Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound   Target Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound

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Query: EC:2.4.2.8 (hypoxanthine-guanine phosphoribosyltransferase)
2,527 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Inosine 5 -monophosphate dehydrogenase (IMPDH) is a rate-limiting enzyme for the synthesis of GTP and dGTP. Two isoforms of IMPDH have been identified. IMPDH Type I is ubiquitous and predominantly present in normal cells, whereas IMPDH Type II is predominant in malignant cells. IMPDH plays an important role in the expression of cellular genes, such as p53, c-myc and Ki-ras. IMPDH activity is transformation and progression linked in cancer cells. IMPDH inhibitors, tiazofurin, selenazofurin, and benzamide riboside share similar mechanism of action and are metabolized to their respective NAD analogues to exert antitumor activity. Tiazofurin exhibits clinical responses in patients with acute myeloid leukemia and chronic myeloid leukemia in blast crisis. These responses relate to the level of the NAD analogue formed in the leukemic cells. Resistance to tiazofurin and related IMPDH inhibitors relate mainly to a decrease in NMN adenylyltransferase activity. IMPDH inhbitors induce apoptosis. IMPDH inhitors are valuable probes for examining biochemical functions of GTP as they selectively reduce guanylate concentration. Incomplete depletion of cellular GTP level seems to down-regulate G-protein function, thereby inhibit cell growth or induce apoptosis. Inosine 5'-monophosphate dehydrogenase (IMPDH, EC 1.1.1.205) catalyzes the dehydrogenation of IMP to XMP utilizing NAD as the proton acceptor. Studies have demonstrated that IMPDH is a rate-limiting step in the de novo synthesis of guanylates, including GTP and dGTP. The importance of IMPDH is central because dGTP is required for the DNA synthesis and GTP plays a major role not only for the cellular activity but also for cellular regulation. Two isoforms of IMPDH have been demonstrated. IMPDH Type I is ubiquitous and predominately present in normal cells, whereas the IMPDH Type II enzyme is predominant in malignant cells. Although guanylates could be salvaged from guanine by the enzyme hypoxanthine-guanine phosphoribosyltransferase (EC 2.4.2.8), the level of circulating guanine is low in dividing cells and this route is probably insufficient to satisfy the needs of guanylates in the cells.
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PMID:Consequences of IMP dehydrogenase inhibition, and its relationship to cancer and apoptosis. 1039 Jun 1

The first p53 gene mutation arising in a human tumor was described a decade ago by Baker et al. [S.J. Baker, E.R. Fearon, J.M. Nigro, S.R. Hamilton, A.C. Preisinger, J.M. Jessup, P. van Tuinen, D.H. Ledbetter, D.F. Barker, Y. Nakamura, R. White, B. Vogelstein, Chromosome 17 deletions and p53 gene mutations in colorectal carcinomas, Science 244 (1989) 217-221]. There are now over 10,000 mutations extracted from the published literature in the IARC database of human p53 tumor mutations [P. Hainaut, T. Hernandez, A. Robinson, P. Rodriguez-Tome, T. Flores, M. Hollstein, C.C. Harris, R. Montesano, IARC database of p53 gene mutations in human tumors and cell lines: updated compilation, revised formats and new visualization tools, Nucleic Acids Res. 26 (1998) 205-213; Version R3, January 1999]. A large and diverse collection of tumor mutations in cancer patients provides important information on the nature of environmental factors or biological processes that are important causes of human gene mutation, since xenobiotic mutagens as well as endogenous mechanisms of genetic change produce characteristic types of patterns in target DNA [J.H. Miller, Mutational specificity in bacteria, Annu. Rev. Genet. 17 (1983) 215-238; T. Lindahl, Instability and decay of the primary structure of DNA, Nature 362 (1993) 709-715; S.P. Hussain, C.C. Harris, Molecular epidemiology of human cancer: contribution of mutation spectra studies of tumor suppressor genes, Cancer Res. 58 (1998) 4023-4037; P. Hainaut, M. Hollstein, p53 and human cancer: the first ten thousand mutations, Adv. Cancer Res. 2000]. P53 gene mutations in cancers can be compared to point mutation spectra at the HPRT locus of human lymphocytes from patients or healthy individuals with known exposure histories, and accumulated data indicate that mutation patterns at the two loci share certain general features. Hypotheses regarding specific cancer risk factors can be tested by comparing p53 tumor mutations typical of a defined patient group against mutations generated experimentally in rodents or in prokaryotic and eukaryotic cells in vitro. Refinements of this approach to hypothesis testing are being explored that employ human p53 sequences introduced artificially into experimental organisms used in laboratory mutagenesis assays. P53-specific laboratory models, combined with DNA microchips designed for high through-put mutation screening promise to unmask information currently hidden in the compilation of human tumor p53 mutations.
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PMID:New approaches to understanding p53 gene tumor mutation spectra. 1063 83

