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

Genotype of Null Polymorphisms in Genes GSTM1, GSTT1, CYP1A1, and CYP1A1*2A (rs4646903 T>C)/CYP1A1*2C (rs1048943 A>G) in Patients with Larynx Cancer in Southeast Spain

Simple Summary

Epidemiological studies have shown that individual susceptibility to cancer is mediated by genetic and environmental factors. The aim of the present study is to evaluate the individuals’ metabolic genetic susceptibility to toxic habits (smoking and alcohol consumption) by detecting polymorphisms CYP1A1 rs1048943 T>C and CYPA1A2 rs4646903 A>G, and null polymorphisms in GSTM1 and GSTT1 genotypes, comparing a group of healthy control subjects with a population of larynx cancer patients from southeastern Spain. As results patients with larynx cancer present more gene GSTM1 and GSTT1 null polymorphisms, and CYP1A1 rs4646903 T>C polymorphisms.

Abstract

Background: some types of cancer have been associated with the presence of single nucleotide polymorphisms (SNPs) of some genes that encode enzymes: glutathione-S transferase (GST), whose alteration leads to loss of function and a lower capacity to eliminate toxic GSTM1 and GSTT1 null genotypes; SNPs causing loss of function of CYP1A1 or CYP1A1–2 cytochrome P450 enzymes related with a lower capacity to deactivate hydrocarbons related to smoking, which involves a higher risk of developing some smoking-dependent cancers including larynx cancer. Objective: to compare the presence of null SNPs in genes GSTM1, GSTT1, and CYP1A1 rs 4646903 T>C, and CYP1A1–2 RS1048943 A>G in patients with hypopharyngeal and larynx cancer with a healthy control group. Materials and method: The study included a total of 80 patients with hypopharyngeal and laryngeal cancer and 23 healthy subjects. Genomic DNA was obtained from saliva samples, determining genotype GSTM1 (present +, or null −), GSTT1 (present + or null −). Polymorphisms (SNP) in CYP1A1 T>C (present + CC, or absent − TC/TT), and CYP1A1–2 A>G (present + GG, or absent − AG/AA). Results: the mean age of patients with larynx cancer was 62 years and of control subjects 63 years. Of the total sample, over 95% were men, and over 90% were smokers. The presence of null genotypes for GTM1 was 50% in patients with larynx cancer (p = 0.042), while GSTT1 was 88.75% (p = 0.002). CYP1A1 rs4646903 T>C polymorphisms were detected in 100% of cases of larynx cancer and 17.39% of healthy subjects (p > 0.001). Conclusions: patients with larynx cancer present more gene GSTM1 and GSTT1 null polymorphisms, and CYP1A1 rs4646903 T>C polymorphisms.

The Role of O6-methylguanine-DNA Methyltransferase Polymorphisms in Prostate Cancer Susceptibility: a Meta-Analysis

To assess the associations between O⁶-methylguanine-DNA methyltransferase(MGMT) polymorphisms and prostate cancer risk. We retrieved PubMed, Cochrane Library and Embase electronic database to search for all eligible studies published from Jan 1, 1970 to Sep 31, 2017 to conduct a Meta-analysis. we identified 11 independent studies in 5 eligible reports, including 5143 cases and 8118 controls. The data suggested that rs12917 was associated with higher PCa risk under the contrast of TT vs CC and recessive model in overall population (TT vs CC: OR = 1.599, 95%CI: 1.007-2.539, P = 0.047; TT vs CC + CT: OR = 1.627, 95%CI: 1.026-2.580, P = 0.038). In subgroup analyses stratified by ethnicity, the remarkable association with higher PCa risk was detected under allelic model, dominant model, the contrast of TC vs CC, and the contrast of TC vs CC + TT in Asian population. (T vs C: OR = 1.911, 95%CI: 1.182-3.090, P = 0.008; TC vs CC: OR = 1.948, 95%CI: 1.152-3.295, P = 0.013; TC + TT vs CC: OR = 1.994, 95%CI: 1.190-3.342, P = 0.009; TC vs CC + TT: OR = 1.926, 95%CI: 1.140-3.255, P = 0.014). However, the data suggest the rs2308327 and rs2308321 polymorphisms of the MGMT gene were nor associated with the susceptibility of prostate cancer. Based on the meta-analysis, MGMT rs12917 polymorphism increase the susceptibility to prostate cancer, which can be taken for a diagnosis and screening molecular biomarker for prostate cancer patients.

