Polymorphism in the human O6-methylguanine-DNA methyltransferase gene detected by PCR-SSCP analysis

Evaluation of mutations on O6-methylguanine methyl transferase structure and its interactions: molecular dynamics simulation study

O6-methylguanine DNA methyl transferase (MGMT) is a significant vehicle for the cellular clearance of alkyl lesions, particularly the methyl group of the O-6 and O-4 positions of guanine and thymine, respectively. Many publications have studied the correlation between polymorphisms in MGMT and susceptibility to various cancers. In the present study, we investigated the consequence of L84F, common single-nucleotide polymorphism, K125E, site-specific mutagenesis, and L84F/K125E on conformation, stability, and behavior of MGMT in the free form and interaction with proliferating cell nuclear antigen (PCNA) and DNA as partners in the biochemical network by using molecular dynamics simulation method. Our results showed that all free variants of MGMT differed from the native form. However, among all free variants of MGMT, the L84F/K125E variant exhibited similar properties compared with the wild-type. In contrast, in complex modes, only amino acid residues of the L84F variant are involved in the interactions with PCNA and DNA somewhat differently relative to the wild-type. Furthermore, L84F SNP showed the highest binding free energy compared to other variants and native forms. These alterations in the amino acids and binding free energy of L84F relative to the native are the reasons for changing its region connection compared to the native form. Therefore, we propose conducting further investigations into the impact of inhibitors or chemotherapeutic agents to assess their effectiveness on MGMT variants compared to the wild-type, aiming to reduce the cost of cancer treatment that will depend on inhibiting native MGMT protein.Communicated by Ramaswamy H. Sarma.

A genotype of the polymorphic DNA repair gene MGMT is associated withde novoglioblastoma

Glioblastoma is one of the most malignant tumors in humans. This tumor is thought to develop as a result of the accumulation of genetic abnormalities, mainly focused on the loss of heterozygosity on chromosome 10. O6-methylguanine-DNA methyltransferase (MGMT), which is one of the most important DNA repair proteins, has also been reported that enzymatic activity, as well as the methylation status of the promoter region of the MGMT gene, contributes to the therapeutic response of alkylating agents. We previously found three allelic variants in the MGMT gene and assayed the characteristics of these polymorphic proteins. We designed a case-control study to investigate the role of MGMT genotypic risk factors for primary brain tumors. We compared the distributions of MGMT genotypes in primary brain tumors and normal controls. The frequencies of MGMT genotypes in examined primary brain tumors were not different from normal subjects. However, the combined heterozygote of V1 and a wild allele (V1/W) was frequently detected in de novo glioblastoma group with significant difference. Interestingly, among glial tumors, the V1/W genotype was dominantly detected in the patients with de novo glioblastoma. This study suggests that the V1/W genotype of the MGMT gene may contribute to the de novo occurrence of glioblastoma.

Polymorphisms of the DNA repair gene MGMT and risk and progression of head and neck cancer

Methylating agents are involved in carcinogenesis, and the DNA repair protein O(6)-methylguanine-DNA methyltransferase (MGMT) removes methyl group from O(6)-methylguanine. Genetic variation in DNA repair genes has been shown to contribute to susceptibility to squamous cell carcinoma of the head and neck (SCCHN). We hypothesize that MGMT polymorphisms are associated with risk of SCCHN. In a hospital-based case-control study of 721 patients with SCCHN and 1234 cancer-free controls frequency-matched by age, sex and ethnicity, we genotyped four MGMT polymorphisms, two in exon 3, 16195C>T and 16286C>T and two in the promoter region, 45996G>T and 46346C>A. We found that none of these polymorphisms alone had a significant effect on risk of SCCHN. However, when these four polymorphisms were evaluated together by the number of putative risk genotypes (i.e. 16195CC, 16286CC, 45996GT+TT, and 46346CA+AA), a statistically significantly increased risk of SCCHN was associated with the combined genotypes with three to four risk genotypes, compared with those with zero to two risk genotypes (adjusted odds ratio (OR)=1.27; 95% confidence interval (CI)=1.05-1.53). This increased risk was also more pronounced among young subjects (OR=1.81; 95% CI=1.11-2.96), men (OR=1.24; 95% CI=1.00-1.55), ever smokers (OR=1.25; 95%=1.01-1.56), ever drinkers (OR=1.29; 95% CI=1.04-1.60), patients with oropharyngeal cancer (OR=1.45; 95% CI=1.12-1.87), and oropharyngeal cancer with regional lymph node metastasis (OR=1.52; 95% CI=1.16-1.89). In conclusion, our results suggest that any one of MGMT variants may not have a substantial effect on SCCHN risk, but a joint effect of several MGMT variants may contribute to risk and progression of SCCHN, particularly for oropharyngeal cancer, in non-Hispanic whites.