Molecular epidemiology is a new and evolving area of research, combining laboratory measurement of internal dose, biologically effective dose, biologic effects, and influence of individual susceptibility with epidemiologic methodologies. Biomarkers evaluated were selected according to basic scheme: biomarkers of exposure--metabolites in urine, DNA adducts, protein adducts, and Comet assay parameters; biomarkers of effect--chromosomal aberrations, sister chromatid exchanges, micronuclei, mutations in the hypoxanthine-guanine phosphoribosyltransferase gene, and the activation of oncogenes coding for p53 or p21 proteins as measured on protein levels; biomarkers of susceptibility--genetic polymorphisms of genes CYP1A1, GSTM1, GSTT1, NAT2. DNA adducts measured by 32P-postlabeling are the biomarker of choice for the evaluation of exposure to polycyclic aromatic hydrocarbons. Protein adducts are useful as a biomarker for exposure to tobacco smoke (4-aminobiphenyl) or to smaller molecules such as acrylonitrile or 1,3-butadiene. Of the biomarkers of effect, the most common are cytogenetic end points. Epidemiologic studies support the use of chromosomal breakage as a relevant biomarker of cancer risk. The use of the Comet assay and methods analyzing oxidative DNA damage needs reliable validation for human biomonitoring. Until now there have not been sufficient data to interpret the relationship between genotypes, biomarkers of exposure, and biomarkers of effect for assessing the risk of human exposure to mutagens and carcinogens.
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PMID:Molecular epidemiology studies on occupational and environmental exposure to mutagens and carcinogens, 1997-1999. 1069 23

Molecular analysis of mutations at the hypoxanthine-guanine phosphoribosyltransferase (HPRT) locus in peripheral blood T-lymphocytes can provide information on mechanisms of somatic in vivo mutation in populations exposed to exogenous carcinogens and in individuals with inherent susceptibility to cancer and other diseases. To study possible mutational changes associated with smoking as a risk factor for lung cancer, we analyzed HPRT mutations in T-cells of newly diagnosed, nonsmoking and smoking lung cancer patients before treatment. Reverse transcriptase polymerase chain reaction (RT-PCR) and DNA sequencing methods were used to identify 146 independent mutations, 73 each from 32 nonsmoking and 31 smoking cases. In 35 T-cell mutants, the HPRT cDNA showed loss of an entire exon, indicating a splicing mutation. Among the remaining 111 fully characterized mutations in the coding region, single base pair (bp) substitutions predominated with 79% (48/61) in nonsmokers and 90% (45/50) in smokers. Frameshift and small deletion (1-24 bp) mutations were found in 18 mutants. The distribution of base pair substitutions was nonrandom, with significant clustering at previously identified hotspot positions 143, 197 and 617 in the HPRT coding sequence (P< or =0.008). One additional hotspot, GC-->TA at position 606, was observed only in smokers (P=0.006). The frequency of GC>TA transversions was higher in smokers (13%) than in nonsmokers (6%). Conversely, smokers had a lower frequency of GC>AT transitions (24%) than nonsmokers (35%). This smoking-associated shift of the HPRT mutational spectrum, although not statistically significant, is consistent with the in vitro mutagenicity of benzo(a)pyrene (BaP), a prominent carcinogen of tobacco smoke, and with known differences in the TP53 mutational spectrum in lung tumors of smokers and nonsmokers. Among nonsmokers, the HPRT mutational spectra in healthy population controls and lung cancer patients were similar, but there was a marginally significant difference (P=0.07) in the distribution of base pair substitutions between smoking controls and patients. These results suggest that (i) general mechanisms of somatic mutagenesis in individuals with possible predisposition to cancer (e.g. nonsmoking lung cancer patients) are not different from those in normal healthy individuals, and (ii) the HPRT gene in T-cells is a useful reporter locus for smoking-associated somatic in vivo mutations occurring early in lung cancer development.
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PMID:Mutational spectra at the hypoxanthine-guanine phosphoribosyltransferase (HPRT) locus in T-lymphocytes of nonsmoking and smoking lung cancer patients. 1086 57

