| 8-oxoguanine DNA glycosylase, N-terminal domain | |||||||||
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 structure of catalytically inactive q315a human 8-oxoguanine glycosylase complexed to 8-oxoguanine dna | |||||||||
| Identifiers | |||||||||
| Symbol | OGG_N | ||||||||
| Pfam | PF07934 | ||||||||
| Pfam clan | CL0407 | ||||||||
| InterPro | IPR012904 | ||||||||
| SCOP2 | 1ebm / SCOPe / SUPFAM | ||||||||
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8-Oxoguanine glycosylase, also known as OGG1, is a DNA glycosylase enzyme that, in humans, is encoded by the OGG1 gene. It is involved in base excision repair. It is found in bacterial, archaeal and eukaryotic species.
OGG1 is the primary enzyme responsible for the excision of 8-oxoguanine (8-oxoG), a mutagenic base byproduct that occurs as a result of exposure to reactive oxygen species (ROS). OGG1 is a bifunctional glycosylase, as it is able to both cleave the glycosidic bond of the mutagenic lesion and cause a strand break in the DNA backbone. Alternative splicing of the C-terminal region of this gene classifies splice variants into two major groups, type 1 and type 2, depending on the last exon of the sequence. Type 1 alternative splice variants end with exon 7 and type 2 end with exon 8. One set of spliced forms are designated 1a, 1b, 2a to 2e. [5] All variants have the N-terminal region in common. Many alternative splice variants for this gene have been described, but the full-length nature for every variant has not been determined. In eukaryotes, the N-terminus of this gene contains a mitochondrial targeting signal, essential for mitochondrial localization. [6] However, OGG1-1a also has a nuclear location signal at its C-terminal end that suppresses mitochondrial targeting and causes OGG1-1a to localize to the nucleus. [5] The main form of OGG1 that localizes to the mitochondria is OGG1-2a. [5] A conserved N-terminal domain contributes residues to the 8-oxoguanine binding pocket. This domain is organised into a single copy of a TBP-like fold. [7]
Despite the presumed importance of this enzyme, mice lacking Ogg1 have been generated and found to have a normal lifespan, [8] and Ogg1 knockout mice have a higher probability to develop cancer, whereas MTH1 gene disruption concomitantly suppresses lung cancer development in Ogg1-/- mice. [9] Mice lacking Ogg1 have been shown to be prone to increased body weight and obesity, as well as high-fat-diet-induced insulin resistance. [10] There is some controversy as to whether deletion of Ogg1 actually leads to increased 8-Oxo-2'-deoxyguanosine (8-oxo-dG) levels: high performance liquid chromatography with electrochemical detection (HPLC-ECD) assay suggests the deletion can lead to an up to 6 fold higher level of 8-oxo-dG in nuclear DNA and a 20-fold higher level in mitochondrial DNA, whereas DNA-fapy glycosylase assay indicates no change in 8-oxo-dG levels.[ citation needed ]
Increased oxidant stress temporarily inactivates OGG1, which recruits transcription factors such as NFkB and thereby activates expression of inflammatory genes. [11]
Mice without a functional OGG1 gene have about a 5-fold increased level of 8-oxo-dG in their livers compared to mice with wild-type OGG1. [9] Mice defective in OGG1 also have an increased risk for cancer. [9] Kunisada et al. [13] irradiated mice without a functional OGG1 gene (OGG1 knock-out mice) and wild-type mice three times a week for 40 weeks with UVB light at a relatively low dose (not enough to cause skin redness). Both types of mice had high levels of 8-oxo-dG in their epidermal cells three hours after irradiation. After 24 hours, over half of the initial amount of 8-oxo-dG was absent from the epidermal cells of the wild-type mice, but 8-oxo-dG remained elevated in the epidermal cells of the OGG1 knock-out mice. The irradiated OGG1 knock-out mice went on to develop more than twice the incidence of skin tumors compared to irradiated wild-type mice, and the rate of malignancy within the tumors was higher in the OGG1 knock-out mice (73%) than in the wild-type mice (50%).
