Nonsense mutations change an amino acid codon to a premature termination codon (PTC) generally through a single‐nucleotide substitution. The generation of a PTC results in a defective truncated protein and often in severe forms of disease. Because of the exceedingly high prevalence of nonsense‐associated diseases and a unifying mechanism, there has been a concerted effort to identify PTC therapeutics. Most clinical trials for PTC therapeutics have been conducted with small molecules that promote PTC read through and incorporation of a near‐cognate amino acid. However, there is a need for PTC suppression agents that recode PTCs with the correct amino acid while being applicable to PTC mutations in many different genomic landscapes. With these characteristics, a single therapeutic will be able to treat several disease‐causing PTCs. In this review, we will focus on the use of nonsense suppression technologies, in particular, suppressor tRNAs (sup‐tRNAs), as possible therapeutics for correcting PTCs. Sup‐tRNAs have many attractive qualities as possible therapeutic agents although there are knowledge gaps on their function in mammalian cells and technical hurdles that need to be overcome before their promise is realized.This article is categorized under: RNA Processing > tRNA Processing Translation > Translation Regulation

Nonsense suppressor transfer RNAs (tRNAs) or AntiCodon-Edited tRNAs (ACE-tRNAs) have long been envisioned as a therapeutic approach to overcome genetic diseases resulting from the introduction of premature termination codons (PTCs). The ACE-tRNA approach for the rescue of PTCs has been hampered by ineffective delivery through available modalities for gene therapy. Here we have screened a series of ACE-tRNA expression cassette sequence libraries containing >1800 members in an effort to optimize ACE-tRNA function and provide a roadmap for optimization in the future. By optimizing PTC suppression efficiency of ACE-tRNAs, we have decreased the amount of ACE-tRNA required by ∼16-fold for the most common cystic fibrosis-causing PTCs.

Nonsense mutations arise from single nucleotide substitutions that result in premature termination codons (PTCs). PTCs result in little to no full-length protein production and decreased mRNA stability due to the nonsense-mediated mRNA decay (NMD) pathway. We provide evidence that anticodon-edited (ACE-) tRNAs efficiently suppress the most prevalent cystic fibrosis (CF)-causing PTCs, promoting significant rescue of endogenous cystic fibrosis transmembrane conductance regulator (CFTR) transcript abundance and channel function in different model systems. We show that our best-performing ACE-tRNA, which decodes all UGA PTCs to a leucine amino acid, markedly rescues CFTR function from the most prevalent CF-causing PTCs, all of which arose from nonleucine encoding codons. Using this single ACE-tRNA variant, we demonstrate significant rescue of CFTR function in an immortalized airway cell line and two different primary CF patient-derived intestinal cell models with CFTR nonsense mutations. Further, we demonstrate that leucine substitution CFTR variants are highly functional. Thus, ACE-tRNAs have promise as a platform therapeutic for CF and other nonsense-associated diseases.

Posttranslational modifications resulting from oxidation of proteins (Ox‐PTMs) are present intracellularly under conditions of oxidative stress as well as basal conditions. In the past, these modifications were thought to be generic protein damage, but it has become increasingly clear that Ox‐PTMs can have specific physiological effects. It is an arduous task to distinguish between the two cases, as multiple Ox‐PTMs occur simultaneously on the same protein, convoluting analysis. Genetic code expansion (GCE) has emerged as a powerful tool to overcome this challenge as it allows for the site‐specific incorporation of an Ox‐PTM into translated protein. The resulting homogeneously modified protein products can then be rigorously characterized for the effects of individual Ox‐PTMs. We outline the strengths and weaknesses of GCE as they relate to the field of oxidative stress and Ox‐PTMs. An overview of the Ox‐PTMs that have been genetically encoded and applications of GCE to the study of Ox‐PTMs, including antibody validation and therapeutic development, is described.

Nonsense mutations, resulting from a premature termination codon (PTC), make up ∼11% of all genetic lesions causing disease, affecting millions of people worldwide. Nonsense suppressor anticodon-edited transfer RNAs (ACE-tRNAs) have emerged as a therapeutic modality for the rescue of PTCs. Delivery of ACE-tRNAs in vivo has been achieved by adeno-associated viral vector and RNA-lipid nanoparticle; however, due to drawbacks associated with these approaches, DNA delivery remains an attractive approach. DNA-based approaches afford ease of manufacturing at a relatively low cost and exhibit improved therapeutic durability and safety as compared to viral vector- or RNA-based approaches. Due to the small size of human tRNA genes employed as ACE-tRNAs, in principle, DNA vectors <200 base pairs (bp) in size (minivectors) could be utilized for delivery of actively transcribed ACE-tRNAs. Here, we demonstrate that linear DNA ACE-tRNA vectors as small as 200 bp effectively suppress several nonsense mutations in CFTR and REP1, and that ACE-tRNA minivectors, when tested in cell or ex vivo models, display significantly improved bioavailability, reduced innate immune burden, and superior biostability as compared to conventional plasmid DNA vectors.

G protein-coupled receptors (GPCRs) are key pharmacological targets, yet many remain underutilized due to unknown activation mechanisms and ligands. Orphan GPCRs, lacking identified natural ligands, are a high priority for research, as identifying their ligands will aid in understanding their functions and potential as drug targets. Most GPCRs, including orphans, couple to G_i/o/z family members, however current assays to detect their activation are limited, hindering ligand identification efforts. We introduce G_zESTY, a sensitive, cell-based assay developed in an easily deliverable format designed to study the pharmacology of G_i/o/z-coupled GPCRs and assist in deorphanization. We optimized assay conditions and developed an all-in-one vector employing cloning methods to ensure the correct expression ratio of G_zESTY components. G_zESTY successfully assessed activation of a library of ligand-activated GPCRs, detecting both full and partial agonism, and responses from endogenous GPCRs. Notably, with G_zESTY we established the presence of endogenous ligands for GPR176 and GPR37 in brain extracts, validating its use in deorphanization efforts. This assay enhances the ability to find ligands for orphan GPCRs, expanding the toolkit for GPCR pharmacologists.