposed to some levels. Recently, Sutherland et al. (2000a, 2000b) developed a molecular biological strategy to identify clustered lesions in DNA resulting from in vitro cellular exposure to gamma radiation. 

Jourdan, M., Garda, J., Defrancq, E., Kotera, M., and Lhomme, J. (1999) 2 -Deoxyribonolactone lesion in DNA refined solution structure determined hy nuclear magnetic resonance and molecular modeling. Biochemistry, 38, 3985-3995. 

Boiteux, S., Gajewski, E., Laval, J., and Dizdaroglu, M. (1992) Substrate specificity of the Escherichia coli Fpg protein (formamidopyrimidine-DNA glycosylase) excision of purine lesions in DNA produced by ionizing radiation or photosensitization. Biochemistry, 31, 106-110. 

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