Differential inactivation of polymorphic variants of human O6-alkylguanine-DNA alkyltransferase

The human DNA repair protein O(6)-alkylguanine-DNA alkyltransferase (hAGT) is an important source of resistance to some therapeutic alkylating agents and attempts to circumvent this resistance by the use of hAGT inhibitors have reached clinical trials. Several human polymorphisms in the MGMT gene that encodes hAGT have been described including L84F and the linked double alteration I143V/K178R. We have investigated the inactivation of these variants and the much rarer variant W65C by O(6)-benzylguanine, which is currently in clinical trials, and a number of other second generation hAGT inhibitors that contain folate derivatives (O(4)-benzylfolic acid, the 3' and 5' folate esters of O(6)-benzyl-2'-deoxyguanosine and the folic acid gamma ester of O(6)-(p-hydroxymethyl)benzylguanine). The I143V/K178R variant was resistant to all of these compounds. The resistance was due solely to the I143V change. These results suggest that the frequency of the I143V/K178R variant among patients in the clinical trials with hAGT inhibitors and the correlation with response should be considered.

Genetic association and functional studies of major polymorphic variants of MGMT

The DNA repair protein, O(6)-methylguanine DNA-methyltransferase (MGMT) prevents mutations and cell death that result from aberrant alkylation of DNA. The polymorphic variants Leu84Phe, Ile143Val, and Lys178Arg are frequent in the human population. We review here studies of these and other MGMT polymorphisms and their association with risk for lung, breast, colorectal and endometrial cancer with a consideration of gene-environment interactions. In addition, we review studies of the effects of polymorphic variation on alkyltransferase activity and expression. It is formally possible that polymorphic variation could modify functions of MGMT other than its alkyltransferase activity. While it was previously reported that an alkylated form of MGMT modifies Estrogen Receptor alpha activity, from our studies we conclude that this regulation is not a major function of MGMT. Overall, the effects of polymorphic variation on protein function are subtle, and further investigation is required to provide a comprehensive mechanism that explains the observed associations of these variants with risk for cancer.

Human variants of O6-alkylguanine-DNA alkyltransferase

This article summarizes the current understanding of known variant forms of the MGMT gene that encode an altered protein. Epidemiological studies have been carried out to test whether these alterations are associated with altered cancer risk. Laboratory studies using recombinant proteins and cells expressing the known variants have investigated the possible effects of these sequence alterations on the ability of the encoded O(6)-alkylguanine-DNA alkyltransferase protein to protect cells from alkylation damage and to respond to therapeutic inactivators currently undergoing trials for cancer chemotherapy.

