Direct assay for O6-methylguanine-DNA methyltransferase and comparison of detection methods for the methylated enzyme in polyacrylamide gels and electroblots

Nanomaterial-based biosensors for DNA methyltransferase assay

We review the recent advances in the development of nanomaterial-based biosensors for DNA methyltransferase assay.

Target-mediated consecutive endonuclease reactions for specific and sensitive homogeneous fluorescence assay of O6-methylguanine-DNA methyltransferase

O(6)-Methylguanine-DNA methyltransferase (MGMT) is one of the most important DNA-repair enzymes. Herein, a simple, sensitive and selective homogeneous fluorescence assay strategy is developed for the detection of MGMT on the basis of target-mediated two consecutive endonuclease reactions. The activity assay of MGMT is firstly accomplished using a hairpin-structured DNA substrate to offer a specific recognition site on the substrate DNA for restriction endonuclease PvuII, and thus to initiate the first endonuclease reaction. The product which activates the second endonuclease reaction allows an efficient amplification approach to create an abundance of fluorescence signal reporters. The first endonuclease reaction offers the method high specificity and the second one furnishes the assay improved sensitivity. The results reveal that the MGMT assay strategy shows dynamic responses in the concentration range from 1 to 120 ng mL(-1) with a detection limit of 0.5 ng mL(-1). By simply altering the alkylated bases, this strategy can also be extended for the detection of other alkyltransferases. Therefore, the developed strategy might provide an intrinsically convenient, sensitive and specific platform for alkyltransferase activate assay and related biochemical studies due to its label-free, homogeneous, and fluorescence-based detection format.

Novel poly(ADP-ribose) polymerase-1 inhibitor, AG14361, restores sensitivity to temozolomide in mismatch repair-deficient cells

PURPOSE

Mismatch repair (MMR) deficiency confers resistance to temozolomide, a clinically active DNA-methylating agent. The purpose of the current study was to investigate the reversal mechanism of temozolomide resistance by the potent novel poly(ADP-ribose) polymerase (PARP)-1 inhibitor, AG14361, in MMR-proficient and -deficient cells.

EXPERIMENTAL DESIGN

The effects of AG14361, in comparison with the methylguanine DNA methyltransferase inhibitor, benzylguanine, on temozolomide-induced growth inhibition were investigated in matched pairs of MMR-proficient (HCT-Ch3, A2780, and CP70-ch3) and -deficient (HCT116, CP70, and CP70-ch2) cells.

RESULTS

AG14361 enhanced temozolomide activity in all MMR-proficient cells (1.5-3.3-fold) but was more effective in MMR-deficient cells (3.7-5.2-fold potentiation), overcoming temozolomide resistance. In contrast, benzylguanine only increased the efficacy of temozolomide in MMR-proficient cells but was ineffective in MMR-deficient cells. The differential effect of AG14361 in MMR-deficient cells was not attributable to differences in PARP-1 activity or differences in its inhibition by AG14361, nor was it attributable to differences in DNA strand breaks induced by temozolomide plus AG14361. MMR-deficient cells are resistant to cisplatin, but AG14361 did not sensitize any cells to cisplatin. PARP-1 inhibitors potentiate topotecan-induced growth inhibition, but AG14361 did not potentiate topotecan in MMR-deficient cells more than in MMR-proficient cells.

CONCLUSIONS

MMR defects are relatively common in sporadic tumors and cancer syndromes. PARP-1 inhibition represents a novel way of selectively targeting such tumors. The underlying mechanism is probably a shift of the cytotoxic locus of temozolomide to N(7)-methylguanine and N(3)-methyladenine, which are repaired by the base excision repair pathway in which PARP-1 actively participates.

Enhancement of fotemustine (Muphoran) cytotoxicity by amifostine in malignant melanoma cell lines

Fotemustine (Muphoran, S10036), a nitrosourea derivative active in the treatment of malignant melanoma and primary brain tumors, was evaluated in combination with the free radicals cytoprotective agent amifostine (Ethyol, WR-2721) and its alkaline phosphatase (AP)-generated active metabolite WR-1065 in four human melanoma (RPMI-7950, SK-MEL2, SK-MEL5 and WM-115) and lung fibroblast (MRC-5) cell lines. No difference in AP activity was found among the melanoma cell lines, but AP was found to be significantly higher in MRC-5. For combination experiments, cell lines were first exposed to amifostine or WR-1065 for 15 min and then exposed to fotemustine for two cell doubling times. Non-cytotoxic amifostine and WR-1065 concentrations used (0.2 and 0.6 and 0.1 and 0.3 mmol/l, respectively) were deduced from clinically achieved plasma values. Interactions were analyzed from the variations in IC(50) of fotemustine induced by pre-exposure of the cells to amifostine or WR-1065. In all melanoma cell lines, amifostine enhanced the cytotoxic activity of fotemustine as a significant decrease in IC(50) was observed. No significant difference was found between synergistic effects achieved with amifostine and WR-1065 given at half concentrations. No differential effect was found in the MRC-5 cell line as compared with the melanoma cell lines. Expression variation of O(6)-methylguanine methyltransferase was not found to be implicated in the interaction. The present results demonstrating that amifostine or its main active metabolite do not impair the cytotoxicity of fotemustine justify an extensive clinical evaluation of this combination in metastatic melanoma.