The potential role of MGMT rs12917 polymorphism in cancer risk: an updated pooling analysis with 21010 cases and 34018 controls

In the present study, we aimed at determining the potential role of rs12917 polymorphism of the O-6-methylguanine-DNA methyltransferase (MGMT) gene in the occurrence of cancer. Based on the available data from the online database, we performed an updated meta-analysis. We retrieved 537 articles from our database research and finally selected a total of 54 case–control studies (21010 cases and 34018 controls) for a series of pooling analyses. We observed an enhanced risk in cancer cases compared with controls, using the genetic models T/T compared with C/C (P-value of association test <0.001; odds ratio (OR) = 1.29) and T/T compared with C/C+C/T (P<0.001; OR = 1.32). We detected similar positive results in the subgroups ‘Caucasian’, and ‘glioma’ (all P<0.05; OR > 1). However, we detected negative results in our analyses of most of the other subgroups (P>0.05). Begg’s and Egger’s tests indicated that the results were free of potential publication bias, and sensitivity analysis suggested the stability of the pooling results. In summary, the T/T genotype of MGMT rs12917 is likely to be linked to an enhanced susceptibility to cancer overall, especially glioma, in the Caucasian population.

Context Matters: Contribution of Specific DNA Adducts to the Genotoxic Properties of the Tobacco-Specific Nitrosamine NNK

The tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is a potent pulmonary carcinogen in laboratory animals. It is classified as a Group 1 human carcinogen by the International Agency for Cancer Research. NNK is bioactivated upon cytochrome P450 catalyzed hydroxylation of the carbon atoms adjacent to the nitrosamino group to both methylating and pyridyloxobutylating agents. Both pathways generate a spectrum of DNA damage that contributes to the overall mutagenic and toxic properties of this compound. NNK is also reduced to form 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL), which is also carcinogenic. Like NNK, NNAL requires metabolic activation to DNA alkylating agents. Methyl hydroxylation of NNAL generates pyridylhydroxybutyl DNA adducts, and methylene hydroxylation leads to DNA methyl adducts. The consequence of this complex metabolism is that NNK generates a vast spectrum of DNA damage, any form of which can contribute to the overall carcinogenic properties of this potent pulmonary carcinogen. This Perspective reviews the chemistry and genotoxic properties of the collection of DNA adducts formed from NNK. In addition, it provides evidence that multiple adducts contribute to the overall carcinogenic properties of this chemical. The adduct that contributes to the genotoxic effects of NNK depends on the context, such as the relative amounts of each DNA alkylating pathway occurring in the model system, the levels and genetic variants of key repair enzymes, and the gene targeted for mutation.

MGMT DNA repair gene promoter/enhancer haplotypes alter transcription factor binding and gene expression

BACKGROUND

The O⁶-methylguanine-DNA methyltransferase (MGMT) protein removes O⁶-alkyl-guanine adducts from DNA. MGMT expression can thus alter the sensitivity of cells and tissues to environmental and chemotherapeutic alkylating agents. Previously, we defined the haplotype structure encompassing single nucleotide polymorphisms (SNPs) in the MGMT promoter/enhancer (P/E) region and found that haplotypes, rather than individual SNPs, alter MGMT promoter activity. The exact mechanism(s) by which these haplotypes exert their effect on MGMT promoter activity is currently unknown, but we noted that many of the SNPs comprising the MGMT P/E haplotypes are located within or in close proximity to putative transcription factor binding sites. Thus, these haplotypes could potentially affect transcription factor binding and, subsequently, alter MGMT promoter activity.

METHODS

In this study, we test the hypothesis that MGMT P/E haplotypes affect MGMT promoter activity by altering transcription factor (TF) binding to the P/E region. We used a promoter binding TF profiling array and a reporter assay to evaluate the effect of different P/E haplotypes on TF binding and MGMT expression, respectively.

