Methylguanine methyltransferase

Cornetta K, Croop J, Dropcho E, Abonour R, Kieran MW, Kreissman S, Reeves L, Erickson LC, Williams DA (2006) A pilot study of dose-intensified procarbazine, CCNU, vincristine for poor prognosis brain tumors utilizing fibronectin-assisted, retroviral-mediated modification of CD34-I- peripheral blood cells with 06-methylguanine DNA methyltransferase. Cancer Gene Ther 13 886-895... 

Abbreviations NMDA N-methyl-D-aspartate GAPDH glycerine aldehyde-3-phosphate dehydrogenase IRE-BP iron responsive element binding protein OMDM transferase O -methylguanine-DNA methyltransferase... 

Esteller M, Hamilton SR, Burger PC, Baylin SB, Herman JG. Inactivation of the DNA repair gene 06-methylguanine-DNA methyltransferase by promoter hypermethylation is a common event in primary human neoplasia. Cancer Res 1999 59 793-797. 

Oligomeric pyruvat-dehydro-genase-complex (PDC), promutagenes DNA base adduct 0(6)-Methylguanosin, 0(6)-Methylguanin-DNA Methyltransferase (MGMT), N-terminal peptide of MGMT 1999 B Factor 100,000,000 (IPCR 1 molecule, ELISA 10-100 amol) Case et al. [29, 40]... 

Despite knowing the structures of all of the DNA lesions, it remains to be determined if any one lesion is more or less toxic to the cells. As discussed above, it was originally thought that DNA interstrand cross-links were the critical lesion. Once it was realized that these lesions are very rare, opinions shifted to suggest that DNA intrastrand cross-links are more cytotoxic. Unfortunately, there is no specific data that implicates either type of lesion in cytotoxicity. For some drugs like nitrosoureas, DNA repair pathways that remove only selected lesions (i.e., 0(6)-methylguanine DNA methyltransferase) have helped to define the role of a particular lesion [24], No separate pathway has been found for repair of a specific cisplatin adduct, so this approach has not been informative. A number of experiments have been performed in which specific adducts on defined DNA sequence, have been transfected into cells. This approach has shown that an adduct inhibits replication or transcription, but this does not directly address the question of mechanism of cytotoxicity. 

Krokan H, Grafstrom RC, Sundqvist K, et al. 1985. Cytotoxicity, thiol depletion and inhibition of O -methylguanine-DNA methyltransferase by various aldehydes in cultured human bronchial fibroblasts. Carcinogenesis (Lond) 6 1755-1760. 

In direct repair the chemical modification that constitutes the lesions is reversed without removing and replacing nucleotides. There are four known direct repair enzymes photolyase, spore photoproduct lyase, methylguanine DNA methyltransferase, and AUcB family oxidative demethylases (1). 

Figure 3 Repair of O methyiguanine by methyiguanine DNA methyltransferase. Methyiguanine DNA methyl transferase has no cofactors and recognizes DNA cooperativeiy. It flips the O methylguanine into its active site and transfers the methyl group to a nucleophilic cysteine in an 2 reaction.

A nearest-neighbour analysis was used to address the contribution of sequence context on O -methylguanine(m G) repair by the E. coli methyltransferases Ada or Ogt, with m G flanked by all permutations of G, A, T, or The Ada methyltransferase demonstrated strong 5 and 3 sequence-specific repair of m G in vivo. The Ada 5 preference decreased in the order Gm GN >Cm GN >... 

Margison, G. P., Cooper, D. P., and Brennand, J. (1985) Cloning of the E. coli (/ -methylguanine and methylphosphotriester methyltransferase gene using a functional DNA repair assay. Nucleic Acids Res. 13, 1939-1952. 

It should be noted that methylation-linked repression of repair genes like that for 06-methylguanine-DNA-methyltransferase will impair DNA repair and will increase mutation rates. Furthermore, methylated CpG sequences are more prone to mutagenesis, because spontaneous deamination of 5-methyl-C will induce a C T transition. Gene silencing by hypermethylation may therefore favor the occurrence of mutational events during tumorigenesis. 

Figure 3-2. Crystals of various proteins from the hyperthermophilic archaeon, Thermococcus kodakaraensis KOD1. A, rod-like crystal of 06-methylguanine-DNA methyltransferase (MGMT) B, plate-like crystal of MGMT C, bar-like crystal of DNA polymerase D, cubic or hexagonal crystals of ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco).

S. Fujiwara, M. Takagi, T. Imanaka, Y. Kai, Hyperthermostable protein structure maintained by intra and inter-helix ion-pairs in archaeal 06-methylguanine-DNA methyltransferase, ]. Mol. Biol. 1999, 292(3), 707-716. 

Base-excision repair requires the recognition and removal of inappropriate bases such as uracil, hypoxanthine, and xanthine from DNA. The DNA glycosylases are enzymes that remove the aberrant base by cleaving the N-glycosyl bond. Direct repair is different than the other repair mechanism in that the mutated base is repaired directly, without removal of the nucleotide. The protein 06-methylguanine DNA methyltransferase, for example, carries our direct repair. 

Yan and his colleagues used a stable isotope labeled proteome internal standard to analyse protein expression alterations during oxidative stress in breast epithelial cells. Affinity chromatography was combined with MS/MS in the proteomic analysis of human 06-methylguanine-DNA methyltransferase. These studies demonstrated that proteomics can elucidate protein expre-ssions associated with the pharmacological and toxic effects of an anticancer drug when integrated with other biochemical methods. 

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