Biomedical subjects

S Boiteux

Publications and source records attributed to S Boiteux.

At least 37 records · Page 2Linked to original sources

PubMed2000

Detection of oxidative base DNA damage by a new biochemical assay.

Reactive oxygen species (ROS) damage DNA which appears to represent the major target involved in mutagenesis, carcinogenesis, and aging cell responses. Various DNA modifications are generated by ROS, but 8-hydroxy-2'-deoxyguanosine (8-oxoG) has retained a lot of attention in the last few years. Therefore, numerous methods have been developed to detect and quantify the extent of 8-oxoG in DNA, most of them requiring a significant amount of DNA that might be limiting in the case of biological samples. 8-oxoG is repaired in Escherichia coli by a specific glycosylase, the Fpg (formamidopyrimidine DNA glycosylase) protein, in a reaction that requires a covalent intermediate favored under reducing conditions. We set up a new assay based on the capture of plasmid DNA into sensitized microplate wells. DNA damaged by photoactivation of methylene blue was adsorbed on a polylysine-treated plastic well. Then the Fpg protein was added, allowed to fix on the damage by taking advantage of minimized glycosylase activity at low temperature and the reductive trapping of the covalent intermediate, yielding to a stable DNA-protein interaction. The trapped protein was subsequently recognized by a specific antibody. A secondary antibody coupled with horseradish peroxidase was used to detect the complex and the measurement was carried out by chemiluminescence. This new assay offers various potentialities, specifically in the field of technology of ROS producers.

8-Hydroxy-2'-Deoxyguanosine↗

PubMed2000

Catalytic and DNA binding properties of the ogg1 protein of Saccharomyces cerevisiae: comparison between the wild type and the K241R and K241Q active-site mutant proteins.

The Ogg1 protein of Saccharomyces cerevisiae belongs to a family of DNA glycosylases and apurinic/apyrimidinic site (AP) lyases, the signature of which is the alpha-helix-hairpin-alpha-helix-Gly/Pro-Asp (HhH-GPD) active site motif together with a conserved catalytic lysine residue, to which we refer as the HhH-GPD/K family. In the yeast Ogg1 protein, yOgg1, the HhH-GPD/K motif spans residues 225-260 and the conserved lysine is K241. In this study, we have purified the K241R and K241Q mutant proteins and compared their catalytic and DNA binding properties to that of the wild-type yOgg1. The results show that the K241R mutation greatly impairs both the DNA glycosylase and the AP lyase activities of yOgg1. Specificity constants for cleavage of a 34mer oligodeoxyribonucleotide containing a 7,8-dihydro-8-oxoguanine (8-OxoG) paired with a cytosine, [8-OxoG.C], are 56 x 10(-)(3) and 5 x 10(-)(3) min(-)(1) nM(-)(1) for the wild-type and the K241R protein, respectively. On the other hand, the K241Q mutation abolishes the DNA glycosylase and AP lyase activities of yOgg1. In contrast, the K241R and K241Q proteins have conserved wild-type DNA binding properties. K(dapp) values for binding of [8-OxoG.C] are 6.9, 7.4, and 4.8 nM for the wild-type, K241R, and K241Q proteins, respectively. The results also show that AP site analogues such as 1, 3-propanediol (Pr), tetrahydrofuran (F), or cyclopentanol (Cy) are not substrates but constitute good inhibitors of the wild-type yOgg1. Therefore, we have used a 59mer [Pr.C] duplex to further analyze the DNA binding properties of the wild-type, K241R, and K241Q proteins. Hydroxyl radical footprints of the wild-type yOgg1 show strong protection of six nucleotides centered around the Pr lesion in the damaged strand. On the complementary strand, only the cytosine placed opposite Pr was strongly protected. The same footprints were observed with the K241R and K241Q proteins, confirming their wild-type DNA binding properties. These results indicate that the K241Q mutant protein can be used to study interactions between yOgg1 and DNA containing metabolizable substrates such as 8-OxoG or an AP site.

Arginine↗

PubMed2000

Expression of the Fpg protein of Escherichia coli in Saccharomyces cerevisiae: effects on spontaneous mutagenesis and sensitivity to oxidative DNA damage.

