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Dept of Pulmonary Medicine, Institute of Clinical Medicine, University of Tsukuba, Ibaraki, Japan
CORRESPONDENCE: Y. Uchida, Dept of Pulmonary Medicine, Institute of Clinical Medicine, University of Tsukuba, 1-;1-;1 Tennnoudai Tsukuba, Ibaraki, 305-;8575, Japan. Fax: 81 298533144
Keywords: chronic obstructive pulmonary disease, polymorphism, tissue inhibitors of metalloproteinases
Received: December 6, 2000
Accepted April 3, 2001
| Abstract |
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Polymorphisms of the TIMP-;2 gene were investigated in 88 COPD patients and 40 control subjects. The variations were examined by single-strand conformational polymorphism analysis followed by sequencing.
Two polymorphisms were identified, +853 G/A and 418 G/C nucleotide substitutions. There was a significant deviation in the genotypic frequencies at +853 and the allele frequencies for G were significantly higher in the COPD patient group than in the control group. For locus 418, the allele frequencies for C in the COPD patient group also tended to be higher than those in the control group. The +853 G/A nucleotide substitution was a silent variant. The 418 G/C substitution was located in the consensus sequence for the Sp1 binding site.
These polymorphisms may be associated with the development of chronic obstructive pulmonary disease, decreasing the transcription and stability of the messenger ribonucleic acid, and available as genetic markers of susceptibility to the disease.
Chronic obstructive pulmonary disease (COPD) is characterized by a slowly progressive airflow limitation caused by emphysema and/or airway disease. The most important risk factor for COPD is cigarette smoke and nearly 90% of COPD patients are smokers 1. Cigarette smoke contains high concentrations of free radicals, which recruit inflammatory cells such as neutrophils and macrophages to the lung. These inflammatory cells also generate reactive oxygen species and release a variety of proteolytic enzymes. The oxidants inactivate antiproteases such as
-;1-;proteinase inhibitor, antileukoprotease and tissue inhibitors of metalloproteinases (TIMPs) 24 as well as directly damage the lung matrix 5. These events generate the proteinase/antiproteinase imbalance that is the most widely accepted theory for the pathogenesis of COPD. However, only
15% of cigarette smokers develop COPD. Therefore, it is suggested that genetic susceptibility to smoking injury may contribute to the development of COPD. This hypothesis is also supported by evidence that there are differences in the prevalence of COPD in smokers between different ethnic groups 6. The genetic factors determining the susceptibility might depend on increased activities of proteolytic enzymes on the one hand, or inactivation of protective enzymes against oxidative or proteolytic injuries on the other. Recently, several studies have demonstrated that expressions or activities of matrix metalloproteinases (MMPs), especially MMP-;1, 2, 8, 9 and membrane-type MMP-;1 (MT1-MMP), are elevated in the lung of COPD patients 710. These findings suggest that upregulated activities of MMPs play a role in the pathogenesis of COPD. One important mechanism for the regulation of the activities of MMPs is the binding to TIMPs. Four members of the TIMP family (TIMP-;1 to TIMP-;4) interact with active forms of MMPs to inhibit their activities. However, it is reported that TIMP-;4 messenger ribonucleic acid (mRNA) cannot be detected in the lung 11. And it is mainly TIMP-;1 and 2 that have been shown to participate in pulmonary diseases characterized by alterations of alveolar structure or abnormal remodelling responses such as emphysema, interstitial fibrosis, acute respiratory distress syndrome and lung cancer. Furthermore, it has been suggested that the MT1-MMP/MMP-;2/TIMP-;2 system plays a significant role in the formation of pulmonary emphysema 8. Therefore, it is possible that mutations of the TIMP-;2 gene that downregulate its own activity result in the progression of the breakdown of lung matrix.
In this study, genetic variations in the promoter and coding regions of the TIMP-;2 gene were investigated. Two polymorphisms were identified; one in the promoter region and the other in exon 3, and an association analysis was carried out for these polymorphisms in COPD patients and healthy controls. Possible mechanisms by which these variants inhibit the activity of TIMP-;2 are discussed.