As reviewed by Valavanidis et al., [14] increased levels of 8-oxo-dG in a tissue can serve as a biomarker of oxidative stress. They also noted that increased levels of 8-oxo-dG are frequently found during carcinogenesis.
In the figure showing examples of mouse colonic epithelium, the colonic epithelium from a mouse on a normal diet was found to have a low level of 8-oxo-dG in its colonic crypts (panel A). However, a mouse likely undergoing colonic tumorigenesis (due to deoxycholate added to its diet [12] ) was found to have a high level of 8-oxo-dG in its colonic epithelium (panel B). Deoxycholate increases intracellular production of reactive oxygen resulting in increased oxidative stress, [15] > [16] and this can lead to tumorigenesis and carcinogenesis.
In a breast cancer study, the methylation level of the OGG1 promoter was found to be negatively correlated with expression level of OGG1 messenger RNA. [17] This means that hypermethylation was associated with low expression of OGG1 and hypomethylation was correlated with over-expression of OGG1. Thus, OGG1 expression is under epigenetic control. Breast cancers with methylation levels of the OGG1 promoter that were more than two standard deviations either above or below the normal were each associated with reduced patient survival. [17]
OGG1 is the primary enzyme responsible for the excision of 8-oxo-dG. Even when OGG1 expression is normal, the presence of 8-oxo-dG is mutagenic, since OGG1 is not 100% effective. Yasui et al. [18] examined the fate of 8-oxo-dG when this oxidized derivative of deoxyguanosine was inserted into a specific gene in 800 cells in culture. After replication of the cells, 8-oxo-dG was restored to G in 86% of the clones, probably reflecting accurate OGG1 base excision repair or translesion synthesis without mutation. G:C to T:A transversions occurred in 5.9% of the clones, single base deletions in 2.1% and G:C to C:G transversions in 1.2%. Together, these mutations were the most common, totalling 9.2% of the 14% of mutations generated at the site of the 8-oxo-dG insertion. Among the other mutations in the 800 clones analyzed, there were also 3 larger deletions, of sizes 6, 33 and 135 base pairs. Thus 8-oxo-dG can directly cause mutations, some of which may contribute to carcinogenesis.
If OGG1 expression is reduced in cells, increased mutagenesis, and therefore increased carcinogenesis, would be expected. The table below lists some cancers associated with reduced expression of OGG1.
| Cancer | Expression | Form of OGG1 | 8-oxo-dG | Evaluation method | Ref. |
|---|---|---|---|---|---|
| Head and neck cancer | Under-expression | OGG1-2a | - | messenger RNA | [19] |
| Adenocarcinoma of gastric cardia | Under-expression | cytoplasmic | increased | immunohistochemistry | [20] |
| Astrocytoma | Under-expression | total cell OGG1 | - | messenger RNA | [21] |
| Esophageal cancer | 48% Under-expression | nuclear | increased | immunohistochemistry | [22] |
| - | 40% Under-expression | cytoplasm | increased | immunohistochemistry | [22] |
OGG1 methylation levels in blood cells were measured in a prospective study of 582 US military veterans, median age 72, and followed for 13 years. High OGG1 methylation at a particular promoter region was associated with increased risk for any cancer, and in particular for risk of prostate cancer. [23]
Enzymatic activity excising 8-oxoguanine from DNA (OGG activity) was reduced in peripheral blood mononuclear cells (PBMCs), and in paired lung tissue, from patients with non–small cell lung cancer. [24] OGG activity was also reduced in PBMCs of patients with head and neck squamous cell carcinoma (HNSCC). [25]
An important effect on cancer is expected to derive from the drastic enhancement of gene expression for certain immunity genes, which OGG1 regulates. [26]
Oxoguanine glycosylase has been shown to interact with XRCC1 [27] and PKC alpha. [28]
The CpG sites or CG sites are regions of DNA where a cytosine nucleotide is followed by a guanine nucleotide in the linear sequence of bases along its 5' → 3' direction. CpG sites occur with high frequency in genomic regions called CpG islands.
