The Genetics of Asthma
http://www.100md.com
《美国胸部学报》
Division of Infection, Inflammation and Repair, School of Medicine, University of Southampton, and Southampton General Hospital, Southampton, United Kingdom
ABSTRACT
The ability to identify novel disease genes by positional cloning led to the identification of a disintegrin and metalloprotease (ADAM)33 gene on chromosome 20p13 as a susceptibility gene for asthma. Case-control and family-based association studies have mostly confirmed a link between ADAM33 and asthma. Its restricted expression to mesenchymal cells as well as its association with bronchial hyperresponsiveness and accelerated decline in lung function over time point strongly to its involvement in the structural airway components of asthma, such as remodeling. Extensive alternative splicing, expression during branching morphogensis in the developing fetus, impaired lung function in childhood, the production of a soluble form linked to chronic asthma, and tight epigenetic regulation indicate a level of complexity in the way ADAM33 influences disease phenotype. Its recent association with chronic obstructive pulmonary disease as well as with asthma and lung development points to functions relating to airway wall modeling and remodeling as a general morphogenetic repair gene rather than being restricted to asthma.
Key Words: ADAMs;asthma;biomarkers;hyperresponsiveness
In 1860, Henry Hyde Salter, in his treaties "On Asthma; Its Pathology and Treatment," drew attention to the spasmodic nature of the airway obstruction and its association with a wide variety of environmental factors. The concept of bronchial hyperresponsiveness (BHR) is now firmly rooted in our understanding of the pathobiology of asthma, with this response relating to a combination of airway inflammation and dysfunctional airway smooth muscle. Although the last two decades have witnessed an enormous increase in our understanding of the cellular and mediator basis of the inflammatory response of asthma, how this relates to the behavior of airway smooth muscle is still largely unknown. From a pathologic standpoint, asthma is characterized by a combination of hyperplasia and hypertrophy of the smooth muscle bundles in both the small and large airways. Although mediators released in response to airway inflammation, such as cysteinyl leukotrienes and prostaglandins, are capable of contracting human airway smooth muscle, it is not clear how airway inflammation relates to the increase in smooth muscle or to its hyperresponsiveness (1).
Since the writings of Salter, a large number of environmental factors have been identified that predispose to asthma, including prenatal maternal influences, allergen exposure, respiratory infection, tobacco smoke, pollutants, prematurity, and dietary factors. Precisely how these factors interact to generate a chronic inflammatory response remains largely speculative. Because asthma, atopy, and BHR cluster in families, there are likely to be powerful genetic factors that predispose toward the development of asthma through gene–environment interactions at specific times as the disease develops and becomes consolidated (2). Beyond inflammation and BHR, a third clinical phenotype that is being increasingly recognized in more severe and chronic asthma is a degree of fixed airflow obstruction that does not respond to bronchodilators or corticosteroids (3). Although chronic inflammation is likely to play an important role in driving chronic persistent asthma, it is becoming increasingly recognized that epithelial damage and activation of underlying airway mesenchymal cells are an important component of remodeled airway in asthma (4). Because communication between the epithelium and underlying mesenchymal is critical for branching morphogenesis of the lung in the developing fetus, it is likely that reactivation of the epithelial mesenchymal trophic unit is involved in the pathogenesis of chronic asthma, particularly that aspect of the disease which is refractory to antiinflammatory therapy (5). Furthermore, genes that have been implicated in lung development may well be involved later in life with the emergence of chronic asthma (6).
