Cystic Fibrosis: There's Hope After All
Cystic fibrosis transmembrane conductance regulator (CFTR) is an N-linked glycoprotein that is most commonly found, transcribed at low concentrations, in the apical membrane of epithelial cells (Collins, 1992). An analysis of the amino acid sequence in the primary structure of the CFTR protein reveals that it is related to that of other transport proteins that belong in the ATP-binding Cassette (ABC) transporter category, thus the CFTR gene is also a member of the ABC group (Sheppard & Welsh, 1999). Experiments with recombinant versions of the CFTR gene have been able to confirm that the CFTR protein is responsible for regulating the Cl- concentration in cells via the activity of cyclic adenosine monophosphate (cAMP) (Sheppard & Welsh, 1999), thus making it an ion-channel protein. Furthermore, there is evidence that the CFTR protein plays a role in the regulation of other ion channels present in the cell membrane (Zielenski, 2000).
Due to the important role of the CFTR protein in the proper functioning of epithelial cells, any disruption in its function would have great implications for an affected individual. This is the case for those individuals suffering from the autosomal recessive genetic disorder known as Cystic Fibrosis (CF). Individuals with CF carry inherited mutations in the CFTR encoding gene, and thus have altered expression of the CFTR protein (Zielenski, 2000). The most common type of mutation that results in CF is the ∆F508 mutation, which resulted from a 3 base pair deletion in the exon transcribed from the CFTR protein. In this mutation, the 3 b.p. deletion corresponds to the deletion of the amino acid phenylalanine at codon 58 in the protein structure, thus disrupting the folding of the CFTR gene (Zielenski, 2000) (Collins, 1992). However, this mutation is only observed in 70% of individuals who suffer from CF (Collins, 1992). A further group of 850 different types of allelic mutations have been identified as potential genetic causes for CF, and also lead to the loss of function of the CFTR protein. However, the mode of action of these mutations can differ. They can result in loss of function by causing the defective synthesis of protein, creating the inability to properly traffic (∆F508) and process the protein into a mature glycosylated protein, causing an incapability of the protein to carry out function due to inability to bind ATP, initiating defective conductance of the protein, resulting in a reduced synthesis of the protein, and finally, by resulting in the production of unstable proteins (Zielenski, 2000).
The main role of the CFTR ion-channel is to selectively allow the passage of ions through the cell membrane (Sheppard & Welsh, 1999). This function is key in maintaining osmolarity in a cell, as osmolarity is dependant on ion concentration. More specifically, CFTR is selective towards anions, and exhibits preferred permeability to anions in the order of I- > Br- > Cl- > F- in solution (Sheppard & Welsh, 1999). The CFTR protein is also permeable to polyatomic anions such as NO3-, as well as water, urea and ATP (Sheppard & Welsh, 1999); but because chloride ions are the most prevalent kinds of anions in the body, the CFTR protein is mainly involved in the transport of Cl-. In normally functioning cells, cAMP stimulates the enzyme known as protein kinase A (PKA), which then phosphorylates the amino acid serine, which is present in the R-domain of the CFTR protein. At this point, CFTR binds to an ATP molecule, and a conformational change is induced that results in the opening of the chloride channel (Collins, 1992). Through this mechanism, and the transport of the Cl- ion secretion out of the cell, the action of the CFTR molecule results in Na+ absorption, and establishes and maintains adequate surface liquid (ASL) (Rubin, 2009). ASL, is the layer of fluid that reaches the height of the cilium of the epithelial cells, and allows their contact with the mucus layer (Rubin, 2009).
In the case of the ∆F508 mutation, the protein is synthesized in the cell, but fails to travel from the endoplasmic reticulum, to the cell membrane; thus, the CFTR protein loses all function. Due to this lack of function, chloride ion concentration is unbalanced, leading to an increase in Na+ absorption, inevitably leading to low amounts of ASL (Rubin, 2009). One theory suggests that this causes the Mucus layer to stick to the surface of the cells, resulting in the establishment of plaque and the entrapment of bacteria (Rubin, 2009). Another theory, stipulates that a lack of properly functioning mucus, that results from the imbalance in water and salt concentrations, make the epithelial cells more susceptible to bacteria, and allow the bacteria to remain directly on the epithelial layer (Rubin, 2009). Thus, the lack of function of the CFTR gene can be directly associated with the clinical features of CF, such as thick mucus and susceptibility to bacterial infections (Zielenski, 2000).
Due to the multi-faceted nature of CF, the diversity of symptoms, and the range in severity of symptoms, many different and personalized approaches need be taken to treat affected individuals. At the ion level of disease manifestation, the drug Amiloride can be used for blocking the excess uptake of Na+ (Collins, 1992) (Brown et al. 1997). While at the mucus level, DNase enzyme can be administered to decrease the viscosity of the mucus (Collins, 1992). Furthermore, antibiotics can be used to decrease the presence of bacteria in the body (Collins, 1992). Sometimes, as a final resort, if infection leads to Bronchiectasis, and causes irreversible lung damage, a lung transplant can be carried out (Collins, 1992). In the case of individuals affected by the ∆F508 version of the disease, the drug Miglustat also looks promising (Norez et al. 2009). The idea behind this type of treatment is based on Miglustat's ability to fix improper trafficking of protein (as is the case with ∆F508 individuals). This is because Miglustat acts as an inhibitor, which blocks the interaction that keeps the CFTR protein stationed at the endoplasmic reticulum, allowing it to unbind, and travel to the cell membrane (Norez et al. 2009). The study by Norez et al. (2009) supports this theory, and specifically shows a change in phenotype from affected, to non-cystic fibrosis-like, in human epithelial cells (Norez et al. 2009).
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