Folic acid deficiency acts synergistically with alkylating agents to increase genetic damage at the HPRT locus in Chinese hamster ovary cells in vitro and in rat splenocytes in vivo. The present studies extend these observations to human cells and, in addition, investigate the role of p53 activity on mutation induction. The human lymphoblastoid cell lines TK6 and WTK1 are derived from the same parental cell line (WI-L2), but WTK1 expresses mutant p53. Treatment of folate-replete or deficient WTK1 and TK6 cells with increasing concentrations (0-50microg/ml) of ethyl methanesulfonate (EMS) resulted in significantly different HPRT mutation dose-response relationships (P<0.01), indicating that folate deficiency increased the EMS-induced mutant frequency in both cell lines, but with a greater effect in TK6 cells. Molecular analyses of 152 mutations showed that the predominant mutation (65%) in both cell types grown in the presence or absence of folic acid was a G>A transition on the non-transcribed strand. These transitions were mainly at non-CpG sites, particularly when these bases were flanked 3' by a purine or on both sides by G:C base pairs. A smaller number of G>A transitions occurred on the transcribed strand (C>T=14%), resulting in 79% total G:C>A:T transitions. There were more genomic deletions in folate-deficient (15%) as compared to replete cells (4%) of both cell types. Mutations that altered RNA splicing were common in both cell types and under both folate conditions, representing 33% of the total mutations. These studies indicate that cells expressing p53 activity exhibit a higher rate of mutation induction but are more sensitive to the toxic effects of alkylating agents than those lacking p53 activity. Folate deficiency tends to reduce toxicity but increase mutation induction after EMS treatment. The p53 gene product did not have a major influence on the molecular spectrum after treatment with EMS, while folate deficiency increased the frequency of deletions in both cell types.
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PMID:The effect of folate deficiency on the cytotoxic and mutagenic responses to ethyl methanesulfonate in human lymphoblastoid cell lines that differ in p53 status. 1116 26

Allelic loss is an important mutational mechanism in human carcinogenesis. Loss of heterozygosity (LOH) at an autosomal locus is one outcome of the repair of DNA double-strand breaks (DSBs) and can occur by deletion or by mitotic recombination. We report that mitotic recombination between homologous chromosomes occurred in human lymphoid cells exposed to densely ionizing radiation. We used cells derived from the same donor that express either normal TP53 (TK6 cells) or homozygous mutant TP53 (WTK1 cells) to assess the influence of TP53 on radiation-induced mutagenesis. Expression of mutant TP53 (Met 237 Ile) was associated with a small increase in mutation frequencies at the hemizygous HPRT (hypoxanthine phosphoribosyl transferase) locus, but the mutation spectra were unaffected at this locus. In contrast, WTK1 cells (mutant TP53) were 30-fold more susceptible than TK6 cells (wild-type TP53) to radiation-induced mutagenesis at the TK1 (thymidine kinase) locus. Gene dosage analysis combined with microsatellite marker analysis showed that the increase in TK1 mutagenesis in WTK1 cells could be attributed, in part, to mitotic recombination. The microsatellite marker analysis over a 64-cM region on chromosome 17q indicated that the recombinational events could initiate at different positions between the TK1 locus and the centromere. Virtually all of the recombinational LOH events extended beyond the TK1 locus to the most telomeric marker. In general, longer LOH tracts were observed in mutants from WTK1 cells than in mutants from TK6 cells. Taken together, the results demonstrate that the incidence of radi-ation-induced mutations is dependent on the genetic background of the cell at risk, on the locus examined, and on the mechanisms for mutation available at the locus of interest.
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PMID:Different mechanisms of radiation-induced loss of heterozygosity in two human lymphoid cell lines from a single donor. 1122 43

