Pharmacological approaches for targeting cystic fibrosis nonsense mutations

Abstract

Cystic fibrosis (CF) is a monogenic autosomal recessive disorder. The clinical manifestations of the disease are caused by ∼2,000 mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) protein. It is unlikely that any one approach will be efficient in correcting all defects. The recent approvals of ivacaftor, lumacaftor/ivacaftor and elexacaftor/tezacaftor/ivacaftor represent the genesis of a new era of precision combination medicine for the CF patient population. In this review, we discuss targeted translational readthrough approaches as mono and combination therapies for CFTR nonsense mutations. We examine the current status of efficacy of translational readthrough/nonsense suppression therapies and their limitations, including non-native amino acid incorporation at PTCs and nonsense-mediated mRNA decay (NMD), along with approaches to tackle these limitations. We further elaborate on combining various therapies such as readthrough agents, NMD inhibitors, and corrector/potentiators to improve the efficacy and safety of suppression therapy. These mutation specific strategies that are directed towards the basic CF defects should positively impact CF patients bearing nonsense mutations.

Keywords: CFTR; Combination therapy; Nonsense mutations; Translational readthrough.

Figure 1.. Translation termination versus readthrough at PTCs:

eRF1 and eRF3 bind to a stop codon when it enters the ribosomal A site. PABPC1 binds to eRF3 and promotes efficiency termination. ABCE1 assists in polypeptide release and mediates ribosomal recycling. When a PTC enters the A site, a truncated polypeptide is generated. At a low frequency, near-cognate tRNAs compete with the eRF1/eRF3 complex for binding to A-site bound PTCs, allowing translation elongation to continue in the original ribosomal reading frame to produce a full-length protein. Several therapeutic approaches are attempting to skew this competition more toward elongation in order to generate more full-length protein.

Figure 2.

Chemical structures of translational readthrough agents, potential nonsense mediated decay inhibitors, and CFTR modulators.

Figure 3.. Schematic illustration of the aberrant translation termination resulting in the activation of nonsense-mediated mRNA decay (NMD).

When a ribosome encounters a PTC, the termination factors eRF1 and eRF3 are recruited to the PTC. UPF1 then binds to the termination complex and becomes phosphorylated, allowing an interaction with UPF2 and/or UPF3. This association recruits decay factors, including the SMG6 endonuclease and the SMG5–7 heterodimer, which subsequently recruit the CCR4/NOT complex to promote decay of the mRNA. Star represents the potential therapeutic target.