8-oxodeoxyguanosine-triphosphate and 8-hydroxyguanine

Accumulation of oxidized bases such as 8-oxoguanine in either nuclear or mitochondrial DNA triggers various cellular dysfunctions including mutagenesis, and programmed cell death or senescence. Recent studies have revealed that oxidized nucleoside triphosphates such as 8-oxo-dGTP in the nucleotide pool are the main source of oxidized bases accumulating in the DNA of cells under oxidative stress. To counteract such deleterious effects of nucleotide pool damage, mammalian cells possess MutT homolog-1 (MTH1) with oxidized purine nucleoside triphosphatase and related enzymes, thus minimizing the accumulation of oxidized bases in cellular DNA. Depletion or increased expression of the MTH1 protein have revealed its significant roles in avoiding programmed cell death or senescence as well as mutagenesis, and accumulating evidences indicate that MTH1 is involved in suppression of degenerative disorders such as neurodegeneration.

Topics: Adenosine Triphosphate; Animals; Apoptosis; Deoxyguanine Nucleotides; DNA Damage; DNA Repair Enzymes; DNA, Mitochondrial; Guanine; Humans; Nerve Degeneration; Neurons; Nucleoside-Triphosphatase; Oxidative Stress; Phosphoric Monoester Hydrolases; Purine Nucleosides; Reactive Oxygen Species

Oxidative stress promotes genomic instability and human diseases. A common oxidized nucleoside is 8-oxo-7,8-dihydro-2'-deoxyguanosine, which is found both in DNA (8-oxo-G) and as a free nucleotide (8-oxo-dGTP). Nucleotide pools are especially vulnerable to oxidative damage. Therefore cells encode an enzyme (MutT/MTH1) that removes free oxidized nucleotides. This cleansing function is required for cancer cell survival and to modulate Escherichia coli antibiotic sensitivity in a DNA polymerase (pol)-dependent manner. How polymerases discriminate between damaged and non-damaged nucleotides is not well understood. This analysis is essential given the role of oxidized nucleotides in mutagenesis, cancer therapeutics, and bacterial antibiotics. Even with cellular sanitizing activities, nucleotide pools contain enough 8-oxo-dGTP to promote mutagenesis. This arises from the dual coding potential where 8-oxo-dGTP(anti) base pairs with cytosine and 8-oxo-dGTP(syn) uses its Hoogsteen edge to base pair with adenine. Here we use time-lapse crystallography to follow 8-oxo-dGTP insertion opposite adenine or cytosine with human pol β, to reveal that insertion is accommodated in either the syn- or anti-conformation, respectively. For 8-oxo-dGTP(anti) insertion, a novel divalent metal relieves repulsive interactions between the adducted guanine base and the triphosphate of the oxidized nucleotide. With either templating base, hydrogen-bonding interactions between the bases are lost as the enzyme reopens after catalysis, leading to a cytotoxic nicked DNA repair intermediate. Combining structural snapshots with kinetic and computational analysis reveals how 8-oxo-dGTP uses charge modulation during insertion that can lead to a blocked DNA repair intermediate.

Topics: Adenine; Base Pairing; Catalytic Domain; Crystallography, X-Ray; Cytosine; Cytotoxins; Deoxyguanine Nucleotides; DNA; DNA Damage; DNA Polymerase beta; DNA Repair; DNA Replication; Guanine; Humans; Hydrogen Bonding; Kinetics; Models, Molecular; Molecular Conformation; Mutagenesis; Neoplasms; Oxidation-Reduction; Oxidative Stress; Static Electricity; Substrate Specificity; Time Factors

A detailed understanding of the mechanisms that underlie antibiotic killing is important for the derivation of new classes of antibiotics and clinically useful adjuvants for current antimicrobial therapies. Our efforts to understand why DinB (DNA polymerase IV) overproduction is cytotoxic to Escherichia coli led to the unexpected insight that oxidation of guanine to 8-oxo-guanine in the nucleotide pool underlies much of the cell death caused by both DinB overproduction and bactericidal antibiotics. We propose a model in which the cytotoxicity of beta-lactams and quinolones predominantly results from lethal double-strand DNA breaks caused by incomplete repair of closely spaced 8-oxo-deoxyguanosine lesions, whereas the cytotoxicity of aminoglycosides might additionally result from mistranslation due to the incorporation of 8-oxo-guanine into newly synthesized RNAs.

Topics: Ampicillin; Anti-Bacterial Agents; Deoxyguanine Nucleotides; DNA Breaks, Double-Stranded; DNA Glycosylases; DNA Polymerase beta; DNA Polymerase I; DNA Polymerase II; DNA-Directed DNA Polymerase; DNA, Bacterial; Escherichia coli; Escherichia coli Proteins; Guanine; Guanine Nucleotides; Hydroxyl Radical; Kanamycin; Microbial Viability; Models, Biological; Norfloxacin; Oxidation-Reduction; Pyrophosphatases; RNA, Bacterial

8-Oxo-7,8-dihydroguanine (8-hydroxyguanine) is oxidized more easily than normal nucleobases, which can produce spiroiminodihydantoin (Sp) and guanidinohydantoin (Gh). These secondary oxidation products of 8-oxo-7,8-dihydroguanine are highly mutagenic when formed within DNA. To evaluate the mutagenicity of the corresponding oxidation products of 8-oxo-7,8-dihydro-2'-deoxyguanosine 5'-triphosphate (8-hydroxy-2'- deoxyguanosine 5'-triphosphate) in the nucleotide pool, Escherichia coli cells deficient in the mutT gene were treated with H(2)O(2), and the induced mutations were analyzed. Moreover, the 2'-deoxyriboside 5'-triphosphate derivatives of Sp and Gh were also introduced into competent E. coli cells. The H(2)O(2) treatment of mutT E. coli cells resulted in increase of G:C → T:A and A:T → T:A mutations. However, the incorporation of exogenous Sp and Gh 2'-deoxyribonucleotides did not significantly increase the mutation frequency. These results suggested that the oxidation product(s) of 8-oxo-7,8-dihydro-2'-deoxyguanosine 5'-triphosphate induces G:C → T:A and A:T → T:A mutations, and that the 2'-deoxyriboside 5'-triphosphate derivatives of Sp and Gh exhibit quite weak mutagenicity, in contrast to the bases in DNA.

