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.

Although most mRNA molecules derived from protein-coding genes are destined to be translated into functional polypeptides, some are eliminated by cellular quality control pathways that collectively perform the task of mRNA surveillance. In the nonsense-mediated mRNA decay (NMD) pathway premature translation termination promotes the recruitment of a set of factors that destabilize a targeted mRNA. The same factors also appear to play key roles in repressing the translation of the mRNA, dissociating its terminating ribosome and mRNP proteins, promoting the degradation of its truncated polypeptide product, and possibly even feeding back to the site of transcription to interfere with splicing of the primary transcript.

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...

Transcripts regulated by the yeast nonsense-mediated and 5' to 3' mRNA decay pathways were identified by expression profiling of wild-type, upf1Delta, nmd2Delta, upf3Delta, dcp1Delta, and xrn1Delta cells. This analysis revealed that inactivation of Upf1p, Nmd2p, or Upf3p has identical effects on global RNA accumulation; inactivation of Dcp1p or Xrn1p exhibits both common and unique effects on global RNA accumulation but causes upregulation of only a small fraction of transcripts; and the majority of transcripts upregulated in upf/nmd strains are also upregulated to similar extents in dcp1Delta and xrn1Delta strains. Our results define the core transcripts regulated by NMD, identify several novel structural classes of NMD substrates, demonstrate that nonsense-containing mRNAs are primarily degraded by the 5' to 3' decay pathway even in the absence of functional NMD, and indicate that 3' to 5' decay, not 5' to 3' decay, may be the major mRNA decay activity in yeast cells.

We developed a procedure to measure mRNA decay rates in the yeast Saccharomyces cerevisiae and applied it to the determination of half-lives for 20 mRNAs encoded by well-characterized genes. The procedure utilizes Northern (RNA) or dot blotting to quantitate the levels of individual mRNAs after thermal inactivation of RNA polymerase II in an rpbl-1 temperature-sensitive mutant. We compared the results of this procedure with results obtained by two other procedures (approach to steady-state labeling and inhibition of transcription with Thiolutin) and also evaluated whether heat shock alters mRNA decay rates. We found that there are no significant differences in the mRNA decay rates measured in heat-shocked and non-heat-shocked cells and that, for most mRNAs, different procedures yield comparable relative decay rates. Of the 20 mRNAs studied, 11, including those encoded by HIS3, STE2, STE3, and MATal, were unstable (t112 < 7 min) and 4, including those encoded by ACT) and PGKI, were stable (t112 > 25 min). We have begun to assess the basis and significance of such differences in the decay rates of these two classes of mRNA. Our results indicate that (i) stable and unstable mRNAs do not differ significantly in their poly(A) metabolism; (ii) deadenylation does not destabilize stable mRNAs; (iii) there is no correlation between mRNA decay rate and mRNA size; (iv) the degradation of both stable and unstable mRNAs depends on concomitant translational elongation; and (v) the percentage of rare codons present in most unstable mRNAs is significantly higher than in stable mRNAs.Differences in the decay rates of individual mRNAs can have profound effects on the overall levels of expression of specific genes (80,93). Although the potential importance of mRNA stability as a mechanism for regulating gene expression has been recognized (7, 86), the structures and mechanisms involved in the determination of individual mRNA decay rates have yet to be elucidated. As an approach to understanding the determinants of mRNA stability, we have begun to compare the properties of mRNAs in Dictyostelium discoideum which differ significantly in their respective decay rates (94). In this report, we describe our initial efforts to perform a similar analysis of mRNAs in the yeast Saccharomyces cerevisiae. Our objective was the identification of both stable and unstable yeast mRNAs that were encoded by genes which had already been well characterized. Success in such an endeavor would make it possible to explore the structural determinants of mRNA stability, for example, by analyzing the decay rates of mRNAs transcribed from chimeric genes (25,42,43,81,96,97).Decay rates for both the poly(A)+ RNA population and for individual yeast mRNAs have been measured previously by several different functional or chemical assays. Half-lives ranging from 16 to 23 min have been measured for the average turnover rate of the poly(A)+ RNA population, whereas half-lives of individual mRNAs span a broader range from 1 to over 100 min (3,18,19,35,36,46,47,50,52,54,...