Multi-forms of human MTH1 polypeptides produced by alternative translation initiation and single nucleotide polymorphism

A profile of 8-oxo-dGTPase activities in the NCI-60 human cancer panel: Meta-analytic insight into the regulation and role of MTH1 (NUDT1) gene expression in carcinogenesis

We measured the specific 8-oxo-dGTPase activity profile of the NCI-60 panel of malignant cell lines, and MTH1 protein levels in a subset of 16 lines. Their 8-oxo-dGTPase activity was compared to twelve publicly accessible MTH1 mRNA expression data bases and their cross-consistency was analyzed. 8-oxo-dGTPase and MTH1 protein levels in these cell lines are generally, but not always, mainly determined by MTH1 mRNA expression levels. The aneuploidy number of MTH1 gene copies only slightly affects its mRNA expression levels. By using the data mining platforms Compare and CellMiner, our 8-oxo-dGTPase profile was compared to five global gene expression datasets to identify genes whose expression levels are directly or inversely associated with 8-oxo-dGTPase. We analyzed effects of SNP within MTH1 on MTH1 mRNA level and enzyme activity. Similar association analysis was performed for five microRNA expression datasets. We identified several proteins and microRNA which might be involved in the regulation of MTH1 expression and we discuss potential mechanisms. Comparison of chemical and natural products sensitivities of the NCI-60 panel suggests seven compounds which are directly or inversely associated with 8-oxo-dGTPase. We provide an integrated picture of MTH1 expression combined from eleven consistent MTH1 mRNA and our 8-oxo-dGTPase activity NCI-60 profiles.

On the epigenetic role of guanosine oxidation

Chemical modifications of DNA and RNA regulate genome functions or trigger mutagenesis resulting in aging or cancer. Oxidations of macromolecules, including DNA, are common reactions in biological systems and often part of regulatory circuits rather than accidental events. DNA alterations are particularly relevant since the unique role of nuclear and mitochondrial genome is coding enduring and inheritable information. Therefore, an alteration in DNA may represent a relevant problem given its transmission to daughter cells. At the same time, the regulation of gene expression allows cells to continuously adapt to the environmental changes that occur throughout the life of the organism to ultimately maintain cellular homeostasis. Here we review the multiple ways that lead to DNA oxidation and the regulation of mechanisms activated by cells to repair this damage. Moreover, we present the recent evidence suggesting that DNA damage caused by physiological metabolism acts as epigenetic signal for regulation of gene expression. In particular, the predisposition of guanine to oxidation might reflect an adaptation to improve the genome plasticity to redox changes.

Mitochondrial maintenance under oxidative stress depends on mitochondrially localised α-OGG1

Accumulation of 8-oxoguanine (8-oxoG) in mitochondrial DNA and mitochondrial dysfunction have been observed in cells deficient for the DNA glycosylase OGG1 when exposed to oxidative stress. In human cells, up to eight mRNAs for OGG1 can be generated by alternative splicing and it is still unclear which of them codes for the protein that ensures the repair of 8-oxoG in mitochondria. Here, we show that the α-OGG1 isoform, considered up to now to be exclusively nuclear, has a functional mitochondrial-targeting sequence and is imported into mitochondria. We analyse the sub-mitochondrial localisation of α-OGG1 with unprecedented resolution and show that this DNA glycosylase is associated with DNA in mitochondrial nucleoids. We show that the presence of α-OGG1 inside mitochondria and its enzymatic activity are required to preserve the mitochondrial network in cells exposed to oxidative stress. Altogether, these results unveil a new role of α-OGG1 in the mitochondria and indicate that the same isoform ensures the repair of 8-oxoG in both nuclear and mitochondrial genomes. The activity of α-OGG1 in mitochondria is sufficient for the recovery of organelle function after oxidative stress.

The roles of human MTH1, MTH2 and MTH3 proteins in maintaining genome stability under oxidative stress

The hydrolysis of nucleotides containing 8-oxo-7,8-dihydroguanine (8-oxoG) is important in the maintenance of genome stability. Human cells possess three types of proteins, MTH1 (NUDT1), MTH2 (NUDT15) and MTH3 (NUDT18), which have the potential to hydrolyze deoxyribonucleoside di- and triphosphates containing 8-oxoG to the monophosphate, the form of which is unusable for DNA synthesis. To elucidate the physiological roles of these enzymes, we constructed single knockout (KO) cell lines for each of the MTH1, MTH2 and MTH3 genes and MTH1 and MTH2-double KO cell lines from the human HeLa S3 line using CRISPR/Cas9. With the exception of MTH3-KO, all of the KO cell lines showed similar proliferation rates to the parental line, HeLa S3, indicating that the MTH1 and MTH2 functions are dispensable for cell growth. On the other hand, the MTH3-KO cells showed a significantly slower growth rate, suggesting that MTH3 has a definite role in cell growth in addition to the cleavage of 8-oxoG-containing deoxyribonucleotide. MTH1-KO, MTH2-KO and MTH1- MTH2-KO cells exhibited increased sensitivity to hydrogen peroxide, whereas MTH3-KO did not. MTH1-KO cells showed only a slight increase in mutant frequency in comparison to the parental HeLa S3 line. The overproduction of MTH1 and MTH2 suppressed the mutator phenotype of mutT-deficient E. coli cells, whereas the overproduction of MTH3 did not show such a suppressive effect. Our findings suggest that both MTH1 and MTH2 are involved in the maintaining genome stability in human cells against oxidative stress, while MTH3 may play some other role(s).

