Organization and expression of the mouse MTH1 gene for preventing transversion mutation

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.

Molecular devices for high fidelity of DNA replication and gene expression

Certain types of DNA lesions, produced through cellular metabolic processes and also by external environmental stresses, are responsible for the induction of mutations as well as of cancer. Most of these lesions can be eliminated by DNA repair enzymes, and cells carrying the remaining DNA lesions are subjected to apoptosis. The persistence of damaged bases in RNA can cause errors in gene expression, and the cells appear to possess a mechanism which can prevent damaged RNA molecules from entering the translation process. We have investigated these processes for high fidelity of DNA replication and gene expression, by using both biochemical and genetic means. We herein describe (1) the molecular mechanisms for accurate DNA synthesis, (2) mammalian proteins for sanitizing the DNA precursor pool, (3) error avoidance mechanisms for gene expression under oxidative stress, and (4) the roles of DNA repair and apoptosis in the prevention of cancer.

A quest to understand molecular mechanisms for genetic stability

In the midst of the post-war turmoil in Japan, I fortunately followed a path to become a scientist. Sometime at an early stage of my career, I encountered the problem of the cellular response to DNA damage and had the chance to discover a DNA repair enzyme. This event greatly influenced the subsequent course of my research, and I extended my studies toward elucidating the molecular mechanisms of mutagenesis as well as of carcinogenesis. Through these studies I came to understand the importance of mechanisms for dealing with the actions of reactive oxygen species to the living systems. These recollections deal with these endeavors with emphasis on the early part of my scientific career.

Cellular 8-oxo-7,8-dihydro-2'-deoxyguanosine 5'-triphosphate pyrophosphohydrolase activity of human and mouse MTH1 proteins does not depend on the proliferation rate

Mammalian MTH1 proteins, homologs of Escherichia coli MutT, are enzymes decomposing 8-oxo-7,8-dihydro-2'-deoxyguanosine 5'-triphosphate (8-oxo-dGTP) to 8-oxo-7,8-dihydro-2'-deoxyguanosine 5'-monophosphate and inorganic pyrophosphate. They play an antimutagenic role by preventing the incorporation of promutagenic 8-oxo-dGTP into DNA. MTH1 gene expression is higher in some physiological types of mammalian cells and in numerous cancer cells, but the mechanism of that upregulation still remains unclear. It has been hypothesized that MTH1 expression might be associated with a proliferation rate of the cells. Therefore, we tested this hypothesis by comparing the functional levels of MTH1 gene expression measured as the 8-oxo-dGTPase activity of its protein products in normal mouse livers and hepatectomized regenerating livers. Although the proliferation rate of the hepatocytes in the regenerating livers was much higher than that in control livers, as confirmed by immunohistochemical assay of proliferating cell nuclear antigen, the 8-oxo-dGTPase activity was not different. In a second approach, we used 57 lines of human cancer cells in which 8-oxo-dGTPase activity was measured and confronted with cell population doubling time. No significant correlations between 8-oxo-dGTPase activity and proliferation rate were observed within groups of six leukemia, eight melanoma, nine lung, seven colon, six central nervous system, six ovarian, eight renal, and seven breast cancer cell lines. Thus, we conclude that the MTH1 expression manifested as the 8-oxo-dGTPase activity of its protein products in mammalian cells is not associated with proliferation rate. Our results will help in further testing of the hypothesis that MTH1 overexpression may be a specific marker of carcinogenesis and/or oxidative stress.