The biological relevance of oxidative DNA damage has been unveiled by the identification of genes such as fpg of E. coli or OGG1 of Saccharomyces cerevisiae. Both Fpg and Ogg1 proteins are DNA glycosylases/AP lyases that excise 7,8-dihydro-8-oxoguanine (8-OxoG) and 2,6-diamino-4-hydroxy-5-N-methylformamidopyrimidine (Me-FapyG) from damaged DNA. Although similar, the enzymatic and biological properties of Fpg and Ogg1 proteins are not identical. Furthermore, the Fpg and Ogg1 proteins do not show significant sequence homologies. In this study, we investigated the ability of the Fpg protein of E. coli to complement phenotypes thought to be due to oxidative DNA damage in Saccharomyces cerevisiae. To express Fpg in yeast, the coding sequence of the fpg gene was placed under the control of a strong yeast promoter in the expression vector pCM190 to generate the pFPG240 plasmid. The Ogg1-deficient yeast strain CD138, ogg1::TRP1, was transformed with pFPG240 and the expression of Fpg was measured. Expression of Fpg in yeast harboring pFPG240 was revealed by efficient release of Me-FapyG and cleavage of 8-OxoG-containing duplexes by cell free protein extracts. The production of the Fpg protein in yeast cells was further demonstrated by immunoblotting analysis using anti-Fpg antibodies. Fpg expression suppresses the spontaneous mutator phenotype of ogg1- yeast for the production of canavanin resistant mutants (CanR) and Lys+ revertants. Fpg expression also restores the capacity of plasmid DNA treated with methylene blue plus visible light (MB-light) to transform the yeast ogg1- rad1- double mutant.

Cross Reactions↗

PubMed2000

Repair and coding properties of 5-hydroxy-5-methylhydantoin nucleosides inserted into DNA oligomers.

1-(2-Deoxy-beta-D-erythro-pentofuranosyl)-5-hydroxy-5-methylhydantoin (5-OH-5-Me-dHyd) (3) has been shown to be a major oxidation product of thymidine formed upon exposure of DNA to (*)OH-radical and excited photosensitizers. To investigate the biological and structural significance of the 5-OH-5-Me-dHyd residue to DNA, the latter modified 2'-deoxyribonucleoside was chemically prepared and then site-specifically incorporated into oligodeoxyribonucleotides. This was efficiently achieved using the phosphoramidite approach that involved mild deprotection conditions. The purity and the integrity of the modified synthetic DNA fragments were checked using different complementary techniques such as HPLC and polyacrylamide gel electrophoresis, together with electrospray ionization and MALDI-TOF mass spectrometry. The piperidine test applied to 5-OH-5-Me-dHyd containing oligonucleotides showed a weak instability of hydantoin nucleoside inserted into the oligonucleotide chain. Several enzymatic experiments aimed at determining the biochemical features of such a DNA lesion were carried out. Thus, processing of 5-OH-5-Me-dHyd by nuclease P(1), snake venom phosphodiesterase, and calf spleen phosphodiesterase was investigated. The specificity and the mechanism of excision of the lesion by several bacterial and yeast DNA N-glycosylases, namely, endonuclease III (endo III), endonuclease VIII (endo VIII), formamidopyrimidine DNA N-glycosylase (Fpg), Ntg1 protein (Ntg1), Ntg2 protein (Ntg2), and Ogg1 protein (yOgg1), were also determined. These repair studies clearly showed that all these enzymes, with the exception of the yOgg1 protein, are able to recognize and remove 5-hydroxy-5-methylhydantoin from the double-stranded DNA fragment. Finally, a 22-mer DNA oligomer bearing a 5-OH-5-Me-dHyd residue was used as a template to study the in vitro nucleotide incorporation opposite the damage by the Klenow fragment of Escherichia coli polymerase I, Taq DNA polymerase, and DNA polymerase beta. Thus, it may be concluded that the oxidized thymine residue is a strongly blocking lesion for the three studied DNA polymerases.

DNA Repair↗

PubMed1999

Excision of oxidatively damaged DNA bases by the human alpha-hOgg1 protein and the polymorphic alpha-hOgg1(Ser326Cys) protein which is frequently found in human populations.