Investigation of micronucleus induction in MTH1 knockdown cells exposed to UVA, UVB or UVC

The longer wave parts of UVR can increase the production of reactive oxygen species (ROS) which can oxidize nucleotides in the DNA or in the nucleotide pool leading to mutations. Oxidized bases in the DNA are repaired mainly by the DNA base excision repair system and incorporation of oxidized nucleotides into newly synthesized DNA can be prevented by the enzyme MTH1. Here we hypothesize that the formation of several oxidized base damages (from pool and DNA) in close proximity, would cause a high number of base excision repair events, leading to DNA double strand breaks (DSB) and therefore giving rise to cytogenetic damage. If this hypothesis is true, cells with low levels of MTH1 will show higher cytogenetic damage after the longer wave parts of UVR. We analyzed micronuclei induction (MN) as an endpoint for cytogenetic damage in the human lymphoblastoid cell line, TK6, with a normal and a reduced level of MTH1 exposed to UVR. The results indicate a higher level of micronuclei at all incubation times after exposure to the longer wave parts of UVR. There is no significant difference between wildtype and MTH1-knockdown TK6 cells, indicating that MTH1 has no protective role in UVR-induced cytogenetic damage. This indicates that DSBs induced by UV arise from damage forms by direct interaction of UV or ROS with the DNA rather than through oxidation of dNTP.

Cellular levels of 8-oxoguanine in either DNA or the nucleotide pool play pivotal roles in carcinogenesis and survival of cancer cells

8-Oxoguanine, a major oxidized base lesion formed by reactive oxygen species, causes G to T transversion mutations or leads to cell death in mammals if it accumulates in DNA. 8-Oxoguanine can originate as 8-oxo-dGTP, formed in the nucleotide pool, or by direct oxidation of the DNA guanine base. MTH1, also known as NUDT1, with 8-oxo-dGTP hydrolyzing activity, 8-oxoguanine DNA glycosylase (OGG1) an 8-oxoG DNA glycosylase, and MutY homolog (MUTYH) with adenine DNA glycosylase activity, minimize the accumulation of 8-oxoG in DNA; deficiencies in these enzymes increase spontaneous and induced tumorigenesis susceptibility. However, different tissue types have different tumorigenesis susceptibilities. These can be reversed by combined deficiencies in the defense systems, because cell death induced by accumulation of 8-oxoG in DNA is dependent on MUTYH, which can be suppressed by MTH1 and OGG1. In cancer cells encountering high oxidative stress levels, a high level of 8-oxo-dGTP accumulates in the nucleotide pool, and cells therefore express increased levels of MTH1 in order to eliminate 8-oxo-dGTP. Suppression of MTH1 may be an efficient strategy for killing cancer cells; however, because MTH1 and OGG1 protect normal tissues from oxidative-stress-induced cell death, it is important that MTH1 inhibition does not increase the risk of healthy tissue degeneration.

First evidence for digenic inheritance in hereditary colorectal cancer by mutations in the base excision repair genes

Biallelic mutations in the base excision repair gene Mut Y homologue (MUTYH) are responsible for variable recessively inherited phenotypes of polyposis. Beside MUTYH, the proteins 8-oxo-guanine DNA glycosylase (OGG1) and MTH1 (or NUDT1) are also involved in the repair of 7,8-dihydro-8-oxoguanine (8-oxo-G), previous studies, however, only found missense mutations of unclear pathogenicity in either MTH1 or OGG1. To investigate the role of a defective 8-oxo-G repair we performed a germline mutation screening in the genes OGG1, MTH1 and MUTYH, in 81 patients with a clinical phenotype ranging from attenuated or atypical adenomatous polyposis coli including hyperplastic polyps to hereditary non-polyposis colorectal cancer (HNPCC) type X syndrome without mono- or biallelic mutations in either APC, MUTYH or the DNA mismatch repair genes. We describe here the first pathogenic germline mutation in OGG1, a splice site mutation affecting exon 1, which was inherited from the father, in combination with a maternal MUTYH missense mutation p.Ile223Val in a female patient with advanced synchronous colon cancer and adenomas at the age of 36 years pointing towards digenic inheritance for colorectal cancer (CRC) predisposition. Monoallelic missense mutations in MTH1 (3x), OGG1 (2x), or MUTYH (3x) were identified in 10 patients (12%), three of them were novel. Our findings indicate that mutations in other genes of the 8-oxo-G repair beside MUTYH are involved in CRC predisposition. Oligogenic inheritance affecting genes of a certain repair pathway might therefore be the missing link between monogenic and polygenic traits.

Involvement of oxidatively damaged DNA and repair in cancer development and aging

DNA damage and DNA repair may mediate several cellular processes, like replication and transcription, mutagenesis and apoptosis and thus may be important factors in the development and pathology of an organism, including cancer. DNA is constantly damaged by reactive oxygen species (ROS) and reactive nitrogen species (RNS) directly and also by products of lipid peroxidation (LPO), which form exocyclic adducts to DNA bases. A wide variety of oxidatively-generated DNA lesions are present in living cells. 8-oxoguanine (8-oxoGua) is one of the best known DNA lesions due to its mutagenic properties. Among LPO-derived DNA base modifications the most intensively studied are ethenoadenine and ethenocytosine, highly miscoding DNA lesions considered as markers of oxidative stress and promutagenic DNA damage. Although at present it is impossible to directly answer the question concerning involvement of oxidatively damaged DNA in cancer etiology, it is likely that oxidatively modified DNA bases may serve as a source of mutations that initiate carcinogenesis and are involved in aging (i.e. they may be causal factors responsible for these processes). To counteract the deleterious effect of oxidatively damaged DNA, all organisms have developed several DNA repair mechanisms. The efficiency of oxidatively damaged DNA repair was frequently found to be decreased in cancer patients. The present work reviews the basis for the biological significance of DNA damage, particularly effects of 8-oxoGua and ethenoadduct occurrence in DNA in the aspect of cancer development, drawing attention to the multiplicity of proteins with repair activities.