Genomic structure and analysis of transcriptional regulation of the mouse zinc-fingers and homeoboxes 1 (ZHX1) gene

The mouse zinc-fingers and homeoboxes 1 (ZHX1) gene was cloned and its transcriptional regulatory mechanism analysed. The mouse ZHX1 gene spans approximately 29 kb and consists of five exons. Exons 1-3 contain the nucleotide sequence of the 5'-noncoding region of mouse ZHX1 cDNA, exon 4 contains a part of the 5'-noncoding region, an entire coding sequence, and a part of the 3'-noncoding sequence, and exon 5 contains the resulting 3'-noncoding sequence. The ZHX1 gene exists as one copy in the haploid mouse genome. Two species of ZHX1 mRNA with or without the nucleotide sequence of the third exon are produced by an alternative splicing. To investigate the regulatory elements involved in the transcription of the ZHX1 gene, transient DNA transfection experiments with ZHX1/firefly luciferase reporter genes were performed using a lipofection method. Functional analyses of a series of 5'- and 3'-deletion constructs of the reporter genes revealed that the nucleotide sequence between -59 and +50 is required for full promoter activity in mouse embryonal carcinoma F9 cells. Two positive regulatory cis-acting elements in the region were identified. These elements, designated as Box A and Box B, are located between nucleotides -47 and -42 and +22 and +27, respectively, and synergistically stimulate transcription of the mouse ZHX1 gene. Electrophoretic mobility shift assays with specific competitors and antibodies show that PEA3 and Yin and Yang 1 (YY1) bind to Box A and Box B, respectively. Thus, we conclude that PEA3 and YY1 synergistically stimulate the transcription of the ZHX1 gene.

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.

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.

Spontaneous tumorigenesis in mice defective in the MTH1 gene encoding 8-oxo-dGTPase

Oxygen radicals, which can be produced through normal cellular metabolism, are thought to play an important role in mutagenesis and tumorigenesis. Among various classes of oxidative DNA damage, 8-oxo-7,8-dihydroguanine (8-oxoG) is most important because of its abundance and mutagenicity. The MTH1 gene encodes an enzyme that hydrolyzes 8-oxo-dGTP to monophosphate in the nucleotide pool, thereby preventing occurrence of transversion mutations. By means of gene targeting, we have established MTH1 gene-knockout cell lines and mice. When examined 18 months after birth, a greater number of tumors were formed in the lungs, livers, and stomachs of MTH1-deficient mice, as compared with wild-type mice. The MTH1-deficient mouse will provide a useful model for investigating the role of the MTH1 protein in normal conditions and under oxidative stress.

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.

Analysis of MTH1 gene function in mice with targeted mutagenesis

Oxidative DNA damage is thought to contribute to carcinogenesis, ageing, and neurological degeneration. Further, the cumulative risk of cancer increases dramatically with age in humans. In general terms, cancer can be regarded as a degenerative disease of ageing. There is evidence for the accumulation of oxidative DNA damage with age based on studies mainly measuring an increase in 8-oxoguanine. 8-Oxo-7,8-dihydro-2'-deoxyguanosine 5'-triphosphate (8-oxo-dGTP) is formed in the nucleotide pool of a cell during normal cellular metabolism. When 8-oxoguanine is incorporated into DNA causes mutation. Organisms possess 8-oxo-dGTPase, an enzyme that specifically degrades 8-oxo-dGTP to 8-oxo-dGMP. To analyze the function of MTH1 with 8-oxo-dGTPase activity in vivo, we generated a mouse line carrying a mutant MTH1 allele created by targeted gene disruption. MTH1 homozygous mutant mice were found to have a physically normal appearance, but seemed to have lost 8-oxo-dGTPase activity in liver extracts. When we examined the susceptibility of the mutant mice to spontaneous tumorigenesis, no significant difference was observed in survival rate of MTH1+/+ and MTH1-/- mice. However, pathological examination revealed a statistically significant difference in the incidence of tumors. More tumors were formed in lungs, livers, and stomachs of MTH1-/- mice than in those of the wild type mice. These studies with MTH1-null mutant mice provided an important insight into the role of this nucleotide sanitization enzyme in terms of the spontaneous tumorigenesis as well as mutagenesis caused by the oxygen-induced DNA damage.

