Targeting a genetic defect: cystic fibrosis transmembrane conductance regulator modulators in cystic fibrosis

Abstract

Cystic fibrosis (CF) is caused by genetic mutations that affect the cystic fibrosis transmembrane conductance regulator (CFTR) protein. These mutations can impact the synthesis and transfer of the CFTR protein to the apical membrane of epithelial cells, as well as influencing the gating or conductance of chloride and bicarbonate ions through the channel. CFTR dysfunction results in ionic imbalance of epithelial secretions in several organ systems, such as the pancreas, gastrointestinal tract, liver and the respiratory system. Since discovery of the CFTR gene in 1989, research has focussed on targeting the underlying genetic defect to identify a disease-modifying treatment for CF. Investigated management strategies have included gene therapy and the development of small molecules that target CFTR mutations, known as CFTR modulators. CFTR modulators are typically identified by high-throughput screening assays, followed by preclinical validation using cell culture systems. Recently, one such modulator, the CFTR potentiator ivacaftor, was approved as an oral therapy for CF patients with the G551D-CFTR mutation. The clinical development of ivacaftor not only represents a breakthrough in CF care but also serves as a noteworthy example of personalised medicine.

Figure 1.

Cystic fibrosis transmembrane conductance regulator (CFTR) gene mutations are categorised into six classes. Mutation classes I, II, V and VI result in an absence or reduced quantity of CFTR protein at the cell membrane, whereas mutation classes III and IV influence the function or activity of CFTR at the cell membrane. Class I mutations are associated with the greatest disruption to CFTR-mediated chloride transport; in general, chloride transport gradually increases through the remaining five classes, with the greatest activity being observed in Class IV–VI mutations.

Figure 2.

Correlation of sweat chloride levels with cystic fibrosis transmembrane conductance regulator (CFTR) protein expression and function. In cystic fibrosis (CF) patients with Class I–III CFTR mutations, absent or minimal CFTR function results in exocrine pancreatic insufficiency and highly elevated levels of sweat chloride concentration. Residual CFTR function in CF patients with Class IV–V CFTR mutations is associated with exocrine pancreatic sufficiency and sweat chloride levels above or below the diagnostic cut-off of 60 mmol·L⁻¹. CFTR-related disorders are non-CF conditions with some CFTR dysfunction, usually presenting with intermediate or normal sweat chloride values and CFTR mutations that are known to affect mRNA production, although to a lesser degree than CF-causing CFTR mutations [23]. CBAVD: congenital bilateral absence of vas deferens. Modified from [22] with permission from the publisher.

Figure 3.

Ivacaftor restores G551D-cystic fibrosis transmembrane conductance regulator (CFTR) function in human bronchial epithelial (HBE) primary cell cultures. a) CFTR activity was evaluated by measuring CFTR-dependent chloride transport (short-circuit current in voltage-clamp mode) relative to wild-type (n=16). Ivacaftor increased the activity of G551D/F508del-CFTR to ∼50% of wild-type (personal communication; F. van Goor, Vertex Pharmaceuticals Inc., Cambridge, MA, USA). Modified from [54]. b) Mean ciliary beat frequency (CBF) after 5 days of treatment with dimethyl sulfoxide (DMSO), 30 nM vasoactive intestinal peptide (VIP) (a naturally occuring CFTR activator), 10 μM ivacaftor (IVA), or 30 nM VIP+10 μM IVA (±sem; n=6). Taken together, the results show that IVA (either alone or in combination with VIP) caused a significant increase in CBF with respect to vehicle control (DMSO). Modified from [52]. *: p<0.05, significant difference between DMSO and G551D/F508del-HBE; ^#: p<0.05, significant difference between DMSO and IVA alone. c) Representative confocal microscopy image of G551D/F508del-HBE cultures treated with vehicle control or ivacaftor. The results show a clear ivacaftor-dependent increase in the depth of airway surface liquid (ASL) relative to control (personal communication; F. van Goor, Vertex Pharmaceuticals Inc., Cambridge, MA, USA). Modified from [52]. d) Dose–response curve of the change in ASL volume in G551D/F508del-HBE after application of ivacaftor at the indicated concentrations (n=3–9). Reproduced from [52].