A comprehensive survey of human polymorphisms at conserved splice dinucleotides and its evolutionary relationship with alternative splicing

BACKGROUND

Alternative splicing (AS) is a key molecular process that endows biological functions with diversity and complexity. Generally, functional redundancy leads to the generation of new functions through relaxation of selective pressure in evolution, as exemplified by duplicated genes. It is also known that alternatively spliced exons (ASEs) are subject to relaxed selective pressure. Within consensus sequences at the splice junctions, the most conserved sites are dinucleotides at both ends of introns (splice dinucleotides). However, a small number of single nucleotide polymorphisms (SNPs) occur at splice dinucleotides. An intriguing question relating to the evolution of AS diversity is whether mutations at splice dinucleotides are maintained as polymorphisms and produce diversity in splice patterns within the human population. We therefore surveyed validated SNPs in the database dbSNP located at splice dinucleotides of all human genes that are defined by the H-Invitational Database.

RESULTS

We found 212 validated SNPs at splice dinucleotides (sdSNPs); these were confirmed to be consistent with the GT-AG rule at either allele. Moreover, 53 of them were observed to neighbor ASEs (AE dinucleotides). No significant differences were observed between sdSNPs at AE dinucleotides and those at constitutive exons (CE dinucleotides) in SNP properties including average heterozygosity, SNP density, ratio of predicted alleles consistent with the GT-AG rule, and scores of splice sites formed with the predicted allele. We also found that the proportion of non-conserved exons was higher for exons with sdSNPs than for other exons.

CONCLUSIONS

sdSNPs are found at CE dinucleotides in addition to those at AE dinucleotides, suggesting two possibilities. First, sdSNPs at CE dinucleotides may be robust against sdSNPs because of unknown mechanisms. Second, similar to sdSNPs at AE dinucleotides, those at CE dinucleotides cause differences in AS patterns because of the arbitrariness in the classification of exons into alternative and constitutive type that varies according to the dataset. Taking into account the absence of differences in sdSNP properties between those at AE and CE dinucleotides, the increased proportion of non-conserved exons found in exons flanked by sdSNPs suggests the hypothesis that sdSNPs are maintained at the splice dinucleotides of newly generated exons at which negative selection pressure is relaxed.

Up-regulation of 8-oxo-dGTPase activity of MTH1 protein in the brain, testes and kidneys of mice exposed to (137)Cs gamma radiation

Abstract Mammalian MTH1 protein is an antimutagenic (2'-deoxy)ribonucleoside 5'-triphosphate pyrophosphohydrolase that prevents the incorporation of oxidatively modified nucleotides into nucleic acids. It decomposes most specifically the miscoding products of oxidative damage to purine nucleic acid precursors (e.g. 8-oxo-dGTP, 2-oxo-dATP, 2-oxo-ATP, 8-oxo-GTP) that may cause point mutations or transcription errors when incorporated into DNA and RNA, respectively. The increased expression of MTH1 mRNA and MTH1 protein was previously proposed as a molecular marker of oxidative stress. Therefore, we hypothesized that increased 8-oxo-dGTPase activity of MTH1 protein in mouse organs could serve as a dose-dependent marker of exposure to ionizing radiation, which is known to induce oxidative stress. To test our hypothesis, we measured 8-oxo-dGTPase activity in six organs of male BL6 mice after exposure to 0, 10, 25 and 50 cGy and 1 Gy of (137)Cs gamma radiation given as a single whole-body dose (1 Gy/min). The mice were killed 4, 8 and 24 h after irradiation. A statistically significant induction of 8-oxo-dGTPase was found in brains, testes and kidneys but not in lungs, hearts or livers. Brains, which demonstrated the highest (4.3-fold) increase of 8-oxo-dGTPase activity, were shown to express approximately 50% higher levels of MTH1 protein. However, due to the lack of a simple positive correlation between the dose and the observed 8-oxo-dGTPase activity in brain, testes and kidneys, we conclude that measurements of 8-oxo-dGTPase activity in these organs may serve as a rough indicator rather than a quantifiable marker of radiation-induced oxidative stress.

Base excision repair and its role in maintaining genome stability

For all living organisms, genome stability is important, but is also under constant threat because various environmental and endogenous damaging agents can modify the structural properties of DNA bases. As a defense, organisms have developed different DNA repair pathways. Base excision repair (BER) is the predominant pathway for coping with a broad range of small lesions resulting from oxidation, alkylation, and deamination, which modify individual bases without large effect on the double helix structure. As, in mammalian cells, this damage is estimated to account daily for 10(4) events per cell, the need for BER pathways is unquestionable. The damage-specific removal is carried out by a considerable group of enzymes, designated as DNA glycosylases. Each DNA glycosylase has its unique specificity and many of them are ubiquitous in microorganisms, mammals, and plants. Here, we review the importance of the BER pathway and we focus on the different roles of DNA glycosylases in various organisms.

Genetic polymorphisms in hMTH1, hOGG1 and hMYH and risk of chronic benzene poisoning in a Chinese occupational population

Oxidative damage to DNA induced by benzene is an important mechanism of its genotoxicity, which leads to chronic benzene poisoning (CBP). Therefore, genetic variation in DNA repair genes may contribute to susceptibility to CBP in the exposed population. We hypothesized that single nucleotide polymorphisms (SNPs) in hMTH1, hOGG1 and hMYH genes are associated with risk of CBP. We genotyped SNPs at codon 83 of hMTH1, codon 326 of hOGG1, and codon 324 of hMYH in 152 CBP patients and 152 healthy workers occupationally exposed to benzene without poisoning manifestations. The genotypes were determined by polymerase chain reaction-restrained fragment length polymorphism (PCR-RFLP) technique. There were 2.51-fold [adjusted odds ratio (OR(adj)), 2.51; 95% CI, 1.14-5.49; P=0.02] and 2.49-fold (OR(adj), 2.49; 95% CI: 1.52-4.07; P<0.01) increased risk of CBP for individuals carrying genotypes of hMTH1 83Val/Met+Met/Met and hOGG1 326Cys/Cys, respectively. Compared with individuals carrying genotypes of hOGG1 326Cys/Cys and hMYH 324His/His at the same time, there was a 0.33-fold (OR(adj), 0.33; 95% CI: 0.15-0.72; P<0.05) decreased risk of CBP for those with genotypes of hOGG1 326Ser/Cys+Ser/Ser and hMYH 324His/Gln+Gln/Gln. In the smoking group, there was a 0.15-fold (OR(adj), 0.15; 95% CI, 0.03-0.68; P=0.01) decreased risk of CBP for subjects carrying genotypes of hMYH 324His/Gln+Gln/Gln compared with those of genotype of hMYH 324His/His. Therefore, our results suggested that polymorphisms at codons 83 of hMTH1 and codon 326 of hOGG1 might contribute to CBP in a Chinese occupational population.

