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. 2016 Feb 1;27(3):424–433. doi: 10.1091/mbc.E14-04-0935

From CFTR biology toward combinatorial pharmacotherapy: expanded classification of cystic fibrosis mutations

Gudio Veit a, Radu G Avramescu a, Annette N Chiang b, Scott A Houck c, Zhiwei Cai d, Kathryn W Peters e, Jeong S Hong f, Harvey B Pollard g, William B Guggino h, William E Balch i, William R Skach j, Garry R Cutting k, Raymond A Frizzell e, David N Sheppard d, Douglas M Cyr c, Eric J Sorscher l, Jeffrey L Brodsky b, Gergely L Lukacs a,m,n,**
Editor: David G Drubino
PMCID: PMC4751594  PMID: 26823392

Abstract

More than 2000 mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) have been described that confer a range of molecular cell biological and functional phenotypes. Most of these mutations lead to compromised anion conductance at the apical plasma membrane of secretory epithelia and cause cystic fibrosis (CF) with variable disease severity. Based on the molecular phenotypic complexity of CFTR mutants and their susceptibility to pharmacotherapy, it has been recognized that mutations may impose combinatorial defects in CFTR channel biology. This notion led to the conclusion that the combination of pharmacotherapies addressing single defects (e.g., transcription, translation, folding, and/or gating) may show improved clinical benefit over available low-efficacy monotherapies. Indeed, recent phase 3 clinical trials combining ivacaftor (a gating potentiator) and lumacaftor (a folding corrector) have proven efficacious in CF patients harboring the most common mutation (deletion of residue F508, ΔF508, or Phe508del). This drug combination was recently approved by the U.S. Food and Drug Administration for patients homozygous for ΔF508. Emerging studies of the structural, cell biological, and functional defects caused by rare mutations provide a new framework that reveals a mixture of deficiencies in different CFTR alleles. Establishment of a set of combinatorial categories of the previously defined basic defects in CF alleles will aid the design of even more efficacious therapeutic interventions for CF patients.

INTRODUCTION

Cystic fibrosis (CF), caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR), is characterized by a multiorgan pathology affecting the upper and lower airway, gastrointestinal and reproductive tracts, and endocrine system (Riordan et al., 1989; Collins, 1992; Rowe et al., 2005; Cutting, 2015). CF is one of the most common lethal autosomal-recessive diseases, with a prevalence of one in 3500 in the United States and one in 2500 in the European Union (Farrell, 2008; Pettit and Fellner, 2014). Lack of functional CFTR expression at the apical membrane of secretory epithelia results in defective Cl− and bicarbonate secretion, coupled to enhanced Na+ absorption and mucus secretion, which in airway epithelia leads to dehydration and acidification of the airway surface liquid (Tarran et al., 2001; Chen et al., 2010; Derichs et al., 2011; Pezzulo et al., 2012). As a consequence, impaired mucociliary clearance provokes recurrent infection and uncontrolled inflammation culminating in lung damage, which is the primary cause of morbidity and mortality in CF (Ratjen and Doring, 2003; Boucher, 2007; Stoltz et al., 2015). CFTR is member of the ATP-binding cassette (ABC) subfamily C (ABCC7) (Kerr, 2002). It consists of two homologous halves, each containing a hexa-helical membrane-spanning domain (MSD1 and MSD2) and a nucleotide-binding domain (NBD1 and NBD2) that are connected by an unstructured regulatory domain (Riordan, 1993; Riordan et al., 1989).

BIOLOGY OF CFTR MUTATION: TRADITIONAL CLASSIFICATION

CF is caused by ∼2000 mutations in the CFTR gene with a wide range of disease severity (www.genet.sickkids.on.ca/home.html; www.cftr2.org; Sosnay et al., 2013), which is further influenced by modifier genes (Collaco and Cutting, 2008; Cutting, 2010) and by the environmental and socioeconomic status of patients (Schechter et al., 2001; Barr et al., 2011; Taylor-Robinson et al., 2014; Kopp et al., 2015). The first classification of CF mutations into four classes according to their primary biological defect was proposed by Welsh and Smith in a landmark paper (Welsh and Smith, 1993). Currently, six major classes are distinguished (Rowe et al., 2005; Zielenski and Tsui, 1995) (Figure 1).

FIGURE 1:

FIGURE 1:

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Traditional classification of CF mutations based on their cellular phenotype. Class I: protein synthesis defect; class II: maturation defect; class III: gating defect; class IV: conductance defect; class V: reduced quantity; and class VI: reduced stability. ER, endoplasmic reticulum; TGN, trans-Golgi network.

Class I encompasses frameshift, splicing, or nonsense mutations that introduce premature termination codons (PTC), resulting in severely reduced or absent CFTR expression.

Class II mutations lead to misfolding, premature degradation by the endoplasmic reticulum (ER) quality-control system, and impaired protein biogenesis, severely reducing the number of CFTR molecules that reach the cell surface.

Class III mutations impair the regulation of the CFTR channel, resulting in abnormal gating characterized by a reduced open probability.

Class IV mutations alter the channel conductance by impeding the ion conduction pore, leading to a reduced unitary conductance (Sheppard et al., 1993; Hammerle et al., 2001).

Class V mutations do not change the conformation of the protein but alter its abundance by introducing promoter or splicing abnormalities (Highsmith et al., 1994, 1997; Zielenski and Tsui, 1995).

Class VI mutations destabilize the channel in post-ER compartments and/or at the plasma membrane (PM), by reducing its conformational stability (Haardt et al., 1999) and/or generating additional internalization signals (Silvis et al., 2003). This results in accelerated PM turnover and reduced apical PM expression (Haardt et al., 1999; Silvis et al., 2003).

For many of the identified mutations, the disease liability is unknown, but efforts are under way to assess their functional consequence and clinical severity (www.cftr2.org; Sosnay et al., 2013).

MUTATION CLASS–SPECIFIC PHARMACOTHERAPY

Defining the cellular and molecular pathology of CFTR mutations proved to be invaluable for development of small-molecule compounds targeting the underlying defect(s) in CF. The fact that some CFTR variants carrying class III or IV mutations can be expressed at the apical membrane of secretory epithelia at a density similar to that of the wild-type protein, although they are functionally impaired (e.g., G551D), led to the development of gating potentiators that increase the open probability and thereby the PM chloride conductance (Yang et al., 2003). VX-770 (ivacaftor) is the first potentiator drug to be U.S. Food and Drug Administration approved for CF treatment; it directly targets the gating defect of the class III mutation G551D-CFTR (Van Goor et al., 2009). This compound was developed by Vertex Pharmaceuticals in conjunction with Cystic Fibrosis Foundation Therapeutics, Inc. (CFFT), and shows remarkable clinical benefit in patients carrying the mutation in either one or two alleles (Van Goor et al., 2009; Accurso et al., 2010; Ramsey et al., 2011). The approval of VX-770 was extended to eight additional class III mutations (G178R, S549N, S549R, G551S, G1244E, S1251N, S1255P, and G1349D) (Yu et al., 2012; Vertex, 2014a) and recently to the class IV mutation R117H (Vertex, 2014b).

The prototypical class II mutation, ΔF508-CFTR (Phe508del), elicits a complex folding defect that compromises both NBD1 stability and the channel’s cooperative domain assembly (Du and Lukacs, 2009; Du et al., 2005; Mendoza et al., 2012; Rabeh et al., 2012). For many years, large-scale efforts have been under way to isolate correctors that act as pharmacological chaperones by directly binding to and promoting the biogenesis of class II CFTR mutations. The most promising corrector compound at present, VX-809 (lumacaftor), partially reverts the ΔF508-CFTR functional expression defect by stabilizing the NBD1-MSD1/2 interface (Farinha et al., 2013; Loo et al., 2013; Okiyoneda et al., 2013; Ren et al., 2013), leading to a marked correction from 3 to 15% of wild-type channel activity in vitro (Van Goor et al., 2011). A clinical trial, however, failed to observe significant clinical benefit in homozygous ΔF508-CFTR patients (Clancy et al., 2012). Acute addition of VX-770 to VX-809-corrected ΔF508-CFTR doubled the PM activity in vitro (Van Goor et al., 2011), and the combination therapy showed modest but significant clinical improvement (Boyle et al., 2014; Wainwright et al., 2015). Based on these results, the combination treatment has been approved for CF patients 12 years and older with two copies of the ΔF508 mutation (Vertex, 2015). Other class II mutations that can be corrected by VX-809 in vitro include E56K, P67L, E92K, R170G, L206W, V232D, F508G, and A561E (Caldwell et al., 2011; Okiyoneda et al., 2013; Ren et al., 2013; Veit et al., 2014; Awatade et al., 2015).

