Mutations in the CFTR gene that lead to defective Cl– and other ion transport through CFTR protein channels are the underlying defect of CF CFTR gene mutations affect the total activity of the CFTR protein channel at the apical cell surface, by affecting the quantity of CFTR channels at the cell surface and/or function of CFTR as an ion channel The reduction in total CFTR protein activity leads to pathophysiological changes in epithelial cells that affect the airway, pancreas, gastrointestinal tract and sweat glands, among other systems References MacDonald KD et al. Paediatr Drugs 2007;9:1–10 Rowe SM et al. N Engl J Med 2005;352:1992–2001 Lommatzsch ST & Aris R. Semin Respir Crit Care Med 2009;30:531–8 Davis PB. Am J Respir Crit Care Med 2006;173:475–82

Key Points Total CFTR activity—the total chloride ion transport mediated by CFTR surface channels—is determined by both CFTR quantity (the number of CFTR channels on the cell surface) and CFTR function (the functional ability of each channel to open and transport chloride)1 The degree to which the CFTR mutation reduces CFTR quantity and/or function determines the total CFTR activity1 Additional Information CFTR quantity is determined by CFTR synthesis (CFTR gene transcription and mRNA translation), CFTR trafficking (delivery of normally processed, mature CFTR protein to the cell surface), and CFTR surface stability (normal CFTR channels are eventually removed from the cell surface)2 CFTR function is determined by channel open probability (the fraction of time that a specific channel is open and transporting chloride) and channel conductance2 References Sheppard DN et al. EMBO J. 1995;14(5):876-883. Derichs N. Eur Respir Rev. 2013;22(127):58-65.

Chloride transport was measured in FRT cells engineered to express one of 63 CFTR mutations (61 missense and 2 in-frame deletion) Ussing chamber studies were used to measure chloride transport, which was expressed as a percentage of the mean transepithelial current in FRT cell lines expressing normal CFTR (% normal CFTR) Reference Sosnay PR et al. Nat Genet. 2013;45(10):1160-1167.

Key Point CFTR genotype of both alleles determines total CFTR activity. Total CFTR activity, in turn, contributes to the expression of the clinical phenotype   Additional Information Both CFTR alleles contribute to the expression of CFTR protein in epithelial cells. Some CFTR mutations result in little to no CFTR activity while other mutations result in some residual CFTR activity The specific mutations on the 2 CFTR alleles and the respective levels of total CFTR activity associated with each mutation together determines the total CFTR activity in a specific individual The total CFTR activity, in turn, influences the expression of the clinical phenotype Reference Zielenski J. Respiration 2000;67:117–33

References Boyle MP & De Boeck K. Lancet Respir Med 2013;1:158–63 Griesenbach U et al. Thorax 1999;54(Suppl 2):S19–23 Zielenski J. Respiration 2000;67:117–33 Davis PB. Am J Respir Crit Care Med 2006;173:475–82 Wilschanski M & Durie PR. Gut 2007;56:1153–63 Castellani C et al. J Cyst Fibros 2008;7:179–96

Comparing clinical data from patients with 6 different residual function mutations show variability in measurements such as: Age of diagnosis ranges from a mean of 61 to 402 Sweat chloride levels range from a mean of 573 to 1034 ppFEV1 levels range from a mean of 595 to 851 Pancreatic insufficiency percentages range from 136 to 501 References De Braekeleer M et al. Hum Genet. 1997;101(2):208-211. Castaldo G et al. J Cystic Fibros. 2006;5(3):193-195. Gilfillan A et al. J Med Genet. 1998;35(2):122-125. Masvidal L et al. Eur J Hum Genet. 2014;22(6):784-791. Antinolo G et al. J Med Genet. 1997;34(2):89-91. The Cystic Fibrosis Genotype –Phenotype Consortium. N Engl J Med. 1993;329(18):1308-1313.

Studies have shown that the increased chloride level in sweat in patients with CF is linked to dysfunction or absence of CFTR protein.1 Generally, the sweat chloride concentration in normal subjects is below 40 mmol/L and in individuals with CF, sweat chloride concentrations are ≥60 mmol/L2 Patients with CF with partial CFTR function and exocrine pancreatic sufficiency tend to have lower sweat chloride levels than patients who have pancreatic insufficiency (PI)1 Individuals with CF and PI have the lowest level of CFTR protein activity and the highest sweat chloride content1 Therefore, sweat chloride level is a biomarker of pancreatic status in CF1 Note for the graph: Three important assumptions were made: (1) Sweat chloride levels are vs predicted CFTR activity; (2) normal individuals are assumed to have 100% CFTR activity; (3) carriers are assumed to have 50% CFTR activity1 References Rowe SM et al. Proc Am Thorac Soc. 2007;4(4):387-398. Farrell PM et al. J Pediatr. 2008;153(2):S4-S14.

This study compared ppFEV1 in patient with CF with CFTR-R117H vs CFTR-F508del A retrospective cohort of patients with CF reported in the US CFF Patient Registry from 2006 to 2010 was used to compare patients with ≥1 R117H mutation with patients homozygous for the F508del mutation In total, 156 R117H and 6251 homozygous F508del patients (from an original 14,450 patients) met the selection criteria and were included in the analysis The ppFEV1 rate of change (slope) was estimated for the R117H and F508del cohorts, adjusted for age group (6-12, 13-17, 18-24, and ≥25 years) Younger patients (6-12 years of age) with an R117H mutation had an overall improving ppFEV1 slope; R117H and F508del patients in the oldest two age groups (18-24 and ≥25 years of age) had similar slopes (rates of decline) Reference Wagener JS et al. NACFC. 2015. Poster 415.

This study focused on survival of patients with CF in the US CFF database based on the severity of their genotypes1 Analysis was limited to CFTR genotypes that were recorded in the database, had a known functional class, and whose allele frequency was >0.1%1 Genotypes were classified as “severe” if the mutations on both alleles fell into class I, II, or III, and classified as “mild” if at least one mutation on one allele fell into class IV or V1 Patients were followed between 1993 and 20021 Median follow-up was 8.6 years for patients with a high-risk CFTR genotype vs 5.1 years for patients with a low-risk CFTR genotype1 There were a total of 1672 deaths during the 10-year follow-up period1 Reference McKone EF et al. Chest. 2006;130(5):1441-1447.