Oxidative damage in nucleic acids and Parkinson's disease

Oxidative DNA lesions, such as 8-oxoguanine (8-oxoG), accumulate in nuclear and mitochondrial genomes during aging, and such accumulation can increase dramatically in patients with Parkinson's disease (PD). To counteract oxidative damage to nucleic acids, human and rodents are equipped with three distinct enzymes. One of these, MTH1, hydrolyzes oxidized purine nucleoside triphosphates, such as 8-oxo-2'-deoxyguanosine triphosphate and 2-hydroxy-2'-deoxyadenosine triphosphate, to their monophosphate forms. The other two enzymes are 8-oxoG DNA glycosylase encoded by the OGG1 gene and adenine/2-hydroxyadenine DNA glycosylase encoded by the MUTYH gene. We have shown a significant increase in 8-oxoG in mitochondrial DNA as well as an elevated expression of MTH1, OGG1, and MUTYH in nigrostriatal dopaminergic neurons of PD patients, suggesting that the buildup of these lesions may cause dopamine neuron loss. We established MTH1-null mice and found that MTH1-null fibroblasts were highly susceptible to cell death caused by H(2)O(2) characterized by pyknosis and electron-dense deposits in the mitochondria, and that this was accompanied by an ongoing accumulation of 8-oxoG in nuclear and mitochondrial DNA. We also showed that MTH1-null mice exhibited an increased accumulation of 8-oxoG in striatal mitochondrial DNA, followed by more extreme neuronal dysfunction after 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine administration than that of wild-type mice. In conclusion, oxidative damage in nucleic acids is likely to be a major risk factor for Parkinson's disease, indicating that a solid understanding of the defense mechanisms involved will enable us to develop new strategies for protecting the brain against oxidative stress.

Analysis of the base excision repair genes MTH1, OGG1 and MUTYH in patients with squamous oral carcinomas

A number of environmental factors, such as tobacco and alcohol, have been implicated, through oxidative DNA damage, in the development of squamous cell carcinomas of the head and neck (SCCHN). Several pathways are involved in the repair of DNA lesions caused by oxidative stress, such as the base excision repair system (BER), which repairs mutation involving 8-oxoguanine and comprises the MUTYH, OGG1 and MTH1 genes. We analysed 29 patients, assessing germline polymorphisms or mutations in these genes by complete genomic sequencing of exons and adjacent intronic regions. Thirty healthy blood donors served as controls. No pathogenic germline mutations were identified. We found common and rare new variants in the coding and adjacent intronic regions. In summary, our data do not support a major role for MUTYH, OGG1 and MTH1 variants in the etiology of sporadic squamous oral/oropharyngeal carcinomas. This does not exclude the involvement of the three BER genes in the tumorigenesis of SCCHN through other mechanisms such as promotor hypermethylation, genomic rearrangements or mutations involving regulatory sequences.

Mutagenesis and carcinogenesis caused by the oxidation of nucleic acids

Genomes and their precursor nucleotides are highly exposed to reactive oxygen species, which are generated both as byproducts of oxygen respiration or molecular executors in the host defense, and by environmental exposure to ionizing radiation and chemicals. To counteract such oxidative damage in nucleic acids, mammalian cells are equipped with three distinct enzymes. MTH1 protein hydrolyzes oxidized purine nucleoside triphosphates, such as 8-oxo-2'-deoxyguanosine triphosphate and 2-hydroxy-2'-deoxyadenosine triphosphate (2-OH-dATP), to the corresponding monophosphates. We observed increased susceptibility to spontaneous carcinogenesis in MTH1-null mice, which exhibit an increased occurrence of A:T-->C:G and G:C-->T:A transversion mutations. 8-Oxoguanine (8-oxoG) DNA glycosylase, encoded by the OGG1 gene, and adenine DNA glycosylase, encoded by the MUTYH gene, are responsible for the suppression of G:C to T:A transversions caused by the accumulation of 8-oxoG in the genome. Deficiency of these enzymes leads to increased tumorigenesis in the lung and intestinal tract in mice, respectively. MUTYH deficiency may also increase G:C to T:A transversions through the misincorporation of 2-OH-dATP, especially in the intestinal tract, since MUTYH can excise 2-hydroxyadenine opposite guanine in genomic DNA and the repair activity is selectively impaired by a mutation found in patients with autosomal recessive colorectal adenomatous polyposis.

