Molecular mechanisms of cystic fibrosis - how mutations lead to misfunction and guide therapy

Abstract

Cystic fibrosis, the most common autosomal recessive disorder in Caucasians, is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene, which encodes a cAMP-activated chloride and bicarbonate channel that regulates ion and water transport in secretory epithelia. Although all mutations lead to the lack or reduction in channel function, the mechanisms through which this occurs are diverse - ranging from lack of full-length mRNA, reduced mRNA levels, impaired folding and trafficking, targeting to degradation, decreased gating or conductance, and reduced protein levels to decreased half-life at the plasma membrane. Here, we review the different molecular mechanisms that cause cystic fibrosis and detail how these differences identify theratypes that can inform the use of directed therapies aiming at correcting the basic defect. In summary, we travel through CFTR life cycle from the gene to function, identifying what can go wrong and what can be targeted in terms of the different types of therapeutic approaches.

Keywords: CFTR; Cystic Fibrosis; Molecular basis of disease; Mutations; Theratypes.

Figure 1. CFTR gene and protein

(A) 3D organization of the CFTR gene – the so-called topologically associated domain – in airway cells, showing the promoter, the boundaries (I and II) and architectural proteins CTCF and cohesin (adapted from [23]). (B) Ribbon representation of the human CFTR 3D structure (ATP-bound, phosphorylated form) in complex with VX-770 (PDB: 6O2P [124]), illustrating striking features of the channel, as discussed in the text. The two ATP molecules are shown at the interface between NBD1 and NBD2. Transmembrane (TM) helices extend in the cytoplasm, forming long intracellular loops (ICLs) which contact the NBDs through coupling helices. ECL4 stands for extracellular loop 4. Fragment of the R domain was modeled as a poly-alanine into a density of the cryo-EM map (yellow). This figure (and Figure 2) was prepared using Chimera https://www.rbvi.ucsf.edu/chimera.

Scheme 1. Classes of mutations in the CFTR gene, according to the molecular and cellular defect caused

Figure 2. Rare mutations affecting the CFTR N-terminal part and rescued by VX-661

Are shown in orange the positions of five amino acids, associated with rare class II mutations, on the 3D structure of human CFTR in complex with VX-661 (colored: experimental 3D structure after blind docking of VX-661 and MD simulation in presence of lipids (tiny sticks) [89], light gray: experimental 3D structure of the protein–drug complex (PDB: 7SV7, [92]). W57 is sandwiched between P67 and L69, in tight contact with the TM6 C-terminal end, thereby forming a lock between the N- and C-terminal ends of MSD1. E56 forms a salt-bridge with R75, itself interacting with E60. These bonds participate in the large network linking MSD1 to the lasso lh2, ICL1 and ICL4 [89].

Figure 3. Mechanisms of disease for CFTR mutations

Mutations can cause different molecular defects and that require specific rescuing strategies.