O6-alkylguanine-DNA alkyltransferase gene polymorphisms and the risk of primary lung cancer

O6-alkylguanine-DNA alkyltransferase (AGT) plays an important role in the repair of O6-alkylguanine adducts, which are major mutagenic lesions produced by environmental carcinogens. Polymorphisms in the AGT gene may affect the capacity to repair DNA damage and thereby have influence on individual's susceptibility to smoking-related cancer. To test this hypothesis, we investigated the potential association of AGT polymorphisms (485C > A, Leu53Leu (C > T) and Leu84Phe] with the risk of lung cancer in a Korean population. The AGT genotypes were determined in 432 lung cancer patients and in 432 healthy controls who were frequency-matched for age and gender. The 485 AA genotype was associated with a significantly increased risk for overall lung cancer as compared with the 485 CC genotype and the combined 485 CC + CA genotype, respectively (adjusted odds ratio (OR) = 1.83, 95% confidence interval (CI) = 1.12-2.99, P = 0.02, and Bonferroni corrected P-value (Pc) = 0.04; and adjusted OR = 1.67, 95% CI = 1.05-2.66, P = 0.03, respectively). When the lung cancer cases were categorized by the tumor histology, the 485 AA genotype was associated with a significantly increased risk of adenocarcinoma (AC) and small cell carcinoma (SmCC), respectively, as compared with the combined 485 CC + CA genotype (adjusted OR = 2.54, 95% CI = 1.39-4.66, P = 0.003; and adjusted OR = 2.19, 95% CI = 1.06-4.55, P = 0.04, respectively). However, the genotype distributions of the Leu53Leu and Leu84Phe polymorphisms were not significantly different between the lung cancer cases and the controls. On a promoter assay, the 485C > A polymorphism did not have an effect on the promoter activity of the AGT gene. These results suggest that the effect of the AGT 485C > A polymorphism on the risk of lung cancer may be secondary to linkage disequilibrium (LD) with either another AGT variant or with a true susceptibility gene, and that the AGT 485C > A polymorphism could be used as a marker for the genetic susceptibility to lung cancer.

DNA repair polymorphisms and risk of colorectal adenomatous or hyperplastic polyps

Genetic variability in DNA repair genes may contribute to differences in DNA repair capacity and susceptibility to cancer, especially in the presence of exposures such as smoking. In a Minnesota-based case-control study of cases with only adenomatous polyps (n = 384), only hyperplastic polyps (n = 191), or both types of polyps (n = 119) versus polyp-free controls (n = 601), we investigated the role of polymorphisms in the DNA repair genes O(6)-methylguanine methyltransferase (MGMT; p.L84F and p.I143V), XPD (p.D312N and p.K751Q), and XPG (p.D1104H). MGMT polymorphisms were not associated with polyp risk. Overall, a homozygous variant XPD-combined genotype was associated with an increased risk of adenomatous polyps [odds ratio (OR), 1.57; 95% confidence interval (95% CI), 1.04-2.38] and an XPGHH1104 genotype with a decreased risk of hyperplastic polyps (OR, 0.36; 95% CI, 0.13-0.98). However, age stratification showed that the XPD association was present only in subjects >/=60 years old (OR, 3.77; 95% CI, 1.94-7.35), whereas the XPG association was observed largely in subjects <60 years old (OR, 0.20; 95% CI, 0.05-0.91). Smokers did not have a significantly increased risk of adenomatous polyps in the absence of synchronous hyperplastic polyps, except for subjects with a homozygous variant XPD genotype or a homozygous wild-type XPG genotype (OR, 3.93; 95% CI, 1.68-9.21 and OR, 1.59; 95% CI, 1.01-2.50, respectively). Smoking was associated with a statistically significant 2.5- to 6-fold increased risk of hyperplastic polyps for individuals with most of the DNA repair genotypes. However, no substantial increase was observed among individuals who were homozygous variant for XPG (1104HH; OR, 1.38; 95% CI, 0.25-7.65). Our data suggest that polymorphisms in DNA repair genes may be risk factors for colorectal neoplasia and that they may exacerbate the effects of exposures to carcinogens.