Ribosomal read-through allows synthesis of full-length CFTR carrying class I mutations. To this end, ataluren (PTC124) was developed as a drug that promotes near-cognate aminoacyl-tRNA incorporation at PTCs (Lentini et al., 2014; Welch et al., 2007). Ataluren partially restores G542X-CFTR (class I) expression in a mouse model and modestly corrects CFTR function in nasal epithelia in patients with class I mutations (Du et al., 2008; Sermet-Gaudelus et al., 2010; Wilschanski et al., 2011). In a recent phase 3 clinical trial, however, ataluren treatment failed to produce significant clinical benefit, perhaps due to an adverse drug–drug interaction with tobramycin, which is a commonly administered, inhaled antibiotic used to treat lung infections in CF patients (Kerem et al., 2014).

LIMITATIONS OF CF MUTATION CLASSIFICATION

The efficacy of available monotherapies for some mutant alleles, which have been designated as class I, class II, or class III/IV mutations, is currently limited. This could be partly explained by the pleiotropic molecular defects caused by single mutations. Thus comprehensive mapping of the multiple molecular defects caused by a single or combination of mutant alleles could offer considerable advantage for improving therapeutic interventions and for future development of drug combinations. In the following list, we present a subset of mutations that display combinatorial molecular defects.

  • ΔF508: The most prevalent class II mutation impairs CFTR conformational maturation and leads to its targeting for premature ER-associated degradation (Cheng et al., 1990; Cyr, 2005; Kim and Skach, 2012; Lukacs et al., 1994). However, ΔF508-CFTR molecules that either constitutively or following rescue procedures escape the ER quality control and accumulate at the PM of airway epithelia exhibit a channel-gating defect, which is a hallmark of class III mutations (Dalemans et al., 1991), as well as accelerated turnover in post ER compartments and at the PM, a class VI mutation characteristic (Lukacs et al., 1993). Unless the folding and conformational dynamics of the rescued ΔF508-CFTR are fully restored to that of the wild-type protein by pharmacological treatment, this mutation remains partially defective and requires correction of its gating and/or peripheral stability defect. Rescue of the gating defect can be achieved with potentiators (e.g., VX-770) (Van Goor et al., 2009). Peripheral stabilization of the ΔF508-CFTR could be attained by 1) the peptide inhibitor iCAL36 (Cushing et al., 2010), 2) preventing post-Golgi ubiquitination (Fu et al., 2015; Okiyoneda et al., 2010), 3) restoring autophagosome formation (Luciani et al., 2012), or 4) modulating cellular protein homeostasis (Hutt et al., 2010). Thus the most common mutant has multiple defects that extend beyond the features of a class II mutation.

  • W1282X: This PTC represents a class I mutation, though recent studies suggest a more complex phenotype. First, the level of the W1282X transcript is reduced by nonsense-mediated RNA decay (Hamosh et al., 1992; Linde et al., 2007). Second, the PTC deletes part of the NBD2, which likely compromises NBD1-NBD2 dimerization and W1282X-CFTR folding and activity. Moreover, if the primary defect is corrected either with spontaneous or drug-induced read-through, some of the fully translated channel will contain nonconservative amino acid substitutions. These missense mutations may cause structural defects (class II characteristic), as suggested by the phenotype of CF patients with a missense mutation at the W1282 residue (Faucz et al., 2007; Ivaschenko et al., 1993; Visca et al., 2008), as well as a gating defect (class III characteristic), which can be inferred based on W1282X-CFTR channel activation after exposure to VX-770 (Xue et al., 2014).

  • P67L: P67L is a mild class II mutation that results in attenuated CFTR biogenesis, as indicated by the reduced ratio between post-ER complex–glycosylated (band C) and ER-resident core-glycosylated protein (band B) (Ren et al., 2013; Sosnay et al., 2013; Van Goor et al., 2014). Treatment with the corrector VX-809 increases the abundance of the complex-glycosylated form and PM density to nearly the level of WT-CFTR (Ren et al., 2013; Veit et al., 2014). However, the mutant channel is also sensitive in vitro to potentiator treatment (a class III characteristic), both in the presence and absence of corrector (Van Goor et al., 2014; Veit et al., 2014). Accordingly, treatment with VX-770 ameliorated the CF lung disease in a heterozygous P67L/ΔF508 patient (Yousef et al., 2015).

  • R117H: This mutation in conjunction with the 5T variant in the polythymidine tract in intron 8 was originally categorized as a class IV mutation, but it also exhibits a gating defect (class III trait) that, at least in part, can be rectified by VX-770 treatment (Sheppard et al., 1993; Van Goor et al., 2014). The R117H mutation also results in reduced complex-glycosylated CFTR expression, which is a class II characteristic (Fanen et al., 1997; Sheppard et al., 1993). This potentially explains the limited success of VX-770 treatment in patients carrying this mutation (Char et al., 2014; Moss et al., 2015).

AN EXPANDED CLASSIFICATION OF MUTANT CFTR BIOLOGY

We propose a modification of the current classification scheme, which would entail permutations of the traditional class I–VI CF mutations. This expanded classification of the major mechanistic categories (Welsh and Smith, 1993; Zielenski, 2000; Rowe et al., 2005) accommodates the unusually complex, combinatorial molecular/cellular phenotypes of CF alleles. It consists of 31 possible classes of mutations, including the original classes I, II, III/IV, V, and VI, as well as their 26 combinations, as depicted in the Venn diagram shown in Figure 2. For the sake of simplicity, class III and IV mutations, representing functional (gating and conductance, respectively) defects, are combined. For example, according to the expanded classification, G551D will be designated as a class III mutation as before (Welsh and Smith, 1993), while ΔF508 will be classified as class II–III–VI, W1282X as class I–II–III–VI, P67L as class II–III, and R117H as class II–III/IV, reflecting the composite defects in mutant CFTR biology (Figure 2 and Table 1).

FIGURE 2:

FIGURE 2:

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Refined classification of CF mutations accounting for complex phenotypes of major CFTR cellular defects. The Venn diagram indicates all combinations of mutation classes with selected examples. Possible combinations without identified mutation are indicated in gray.

TABLE 1:

Examples for CF mutations with complex or classical cellular phenotypes.