RESULTS

Our data revealed a significant difference in TF binding profiles between the different haplotypes evaluated. We identified TFs that consistently showed significant haplotype-dependent binding alterations (p ≤ 0.01) and revealed their role in regulating MGMT expression using siRNAs and a dual-luciferase reporter assay system.

CONCLUSIONS

The data generated support our hypothesis that promoter haplotypes alter the binding of TFs to the MGMT P/E and, subsequently, affect their regulatory function on MGMT promoter activity and expression level.

Genetic Association Between Angiotensinogen Polymorphisms and Lung Cancer Risk

Earlier published studies investigating the association between polymorphisms in the angiotensinogen gene and lung cancer risk showed no consistent results. In this study, we have summarized all currently available data to examine the correlation by meta-analysis. Case-control studies addressing the association being examined were identified through Embase, the Cochrane Library, ISI Web of Science (Web of Knowledge), Google Scholar, PubMed, and CNKI databases. Risk of lung cancer (odds ratio [OR] and 95% confidence interval [CI]) was estimated with the fixed or the random effects model assuming homozygous, allele, heterozygous, dominant, and recessive models for all angiotensinogen polymorphisms. We identified a total of 10 articles in this meta-analysis, including 7 for Leu84Phe, 4 for Ile143Val, and 3 for Leu53Leu. In the meta-analysis of Leu84Phe polymorphism, the homozygous model provided an OR of 1.44 (Phe/Phe vs Ile/Ile: OR = 1.44, 95% CI = 1.04-1.99, P values for heterogeneity test (Q-test) [P(Het)] = 0.382). The significantly increased risk was similarly indicated in the recessive model (Phe/Phe vs Phe/Ile + Ile/Ile: OR = 1.41, 95% CI = 1.02-1.95, P(Het) = 0.381). We also observed a positive association in the Caucasian subgroup. The heterozygous model and the dominant model tested for the Ile143Val polymorphism showed a marginally increased risk (Ile/Val vs Ile/Ile: OR = 1.16, 95% CI = 1.00-1.36, P(Het) = 0.323; Val/Val + Ile/Val vs Ile/Ile: OR = 1.15, 95% CI = 0.99-1.34, P(Het) = 0.253). These data suggest that Leu84Phe and Ile143Val polymorphisms in the angiotensinogen gene may be useful biomarkers for lung cancer in some specific populations.

Role of O6-methylguanine-DNA methyltransferase in pathogenesis of colorectal cancer

As the fourth most common malignancy, colorectal cancer poses a serious threat to human health. With the changes in lifestyle and diet, the incidence of colorectal cancer is increasing year by year. Inactivation of tumor suppressor genes, activation of oncogenes and abnormal overexpression of DNA repair genes have been known to be responsible for the pathogenesis of colorectal cancer. O⁶-methylguanine DNA methyltransferase (MGMT) is a DNA repair enzyme that can remove the adducts on DNA and repair the DNA damage. Studies have found that methylation of the MGMT gene is closely related to the occurrence of colorectal cancer.

Influence of promoter/enhancer region haplotypes on MGMT transcriptional regulation: a potential biomarker for human sensitivity to alkylating agents

The O6-methylguanine-DNA methyltransferase gene (MGMT) encodes the direct reversal DNA repair protein that removes alkyl adducts from the O6 position of guanine. Several single-nucleotide polymorphisms (SNPs) exist in the MGMT promoter/enhancer (P/E) region. However, the haplotype structure encompassing these SNPs and their functional/biological significance are currently unknown. We hypothesized that MGMT P/E haplotypes, rather than individual SNPs, alter MGMT transcription and can thus alter human sensitivity to alkylating agents. To identify the haplotype structure encompassing the MGMT P/E region SNPs, we sequenced 104 DNA samples from healthy individuals and inferred the haplotypes using the data generated. We identified eight SNPs in this region, namely T7C (rs180989103), T135G (rs1711646), G290A (rs61859810), C485A (rs1625649), C575A (rs113813075), G666A (rs34180180), C777A (rs34138162) and C1099T (rs16906252). Phylogenetics and Sequence Evolution analysis predicted 21 potential haplotypes that encompass these SNPs ranging in frequencies from 0.000048 to 0.39. Of these, 10 were identified in our study population as 20 paired haplotype combinations. To determine the functional significance of these haplotypes, luciferase reporter constructs representing these haplotypes were transfected into glioblastoma cells and their effect on MGMT promoter activity was determined. Compared with the most common (reference) haplotype 1, seven haplotypes significantly upregulated MGMT promoter activity (18-119% increase; P < 0.05), six significantly downregulated MGMT promoter activity (29-97% decrease; P < 0.05) and one haplotype had no effect. Mechanistic studies conducted support the conclusion that MGMT P/E haplotypes, rather than individual SNPs, differentially regulate MGMT transcription and could thus play a significant role in human sensitivity to environmental and therapeutic alkylating agents.

