Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: UMLS:C0596263 (carcinogenesis)
 64,820 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Nitrosoureas form O6-alkylguanine-DNA adducts that are converted to G to A transitions, the mutation found in the activated ras oncogenes of nitrosourea-induced mouse lymphomas and rat mammary tumors. These adducts are removed by the DNA repair protein O6-alkylguanine-DNA alkyltransferase. Transgenic mice that express the human homolog of this protein in the thymus were found to be protected from developing thymic lymphomas after exposure to N-methyl-N-nitrosourea. Thus, transgenic expression of a single human DNA repair gene is sufficient to block chemical carcinogenesis. The transduction of DNA repair genes in vivo may unravel mechanisms of carcinogenesis and provide therapeutic protection from known carcinogens.
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PMID:The prevention of thymic lymphomas in transgenic mice by human O6-alkylguanine-DNA alkyltransferase. 842 82

O6-Methylguanine-DNA methyltransferase (MGMT) is a DNA repair protein that plays an important role in chemotherapy, mutagenesis, and carcinogenesis. Recombinant human MGMT was isolated from an Escherichia coli high performance expression system and purified to homogeneity. The kinetic and DNA-binding properties of the recombinant human MGMT were studied. The purified human MGMT reacted stoichiometrically with methylated DNA under second-order rate kinetics. The rate constant with normal methylated DNA was 1 x 10(9) M-1 min-1 at 37 degrees C. The binding to DNA was the rate determining step in the repair process. Approximately eight base pairs of the DNA substrate were covered by the human MGMT protein. The affinity constant for interaction of DNA to MGMT was approximately 4.7 x 10(5) M-1. The binding to methylated DNA was also examined; the binding affinity to methylated DNA was two times higher than that to unmodified DNA. The interaction with DNA induced a conformational change in the human MGMT protein as monitored by circular dichroism and fluorescence analysis. A similar conformational change was induced by both methylated and unmodified DNA.
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PMID:Kinetic and DNA-binding properties of recombinant human O6-methylguanine-DNA methyltransferase. 842 52

The targeting of mRNA with antisense oligonucleotides is increasingly employed to inhibit the expression of gene function. Since the level of the DNA repair protein O6-methylguanine-DNA methyltransferase (MGMT) is decisive in protection of cells against damage produced by alkylating agents, including cytostatic drugs, the targeted inhibition of this repair activity might be of importance for therapeutic approaches. In order to investigate whether antisense targeted MGMT depletion is feasible to transiently modify the sensitivity of cells to anticancer drugs, we studied the expression of MGMT and cellular sensitivity upon inhibitor and antisense treatment using CHO transfectants expressing human MGMT. It was shown by polymerase chain reaction that antisense oligonucleotides specifically inhibited MGMT mRNA level. Nevertheless, MGMT protein was found not to be reduced significantly, as demonstrated by Western blotting. Correspondingly, no significant decrease in MGMT activity was observed, as measured 36 h after MGMT antisense oligonucleotide administration. Given together with the MGMT depleting agent O6-methylguanine, reduction in MGMT protein as well as activity was found. MGMT antisense oligonucleotide enhanced the sensitivity of cells to the tumor therapeutic drug mitozolomide, as measured by sister chromatid exchange formation. This sensitization was further enhanced by combined treatment with antisense oligonucleotide and O6-methylguanine, indicating that MGMT antisense can be supportive in sensitization of cells to an alkylating drug.
Carcinogenesis 1996 Jan
PMID:Targeting of O6-methylguanine-DNA methyltransferase (MGMT) activity by antimessenger oligonucleotide sensitizes CHO/Mex+ transfected cells to mitozolomide. 856 32

The inter- and intracellular distribution of the DNA repair protein O6-alkylguanine-DNA alkyltransferase (ATase) may be an important factor in the sensitivity or resistance of tumours to treatment with certain alkylating agents, including the methyltriazenes and nitrosoureas. In order to examine this issue 26 human brain tumour sections (23 high grade gliomas and three low grade gliomas) were examined for ATase expression by immunohistochemistry using a rabbit anti-human ATase polyclonal antibody. Positive staining, seen as fine black granules mainly confined to the nucleus, was observed in all the glioma sections examined. There was marked cellular heterogeneity, ranging from cells completely devoid of staining to cells with very intense staining. Semi-quantitatively, in the 23 high grade gliomas examined six had 1+ staining, seven had 2+ staining and 10 had 3+ staining, whereas all three low grade gliomas had 1+ staining. These results are in contrast to published reports showing that approximately 35% of human brain tumour-derived cell lines and xenografts had very low levels of ATase activity and suggest that the complete lack of ATase is not a common occurrence in high grade glioma.
Carcinogenesis 1996 Apr
PMID:Inter- and intracellular heterogeneity of O6-alkylguanine-DNA alkyltransferase expression in human brain tumors: possible significance in nitrosourea therapy. 862 71

