CFTR Mutation

Figure 5. The six classes of CFTR mutations with percentage of people with CF who has at least one mutation 4

Mutation: 

There are over 2,500 mutations that have been identified that can be divided into six classes. 1 The most frequent mutation is a class two mutation where three base pairs of the CFTR gene are deleted leading to the loss of phenylalanine, at amino acid position 508 (F508). 1, 3, 7, 9 The F508 is located in a loop on the surface of the NBD with the hydrophobic aromatic side chain projecting at the end of TM helix 11, as seen in Figure 6. Deletion of the F508 (F508del) would shorten the loop, which leaves a crevice at the NBD1 and TMD interface. 7

This leaves the crevice to be “partially filled by R1070, whose side chain swings into contact with the main chain atoms in NBD1” causing misfolding and interface disruption. 7

The F508 mutation causes CFTR protein misfolding and retention in the endoplasmic reticulum as well as defective channel gating. 5 The protein misfolding is a result of interface disruption between domains, which decreases stability of the CFTR protein. Studies show that the disruption occurs at the folding kinetics of NBD1 and the interdomain interactions between the transmembrane domains and the nucleotide-binding domains. 8 At the interface between the transmembrane domains and the nucleotide-binding domains are cytosolic loops, which are characterized by the presence of coupling helices, which “is a short α helix in one of the cytoplasmic protrusions of the TMD that fits into a groove of an NBD monomer” to relay conformational changes between the two domains. 5, 11 At the interface of NBD1 with CL4, F508 interacts with a cluster of aromatic residues in CL4 (F1068, Y1073, and F1074) to stabilize this interface. More specifically, Julie Forman-Kay at the Hospital for Sick Children in Canada believes that the F508del forces NBD1 “into a conformation that inhibits dimerization,” which in turn “reduces the channel production and function”. 6 As a result, anion transport of chloride, a component of salt, is limited, and the quantity of CFTR protein at the apical surface of epithelial cells is significantly reduced. 9 When chloride is trapped in cells, water is unable to hydrate the cellular surface, which causes the mucus covering the cells to become thick and sticky, thus generating many of the symptoms associated with cystic fibrosis such as wheezing, salty tasting skin, persistent coughing with phlegm, etc. 2 

Figure 6. The orange loop represents where F508 amino acid would position 10

*CFTR mutation: 0:30

*How specific drugs changes CFTR: 1:26

https://www.youtube.com/watch?v=6IbP1ASGv9w

References