Refined classification Mutation I II III/IV V VI Model Reference
I–II–III–VI W1282X X1,2,3 X2,5 X2,4,5 X5 1HNE 1Hamosh et al., 1992
2HBE 2Cyr lab, unpublisheda
3CFBE 3Frizzell lab, unpublishedb
4CFBE 4Xue et al., 2014
5CFBE 5Lukacs lab, unpublishedc
II–III M1V X6 X6 6FRT 6Van Goor et al., 2014
II–III E56K X5,6 X6 5CFBEd 5Lukacs lab, unpublished
6FRT 6Van Goor et al., 2014
II–III P67L X3,6,7,8,9,10 X6,7,10 3CFBE 3Frizzell lab, unpublished
6FRT 6Van Goor et al., 2014
7CFBE 7Veit et al., 2014
8Hek293 8Ren et al., 2013
9HeLa 9Sosnay et al., 2013
10FRT 10Sorscher lab, unpublishede
II–III R74W X6 X6 6FRT 6Van Goor et al., 2014
II-III E92K X3,5,6,8,16 X5 3CFBE 3Frizzell lab, unpublished
5CFBE 5Lukacs lab, unpublished
6FRT 6Van Goor et al., 2014
8HEK293 8Ren et al., 2013
16HEK293 16Brodsky lab, unpublished
II–III P99L X11 X11 11HeLa 11Sheppard et al., 1996
II–III D110H X6 X6 6FRT 6Van Goor et al., 2014
II–III R117C X6 X6 6FRT 6Van Goor et al., 2014
II–III R117H X2,3,12,13 X2,6,12 2HBE 2Cyr lab, unpublished
3CFBE 3Frizzell lab, unpublished
6FRT 6Van Goor et al., 2014
12FRT, HeLa 12Sheppard et al., 1993
13HeLa 13Fanen et al., 1997
II–III R170G X7,14 X7 7CFBE 7Veit et al., 2014
14BHK 14Okiyoneda et al., 2013
II–III E193K X5 X5,6 5CFBE 5Lukacs lab, unpublished
6FRTf 6Van Goor et al., 2014
II–III P205S X11 X11 11HeLa 11Sheppard et al., 1996
II–III L206W X5,6,8 X5,6 5CFBE 5Lukacs lab, unpublished
6FRT 6Van Goor et al., 2014
8HEK293 8Ren et al., 2013
II–III V232D X15 X15 15HEK293 15Caldwell et al., 2011
II–III R334W X2,3,5 X2,5,6,12 2COS-7 2Cyr lab, unpublished
3CFBE 3Frizzell lab, unpublished
5CFBE 5 Lukacs lab, unpublished
II–III R334W X2,3,5 X2,5,6,12 2COS-7 2Cyr lab, unpublished
3CFBE 3Frizzell lab, unpublished
5CFBE 5 Lukacs lab, unpublished
6FRTf 6Van Goor et al., 2014
12HeLa 12Sheppard et al., 1993
II–III I336K X6 X6 6FRT 6Van Goor et al., 2014
II–III T338I X5,6 X5,6 5CFBE 5Lukacs lab, unpublished
6FRT 6Van Goor et al., 2014
II–III S341P X5,6 X5,6 5CFBE 5Lukacs lab, unpublished
6FRT 6Van Goor et al., 2014
II–III A455E X3,6,16, 17 X6 3CFBE 3Frizzell lab, unpublished
6FRT 6Van Goor et al., 2014
16HEK293 16Brodsky lab, unpublishedg
17FRT, HeLad 17Sheppard et al., 1995
II–III S549R X3,5,18 X5,18 3CFBE 3Frizzell lab, unpublished
5CFBE 5Lukacs lab, unpublished
15FRT 18Yu et al., 2012
II–III D579G X5,6 X5,6 5CFBE 5Lukacs lab, unpublished
6FRT 6Van Goor et al., 2014
II–III R668C X6 X6 6FRT 6Van Goor et al., 2014
II–III L927P X6 X6 6FRT 6Van Goor et al., 2014
II–III S945L X6 X6 6FRT 6Van Goor et al., 2014
II–III S977F X6 X6 6FRT 6Van Goor et al., 2014
II–III L997F X6 X6 6FRT 6Van Goor et al., 2014
II–III H1054D X6 X6 6FRT 6Van Goor et al., 2014
II–III R1066H X6 X6 6FRT 6Van Goor et al., 2014
II–III A1067T X6 X6 6FRT 6Van Goor et al., 2014
II–III R1070Q X6 X6 6FRT 6Van Goor et al., 2014
II–III R1070W X6,14 X6 6FRT 6Van Goor et al., 2014
14BHK 14Okiyoneda et al., 2013
II–III F1074L X6 X6 6FRT 6Van Goor et al., 2014
II–III D1270N X6 X6 6FRT 6Van Goor et al., 2014
II–VI S492F X3,5,6 X5 3CFBE 3Frizzell lab, unpublished
5CFBE 5Lukacs lab, unpublished
6FRT 6Van Goor et al., 2014
II–III–VI R347P X3,5,6 X5,6,12 X5 3CFBE 3Frizzell lab, unpublished
5CFBE 5Lukacs lab, unpublished
6FRT 6Van Goor et al., 2014
12HeLa 12Sheppard et al., 1993
II–III–VI ΔF508 X19 X20 X21 19COS 19Cheng et al., 1990
20Vero 20Dalemans et al., 1991
21CHO 21Lukacs et al., 1993
II–III–VI A561E X6,22,23 X23 X23 6FRTd 6Van Goor et al., 2014
22HBEd 22Awatade et al., 2015
23BHK 23Wang et al., 2014
II–III–VI L1077P X3,6,16,24 X24 X24 3CFBE 3Frizzell lab, unpublished
6FRT 6Van Goor et al., 2014
16HEK293 16Brodsky lab, unpublished
24CHO 24Sheppard lab, unpublishedh
II–III–VI N1303K X2,3,5,6,16,22,24 X24 X5 2HBE 2Cyr lab, unpublished
3CFBE 3Frizzell lab, unpublished
5CFBE 5Lukacs lab, unpublished
6FRT 6Van Goor et al., 2014
16HEK293 16Brodsky lab, unpublished
22HBE 22Awatade et al., 2015
24CHOi 24Sheppard lab, unpublished
III–VI Q1411X X25 X26 25BHK 25Gentzsch et al., 2002
26Cos, BHK 26Haardt et al., 1999
II A46D X6 6FRT 6Van Goor et al., 2014
II G85E X3,6,16,24 3CFBE 3Frizzell lab, unpublished
6FRT 6Van Goor et al., 2014
16HEK293 16Brodsky lab, unpublished
24CHO 24Sheppard lab, unpublished
III R352Q X5,6 5CFBE 5Lukacs lab, unpublished
6FRTf 6Van Goor et al., 2014
II L467P X6 6FRT 6Van Goor et al., 2014
II V520F X3,6,16 3CFBE 3Frizzell lab, unpublished
6FRT 6Van Goor et al., 2014
16HEK293 16Brodsky lab, unpublished
II A559T X6 6FRT 6Van Goor et al., 2014
II R560S X6 6FRT 6Van Goor et al., 2014
II R560T X3,6,16 3CFBE 3Frizzell lab, unpublished
6FRT 6Van Goor et al., 2014
16HEK293 16Brodsky lab, unpublished
II R560K X3 3CFBE 3Frizzell lab, unpublished
II Y569D X6 6FRT 6Van Goor et al., 2014
II D614G X3 3CFBE 3Frizzell lab, unpublished
II L1065P X3,6 3CFBE 3Frizzell lab, unpublished
6FRT 6Van Goor et al., 2014
II R1066C X3,6 3CFBE 3Frizzell lab, unpublished
6FRT 6Van Goor et al., 2014
II R1066M X6 6FRT 6Van Goor et al., 2014
II H1085R X6 6FRT 6Van Goor et al., 2014
II M1101K X6 6FRT 6Van Goor et al., 2014
III D110E X6 6FRT 6Van Goor et al., 2014
III G178R X18 18FRT 18Yu et al., 2012
III R347H X6,7 6FRT 7Veit et al., 2014
7CFBE 7Veit et al., 2014
III S549N X18 18FRT 18Yu et al., 2012
III G551D X27, 28 27CHO 27Bompadre et al., 2007
28L 28Yang et al., 1993
III G551S X18 18FRT 18Yu et al., 2012
III F1052V X6 6FRT 6Van Goor et al., 2014
III K1060T X6 6FRT 6Van Goor et al., 2014
III D1152H X6 6FRT 6Van Goor et al., 2014
III S1235R X6 6FRT 6 Van Goor et al., 2014
III G1244E X18 18FRT 18 Yu et al., 2012
III S1251N X18 18FRT 18Yu et al., 2012
III S1255P X18 18FRT 18Yu et al., 2012
III G1349D X18,27 18FRT 18Yu et al., 2012
27CHO 27Bompadre et al., 2007
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Superscript numbers refer to references in far-right column.

aS.A.H. and D.M.C., unpublished observations

bK.W.P. and R.A.F., unpublished observations.

cR.G.A., H.Xu, and G.L.L., unpublished observations.

dDoes not exhibit a gating or conductance defect in this cell model.

eJ.S.H. and E.J.S., unpublished observations.

fDoes not exhibit a biogenesis defect in this cell model.

gA.N.C. and J.L.B., unpublished observations.

hZ.C. and D.N.S., unpublished observations.

iDoes not exhibit a peripheral stability defect in this cell model.