We have investigated the substrate specificity of the major nuclear form of the human Ogg1 protein, referred as alpha-hOgg1, for excision of damaged bases from DNA exposed to gamma-irradiation. Excision products were identified and quantified using gas chromatography/isotope dilution mass spectrometry (GC/IDMS). The GST-alpha-hOgg1 protein used in this study is a fusion of alpha-hOgg1 to the C-terminus of the GST protein. The results show that GST-alpha-hOgg1 protein excises 8-hydroxyguanine (8-OH-Gua) and 2,6-diamino-4-hydroxy-5-formamidopyrimidine (FapyGua) from DNA exposed to gamma-irradiation in a solution saturated with N(2)O or air. Fourteen other lesions, including oxidised purines and pyrimidines, were not excised from these substrates. Catalytic constants were measured for the excision of 8-OH-Gua and FapyGua from DNA gamma-irradiated under N(2)O. The k (cat)/ K (m)values for excision of 8-OH-Gua and FapyGua were 4.47 x 10(-5)and 8.97 x 10(-5)(min(-1)nM(-1)), respectively. The substrate specificity and the catalytic parameters of the wild-type GST-alpha-hOgg1 protein were compared to that of a polymorphic form of alpha-hOgg1 harbouring a Ser-->Cys mutation at codon 326. In the Japanese population, 47.6% of individuals possess both alleles coding for the wild-type alpha-hOgg1-Ser(326)and mutant alpha-hOgg1-Cys(326)proteins. The GST-alpha-hOgg1-Cys(326)protein was purified and its substrate specificity was determined by GC/IDMS analysis. The results show that the GST-alpha-hOgg1-Cys(326)protein efficiently excises 8-OH-Gua and FapyGua from gamma-irradiated DNA. The k (cat)/ K (m)values for excision of 8-OH-Gua and FapyGua were 2. 82 x 10(-5)and 4.43 x 10(-5)(min(-1)nM(-1)), respectively. Furthermore, we compared the capacity of these two forms of alpha-hOgg1 to act on substrates containing 2,6-diamino-4-hydroxy-5- N -methylformamidopyrimidine (Me-FapyGua). The k (cat)/ K (m)values for excision of Me-FapyGua were 278 x 10(-5)and 319 x 10(-5)(min(-1)nM(-1)), respectively. Cleavage of 34mer oligodeoxyribonucleotides containing 8-OH-Gua, 8-hydroxyadenine or an apurinic/apyrimidinic site paired with a cytosine was also investigated. The results show that both GST-alpha-hOgg1-Ser(326)and GST-alpha-hOgg1-Cys(326)catalyse the various cleavage reactions at very similar rates. Furthermore, both proteins efficiently complement the mutator phenotype of the fpg mutY mutant of Escherichia coli.

Amino Acid Substitution↗

PubMed1999

Saccharomyces cerevisiae Ntg1p and Ntg2p: broad specificity N-glycosylases for the repair of oxidative DNA damage in the nucleus and mitochondria.

Saccharomyces cerevisiae possesses two functional homologues (Ntg1p and Ntg2p) of the Escherichia coli endonuclease III protein, a DNA base excision repair N-glycosylase with a broad substrate specificity directed primarily against oxidatively damaged pyrimidines. The substrate specificities of Ntg1p and Ntg2p are similar but not identical, and differences in their amino acid sequences as well as inducibility by DNA damaging agents suggest that the two proteins may have different biological roles and subcellular locations. Experiments performed on oligonucleotides containing a variety of oxidative base damages indicated that dihydrothymine, urea, and uracil glycol are substrates for Ntg1p and Ntg2p, although dihydrothymine was a poor substrate for Ntg2p. Vectors encoding Ntg1p-green fluorescent protein (GFP) and Ntg2p-GFP fusions under the control of their respective endogenous promoters were utilized to observe the subcellular targeting of Ntg1p and Ntg2p in S. cerevisiae. Fluorescence microscopy of pNTG1-GFP and pNTG2-GFP transformants revealed that Ntg1p localizes primarily to the mitochondria with some nuclear localization, whereas Ntg2p localizes exclusively to the nucleus. In addition, the subcellular location of Ntg1p and Ntg2p confers differential sensitivities to the alkylating agent MMS. These results expand the known substrate specificities of Ntg1p and Ntg2p, indicating that their base damage recognition ranges show distinct differences and that these proteins mediate different roles in the repair of DNA base damage in the nucleus and mitochondria of yeast.

