Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: UNIPROT:P00492 (hypoxanthine-guanine phosphoribosyltransferase)
 2,385 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

O6-Methylguanine (m6G) is an altered base produced in DNA by SN1 methylating agents such as N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). This lesion is repaired by the protein O6-methylguanine-DNA methyltransferase (MGMT) in normal human cell lines, but is not repaired in certain human tumor lines that are termed Mex- or Mer-. Compared with repair-proficient cell lines, such repair-deficient tumor lines are hypersensitive to the production by MNNG of sister-chromatid exchanges (SCE), mutations and lethality. We report here that MNNG treatment produces 1 SCE for every 42 +/- 10 m6G formed in the genome of Mer- tumor cells, 1 6TG-resistant mutant for every 8 (range of 5-14) m6G produced statistically in the coding region of the hypoxanthine phosphoribosyltransferase gene, and 1 lethal event per 6650 +/- 1200 m6G. In addition, in vitro base mismatch incision at m6G: BrU pairs was similar to that at m6G: T pairs, the lesions that likely initiate SCE production. We conclude that m6G residues in genomic DNA are very recombinogenic as well as highly mutagenic in Mer- human tumor cells. The results are interpreted in terms of the relationship between methylation-induced SCE and G: T mismatch recognition.
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PMID:On the quantitative relationship between O6-methylguanine residues in genomic DNA and production of sister-chromatid exchanges, mutations and lethal events in a Mer- human tumor cell line. 751 Mar 69

The effect of human O6-methylguanine-DNA methyltransferase (MGMT) on the cytotoxicity, the mutagenicity, and the specific kinds of base substitutions induced by N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) were examined in non-MGMT transfected Chinese hamster ovary cells (CHOM cells) and in those cells which had been transfected with human MGMT complementary DNA (AGT cells). AGT cells containing a high level of human MGMT activity were markedly more resistant to the cytotoxic and mutagenic effects of MNNG than CHOM cells which had no detectable MGMT activity. The dosages of MNNG which reduced to 50% of colony forming ability were estimated to be 0.8 microM for CHOM and 10 microM for AGT cells. The induction frequency of 6-thioguanine-resistant cells was significantly declined in AGT cells. At 4 microM MNNG, this frequency was declined from 273 mutants/10(6) viable CHOM cells to 13 mutants/10(6) viable AGT cells. The entire coding region of the hypoxanthine (guanine) phosphoribosyltransferase (hprt) gene in 37 AGT and 22 CHOM mutants was characterized by direct sequencing of the mRNA-polymerase chain reaction-amplified complementary DNA. Base changes at the intron-exon boundaries of the hprt DNA in the splicing mutants were further examined. Those results indicated that G to A transitions were significantly reduced in MNNG-treated AGT cells (chi 2 test, P < 0.001), suggesting that O6-methylguanine was repaired error free by human MGMT. In contrast, no difference arose in the frequencies of T to C transitions induced by MNNG in these two populations. All of the G to A transitions induced in AGT cells were located on the nontranscribed strand, assuming that the causative lesion was O6-methylguanine (P < 0.05). Such a strand specificity was not observed in CHOM mutants. Most of the G to A transitions observed in CHOM mutants were located at the middle guanine of 5'-GGPu sequences. Transitions observed at these sites, particularly 5'-GGG, were significantly reduced in AGT mutants (P < 0.05). Our results have suggested that human MGMT specifically repairs O6-methylguanine with a preference to remove those located on the transcribed strand and middle guanine of 5'-GGG.
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PMID:Strand- and sequence-specific attenuation of N-methyl-N'-nitro-N-nitrosoguanidine-induced G.C to A.T transitions by expression of human 6-methylguanine-DNA methyltransferase in Chinese hamster ovary cells. 803 7