Exon 3 polymorphisms and haplotypes of O6-methylguanine-DNA methyltransferase and risk of bladder cancer in southern China: A case–control analysis

Methylating agents are involved in bladder carcinogenesis. O6-Methylguanine-DNA methyltransferase (MGMT) is a DNA repair protein that removes methyl group from O6-methylguanine and thus plays an important role in the etiology of cancer. We hypothesized that two MGMT polymorphisms in exon 3, C16195T (or MGMT L53L) and C16286T (or MGMT L84F) are associated with risk of bladder cancer. In a hospital-based case-control study of 167 patients with bladder cancer and 204 cancer-free controls frequency-matched by age, sex, smoking status, and alcohol use, we genotyped these two MGMT polymorphisms. We found that these two polymorphisms alone had a non-significant main effect on risk of bladder cancer. However, when these two polymorphisms were evaluated together, individuals with the combined genotypes or haplotypes with one or more variant alleles (i.e. the 16195T and 16286T alleles) had statistically significantly increased risk of bladder cancer (adjusted odd ratio [OR]=1.67, 95% confidence interval [CI], 1.01-2.77) compared with those with no variant allele. In the stratification analysis, the risk of bladder cancer was increased in a dose-response manner as the age increased (P(trend)=0.010), and the increased risk was more pronounced among old subjects (>65 years) (adjusted OR=2.51, 95% CI, 1.05-6.04), men (1.76, 1.00-3.10), and non-drinkers (1.91, 1.08-3.36). In conclusion, these two MGMT polymorphisms may jointly play a role, in the etiology of bladder cancer in southern Chinese population. Larger studies are warranted to validate our findings.

Polymorphisms in TDG and MGMT genes - epidemiological and functional study in lung cancer patients from Poland

Functional genetic polymorphisms of DNA repair genes are good candidates for cancer susceptibility markers. We studied two genes coding for proteins removing small DNA adducts by direct repair (MGMT), or mispaired DNA bases by base excision repair (TDG). The non-silent polymorphisms of MGMT (84:Phe, 143:Val, 178:Arg) and TDG (199:Ser, 367:Met), and the functional MGMT enhancer polymorphism, did not show any statistically significant association with lung cancer risk in our case-control analysis, but due to the relatively small number of individuals, strong conclusions on cancer risk association or lack thereof cannot be made. Sequencing of the TDG cDNA has not revealed any novel polymorphism, but did find an alternatively spliced mRNA missing exon 2. Our search for polymorphisms within the promoter-enhancer region of MGMT revealed three novel sequence variants. The functional significance of the previously published MGMT enhancer polymorphism (1099C->T) was assessed. The less frequent sequence variant of the enhancer was associated with a modest (16-64%), but statistically significant, increase of MGMT promoter-enhancer activity in the studied cell lines. This work points to the importance of studying the expression-regulating elements of genes, as they may contain functional polymorphisms with the potential for modulating risk of various diseases, including cancer.

Lack of association between Caucasian lung cancer risk and O6-methylguanine-DNA methyltransferase-codon 178 genetic polymorphism

The formation of DNA adducts is thought to be a critical step for the induction of chemically induced cancer. O(6)-Methylguanine-DNA methyltransferase (MGMT) is a ubiquitously expressed enzyme that repairs DNA adducts formed by alkylating carcinogens. Thus, genetic polymorphisms of the MGMT that could result in differences in MGMT activity are potential risk factors for cancer. In the present study, we established a convenient and reliable genotyping method for the MGMT codon 178 polymorphism, a Lys (AAG) to Arg (AGG) substitution, using restriction fragment length polymorphism (RFLP), and studied differences in the distribution of this polymorphism in 92 Caucasian lung cancer patients and 85 controls. Frequencies of the "A" and "G" alleles (MGMT codon 178, AAG and AGG, respectively) were 0.91 and 0.09, respectively. The genetic polymorphism of the MGMT codon 178 was linked with that of the MGMT codon 143 (P < 0.05). The distribution of the MGMT codon 178 genetic polymorphism was not significantly different between lung cancer patients and controls. Thus, our study suggests that the MGMT codon 178 (and possibly 143) polymorphisms do not appear to markedly affect lung cancer risk for this population. In addition, we found an apparent 10bp-deletion in the intron before exon 5 by DNA sequencing. Because this "deletion" was observed in all sequenced samples (N = 20), the previously reported human (Caucasian) MGMT gene sequence should be revised to exclude this 10bp segment.