A recent study by Vertex Pharmaceuticals successfully demonstrated that 24 of 54 tested missensse mutations display both a processing (class II) and gating (class III) defect in the Fischer rat thyroid epithelial expression system (Van Goor et al., 2014). Characterization of several rare CF mutations is ongoing in laboratories of the CFTR2 Consortium, the CFTR Folding Consortium, CFFT, Vertex Pharmaceuticals, and many others (Caldwell et al., 2011; Yu et al., 2012; Sosnay et al., 2013; Harness-Brumley et al., 2014; Hong et al., 2014; Van Goor et al., 2014; Wang et al., 2014; Awatade et al., 2015). This work will likely provide further examples of combinatorial mechanistic defects exhibited by CF mutants.

THERAPEUTIC SUSCEPTIBILITY OF CF MUTATIONS WITH COMPLEX BIOLOGICAL DEFECTS

In-depth analysis of the biology of CF mutants distinguishes them according to their complex molecular pathology and suggests drug combinations for treatment of different patient populations. This process, called “theratyping” (Cutting, 2015), will pave the way to personalized medicine in CF. However, reliable prediction of the responsiveness of a mutant phenotype to pharmacotherapy could be challenging and is dependent on the cellular model system (Pedemonte et al., 2010).

Emerging evidence also suggests that the efficacy of approved and preclinical drugs may vary with different mutations within the same class. For example, while nearly complete processing correction of P67L- and R170G-CFTR (class II) was achieved with VX-809 treatment (Okiyoneda et al., 2013; Ren et al., 2013; Veit et al., 2014), VX-809 only partially reversed the folding defect of some other class II mutants; for example, N1303K and ΔF508 (Okiyoneda et al., 2013; Awatade et al., 2015). This differential susceptibility to correction is attributed to the nature of the primary folding/structural defect. According to one hypothesis, robust folding correction of ΔF508-CFTR requires corrector combinations to avert its NBD1-MSD1/2 interface and NBD1 stability defects (Mendoza et al., 2012; Rabeh et al., 2012; He et al., 2013; Okiyoneda et al., 2013). The N1303K mutation in NBD2 was not rescued by VX-809, and only modest processing was observed by targeting both the NBD1/MSDs and NBD2 interfaces with C4 and C18 (a VX-809 analogue) (Okiyoneda et al., 2013; Rapino et al., 2015).

Some of the class III mutations also respond differently to the gating potentiator VX-770. Although R347H- and T338I-CFTR cause severe functional defects with no or modest loss of protein expression, only R347H-CFTR is potentiated by VX-770 to near wild type–like conductance (Van Goor et al., 2014). Likewise, the P5 potentiator activates ΔF508-CFTR, but it has no effect on G551D-CFTR chloride permeation (Yang et al., 2003). Thus identification of mutation-specific novel potentiators or their combinations may further optimize channel rescue for specific class III/IV mutations. Additive enhancement of G551D-CFTR activity by the combination of the potentiators genistein and curcumin supports the feasibility of combining potentiators (Yu et al., 2011). Likewise, we envision that mutation-specific read-through drugs will ultimately need to be combined with other correctors and potentiators, based on the pleiotropic defects associated with this class of mutations (as illustrated for W1282X above).

CONCLUDING REMARKS

The ultimate goal of theratyping is to achieve optimal correction of a specific mutant defect by selecting the most efficacious CFTR modulator(s), including correctors(s), potentiator(s), and/or read-through drugs, or a combination of these drugs. Based on accumulating observations, however, mechanistic subdivisions of some of the major classes of mutations (classes I, II, and III) may be necessary to further improve the success of drug-selection strategies. This will facilitate the theratyping of CF alleles and their combinations and expedite the identification and approval process for combination therapies. Theratyping has already proven successful in identifying class III mutations that are responsive to VX-770 (Yu et al., 2012), leading to the approval of this drug for eight rare mutations besides G551D (Vertex, 2014a). In fact, the results of large-scale theratyping could be overlaid as a third dimension on the Venn diagram presented in Figure 2.

Thus, during the 22 years following the initial classification of CF mutations (Welsh and Smith, 1993), our understanding of the molecular complexity of CF alleles has evolved remarkably, establishing the need for an advanced mutation classification scheme in conjunction with personalized CF therapy.

Acknowledgments

We thank the members of the CFTR Folding Consortium, the CFTR Theratype Group, C. M. Penland, and K. Tuggle (Cystic Fibrosis Foundation, Bethesda, MD) for their valuable support. The work described here was supported by the following institutions and grants: National Institutes of Health (NIH) NO1-HL28187 and IAA-A-HL-14-007.001 to H.B.P.; Cystic Fibrosis Foundation (CFF), NIH DK51870, TRDRP23RT-0012, and HL095524 to W.E.B.; NIH R01 DK, CFF CUTT13A1, and CUTTXX0 to G.R.C.; CFFT SHEPPA14XX0 and Cystic Fibrosis Trust to D.N.S.; NIH R01-DK068196, P30-DK072506, and CFFT FRIZZE05X0 to R.A.F.; NIH RO1 GM56981 and CFFT CYR13XX0 to D.M.C.; the CFF Research Development Program, CFFT SORSCH05XXO, and SORSCH14XXO to E.J.S.; CFFT BRODSK13XX0 and NIH GM75061 to J.L.B.; CF Canada, CFFT Lukacs13XXO, NIH DK075302, and Canadian Institutes of Health Research to G.L.L. R.G.A. was supported by CF Canada Studentship; G.L.L. is a recipient of a Canada Research Chair.

Abbreviations used:

ABC

ATP-binding cassette

CF

cystic fibrosis

CFFT

Cystic Fibrosis Foundation Therapeutics, Inc.

CFTR

cystic fibrosis transmembrane conductance regulator

ER

endoplasmic reticulum

MSD

membrane-spanning domain

NBD

nucleotide-binding domain

PM

plasma membrane

PTC

premature termination codon.