THE DISCOVERY OF NOVEL ASTHMA GENES
On the basis of current understanding of asthma, it is clear that asthma, atopy, and BHR have strong genetic components, but for these to manifest as chronic asthma, important environmental factors are also required, often beginning in pregnancy and early infancy, such as respiratory infections, environmental tobacco smoke exposure, prematurity, and a variety of dietary factors. Using doctor-diagnosed asthma as the clinical phenotype, linkage analysis in 260 families in the United Kingdom and United States led to a region on chromosome 20p13 containing a putative asthma susceptibility gene (7). Conditioning of the phenotype for BHR strengthened the linkage signal, whereas incorporation of total or specific serum IgE into the phenotype weakened the linkage signal (7). Subsequently, positional cloning identified a disintegrin and metalloprotease 33 (ADAM33) as the candidate gene accounting for the linkage signal. Subsequently, there have been a large number of case-control and family-based association studies focused on ADAM33, with the majority (8), but not all (9, 10), confirming the original finding. For example, the Collaborative Study on the Genetics of Asthma, comprising eight U.S. genetic centers, demonstrated positive association between single nucleotide polymorphisms (SNPs) of ADAM33 and asthma and BHR in African-American, Hispanic, and white populations (11). In a further study involving 1,299 asthma cases, 1,665 control subjects, and 4,561 family members, Blakey and colleagues (12) applied a literature-based meta-analysis, and supplemented the database with new asthma cases from populations in Iceland and the United Kingdom, and demonstrated significant association for 4 of the 13 SNPs tested. The authors stated that the polymorphic variation in ADAM33 that they had identified with asthma could account for 50,000 excess cases of asthma in the United Kingdom alone.
THE STRUCTURE AND CELLULAR EXPRESSION OF ADAM33
ADAM33 consists of 22 exons that encode a signal sequence, prodomain, catalytic domain, disintegrin domain, cysteine-rich domain, EGF domain, transmembrane domain, and cytoplasmic domain with a long 3'-untranslated region (7). From a functional standpoint, these different domains translate into different functions of ADAM33, which include activation, proteolysis, adhesion, fusion, and intracellular signaling (13). ADAM33 belongs to a family of 40 ADAM proteins and, being a metalloprotease, the catalytic domain has a zinc binding site. ADAM33 mRNA is preferentially expressed in smooth muscle, fibroblasts, and myofibroblasts, but not in the bronchial epithelium or in inflammatory or immune cells (7, 11). An antibody directed to the cytoplasmic domain of ADAM33 when applied to sections of bronchial biopsies from patients with asthma demonstrates preferential immunostaining of airway smooth muscle with occasional positive fibroblasts. The crystal structure of the catalytic domain of ADAM33 has been resolved around the nonselective matrix metalloproteinase inhibitor (marimastat) in addition to the zinc binding site. ADAM33 also has a calcium binding site and a structure at the entrance of the active site of the catalytic domain strongly suggestive of highly discriminate substrate specificity (15). Using a range of peptides known to be cleaved by ADAM proteins, Zou and coworkers (16) have shown stem cell factor, amyloid precursor peptide (APP), insulin B chain, and tumor necrosis factor–related activation-induced cytokine (TRANCE) to be substrates of ADAM33, but, based on enzyme kinetics, these are unlikely to be the natural substrates. Although the natural substrates for ADAM33 are not yet known, a single alanine substitution at position 2 of a 10-residue APP peptide yielded a 20-fold more efficient substrate (17). Terminal truncation studies identified a minimal 9-residue core important for ADAM33 recognition and cleavage. Further modification of APP peptide has resulted in the identification of a sequence that is cleaved a 100 times more effectively than the native APP peptide (17).
ALTERNATIVELY SPLICED VARIANTS OF ADAM33
Although it has been possible to detect and overexpress full-length ADAM33, analysis of human airway fibroblasts have identified at least six alternatively spliced variants, none of which contain the catalytic domain (18). Indeed, the metalloprotease domain is present in less than 2% of mRNA transcripts. Further analysis has shown that there is selective nuclear transport of ADAM33 mRNA spliced variants into the cytoplasm of airway fibroblast in favor of the full-length molecule, but this still forms the minority of cytoplasmic mRNA for ADAM33, with the shortened forms accounting for more than 95% of sequences identified with ADAM33. Analysis of ADAM33 splice variant expression in bronchial biopsies from normal subjects and subjects with asthma has again shown that the full-length molecule containing the catalytic domain is the minority transcript with the alternatively spliced variants dominating, but there was no clear difference between asthmatic and normal biopsies in the expression of each of the variants. Using an antibody directed to the cytoplasmic domain, Western blotting has shown that human airway fibroblasts also contain a large number of protein variants that probably represent the products of alternative splicing (19). Multiple ADAM33 protein isoforms have also been identified in airway smooth muscle cells but not in epithelial cells obtained by bronchial brushing.
