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1 Occupational Health Program, Departments of Environmental Health and 2 Biostatistics, Harvard School of Public Health, Boston, Massachusetts; 3 Thoracic Surgery Unit, Departments of Surgery and 4 Hematology-Oncology, and 5 Pulmonary and Critical Care Unit, Department of Medicine, Massachusetts General Hospital, Harvard Medical School, Boston, Massachusetts
Requests for reprints: David C. Christiani, Harvard School of Public Health, 665 Huntington Avenue, Boston, MA 02115. Tel: 617-432-3323; Fax: 617-432-6981. E-mail: dchristi{at}hsph.harvard.edu
| Abstract |
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| Introduction |
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| Materials and Methods |
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Statistical Analysis
Although individuals of all races were recruited for this study, we restricted our analyses to Caucasians (97%) in order to minimize confounding due to allele frequency variation by ethnicity. Logistic regression models were fit to examine the relationship between the log odds of lung cancer and each covariate, after adjusting for possible confounding factors such as age, gender, smoking status, pack-years of smoking, and years since smoking cessation (if ex-smoker, defined as those who quit smoking more than a year before recruitment). As suggested before, square root transformed pack-years of smoking were used in the analyses instead of the original untransformed variable (7). In addition to the overall association analysis, we did a stratified analysis by gender, age, smoking status, histology, and clinical stages, to further explore the association between MMP-1 polymorphism and the risk of lung cancer in each stratum. Case-only analyses were done to investigate the effect of MMP-1 polymorphism on different histologic subtypes or clinical stages among lung cancer patients. Statistical analyses were undertaken using SAS statistical packages (SAS Institute, Cary, NC).
| Results |
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2 goodness of fit). MMP-1 genotype frequencies were very similar between cases and controls, with the frequencies of 1G/1G, 1G/2G, and 2G/2G genotypes of 26%, 51%, and 23% in cases, and 27%, 49% and 24% in controls, respectively. There is no statistical difference in genotype distributions between cases and controls for different strata of age, gender, smoking status, pack-years of smoking, and clinical stage subgroups (P > 0.10; Table 2).
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No statistically significant association was found between MMP-1 polymorphism and lung cancer risk in younger or older subjects (Table 2). Statistically significant higher lung cancer risk (only for the 1G/2G genotype) was found in males but not in females, with the adjusted ORs for 1G/2G and 2G/2G genotypes (when using the 1G/1G genotype as reference) of 1.30 (95% CI, 1.00-1.75) and 1.23 (95% CI, 0.88-1.73) in males, and 0.99 (95% CI, 0.74-1.32) and 1.05 (95% CI, 0.75-1.46) in females, respectively. For different smoking categories, statistically higher lung cancer risk (only for the 1G/2G genotype) was found in never-smokers, with the adjusted ORs for 1G/2G and 2G/2G genotypes (when versus the 1G/1G genotype) of 1.67 (95% CI, 1.02-2.76) and 1.50 (95% CI, 0.86-2.62), respectively. No associations were found in ex-smokers or current smokers, or in different pack-years of smoking (Table 2).
Association Between MMP-1 Genotypes and Lung Cancer Risk in Different Histologic and Disease Stages
For different histologic subtypes of lung cancer, higher lung cancer risk of MMP-1 polymorphism was found for patients with adenocarcinoma than for patients with squamous cell carcinoma (when each was compared with all controls), with the adjusted ORs for 1G/2G and 2G/2G genotypes (when versus the 1G/1G genotype) of 1.20 (95% CI, 0.95-1.52) and 1.21 (95% CI, 0.92-1.59), respectively, for patients with adenocarcinoma; and 0.93 (95% CI, 0.66-1.31), and 0.72 (95% CI, 0.47-1.11), respectively, for squamous cell carcinoma patients (Table 3). In case-only analysis, the adjusted ORs of adenocarcinoma versus squamous cell carcinoma for 1G/2G and 2G/2G genotypes were 1.12 (95% CI, 0.83-1.51) and 1.51 (95% CI, 1.04-2.20), respectively. By gender, the histologic difference of MMP-1 polymorphism was found in males only, and not in females, with the case-only adjusted ORs (adenocarcinoma versus squamous cell carcinoma) for the 1G/2G and 2G/2G genotypes were 1.25 (95% CI, 0.85-1.85) and 1.88 (95% CI, 1.16-3.07) in males; and 0.90 (95% CI, 0.55-1.48) and 1.05 (95% CI, 0.58-1.91) in females.