To examine the miscoding properties of modified guanine residues bearing increasingly bulky O6-substituents, Rat4 cells, grown in the presence of O6-benzylguanine to deplete the DNA repair protein O6-alkylguanine-DNA alkyltransferase, were transfected with plasmids carrying H-ras genes in which O6-methyl, O6-ethyl- and O6-benzylguanine were substituted for the first, second or both the first and second guanine residues of codon 12 (GGA). DNA from isolated transformed colonies was amplified by PCR and directly sequenced by high-temperature manual and automated methods. The results show that O6-ethylguanine and O6-benzylguanine induced semi-targeted as well as targeted mutations, in contrast to O6-methylguanine, which induced only targeted mutations. When incorporated in place of the first guanine of H-ras codon 12, the targeted mutations induced by all these modified guanines were exclusively G-->A transitions. When incorporated at the second position of codon 12, O6-benzylguanine induced G-->A, G-->T and G-->C mutations. O6-Ethylguanine at the second position induced chiefly G-->A transitions, and O6-methylguanine induced G-->A transitions exclusively. Semi-targeted mutations were strictly G-->A at the base 3' to a position 1 adduct or 5' to a position 2 adduct. The mechanism for induction of targeted mutations probably involves decreasing preference to thymidine incorporation opposite an O6-modified guanine as the size of the O6-substituent increases, while the mechanism for non-targeted mutations may be related to abasic site formation or to translesion synthesis which might be made error-prone by obstructive DNA lesions in this context.
Carcinogenesis 1996 Apr
PMID:O6-ethylguanine and O6-benzylguanine incorporated site-specifically in codon 12 of the rat H-ras gene induce semi-targeted as well as targeted mutations in Rat4 cells. 862

Exposure to exogenous alkylating agents, particularly N-nitroso compounds, has been associated with increased incidence of primary human brain tumors, while intrinsic risk factors are currently unknown. The DNA repair protein O6-methylguanine-DNA methyltransferase (MGMT) is a major defense against the carcinogenicity of N-nitroso compounds and other alkylators. We report here that in 55% (64/117) of cases, histologically normal brain tissue adjacent to primary human brain tumors lacked detectable MGMT activity [methyl excision repair-defective (Mer-) status]. The incidence of Mer- status in normal brain tissue from brain tumor patients was age-dependent, increasing from 21% in children 0.25-19 years of age to 75% in adults over 50. In contrast, Mer- status was found in 12% (5/43) of normal brain specimens from patients operated for conditions other than primary brain tumors and was not age-dependent. The 4.6-fold elevation in incidence of Mer- status in brain tumor patients is highly significant (chi2 = 24; p < or = 0.001). MGMT activity was independent of age in the lymphocytes of brain tumor patients and was present in lymphocytes from six of nine tumor patients whose normal brain specimen was Mer-. DNA polymerase beta, apurinic/apyrimidinic endonuclease, and lactate dehydrogenase activities were present in all specimens tested, including Mer- specimens from brain tumor patients. Our data are consistent with a model of carcinogenesis in human brain in which epigenetically regulated lack of MGMT is a predisposing factor and alkylation-related mutagenesis is a driving force.
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PMID:Lack of the DNA repair protein O6-methylguanine-DNA methyltransferase in histologically normal brain adjacent to primary human brain tumors. 869 23

Carcinogenesis proceeds in discrete steps involving initiation and promotion. There is ample evidence that the underlying cause of initiation is mutation, whereas for tumor promotion different hypotheses exist postulating the involvement of both epigenetic and genetic changes. DNA repair protects against tumor formation, but it has not been proven whether protection occurs at the level of tumor initiation or promotion. Since the most advanced experimental system for studying multistep carcinogenesis is the mouse skin, we generated transgenic mice that overexpress the human DNA repair protein O6-methylguanine-DNA methyltransferase (MGMT) in their epidermal cells by virtue of cytokeratin (Ck) promoters. Total cellular methyltransferase activity was found to be significantly higher in skin protein extracts of transgenic as compared to nontransgenic mice. CkMGMT transgenic mice along with nontransgenic controls were treated according to the multistage skin carcinogenesis protocol. For initiation, a single subthreshold dose of N-nitroso-N-methylurea (MNU) or 7,12-dimethylbenz(a)anthracene (DMBA) was topically applied to the dorsal skin of the mice. Tumor promotion was carried out by repeated 12-O-tetradecanoylphorbol-13-acetate application. Our results clearly show that CkMGMT transgenic mice are strongly protected against MNU- but not DMBA-initiated skin tumor formation. As compared to nontransgenic controls, transgenic mice exhibited an approximately 6-fold reduction of skin tumor incidence after treatment with 20 micromol or 50 micromol MNU followed by 12-O-tetradecanoylphorbol-13-acetate. These results provide direct and the most compelling evidence to date that the DNA lesion O6-methylguanine is of decisive importance in tumor initiation, and that the protective effect of the repair protein MGMT in carcinogenesis is due to prevention of initiation without affecting tumor promotion.
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PMID:Targeted expression of human O(6)-methylguanine-DNA methyltransferase (MGMT) in transgenic mice protects against tumor initiation in two-stage skin carcinogenesis. 876 16