MGMT Leu84Phe polymorphism contributes to cancer susceptibility: evidence from 44 case-control studies

Background

O⁶-methylguanine-DNA methyltransferase is one of the few proteins to directly remove alkylating agents in the human DNA direct reversal repair pathway. A large number of case-control studies have been conducted to explore the association between MGMT Leu84Phe polymorphism and cancer risk. However, the results were not consistent.

Methods

We carried out a meta-analysis of 44 case-control studies to clarify the association between the Leu84Phe polymorphism and cancer risk.

Results

Overall, significant association of the T allele with cancer susceptibility was verified with meta-analysis under a recessive genetic model (P<0.001, OR=1.30, 95%CI 1.24-1.50) and TT versus CC comparison (P=0.001, OR=1.29, 95% CI 1.12-1.50). In subgroup analysis, a significant increased risk was found for lung cancer (TT versus CC, P=0.027, OR=1.67, 95% CI 1.06-2.63; recessive genetic model, P=0.32, OR=1.64, 95% CI 1.04-2.58), whereas risk of colorectal cancer was significantly low under a dominant genetic model (P=0.019, OR=0.84, 95% CI 0.72-0.97). Additionally, a significant association between TT genetic model and total cancer risk was found in the Caucasian population (TT versus CC, P=0.014, OR=1.29, 95% CI 1.05-1.59; recessive genetic model, P=0.009, OR=1.31, 95% CI 1.07-1.61), but not in the Asian population. An increased risk for lung cancer was also verified in the Caucasian population (TT versus CC, P=0.035, OR=1.62, 95% CI 1.04-2.53; recessive genetic model, P=0.048, OR=1.57, 95% CI 1.01-2.45).

Conclusions

These results suggest that MGMT Leu84Phe polymorphism might contribute to the susceptibility of certain cancers.

The polymorphisms in the MGMT gene and the risk of cancer: a meta-analysis

Polymorphisms in the MGMT gene have been implicated in susceptibility to cancer, but the published studies have reported inconclusive results. The objective of the current study was to investigate the genetic risk of polymorphisms in the MGMT gene for cancer. A meta-analysis was carried out to analyze the association between polymorphisms in the MGMT gene and cancer risk. Five polymorphisms (Leu84Phe, Leu53Leu, Ile143Val, Lys178Arg, and -485C/A) with 98 case-control studies from 49 articles were analyzed. The results indicated that individuals who carried the Phe/Phe homozygote genotype of Leu84Phe had a 31 % increased risk of cancer compared with the Leu allele (Leu + Leu/Phe) carriers (odds ratio [OR] = 1.32, 95 % confidence interval [CI] = 1.15-1.52, P < 0.0001 for Phe/Phe vs. Phe/Leu + Leu/Leu). However, there was no significant association between the risk of cancer and the other four polymorphisms (Leu53Leu, Ile143Val, Lys178Arg, and -485C/A). In further stratified analyses for the Leu84Phe and Ile143Val polymorphisms, the increased risk of cancer remained in subgroups of Caucasians, patients with esophageal cancer for the Leu84Phe polymorphism, and patients with lung cancer for the Ile143Val polymorphism. Results from the current meta-analysis suggested that Leu84Phe and Ile143Val in the MGMT gene are risk factors for cancer. In the future, more studies should be performed to validate our results.

