Asthma is a major cause of chronic lung disease world-wide and has increased in prevalence by more than 80% in all age and ethnic groups, particularly in urban areas. It is characterized by airway inflammation, hyperresponsiveness, and mucus obstruction, which reflect airway remodeling. Asthmatic patients are at risk of acute and sudden increases in airflow resistance due to airway obstruction, caused by excess mucus. Mucin glycoproteins, the major macromolecular component of mucus, are hypersecreted/overproduced during acute and chronic lung diseases, and are now considered as part of the innate immune response system.
MUC7, the smallest known mucin, contributes to the innate immune response in salivary secretions as it binds bacteria and MUC7 peptides exhibit anti-bacterial, anti-fungal and possibly anti-viral properties. We hypothesize that MUC7 mucin glycoprotein may be a key reactive protein in the innate immune response to inflammation in the lower respiratory tract. MUC7 mucin is minimally expressed in lung tissues of healthy individuals and when detected is expressed in a subset of serous cells in the submucosal glands. This dissertation focuses on investigating whether MUC7 mucin glycoproteins has a role the innate immune response of lungs chronic disease and whether there are genetic modifications in MUC7 that could lead to an altered risks of being diagnosed with asthma in a population pre-disposed to asthma.
MUC7 mucin glycoprotein expression was identified in lung tissue and secretions of patients with asthma, infants with Respiratory Syncytial Virus-induced bronchiolitis of the lower respiratory tract and infants with Chronic Lung Disease. We also identified a MUC7 fragmentation that may be linked with disease severity and began to evaluate which lung protease may fragment MUC7. We show that matrix metalloproteases-9 is secreted apically from airway epithelial cells in vitro, and does not respond to cigarette smoke (CS) and H2O2. We also show while the matrix metalloproteases-9 is activated, it does not cleave MUC7.
The effect of inflammatory mediators and acute-on chronic-triggers on MUC7 gene regulation was investigated. It had been reported (Li & Bobek. 2006. Am. J. Respir. Cell Mol.Biol. 35:593-601) that MUC7 mRNA and protein expression is increased in lung cells following exposure to various stimuli, including Tumor Necrosis Factor α, Epidermal Growth Factor and lipopolysaccharide and that Tumor Necrosis Factor α transcriptionally regulates MUC7 though the NFκB cis-sequence in the MUC7 promoter. My data show that MUC7 is not transcriptionally upregulated, by tumor necrosis factor α, cigarette smoke condensate, interleukin 8, hydrogen peroxide and interferon γ, nor was the abundance of endogenous mRNA increased following exposure to these inflammatory mediators and additional acute-on-chronic triggers, epidermal growth factor, lipopolysaccharide and interleukin 1β.
The only genetic association of mucin genes with asthma occurs with the MUC7 gene. A previously identified polymorphism that encodes a decreased number of tandem repeats (TR), MUC7*5 allele encoding for 5 rather than 6 TR, was associated with a decreased risk of being asthmatic in a Northern European cohort (Kirkbride et al. 2001. Eur. J. Hum. Gen. 9:347-35). African Americans have a higher prevalence of asthma, we hypothesized that they would have a lower prevalence of the apparently protective MUC7*5 allele. Our data clearly show a reduction in the cohort frequency of African American asthmatics. Additionally, our data show a significant increase in the frequency of the MUC7*5 allele in the African American non-asthmatic control group compared to the asthmatic group. We also identified a novel MUC7*4 polymorphism in an African American non-asthmatic patient. This novel polymorphism resulted in a reduction in the number and rearrangement of the encoded TR.
There is an altered expression of MUC7 mucin and smaller molecular weight fragments in the asthmatic airways. Furthermore, there is an association between a shorter protein length and a decreased risk of being asthmatic. We would postulate that these data could support the role of MUC7 as a protective protein in the innate immune response in the asthmatic airway. In future experiments, we would aim to further investigate the molecular mechanisms of MUC7 mucin in the innate immune response and the effects of alterations within the TR domain on MUC7 mucins biological function.
|Advisor:||Rose, Mary C.|
|Commitee:||Colberg-Poley, Anamaris M., Constant, Stephanie L., Freishtat, Robert J., Hoffman, Eric P., Smith, Michael D.|
|School:||The George Washington University|
|School Location:||United States -- District of Columbia|
|Source:||DAI-B 70/02, Dissertation Abstracts International|
|Subjects:||Molecular biology, Genetics, Cellular biology|
|Keywords:||Airway epithelium, Asthma, Chronic airway diseases, Glycoprotein, Innate immunity, MUC7, Mucin|
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