Nonsense suppression therapies in human genetic diseases

Abstract

About 11% of all human disease-associated gene lesions are nonsense mutations, resulting in the introduction of an in-frame premature translation-termination codon (PTC) into the protein-coding gene sequence. When translated, PTC-containing mRNAs originate truncated and often dysfunctional proteins that might be non-functional or have gain-of-function or dominant-negative effects. Therapeutic strategies aimed at suppressing PTCs to restore deficient protein function-the so-called nonsense suppression (or PTC readthrough) therapies-have the potential to provide a therapeutic benefit for many patients and in a broad range of genetic disorders, including cancer. These therapeutic approaches comprise the use of translational readthrough-inducing compounds that make the translational machinery recode an in-frame PTC into a sense codon. However, most of the mRNAs carrying a PTC can be rapidly degraded by the surveillance mechanism of nonsense-mediated decay (NMD), thus decreasing the levels of PTC-containing mRNAs in the cell and their availability for PTC readthrough. Accordingly, the use of NMD inhibitors, or readthrough-compound potentiators, may enhance the efficiency of PTC suppression. Here, we review the mechanisms of PTC readthrough and their regulation, as well as the recent advances in the development of novel approaches for PTC suppression, and their role in personalized medicine.

Keywords: Nonsense mutation; Premature termination codon (PTC); Readthrough therapy; Stop codon readthrough; Translation termination.

Fig. 1

Translation termination (a) or basal readthrough (b) at a normal stop codon. a In eukaryotic cells, all three stop codons are decoded by the eukaryotic release factor 1 (eRF1) when it enters the ribosomal A site. eRF1 also binds to and activates eRF3, which is a GTPase that hydrolyses GTP. Hydrolysis of GTP by eRF3 allows the accommodation of the eRF1 in the peptidyl transferase center. This promotes the release of the polypeptide from the ribosome. The productive binding of the eRF1–eRF3 complex to the ribosome is enhanced by the cytoplasmic 1 poly(A)-binding proteins (PABPC1), inducing a proper translation termination reaction. The ATPase ABCE1 is also able to interact with eRF1. b Basal stop codon readthrough occurs when normal translation termination is not efficient. In this case, a near-cognate tRNA is indeed able to outcompete eRF1 (which remains bound to DDX19). In this condition, the near-cognate tRNA accommodates in the A site of the ribosome, leading to the misreading of the stop codon into a sense codon, and an amino acid is incorporated into the nascent polypeptide. Consequently, translation resumes until the ribosome reaches another stop codon in the same open reading frame and a C-terminal elongated protein is synthesized

Fig. 2

Translation termination (a) or nonsense-mediated decay (NMD) (b) at a premature termination codon (PTC). a If the PTC is located downstream of the last exon–exon junction, a proper translation termination reaction occurs and there is synthesis of a truncated protein. b If the mRNA carries a PTC located more than 55 nucleotides upstream of the last exon–exon junction, when the ribosome reaches the PTC, the termination complex that is formed cannot interact with the cytoplasmic poly(A)-binding protein (PABPC) and instead it interacts with the NMD-factor UPF1. Under these conditions, the mRNA is rapidly degraded by the process of NMD

Fig. 3

Basal readthrough (a) or pharmacologically induced readthrough (b) at a premature termination codon (PTC). a When an aminoacyl-tRNA is accommodated in the A site of the ribosome instead of eRF1–eRF3 at a PTC, an amino acid is incorporated into the nascent polypeptide. Consequently, translation resumes until the ribosome reaches another stop codon in the same open reading frame. b Some compounds have the ability to increase the binding of near-cognate aminoacyl-tRNAs to the PTC, which allows a more efficient PTC readthrough. This approach constitutes what is called nonsense suppression therapy