The GT to GC single nucleotide polymorphism at the beginning of an alternative exon 2C of human MTH1 gene confers an amino terminal extension that functions as a mitochondrial targeting signal

Human MTH1 protein hydrolyzes oxidized purine nucleotides 8-oxo-2'-deoxyguanosine triphosphate (8-oxo-dGTP), 2-OH-dATP or their ribo-forms to their monophosphates, thus minimizing replicational and transcriptional errors both in the nuclei and mitochondria. MTH1 suppresses mitochondrial dysfunction and cell death caused by H(2)O(2). Furthermore, MTH1 suppresses the transient increase in 8-oxoguanine in mitochondrial DNA in the dopaminergic nerve terminals in mouse striatum after 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine administration, and it protects the nerve terminals. We previously reported that a novel MTH1 allele with a single nucleotide polymorphism (SNP) in its exon 2c segment encodes the fourth MTH1 isoform, namely, MTH1a (p26), in addition to the three known isoforms, MTH1b (p22), c (p21), and d (p18). Another SNP located in exon 4 of the MTH1 gene, which is closely linked to the SNP in exon 2c, substitutes the Val83 residue in MTH1d with Met83. We herein show that all MTH1 isoforms efficiently hydrolyzed 2-OH-dATP and 8-oxo-dGTP. The amino terminal region of MTH1a functioned as a mitochondrial targeting signal when it was expressed in the HeLa cells as a fusion protein with enhanced green fluorescent protein. The cellular fractionation revealed that MTH1a(Met83) was localized in the mitochondria to the same extent as was MTH1d(Val83). However, the mitochondrial translocation of MTH1d(Met83) was less efficient than that of MTH1d(Val83).

Modulation of oxidative mutagenesis and carcinogenesis by polymorphic forms of human DNA repair enzymes

Chromosome DNA is continuously exposed to various endogenous and exogenous mutagens. Among them, oxidation is one of the most common threats to genetic stability, and multiple DNA repair enzymes protect chromosome DNA from the oxidative damage. In Escherichia coli, three repair enzymes synergistically reduce the mutagenicity of oxidized base 8-hydroxy-guanine (8-OH-G). MutM DNA glycosylase excises 8-OH-G from 8-OH-G:C pairs in DNA and MutY DNA glycosylase removes adenine incorporated opposite template 8-OH-G during DNA replication. MutT hydrolyzes 8-OH-dGTP to 8-OH-dGMP in dNTP pool, thereby reducing the chance of misincorporation of 8-OH-dGTP by DNA polymerases. Simultaneous inactivation of MutM and MutY dramatically increases the frequency of spontaneous G:C to T:A mutations, and the deficiency of MutT leads to the enhancement of T:A to G:C transversions more than 1000-fold over the control level. In humans, the functional homologues of MutM, MutY and MutT, i.e., OGG1, MUTYH (MYH) and MTH1, contribute to the protection of genomic DNA from oxidative stress. Interestingly, several polymorphic forms of these proteins exist in human populations, and some of them are suggested to be associated with cancer susceptibility. Here, we review the polymorphic forms of OGG1, MUTYH and MTH1 involved in repair of 8-OH-G and 8-OH-dGTP, and discuss the significance of the polymorphisms in the maintenance of genomic integrity. We also summarize the polymorphic forms of human DNA polymerase eta, which may be involved in damage tolerance and mutagenesis induced by oxidative stress.

Association study of human MTH1 gene polymorphisms with type 1 diabetes mellitus

Reactive oxygen species are considered to play a role in the development of diabetes mellitus and its complications. Human MTH1 (mutT homologue 1) has 8-oxo-7,8-dihydrodeoxyguanosine triphosphatase activity, which repairs oxidized forms of dGTP. This enzyme is known to have a thermolabile Met83 variant. We examined whether Val83Met polymorphism of human MTH1 gene is associated with type 1 diabetes mellitus. We recruited 156 type 1 diabetic patients (59 males and 97 females). The polymorphism was analyzed by restriction fragment length polymorphism analysis with Nsi I. The Met/Met genotype at codon 83 was very rare in both control and patient groups. Val/Met genotype tended to be more frequent in the whole type 1 diabetic patients than in controls. When subjects were divided into subgroups according to gender, there were no differences in the genotype and allele frequencies between patients and controls in males. On the other hand, in female type 1 diabetic patients, the Val/Met genotype was more frequent than in female controls (corrected P = 0.102). The Met allele was significantly more frequent in female type 1 diabetic patients than in female controls (corrected P = 0.022). Our results suggested that the Met allele at codon 83 of MTH1 gene might be involved in the development of type 1 diabetes mellitus in the Japanese female population.

Oxidative DNA damage and disease: induction, repair and significance

The generation of reactive oxygen species may be both beneficial to cells, performing a function in inter- and intracellular signalling, and detrimental, modifying cellular biomolecules, accumulation of which has been associated with numerous diseases. Of the molecules subject to oxidative modification, DNA has received the greatest attention, with biomarkers of exposure and effect closest to validation. Despite nearly a quarter of a century of study, and a large number of base- and sugar-derived DNA lesions having been identified, the majority of studies have focussed upon the guanine modification, 7,8-dihydro-8-oxo-2'-deoxyguanosine (8-OH-dG). For the most part, the biological significance of other lesions has not, as yet, been investigated. In contrast, the description and characterisation of enzyme systems responsible for repairing oxidative DNA base damage is growing rapidly, being the subject of intense study. However, there remain notable gaps in our knowledge of which repair proteins remove which lesions, plus, as more lesions identified, new processes/substrates need to be determined. There are many reports describing elevated levels of oxidatively modified DNA lesions, in various biological matrices, in a plethora of diseases; however, for the majority of these the association could merely be coincidental, and more detailed studies are required. Nevertheless, even based simply upon reports of studies investigating the potential role of 8-OH-dG in disease, the weight of evidence strongly suggests a link between such damage and the pathogenesis of disease. However, exact roles remain to be elucidated.