Footnotes

REFERENCES

  1. Accurso FJ, Rowe SM, Clancy JP, Boyle MP, Dunitz JM, Durie PR, Sagel SD, Hornick DB, Konstan MW, Donaldson SH, et al. Effect of VX-770 in persons with cystic fibrosis and the G551D-CFTR mutation. N Engl J Med. 2010;363:1991–2003. doi: 10.1056/NEJMoa0909825. [DOI] [PMC free article] [PubMed] [Google Scholar]
  2. Awatade NT, Uliyakina I, Farinha CM, Clarke LA, Mendes K, Sole A, Pastor J, Ramos MM, Amaral MD. Measurements of functional responses in human primary lung cells as a basis for personalized therapy for cystic fibrosis. EBioMedicine. 2015;2:147–153. doi: 10.1016/j.ebiom.2014.12.005. [DOI] [PMC free article] [PubMed] [Google Scholar]
  3. Barr HL, Britton J, Smyth AR, Fogarty AW. Association between socioeconomic status, sex, and age at death from cystic fibrosis in England and Wales (1959 to 2008): cross sectional study. Br Med J. 2011;343:d4662. doi: 10.1136/bmj.d4662. [DOI] [PMC free article] [PubMed] [Google Scholar]
  4. Bompadre SG, Sohma Y, Li M, Hwang TC. G551D and G1349D, two CF-associated mutations in the signature sequences of CFTR, exhibit distinct gating defects. J Gen Physiol. 2007;129:285–298. doi: 10.1085/jgp.200609667. [DOI] [PMC free article] [PubMed] [Google Scholar]
  5. Boucher RC. Airway surface dehydration in cystic fibrosis: pathogenesis and therapy. Annu Rev Med. 2007;58:157–170. doi: 10.1146/annurev.med.58.071905.105316. [DOI] [PubMed] [Google Scholar]
  6. Boyle MP, Bell SC, Konstan MW, McColley SA, Rowe SM, Rietschel E, Huang X, Waltz D, Patel NR, Rodman D. A CFTR corrector (lumacaftor) and a CFTR potentiator (ivacaftor) for treatment of patients with cystic fibrosis who have a phe508del CFTR mutation: a phase 2 randomised controlled trial. Lancet Respir Med. 2014;2:527–538. doi: 10.1016/S2213-2600(14)70132-8. [DOI] [PubMed] [Google Scholar]
  7. Caldwell RA, Grove DE, Houck SA, Cyr DM. Increased folding and channel activity of a rare cystic fibrosis mutant with CFTR modulators. Am J Physiol Lung Cell Mol Physiol. 2011;301:L346–L352. doi: 10.1152/ajplung.00044.2011. [DOI] [PMC free article] [PubMed] [Google Scholar]
  8. Char JE, Wolfe MH, Cho HJ, Park IH, Jeong JH, Frisbee E, Dunn C, Davies Z, Milla C, Moss RB, et al. A little CFTR goes a long way: CFTR-dependent sweat secretion from G551D and R117H-5T cystic fibrosis subjects taking ivacaftor. PLoS One. 2014;9:e88564. doi: 10.1371/journal.pone.0088564. [DOI] [PMC free article] [PubMed] [Google Scholar]
  9. Chen JH, Stoltz DA, Karp PH, Ernst SE, Pezzulo AA, Moninger TO, Rector MV, Reznikov LR, Launspach JL, Chaloner K, et al. Loss of anion transport without increased sodium absorption characterizes newborn porcine cystic fibrosis airway epithelia. Cell. 2010;143:911–923. doi: 10.1016/j.cell.2010.11.029. [DOI] [PMC free article] [PubMed] [Google Scholar]
  10. Cheng SH, Gregory RJ, Marshall J, Paul S, Souza DW, White GA, O’Riordan CR, Smith AE. Defective intracellular transport and processing of CFTR is the molecular basis of most cystic fibrosis. Cell. 1990;63:827–834. doi: 10.1016/0092-8674(90)90148-8. [DOI] [PubMed] [Google Scholar]
  11. Clancy JP, Rowe SM, Accurso FJ, Aitken ML, Amin RS, Ashlock MA, Ballmann M, Boyle MP, Bronsveld I, Campbell PW, et al. Results of a phase IIa study of VX-809, an investigational CFTR corrector compound, in subjects with cystic fibrosis homozygous for the F508del-CFTR mutation. Thorax. 2012;67:12–18. doi: 10.1136/thoraxjnl-2011-200393. [DOI] [PMC free article] [PubMed] [Google Scholar]
  12. Collaco JM, Cutting GR. Update on gene modifiers in cystic fibrosis. Curr Opin Pulm Med. 2008;14:559–566. doi: 10.1097/MCP.0b013e3283121cdc. [DOI] [PMC free article] [PubMed] [Google Scholar]
  13. Collins FS. Cystic fibrosis: molecular biology and therapeutic implications. Science. 1992;256:774–779. doi: 10.1126/science.1375392. [DOI] [PubMed] [Google Scholar]
  14. Cushing PR, Vouilleme L, Pellegrini M, Boisguerin P, Madden DR. A stabilizing influence: CAL PDZ inhibition extends the half-life of ΔF508-CFTR. Angew Chem Int Ed Engl. 2010;49:9907–9911. doi: 10.1002/anie.201005585. [DOI] [PMC free article] [PubMed] [Google Scholar]
  15. Cutting GR. Modifier genes in Mendelian disorders: the example of cystic fibrosis. Ann NY Acad Sci. 2010;1214:57–69. doi: 10.1111/j.1749-6632.2010.05879.x. [DOI] [PMC free article] [PubMed] [Google Scholar]
  16. Cutting GR. Cystic fibrosis genetics: from molecular understanding to clinical application. Nat Rev Genet. 2015;16:45–56. doi: 10.1038/nrg3849. [DOI] [PMC free article] [PubMed] [Google Scholar]
  17. Cyr DM. Arrest of CFTR ΔF508 folding. Nat Struct Mol Biol. 2005;12:2–3. doi: 10.1038/nsmb0105-2. [DOI] [PubMed] [Google Scholar]
  18. Dalemans W, Barbry P, Champigny G, Jallat S, Dott K, Dreyer D, Crystal RG, Pavirani A, Lecocq JP, Lazdunski M. Altered chloride ion channel kinetics associated with the ΔF508 cystic fibrosis mutation. Nature. 1991;354:526–528. doi: 10.1038/354526a0. [DOI] [PubMed] [Google Scholar]
  19. Derichs N, Jin BJ, Song Y, Finkbeiner WE, Verkman AS. Hyperviscous airway periciliary and mucous liquid layers in cystic fibrosis measured by confocal fluorescence photobleaching. FASEB J. 2011;25:2325–2332. doi: 10.1096/fj.10-179549. [DOI] [PMC free article] [PubMed] [Google Scholar]
  20. Du K, Lukacs GL. Cooperative assembly and misfolding of CFTR domains in vivo. Mol Biol Cell. 2009;20:1903–1915. doi: 10.1091/mbc.E08-09-0950. [DOI] [PMC free article] [PubMed] [Google Scholar]
  21. Du K, Sharma M, Lukacs GL. The ΔF508 cystic fibrosis mutation impairs domain-domain interactions and arrests post-translational folding of CFTR. Nat Struct Mol Biol. 2005;12:17–25. doi: 10.1038/nsmb882. [DOI] [PubMed] [Google Scholar]
  22. Du M, Liu X, Welch EM, Hirawat S, Peltz SW, Bedwell DM. PTC124 is an orally bioavailable compound that promotes suppression of the human CFTR-G542X nonsense allele in a CF mouse model. Proc Natl Acad Sci USA. 2008;105:2064–2069. doi: 10.1073/pnas.0711795105. [DOI] [PMC free article] [PubMed] [Google Scholar]
  23. Fanen P, Labarthe R, Garnier F, Benharouga M, Goossens M, Edelman A. Cystic fibrosis phenotype associated with pancreatic insufficiency does not always reflect the cAMP-dependent chloride conductive pathway defect. Analysis of C225R-CFTR and R1066C-CFTR. J Biol Chem. 1997;272:30563–30566. doi: 10.1074/jbc.272.48.30563. [DOI] [PubMed] [Google Scholar]
  24. Farinha CM, King-Underwood J, Sousa M, Correia AR, Henriques BJ, Roxo-Rosa M, Da Paula AC, Williams J, Hirst S, Gomes CM, Amaral MD. Revertants, low temperature, and correctors reveal the mechanism of F508del-CFTR rescue by VX-809 and suggest multiple agents for full correction. Chem Biol. 2013;20:943–955. doi: 10.1016/j.chembiol.2013.06.004. [DOI] [PubMed] [Google Scholar]
