Repairing the basic defect in cystic fibrosis – one approach is not enough

Abstract

Repairing the basic defect in cystic fibrosis folding of these mutant proteins, allowing their escape from ER-associated degradation and their expression at the cell surface [4].Class III: mutations that include those that disrupt channel regulation through impaired gating. These are mainly mutations located in the NBDs, affecting ATP binding and/or hydrolysis, thus precluding the channel from functioning properly. The prototypical example is G551D (the third most common disease causing mutation present in 4-5% of CF patients). F508del-CFTR can also be included in class III because channels with this mutation that are coaxed to the plasma membrane also exhibit a gating defect.Class IV: mutations that decrease Cl À ion conductance, i.e. delay the flow of Cl À ions through CFTR upon cAMP stimulation (e.g. R334W and R347P). These mutations localize mostly to the MSD1 and MSD2 regions, thus preventing a correct flow of ions through the channel pore. Class V: mutations that reduce CFTR protein levels, often by affecting splicing and generating both aberrant mRNA transcripts and a reduced amount of normal mRNA transcripts [4,5]. With this type of mutation it is sometimes challenging to establish their effect on the overall CFTR function and therefore their involvement in the disease, which has a direct and significant impact on genetic counseling [7].Class VI: mutations that include those that decrease the retention and stability of CFTR at the cell surface. F508del-CFTR rescued to the membrane (rF508del-CFTR), either by incubation at low temperatures (26-30°C) or by treatment with chemical correctors, also belongs to this class since it shows a significantly decreased half-life at the plasma membrane [8][9][10]. Another example is the decreased cell surface stability of a deletion mutant that takes out the CFTR initiation codon, so that the resultant protein lacks the N-terminal tail required to prevent its rapid internalization [11]. The obvious approach: gene therapy for CFBeing a monogenic disorder, CF is an obvious candidate for gene therapy -which will simply consist in the introduction of exogenous CFTR gene or cDNA into the airways of CF patients.Early findings reported that delivery of CFTR to a small percentage (6-10%) of human CF airway epithelial cells was able to restore the chloride transport to levels comparable to those observed in non-CF cells [12]. However, the experimental conditions used were far from representing the morphological characteristics of the airway epithelium. More recent studies used a recombinant human parainfluenza virus to introduce the CFTR gene into ciliated cells (a model much closer to human airway epithelium), and were able to also restore airway surface liquid (ASL) volume and mucus transport in about 25% of the cells [13].Given that CF, at least in the early stages, is mostly a disease of the small airways, airway epithelial cells are in fact the appropriate target cell for CF gene therapy [14]. At first, one might think that the airways are an easy target, accessible by the...

The cystic fibrosis transmembrane conductance regulator (CFTR) and its stability

“…It is now 20 years after these studies, and there has been the discovery of a number of pharmacological correctors aimed at treatment of the F508del mutation [25]. For example, compounds have been produced that affect the F508del-CFTR interactome including the proteasome complex that leads to degradation of the protein and chaperones that specifically bind to the F508del-CFTR peptide [13, 26, 27]. …”

Bactericidal and Fungicidal Activity of N -Chlorotaurine Is Enhanced in Cystic Fibrosis Sputum Medium

“…More than 120 different mutations of the CFTR gene are known, definitely causing milder or more severe disease (2). The most abundant mutation is the ΔF508 mutation (lack of phenylalanine at position 508) (2,3). The disturbed chloride transport causes less water secretion of the exocrine glands and thus less hydration of all of their products.…”

Rab GTPases regulate the trafficking of channels and transporters – a focus on cystic fibrosis

“…This study also reported that blocking of endocytosis with DN Rab5a and enhancement of recycling with overexpression of Rab11 is able to promote accumulation of rescued F508del-CFTR at the PM -one of the first studies to report that the late steps of trafficking are amenable to therapeutic modulation to promote mutant protein stability (reviewed in ref. 16). …”

“…In fact, the latest evidence with this latter class of molecules agrees with the concept that the treatment of a pathology as complex as CF will require the combination several therapeutic approaches directed at the several biochemical and physiologic processes that are compromised in CF disease. 16 …”