Biological significance of the defense mechanisms against oxidative damage in nucleic acids caused by reactive oxygen species: from mitochondria to nuclei

In mammalian cells, more than one genome in a single cell has to be maintained throughout the entire life of the cell, namely, one in the nucleus and the other in the mitochondria. The genomes and their precursor nucleotides are highly exposed to reactive oxygen species, which are inevitably generated as a result of the respiratory function in mitochondria. To counteract such oxidative damage in nucleic acids, cells are equipped with several defense mechanisms. Modified nucleotides in the nucleotide pools are hydrolyzed, thus avoiding their incorporation into DNA or RNA. Damaged bases in DNA with relatively small chemical alterations are mainly repaired by the base excision repair (BER) system, which is initiated by the excision of damaged bases by specific DNA glycosylases. MTH1 protein hydrolyzes oxidized purine nucleoside triphosphates, such as 8-oxo-dGTP, 8-oxo-dATP, and 2-hydroxy (OH)-dATP to the monophosphates, and MTH1 are located in the cytoplasm, mitochondria, and nucleus. We observed an increased susceptibility to spontaneous carcinogenesis in Mth1-deficient mice and an alteration of MTH1 expression along with the accumulation of 8-oxo-dG in patients with various neurodegenerative diseases. Enzymes for the BER pathway, namely, 8-oxoG DNA glycosylase (OGG1), 2-OH-A/adenine DNA glycosylase (MUTYH), and AP endonuclease (APEX2) are also located both in the mitochondria and in the nuclei, and the expression of mitochondrial OGG1 is altered in patients with various neurodegenerative diseases. We also observed increased susceptibilities to spontaneous carcinogenesis in OGG1 and MUTYH-deficient mice. The increased occurrence of lung tumor in OGG1-deficient mice was completely abolished by the concomitant disruption of the Mth1 gene.

An oxidized purine nucleoside triphosphatase, MTH1, suppresses cell death caused by oxidative stress

MTH1 hydrolyzes oxidized purine nucleoside triphosphates such as 8-oxo-2'-deoxyguanosine 5'-triphosphate (8-oxo-dGTP) and 2-hydroxy-2'-deoxyadenosine 5'-triphosphate (2-OH-dATP) and thus protects cells from damage caused by their misincorporation into DNA. In the present study, we established MTH1-null mouse embryo fibroblasts that were highly susceptible to cell dysfunction and death caused by exposure to H2O2, with morphological features of pyknosis and electron-dense deposits accumulated in mitochondria. The cell death observed was independent of both poly(ADP-ribose) polymerase and caspases. A high performance liquid chromatography tandem mass spectrometry analysis and immunofluorescence microscopy revealed a continuous accumulation of 8-oxo-guanine both in nuclear and mitochondrial DNA after exposure to H2O2. All of the H2O2-induced alterations observed in MTH1-null mouse embryo fibroblasts were effectively suppressed by the expression of wild type human MTH1 (hMTH1), whereas they were only partially suppressed by the expression of mutant hMTH1 defective in either 8-oxo-dGTPase or 2-OH-dATPase activity. Human MTH1 thus protects cells from H2O2-induced cell dysfunction and death by hydrolyzing oxidized purine nucleotides including 8-oxo-dGTP and 2-OH-dATP, and these alterations may be partly attributed to a mitochondrial dysfunction.

Inhibition of 8-oxo-2'-deoxyguanosine 5'-triphosphate pyrophosphohydrolase (8-oxo-dGTPase) activity of the antimutagenic human MTH1 protein by nucleoside 5'-diphosphates

The hMTH1 protein, a human homologue of E. coli MutT protein, is an enzyme converting 8-oxo-2'-deoxyguanosine 5'-triphosphate (8-oxo-dGTP) to 8-oxo-2'-deoxyguanosine 5'-monophosphate (8-oxo-dGMP) and inorganic pyrophosphate. It is thought to play an antimutagenic role by preventing the incorporation of promutagenic 8-oxo-dGTP into DNA. As found in our previous investigations, 8-oxo-2'-deoxyguanosine 5'-diphosphate (8-oxo-dGDP) strongly inhibited 8-oxo-dGTPase activity of MTH1. Following this finding, in the present study we have tested the canonical ribo- and deoxyribonucleoside 5'-diphosphates (NDPs and dNDPs) for possible inhibition of 8-oxo-dGTP hydrolysis by hMTH1 extracted from CCRF-CEM cells (a human leukemia cell line). Among them, the strongest inhibitors appeared to be dGDP (Ki=74 microM), dADP (Ki=147 microM), and GDP (Ki=502 microM). Other dNDPs and NDPs, such as dCDP, dTDP, ADP, CDP, and UDP were much weaker inhibitors, with Ki in the millimolar range. Based on the present results and published data, we estimate that the strongest inhibitors, dGDP and dADP, at physiological concentrations not exceeding 5 microM and GDP at mean concentration of 30 microM, taken together, can decrease the cellular hMTH1 enzymatic activity vs. 8-oxo-dGTP (expected to remain below 500 pM) by up to 15%. The other five NDPs and dNDPs tested cannot markedly affect this activity.

Enzymology of the repair of free radicals-induced DNA damage

A number of intrinsic and extrinsic mutagens induce structural damage in cellular DNA. These DNA damages are cytotoxic, miscoding or both and are believed to be at the origin of cell lethality, tissue degeneration, ageing and cancer. In order to counteract immediately the deleterious effects of such lesions, leading to genomic instability, cells have evolved a number of DNA repair mechanisms including the direct reversal of the lesion, sanitation of the dNTPs pools, mismatch repair and several DNA excision pathways including the base excision repair (BER) nucleotide excision repair (NER) and the nucleotide incision repair (NIR). These repair pathways are universally present in living cells and extremely well conserved. This review is focused on the repair of lesions induced by free radicals and ionising radiation. The BER pathway removes most of these DNA lesions, although recently it was shown that other pathways would also be efficient in the removal of oxidised bases. In the BER pathway the process is initiated by a DNA glycosylase excising the modified and mismatched base by hydrolysis of the glycosidic bond between the base and the deoxyribose of the DNA, generating a free base and an abasic site (AP-site) which in turn is repaired since it is cytotoxic and mutagenic.