REGULATION OF ADAM33 EXPRESSION
To gain further insight into the mechanisms that control ADAM33 expression, chromatin accessibility, histone modification, and DNA methylation at the ADAM33 promoter have been compared in fibroblast and epithelial cells (20). The CpG island identified in the promoter seems particularly critical in regulating ADAM33 expression because this island is hypermethylated in epithelial cells but not in fibroblasts. Demethylation of CpG islands by 5-aza-deoxy cytosine leads to the ADAM33 gene being expressed in H292 bronchial epithelial cells, reinforcing the importance of epigenetic regulation of this asthma susceptibility gene.
ASSOCIATION OF ADAM33 WITH ASTHMA SUBPHENOTYPES
It is well recognized that, in patients with chronic persistent asthma, baseline lung function declines more rapidly over time when compared with that of normal individuals (21). Jongepier and colleagues investigated genetic and environmental factors that may contribute to this accelerated decline in 200 patients with chronic asthma studied annually for 25 yr (22). They showed that the rare allele of the S_2 polymorphism was significantly associated with excess decline in FEV1 over time and concluded that this variant of ADAM33 was not only important in the development of asthma but also in disease progression, possibly related to enhanced airway remodeling. A further study by the same group (23) investigated whether polymorphic variation of ADAM33 could also predict an accelerated decline in baseline lung function at a population level. A total of 1,390 subjects from a Dutch cohort were genotyped for eight asthma-associated single nucleotide polymorphisms (SNPs) and these were analyzed in relation to baseline FEV1 measured every 3 yr for 25 yr. Individuals homozygous for the minor alleles of SNPs S_2 and Q-1 and heterozygous for the SNP S_1 had a significantly accelerated decline in FEV1 of 4.9, 9.6, and 3.6 ml/yr, respectively, when compared with the wild-type allele. A further analysis demonstrated a higher prevalence of the SNPs F+1, S_1, S_2, and T_2 in subjects with chronic obstructive pulmonary disease at GOLD (Global Initiative for Chronic Obstructive Lung Disease) stage 2 or higher. Thus, in addition to asthma, it seems that polymorphic variation in ADAM33 also influences the rate of decline of lung function at a population level and more specifically in chronic obstructive pulmonary disease.
ADAM33 AS A MORPHOGENETIC GENE
Metalloprotease and ADAM proteins play an important role in branching morphogenesis of the lung by influencing the balance of growth factors at specific stages during development (24). As in adult airways, multiple forms of ADAM33 protein isoforms exist in human embryonic lung when assessed at 8 to 12 wk of development (19). Although many of these variants were similar in size to those identified in adult airways and airway smooth muscle, fetal lung also expressed a unique, small 25-kD variant. Immunohistochemistry applied to tissue sections of human fetal lung demonstrated ADAM33 immunostaining not only in airway smooth muscle but also in primitive mesenchymal cells that formed a cuff at the end of the growing lung bud. Thus, ADAM33 may well play a role in orchestrating branching morphogenesis in the human lung and minor polymorphic variations may influence the subsequent susceptibility of the lung to asthma.
Recent longitudinal cohort studies have established that reduced baseline lung function in infancy is a risk factor for the development of persistent childhood wheeze, BHR, and asthma. In a prospective cohort study of the development of asthma and allergies in children (the Manchester Asthma and Allergy Study, MAAS), we recently investigated whether polymorphic variation in ADAM33 could influence lung function in infancy (25). Three hundred children recruited before birth from parents with asthma and allergy were genotyped for 16 asthma-associated SNPs of ADAM33 and assessed for baseline lung function at the ages of 3 and 5 yr using body plethysmography. Five SNPs were associated with wheeze in early life (F+1, N+1, ST+5, T1, and T2). Children homozygous for the A allele of F+1 also had an increased risk of transient early wheeze (odds ratio, 2.39; 95% CI, 1.18–4.86; p = 0.02). However, no association was found between ADAM33 SNPs and allergic sensitization. At the age of 5 yr, four of the SNPs where associated with reduced FEV1 (F+1, N+1, T1, and T2; p < 0.04). Linkage disequilibrium mapping of ADAM33 pointed to functional SNPs lying between F+1 and B+1. The authors concluded that poor early-life lung function is in part a genetically determined trait and this may increase the risk of chronic asthma.
