Phenotyping of Rare CFTR Mutations Reveals Distinct Trafficking and Functional Defects.

Background. The most common CFTR mutation, F508del, presents with multiple cellular defects. However, the possible multiple defects caused by many rarer CFTR mutations are not well studied. We investigated four rare CFTR mutations E60K, G85E, E92K and A455E against well-characterized mutations, F508del and G551D, and their responses to corrector VX-809 and/or potentiator VX-770. Methods. Using complementary assays in HEK293T stable cell lines, we determined maturation by Western blotting, trafficking by flow cytometry using extracellular 3HA-tagged CFTR, and function by halide-sensitive YFP quenching. In the forskolin-induced swelling assay in intestinal organoids, we validated the effect of tagged versus endogenous CFTR. Results. Treatment with VX-809 significantly restored maturation, PM localization and function of both E60K and E92K. Mechanistically, VX-809 not only raised the total amount of CFTR, but significantly increased the traffic efficiency, which was not the case for A455E. G85E was refractory to VX-809 and VX-770 treatment. Conclusions. Since no single model or assay allows deciphering all defects at once, we propose a combination of phenotypic assays to collect rapid and early insights into the multiple defects of CFTR variants.

What can the CF registry tell us about rare CFTR-mutations? A Belgian study.

BACKGROUND: CFTR2 provides clinical and functional information of the most common CFTR-mutations. Rare mutations (RMs) occur in only a few patients with limited reported clinical data. Their role in CF-disease liability is hardly documented. METHODS: Belgian CF-Registry 2013 data were analyzed to identify CF with at least 1 RM (CF+RM). Clinical data and sweat chloride of CF+RM were compared to CF-controls, carrying 2 class 1 to 3 mutations (CFclassic). Disease severity was compared between both groups. To avoid bias in the comparison, transplanted patients were excluded from each group. RESULTS: Seventy-seven CF+RM were identified (77/1183 = 6.5%). Sixty-four different RM were detected, of which 21 had not been previously reported. All RMs, corresponding to HGVS (Human Genome Variation Society) nomenclature, were listed in supplementary data. Seven transplanted CF+RM were excluded for further analysis. CF+RM had higher age at diagnosis [median (IQR)] [3.7 y (0.3-18.3) vs. 0.3y (0.1-2,0) (p < 0.0001)], lower sweat chloride [96 mmol/L (64-107) vs. 104 mmol/L (97-115) (p < 0.0001)], higher FEV1%pred [77%pred (58-96) vs. 68%pred (48-86) (p = 0.017)], were less frequently pancreatic insufficient [56% vs. 98% (p < 0.0001)], Pseudomonas aeruginosa colonized [24% vs. 44% (p = 0.0093)] and needed fewer IV antibiotics [36% vs. 51% (p = 0.041)] than CFclassic. However, a wide spectrum of disease severity was seen amongst CF+RM. CONCLUSIONS: CF-patients with a RM cover 6.5% of the Belgian CF-population. Rare mutations can be found in severely ill patients, but more often in late diagnosed, pancreatic sufficient patients.