Oxidative nucleotide damage: consequences and prevention

8-Oxoguanine (8-oxo-7,8-dihydroguanine) is produced in DNA, as well as in nucleotide pools of cells, by reactive oxygen species normally formed during cellular metabolic processes. 8-Oxoguanine nucleotide can pair with cytosine and adenine nucleotides with an almost equal efficiency, then transversion mutation ensues. MutT protein of Escherichia coli and related mammalian protein MTH1 specifically degrade 8-oxo-dGTP to 8-oxo-dGMP, thereby preventing misincorporation of 8-oxoguanine into DNA. The bacterial and mammalian enzymes are close in their size and share a highly conserved region consisting of 23 residues with 14 identical amino acids. Following saturation mutagenesis of this region, most of these residues proved to be essential to exert 8-oxo-dGTPase activity. Gene targeting was done to establish MTH1-deficient cell lines and mice for study. When examined 18 months after birth, a greater number of tumors were formed in the lungs, livers, and stomachs of MTH1(-/-) mice, as compared with findings in wild-type mice. These proteins protect genetic information from untoward effects of threats of endogenous oxygen.

Clonal proteomics: one gene - family of proteins

The work presented here attempts to consolidate our knowledge on cellular transcriptome and proteome. It takes into account that a typical activated cell (lymphocyte) contains 40 000 mRNA molecules at any time, and it represents about 5000 different molecular species of transcripts. Such a cell has about 1 000 000 000 protein molecules, some of them being present at 10 000 000 copies while others at a very low copy number (say 1 to 10 copies per cell). By studying cell free expression of individual cDNA clones (or pools of known complexity) we address to those rare molecular components that will remain undetected by the current analytical means. For our analysis we use cell free translation systems (wheat germ or rabbit reticulocyte origin) and we study polypeptide products originating from intact, or restriction endonuclease-treated cDNA clones. We conclude that in most instances expressed genes yield transcript(s) that translate into several, and often very numerous families of polypeptide species. In our ISODALT two-dimensional gel system we characterize the proteomic profile of the clonal polypeptide families in terms of their molecular mass, charge, multiple products, and appearance.

A molecular basis for the selective recognition of 2-hydroxy-dATP and 8-oxo-dGTP by human MTH1

MTH1 hydrolyzes oxidized purine nucleoside triphosphates such as 8-oxo-dGTP, 8-oxo-dATP, 2-hydroxy-dATP, and 2-hydroxy rATP to monophosphates, and thus avoids errors caused by their misincorporation during DNA replication or transcription, which may result in carcinogenesis or neurodegeneration. This substrate specificity for oxidized purine nucleoside triphosphates was investigated by mutation analyses based on the sequence comparison with the Escherichia coli homolog, MutT, which hydrolyzes only 8-oxo-dGTP and 8-oxo-rGTP but not oxidized forms of dATP or ATP. Neither a replacement of the phosphohydrolase module of MTH1 with that of MutT nor deletions of the C-terminal region of MTH1, which is unique for MTH1, altered the substrate specificity of MTH1. In contrast, the substitution of residues at position Trp-117 and Asp-119 of MTH1, which showed apparent chemical shift perturbations with 8-oxo-dGDP in NMR analyses but are not conserved in MutT, affected the substrate specificity. Trp-117 is essential for MTH1 to recognize both 8-oxo-dGTP and 2-hydroxy-dATP, whereas Asp-119 is only essential for recognizing 2-hydroxy-dATP, thus suggesting that origins of the substrate-binding pockets for MTH1 and MutT are different.

Regulation of intracellular localization of human MTH1, OGG1, and MYH proteins for repair of oxidative DNA damage

In mammalian cells, more than one genome has to be maintained throughout the entire life of the cell, one in the nucleus and the other in mitochondria. It seems likely that the genomes in mitochondria are highly exposed to reactive oxygen species (ROS) as a result of their respiratory function. Human MTH1 (hMTH1) protein hydrolyzes oxidized purine nucleoside triphosphates, such as 8-oxo-dGTP, 8-oxo-dATP, and 2-hydroxy (OH)-dATP, thus suggesting that these oxidized nucleotides are deleterious for cells. Here, we report that a single-nucleotide polymorphism (SNP) in the human MTH1 gene alters splicing patterns of hMTH1 transcripts, and that a novel hMTH1 polypeptide with an additional mitochondrial targeting signal is produced from the altered hMTH1 mRNAs; thus, intracellular location of hMTH1 is likely to be affected by a SNP. These observations strongly suggest that errors caused by oxidized nucleotides in mitochondria have to be avoided in order to maintain the mitochondrial genome, as well as the nuclear genome, in human cells. Based on these observations, we further characterized expression and intracellular localization of 8-oxoG DNA glycosylase (hOGG1) and 2-OH-A/adenine DNA glycosylase (hMYH) in human cells. These two enzymes initiate base excision repair reactions for oxidized bases in DNA generated by direct oxidation of DNA or by incorporation of oxidized nucleotides. We describe the detection of the authentic hOGG1 and hMYH proteins in mitochondria, as well as nuclei in human cells, and how their intracellular localization is regulated by alternative splicing of each transcript.

Intracellular distribution of the antimutagenic enzyme MTH1 in the liver, kidney and testis of F344 rats and its modulation by cadmium

Cellular distribution of the antimutagenic MTH1protein in the liver, kidney, and testis of Fischer rat was evaluated using the immunohistochemical staining with anti-MTH1 polyclonal antibody. The present investigation revealed a non-uniform distribution of MTH1 among cells and among the cytoplasmic, nuclear, and membranal structures of cells within a given tissue. A particularly strong expression of MTH1 was observed for the first time in the perinuclear acrosomic bodies of spermatocytes and in the acrosomic vesicles of sperm heads. Treatment of rats with a single sc dose of 20 micromol Cd(II)/kg body wt. produced histopathologic changes in these organs accompanied by redistribution of the cellular MTH1 protein between the cytoplasm and nuclei. The acute phase of Cd(II) toxicity, that in the liver and especially in the testes (but not in kidneys) led to cell necrosis, was accompanied by a characteristic decrease in the abundance of MTH1-expressing nuclei. Chronic toxicity without necrosis, persisting in the kidney over the entire 14-day study, as well as the survival and proliferation of cells, observed in the liver and testis after the necrotizing phase, were signified by increased number of nuclei expressing MTH1. Thus, unlike previous biochemical studies, immunohistochemistry managed to reveal alterations in the patterns of inter- and intracellular distribution of MTH1, associated apparently with the conditional changes in the dynamics of synthesis of nucleic acids, assisted by this protein.