  25. Farrell PM. The prevalence of cystic fibrosis in the European Union. J Cyst Fibros. 2008;7:450–453. doi: 10.1016/j.jcf.2008.03.007. [DOI] [PubMed] [Google Scholar]
  26. Faucz FR, Gimenez J, Ramos MD, Pereira-Ferrari L, Estivill X, Raskin S, Casals T, Culpi L. Cystic fibrosis in a southern Brazilian ­ population: characteristics of 90% of the alleles. Clin Genet. 2007;72:218–223. doi: 10.1111/j.1399-0004.2007.00854.x. [DOI] [PubMed] [Google Scholar]
  27. Fu L, Rab A, Tang L, Bebok Z, Rowe SM, Bartoszewski R, Collawn JF. ΔF508 CFTR surface stability is regulated by DAB2 and CHIP-mediated ubiquitination in post-endocytic compartments. PLoS One. 2015;10:e0123131. doi: 10.1371/journal.pone.0123131. [DOI] [PMC free article] [PubMed] [Google Scholar]
  28. Gentzsch M, Aleksandrov A, Aleksandrov L, Riordan JR. Functional analysis of the C-terminal boundary of the second nucleotide binding domain of the cystic fibrosis transmembrane conductance regulator and structural implications. Biochem J. 2002;366:541–548. doi: 10.1042/BJ20020511. [DOI] [PMC free article] [PubMed] [Google Scholar]
  29. Haardt M, Benharouga M, Lechardeur D, Kartner N, Lukacs GL. C-terminal truncations destabilize the cystic fibrosis transmembrane conductance regulator without impairing its biogenesis. A novel class of mutation. J Biol Chem. 1999;274:21873–21877. doi: 10.1074/jbc.274.31.21873. [DOI] [PubMed] [Google Scholar]
  30. Hammerle MM, Aleksandrov AA, Riordan JR. Disease-associated mutations in the extracytoplasmic loops of cystic fibrosis transmembrane conductance regulator do not impede biosynthetic processing but impair chloride channel stability. J Biol Chem. 2001;276:14848–14854. doi: 10.1074/jbc.M011017200. [DOI] [PubMed] [Google Scholar]
  31. Hamosh A, Rosenstein BJ, Cutting GR. CFTR nonsense mutations G542X and W1282X associated with severe reduction of CFTR mRNA in nasal epithelial cells. Hum Mol Genet. 1992;1:542–544. doi: 10.1093/hmg/1.7.542. [DOI] [PubMed] [Google Scholar]
  32. Harness-Brumley C, Millen L, Huerta C, Schmidt A, Wigley W, Sosnay PR, Cutting GR, Thomas PJ. Groups of Cftr2 disease-causing mutations that respond to specific modulators. Pediatr Pulmonol. 2014;49:220–220. [Google Scholar]
  33. He L, Kota P, Aleksandrov AA, Cui L, Jensen T, Dokholyan NV, Riordan JR. Correctors of ΔF508 CFTR restore global conformational maturation without thermally stabilizing the mutant protein. FASEB J. 2013;27:536–545. doi: 10.1096/fj.12-216119. [DOI] [PMC free article] [PubMed] [Google Scholar]
  34. Highsmith WE, Burch LH, Zhou Z, Olsen JC, Boat TE, Spock A, Gorvoy JD, Quittel L, Friedman KJ, Silverman LM, et al. A novel mutation in the cystic fibrosis gene in patients with pulmonary disease but normal sweat chloride concentrations. N Engl J Med. 1994;331:974–980. doi: 10.1056/NEJM199410133311503. [DOI] [PubMed] [Google Scholar]
  35. Highsmith WE, Jr., Burch LH, Zhou Z, Olsen JC, Strong TV, Smith T, Friedman KJ, Silverman LM, Boucher RC, Collins FS, Knowles MR. Identification of a splice site mutation (2789 +5 G > A) associated with small amounts of normal CFTR mRNA and mild cystic fibrosis. Hum Mutat. 1997;9:332–338. doi: 10.1002/(SICI)1098-1004(1997)9:4<332::AID-HUMU5>3.0.CO;2-7. [DOI] [PubMed] [Google Scholar]
  36. Hong JS, Mahiou J, Liang F, Bihler HJ, Mense M, Lukacs GL, Wen H, Sorscher EJ. Epithelial models encoding diverse Cftr2 mutations for studies of disease mechanism and drug discovery. Pediatr Pulmonol. 2014;49:277–278. [Google Scholar]
  37. Hutt DM, Herman D, Rodrigues AP, Noel S, Pilewski JM, Matteson J, Hoch B, Kellner W, Kelly JW, Schmidt A, et al. Reduced histone deacetylase 7 activity restores function to misfolded CFTR in cystic fibrosis. Nat Chem Biol. 2010;6:25–33. doi: 10.1038/nchembio.275. [DOI] [PMC free article] [PubMed] [Google Scholar]
  38. Ivaschenko TE, Baranov VS, Dean M. Two new mutations detected by single-strand conformation polymorphism analysis in cystic fibrosis from Russia. Hum Genet. 1993;91:63–65. doi: 10.1007/BF00230224. [DOI] [PubMed] [Google Scholar]
  39. Kerem E, Konstan MW, De Boeck K, Accurso FJ, Sermet-Gaudelus I, Wilschanski M, Elborn JS, Melotti P, Bronsveld I, Fajac I, et al. Ataluren for the treatment of nonsense-mutation cystic fibrosis: a randomised, double-blind, placebo-controlled phase 3 trial. Lancet Respir Med. 2014;2:539–547. doi: 10.1016/S2213-2600(14)70100-6. [DOI] [PMC free article] [PubMed] [Google Scholar]
  40. Kerr ID. Structure and association of ATP-binding cassette transporter nucleotide-binding domains. Biochim Biophys Acta. 2002;1561:47–64. doi: 10.1016/s0304-4157(01)00008-9. [DOI] [PubMed] [Google Scholar]
  41. Kim SJ, Skach WR. Mechanisms of CFTR Folding at the endoplasmic reticulum. Front Pharmacol. 2012;3:201. doi: 10.3389/fphar.2012.00201. [DOI] [PMC free article] [PubMed] [Google Scholar]
  42. Kopp BT, Sarzynski L, Khalfoun S, Hayes D, Jr., Thompson R, Nicholson L, Long F, Castile R, Groner J. Detrimental effects of secondhand smoke exposure on infants with cystic fibrosis. Pediatr Pulmonol. 2015;50:25–34. doi: 10.1002/ppul.23016. [DOI] [PubMed] [Google Scholar]
  43. Lentini L, Melfi R, Di Leonardo A, Spinello A, Barone G, Pace A, Palumbo Piccionello A, Pibiri I. Toward a rationale for the PTC124 (Ataluren) promoted readthrough of premature stop codons: a computational approach and GFP-reporter cell-based assay. Mol Pharm. 2014;11:653–664. doi: 10.1021/mp400230s. [DOI] [PMC free article] [PubMed] [Google Scholar]
  44. Linde L, Boelz S, Nissim-Rafinia M, Oren YS, Wilschanski M, Yaacov Y, Virgilis D, Neu-Yilik G, Kulozik AE, Kerem E, Kerem B. Nonsense-mediated mRNA decay affects nonsense transcript levels and governs response of cystic fibrosis patients to gentamicin. J Clin Invest. 2007;117:683–692. doi: 10.1172/JCI28523. [DOI] [PMC free article] [PubMed] [Google Scholar]
  45. Loo TW, Bartlett MC, Clarke DM. Corrector VX-809 stabilizes the first transmembrane domain of CFTR. Biochem Pharmacol. 2013;86:612–619. doi: 10.1016/j.bcp.2013.06.028. [DOI] [PubMed] [Google Scholar]
  46. Luciani A, Villella VR, Esposito S, Gavina M, Russo I, Silano M, Guido S, Pettoello-Mantovani M, Carnuccio R, Scholte B, et al. Targeting autophagy as a novel strategy for facilitating the therapeutic action of potentiators on ΔF508 cystic fibrosis transmembrane conductance regulator. Autophagy. 2012;8:1657–1672. doi: 10.4161/auto.21483. [DOI] [PMC free article] [PubMed] [Google Scholar]
  47. Lukacs GL, Chang XB, Bear C, Kartner N, Mohamed A, Riordan JR, Grinstein S. The ΔF508 mutation decreases the stability of cystic fibrosis transmembrane conductance regulator in the plasma membrane. Determination of functional half-lives on transfected cells. J Biol Chem. 1993;268:21592–21598. [PubMed] [Google Scholar]