Base Sequence↗

PubMed1999

AP site structural determinants for Fpg specific recognition.

The binding of Escherichia coli and Lactococcus lactis Fapy-DNA glyosylase (Fpg) proteins to DNA containing either cyclic or non-cyclic abasic (AP) site analogs was investigated by electrophoretic mobility shift assay (EMSA) and by footprinting experiments. We showed that the reduced AP site is the best substrate analog for the E.coli and L.lactis enzymes ( K Dapp = 0.26 and 0.5 nM, respectively) as compared with the other analogs tested in this study ( K Dapp >2.8 nM). The 1,3-propanediol (Pr) residue-containing DNA seems to be the minimal AP site structure allowing a Fpg specific DNA binding, since the ethyleneglycol residue is not specifically bound by these enzymes. The newly described cyclopentanol residue is better recognized than tetrahydrofuran (for the E.coli Fpg, K Dapp = 2.9 and 25 nM, respectively). These results suggest that the hemiacetal form of the AP site is negatively discriminated by the Fpg protein suggesting a hydrogen bond between the C4'-hydroxyl group of the sugar and a Fpg residue. High-resolution hydroxyl radical footprinting using a duplex containing Pr shows that Fpg binds to six nucleotides on the strand containing the AP site and only the base opposite the lesion on the undamaged complementary strand. This comparative study provides new information about the molecular mechanism involved in the Fpg AP lyase activity.

Cyclopentanes↗

PubMed1999

Spontaneous mutation, oxidative DNA damage, and the roles of base and nucleotide excision repair in the yeast Saccharomyces cerevisiae.

The OGG1 gene of Saccharomyces cerevisiae encodes a DNA glycosylase that excises 7,8-dihydro-8-oxoguanine (8-OxoG). When compared to wild-type, ogg1 mutants show an increase in the frequency of GC to TA transversions, indicating a role for Ogg1 in the repair of 8-OxoG. Here we report an increased frequency of forward mutation to canavanine resistance in mutants defective in the nucleotide excision repair (NER) gene RAD14. This was not increased further in strains additionally defective in OGG1. However, when compared to strains solely defective in OGG1, ogg1radl4 mutants displayed an increase in spontaneous GC to TA transversions. Intriguingly, reversion of the lys1-1 ochre allele was not increased in rad14 mutants, suggesting that oxidative base damage may only represent a substrate for NER in certain regions of the genome. We also examined repair of oxidative DNA damage by transforming mutant strains with plasmid DNA treated with methylene blue plus visible light. Mutants defective in OGG1 showed no significant reduction in transformation efficiency compared with wild-type strains. In contrast, disruption of RAD14 reduced the efficiency of transformation, yet there was no further decrease in an ogg1rad14 mutant. This strongly supports a role for NER in the repair of oxidative base damage in yeast, and differs from similar experiments carried out in E. coli, where transformation efficiency is only reduced in mutants defective in both fpg and uvrA. Finally, the repair of Fpg-sensitive sites was examined at the MATalpha and HMLalpha mating type loci, and NER was found to play a role in their removal.

Canavanine↗

PubMed1999

Photodynamic DNA damage induced by phycocyanin and its repair in Saccharomyces cerevisiae.

In the present study, we analyzed DNA damage induced by phycocyanin (PHY) in the presence of visible light (VL) using a set of repair endonucleases purified from Escherichia coli. We demonstrated that the profile of DNA damage induced by PHY is clearly different from that induced by molecules that exert deleterious effects on DNA involving solely singlet oxygen as reactive species. Most of PHY-induced lesions are single strand breaks and, to a lesser extent, base oxidized sites, which are recognized by Nth, Nfo and Fpg enzymes. High pressure liquid chromatography coupled to electrochemical detection revealed that PHY photosensitization did not induce 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodGuo) at detectable levels. DNA repair after PHY photosensitization was also investigated. Plasmid DNA damaged by PHY photosensitization was used to transform a series of Saccharomyces cerevisiae DNA repair mutants. The results revealed that plasmid survival was greatly reduced in rad14 mutants, while the ogg1 mutation did not modify the plasmid survival when compared to that in the wild type. Furthermore, plasmid survival in the ogg1 rad14 double mutant was not different from that in the rad14 single mutant. The results reported here indicate that lethal lesions induced by PHY plus VL are repaired differently by prokaryotic and eukaryotic cells. Moreover, nucleotide excision repair seems to play a major role in the recognition and repair of these lesions in Saccharomyces cerevisiae.