Cystemustine (N'-(2-chloroethyl)-N-(2-(methylsulphonyl)ethyl)-N'-nitrosourea) is a new chloroethylnitrosourea (CENU) being used in phase II clinical trials of disseminated melanoma. Clinical results show that tumour regression has only been observed in 25% of melanomas treated by CENUs. Tumour resistance to CENU is known to be mainly due to a DNA repair protein, O6-methylguanine-DNA methyltransferase (MGMT). The poor remission rate of melanoma with CENUs is attributed to the fact that metastases contain high MGMT levels. Previously, we have shown that O6-benzyl-N2-acetylguanosine (BNAG), an MGMT inhibitor, can be combined with cystemustine by intravenous administration, and increases the antitumour effect of cystemustine in resistant human melanoma. In the work presented here, we investigated the in vitro pharmacological effect of this combination on the DNA of human melanoma cells (M3Dau cells). A quantitative polymerase chain reaction (QPCR) assay was used to measure DNA damage in a fragment (2.7 kb) of the hprt gene. The results show that treatment with BNAG enhances the number of lesions in the DNA of cystemustine-treated resistant malignant melanocytes, which may account for the high tumour-cell toxicity of the combination of cystemustine and BNAG.
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PMID:Melanoma-cell toxicity of cystemustine combined with O6-benzyl-N2-acetylguanosine. 961 Aug 64

The antioxidant butylated hydroxyanisole (BHA) is a rodent carcinogen that also reduces the mutagenicity and carcinogenicity of other agents. In this study, we have evaluated possible mechanisms for the antimutagenicity of BHA by investigating its effects on N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)-treated Chinese hamster V79 cells. Mutant frequency was determined using the hprt/V79 assay, while plating efficiency was used to measure cytotoxicity, and apoptosis was measured by flow immunofluorocytometry. In addition, DNA strand breaks and the kinetics of strand-break rejoining were investigated by the alkaline elution of DNA and by single-cell gel electrophoresis (SCGE). Although the higher concentration of BHA (0.5 mM) increased the cytotoxicity of MNNG and the lower concentration of BHA (0.25 mM) did not change it, both concentrations were antimutagenic in MNNG-treated cells, with the greater effect occurring at the lower BHA concentration. Neither BHA nor MNNG nor BHA + MNNG increased the level of apoptotic nuclei, and BHA did not change the level of MNNG-induced DNA strand breaks, though it did inhibit their rejoining. Determination of O(6)-methylguanine-DNA-methyltransferase (MGMT) activity confirmed that V79 cells do not synthesize active MGMT protein; MGMT activity was also undetectable after MNNG and BHA + MNNG treatment. The ability of BHA to reduce the level of MNNG-induced mutations did not correlate with cytotoxicity, induction of apoptosis, the level of DNA strand break induction, or MGMT activity. A modified SCGE assay showed that BHA significantly reduced the level of formamidopyrimidine-DNA-glycosylase + endonucleaseIII-sensitive sites, which at least partially are caused by oxidative DNA lesions. The results suggest that the protective effect of BHA on MNNG-induced mutagenicity is best explained by the antioxidative activity of BHA, which may scavenge free radicals that participate in MNNG-induced mutagenicity.
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PMID:Molecular and cellular influences of butylated hydroxyanisole on Chinese hamster V79 cells treated with N-methyl-N'-nitro-N-nitrosoguanidine: antimutagenicity of butylated hydroxyanisole. 1255 89

The DNA repair protein O(6)-methylguanine-DNA methyltransferase (MGMT) protects from toxicity and mutations incurred following alkylating agents by removing O(6)-alkylguanine lesions. Using Mgmt-/- mice, we examined MGMT's role in protecting from in vivo mutations induced by three different alkylating agents, temozolomide (TMZ), 1,3-bis (2-chloroethyl)-1-nitrosourea (BCNU) and cyclophosphamide. Mutant frequencies were determined in the hypoxanthine-guanine phosphoribosyltransferase gene of splenic T-lymphocytes from C57BL/6 mice (Mgmt+/+ and Mgmt-/-) following TMZ, BCNU or cyclophosphamide. Following TMZ, the mutation frequency was significantly greater in Mgmt-/- mice (5.5 and 9.8 x 10(-6) for 7 and 10 mg/kg TMZ, respectively) compared with vehicle-treated mice (1.0 x 10(-6), P <or= 0.05). In contrast, TMZ-induced mutations were not increased over vehicle in Mgmt+/+ mice. The mutation frequency of mice treated with BCNU (7.5 mg/kg) was the same regardless of Mgmt status. Similarly, pretreatment of Mgmt+/+ mice with 30 mg/kg O(6)-benzylguanine, a potent inactivator of MGMT, prior to BCNU (15 mg/kg) did not result in significantly more mutations than mice treated with BCNU alone. Following cyclophosphamide, mutation frequencies significantly increased from 1.8 x 10(-6) in control-treated mice to 12.9 x 10(-6) in Mgmt+/+ and 18.1 x 10(-6) in Mgmt-/- mice, although the difference in Mgmt-/- compared with Mgmt+/+ was not significant. Acrolein and chloroacetaldehyde, metabolites of cyclophosphamide, were not mutagenic in Mgmt+/+ and Mgmt-/- mice. These results demonstrate that MGMT significantly protects against in vivo TMZ-induced mutations and that MGMT deficiency does not result in greater mutation frequency following cyclophosphamide or BCNU compared with wild-type mice.
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PMID:Role of O6-methylguanine-DNA methyltransferase in protecting from alkylating agent-induced toxicity and mutations in mice. 1711 24