ADAM33 AS A BIOMARKER OF SEVERE ASTHMA
Using an antibody directed to sequences in the catalytic domain, Lee and colleagues (26) recently described the presence of a soluble form of ADAM33 of approximately 55 kD (sADAM33). The sADAM33 protein was identified in bronchoalveolar lavage fluids of subjects with asthma and normal control subjects using Western blotting. Using the antibody directed to the catalytic domain, immunohistochemistry localized ADAM33 to airway smooth muscle and the subepithelial basement membrane region, suggesting entrapment. In contrast to the cellular provenance of ADAM33, identified with an antibody directed to the cytoplasmic domain (which would identify all forms of ADAM33), the soluble form was reported to be overexpressed in airway smooth muscle and basement membrane in subjects with asthma but not in normal control subjects. Furthermore, applying an immunoassay for sADAM33 in bronchoalveolar lavage fluid, levels increased significantly in proportion to asthma severity, with ADAM33 protein levels correlating inversely with the FEV1% predicted. These exciting observations raise the possibility that sADAM33 is a biomarker of asthma severity and chronicity and, in its soluble form, could play an important role in asthma pathogenesis.
CONCLUSIONS
ADAM33 is only one of many genes that influence the onset and progression of asthma. Its complexity and potential for considerable variations may well be important in shaping multiple phenotypes and partial phenotypes, including early-life lung function, asthma, and chronic obstructive pulmonary disease. It is still not clear which of the SNPs identified relate causatively to asthma or whether the functional SNPs have yet to be discovered. Effort is underway to comprehensively SNP-type ADAM33 in an attempt to identify single or multiple abnormalities within the gene that translate into disease phenotype. A key next step will be to define what functional aspects of ADAM33 map onto these phenotypes.
FOOTNOTES
Conflict of Interest Statement: S.T.H. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. Y.Y. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. H.-M. H. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. R.M.P. holds no awards and has no independent grants. He is funded as a Research Fellow by the Wellcome Trust. J.W.H. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. H.Y. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. Y.Y.P. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. J.C. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. D.E.D. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript.
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Raby BA, Silverman EK, Kwiatkowski DJ, Lange C, Lazarus R, Weiss ST. ADAM33 polymorphisms and phenotype associations in childhood asthma. J Allergy Clin Immunol 2004;113:1071–1078.
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ABSTRACT
The ability to identify novel disease genes by positional cloning led to the identification of a disintegrin and metalloprotease (ADAM)33 gene on chromosome 20p13 as a susceptibility gene for asthma. Case-control and family-based association studies have mostly confirmed a link between ADAM33 and asthma. Its restricted expression to mesenchymal cells as well as its association with bronchial hyperresponsiveness and accelerated decline in lung function over time point strongly to its involvement in the structural airway components of asthma, such as remodeling. Extensive alternative splicing, expression during branching morphogensis in the developing fetus, impaired lung function in childhood, the production of a soluble form linked to chronic asthma, and tight epigenetic regulation indicate a level of complexity in the way ADAM33 influences disease phenotype. Its recent association with chronic obstructive pulmonary disease as well as with asthma and lung development points to functions relating to airway wall modeling and remodeling as a general morphogenetic repair gene rather than being restricted to asthma.
Key Words: ADAMs;asthma;biomarkers;hyperresponsiveness
In 1860, Henry Hyde Salter, in his treaties "On Asthma; Its Pathology and Treatment," drew attention to the spasmodic nature of the airway obstruction and its association with a wide variety of environmental factors. The concept of bronchial hyperresponsiveness (BHR) is now firmly rooted in our understanding of the pathobiology of asthma, with this response relating to a combination of airway inflammation and dysfunctional airway smooth muscle. Although the last two decades have witnessed an enormous increase in our understanding of the cellular and mediator basis of the inflammatory response of asthma, how this relates to the behavior of airway smooth muscle is still largely unknown. From a pathologic standpoint, asthma is characterized by a combination of hyperplasia and hypertrophy of the smooth muscle bundles in both the small and large airways. Although mediators released in response to airway inflammation, such as cysteinyl leukotrienes and prostaglandins, are capable of contracting human airway smooth muscle, it is not clear how airway inflammation relates to the increase in smooth muscle or to its hyperresponsiveness (1).