Culture Media↗

PubMed1999

Frequent allelic loss at chromosome 3p distinct from genetic alterations of the 8-oxoguanine DNA glycosylase 1 gene in head and neck cancer.

Cigarette smoking is the major known risk factor for head and neck cancer. Tobacco promotes oxidative stress and enhances tissue levels of 8-hydroxyguanine (8-OH-G) in smokers. The presence of 8-OH-G does not impede replication but leads to an accumulation of G-->T transversions. Recently, the gene for human 8-oxoguanine DNA glycosylase 1 (hOGG1), an enzyme involved in the repair of 8-OH-G in humans, was cloned and mapped to chromosome 3p. In head and neck tumors, the hOGG1 gene locus is often targeted by loss of heterozygosity (LOH), and the spectrum of mutations in the p53 gene shows a bias in favor of G:C-->T:A transversions, as would be expected if HOGG1 repair functions were disabled. To test the involvement of hOGG1 in head and neck carcinogenesis, we had previously screened 56 tumors for LOH at 3p. From these tumors and two others, we selected 33 tumors demonstrating LOH for further mutational analysis of this gene. No somatic inactivating mutation was found in hOGG1. Polymorphisms involving intron 4 and exon 7 were present in 30% of the patients. A new polymorphism was identified in one patient in exon 6 and led to the amino-acid change G308E. Similar repair activities were found for the wild-type and exon 6-variant enzymes. Therefore, the involvement of hOGG1 in head and neck carcinogenesis is not strongly supported by this work.

Alleles↗

PubMed1998

Substrate specificities of the ntg1 and ntg2 proteins of Saccharomyces cerevisiae for oxidized DNA bases are not identical.

Two genes of Saccharomyces cerevisiae, NTG1 and NTG2, encode proteins with a significant sequence homology to the endonuclease III of Escherichia coli. The Ntg1 and Ntg2 proteins were overexpressed in E.coli and purified to apparent homogeneity. The substrate specificity of Ntg1 and Ntg2 proteins for modified bases in oxidatively damaged DNA was investigated using gas chromatography/isotope-dilution mass spectrometry. The substrate used was calf-thymus DNA exposed to gamma-radiation in N2O-saturated aqueous solution. The results reveal excision by Ntg1 and Ntg2 proteins of six pyrimidine-derived lesions, 5-hydroxy-6-hydrothymine, 5-hydroxy-6-hydrouracil, 5-hydroxy-5-methylhydantoin, 5-hydroxyuracil, 5-hydroxycytosine and thymine glycol, and two purine-derived lesions, 2,6-diamino-4-hydroxy-5-formamidopyrimidine and 4,6-diamino-5-formamidopyrimidine from gamma-irradiated DNA. In contrast, Ntg1 and Ntg2 proteins do not release 8-hydroxyguanine or 8-hydroxyadenine from gamma-irradiated DNA. The Ntg1 and Ntg2 proteins also release 2, 6-diamino-4-hydroxy-5-N-methylformamido-pyrimidine from damaged poly(dG-dC).poly(dG-dC). Excision was measured as a function of enzyme concentration and time. Furthermore, kinetic parameters were determined for each lesion. The results show that kinetic constants varied among the different lesions for the same enzyme. We also investigated the capacity of the Ntg1 and Ntg2 proteins to cleave 34mer DNA duplexes containing a single 8-OH-Gua residue mispaired with each of the four DNA bases. The results show that the Ntg1 protein preferentially cleaves a DNA duplex containing 8-OH-Gua mispaired with a guanine. Moreover, the Ntg1 protein releases free 8-OH-Gua from 8-OH-Gua/Gua duplex but not from duplexes containing 8-OH-Gua mispaired with adenine, thymine or cytosine. In contrast, the Ntg2 protein does not incise duplexes containing 8-OH-Gua mispaired with any of the four DNA bases. These results demonstrate that substrate specificities of the Ntg1 and Ntg2 proteins are similar but not identical and clearly different from that of the endonuclease III of E.coli and its homologues in Schizosaccharomyces pombe or human cells.