O6-methylguanine-DNA-methyltransferase expression and gene polymorphisms in relation to chemotherapeutic response in metastatic melanoma

In a retrospective study, O⁶-methylguanine-DNA-methyltransferase (MGMT) expression was analysed by immunohistochemistry using monoclonal human anti-MGMT antibody in melanoma metastases in patients receiving dacarbazine (DTIC) as single-drug therapy or as part of combination chemotherapy with DTIC–vindesine or DTIC–vindesine–cisplatin. The correlation of MGMT expression levels with clinical response to chemotherapy was investigated in 79 patients with metastatic melanoma. There was an inverse relationship between MGMT expression and clinical response to DTIC-based chemotherapy (P=0.05). Polymorphisms in the coding region of the MGMT gene were also investigated in tumours from 52 melanoma patients by PCR/SSCP and nucleotide sequence analyses. Single-nucleotide polymorphisms (SNPs) in exon 3 (L53L and L84F) and in exon 5 (I143V/K178R) were identified. There were no differences in the frequencies of these polymorphisms between these melanoma patients and patients with familial melanoma or healthy Swedish individuals. Functional analysis of variants MGMT-I143V and -I143V/K178R was performed by in vitro mutagenesis in Escherichia coli. There was no evidence that these variants decreased the MGMT DNA repair activity compared to the wild-type protein. All melanoma patients with the MGMT 53/84 polymorphism except one had tumours with high MGMT expression. There was no significant correlation between any of the MGMT polymorphisms and clinical response to chemotherapy, although an indication of a lower response rate in patients with SNPs in exon 5 was obtained. Thus, MGMT expression appears to be more related to response to chemotherapy than MGMT polymorphisms in patients with metastatic melanoma.

Polymorphisms in DNA repair and environmental interactions

The repair of damage to DNA is critical to the survival of a cell. However, not all organisms nor all individuals express a similar response to challenges to their genetic material. Numerous polymorphisms in genes involved in DNA repair have been found in individuals with DNA repair-related disease as well as in the general population. Studies of these variants are critical in understanding the response of the cell to DNA damage. In some cases, these changes predispose the carrier to a greatly increased risk of cancer. In other cases, the effects are subtler and depend on interactions between the alleles of several genes, or with environmental factors. Consequently, the health effects of exposure to genotoxic or carcinogenic compounds or agents can depend on the variations in these genes. This review will highlight some of the effects that variants, found in many of the genes involved in human DNA repair pathways, have on the response to damage, and their role in susceptibility of the cell and organism to environmental genotoxins. This review will concentrate on the mismatch repair, nucleotide repair, base excision repair, strand break repair, and direct alkyl repair pathways.

Analysis of O(6)-methylguanine-DNA methyltransferase in melanoma tumours in patients treated with dacarbazine-based chemotherapy

In a retrospective study we analysed the levels of the DNA repair protein O(6)-methylguanine-DNA methyltransferase (MGMT) in melanoma metastases in patients receiving dacarbazine (DTIC) either as a single drug or as part of combination chemotherapy regimens, and related the expression levels to the clinical response to treatment. Biopsies of subcutaneous and lymph node metastases obtained before chemotherapy in 65 patients with disseminated malignant melanoma were examined for MGMT protein levels by immunohistochemistry using a monoclonal anti-human MGMT antibody. All patients received chemotherapy with DTIC, given either as a single drug or in combination with vindesine and in some cases cisplatin. DTIC as single agent was given to 44 patients, while 21 received combination chemotherapy. Objective responses to chemotherapy were seen in 12 patients, while 53 patients failed to respond to treatment. The expression of MGMT was determined according to the proportion of antibody-stained tumour cells, using a cut-off level of 50%. In 12 of the patients more than one metastasis was analysed, and in seven of these cases the MGMT expression differed between tumours in the same individual. Among the responders a larger proportion (six out of 12, 50%) had tumours containing less than 50% MGMT-positive tumour cells than among the non-responders (12 out of 53, 23%). These data are consistent with the hypothesis that MGMT contributes to resistance to DTIC-based treatment, although the difference between responders and non-responders with respect to the proportion of MGMT-positive tumour cells was not statistically significant (P = 0.077).

