CFTR Modulators: Shedding Light on Precision Medicine for Cystic Fibrosis

Abstract

Cystic fibrosis (CF) is the most common life-threatening monogenic disease afflicting Caucasian people. It affects the respiratory, gastrointestinal, glandular and reproductive systems. The major cause of morbidity and mortality in CF is the respiratory disorder caused by a vicious cycle of obstruction of the airways, inflammation and infection that leads to epithelial damage, tissue remodeling and end-stage lung disease. Over the past decades, life expectancy of CF patients has increased due to early diagnosis and improved treatments; however, these patients still present limited quality of life. Many attempts have been made to rescue CF transmembrane conductance regulator (CFTR) expression, function and stability, thereby overcoming the molecular basis of CF. Gene and protein variances caused by CFTR mutants lead to different CF phenotypes, which then require different treatments to quell the patients' debilitating symptoms. In order to seek better approaches to treat CF patients and maximize therapeutic effects, CFTR mutants have been stratified into six groups (although several of these mutations present pleiotropic defects). The research with CFTR modulators (read-through agents, correctors, potentiators, stabilizers and amplifiers) has achieved remarkable progress, and these drugs are translating into pharmaceuticals and personalized treatments for CF patients. This review summarizes the main molecular and clinical features of CF, emphasizes the latest clinical trials using CFTR modulators, sheds light on the molecular mechanisms underlying these new and emerging treatments, and discusses the major breakthroughs and challenges to treating all CF patients.

Keywords: ABC transporters; CFTR; cystic fibrosis; intracellular trafficking; personalized medicine; protein misfolding; proteostasis network.

FIGURE 1

CFTR schematic structure – Cystic fibrosis transmembrane conductance regulator (CFTR) is a 1,480-amino acids protein inserted into the cell surface. CFTR possesses five domains: two transmembrane domains (TMD1/2), containing six hydrophobic alpha-helices, which cross the cell surface lipid bilayer, and are joined by two intracellular loops and three extracellular loops, and with glycosylated residues linked in the extracellular loop 4 (N894, N900); two nucleotide-binding domains (NBD1/2) with highly conserved sequenced for ATP-binding, where occur hydrolysis; and one regulatory domain (RD) with multiple phosphorylation sites. CFTR channel open when protein kinase A (PKA) and protein kinase C (PKC) phosphorylate RD and ATPs bind to side chain charged amino acids in NBDs, thereby activating CFTR function. TMDs form the gate where occurs chloride conductance. The positions denoted into the boxes correspond to the first and last amino acid of each fragment and CFTR sequence was obtained in the Cystic Fibrosis Mutation Database (CFTR1 database; http://www.genet.sickkids.on.ca/Home.html).

FIGURE 2

Estimated prevalence of cystic fibrosis per 100,000 habitants – Data compiled from the latest registry reports of Europe (European Cystic Fibrosis Society [ECFS], 2016), United States (Cystic Fibrosis Foundation [CFF], 2015), Canada (Cystic Fibrosis Canada [CFC], 2016), Australia (Cystic Fibrosis Federation Australia [CFFA], 2016) and Brazil (Brazilian Cystic Fibrosis Study Group [GBEFC], 2016).

FIGURE 3

Demography of cystic fibrosis in a sample of 78,627 patients in different countries or demographic regions – (A) Prevalence by gender: average of 52% male and 48% female. (B) Within the sample group, 86% have been genotyped and approximately 38% are ΔF508-homozygous, 35% ΔF508-heterozygous and 13% bearing other CFTR (cystic fibrosis transmembrane conductance regulator) mutants in both alleles. (C) About 50% of patients are under 18 years and 50% are 18 years or older. Data compiled from the latest registry reports of Europe (EU; European Cystic Fibrosis Society [ECFS], 2016), United States (US; Cystic Fibrosis Foundation [CFF], 2015), Canada (CA; Cystic Fibrosis Canada [CFC], 2016), Australia (AU; Cystic Fibrosis Federation Australia [CFFA], 2016) and Brazil (BR; Brazilian Cystic Fibrosis Study Group [GBEFC], 2016).

FIGURE 4

Pathophysiological cascade of respiratory disorder in cystic fibrosis – Cellular mechanism of cystic fibrosis begins with the defective CFTR (cystic fibrosis transmembrane conductance regulator) gene and shortage of CFTR channel at the plasma membrane. A vicious cycle of airways obstruction, inflammation and infection leads to epithelial damage, lung remodeling and end-stage lung disease. ENaC, epithelial Na⁺ channel; Aqp, aquaporin.

FIGURE 5

Effect of novel therapies on life expectancy of cystic fibrosis patients – Schematic illustration of how the discovery and introduction of novel cystic fibrosis (CF) treatments have influenced the patients’ survival over the decades. HTS: high throughput screening, AZLI, aztreonam for inhalation solution; TIP, tobramycin inhalation solution; KALY, Kalydeco^TM; ORK, Orkambi^TM. ^∗enteric-coated pancreatic enzymes. (Reproduced and adapted with permission of European Respiratory Society©: The European Lung White Book Respiratory Health and Disease in Europe, 2^nd Ed. © 2013 European Respiratory Society, Sheffield, UK. Print ISBN: 978-1-84984-042-2, Online ISBN: 978-1-84984-043-9).

FIGURE 6

A subset of proteostasis network engaged to CFTR degradation – CFTR interactome involves several quality control proteins that directly or indirectly target CFTR to degradation. Proteasomes and aggresomes eliminate CFTR that fails in acquire the native conformation. Lysosomes degrade CFTR removed from the cell surface during the recycling. The black lines denoted the interaction between CFTR and proteostasis components. AHSA1, activator of 90 kDA Hsp ATPase homolog 1; CAL, CFTR-associated ligand; CHIP, carboxyl terminus of Hsc70-interacting protein; CFTR, cystic fibrosis transmembrane conductance regulator; HDAC, histone deacetylase; Hsc, heat-shock cognate; Hsp, heat-shock protein; NHERF, Na⁺/H⁺ exchanger regulatory factor; SUMO, small ubiquitin-like modifier; STX, syntaxin; Ub, ubiquitin; and VCP, vasolin-containing protein.

FIGURE 7

Classes of CFTR mutations – Distribution of CFTR mutations into six functional classes according to the primary molecular defect: Class I mutants are no protein synthesis, since the presence of premature stop codons (class Ia) or frameshifts for deletions or insertions (class Ib) preclude translation of full-length CFTR. Class II mutants are impaired trafficking protein, since CFTR fails to acquire complete folding and ER-associated degradation (ERAD) machinery eliminate the protein. Class III mutants are defective channel gating, since CFTR reach the cell surface, but it does not exhibit channel gating due to diminished ATP binding and hydrolysis. Class IV mutants are less functional proteins, since channel amount that achieve the plasma membrane could be similar to wt-CFTR, but it presents reduced chloride conductance. Class V mutants are less protein maturation caused by amino acid substitution or alternative splicing, since the protein amount that reaches the cell surface is reduced and it also leads to loss of chloride transport due to reduction in the quantity of CFTR channels. Class VI mutants are less stable protein, since CFTR at the plasma membrane is removed during the recycling and it is sent for lysosome degradation. wt, wild type; CFTR, cystic fibrosis transmembrane conductance regulator; rΔF508, rescued ΔF508 by low-temperature incubation; and ER, endoplasmic reticulum.