Since the writings of Salter, a large number of environmental factors have been identified that predispose to asthma, including prenatal maternal influences, allergen exposure, respiratory infection, tobacco smoke, pollutants, prematurity, and dietary factors. Precisely how these factors interact to generate a chronic inflammatory response remains largely speculative. Because asthma, atopy, and BHR cluster in families, there are likely to be powerful genetic factors that predispose toward the development of asthma through gene–environment interactions at specific times as the disease develops and becomes consolidated (2). Beyond inflammation and BHR, a third clinical phenotype that is being increasingly recognized in more severe and chronic asthma is a degree of fixed airflow obstruction that does not respond to bronchodilators or corticosteroids (3). Although chronic inflammation is likely to play an important role in driving chronic persistent asthma, it is becoming increasingly recognized that epithelial damage and activation of underlying airway mesenchymal cells are an important component of remodeled airway in asthma (4). Because communication between the epithelium and underlying mesenchymal is critical for branching morphogenesis of the lung in the developing fetus, it is likely that reactivation of the epithelial mesenchymal trophic unit is involved in the pathogenesis of chronic asthma, particularly that aspect of the disease which is refractory to antiinflammatory therapy (5). Furthermore, genes that have been implicated in lung development may well be involved later in life with the emergence of chronic asthma (6).
THE DISCOVERY OF NOVEL ASTHMA GENES
On the basis of current understanding of asthma, it is clear that asthma, atopy, and BHR have strong genetic components, but for these to manifest as chronic asthma, important environmental factors are also required, often beginning in pregnancy and early infancy, such as respiratory infections, environmental tobacco smoke exposure, prematurity, and a variety of dietary factors. Using doctor-diagnosed asthma as the clinical phenotype, linkage analysis in 260 families in the United Kingdom and United States led to a region on chromosome 20p13 containing a putative asthma susceptibility gene (7). Conditioning of the phenotype for BHR strengthened the linkage signal, whereas incorporation of total or specific serum IgE into the phenotype weakened the linkage signal (7). Subsequently, positional cloning identified a disintegrin and metalloprotease 33 (ADAM33) as the candidate gene accounting for the linkage signal. Subsequently, there have been a large number of case-control and family-based association studies focused on ADAM33, with the majority (8), but not all (9, 10), confirming the original finding. For example, the Collaborative Study on the Genetics of Asthma, comprising eight U.S. genetic centers, demonstrated positive association between single nucleotide polymorphisms (SNPs) of ADAM33 and asthma and BHR in African-American, Hispanic, and white populations (11). In a further study involving 1,299 asthma cases, 1,665 control subjects, and 4,561 family members, Blakey and colleagues (12) applied a literature-based meta-analysis, and supplemented the database with new asthma cases from populations in Iceland and the United Kingdom, and demonstrated significant association for 4 of the 13 SNPs tested. The authors stated that the polymorphic variation in ADAM33 that they had identified with asthma could account for 50,000 excess cases of asthma in the United Kingdom alone.
THE STRUCTURE AND CELLULAR EXPRESSION OF ADAM33
ADAM33 consists of 22 exons that encode a signal sequence, prodomain, catalytic domain, disintegrin domain, cysteine-rich domain, EGF domain, transmembrane domain, and cytoplasmic domain with a long 3'-untranslated region (7). From a functional standpoint, these different domains translate into different functions of ADAM33, which include activation, proteolysis, adhesion, fusion, and intracellular signaling (13). ADAM33 belongs to a family of 40 ADAM proteins and, being a metalloprotease, the catalytic domain has a zinc binding site. ADAM33 mRNA is preferentially expressed in smooth muscle, fibroblasts, and myofibroblasts, but not in the bronchial epithelium or in inflammatory or immune cells (7, 11). An antibody directed to the cytoplasmic domain of ADAM33 when applied to sections of bronchial biopsies from patients with asthma demonstrates preferential immunostaining of airway smooth muscle with occasional positive fibroblasts. The crystal structure of the catalytic domain of ADAM33 has been resolved around the nonselective matrix metalloproteinase inhibitor (marimastat) in addition to the zinc binding site. ADAM33 also has a calcium binding site and a structure at the entrance of the active site of the catalytic domain strongly suggestive of highly discriminate substrate specificity (15). Using a range of peptides known to be cleaved by ADAM proteins, Zou and coworkers (16) have shown stem cell factor, amyloid precursor peptide (APP), insulin B chain, and tumor necrosis factor–related activation-induced cytokine (TRANCE) to be substrates of ADAM33, but, based on enzyme kinetics, these are unlikely to be the natural substrates. Although the natural substrates for ADAM33 are not yet known, a single alanine substitution at position 2 of a 10-residue APP peptide yielded a 20-fold more efficient substrate (17). Terminal truncation studies identified a minimal 9-residue core important for ADAM33 recognition and cleavage. Further modification of APP peptide has resulted in the identification of a sequence that is cleaved a 100 times more effectively than the native APP peptide (17).