Interaction of mammalian O(6)-alkylguanine-DNA alkyltransferases with O(6)-benzylguanine

Human O(6)-alkylguanine-DNA alkyltransferase (hAGT) activity is a major factor in providing resistance to cancer chemotherapeutic alkylating agents. Inactivation of hAGT by O(6)-benzylguanine (BG) is a promising strategy for overcoming this resistance. Previous studies, which have focused on the region encompassed by residues Pro138 to Gly173, have identified more than 100 individual mutations located at 23 discrete sites at which alterations can render AGT less sensitive to BG. We have now extended the examination of possible sites in hAGT at which alterations might lead to BG resistance to include the residues from Val130 to Asn137, which also make up part of the binding pocket into which BG is postulated to fit. A further 21 mutations located at positions Gly132, Met134, Arg135, and Gly136 were found to lower sensitivity to BG. Mutants R135L, R135Y, and G136P were the most strikingly resistant, with a 50-fold increase in the amount of BG needed to obtain 50% inactivation. These results therefore increase the number of sites at which BG resistance can occur in response to a single amino acid change to 27. Although mammalian AGTs are very similar in amino acid sequence, mouse AGT (mAGT) is significantly less sensitive to BG than rat AGT (rAGT) or hAGT. Construction of chimeric proteins in which portions came from the rAGT and the mAGT indicated that the difference in inactivation resided solely in the amino acids located in the sequence from residues 150 to 188. Individual mutations of the three residues where rAGT and mAGT differ in this region showed that the principal reason for the reduced ability of the mAGT to react with BG was the presence of a histidine residue at position 161, which is occupied by asparagine in rAGT and hAGT. These experiments indicate that many minor changes in amino acids forming all parts of the nucleoside binding pocket of AGT can alter its ability to react with BG and that the possibility that polymorphisms or variants may occur reducing the effectiveness of combination therapy with BG and alkylating agents must be considered.

Expression and prognostic significance of O6-methylguanine-DNA methyltransferase in hepatocellular, gastric, and breast cancers

BACKGROUND

O6-Methylguanine-DNA methyltransferase (MGMT) is an enzyme that repairs O6-methylguanine, a promutagenic DNA base damaged by endogenous and environmental alkylating agents. There are few reports that describe whether or not abnormal MGMT expression correlates with the prognosis in human solid cancers.

METHODS

The expression of MGMT was immunohistochemically evaluated in 60, 62, 105, and 46 paraffin-embedded samples from patients with curatively resected hepatocellular, gastric, colorectal, and breast cancers, respectively.

RESULTS

The expression of MGMT was a positive predictive factor for overall survival in hepatocellular (P = .005) and gastric cancers (P < .001) and for relapse-free survival in breast cancers (P < .001). MGMT-positive gastric tumors (n = 42) were correlated with the absence of serosal invasion (P = .045), lymph node metastasis (P = .006), intestinal type (P = .018), and low pathological tumor, node, metastasis stage (P < .001). All breast tumors that recurred locally after operation were MGMT negative (P = .004). The clinicopathologic characteristics of colorectal cancers with respect to MGMT expression did not significantly differ.

CONCLUSIONS

The expression of MGMT is a predictive prognostic marker in patients with hepatocellular, gastric, and breast cancers. These findings may help to establish therapeutic strategies for patients with these types of solid cancer.