We have studied the effects of a defect in the p53 gene on spontaneous and radiation-induced somatic mutation frequencies in vivo by measuring T-cell receptor (TCR) and hypoxanthine phosphoribosyltransferase (HPRT) mutant frequencies (MFs) in p53 deficient mice both before and after exposure to X-irradiation. In the absence of irradiation, the TCR and HPRT mutant frequencies were roughly two-fold higher in p53 null (-/-) mice than in wild-type (+/+) mice. Unexpectedly, the TCR and HPRT MFs were slightly lower in heterozygote p53 (+/-) than in wild-type (+/+) mice, however. After 2 weeks 2Gy whole body irradiation the TCR and HPRT MFs were about two-fold higher in the p53 null (-/-) and p53 (+/-) mice than in the wild-type. Taken together, these findings suggest that a defect in the p53 gene may lead to TCR and HPRT mutants being recovered at higher frequencies in both irradiated and unirradiated mice, but it should be emphasized that the effects we have observed are not particularly strong, albeit that they are statistically significant. Interestingly, several of the highest TCR MF values that we observed in the course of our experiments were recorded in p53 (-/-) animals that had developed thymomas and hence appeared to be cancer prone.
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PMID:Elevated in vivo frequencies of mutant T cells with altered functional expression of the T-cell receptor or hypoxanthine phosphoribosyltransferase genes in p53-deficient mice. 1160 Jan 27

As the primary metabolite of alcohol, acetaldehyde (AA) may be responsible for many pathological effects related to consumption of alcohol, such as esophageal cancer. The spectrum of p53 mutations in esophageal tumors is indicative of the involvement of exogenous agents, such as tobacco smoke. There is, however, no experimental proof for the involvement of alcohol as data on mutational spectrum induced by AA in human genes is completely lacking. The aim of this study is to investigate whether AA leaves mutational fingerprint in the HPRT reporter gene in human peripheral T cells. Pre-existing in vivo HPRT mutants were removed from PHA-stimulated T lymphocytes before in vitro treatment with 2.4 mM AA for 24 h. Following cell growth to allow mutation expression, independent 6-thioguanine-resistant mutants were selected from large numbers of subcultures showing a 3-fold induction of mutant frequency on average. A total of 73 induced and 36 spontaneous mutants were found to carry a missense, nonsense, frameshift or splice mutation. Base substitutions were identified in the coding or splicing sequences of 55 induced and 26 control mutants. The induced base changes were mainly G > A transition (40%, G on non-transcribed strand) followed by A > T transversions (14.5%, A on non-transcribed strand). The control mutants had significantly (P = 0.04) less G > A transition (15.4%) and completely lacked A > T transversions. We also identified 5'-AGG-3' or 5'-AAG-3' as potential target sequences for AA-induced G > A transitions. This specific mutational spectrum induced by AA is consistent with the known formation and persistency of N(2)-ethyl-2'-guanosine adduct and with the predominance of G > A transitions and mutations at A:T base pairs in the p53 gene of esophageal tumors. We conclude that AA may be involved in the pathogenesis of esophageal cancer.
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PMID:Mutational spectrum induced by acetaldehyde in the HPRT gene of human T lymphocytes resembles that in the p53 gene of esophageal cancers. 1169 45