ALTERNATIVELY SPLICED VARIANTS OF ADAM33
Although it has been possible to detect and overexpress full-length ADAM33, analysis of human airway fibroblasts have identified at least six alternatively spliced variants, none of which contain the catalytic domain (18). Indeed, the metalloprotease domain is present in less than 2% of mRNA transcripts. Further analysis has shown that there is selective nuclear transport of ADAM33 mRNA spliced variants into the cytoplasm of airway fibroblast in favor of the full-length molecule, but this still forms the minority of cytoplasmic mRNA for ADAM33, with the shortened forms accounting for more than 95% of sequences identified with ADAM33. Analysis of ADAM33 splice variant expression in bronchial biopsies from normal subjects and subjects with asthma has again shown that the full-length molecule containing the catalytic domain is the minority transcript with the alternatively spliced variants dominating, but there was no clear difference between asthmatic and normal biopsies in the expression of each of the variants. Using an antibody directed to the cytoplasmic domain, Western blotting has shown that human airway fibroblasts also contain a large number of protein variants that probably represent the products of alternative splicing (19). Multiple ADAM33 protein isoforms have also been identified in airway smooth muscle cells but not in epithelial cells obtained by bronchial brushing.
REGULATION OF ADAM33 EXPRESSION
To gain further insight into the mechanisms that control ADAM33 expression, chromatin accessibility, histone modification, and DNA methylation at the ADAM33 promoter have been compared in fibroblast and epithelial cells (20). The CpG island identified in the promoter seems particularly critical in regulating ADAM33 expression because this island is hypermethylated in epithelial cells but not in fibroblasts. Demethylation of CpG islands by 5-aza-deoxy cytosine leads to the ADAM33 gene being expressed in H292 bronchial epithelial cells, reinforcing the importance of epigenetic regulation of this asthma susceptibility gene.
ASSOCIATION OF ADAM33 WITH ASTHMA SUBPHENOTYPES
It is well recognized that, in patients with chronic persistent asthma, baseline lung function declines more rapidly over time when compared with that of normal individuals (21). Jongepier and colleagues investigated genetic and environmental factors that may contribute to this accelerated decline in 200 patients with chronic asthma studied annually for 25 yr (22). They showed that the rare allele of the S_2 polymorphism was significantly associated with excess decline in FEV1 over time and concluded that this variant of ADAM33 was not only important in the development of asthma but also in disease progression, possibly related to enhanced airway remodeling. A further study by the same group (23) investigated whether polymorphic variation of ADAM33 could also predict an accelerated decline in baseline lung function at a population level. A total of 1,390 subjects from a Dutch cohort were genotyped for eight asthma-associated single nucleotide polymorphisms (SNPs) and these were analyzed in relation to baseline FEV1 measured every 3 yr for 25 yr. Individuals homozygous for the minor alleles of SNPs S_2 and Q-1 and heterozygous for the SNP S_1 had a significantly accelerated decline in FEV1 of 4.9, 9.6, and 3.6 ml/yr, respectively, when compared with the wild-type allele. A further analysis demonstrated a higher prevalence of the SNPs F+1, S_1, S_2, and T_2 in subjects with chronic obstructive pulmonary disease at GOLD (Global Initiative for Chronic Obstructive Lung Disease) stage 2 or higher. Thus, in addition to asthma, it seems that polymorphic variation in ADAM33 also influences the rate of decline of lung function at a population level and more specifically in chronic obstructive pulmonary disease.