Base Pair Mismatch↗

PubMed1998

Fanconi's anaemia cells have normal steady-state levels and repair of oxidative DNA base modifications sensitive to Fpg protein.

Cells from Fanconi's anaemia (FA) patients are abnormally sensitive to oxygen. However, a distinct genetic defect in either the cellular defence against reactive oxygen species (ROS) or in their metabolic generation has not been identified to date. Recently, the gene for the human 8-hydroxyguanine (8-oxoG) glycosylase, which removes this oxidative base modification from the genome, has been localized on chromosome 3p25, i.e., in the same region as the FA complementation group D (FAD) gene. We therefore studied the removal of photosensitization-induced 8-oxoG residues from the DNA of FA cells, using Fpg protein, the bacterial 8-oxoG glycosylase, to quantify the lesions by alkaline elution. Similar repair kinetics (approx. 50% removal within 2 h) were observed in Epstein-Barr virus (EBV) immortalized lymphoid cells from FA complementation groups A, B, C and D and in control cells from normal donors, as well as in primary fetal lung fibroblasts not yet assigned to a specific complementation group. The susceptibility for the induction of oxidative DNA modifications by photosensitization was similar in all cells. In addition, the background (steady-state) levels of Fpg-sensitive oxidative DNA base modifications, which reflect the balance between generation and removal of the lesions, were similar in control and FA cells. It is concluded that both the generation and the overall removal of 8-oxoG residues in nuclear DNA is not impaired in FA cells.

Cell Division↗

PubMed1998

Mutations in OGG1, a gene involved in the repair of oxidative DNA damage, are found in human lung and kidney tumours.

The human OGG1 gene encodes a DNA glycosylase activity catalysing the excision of the mutagenic lesion 7,8-dihydro-8-oxoguanine from oxidatively damaged DNA. The OGG1 gene was localized to chromosome 3p25, a region showing frequent loss of heterozygosity (LOH) in lung and kidney tumours. In this study, we have analysed by RT-PCR the expression of OGG1 in 25 small cell lung cancers, in 15 kidney carcinomas and the 15 normal kidney counterparts. The results show that OGG1 messenger RNA can be detected in all tumours tested and that no significant difference was observed in the level of expression between normal and tumoral kidney tissues. Denaturing gradient gel electrophoresis (DGGE) was used to screen this series of human tumours for alterations in the OGG1 cDNA. The study revealed homozygous mutations in three tumours, two from lung and one from kidney. Sequencing analysis of the mutants identified a single base substitution in each of the three cases: two transversions (GC to TA and TA to AT) and one transition (GC to AT). All three substitutions cause an amino acid change in the hOgg1 protein. For the mutant kidney tumour, the normal tissue counterpart shows a wild-type profile. These results suggest a role for OGG1 mutations in the course of the multistage process of carcinogenesis in lung or kidney.

Carcinoma, Small Cell↗

PubMed1998

Substrate specificity of the Ogg1 protein of Saccharomyces cerevisiae: excision of guanine lesions produced in DNA by ionizing radiation- or hydrogen peroxide/metal ion-generated free radicals.

We have investigated the substrate specificity of the Ogg1 protein of Saccharomyces cerevisiae (yOgg1 protein) for excision of modified DNA bases from oxidatively damaged DNA substrates using gas chromatography/isotope dilution mass spectrometry. Four DNA substrates prepared by treatment with H2O2/Fe(III)-EDTA/ascorbic acid, H2O2/Cu(II) and gamma-irradiation under N2O or air were used. The results showed that 8-hydroxyguanine (8-OH-Gua) and 2,6-diamino-4-hydroxy-5-formamidopyrimidine (FapyGua) were efficiently excised from DNA exposed to ionizing radiation in the presence of N2O or air. On the other hand, 8-OH-Gua and FapyGua were not excised from H2O2/Fe(III)-EDTA/ascorbic acid-treated and H2O2/Cu(II)-treated DNA respectively. Fourteen other lesions, including the adenine lesions 8-hydroxyadenine and 4,6-diamino-5-formamidopyrimidine, were not excised from any of the DNA substrates. Kinetics of excision significantly depended on the nature of the damaged DNA substrates. The findings suggest that, in addition to 8-OH-Gua, FapyGua may also be a primary substrate of yOgg1 in cells. The results also show significant differences between the substrate specificities of yOgg1 protein and its functional analog Fpg protein in Escherichia coli.