Molecular epidemiology of female lung cancer

Lung cancer is still a leading cause of cancer mortality in the world. The incidence of lung cancer in developed countries started to decrease mainly due to global anti-smoking campaigns. However, the incidence of lung cancer in women has been increasing in recent decades for various reasons. Furthermore, since the screening of lung cancer is not as yet very effective, clinically applicable molecular markers for early diagnosis are much required. Lung cancer in women appears to have differences compared with that in men, in terms of histologic types and susceptibility to environmental risk factors. This suggests that female lung cancer can be derived by carcinogenic mechanisms different from those involved in male lung cancer. Among female lung cancer patients, many are non-smokers, which could be studied to identify alternative carcinogenic mechanisms independent from smoking-related ones. In this paper, we reviewed molecular susceptibility markers and genetic changes in lung cancer tissues observed in female lung cancer patients, which have been validated by various studies and will be helpful to understand the tumorigenesis of lung cancer.

Associations between polymorphisms in DNA repair genes and TP53 mutations in non-small cell lung cancer

This study was conducted to identify genetic factors predisposing to TP53 mutations in patients with non-small cell lung cancer (NSCLC). A comprehensive panel of potentially functional single nucleotide polymorphisms (SNPs) in DNA repair genes was evaluated in relation to TP53 mutations. Thirty-seven SNPs in 28 DNA repair genes were genotyped by a sequenome mass spectrometry-based genotyping assay in 173 NSCLCs and the associations with TP53 mutations in the entire coding exons (exons 2-11), including splicing sites of the gene, were analyzed. Four SNPs (XPA rs1800975, OGG1 rs1052133, ADPRT rs1136410, and NBS1 rs1805794) were significantly associated with the prevalence of TP53 mutations in multivariate analysis for each SNP. When the 4 SNPs were combined, the prevalence of TP53 mutations was increased as the number of bad genotypes increased (P(trend)=0.001). Patients with 3 and 4 bad genotypes had a significantly higher frequency of TP53 mutations than those with 0-1 bad genotypes (adjusted odds ratio=5.18, 95% confidence interval=1.51-17.81, P=0.01 and adjusted odds ratio=18.26, 95% confidence interval=2.87-116.09, P=0.002, respectively). These findings suggest that the 4 SNPs may modulate the occurrence of TP53 mutations and contribute to lung carcinogenesis. However, larger studies are required to confirm our findings in other ethnic populations.

MGMT -535G>T polymorphism is associated with prognosis for patients with metastatic colorectal cancer treated with oxaliplatin-based chemotherapy

PURPOSE

The present study analyzed the polymorphisms of DNA repair genes and their impact on the response to chemotherapy and survival of patients with colorectal cancer.

PATIENTS AND METHODS

A total of 94 patients with recurrent or metastatic colorectal cancer treated with oxaliplatin-based combination chemotherapy were enrolled in the present study. The single nucleotide polymorphisms of 16 DNA repair genes were determined using a PCR-RFLP assay.

RESULTS

During the median follow-up duration of 15.9 (2.1-53.0) months, 67 (71.3%) progressions and 29 (30.9%) deaths were observed. Among the 60 patients assessable for response, response to the oxaliplatin-based regimens was found in 27 (45%) patients (9 CR and 18 PR). In a logistic regression analysis adjusted to age, sex, primary site, disease status, and regimen, the POLR2C rs4937 and MSH2 rs3732183 polymorphisms were statistically associated with the response to the oxaliplatin-based chemotherapy. A multivariate survival analysis showed that the TT genotype of the MGMT (rs1625649) -535G>T polymorphism was found to correlate with a worse progression-free survival (PFS) than the combined GG + GT genotypes (HR = 3.137; 95% CI = 1.423-6.914; P = 0.005), which was also observed among the 60 evaluable patients (HR = 2.653; 95% CI = 1.101-6.392; P = 0.030) For the clinical parameters, curative resection was the most significant prognostic factor in a Cox model for PFS and overall survival (HR = 0.229 and 0.205; P < 0.001 and 0.001, respectively).

CONCLUSION

The MGMT -535G>T polymorphism (rs1625649) was found to be correlated with PFS in patients with advanced colorectal cancer treated with oxaliplatin-based chemotherapy.