DNA repair is a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolic activities and environmental factors such as radiation can cause DNA damage, resulting in tens of thousands of individual molecular lesions per cell per day. Many of these lesions cause structural damage to the DNA molecule and can alter or eliminate the cell's ability to transcribe the gene that the affected DNA encodes. Other lesions induce potentially harmful mutations in the cell's genome, which affect the survival of its daughter cells after it undergoes mitosis. As a consequence, the DNA repair process is constantly active as it responds to damage in the DNA structure. When normal repair processes fail, and when cellular apoptosis does not occur, irreparable DNA damage may occur. This can eventually lead to malignant tumors, or cancer as per the two-hit hypothesis.
DNA glycosylases are a family of enzymes involved in base excision repair, classified under EC number EC 3.2.2. Base excision repair is the mechanism by which damaged bases in DNA are removed and replaced. DNA glycosylases catalyze the first step of this process. They remove the damaged nitrogenous base while leaving the sugar-phosphate backbone intact, creating an apurinic/apyrimidinic site, commonly referred to as an AP site. This is accomplished by flipping the damaged base out of the double helix followed by cleavage of the N-glycosidic bond.
Base excision repair (BER) is a cellular mechanism, studied in the fields of biochemistry and genetics, that repairs damaged DNA throughout the cell cycle. It is responsible primarily for removing small, non-helix-distorting base lesions from the genome. The related nucleotide excision repair pathway repairs bulky helix-distorting lesions. BER is important for removing damaged bases that could otherwise cause mutations by mispairing or lead to breaks in DNA during replication. BER is initiated by DNA glycosylases, which recognize and remove specific damaged or inappropriate bases, forming AP sites. These are then cleaved by an AP endonuclease. The resulting single-strand break can then be processed by either short-patch or long-patch BER.
In biology, reprogramming refers to erasure and remodeling of epigenetic marks, such as DNA methylation, during mammalian development or in cell culture. Such control is also often associated with alternative covalent modifications of histones.
MUTYH is a human gene that encodes a DNA glycosylase, MUTYH glycosylase. It is involved in oxidative DNA damage repair and is part of the base excision repair pathway. The enzyme excises adenine bases from the DNA backbone at sites where adenine is inappropriately paired with guanine, cytosine, or 8-oxo-7,8-dihydroguanine, a common form of oxidative DNA damage.
DNA oxidation is the process of oxidative damage of deoxyribonucleic acid. As described in detail by Burrows et al., 8-oxo-2'-deoxyguanosine (8-oxo-dG) is the most common oxidative lesion observed in duplex DNA because guanine has a lower one-electron reduction potential than the other nucleosides in DNA. The one electron reduction potentials of the nucleosides are guanine 1.29, adenine 1.42, cytosine 1.6 and thymine 1.7. About 1 in 40,000 guanines in the genome are present as 8-oxo-dG under normal conditions. This means that >30,000 8-oxo-dGs may exist at any given time in the genome of a human cell. Another product of DNA oxidation is 8-oxo-dA. 8-oxo-dA occurs at about 1/10 the frequency of 8-oxo-dG. The reduction potential of guanine may be reduced by as much as 50%, depending on the particular neighboring nucleosides stacked next to it within DNA.
DNA excision repair protein ERCC-1 is a protein that in humans is encoded by the ERCC1 gene. Together with ERCC4, ERCC1 forms the ERCC1-XPF enzyme complex that participates in DNA repair and DNA recombination.