ADAM33 AS A MORPHOGENETIC GENE
Metalloprotease and ADAM proteins play an important role in branching morphogenesis of the lung by influencing the balance of growth factors at specific stages during development (24). As in adult airways, multiple forms of ADAM33 protein isoforms exist in human embryonic lung when assessed at 8 to 12 wk of development (19). Although many of these variants were similar in size to those identified in adult airways and airway smooth muscle, fetal lung also expressed a unique, small 25-kD variant. Immunohistochemistry applied to tissue sections of human fetal lung demonstrated ADAM33 immunostaining not only in airway smooth muscle but also in primitive mesenchymal cells that formed a cuff at the end of the growing lung bud. Thus, ADAM33 may well play a role in orchestrating branching morphogenesis in the human lung and minor polymorphic variations may influence the subsequent susceptibility of the lung to asthma.
Recent longitudinal cohort studies have established that reduced baseline lung function in infancy is a risk factor for the development of persistent childhood wheeze, BHR, and asthma. In a prospective cohort study of the development of asthma and allergies in children (the Manchester Asthma and Allergy Study, MAAS), we recently investigated whether polymorphic variation in ADAM33 could influence lung function in infancy (25). Three hundred children recruited before birth from parents with asthma and allergy were genotyped for 16 asthma-associated SNPs of ADAM33 and assessed for baseline lung function at the ages of 3 and 5 yr using body plethysmography. Five SNPs were associated with wheeze in early life (F+1, N+1, ST+5, T1, and T2). Children homozygous for the A allele of F+1 also had an increased risk of transient early wheeze (odds ratio, 2.39; 95% CI, 1.18–4.86; p = 0.02). However, no association was found between ADAM33 SNPs and allergic sensitization. At the age of 5 yr, four of the SNPs where associated with reduced FEV1 (F+1, N+1, T1, and T2; p < 0.04). Linkage disequilibrium mapping of ADAM33 pointed to functional SNPs lying between F+1 and B+1. The authors concluded that poor early-life lung function is in part a genetically determined trait and this may increase the risk of chronic asthma.
ADAM33 AS A BIOMARKER OF SEVERE ASTHMA
Using an antibody directed to sequences in the catalytic domain, Lee and colleagues (26) recently described the presence of a soluble form of ADAM33 of approximately 55 kD (sADAM33). The sADAM33 protein was identified in bronchoalveolar lavage fluids of subjects with asthma and normal control subjects using Western blotting. Using the antibody directed to the catalytic domain, immunohistochemistry localized ADAM33 to airway smooth muscle and the subepithelial basement membrane region, suggesting entrapment. In contrast to the cellular provenance of ADAM33, identified with an antibody directed to the cytoplasmic domain (which would identify all forms of ADAM33), the soluble form was reported to be overexpressed in airway smooth muscle and basement membrane in subjects with asthma but not in normal control subjects. Furthermore, applying an immunoassay for sADAM33 in bronchoalveolar lavage fluid, levels increased significantly in proportion to asthma severity, with ADAM33 protein levels correlating inversely with the FEV1% predicted. These exciting observations raise the possibility that sADAM33 is a biomarker of asthma severity and chronicity and, in its soluble form, could play an important role in asthma pathogenesis.
CONCLUSIONS
ADAM33 is only one of many genes that influence the onset and progression of asthma. Its complexity and potential for considerable variations may well be important in shaping multiple phenotypes and partial phenotypes, including early-life lung function, asthma, and chronic obstructive pulmonary disease. It is still not clear which of the SNPs identified relate causatively to asthma or whether the functional SNPs have yet to be discovered. Effort is underway to comprehensively SNP-type ADAM33 in an attempt to identify single or multiple abnormalities within the gene that translate into disease phenotype. A key next step will be to define what functional aspects of ADAM33 map onto these phenotypes.
FOOTNOTES
Conflict of Interest Statement: S.T.H. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. Y.Y. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. H.-M. H. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. R.M.P. holds no awards and has no independent grants. He is funded as a Research Fellow by the Wellcome Trust. J.W.H. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. H.Y. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. Y.Y.P. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. J.C. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. D.E.D. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript.
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