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| Discussion |
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In this study, the MMP-1 2G allele was associated with higher lung cancer risk in never-smokers, but not in ex- or current smokers, nor in any of the smoking subgroups. One possible explanation is that although cigarette smoking may increase MMP-1 mRNA levels and MMP-1 activity (12, 13), smoking may also increase the mRNA levels of tissue inhibitors of metalloprotease (14, 15). Therefore, the effect of MMP-1 polymorphism in smokers may depend on the balances between MMPs and tissue inhibitors of metalloprotease. The results are inconsistent with a previous study where the 2G/2G genotype was found to be associated with higher lung cancer risk in current smokers and heavy smokers, and not in never-smokers (3). However, the genotype distribution of this polymorphism was out of Hardy-Weinberg equilibrium in the control population for the previous study, and the 1G/2G genotype was combined with the 1G/1G genotype in all analyses. Further studies are needed to investigate the effect of cigarette smoking on the expression levels of different genotypes of MMP-1.
Consistent with the previous study (3), we found that lung cancer risk conferred by the 2G allele was higher in males than females. Similar gender differences in risk for the MMP-1 polymorphism were also found for renal cell and gastric cancer patients (16, 17). Sex hormones including progesterone, estradiol, and ovarian steroids levels may decrease the MMP-1 expression levels and increase the tissue inhibitor levels of MMP-1 proteins (18-20). Therefore, females may have lower MMP-1 levels than males, and estrogen levels may be more important in the influence on the expression levels of MMP-1 than the polymorphism (16).
For different histologic subtypes of patients, we observed a higher risk of the 2G allele for patients with adenocarcinoma when compared with patients with squamous cell carcinoma, specifically for males. This finding is echoed in a study that reported adenocarcinoma overexpressed MMP-1 more frequently than squamous cell carcinoma in nonsmall cell lung cancer tissues (21). The mechanism for this histologic difference is still under investigation, but there are two distinct theories. The MMP-1 1607 1G/2G polymorphism is adjacent to an activating protein-1 site at 1,602 bp, which may cooperate with the 2G allele (Ets site) to induce higher levels of transcription (5), and the activator protein-1 site is inhibitory in the context of the 1G allele, but activating the 2G allele (6). If this activator protein-1 site is mutated, it could lead to a substantial increase in expression of the 1G allele where the difference in transcription between 1G and 2G alleles is abolished and the MMP-1 expression is similar between the two alleles (6). It is possible that activator protein-1 mutant frequencies are different between different histologic subtypes of lung cancer, which may induce the differential association between MMP-1 polymorphism and lung cancer risk by histologic subtypes. An alternative explanation is that squamous cell carcinoma tumor tissues have been found to have higher rates of p53 mutations than adenocarcinoma tissues (22). MMP-1 is a target of the p53 protein, and wild-type p53 can exert a strong inhibitory effect on the human MMP-1 promoter by disrupting communications between the transactivator 72AP-1 and the basal transcriptional complex, whereas p53 mutants lose such repressive activity (23, 24). The down-regulation of the human MMP1 promoter by p53 was abolished after the proximal 72AP-1 site was deleted or mutated (24). Therefore, mutated p53 may lose the ability of regulating MMP-1 transcription.
There are several limitations to this study. Firstly, this is a hospital-based case-control study, where a subset of the controls included healthy spouses and friends of patients with lung cancer. Secondly, we only evaluated the MMP-1 1607 1G/2G polymorphism that has been suggested to be associated with MMP-1 expression levels, which may result in some misclassification in MMP-1 action. It is possible that other polymorphisms might also be associated with MMP-1 expression levels, which may be one reason why we observed a stronger association for the 1G/2G genotype than the 2G/2G genotype in most of the comparisons. Thirdly, although we adjusted for various smoking variables in all of the analyses, secondhand smoke exposure, alcohol consumption, diet, and environmental and occupational exposure data were not adjusted in our logistic regression models because of missing or incomplete information. Lastly, we did multiple comparisons through subgroup analyses. It is possible that the borderline significant results could be due to multiple comparisons. Given our sample size, we have tried as much as possible to avoid the bias by careful a priori specification of our primary hypotheses.
In conclusion, the 2G allele of the MMP-1 1607 1G/2G polymorphism is associated with higher lung cancer risk in never-smokers and males, where the risk is mostly attributable to patients with adenocarcinoma.
| Acknowledgments |
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| Footnotes |
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The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Note: L. Su and W. Zhou contributed equally to this work.
Received 6/29/04; revised 9/ 9/04; accepted 10/27/04.
| References |
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and sex steroids. Ceska Gynekol 2000;65:2115.[Medline]
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