The tumor suppressor p53 plays an important role in guarding the genomic integrity of the cells. The 3'-->5' exonuclease activity of p53 has recently been recognized as a novel biochemical function of this molecule, and has been shown to preferentially excise mismatched nucleotides from DNA and enhance the DNA replication fidelity of polymerase alpha in vitro. The present study further investigated the role of this biochemical function in whole cells by testing the possibility that p53 may reduce mismatched mutations in cells under a stress of DNA replication errors. Cells with different states of p53 expression, either endogenously or ectopically, were exposed to hydroxyurea to induce an imbalance of cellular dNTP pools and cause replication errors. The rates of mutation at the hypoxanthine guanine phosphoribosyltransferase ( HPRT) gene were determined by selecting colonies of HPRT- mutants. Incubation of cells with hydroxyurea induced a similar degree of dNTP pool imbalance in each cell line, but caused significantly more mutations in cells lacking p53 protein expression. The mutation frequency was significantly reduced by introduction of a wild-type p53 expression vector into the p53-null cells. Analysis of the mutants demonstrated that the clones were devoid of HPRT enzyme activity, but appeared to transcribe full-length HPRT mRNA. These data suggest that p53 is able to reduce mutations caused by misincorporation of deoxynucleotides. Thus, the preferential removal of mismatched nucleotides from DNA by p53 may be a mechanism to maintain genomic integrity. Defect in this biochemical function of p53 may contribute to genetic instability associated with cancer development and progression.
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PMID:Suppression of mismatched mutation by p53: a mechanism for guarding genomic integrity. 1186 21

SIN-1 (3-morpholinosydnonimine), the active metabolite of the vasodilator drug molsidomine, decomposes spontaneously in solution. In the presence of oxygen, NO* and O(2)(*-) are released, generating peroxynitrite, a potent oxidizing agent, at a constant rate over a 2 h period. We utilized this system to investigate mechanisms of peroxynitrite-induced cytotoxicity, genotoxicity, apoptosis, and mitochondrial damage in two human lymphoblastoid cell lines carrying either wild-type (TK6 cells) or mutant p53 (WTK-1 cells) genes. Treatment of TK6 cells with 5 mM SIN-1 for 1.5 h resulted in 28 +/- 6% survival 24 h later. Exposure in the presence of different radical scavengers significantly increased survival, as follows: cytochrome c, 96 +/- 3%; Tiron, 69 +/- 0%; SOD plus catalase, 83 +/- 5%; carboxy-PTIO, 87 +/- 3%; and uric acid, 87 +/- 2%. D-mannitol was ineffective in reducing lethality, as were SOD and catalase when added individually or in heat-inactivated form. Spontaneous as well as SIN-1-induced mutant fractions (MF) in both HPRT and TK genes were significantly higher in WTK-1 cells than in TK6 cells (p < 0.05-0.01). Exposure to 2.5 mM SIN-1 induced time-dependent apoptosis in TK6 cells, but not in WTK-1 cells. Mitochondrial membrane depolarization was also observed in both cell lines after SIN-1 treatment. Neutral comet assay demonstrated that SIN-1 treatment resulted in higher levels of DNA double-strand breaks in TK6 cells than in WTK-1 cells. Collectively, these data show that SIN-1 can be used as an effective peroxynitrite generator in cell culture experiments under these experimental conditions, in which it induced a greater apoptotic response but was less potent as a mutagen in TK6 cells compared with WTK-1 cells. Thus, p53 status was an important determinant of SIN-1 induced mutagenesis and apoptosis in these two human lymphoblastoid cell lines.
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PMID:Genotoxicity, mitochondrial damage, and apoptosis in human lymphoblastoid cells exposed to peroxynitrite generated from SIN-1. 1195 39


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