Repair of O(6)-alkylguanine by alkyltransferases

The predominant pathway for the repair of O(6)-methylguanine in DNA is via the activity of an alkyltransferase protein that transfers the methyl group to a cysteine acceptor site on the protein itself. This review article describes recent studies on this alkyltransferase. The protein repairs not only methyl groups but also 2-chloroethyl-, benzyl- and pyridyloxobutyl-adducts. It acts on double-stranded DNA by flipping the O(6)-guanine adduct out of the DNA helix and into a binding pocket. The free base, O(6)-benzylguanine, is able to bind in this pocket and react with the cysteine, rendering it an effective inactivator of mammalian alkyltransferases. The alkylated form of the protein is rapidly degraded by the ubiquitin/proteasomal system. Some tumor cells do not express alkyltransferase despite having an intact gene. Methylation of key sites in CpG-rich islands in the promoter region are involved in this silencing and a change in the nuclear localization of an enhancer binding protein may also contribute. The alkyltransferase promoter contains Sp1, GRE and AP-1 sites and is slightly inducible by glucocorticoids and protein kinase C activators. There is a complex relationship between p53 and alkyltransferase expression with p53 mediating a rise in alkyltransferase in response to ionizing radiation but having no clear effect on basal levels. DNA adducts at the O(6)-position of guanine are a major factor in the carcinogenic, mutagenic, apoptopic and clastogenic actions of methylating agents and chloroethylating agents. Studies with transgenic mice in which alkyltransferase levels are increased or decreased confirm the importance of this repair pathway in protecting against carcinogenesis. Alkyltransferase activity in tumors protects them from therapeutic agents such as temozolomide and BCNU. This resistance is abolished by O(6)-benzylguanine and this drug is currently in clinical trials to enhance cancer chemotherapy by these agents. Studies are in progress to reduce the toxicity of such therapy towards the bone marrow by gene therapy to express alkyltransferases with mutations imparting resistance to O(6)-benzylguanine at high levels in marrow stem cells. Several polymorphisms in the human alkyltransferase gene have been identified but the significance of these in terms of alkyltransferase action is currently unknown.

Characterization of human polymorphic DNA repair methyltransferase

The O6-methylguanine-DNA methyltransferase (MGMT) is a critical defence against alkylation-induced mutagenesis and carcinogenesis. More than a 20-fold interindividual difference in the MGMT activity is known to exist among human cultured fibroblasts. We previously reported three allelic variants of the human MGMT gene, namely V1, V2, and V3. Both V1 and V2 carry amino acid substitutions, Leu84Phe and Trp65Cys, respectively, while V3 has a silent mutation. In order to reveal the pharmacogenetic and ecogenetic significance of polymorphism in the human MGMT gene, we investigated the in-vivo characteristics of V1 and V2 methyltransferase enzyme. Escherichia coli strain KT233 (ogt-, ada-) and mer- HeLa MR cells carrying a V1 sequence exhibited almost the same level of sensitivity against N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), as did those with a wild-type sequence. The level of methyltransferase protein in those cells was essentially the same as for the wild-type and V1 samples. On the other hand, E. coli and human cells expressing V2 cDNA showed a significantly reduced level of survival. In these cells, V2 protein was hardly detected, even though mRNA was produced normally. An in-vitro translation experiment revealed that the V2 sequence had the potential to produce methyltransferase protein, as did the wild-type and V1 sequences. There was also evidence for a small amount of V2 protein being produced but rapidly degraded, thus implying that the V2 molecule is unstable in vivo. Using purified recombinant proteins, we estimated the kinetic values of wild-type and variant form of enzymes, which would support these views. From these results, we concluded that the wild-type and V1 protein have similar enzymatic and physicochemical properties, while V2 protein is considered to be unstable and rare.

A convenient method for genotyping of human O6-methylguanine-DNA methyltransferase polymorphism

O6-methylguanine-DNA methyltransferase (MGMT) is one of the DNA repair enzymes in mammals. We previously screened the variant alleles for the MGMT gene in the general population, and found three variants (V1, V2, V3), two of which caused amino acid substitutions (Leu84Phe for V1, and Trp65Cys for V2). In order to accelerate the ecogenetic and pharmacogenetic studies on MGMT polymorphism, we therefore developed a new PCR-based RFLP method for genotyping. The present method has some advantages over the initial PCR-single strand conformation polymorphism (SSCP) method, particularly regarding its simplicity, rapidity and specificity.