DNA-binding mechanism of the Escherichia coli Ada O(6)-alkylguanine-DNA alkyltransferase

The C-terminal domain of the Escherichia coli Ada protein (Ada-C) aids in the maintenance of genomic integrity by efficiently repairing pre-mutagenic O:(6)-alkylguanine lesions in DNA. Structural and thermodynamic studies were carried out to obtain a model of the DNA-binding process. Nuclear magnetic resonance (NMR) studies map the DNA-binding site to helix 5, and a loop region (residues 151-160) which form the recognition helix and the 'wing' of a helix-turn-wing motif, respectively. The NMR data also suggest the absence of a large conformational change in the protein upon binding to DNA. Hence, an O:(6)-methylguanine (O:(6)meG) lesion would be inaccessible to active site nucleophile Cys146 if the modified base remained stacked within the DNA duplex. The experimentally determined DNA-binding face of Ada-C was used in combination with homology modelling, based on the catabolite activator protein, and the accepted base-flipping mechanism, to construct a model of how Ada-C binds to DNA in a productive manner. To complement the structural studies, thermodynamic data were obtained which demonstrate that binding to unmethylated DNA was entropically driven, whilst the demethylation reaction provoked an exothermic heat change. Methylation of Cys146 leads to a loss of structural integrity of the DNA-binding subdomain.

Dominant sensitization variants of human O(6)-methylguanine-DNA-methyltransferase obtained by a mutational screen of surface residues

A scanning mutagenesis experiment was performed on human O(6)-methylguanine methyltransferase (hMGMT), directed largely at non-conserved surface residues that have not previously been studied. Variants typically contained two or more substitutions. Two of the 16 variants characterized in detail are inactive for methyltransfer, but increase the cytotoxicity and mutagenic effects of methylating agents. This phenotype is reminiscent of a variant (C145A) that has a mutation in the methyl-accepting cysteine. C145A is inactive, but reportedly binds methylated DNA and confers sensitivity to methylating agents. The sensitization phenotype of the two new variants is more striking in strains that are wild-type for DNA repair than in strains that are deficient for repair, suggesting that these proteins inhibit functional DNA repair proteins by competitively binding to methylated DNA. Both variants have multiple substitutions in the last helix of the protein. These results suggest that the C-terminal helix is necessary for methyltransfer activity, but not for methylguanine-specific binding.

Characterisation of the DNA repair enzyme for O(6)-methylguanine in cirrhosis

BACKGROUND/AIMS

The reason(s) why cirrhosis is a major risk factor for the development of hepatocellular carcinoma worldwide are largely unknown. In a previous preliminary study, we reported deficiency of the repair enzyme for the highly promutagenic and potentially carcinogenic DNA base lesion, O(6)-methylguanine, in cirrhotic human liver. The aims of the present study were: (i) to confirm this observation in an extended series of cirrhosis patients, using a new DNA repair enzyme assay approach, (ii) to characterise the enzyme, in particular to seek physicochemical differences between control and cirrhotic liver that might account for the enzyme defect in cirrhosis, and (iii) to examine the relationship between magnitude of DNA repair deficiency in cirrhotic liver and aetiology of cirrhosis or male sex, both of which are independent risk factors of hepatocellular carcinoma.

METHODS

Tissue extracts containing DNA repair enzyme, O(6)-methylguanine-DNA methyltransferase, were prepared from liver biopsy samples from 41 patients with cirrhosis (10 viral (HBV and HCV), 17 alcohol and 14 autoimmune), 17 patients with non-cirrhotic diseased liver and 4 patients with histologically normal liver. Repair enzyme levels and electrophoretic profiles were both obtained using sodium dodecyl sulphate-polyacrylamide gel electrophoresis in conjunction with fluorography and densitometry.

