MTH1, an oxidized purine nucleoside triphosphatase, prevents the cytotoxicity and neurotoxicity of oxidized purine nucleotides

AtNUDX1, an 8-oxo-7,8-dihydro-2'-deoxyguanosine 5'-triphosphate pyrophosphohydrolase, is responsible for eliminating oxidized nucleotides in Arabidopsis

Cellular DNA, RNA and their precursor nucleotides are at high risk of being oxidized by reactive oxygen species. An oxidized base, 8-oxo-7,8-dihydro-2'-(deoxy)guanosine, can pair with both adenine and cytosine, and thus would cause both replicational and translational errors. Previously, we have reported that an Arabidopsis Nudix hydrolase, AtNUDX1, acts to hydrolyze an oxidized deoxyribonucleotide, 8-oxo-7,8-dihydro-2'-deoxyguanosine 5'-triphosphate (8-oxo-dGTP). Here we showed that 8-oxo-dGTP pyrophosphohydrolase activity is not exhibited by any other Arabidopsis Nudix hydrolase. AtNUDX1 acted on an oxidized ribonucleotide, 8-oxo-GTP, with high affinity (K(m) 28.1 microM). In a transcriptional mutational analysis using the lacZ reporter gene, the phenotypic suppression of the lacZ amber mutation in a mutT-deficient Escherichia coli strain caused by the misincorporation of 8-oxo-GTP into the mRNA was significantly diminished by expression of AtNUDX1. These findings suggest that AtNUDX1 prevents transcriptional errors in vivo. A confocal microscopic analysis using a green fluorescent protein (GFP) fusion protein demonstrated that AtNUDX1 is distributed in the cytosol, where the main pool of nucleotides in the cells exists. The level of 8-oxo-guanosine in genomic DNA was significantly increased in knockout nudx1 plants compared with wild-type plants under normal and oxidative stress (3 microM paraquat) conditions. The results obtained here indicate that AtNUDX1 functions in cellular defense against oxidative DNA and RNA damage through the sanitization of their precursor pools in the cytosol in Arabidopsis cells.

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.

Nonpolar isosteres of damaged DNA bases: effective mimicry of mutagenic properties of 8-oxopurines

A substantial fraction of mutations that arise in the cell comes from oxidative damage to DNA bases. Oxidation of purine bases at the 8-position, yielding 8-oxo-G and 8-oxo-A, results in conformational changes (from anti to syn) that cause miscoding during DNA replication. Here we describe the synthesis and biophysical and biochemical properties of low-polarity shape mimics of 8-oxopurines, and we report that these new analogues exhibit remarkable mimicry of the mutagenic properties of the natural damaged bases. A 2-chloro-4-fluoroindole nucleoside (1) was designed as an isosteric analogue of 8-oxo-dG, and a 2-chloro-4-methylbenzimidazole nucleoside (2) as a mimic of 8-oxo-dA. The nucleosides were prepared by reaction of the parent heterocycles with Hoffer's chlorodeoxyribose derivative. Structural studies of the free nucleosides 1 and 2 revealed that both bases are oriented syn, thus mimicking the conformation of the oxopurine nucleosides. Suitably protected phosphoramidite derivatives were prepared for incorporation into synthetic DNAs, to be used as probes of DNA damage responses, and 5'-triphosphate derivatives (3 and 4) were synthesized as analogues of damaged nucleotides in the cellular nucleotide pool. Base pairing studies in 12-mer duplexes showed that 1 and 2 have low affinity for polar pairing partners, consistent with previous nonpolar DNA base analogues. However, both compounds pair with small but significant selectivity for purine partners, consistent with the idea that the syn purine geometry leads to pyrimidine-like shapes. Steady-state kinetics studies of 1 and 2 were carried out with the Klenow fragment of Escherichia coli DNA Pol I (exo-) in single-nucleotide insertions. In the DNA template, the analogues successfully mimicked the mutagenic behavior of oxopurines, with 1 being paired selectively with adenine and 2 pairing selectively with guanine. The compounds showed similar mutagenic behavior as nucleoside triphosphate analogues, being preferentially inserted opposite mutagenic purine partners. The results suggest that much of the mutagenicity of oxopurines arises from their shapes in the syn conformation rather than from electrostatic and hydrogen-bonding effects. The new analogues are expected to be generally useful as mechanistic probes of cellular responses to DNA damage.

Accumulation of 8-oxo-deoxyguanosine in cardiovascular tissues with the development of hypertension

Accumulation of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxo-dG) in DNA is associated with mutagenesis and cell death. Little attention has been given to the biological significance of 8-oxo-dG accumulation in cardiovascular tissues during the different stage of hypertension and its prevention. We thus investigated the levels and localization of both 8-oxo-dG accumulation and expression of MTH1, which hydrolyzes 8-oxo-dGTP to prevent its incorporation into DNA, in the thoracic aorta prepared from stroke-prone spontaneously hypertensive rats (SHRSP) and age-matched Wister-Kyoto rats (WKY), aged 5-32 weeks. HPLC-MS/MS analysis revealed that the levels of nuclear 8-oxo-dG in the aorta increased significantly in SHRSP, but not WKY, with aging. Immunohistochemical study revealed that both TUNEL reactivity and 8-oxo-dG immunoreactivity were increased in smooth muscle cells (SMC) and endothelial cells (EC) of the aorta with aging, and they exhibited similar distributions in serial sections. The number of 8-oxo-dG and TUNEL positive cells in EC, but not in SMC, was significantly higher in SHRSP than WKY at 32 weeks of age. In contrast, the expression levels of Mth1mRNA and MTH1 protein in the aorta were similarly decreased both in SHRSP and WKY with aging. However, the number of MTH1 expressing EC was remarkably increased in the older SHRSP compared to the younger ones or age-matched WKY. Hypertension significantly increased not only 8-oxo-dG accumulation but also the expression of MTH1 in EC of the aorta during aging. While accumulation of 8-oxo-dG in SMC of the aorta was slightly increased, the expression of MTH1 protein in SMC was rather decreased by hypertension. We thus suggest that MTH1 may protect EC in the aorta from the oxidative damage increased by hypertension.