DNA Damage↗

PubMed1998

Excision repair of 8-hydroxyguanine in mammalian cells: the mouse Ogg1 protein as a model.

8-Hydroxyguanine (8-OH-Gua) is a major mutagenic lesion produced on DNA by the oxidative stress induced by either the endogen metabolism or the exposure to external agents. In bacteria and yeast this modified base can be removed by specific DNA glycosylases. Recently a human gene coding for an 8-OH-Gua DNA glycosylase/AP lyase has been identified by its homology to the yeast OGG1. This gene is located in human chromosome 3p25, a region commonly rearranged in various cancers, specially in lung tumor cells. We report here the cloning, by sequence homology to the yeast OGG1, of a mouse cDNA coding for a 8-OH-Gua DNA glycosylase with 84% and 38% identity to the human and yeast relevant proteins, respectively. The Ogg1 gene is localized to the mouse chromosome 6E. The mouse Qgg1 cDNA, when expressed in Eschierichia coli, is capable of suppressing the spontaneous mutator phenotype of a DNA repair deficient fpg mutgamma strain. The mouse Ogg1 protein acts efficiently on duplexes in which the 8-OH-Gua is paired with a cytosine but is inactive on 8-OH-Gua: Ade pair, consistently with its proposed biological role in the avoidance of mutations. A comparison of the mouse enzyme with other eukaryotic Ogg1 enzymes is also presented. The isolation of this gene will allow the development of an animal model to study the effects of oxidative stress on carcinogenesis and degenerative diseases.

Amino Acid Sequence↗

PubMed1998

Opposite base-dependent excision of 7,8-dihydro-8-oxoadenine by the Ogg1 protein of Saccharomyces cerevisiae.

The yOgg1 protein of Saccharomyces cerevisiae is a DNA glycosylase/AP lyase that excises guanine lesions such as 7,8-dihydro-8-oxoguanine (8-OxoG) and 2,6-diamino-4-hydroxy-5-N-methylformamidopyrimidine (me-Fapy-G) and incises apurinic/apyrimidinic sites (AP sites) in damaged DNA. The yOgg1 protein displays a marked preference for DNA duplexes containing 8-OxoG or AP sites placed opposite cytosine. In this paper, we show that yOgg1 can also excise an adenine lesion, 7,8-dihydro-8-oxoadenine (8-OxoA), when paired with cytosine or 5-methylcytosine. In contrast, yOgg1 does not release 8-OxoA when placed opposite thymine, adenine, guanine or uracil. The specificity constants (Kcat/Km) for repair of 8-OxoG/C and 8-OxoA/C duplexes are (50 +/- 18) x 10(-3) and (13 +/- 3) x 10(-3)/min/nM, respectively. The catalytic mechanism for strand cleavage at 8-OxoA/C involves excision of 8-OxoA by the DNA glycosylase activity of yOgg1, followed by incision at the newly formed AP site via a beta-elimination reaction. Furthermore, cleavage of 8-OxoA/C involves formation of a reaction intermediate that is converted into a stable covalent adduct in the presence of sodium borohydride (NaBH4). The yOgg1 protein binds strongly to the 8-OxoA/C duplex, as demonstrated by an apparent dissociation constant (Kdapp) value of 45 nM, as determined by gel mobility shift assay. In contrast, the yOgg1 protein has a very low binding affinity for the 8-OxoA/T duplex, a Kdapp value of 680 nM, which in turn can explain the lack of repair of 8-OxoA in this duplex. The capacity of other DNA glycosylases/AP lyases to repair 8-OxoA has also been investigated. The results show that human hOgg1 protein efficiently repairs 8-OxoA placed opposite cytosine or 5-methylcytosine. On the other hand, the Fpg protein of Escherichia coli cleaves 8-OxoA/C at a very slow rate as compared with yOgg1.

