DNA oxidation is the process of oxidative damage on Deoxyribonucleic Acid. It occurs most readily at guanine residues due to the high oxidation potential of this base relative to cytosine, thymine, and adenine. It is widely believed to be linked to certain disease and cancers.
RNAs in native milieu are exposed to various insults. Among these threats, oxidative stress is one of the major causes of damage to RNAs. The level of oxidative stress that a cell endures is reflected by the quantity of Reactive oxygen species (ROS). ROS are generated from normal oxygen metabolism in cells and are recognized as a list of active molecules, such as O2•−, 1O2, H2O2 and, •OH . A nucleic acid can be oxidized by ROS through a Fenton reaction. To date, around 20 oxidative lesions have been discovered in DNA. RNAs are likely to be more sensitive to ROS for the following reasons: i) the basically single-stranded structure exposes more sites to ROS; ii) compared with nuclear DNA, RNAs are less compartmentalized; iii) RNAs distribute broadly in cells not only in the nucleus as DNAs do, but also in large portions in the cytoplasm. This theory has been supported by a series of discoveries from rat livers, human leukocytes, etc. Actually, monitoring a system by applying the isotopical label [18O]-H2O2 shows greater oxidation in cellular RNA than in DNA. Oxidation randomly damages RNAs, and each attack bring problems to the normal cellular metabolism. Although alteration of genetic information on mRNA is relatively rare, oxidation on mRNAs in vitro and in vivo results in low translation efficiency and aberrant protein products. Though the oxidation strikes the nucleic strands randomly, particular residues are more susceptible to ROS, such hotspot sites being hit by ROS at a high rate. Among all the lesions discovered thus far, one of the most abundant in DNA and RNA is the 8-hydroxyguanine. Moreover, 8-hydroxyguanine is the only one measurable among all the RNA lesions. Besides its abundance, 8-hydroxydeoxyguanosine (8-oxodG) and 8-hydroxyguanosine (8-oxoG) are identified as the most detrimental oxidation lesions for their mutagenic effect, in which this non-canonical counterpart can faultily pair with both adenine and cytosine at the same efficiency. This mis-pairing brings about the alteration of genetic information through the synthesis of DNA and RNA. In RNA, oxidation levels are mainly estimated through 8-oxoG-based assays. So far, approaches developed to directly measure 8-oxoG level include HPLC-based analysis and assays employing monoclonal anti-8-oxoG antibody. The HPLC-based method measures 8-oxoG with an electrochemical detector (ECD) and total G with a UV detector. The ratio that results from comparing the two numbers provides the extent that the total G is oxidized. Monoclonal anti-8-oxoG mouse antibody is broadly applied to directly detect this residue on either tissue sections or membrane, offering a more visual way to study its distribution in tissues and in discrete subsets of DNA or RNA. The established indirect techniques are mainly grounded on this lesion’s mutagenic aftermath, such as the lacZ assay. This method was first set up and described by Taddei and was a potentially powerful tool to understand the oxidation situation at both the RNA sequence level and single nucleotide level. Another source of oxidized RNAs is mis-incorporation of oxidized counterpart of single nucleotides. Indeed, the RNA precursor pool size is hundreds of sizes bigger than DNA’s.
Potential factors for RNA quality control
There have been furious debates on whether the issue of RNA quality control does exist. However, with the concern of various length of half life of diverse RNA species ranging from several minutes to hours, degradation of defective RNA can not easily be attributed to its transient character anymore. Indeed, reaction with ROS takes only few minutes, which is even shorter than average life-span of the most unstable RNAs. Adding the fact that stable RNA take the lion’s share of total RNA, RNA error deleting become hypercritical and should not be neglected anymore .This theory is upheld by the fact that level of oxidized RNA decreases after removal the oxidative challenge . Some potential facors include ribonucleases, which are suspected to selectively degrade damaged RNAs under stresses. Also enzymes working at RNA precursor pool level,are known to control quality of RNA sequence by changing error precursor to the form that can't be included directly into nascent strand.
- Buechter, DD. (1988) Free radicals and oxygen toxicity.Pharm Res. 5:253-60.
- Wardman, P. and Candeias, L.P. (1996). Fenton chemistry: an introduction. Radiat. Res. 145, 523–531.
- Cooke, M. S., Evans, M. D., Dizdaroglu, M., Lunec, J. (2003) Oxidative DNA damage: mechanisms, mutation, and disease. FASEB J. 17,195–1214
- Li, Z., Wu, J. and Deleo, C.J. (2006) RNA Damage and Surveillance under Oxidative Stress. IUBMB Life, 58(10): 581-588
- Hofer, T., Seo, A. Y., Prudencio, M., and Leeuwenburgh, C. (2006) A method to determine RNA and DNA oxidation simultaneously by HPLC-ECD: greater RNA than DNA oxidation in rat liver after doxorubicin administration. Biol. Chem. 387, 103 – 111
- Dukan,S., Farwell, A., Ballesteros, M., Taddei, F., Radman,M. and Nystrom,T. (2000) Protein oxidation in response to increased transcriptional and translational errors. Proc. Natl. Acad. Sci. USA, 97 No.11 5746-5749
- Gajewski, E., Rao, G., Nackerdien, Z., and Dizdaroglu, M. (1990) Modification of DNA bases in mammalian chromatin by radiationgenerated free radicals. Biochemistry 29, 7876 – 7882.
- Ames, B. N., and Gold, L. S. (1991) Endogenous mutagens and the causes of aging and cancer. Mutat. Res. 250, 3 – 16.
- Shibutani, S., Takeshita, M., and Grollman, A. P. (1991) Insertion of specific bases during DNA synthesis past the oxidation-damaged base 8-oxodG. Nature 349, 431–434.
- Taddei, F., Hayakawa, H., Bouton, M., Cirinesi, A., Matic, I., Sekiguchi, M., and Radman, M. (1997) Counteraction by MutT protein of transcriptional errors caused by oxidative damage. Science 278, 128 – 130.
- Weimann, A., Belling, D., and Poulsen, H. E. (2002) Quantification of 8-oxoGuanine and guanine as the nucleobase, nucleoside and deoxynucleoside forms in human urine by high-performance liquid chromatography-electrospray tandem mass spectrometry. Nucleic Acids Res. 30, E7.
- Park, E. M., Shigenaga, M. K., Degan, P., Korn, T. S., Kitzler, J. W., Wehr, C. M., Kolachana, P., and Ames, B. N. (1992) Assay of excised oxidative DNA lesions: isolation of 8-oxoguanine and its nucleoside derivatives from biological fluids with a monoclonal antibody column. Proc. Natl. Acad. Sci. USA 89, 3375 – 3379.
- Shen, Z., Wu, W., and Hazen, S. L. (2000) Activated leukocytes oxidatively damage DNA, RNA, and the nucleotide pool through halide-dependent formation of hydroxyl radical. Biochemistry 39, 5474 – 5482.
- Kajitani, K., Yamaguchi, H., Dan Y., Furuichi, M., Kang D., and Nakabeppu, Y. (2006) MTH1, and oxidized purine nucleoside triphosphatase, suppresses the accumulation of oxidative damage of nucleic acids in the hippocampalmicroglia during kainite-induced excitotoxicity. J.Neurosci. 26, 1688-1689.