  48. Lukacs GL, Mohamed A, Kartner N, Chang XB, Riordan JR, Grinstein S. Conformational maturation of CFTR but not its mutant counterpart (ΔF508) occurs in the endoplasmic reticulum and requires ATP. EMBO J. 1994;13:6076–6086. doi: 10.1002/j.1460-2075.1994.tb06954.x. [DOI] [PMC free article] [PubMed] [Google Scholar]
  49. Mendoza JL, Schmidt A, Li Q, Nuvaga E, Barrett T, Bridges RJ, Feranchak AP, Brautigam CA, Thomas PJ. Requirements for efficient correction of ΔF508 CFTR revealed by analyses of evolved sequences. Cell. 2012;148:164–174. doi: 10.1016/j.cell.2011.11.023. [DOI] [PMC free article] [PubMed] [Google Scholar]
  50. Moss RB, Flume PA, Elborn JS, Cooke J, Rowe SM, McColley SA, Rubenstein RC, Higgins M. Efficacy and safety of ivacaftor in patients with cystic fibrosis who have an Arg117His-CFTR mutation: a double-blind, randomised controlled trial. Lancet Respir Med. 2015;3:524–533. doi: 10.1016/S2213-2600(15)00201-5. [DOI] [PMC free article] [PubMed] [Google Scholar]
  51. Okiyoneda T, Barriere H, Bagdany M, Rabeh WM, Du K, Hohfeld J, Young JC, Lukacs GL. Peripheral protein quality control removes unfolded CFTR from the plasma membrane. Science. 2010;329:805–810. doi: 10.1126/science.1191542. [DOI] [PMC free article] [PubMed] [Google Scholar]
  52. Okiyoneda T, Veit G, Dekkers JF, Bagdany M, Soya N, Xu H, Roldan A, Verkman AS, Kurth M, Simon A, et al. Mechanism-based corrector combination restores ΔF508-CFTR folding and function. Nat Chem Biol. 2013;9:444–454. doi: 10.1038/nchembio.1253. [DOI] [PMC free article] [PubMed] [Google Scholar]
  53. Pedemonte N, Tomati V, Sondo E, Galietta LJ. Influence of cell background on pharmacological rescue of mutant CFTR. Am J Physiol Cell Physiol. 2010;298:C866–C874. doi: 10.1152/ajpcell.00404.2009. [DOI] [PubMed] [Google Scholar]
  54. Pettit RS, Fellner C. CFTR modulators for the treatment of cystic fibrosis. P T. 2014;39:500–511. [PMC free article] [PubMed] [Google Scholar]
  55. Pezzulo AA, Tang XX, Hoegger MJ, Alaiwa MH, Ramachandran S, Moninger TO, Karp PH, Wohlford-Lenane CL, Haagsman HP, van Eijk M, et al. Reduced airway surface pH impairs bacterial killing in the porcine cystic fibrosis lung. Nature. 2012;487:109–113. doi: 10.1038/nature11130. [DOI] [PMC free article] [PubMed] [Google Scholar]
  56. Rabeh WM, Bossard F, Xu H, Okiyoneda T, Bagdany M, Mulvihill CM, Du K, di Bernardo S, Liu Y, Konermann L, et al. Correction of both NBD1 energetics and domain interface is required to restore ΔF508 CFTR folding and function. Cell. 2012;148:150–163. doi: 10.1016/j.cell.2011.11.024. [DOI] [PMC free article] [PubMed] [Google Scholar]
  57. Ramsey BW, Davies J, McElvaney NG, Tullis E, Bell SC, Drevinek P, Griese M, McKone EF, Wainwright CE, Konstan MW, et al. A CFTR potentiator in patients with cystic fibrosis and the G551D mutation. N Engl J Med. 2011;365:1663–1672. doi: 10.1056/NEJMoa1105185. [DOI] [PMC free article] [PubMed] [Google Scholar]
  58. Rapino D, Sabirzhanova I, Lopes-Pacheco M, Grover R, Guggino WB, Cebotaru L. Rescue of NBD2 mutants N1303K and S1235R of CFTR by small-molecule correctors and transcomplementation. PLoS One. 2015;10:e0119796. doi: 10.1371/journal.pone.0119796. [DOI] [PMC free article] [PubMed] [Google Scholar]
  59. Ratjen F, Doring G. Cystic fibrosis. Lancet. 2003;361:681–689. doi: 10.1016/S0140-6736(03)12567-6. [DOI] [PubMed] [Google Scholar]
  60. Ren HY, Grove DE, De La Rosa O, Houck SA, Sopha P, Van Goor F, Hoffman BJ, Cyr DM. VX-809 corrects folding defects in cystic fibrosis transmembrane conductance regulator protein through action on membrane-spanning domain 1. Mol Biol Cell. 2013;24:3016–3024. doi: 10.1091/mbc.E13-05-0240. [DOI] [PMC free article] [PubMed] [Google Scholar]
  61. Riordan JR. The cystic fibrosis transmemebrane conductance regulator. Annu Rev Physiol. 1993;55:609–630. doi: 10.1146/annurev.ph.55.030193.003141. [DOI] [PubMed] [Google Scholar]
  62. Riordan JR, Rommens JM, Kerem B, Alon N, Rozmahel R, Grzelczak Z, Zielenski J, Lok S, Plavsic N, Chou JL, et al. Identification of the cystic fibrosis gene: cloning and characterization of complementary DNA. Science. 1989;245:1066–1073. doi: 10.1126/science.2475911. [DOI] [PubMed] [Google Scholar]
  63. Rowe SM, Miller S, Sorscher EJ. Cystic fibrosis. N Engl J Med. 2005;352:1992–2001. doi: 10.1056/NEJMra043184. [DOI] [PubMed] [Google Scholar]
  64. Schechter MS, Shelton BJ, Margolis PA, Fitzsimmons SC. The ­association of socioeconomic status with outcomes in cystic fibrosis ­patients in the United States. Am J Respir Crit Care Med. 2001;163:1331–1337. doi: 10.1164/ajrccm.163.6.9912100. [DOI] [PubMed] [Google Scholar]
  65. Sermet-Gaudelus I, Boeck KD, Casimir GJ, Vermeulen F, Leal T, Mogenet A, Roussel D, Fritsch J, Hanssens L, Hirawat S, et al. Ataluren (PTC124) induces cystic fibrosis transmembrane conductance regulator protein expression and activity in children with nonsense mutation cystic fibrosis. Am J Respir Crit Care Med. 2010;182:1262–1272. doi: 10.1164/rccm.201001-0137OC. [DOI] [PubMed] [Google Scholar]
  66. Sheppard DN, Ostedgaard LS, Winter MC, Welsh MJ. Mechanism of dysfunction of two nucleotide binding domain mutations in cystic fibrosis transmembrane conductance regulator that are associated with pancreatic sufficiency. EMBO J. 1995;14:876–883. doi: 10.1002/j.1460-2075.1995.tb07069.x. [DOI] [PMC free article] [PubMed] [Google Scholar]
  67. Sheppard DN, Rich DP, Ostedgaard LS, Gregory RJ, Smith AE, Welsh MJ. Mutations in CFTR associated with mild-disease-form Cl– channels with altered pore properties. Nature. 1993;362:160–164. doi: 10.1038/362160a0. [DOI] [PubMed] [Google Scholar]
  68. Sheppard DN, Travis SM, Ishihara H, Welsh MJ. Contribution of proline residues in the membrane-spanning domains of cystic fibrosis transmembrane conductance regulator to chloride channel function. J Biol Chem. 1996;271:14995–15001. doi: 10.1074/jbc.271.25.14995. [DOI] [PubMed] [Google Scholar]
  69. Silvis MR, Picciano JA, Bertrand C, Weixel K, Bridges RJ, Bradbury NA. A mutation in the cystic fibrosis transmembrane conductance regulator generates a novel internalization sequence and enhances endocytic rates. J Biol Chem. 2003;278:11554–11560. doi: 10.1074/jbc.M212843200. [DOI] [PubMed] [Google Scholar]
  70. Sosnay PR, Siklosi KR, Van Goor F, Kaniecki K, Yu H, Sharma N, Ramalho AS, Amaral MD, Dorfman R, Zielenski J, et al. Defining the disease liability of variants in the cystic fibrosis transmembrane conductance regulator gene. Nat Genet. 2013;45:1160–1167. doi: 10.1038/ng.2745. [DOI] [PMC free article] [PubMed] [Google Scholar]
  71. Stoltz DA, Meyerholz DK, Welsh MJ. Origins of cystic fibrosis lung disease. N Engl J Med. 2015;372:351–362. doi: 10.1056/NEJMra1300109. [DOI] [PMC free article] [PubMed] [Google Scholar]
