Nonsense mutations promote premature translational termination and cause anywhere from 5-70% of the individual cases of most inherited diseases. Studies on nonsense-mediated cystic fibrosis have indicated that boosting specific protein synthesis from <1% to as little as 5% of normal levels may greatly reduce the severity or eliminate the principal manifestations of disease. To address the need for a drug capable of suppressing premature termination, we identified PTC124-a new chemical entity that selectively induces ribosomal readthrough of premature but not normal termination codons. PTC124 activity, optimized using nonsense-containing reporters, promoted dystrophin production in primary muscle cells from humans and mdx mice expressing dystrophin nonsense alleles, and rescued striated muscle function in mdx mice within 2-8 weeks of drug exposure. PTC124 was well tolerated in animals at plasma exposures substantially in excess of those required for nonsense suppression. The selectivity of PTC124 for premature termination codons, its well characterized activity profile, oral bioavailability and pharmacological properties indicate that this drug may have broad clinical potential for the treatment of a large group of genetic disorders with limited or no therapeutic options.
The levels of cellular messenger RNA transcripts can be regulated by controlling the rate at which the mRNA decays. Because decay rates affect the expression of specific genes, they provide a cell with flexibility in effecting rapid change. Moreover, many clinically relevant mRNAs--including several encoding cytokines, growth factors and proto-oncogenes--are regulated by differential RNA stability. But what are the sequence elements and factors that control the half-lives of mRNAs?
mRNA decay rates often increase when translation is terminated prematurely due to a frameshift or nonsense mutation. We have identified a yeast gene, UPF1, that codes for a trans-acting factor whose function is necessary for enhanced turnover of mRNAs containing a premature stop codon. In the absence of UPF1 function, frameshift or nonsense mutations in the HIS4 or LEU2 genes that normally cause rapid mRNA decay fail to have this effect. Instead, the mRNAs decay at rates similar to the corresponding wild-type mRNAs. The stabilization of frameshift or nonsense mRNAs observed in upfl-strains does not appear to result from enhanced readthrough of the termination signal. Loss of UPF1 function has no effect on the accumulation or stability of HIS4 + or LEU2 + mRNA, suggesting that the UPF1 product functions only in response to a premature termination signal. When we examined the accumulation and stability of other wild-type mRNAs in the presence or absence of UPF1, including MAT~I, STE3, ACT1, PGK1, PAB1, and URA3 mRNAs, only the URA3 transcript was affected. On the basis of these and other results, the UPF1 product appears to participate in a previously uncharacterized pathway leading to the degradation of a limited class of yeast transcripts. Nonsense mutations that generate a premature translational termination signal often reduce the steady-state accumulation of the corresponding mRNA (Brown 1989;Peltz et al. 1990). In a study of the yeast URA3 gene, it was shown that the extent of reduced mRNA accumulation depends on the position of the nonsense mutation (Losson and Lacroute 1979). Mutations near the 5' end of the transcript were shown to have a greater destabilizing effect than mutations near the 3' end. Furthermore, introduction of an amber tRNA suppressor restabilized ura3 nonsense mRNA, indicating that the turnover rate is determined in part by the relative efficiencies of termination versus readthrough of the stop codon. These studies suggested that the turnover rate of nonsense mRNA is probably related to some aspect of its translation rather than to a potential change in mRNA structure that might result from the presence of a nonsense mutation.Similar studies in higher eukaryotes have proven more difficult to interpret. In some cases, the introduction of a premature stop codon into a gene has been linked to 3Corresponding author. increased cytoplasmic turnover (Maquat et al. 1981;Barker and Beemon 1991). However, other studies suggest that nonsense mutations may cause changes in nuclear processing and/or transport, and these changes, rather than cytoplasmic mRNA degradation, may be primarily responsible for decreased steady-state mRNA levels (Humphries et al. 1984;Takeshita et al. 1984;Urlaub et al. 1989;Cheng et al. 1990).Here, we report the characterization of mutations in the yeast Saccharomyces cerevisiae that specifically stabilize mRNAs containing a premature translational termination signal. The mutations arose in a strain containing his4-38, a + 1 frameshift mutation near the 5' end of the HIS4 tr...
Spinal muscular atrophy (SMA) is a genetic disease caused by mutation or deletion of the survival of motor neuron 1 (SMN1) gene. A paralogous gene in humans, SMN2, produces low, insufficient levels of functional SMN protein due to alternative splicing that truncates the transcript. The decreased levels of SMN protein lead to progressive neuromuscular degeneration and high rates of mortality. Through chemical screening and optimization, we identified orally available small molecules that shift the balance of SMN2 splicing toward the production of full-length SMN2 messenger RNA with high selectivity. Administration of these compounds to Δ7 mice, a model of severe SMA, led to an increase in SMN protein levels, improvement of motor function, and protection of the neuromuscular circuit. These compounds also extended the life span of the mice. Selective SMN2 splicing modifiers may have therapeutic potential for patients with SMA.
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