Effects of O6-methylguanine-DNA methyltransferase (MGMT) polymorphisms on cancer: a meta-analysis

O(6)-methylguanine-DNA methyltransferase is one of the rare proteins to directly remove alkylating agents in the human DNA direct reversal repair pathway. Its two common single-nucleotide polymorphisms, Leu84Phe and Ile143Val, had previously been identified to contribute to susceptibility of cancer. However, there are conflicting results in studies on the association of the two polymorphisms with cancer. Therefore, we conducted a meta-analysis to clarify the paradox with a large collected sample (13,069 cancer patients and 20,290 controls). We found significant association between the T allele (84Phe) and cancer risk, under the recessive genetic model [P = 0.023, odds ratio (OR) = 1.251, 95% confidence interval (CI) 1.031-1.517, P(heterogeneity) = 0.270], TT versus CC comparison (P = 0.035, OR = 1.239, 95% CI 1.015-1.511, P(heterogeneity) = 0.225) and TT versus CT comparison (P = 0.007, OR = 1.292, 95% CI 1.071-1.559, P(heterogeneity) = 0.374), using the random-effect model. In the ethnicity subgroup analysis, a significant association with cancer among Caucasians was found under the recessive genetic model, homozygote comparison and TT versus TC comparison. In the tumour sites subgroup analysis, only the protective effects of Leu84Phe polymorphism were found in colorectal cancer, under CT versus CC comparison. No significant association between the G allele of Ile143Val and cancer risk was found. The G allele showed an increased lung cancer risk under the dominant genetic model and AG versus AA comparison in all Hardy-Weinberg equilibrium subjects, only when the fixed-effect model was used. However, it was insignificant in the random-effect model.

Inherited predisposition of lung cancer: a hierarchical modeling approach to DNA repair and cell cycle control pathways

The DNA repair systems maintain the integrity of the human genome and cell cycle checkpoints are a critical component of the cellular response to DNA damage. We hypothesized that genetic variants in DNA repair and cell cycle control pathways will influence the predisposition to lung cancer, and studied 27 variants in 17 DNA repair enzymes and 10 variants in eight cell cycle control genes in 1,604 lung cancer patients and 2,053 controls. To improve the estimation of risks for specific variants, we applied a Bayesian approach in which we allowed the prior knowledge regarding the evolutionary biology and physicochemical properties of the variant to be incorporated into the hierarchical model. Based on the estimation from the hierarchical modeling, subjects who carried OGG1 326C/326C homozygotes, MGMT 143V or 178R, and CHEK2 157I had an odds ratio of lung cancer equal to 1.45 [95% confidence interval (95% CI), 1.05-2.00], 1.18 (95% CI, 1.01-1.40), and 1.58 (95% CI, 1.14-2.17). The association of CHEK2 157I seems to be overestimated in the conventional analysis. Nevertheless, this association seems to be robust in the hierarchical modeling. None of the pathways seem to have a prominent effect. In general, our study supports the notion that sequence variation may explain at least some of the variation of inherited susceptibility. In particular, further investigation of OGG1, MGMT, and CHEK2 focusing on the genetic regions where the present markers are located or the haplotype blocks tightly linked with these markers might be warranted.

Correlating observed odds ratios from lung cancer case-control studies to SNP functional scores predicted by bioinformatic tools

Bioinformatic tools are widely utilized to predict functional single nucleotide polymorphisms (SNPs) for genotyping in molecular epidemiological studies. However, the extent to which these approaches are mirrored by epidemiological findings has not been fully explored. In this study, we first surveyed SNPs examined in case-control studies of lung cancer, the most extensively studied cancer type. We then computed SNP functional scores using four popular bioinformatics tools: SIFT, PolyPhen, SNPs3D, and PMut, and determined their predictive potential using the odds ratios (ORs) reported. Spearman's correlation coefficient (r) for the association with SNP score from SIFT, PolyPhen, SNPs3D, and PMut, and the summary ORs were r=-0.36 (p=0.007), r=0.25 (p=0.068), r=-0.20 (p=0.205), and r=-0.12 (p=0.370), respectively. By creating a combined score using information from all four tools we were able to achieve a correlation coefficient of r=0.51 (p<0.001). These results indicate that scores of predicted functionality could explain a certain fraction of the lung cancer risk detected in genetic association studies and more accurate predictions may be obtained by combining information from a variety of tools. Our findings suggest that bioinformatic tools are useful in predicting SNP functionality and may facilitate future genetic epidemiological studies.