| Subjects and methods |
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Deoxyribonucleic acid samples and polymerase chain reaction conditions
Genomic deoxyribonucleic acid (DNA) samples were prepared from whole blood using a QIAGEN DNA blood kit (QIAGEN, Hilden, Germany). The nucleotide sequence and the genomic structure of the human TIMP-;2 gene have been reported previously 12, (Genbank accession number U44381
[GenBank]
-U44385). The positions of nucleotides in this study are given relative to the major transcription initiation site. Eleven sets of oligonucleotide primers were prepared to produce polymerase chain reaction (PCR) DNA fragments covering the promoter region and the whole coding region (table 2
). The 5' terminus of each primer was labelled with fluorescein isothiocyanate for fluorescence-based single-strand conformational polymorphism (SSCP) analysis. PCR amplification was performed with 100 ng of genomic DNA in a total volume of 50 µL containing 200 µM of each deoxyribonucleoside triphosphate (dNTP), 100 nM of each primer, and 1.25 U of Hotstar Taq DNA polymerase (QIAGEN). The annealing temperatures for PCR are given in table 2
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Statistical analysis
The values in the profile shown in table 1
are the mean±sem. Statistical analysis of age, Brinkman's index (the number of cigarettes·day1xthe number of yrs) and results of the pulmonary function test was performed by unpaired t-;test. Significance was accepted at p<0.05. Significant differences in both genotypic and allele frequencies between patients and controls were analysed by using Fisher's exact test (2x3 table and 2x2 table). Odds ratios and 95% confidence intervals (CIs) were calculated to quantitatively assess the degree of association between particular alleles and COPD.
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| Results |
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Two variants for the TIMP-;2 gene were identified by PCR-SSCP analysis followed by direct DNA sequencing (fig.1
). One was a G/A nucleotide substitution at position +853 in exon 3 that was a silent variant. The other was characterized by a single base G/C nucleotide substitution at position 418, which was located in the consensus sequence for Sp1 (GAGGCTGGG) in the promoter region. The homozygous wild-type, the homozygous variant-type, and the heterozygous genotype were identified for these two variants. Since no restriction enzyme was available for these polymorphisms, SSCP analysis was used to screen for the presence or absence of the nucleotide substitution.
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| Discussion |
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Another polymorphism, the single base G/C substitution at position 418, is located in the consensus sequence for the Sp1 binding site 16 in the promoter region. Sp1 protein binds to the consensus sequence in a variety of cells and stimulates transcriptional activity. A study of the promoter activity of the TIMP-;2 gene has demonstrated that a 519-bp segment upstream from the major transcription initiation site contains active promoter elements 17. Therefore, it is possible that the 418 nucleotide substitution in the consensus sequence for Sp1 results in downregulation of the transcriptional activity of the TIMP-;2 gene.
There is evidence that MMPs, especially MMP-;1, 2, 8, 9 and MT1-MMP, play a role in the development of COPD. The mRNA expressions or the enzymatic activities of the MMPs are elevated in alveolar macrophages, the lung tissue or bronchoalveolar lavage fluid from COPD patients 79. Immunoreactivity for the MMPs is observed in the lung tissue of COPD patients 8, 10. Furthermore, it has been reported that macrophage elastase (MME; MMP-12) gene-knockout mice (MME/) do not develop emphysema in response to exposure to cigarette smoke, while wild-type (MME+/+) mice do 18. Their study directly demonstrated the contribution of MMP-12 to the pathogenesis of cigarette smoke-induced emphysema. All of these MMPs are counteracted by TIMPs. Until now, no comprehensive quantitative comparison has been made of the inhibitory selectivity of different TIMPs with different MMPs. However, it is suggested that the MT1-MMP/MMP-;2/TIMP-;2 system is important in extracellular matrix degradation of emphysematous lung 8. In addition, TIMP-;2 is a more effective inhibitor of MMP-;2 and MMP-;9 than TIMP-;1 19. It seems likely that downregulation of TIMP-;2 leads to increased activities of MMPs and results in the development of COPD.
COPD is a heterogeneous disease including emphysema and chronic bronchitis. MMPs break down connective tissue in the lung parenchyma resulting in emphysema. In contrast, chronic bronchitis is an inflammatory disease characterized by hypersecretion from hypertrophic bronchial glands. It is suggested that the secretion and activation of MMP-;2 is correlated with gland morphogenesis 20. In addition, MMPs recruit neutrophils to the lung 21, resulting in mucous hypersecretion and inflammation of the bronchial trees. Therefore, it is possible that the inhibition of TIMP-;2 activity is related to chronic bronchitis as well as emphysema.
In conclusion, two polymorphisms for the tissue inhibitor of metalloproteinases-;2 gene were associated with chronic obstructive pulmonary disease susceptibility caused by cigarette smoke in a Japanese population. Although chronic obstructive pulmonary disease is thought to be caused by a complex interaction of environmental and genetic factors, the polymorphisms identified in this study might be useful for predicting the susceptibility to the disease. Quitting smoking may be critical for the prevention of chronic obstructive pulmonary disease in people with either of these polymorphisms. It is also expected that these polymorphisms will contribute to the elucidation of mechanisms for the development of chronic obstructive pulmonary disease. It would be interesting to investigate the prevalence of these polymorphisms in chronic obstructive pulmonary disease patients among different ethnic groups.
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