Combating Cystic Fibrosis: In Search for CF Transmembrane Conductance Regulator (CFTR) Modulators

Cystic Fibrosis (CF) is the most common lethal monogenic disease among Caucasians with an estimated worldwide patient population of 70 000 and a median survival age of only 37 years. CF results in chronic obstruction in multiple organs and consequently in an array of pathologies, including depressed lung function, inflammation and chronic lung disease, pancreatic and digestive insufficiency, bowel obstruction, diabetes, hepatic damage and male sterility. CF is caused by mutations to the gene coding the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel. CFTR is a member of the ATP binding cassette (ABC) transporter family, and its architecture is similar to that of other ABC transporters. The structure of CFTR is comprised of two membrane spanning domains (MSDs), each made of six transmembrane a-helices, which form the chloride conductance pathway, and two nucleotide binding domains (NBDs), which control the gating of the channel through the binding and hydrolysis of ATP. 3] The MSDs and NBDs are connected through four long intracellular loops (ICLs), which are believed to transmit the conformational changes brought about by the binding and hydrolysis of ATP to the NBDs (Figure 1). Nevertheless, CFTR is different from other members of the ABC transporter family in two important aspects; namely, it is the only ion channel in the family and features a unique structural motif termed the R-domain, which was recently shown to be largely disordered. More than 1 800 mutations to the gene coding for CFTR are reported in the literature. These mutations are classified according to the different mechanism by which they are known to disrupt CFTR production and function. 12] The most common disease-causing mutation, the deletion of the codon for phenylalanine 508 (DF508) in the primary sequence of wild-type (wt) CFTR, is categorized as a class II mutation (defective protein processing). Over 90 % of patients carry at least one allele of the DF508 mutant gene whose product is a CFTR channel that is poorly processed within the cell. 13] Most of the mutant protein molecules are incorrectly or incompletely folded and become targeted to endoplasmic reticulum (ER)-associated degradation. The few mutant chloride channels that pass the quality control or simply escape the ER before degradation are eventually incorporated into the plasma membrane. However, these are thought to represent less than 5 % of the level observed in cells expressing wt-CFTR and in addition are characterized by an open probability about 15-fold lower than that of wt-CFTR 18] and by impaired membrane stability, resulting in a commensurate low total whole-cell chloride conductance. Another common mutation is glycineto-aspartate at position 551 (G551D) in the signature motif of NBD1. This mutation is categorized as a class III mutation (defective regulation) and appears in ~4 % of CF patients. The resulting mutant can traffic normally to the apical membrane and is normally phosphorylated, but is characterized by a drastically reduced channel activity due to an extremely low open probability: ~1 % compared to that of wt-CFTR. Class I mutations (defective protein production) are nonsense mutations that create a false stop signal in the mRNA. This signal causes the ribosome to halt translation prematurely leading to a shortened, nonfunctioning protein. Class I mutations are found in ~10 % of CF patients, and the most common ones are G542X and W1282X, the latter being particularly common in Ashkenazi Jews. Understanding the different molecular mechanisms of CFTR dysfunction is necessary in order to provide the scientific basis for the development of drugs for mutation-specific therapy of CF.