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.

Lung cancer risk and variation in MGMT activity and sequence

O(6)-Alkylguanine-DNA alkyltransferase (MGMT) repairs DNA adducts that result from alkylation at the O(6) position of guanine. These lesions are mutagenic and toxic and can be produced by a variety of agents including the tobacco-specific nitrosamines, carcinogens present in cigarette smoke. Here, we review some of our work in the context of inter-individual differences in MGMT expression and their potential influence on lung cancer risk. In humans there are marked inter-individual differences in not only levels of DNA damage in the lung (N7-methylguanine) that can arise from exposure to methylating agents but also in MGMT activity in lung tissues. In the presence of such exposure, this variability in MGMT activity may alter cancer susceptibility, particularly as animal models have demonstrated that the complete absence of MGMT activity predisposes to alkylating-agent induced cancer while overexpression is protective. Recent studies have uncovered a series of polymorphisms that affect protein activity or are associated with differences in expression levels. The associations between these (and other) polymorphisms and cancer risk are inconsistent, possibly because of small sample sizes and inter-study differences in lung cancer histology. We have recently analysed a consecutive series of case-control studies and found evidence that lung cancer risk was lower in subjects with the R178 allele.

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.

Association of Genetic Variants of O6-Methylguanine-DNA Methyltransferase with Risk of Lung Cancer in Non-Hispanic Whites

O6-methylguanine, a methylated damage lesion in DNA, correlates with spontaneous G:C --> A:T transition mutations and leads to activation of oncogene K-ras or dysfunction of the tumor suppressor gene p53. O6-methylguanine-DNA methyltransferase (MGMT) is critical for repairing damage to the O6-position of guanine. Therefore, we tested our hypothesis that genetic variants of MGMT are associated with increased lung cancer risk in a Caucasian population of 1,121 lung cancer patients and 1,163 matched cancer-free controls. We genotyped four potentially functional single nucleotide polymorphisms (SNPs) of MGMT: exon 3 codon 84C --> T (L84F), exon 5 codon 143A --> G (I143V), and two promoter SNPs 135G --> T and 485C --> A. The allele frequency distributions of the SNPs of codon 84C --> T and the promoter 135G --> T in the cases were borderline different from that in the controls. After defining the minor allele (T for codon 84C --> T and G for codon 143A --> G) as the variant allele, we categorized the MGMT genotypes as either 0 variants (84CC-143AA) or 1-4 variants. Compared with 0 variants, those with 1-4 variants showed a statistically significantly increased risk of lung cancer (P = 0.040). Further stratification analysis showed that this increased risk was more pronounced in women, current smokers, and non-small cell lung cancer. We did not find any association between the MGMT promoter SNPs and lung cancer risk. Our findings suggest that non-synonymous SNPs in MGMT are associated with modestly increased risk of lung cancer in Caucasians and need to be further investigated.

MDM2 309T>G polymorphism and risk of lung cancer in a Korean population

BACKGROUND

The MDM2 protein plays an important role in regulating cell proliferation and apoptosis by interaction with multiple proteins including p53 and Rb. A polymorphism (309T>G) in the MDM2 promoter has been shown to result in higher levels of MDM2 RNA and protein. In order to evaluate the association of the MDM2 309T>G polymorphism and lung cancer risk, we carried out a case-control study in a Korean population.

METHODS

The MDM2 genotypes were determined in 582 lung cancer patients and in 582 healthy control subjects who were frequency matched for age and gender.

RESULTS

The distribution of the MDM2 309T>G genotypes was not significantly different between overall lung cancer cases and controls. However, when the cases were categorized by tumor histology, the 309GG genotype was associated with a significantly increased risk of adenocarcinoma (adjusted OR=1.91, 95% CI=1.16-3.14, P=0.01) compared to the 309TT genotype. In addition, the risk of adenocarcinoma increased as the number of 309G alleles increased (P(trend)=0.01).

CONCLUSION

Our findings suggest that the MDM2 309T>G polymorphism may be used as a marker for genetic susceptibility to adenocarcinoma of the lung.