Estimates of genotoxic effects of mutagens at low and protracted doses are often based on linear extrapolation of data obtained at relatively high doses. To test the validity of such an approach, a comparison was made between the mutagenicity of N-methyl-N-nitrosourea (MNU) in T-lymphocytes of the rat following two treatment protocols, i.e. sub-chronic exposure to a low dose (15-45 repeated exposures to 1mg/kg of MNU) or acute exposure to a single high dose (15, 30 or 45 mg/kg of MNU). Mutation induction appeared dramatically lower following sub-chronic treatment compared to treatment with a single high exposure. Furthermore, DNA sequence analysis of the coding region of the hprt gene in MNU-induced mutants showed that acute high dose treatment causes mainly GC-->AT base pair changes, whereas sub-chronic treatment results in a significant contribution of AT base pair changes to mutation induction. We hypothesize that O(6)-methylguanine-DNA methyltransferase is saturated after acute treatments, while after sub-chronic treatment most O(6)-methylguanine is efficiently repaired. These data suggest (i) that risk estimations at low and protracted doses of MNU on the basis of linear extrapolation of effects measured at high dose are too high and (ii) that the protective effects of DNA repair processes are relatively strong at low sub-chronic exposure.
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PMID:Reduced methylation-induced mutagenesis in rat splenocytes in vivo by sub-chronic low dose exposure to N-metyl-N-nitrosourea. 1824 17

The primary function of O(6)-alkylguanine-DNA alkyltransferase (AGT) is to maintain genomic integrity in the face of damage by both endogenous and exogenous alkylating agents. However, paradoxically, bacterial and mammalian AGTs have been shown to increase cytotoxicity and mutagenicity of dihaloalkanes and other bis-electrophiles when expressed in bacterial cells. We have extended these studies to mammalian cells using CHO cells that lack AGT expression and CHO cells stably transfected with a plasmid that expresses human AGT. The cytotoxicity of 1,2-dibromoethane, dibromomethane and epibromohydrin was significantly increased by the presence of AGT but cytotoxicity of butadiene diepoxide was not affected. Mutations caused by these agents were assessed using hypoxanthine-guanine phosphoribosyltransferase (HPRT) as a reporter gene. There was a small (c. 2-3-fold) but statistically significant AGT-mediated increase in mutations caused by 1,2-dibromoethane, dibromomethane and epibromohydrin. Analysis of the mutation spectrum induced by 1,2-dibromoethane showed that the presence of AGT also altered the types of mutations with an increase in total base substitution mutants due to a rise in transversions at both G:C and A:T sites. AGT expression also led to mutations arising from the transcribed strand, which were not seen in cells lacking AGT. Although the frequency of deletion mutations was decreased by AGT expression, the formation of large deletions (> or = 3 exons) was increased. This work demonstrates that interaction of AGT with some bis-electrophiles can cause mutagenicity and diminished cell survival in mammalian cells. It is consistent with the hypothesis that DNA-AGT cross-links, which have been characterized in experiments with purified AGT protein and such bis-electrophiles, can be formed in mammalian cells.
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PMID:Alkyltransferase-mediated toxicity of bis-electrophiles in mammalian cells. 1994 75