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

The CHO-UV-1 mutant, a Chinese hamster ovary cell with defective postreplication recovery of DNA, is 2- to 4-fold more sensitive than its wild-type counterpart (CHO-77256) to the lethal effects of ethylating agents and UV radiation; it is also hypersensitive (10- to 20-fold) to some DNA-methylating and -cross-linking agents. We studied the CHO-UV-1 mutant further to define its phenotype in terms of DNA damage induction and repair, methyltransferase activity, and effects of caffeine on mutational and lethal responses. Both wild-type and CHO-UV-1 cells incurred similar levels and types of damage when exposed to UV radiation, N-methyl-N'-nitro-N-nitrosoguanidine, or N-methyl-N-nitrosourea. The rate and extent of repair of Micrococcus luteus endonuclease-sensitive sites after UV irradiation or treatment with N-methyl-N'-nitro-N-nitrosoguanidine were also equivalent in these two cell types. Twenty % of the initial endonuclease-sensitive sites induced in either cell line remained at 18 h after UV irradiation; approximately 8% of the sites after N-methyl-N'-nitro-N-nitrosoguanidine exposure were present in both parental and CHO-UV-1 cells after a 17-h repair period. Moreover, the ability of CHO-UV-1 to resynthesize and ligate DNA during excision repair was similar to that of its parent. Neither CHO-UV-1 nor CHO-77256 had appreciable levels of O6-methylguanine-DNA methyltransferase activity which ameliorates the cytotoxicity of alkylating agents. Caffeine, a known inhibitor of postreplication repair, decreased the frequency of mutation induction at the hypoxanthine-guanine phosphoribosyltransferase locus by 40-55% in CHO-77256 but not in CHO-UV-1. These results rule out defective excision repair as a factor in the hypersensitivity of the CHO-UV-1 mutant to DNA-damaging agents. Hence, this cell line appears to derive from a mutation affecting nonexcision repair processes and should be useful in clarifying the mechanism(s) of postreplication recovery of DNA in mammalian cells.
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PMID:Genetic and biochemical characterization of the CHO-UV-1 mutant defective in postreplication recovery of DNA. 231 21

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

Fifteen variants with >/=30-fold resistance to N-methyl-N-nitrosourea were isolated from the Burkitt's lymphoma Raji cell line. Eight had received a single treatment with a highly cytotoxic dose. The remainder, including the previously described RajiF12 cell line, arose following multiple exposures to initially moderate but escalating doses. Surprisingly, methylation resistance arose in three clones by reactivation of a previously silent O6-methylguanine-DNA methyltransferase gene. Five clones, including RajiF12, displayed the microsatellite instability and increased spontaneous mutation rates at the hypoxanthine-guanine phosphoribosyltransferase locus, consistent with deficiencies in mismatch repair. Defects in either the hMutSalpha or hMutLalpha mismatch repair complexes were identified in extracts of these resistant clones by in vitro complementation using extracts from colorectal carcinoma cell lines. Defects in hMutLalpha were confirmed by Western blot analysis. Remarkably, five methylation-resistant clones in which mismatch repair defects were demonstrated by biochemical assays did not exhibit significant microsatellite instability.
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PMID:Mismatch repair defects and O6-methylguanine-DNA methyltransferase expression in acquired resistance to methylating agents in human cells. 935 25

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