  72. Tarran R, Grubb BR, Parsons D, Picher M, Hirsh AJ, Davis CW, Boucher RC. The CF salt controversy: in vivo observations and therapeutic approaches. Mol Cell. 2001;8:149–158. doi: 10.1016/s1097-2765(01)00286-6. [DOI] [PubMed] [Google Scholar]
  73. Taylor-Robinson DC, Thielen K, Pressler T, Olesen HV, Diderichsen F, Diggle PJ, Smyth R, Whitehead M. Low socioeconomic status is associated with worse lung function in the Danish cystic fibrosis population. Eur Respir J. 2014;44:1363–1366. doi: 10.1183/09031936.00063714. [DOI] [PMC free article] [PubMed] [Google Scholar]
  74. Van Goor F, Hadida S, Grootenhuis PD, Burton B, Cao D, Neuberger T, Turnbull A, Singh A, Joubran J, Hazlewood A, et al. Rescue of CF airway epithelial cell function in vitro by a CFTR potentiator, VX-770. Proc Natl Acad Sci USA. 2009;106:18825–18830. doi: 10.1073/pnas.0904709106. [DOI] [PMC free article] [PubMed] [Google Scholar]
  75. Van Goor F, Hadida S, Grootenhuis PD, Burton B, Stack JH, Straley KS, Decker CJ, Miller M, McCartney J, Olson ER, et al. Correction of the F508del-CFTR protein processing defect in vitro by the investigational drug VX-809. Proc Natl Acad Sci USA. 2011;108:18843–18848. doi: 10.1073/pnas.1105787108. [DOI] [PMC free article] [PubMed] [Google Scholar]
  76. Van Goor F, Yu H, Burton B, Hoffman BJ. Effect of ivacaftor on CFTR forms with missense mutations associated with defects in protein processing or function. J Cyst Fibros. 2014;13:29–36. doi: 10.1016/j.jcf.2013.06.008. [DOI] [PubMed] [Google Scholar]
  77. Veit G, Avramescu RG, Perdomo D, Phuan PW, Bagdany M, Apaja PM, Borot F, Szollosi D, Wu YS, Finkbeiner WE, et al. Some gating potentiators, including VX-770, diminish ΔF508-CFTR functional expression. Sci Transl Med. 2014;6:246ra97. doi: 10.1126/scitranslmed.3008889. [DOI] [PMC free article] [PubMed] [Google Scholar]
  78. Vertex 2014a. U.S. Food and Drug Administration approves KALYDECO™ (ivacaftor) for use in eight additional mutations that cause cystic fibrosis. Press release, February 21, 2014. http://investors.vrtx.com/releasedetail.cfm?ReleaseID=827435 (accessed 1 October 2015).
  79. Vertex 2014b. U.S. Food and Drug Administration approves KALYDECO® (ivacaftor) for use in people with cystic fibrosis ages 6 and older who have the R117H Mutation. Press release, December 29, 2014. http://investors.vrtx.com/releasedetail.cfm?ReleaseID=889027 (accessed 1 October 2015)
  80. Vertex 2015. FDA approves ORKAMBI™ (lumacaftor/ivacaftor)—the first medicine to treat the underlying cause of cystic fibrosis for people ages 12 and older with two copies of the F508del mutation. Press release, July 2, 2015. http://investors.vrtx.com/releasedetail.cfm?ReleaseID=920512 (accessed 1 October 2015).
  81. Visca A, Bishop CT, Hilton SC, Hudson VM. Improvement in clinical markers in CF patients using a reduced glutathione regimen: an uncontrolled, observational study. J Cyst Fibros. 2008;7:433–436. doi: 10.1016/j.jcf.2008.03.006. [DOI] [PubMed] [Google Scholar]
  82. Wainwright CE, Elborn JS, Ramsey BW, Marigowda G, Huang X, Cipolli M, Colombo C, Davies JC, De Boeck K, Flume PA, et al. ­Lumacaftor-ivacaftor in patients with cystic fibrosis homozygous for Phe508del CFTR. N Engl J Med. 2015;373:220–231. doi: 10.1056/NEJMoa1409547. [DOI] [PMC free article] [PubMed] [Google Scholar]
  83. Wang Y, Liu J, Loizidou A, Bugeja LA, Warner R, Hawley BR, Cai Z, Toye AM, Sheppard DN, Li H. CFTR potentiators partially restore channel function to A561E-CFTR, a cystic fibrosis mutant with a similar mechanism of dysfunction as F508del-CFTR. Br J Pharmacol. 2014;171:4490–4503. doi: 10.1111/bph.12791. [DOI] [PMC free article] [PubMed] [Google Scholar]
  84. Welch EM, Barton ER, Zhuo J, Tomizawa Y, Friesen WJ, Trifillis P, Paushkin S, Patel M, Trotta CR, Hwang S, et al. PTC124 targets genetic disorders caused by nonsense mutations. Nature. 2007;447:87–91. doi: 10.1038/nature05756. [DOI] [PubMed] [Google Scholar]
  85. Welsh MJ, Smith AE. Molecular mechanisms of CFTR chloride channel dysfunction in cystic fibrosis. Cell. 1993;73:1251–1254. doi: 10.1016/0092-8674(93)90353-r. [DOI] [PubMed] [Google Scholar]
  86. Wilschanski M, Miller LL, Shoseyov D, Blau H, Rivlin J, Aviram M, Cohen M, Armoni S, Yaakov Y, Pugatsch T, et al. Chronic ataluren (PTC124) treatment of nonsense mutation cystic fibrosis. Eur Respir J. 2011;38:59–69. doi: 10.1183/09031936.00120910. [DOI] [PubMed] [Google Scholar]
  87. Xue X, Mutyam V, Tang L, Biswas S, Du M, Jackson LA, Dai Y, Belakhov V, Shalev M, Chen F, et al. Synthetic aminoglycosides efficiently suppress cystic fibrosis transmembrane conductance regulator nonsense mutations and are enhanced by ivacaftor. Am J Respir Cell Mol Biol. 2014;50:805–816. doi: 10.1165/rcmb.2013-0282OC. [DOI] [PMC free article] [PubMed] [Google Scholar]
  88. Yang H, Shelat AA, Guy RK, Gopinath VS, Ma T, Du K, Lukacs GL, Taddei A, Folli C, Pedemonte N, et al. Nanomolar affinity small molecule correctors of defective ΔF508-CFTR chloride channel gating. J Biol Chem. 2003;278:35079–35085. doi: 10.1074/jbc.M303098200. [DOI] [PubMed] [Google Scholar]
  89. Yang Y, Devor DC, Engelhardt JF, Ernst SA, Strong TV, Collins FS, Cohn JA, Frizzell RA, Wilson JM. Molecular basis of defective anion transport in L cells expressing recombinant forms of CFTR. Hum Mol Genet. 1993;2:1253–1261. doi: 10.1093/hmg/2.8.1253. [DOI] [PubMed] [Google Scholar]
  90. Yousef S, Solomon GM, Brody A, Rowe SM, Colin AA. Improved clinical and radiographic outcomes after treatment with ivacaftor in a young adult with cystic fibrosis with the P67L CFTR mutation. Chest. 2015;147:e79–e82. doi: 10.1378/chest.14-1198. [DOI] [PubMed] [Google Scholar]
  91. Yu H, Burton B, Huang CJ, Worley J, Cao D, Johnson JP, Jr., Urrutia A, Joubran J, Seepersaud S, Sussky K, Hoffman BJ, Van Goor F. Ivacaftor potentiation of multiple CFTR channels with gating mutations. J Cyst Fibros. 2012;11:237–245. doi: 10.1016/j.jcf.2011.12.005. [DOI] [PubMed] [Google Scholar]
  92. Yu YC, Miki H, Nakamura Y, Hanyuda A, Matsuzaki Y, Abe Y, Yasui M, Tanaka K, Hwang TC, Bompadre SG, Sohma Y. Curcumin and genistein additively potentiate G551D-CFTR. J Cyst Fibros. 2011;10:243–252. doi: 10.1016/j.jcf.2011.03.001. [DOI] [PMC free article] [PubMed] [Google Scholar]
  93. Zielenski J. Genotype and phenotype in cystic fibrosis. Respiration. 2000;67:117–133. doi: 10.1159/000029497. [DOI] [PubMed] [Google Scholar]
  94. Zielenski J, Tsui LC. Cystic fibrosis: genotypic and phenotypic variations. Annu Rev Genet. 1995;29:777–807. doi: 10.1146/annurev.ge.29.120195.004021. [DOI] [PubMed] [Google Scholar]

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