Multiple forms of formamidopyrimidine-DNA glycosylase produced by alternative splicing in Arabidopsis thaliana

Formamidopyrimidine-DNA glycosylase (FPG) catalyzes the initial steps in the repair of DNA containing oxidized purines. Two cDNA clones from Arabidopsis thaliana encoding homologs of bacterial FPG have previously been described. We now report that there are at least five additional variants of FPG mRNA in Arabidopsis, each apparently produced from the same gene (AtMMH) by alternative splicing. Thus, AtMMH, like at least four other genes in the base excision repair pathway of human cells, produces multiple forms of protein product through alternative splicing. The variant forms of Arabidopsis FPG may be localized in different locations in the cells, may have different preferences for oxidized substrates, and/or may recruit different proteins that guide the subsequent steps of base excision repair.

Molecular genetics and structural biology of human MutT homolog, MTH1

The human MTH1 gene located on chromosome 7p22 consists of 5 major exons. MTH1 gene produces seven types of mRNAs and the B-type mRNAs with exon 2b-2c segments direct synthesis of three forms of MTH1 polypeptides (p22, p21, and p18) by alternative initiation of translation, while the others encode only p18. In human cells, p18, the major form is mostly localized in the cytoplasm with some in the mitochondria. A single nucleotide polymorphism (SNP) in exon 2, which is tightly liked to another SNP (GTG83/ATG83), creates an additional alternative in-frame AUG in B-type MTH1 mRNAs yielding the fourth MTH1 polypeptide, p26 that possesses an additional mitochondrial targeting signal. These SNPs are likely to be one of the risk factors for cancer or for neuronal degeneration. The 30 amino acid residues are identical between MTH1 and MutT, and there is a highly conserved region consisting of 23 residues (MTH1: Gly36 to Gly58), with 14 identical residues. A chimeric protein in which the 23 residue sequence of MTH1 was replaced with that of MutT, retains the capability to hydrolyze 8-oxo-dGTP, indicating that the 23 residue sequences of MTH1 and MutT are functionally and structurally equivalent, and constitute a functional phosphohydrolase module. Saturated mutagenesis of the module in MTH1 indicated that an amphipathic property of the alpha-helix I consisting of 14 residues of the module (Thr44 to Gly58) is essential to maintain the stable catalytic surface for 8-oxo-dGTPase. MTH1 but not MutT efficiently hydrolyzes two forms of oxidized dATP, 2-hydroxy-dATP and 8-oxo-dATP, as well as 8-oxo-dGTP and 8-oxo-GTP. Thus, MTH1 is designated as the oxidized purine nucleoside triphosphatase and has a much wider substrate specificity than MutT. There is a significant homology between MTH1 protein and the C-terminal half of human MYH protein, which may be involved in the recognition of 8-oxoguanine and 2-hydroxyadenine.

Alternative forms of formamidopyrimidine-DNA glycosylase from Arabidopsis thaliana

Formamidopyrimidine-DNA glycosylase (FPG) catalyzes the initial steps in the repair of DNA containing oxidized purines. Two complementary DNA clones encoding homologs of bacterial FPG, designated Atfpg-1 and Atfpg-2, have been isolated from Arabidopsis thaliana. They are products of alternative splicing of the transcript of a single gene. Proteins encoded by both clones, AtFPG-1 and AtFPG-2, engineered to contain oligohistidine sequences on their C-terminal ends, were expressed in Escherichia coli and purified, and their activities were assayed. Both proteins cleaved DNA that contained apurinic sites, indicating that they have abasic lyase activity. AtFPG-1, but not AtFPG-2, showed significant cleavage of a double-stranded oligonucleotide that contained 8-oxo-guanine, indicating that the structural differences between the two proteins influence their enzymatic activities. However, both proteins were able to cleave the same sites in DNA that was treated with visible light in the presence of methylene blue.

A valine to methionine polymorphism at codon 83 in the 8-oxo-dGTPase gene MTH1 is not associated with sporadic Parkinson's disease

Recently, 8-oxo-7,8-dihydrodeoxyguanosine triphosphatase (8-oxo-dGTPase; MTH1), a key enzyme for preventing oxidative stress-induced DNA damage, has been found to be expressed aberrantly in the nigrostriatal dopaminergic neurones in the brains of those with Parkinson's disease (PD). A valine (Val) to methionine (Met) polymorphism at codon 83 in exon 4 of the MTH1 gene was studied in 73 patients with sporadic PD and 151 age-matched non-PD controls by PCR-RFLP analysis, to determine a possible association of this polymorphism with development of PD. The frequency of either 83Val or 83Met allele was not statistically different between PD patients (92.5% or 7.5%) and the controls (88.7% or 11.3%) (chi(2) = 1.511, P = 0.2190). The 83Met/Met homozygotes consisting of an infrequent genotype in the control population (1.3%) were not found in the PD group. The frequency of both 83Val/Met heterozygotes and 83Met/Met homozygotes combined was not statistically different between PD patients (15.1%) and the controls (21.2%), compared with that of the 83Val/Val homozygotes (chi(2) = 1.190, P = 0.2754). These results indicate that the 83Val/Met polymorphism in the MTH1 gene is not associated with an increased risk for development of sporadic PD.