mRNA, the protein product is either incomplete or completely absent. Class II mutations are caused by the deletion of individual amino acid residues. With this type of mutation, CFTR is still produced but is defective and therefore not properly glycosylated. Consequently, it is degraded in the endoplasmic reticulum before it can reach the cell membrane. The delta-F508 mutation, a class II mutation, is by far the most common mutation, accounting for 88.5% of all CF mutations (11). The nomenclature of the delta-F508 mutation indicates that the amino acid phenylalanine (F) is missing (delta) at position 508 in the CFTR gene. Mutations in classes III through V all result in CFTR proteins that are able to reach the cell membrane, but cannot function properly once they are there. The last category of mutations, class VI, is the least understood. It involves frameshift mutations that cause a truncation of the CFTR protein, impairing the protein’s ability to regulate other types of ion channels (1).
“Patients with CF often undergo lung transplants when their own lungs become too damaged. However, this procedure only ‘buys time’ for the patient...”
Current Treatments, Future Research & Development Lung Transplants Patients with CF often undergo lung transplants when their own lungs become too damaged. However, this procedure only “buys time” for the patient and does not address the root of the problem, since faulty CFTR channels still exist in the pancreas, intestines, sinuses, and reproductive tract (11). In addition, many complications may arise from this procedure. In order to prevent the immune system from rejecting the lungs, immunosuppressive drugs must be taken every day for the rest of the patient’s life. These drugs have been known to cause side effects, including diabetes, kidney problems, and cancer-like tumors. Furthermore, only about 50% of patients receiving a lung transplant survive after 5 years (11).
Targeted Drugs Rather than treating the symptoms as they arise, another course of treatment involves drugs that target the mutated CFTR channels themselves. Currently, the most promising drugs being researched are Kalydeco (VX-770) and Lumacaftor (VX-809), both developed by Vertex Pharmaceuticals (11). VX-809, which is currently undergoing clinical trials, was developed specifically for patients with the homozygous delta-F508 mutation, which results in defective trafficking of CFTR to the apical cell membrane. VX-809 is a corrector, a category of drugs that increase the amount of mutant CFTR transported to the cell membrane by interfering with the protein degradation pathway that targets the misfolded CFTR proteins. Although this drug 28
does not correct the mutation in the CFTR channels, the drug increases the presence of CFTR at the cell membrane, which still significantly increases ion transport (11). Kalydeco (or VX-770) is a potentiator, another class of drugs which, instead of targeting the transportation of the protein to the membrane, improves the activity of mutant CFTR already at the cell membrane. This drug was developed for class IV mutations, in which the CFTR can still reach the cell membrane, but the gating of the channel is faulty, resulting in insufficient amounts of ions flowing through the channel. It is thought that a combination of correctors and potentiators would be the most effective in treating CF. A corrector could bring more of the mutated CFTR to the cell membrane, and then a potentiator could increase its activity (11).
Pharmacogenomics Approach Aminoglycosides, a group of antibiotics, have been used to repair stop codon mutations (class I mutations). These drugs bind to a specific site in ribosomal RNA and disrupt codonanticodon recognition at the aminoacyl-tRNA site (1). In cultured cells, this method has been shown to restore CFTR synthesis up to 10-20% of normal levels. Genistein, a flavonoid compound, can be used to increase the channel open time of any wild type CFTR that remain in patients with CF, and consequently restore (to some degree) chloride transport. (1)
RNA Editing A recent study published in April 2013 by Montiel-Gonzalez, Vallecillo, and Ydowski describes a new technique that uses site-directed RNA editing to correct the genetic mutation of the CFTR gene. Adenosine deaminase is a type of enzyme that acts on RNA by catalyzing the natural process of site-directed mutations. These enzymes convert adenosine to the nucleoside inosine, which is read as guanosine during translation. As a result, during mRNA editing, codons can be “recoded,” changing the function of the resulting proteins (12). In this study, Montiel-Gonzales et al. were able to engineer recombinant enzymes that could be directed to edit anywhere along the entire RNA registry. They accomplished this by replacing endogenous targeting domains from human adenosine deaminase with a different complementary strand of RNA. The resulting enzyme is capable of selectively editing a single adenosine. The W496X mutation of the CFTR gene, which is caused by a premature stop codon, was chosen in this study for targeting by adenosine deaminase. By developing a “guiding” strand of RNA that DARTMOUTH UNDERGRADUATE JOURNAL OF SCIENCE