N-Methylpurine-DNA glycosylase (MPG), a ubiquitous DNA repair protein, removes several N-alkylpurine adducts, hypoxanthine, cyclic ethenoadducts of adenine, guanine and cytosine and 8-oxoguanine from DNA. The recombinant human and mouse MPGs, purified from Escherichia coli, show a significant difference in substrate preference. While both proteins prefer 3-methyladenine over other N-alkylpurines in DNA, the mouse MPG removes 7-methylguanine and 3-methylguanine at an approximately 2- to 3-fold higher rate than the human protein when adjusted for equal activity for the release of 3-methyladenine from DNA. Hybrid recombinant proteins containing N-terminal and C-terminal halves of the human and mouse glycosylases were partially purified from MPG-negative E.coli. Their substrate preferences suggest that the N-terminal half is more critical for the recognition of 3-methylguanine and 7-methylguanine.
Carcinogenesis 1996 Oct
PMID:Distinct substrate preference of human and mouse N-methylpurine-DNA glycosylases. 889 86

O6-benzylguanine (BG) is a potent, specific inactivator of the DNA repair protein O6-alkylguanine-DNA alkyltransferase (AGT), which enhances sensitivity to nitrosoureas in cells and tumor-bearing animals. BG is presently undergoing clinical trials for development as an agent to enhance the therapeutic index of alkylating agent chemotherapy. It has been reported that a polymorphism exists in the human agt gene, with about 15% of the Japanese population having arginine at codon 160 instead of glycine on the polypeptide (Y. Imai et al., Carcinogenesis, 16: 2441-2445, 1995). The resultant mutant AGT protein is equally effective against both methylated DNA as compared with wild type protein. However, this mutant AGT protein was less sensitive to inactivation by BG with a 20-fold increase in the ED50 value. This observation raised the possibility that a subpopulation of patients may be resistant to BG due to a single base change. We have demonstrated that this alteration also reduces the sensitivity to O6-benzyl-8-oxoguanine, an equally potent, yet much longer-lived human metabolite of BG. To test the possibility that this germ-line mutation of the agt gene might explain resistance to BG and O6-benzyl-8-oxoguanine of patients on our Phase I clinical trials, we evaluated the DNA from lymphocytes of 18 patients. The G160R mutation was not found in any of the 18 patients. To determine the frequency of this mutation in the United States population, DNA from 181 healthy individuals were investigated and, again, the mutation was not observed in this cohort. Therefore, if the mutation exists, it is in statistically <1.6% of the United States noncancerous population. To investigate the possibility that this mutation might be somatic, we evaluated genomic DNA samples from 94 human primary cancers of four different histological subtypes (brain, colon, esophageal, and head and neck). Again, none were found to have the G160R mutation.
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PMID:Lack of evidence for a polymorphism at codon 160 of human O6-alkylguanine-DNA alkyltransferase gene in normal tissue and cancer. 991 21

Microsatellite instability (MSI) is a form of genomic instability in tumors that may reflect mechanisms underlying carcinogenesis. Assessment of MSI in various types of sporadic tumors is therefore relevant to an understanding of molecular pathogenesis. In the case of sporadic adult gliomas, destabilization of mononucleotide, dinucleotide, and longer repeat sequences has been reported in high-grade tumors, though published estimates of the frequency of MSI vary widely. In the present work, we quantitated the frequency of length alterations at three microsatellite loci in 26 glioma/normal tissue pairs and at nine additional loci in 16 of the pairs. We analyzed di- and tetranucleotide markers, including five previously reported to be unstable in gliomas. and examined mostly high-grade tumors, both diploid and aneuploid. A large proportion of the tumor and normal brain specimens had no detectable activity of the DNA repair protein O6-methylguanine-DNA methyltransferase, a prevalent phenotypic trait in these tissues that we thought might be associated with MSI. We observed no length alterations in 222 sequence analyses, and estimate the frequency of MSI in our tumor sample as < 0.45% unstable sequences among all sequences examined, or < 3.9% gliomas with unstable sequences. We conclude that microsatellite length alterations are infrequent in our tumor population, and interpret currently available literature to indicate that the frequency of MSI is low in sporadic adult gliomas.
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PMID:Microsatellite instability is infrequent in sporadic adult gliomas. 1010 Jul 59


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