N-terminal CFTR missense variants severely affect the behavior of the CFTR chloride channel

Over 1,500 cystic fibrosis transmembrane conductance regulator (CFTR) gene sequence variations have been identified in patients with cystic fibrosis (CF) and related disorders involving an impaired function of the CFTR chloride channel. However, detailed structure-function analyses have only been established for a few of them. This study aimed evaluating the impact of eight N-terminus CFTR natural missense changes on channel behavior. By site-directed mutagenesis, we generated four CFTR variants in the N-terminal cytoplasmic tail (p.P5L, p.S50P, p.E60K, and p.R75Q) and four in the first transmembrane segment of membrane-spanning domain 1 (p.G85E/V, p.Y89C, and p.E92K). Immunoblot analysis revealed that p.S50P, p.E60K, p.G85E/V, and p.E92K produced only core-glycosylated proteins. Immunofluorescence and whole cell patch-clamp confirmed intracellular retention, thus reflecting a defect of CFTR folding and/or trafficking. In contrast, both p.R75Q and p.Y89C had a glycosylation pattern and a subcellular distribution comparable to the wild-type CFTR, while the percentage of mature p.P5L was considerably reduced, suggesting a major biogenesis flaw on this channel. Nevertheless, whole-cell chloride currents were recorded for all three variants. Single-channel patch-clamp analyses revealed that the channel activity of p.R75Q appeared similar to that of the wild-type CFTR, while both p.P5L and p.Y89C channels displayed abnormal gating. Overall, our results predict a major impact of the CFTR missense variants analyzed, except p.R75Q, on the CF phenotype and highlight the importance of the CFTR N-terminus on channel physiology.