Sunday, May 01, 2011

Role of mucin expression in the pathology of nasal polyposis


Rhinorrhea due to increased mucous secretion is one of the cardinal symptoms of nasal polyposis. Nasal polypi not only causes an increase in the amount mucin secretion but also changes its physical properties. It makes the mucin secretion thicker than usual hampering the normal mucosal transport mechanism. Changes in the physical properties of mucin involve changes in the relative amounts of sol and gel components. Nasal polypi expresses a wide spectrum of mucin genes. This genetic expression of mucin genes have been thought to play a role in the pathophysiology of nasal polyposis.

Causes of increased mucous secretion in patients with nasal polyposis:

1. Sinus infection coexisting / complicating nasal polyposis can cause increased mucous secretion. Studies have shown that 3 types of mucin have been expressed in these patients. They include MUC2, MUC5AC, and MUC5B. In patients with nasal polyposis there was inverse relationship between the levels of MUC2 and MUC5AC levels in the sinus secretion.
2. Increase in the surface epithelial area due to the presence of polypi could be one of the causes of increased mucous secretion. Even when squamous metaplasia is present due to exposure these squamous cells also express mucin.
3. Due to inflammation there is an increase in the number of mucous secretory elements like goblet cells / submucosal glandular cells. This could be due to the presence of increased levels of inflammatory mediators like IL-9, IL-13. These mediators increase the amount of secretion of mucin by goblet and submucosal glandular elements.

Mucin genes:

This gene has been demonstrated in normal inferior turbinate epithelium and nasal polypoidal tissue. Infact studies have not demonstrated any appreciable changes in this gene expression between these two tissue. One important issue that deserves mention is that this gene is found to be downregulated in the inferior turbinate of patients with vasomotor rhinitis when compared to normal individuals. This has led authors to postulate that decreased amounts of MUC1 expression in the mucosa over inferior turbinate some how trigger off neurogenic stimuli leading on to production of copious watery secretion from the nasal cavity. This feature is classically seen in patients with vasomotor rhinitis.

This gene is responsible for the production of large secretoy mucin. This mucin is commonly seen in intestinal epithelium serving to protect it from the luminal contents. MUC2 upregulation is seen in the mucosa of maxillary sinus in patients with nasal polyposis.

MUC 3 & MUC 6:
MUC3 gene encodes membrane bound mucin. This type of membrane bound mucin is seen in intestinal secretions. MUC6 gene encodes for secretory mucin which is commonly present in gastric secretions. It is this mucin which prevents self digestion of gastric mucosa by the HCL present in the stomach. Patients with nasal polyposis demonstrated MUC 3 upregulation with no increase in the amount of MUC6.

Studies have shown that this gene is the most predominantly expressed one in nasal polyposis. This gene is responsible for encoding membrane bound mucin which is commonly present in patients with nasal polyposis. This gene is also postulated to be involved in the process of epithelial hyperplasia and metaplasia.

A lot of work needs to be done in this direction to throw more light on this issue.


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