After many studies of chloride transport developed, the gene responsible for cystic fibrosis was finally isolated. The protein product of the gene, which was somehow influenced by chloride movement, was named the cystic fibrosis transmembrane conductance regulator (CFTR). Cystic fibrosis is caused by mutations in the gene encoding of the CFTR, yet there are multiple lesions within the gene that can cause the disease.
Phenylalanine 508 Mutation
The most common cause of cystic fibrosis is the phenylalanine 508 mutation which is responsible for 70% of cases of cystic fibrosis. In a comparison of the cDNA sequence in clones of the CFTR gene isolated from the sweat gland libraries constructed from the RNA of normal individuals and the clones from CF patients, it was evident that three nucleotides were missing from the CF patientsí CFTR gene. These missing nucleotides corresponded to the deletion of a single amino acid, phenylalanine, at position 508 of the protein product. It is this protein product which functions as the gateway for chloride into the cell, yet the malfunctioning channel results in a membrane that is impermeable to chloride and little to no chloride movement across the cell membrane. When a gene (for CFTR) was transferred into chloride-impermeable cells, the ion was forced to move-- the channelís likeness for chloride decreased if the gene was changed in ways that affected the CFTR protein. The deletion that leads to the exclusion of phenylalanine 508 from the CFTR protein is what leads to the loss of chloride transport in most cases. However, different CFTR mutations lead to varied molecular consequences.
How the Chloride Channel Works
For the chloride channel to function, it is crucial that two domains of the channel function. The first domain is actually two nucleotide binding domains which take up ATP and serves to regulate activity of the channel (i.e., the channel will not open unless ATP is bound to these sites). Importantly, the flow of chloride through the open channel does not require input of metabolic energy (ATP)because the net movement of chloride is down its electrochemical gradient. this is true for all channels, not just CFTR. The second domain is a regulatory domain that is studded with phosphate groups when chloride is passed through the channel. If the phosphate groups are not present or ATP has not been taken up and cleaved, the gateway for chlorideís passage into the cell is blocked. In other cases, the defective protein fails to leave the endoplasmic reticulum resulting in the absence of the chloride channel on the outer membrane of the cell.
Information compiled from "Gene therapy for Cystic Fibrosis: A Potential Cure" in the April 1996 issue of the Maryland Medical Journal
Points where the problems occur courtesy of Dr. William J. Grimes