Methylated-DNA--protein-cysteine methyltransferase(MGMT), also known as O6-alkylguanine DNA alkyltransferaseAGT, is a protein that in humans is encoded by the MGMT gene. MGMT is crucial for genome stability. It repairs the naturally occurring mutagenic DNA lesion O6-methylguanine back to guanine and prevents mismatch and errors during DNA replication and transcription. Accordingly, loss of MGMT increases the carcinogenic risk in mice after exposure to alkylating agents. The two bacterial isozymes are Ada and Ogt.
Endonuclease VIII-like 1 is an enzyme that in humans is encoded by the NEIL1 gene.
Single-strand selective monofunctional uracil DNA glycosylase is an enzyme that in humans is encoded by the SMUG1 gene. SMUG1 is a glycosylase that removes uracil from single- and double-stranded DNA in nuclear chromatin, thus contributing to base excision repair.
8-Oxoguanine (8-hydroxyguanine, 8-oxo-Gua, or OH8Gua) is one of the most common DNA lesions resulting from reactive oxygen species modifying guanine, and can result in a mismatched pairing with adenine resulting in G to T and C to A substitutions in the genome. In humans, it is primarily repaired by DNA glycosylase OGG1. It can be caused by ionizing radiation, in connection with oxidative metabolism.
The DNA damage theory of aging proposes that aging is a consequence of unrepaired accumulation of naturally occurring DNA damage. Damage in this context is a DNA alteration that has an abnormal structure. Although both mitochondrial and nuclear DNA damage can contribute to aging, nuclear DNA is the main subject of this analysis. Nuclear DNA damage can contribute to aging either indirectly or directly.
For molecular biology in mammals, DNA demethylation causes replacement of 5-methylcytosine (5mC) in a DNA sequence by cytosine (C). DNA demethylation can occur by an active process at the site of a 5mC in a DNA sequence or, in replicating cells, by preventing addition of methyl groups to DNA so that the replicated DNA will largely have cytosine in the DNA sequence.
8-Oxo-2'-deoxyguanosine (8-oxo-dG) is an oxidized derivative of deoxyguanosine. 8-Oxo-dG is one of the major products of DNA oxidation. Concentrations of 8-oxo-dG within a cell are a measurement of oxidative stress.
While the cellular and molecular mechanisms of learning and memory have long been a central focus of neuroscience, it is only in recent years that attention has turned to the epigenetic mechanisms behind the dynamic changes in gene transcription responsible for memory formation and maintenance. Epigenetic gene regulation often involves the physical marking of DNA or associated proteins to cause or allow long-lasting changes in gene activity. Epigenetic mechanisms such as DNA methylation and histone modifications have been shown to play an important role in learning and memory.
DNA damage is an alteration in the chemical structure of DNA, such as a break in a strand of DNA, a nucleobase missing from the backbone of DNA, or a chemically changed base such as 8-OHdG. DNA damage can occur naturally or via environmental factors, but is distinctly different from mutation, although both are types of error in DNA. DNA damage is an abnormal chemical structure in DNA, while a mutation is a change in the sequence of base pairs. DNA damages cause changes in the structure of the genetic material and prevents the replication mechanism from functioning and performing properly. The DNA damage response (DDR) is a complex signal transduction pathway which recognizes when DNA is damaged and initiates the cellular response to the damage.
2-hydroxy-dATP diphosphatase is an enzyme that in humans is encoded by the NUDT1 gene. During DNA repair, the enzyme hydrolyses oxidized purines and prevents their addition onto the DNA chain. As such it has important role in aging and cancer development.
Antimutagens are the agents that interfere with the mutagenicity of a substance. The interference can be in the form of prevention of the transformation of a promutagenic compound into actual active mutagen, inactivation, or otherwise the prevention of Mutagen-DNA reaction.
The TET enzymes are a family of ten-eleven translocation (TET) methylcytosine dioxygenases. They are instrumental in DNA demethylation. 5-Methylcytosine is a methylated form of the DNA base cytosine (C) that often regulates gene transcription and has several other functions in the genome.
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