5-Methylcytosine↗

PubMed1997

The Ogg1 protein of Saccharomyces cerevisiae: a 7,8-dihydro-8-oxoguanine DNA glycosylase/AP lyase whose lysine 241 is a critical residue for catalytic activity.

The OGG1 gene of Saccharomyces cerevisiae codes for a DNA glycosylase that excises 7,8-dihydro-8- oxoguanine (8-OxoG) and 2,6-diamino-4-hydroxy-5- N -methylformamidopyrimidine (Fapy) from damaged DNA. In this paper, we have analysed the substrate specificity and the catalytic mechanism of the Ogg1 protein acting on DNA subtrates containing 8-OxoG residues or apurinic/apyrimidinic (AP) sites. The Ogg1 protein displays a marked preference for DNA duplexes containing 8-OxoG placed opposite a cytosine, the rank order for excision of 8-OxoG and cleavage efficiencies being 8-OxoG/C >8-OxoG/T >>8-OxoG/G and 8-OxoG/A. The cleavage of the DNA strand implies the excision of 8-OxoG followed by abeta-elimination reaction at the 3'-side of the resulting AP site. The Ogg1 protein efficiently cleaves a DNA duplex where a preformed AP site is placed opposite a cytosine (AP/C). In contrast, AP/T, AP/A or AP/G substrates are incised with a very low efficiency. Furthermore, cleavage of 8-OxoG/C or AP/C substrates implies the formation of a reaction intermediate that is converted into a stable covalent adduct in the presence of sodium borohydre (NaBH4). Therefore, the Ogg1 protein is a eukaryotic DNA glycosylase/AP lyase. Sequence homology searches reveal that Ogg1 probably shares a common ancestor gene with the endonuclease III of Escherichia coli. A consensus sequence indicates a highly conserved lysine residue, K120 of endonuclease III or K241 of Ogg1, respectively. Mutations of K241 to Gln (K241Q) and Arg (K241R) have been obtained after site directed mutagenesis of OGG1. Mutation K241Q completely abolishes DNA glycosylase activity and covalent complex formation in the presence of NaBH4. However, the K241Q mutant still binds DNA duplexes containing 8-OxoG/C. In contrast, K241R mutation results in a catalytically active form of Ogg1. These results strongly suggest that the free amino group of Lys241 is involved in the catalytic mechanism of the Ogg1 protein.

Binding Sites↗

PubMed1997

Cloning and characterization of hOGG1, a human homolog of the OGG1 gene of Saccharomyces cerevisiae.

The OGG1 gene of Saccharomyces cerevisiae encodes a DNA glycosylase activity that is a functional analog of the Fpg protein from Escherichia coli and excises 7,8-dihydro-8-oxoguanine (8-oxoG) from damaged DNA. The repair of this ubiquitous kind of oxidative damage is essential to prevent mutations both in bacteria and in yeast. A human cDNA clone carrying an ORF displaying homology to the yeast protein was identified. The predicted protein has 345 amino acids and a molecular mass of 39 kDa. This protein shares a 38% sequence identity with the yeast Ogg1 protein, adding this novel human gene product to the growing family of enzymes that the repair of oxidatively damaged bases and are related to the E. coli endonuclease III. Northern blot analysis indicates that this gene, localized to chromosome 3p25, is ubiquitously expressed in human tissues. The cloned coding sequence was expressed in an E. coli strain that carried a disrupted fpg gene, the bacterial functional analog of OGG1. Cell-free extracts from these cultures displayed a specific lyase activity on duplex DNA that carried an 8-oxoG/C base pair. The products of the reaction are consistent with an enzymatic activity like the one displayed by the yeast Ogg1. Analysis of the substrate specificity reveals a very strong preference for DNA fragments harboring 8-oxoG/C base pairs. The pattern of specificity correlates well with the one found for the yeast enzyme. Moreover, when the human coding sequence was expressed in a yeast strain mutant in OGG1 it was able to complement the spontaneous mutator phenotype. These results make this novel gene (hOGG1) a strong candidate for the human homolog of the yeast OGG1 and suggest an important role of its product in the protection of the genome from the mutagenic effects of the oxidatively damaged purines.

Amino Acid Sequence↗