RESULTS

We have demonstrated the feasibility of both analysing and reproducibly measuring DNA repair enzyme in small liver biopsy samples using the gel electrophoresis/densitometry approach for enzyme assay. Using the new densitometric assay approach, levels of O(6)-methylguanine-DNA methyltransferase were found to be significantly lower in cirrhotic compared to non-cirrhotic liver, with very low values obtained for two individuals who were incidentally found to have small hepatocellular carcinomas at the time of liver transplantation. There were no significant differences in enzyme levels between patients with cirrhosis of different aetiology, and between male and female patients. Methyltransferase was found to be present in all liver extracts as a major 23.1 kDA protein, along with other less abundant enzyme forms of both slightly higher and lower molecular weight; there were no obvious differences in size and relative abundance of these enzyme forms between liver samples from any of the patient and control groups. Our findings of enzyme instability and multiple forms led us to examine methyltransferase's amino acid sequence for the presence of primary structure motifs indicative of targeted degradation of the enzyme by intracellular proteases. We have identified a eukaryotic thiol protease active site motif and two cyclin-like "destruction box" motifs in the N-terminal half of the enzyme.

CONCLUSIONS

Our findings provide further evidence suggesting that deficiency in the ability to repair O(6)-methylguanine-DNA underlies the increased risk of hepatocellular carcinoma seen in patients with cirrhosis. The degree of DNA repair deficiency not varying between cirrhotic patients subgrouped according to other risk factors for hepatocellular carcinoma, suggests that DNA repair deficiency is only one of several contributory factors in liver carcinogenesis. The presence in methyltransferase of thiol protease active site and "destruction box" motifs may be of relevance to observed repair enzyme instability and presence of multiple enzyme forms in human liver extracts.

Contribution of O6-methylguanine-DNA methyltransferase to monofunctional alkylating-agent resistance in human brain tumor-derived cell lines

The DNA repair protein O6-methylguanine-DNA methyltransferase (MGMT) has been implicated in resistance of human brain tumors to alkylating agents. We observed that 14 human medulloblastoma- and glioma-derived cell lines differ in sensitivity to the methylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), as shown by their 28-fold range in 10% survival dose (LD10). By using the substrate analogue inhibitor O6-benzylguanine (O6-BG), we showed that the contribution of MGMT to resistance varies widely, as evidenced by 3- to 30-fold reductions in LD10 among the lines, and varies up to 20-fold among subpopulations of individual lines. Importantly, variability in resistance, manifested as a 20-fold range in LD10, persists after measurable MGMT is eliminated, disclosing differential contributions of other resistance mechanisms to survival. Cells exposed to MNNG while suspended in growth medium are more resistant than cells alkylated as subconfluent monolayers, and MGMT accounts for a smaller proportion of their resistance. Notably, the MGMT content of the lines is not statistically correlated with MNNG resistance or with potentiation of killing by O6-BG, even though MGMT is a biochemically demonstrated determinant of resistance. In contrast, the same lines vary less in resistance to the ethylating agent N-ethylnitrosourea (ENU), and MGMT makes only a small contribution to resistance. Our results strongly indicate that resistance to both MNNG and ENU is multifactorial.

Defective repair of O6-methylguanine-DNA in primary Sjögren's syndrome patients predisposed to lymphoma

OBJECTIVE

To investigate a role for mutation in the aetiogenesis of autoimmune disease by examining levels of repairing enzyme for the promutagenic DNA base lesion, O6-methylguanine, in lymphocyte extracts from patients with autoimmune diseases. We included primary Sjögrens syndrome (PSS) patients because of the additional relevance of their being at increased risk (> 40-fold) of developing lymphoma.

METHODS

Lymphocytes were prepared from patients with PSS (n = 22) (12 with parotid gland enlargement, an indicator of extensive lymphoproliferation), rheumatoid arthritis (n = 12), primary biliary cirrhosis (n = 11), osteoarthritis (n = 12), and healthy individuals (n = 11). MGMT amounts were determined in lymphocyte extracts by direct enzyme assay and expressed in relation to total extract DNA, protein, or cell number.

RESULTS

We found no defect in the repairing methyltransferase enzyme between any of the groups, except in PSS patients at increased risk of developing lymphoma (those with enlarged parotid glands): p < 0.0001 and p = 0.0056, compared with healthy controls and PSS patients without parotid gland swelling, respectively.

CONCLUSIONS

Our findings implicate persistence of O6-methylguanine-DNA in the aetiology of lymphoma associated with PSS, and raise the possibility that an alternative repair process for O6-methylguanine-DNA, nucleotide excision repair, might be defective in autoimmune disease.

Deficient repair of DNA lesion O6-methylguanine in cirrhosis

Cirrhosis is a risk factor for hepatocellular carcinoma. O6-methylguanine is a promutagenic and potentially carcinogenic DNA lesion produced by environmental alkylating agents. If it is not repaired, DNA replication can lead to a G-to-A transition mutation, which is a known mechanism of oncogene activation. We have found that the activity of the repairing methyltransferase enzyme is significantly lower in cirrhotic tissue than in non-cirrhotic diseased liver or in normal liver. This finding suggests a mechanism for cirrhosis being a risk factor for cancer of the liver: increased cellular proliferation together with persistence of O6-methylguanine might lead to malignant transformation of liver cells through mutation and oncogene activation.