Topics: Deoxyguanine Nucleotides; Escherichia coli; Escherichia coli Proteins; Guanidines; Guanine; Guanosine; Guanosine Triphosphate; Hydantoins; Hydrogen Peroxide; Mutagens; Oxidation-Reduction; Spiro Compounds

8-Oxo-7,8-dihydroguanine (8-oxo-Gua, also known as 8-hydroxyguanine) is a major base lesion that is generated by reactive oxygen species in both the DNA and nucleotide pool. The role of DNA glycosylases, which initiate base excision repair, in the mutagenic processes of 8-oxo-Gua in DNA and 8-oxo-7,8-dihydro-2'-deoxyguanosine 5'-triphosphate (8-oxo-dGTP, also known as 8-hydroxy-2'-deoxyguanosine 5'-triphosphate) were investigated using supF shuttle plasmids propagated in human cells. The DNA glycosylases, OGG1, MUTYH, NTH1, and NEIL1, in 293T cells were individually knocked-down by siRNAs and plasmid DNAs containing an 8-oxo-Gua:C/8-oxo-Gua:A pair, and 8-oxo-dGTP plus unmodified plasmid DNA were then introduced into the knocked-down cells. The knock-down of OGG1, MUTYH, NTH1, and NEIL1 resulted in a significant increase in G:C-->T:A transversions caused by the 8-oxo-Gua:C pair in the shuttle plasmid. The knock-down of MUTYH resulted in a reduction in A:T-->C:G transversions induced by 8-oxo-dGTP and the 8-oxo-Gua:A pair, but the knockdown of OGG1, NTH1, and NEIL1 had no effect on mutagenesis. These results indicate that all of the above DNA glycosylases suppress mutations caused by 8-oxo-Gua:C in DNA. In contrast, it appears that MUTYH enhances A:T-->C:G mutations caused by 8-oxo-dGTP.

Topics: Adenine; Base Sequence; Cell Line; Cytosine; Deoxyguanine Nucleotides; DNA; DNA Glycosylases; DNA Repair; Gene Knockdown Techniques; Guanine; Humans; Mutagenesis; Mutation; Plasmids; RNA, Small Interfering

8-OxoGua (8-oxo-7,8-dihydroguanine) is produced in nucleic acids as well as in nucleotide pools of cells, by reactive oxygen species normally formed during cellular metabolic processes. MutT protein of Escherichia coli specifically degrades 8-oxoGua-containing deoxyribo- and ribonucleoside triphosphates to corresponding nucleoside monophosphates, thereby preventing misincorporation of 8-oxoGua into DNA and RNA, which would cause mutation and phenotypic suppression, respectively. Here, we report that the MutT protein has additional activities for cleaning up the nucleotide pools to ensure accurate DNA replication and transcription. It hydrolyzes 8-oxo-dGDP to 8-oxo-dGMP with a K(m) of 0.058 microM, a value considerably lower than that for its normal counterpart, dGDP (170 microM). Furthermore, the MutT possesses an activity to degrade 8-oxo-GDP to the related nucleoside monophosphate, with a K(m) value 8000 times lower than that for GDP. These multiple enzyme activities of the MutT protein would facilitate the high fidelity of DNA and RNA syntheses.

Topics: Deoxyadenine Nucleotides; Deoxycytosine Nucleotides; Deoxyguanine Nucleotides; DNA Replication; DNA, Bacterial; Escherichia coli Proteins; Guanine; Guanosine Triphosphate; Hydrolysis; Kinetics; Multienzyme Complexes; Phosphoric Monoester Hydrolases; Pyrophosphatases; RNA, Bacterial; Thymine Nucleotides; Transcription, Genetic

DNA 8-oxoguanine (8-oxoG) causes transversions and is also implicated in frameshifts. We previously identified the dNTP pool as a likely source of mutagenic DNA 8-oxoG and demonstrated that DNA mismatch repair prevented oxidation-related frameshifts in mononucleotide repeats. Here, we show that both Klenow fragment and DNA polymerase alpha can utilize 8-oxodGTP and incorporate the oxidized purine into model frameshift targets. Both polymerases incorporated 8-oxodGMP opposite C and A in repetitive DNA sequences and efficiently extended a terminal 8-oxoG. The human MutSalpha mismatch repair factor recognized DNA 8-oxoG efficiently in some contexts that resembled frameshift intermediates in the same C or A repeats. DNA 8-oxoG in other slipped/mispaired structures in the same repeats adopted configurations that prevented recognition by MutSalpha and by the OGG1 DNA glycosylase thereby rendering it invisible to DNA repair. These findings are consistent with a contribution of oxidative DNA damage to frameshifts. They also suggest how mismatch repair might reduce the burden of DNA 8-oxoG and prevent frameshift formation.

Topics: Adenosine; Base Pair Mismatch; Cytosine; Deoxyguanine Nucleotides; DNA; DNA Glycosylases; DNA Repair; DNA-Binding Proteins; Guanine; Guanosine Monophosphate; MutS Homolog 2 Protein; Proto-Oncogene Proteins; Repetitive Sequences, Nucleic Acid