Activity of the antimutagenic enzyme 8-oxo-2'-deoxyguanosine 5'-triphosphate pyrophosphohydrolase (8-oxo-dGTPase) in cultured chinese hamster ovary cells: effects of cell cycle, proliferation rate, and population density

Mammalian 8-oxo-2'-deoxyguanosine 5'-triphosphate pyrophosphohydrolases (8-oxo-dGTPases), such as MTH1, are believed to play the same antimutagenic role as their bacterial homologues, like MutT. Both decompose promutagenic 8-oxo-dGTP, a product of active oxygen's attack on dGTP. It is not known how 8-oxo-dGTPase expression and function are regulated. Therefore, we investigated the effect of cell population density, proliferation rate, and cell cycle phase on 8-oxo-dGTPase specific activity in cultured Chinese hamster ovary K1-BH4 (CHO) cells. With increasing cell population density (from 30 to 95% confluence), the activity of 8-oxo-dGTPase per milligram protein decreased by 33% (p =.007 by ANOVA) while cells shifted by 9% into the G(0)/G(1) phase, with a 5% drop in cells in S phase. Importantly, inhibition of the cells' proliferation rate by calf serum deprivation caused a more dramatic 23% shift toward the G(0)/G(1) phase and a 25% drop in S phase, but had no effect on 8-oxo-dGTPase activity. Likewise, no differences in the enzyme activity were observed within cell populations of different cell cycle phases separated by centrifugal elutriation. Thus, the present results exclude cell cycle-dependent regulation of 8-oxo-dGTPase activity in CHO cells or its simple dependence on proliferation rate. The observed decrease of 8-oxo-dGTPase activity with increasing cell population density might be related to augmentation of cell-to-cell contact.

Expression and differential intracellular localization of two major forms of human 8-oxoguanine DNA glycosylase encoded by alternatively spliced OGG1 mRNAs

We identified seven alternatively spliced forms of human 8-oxoguanine DNA glycosylase (OGG1) mRNAs, classified into two types based on their last exons (type 1 with exon 7: 1a and 1b; type 2 with exon 8: 2a to 2e). Types 1a and 2a mRNAs are major in human tissues. Seven mRNAs are expected to encode different polypeptides (OGG1-1a to 2e) that share their N terminus with the common mitochondrial targeting signal, and each possesses a unique C terminus. A 36-kDa polypeptide, corresponding to OGG1-1a recognized only by antibodies against the region containing helix-hairpin-helix-PVD motif, was copurified from the nuclear extract with an activity introducing a nick into DNA containing 8-oxoguanine. A 40-kDa polypeptide corresponding to a processed form of OGG1-2a was detected in their mitochondria using antibodies against its C terminus. Electron microscopic immunocytochemistry and subfractionation of the mitochondria revealed that OGG1-2a locates on the inner membrane of mitochondria. Deletion mutant analyses revealed that the unique C terminus of OGG1-2a and its mitochondrial targeting signal are essential for mitochondrial localization and that nuclear localization of OGG1-1a depends on the NLS at its C terminus.

Regulation of expression of the human MTH1 gene encoding 8-oxo-dGTPase. Alternative splicing of transcription products

The enzyme 8-oxo-7,8-dihydrodeoxyguanosine triphosphatase (8-oxo-dGTPase) hydrolyzes 8-oxo-dGTP to 8-oxo-dGMP, thereby preventing misincorporation of 8-oxo-dGTP into DNA. We investigated expression of MTH1 gene encoding 8-oxo-dGTPase. Large amounts of MTH1 mRNA were present in thymus and testis, embryonic tissues, and certain cell lines. In peripheral blood lymphocytes, the level of MTH1 mRNA was significantly increased after concomitant treatment with phytohemagglutinin and interleukin-2. Analyses of the 5' regions of the MTH1 transcripts revealed that 7 types of MTH1 mRNAs, which may be produced by transcription initiation at different sites and/or alternative splicing. The MTH1 gene consists of 5 major exons, some of which are composed of differentially processed segments. All types of MTH1 mRNAs carry the entire coding region, and may be functional. Three ATG initiation codons in-frame were found in the 5' regions of some of the MTH1 mRNAs. There is a polymorphic alteration at the 5' splicing site (GT to GC) located in exon 2, an event which affects splicing patterns of the MTH1 transcript. Allele frequency of this polymorphism is about 20% among healthy volunteers.