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Nonsense-mediated mRNA decay (NMD) is a cellular surveillance pathway that recognizes and degrades mRNAs with premature termination codons (PTCs). The mechanisms underlying translation termination are key to the understanding of RNA surveillance mechanisms such as NMD and crucial for the development of therapeutic strategies for NMD-related diseases. Here, we have used a fully reconstituted in vitro translation system to probe the NMD proteins for interaction with the termination apparatus. We discovered that UPF3B (i) interacts with the release factors, (ii) delays translation termination and (iii) dissociates post-termination ribosomal complexes that are devoid of the nascent peptide. Furthermore, we identified UPF1 and ribosomes as new interaction partners of UPF3B. These previously unknown functions of UPF3B during the early and late phases of translation termination suggest that UPF3B is involved in the crosstalk between the NMD machinery and the PTC-bound ribosome, a central mechanistic step of RNA surveillance. [ABSTRACT FROM AUTHOR]

Nonsense-mediated mRNA decay (NMD) is a mechanism, which selectively degrades transcripts carrying premature termination codons (PTCs) and a variety of physiologic transcripts containing NMD-inducing features. In a recent study, we have found variable NMD efficiency among nasal epithelial cells obtained from cystic fibrosis (CF) patients. This variability was found for CF transmembrane conductance regulator (CFTR) transcripts carrying the W1282X PTC, as well as for several NMD physiologic substrates. Here, we aimed to investigate the possibility that variability in NMD efficiency is a more generalized phenomenon and is not restricted to nasal epithelial cells. To investigate this possibility, we analyzed the NMD efficiency of both a CFTR constructs carrying the W1282X PTC and β-globin constructs carrying the NS39 PTC, in HeLa and MCF7 cells. Variability in NMD efficiency was found for both constructs between the cells, such that in HeLa cells the NMD was highly efficient and in MCF7 the efficiency was significantly lower. Moreover, similar differences in the efficiency of NMD were found for five endogenous NMD physiologic transcripts. Altogether, our results demonstrate existence of cells in which NMD of all transcripts is efficient, whereas others in which the NMD is less efficient, suggesting that the efficiency of NMD is an inherent character of cells. Our results also suggest that variability in the efficiency of NMD is a general phenomenon and is not restricted to nasal epithelial cells. As NMD affects the level of many transcripts, variability in the NMD efficiency might play a role as a genetic modifier of different cellular functions.European Journal of Human Genetics (2007) 15, 1156–1162; doi:10.1038/sj.ejhg.5201889; published online 11 July 2007 [ABSTRACT FROM AUTHOR]

Abstract: Nonsense-mediated mRNA decay (NMD) is a surveillance pathway that mediates rapid degradation of transcripts bearing premature translation termination codons (PTCs) and thereby limits the expression of unproductively processed mRNAs and the synthesis of C-terminally truncated peptides. Both its importance as a means to control gene expression and in the context of genetic and acquired human diseases call for an exploration of the mammalian NMD pathway using chemical biology approaches. Here, we describe a novel cell-based chemiluminescence reporter system that recapitulates the hallmark features of mammalian NMD. The assay is characterized by its high sensitivity, robustness, and its potential for automated handling. Limiting NMD efficiency by RNAi-mediated depletion of the essential NMD factor UPF1 markedly and specifically increased the NMD reporter mRNA level and resulted in a proportional increase in protein expression reflected by Renilla luminescence. The PI 3-kinase inhibitor wortmannin has previously been found to up-modulate PTC-containing transcripts by inhibiting the UPF1 kinase SMG1. Wortmannin treatment enhanced NMD reporter expression in our system in a dose-dependent way, illustrating its utility for small molecule screening. [Copyright &y& Elsevier]

In eukaryotes, a surveillance pathway known as nonsense-mediated decay (NMD) regulates the abundance of messenger RNAs containing premature termination codons (PTCs). In mammalian cells, it has been asserted that the NMD-relevant first round of translation is special and involves initiation by a specific protein heterodimer, the nuclear cap-binding complex (CBC). Arguing against a requirement for CBC-mediated translation initiation, we show that ribosomal recruitment by the internal ribosomal entry sequence of the encephalomyocarditis virus triggers NMD of a PTC-containing transcript under conditions in which ribosome entry from the cap is prohibited. These data generalize the previous model and suggest that translation per se, irrespective of how it is initiated, can mediate NMD. [ABSTRACT FROM AUTHOR]

Nonsense-mediated decay (NMD) eliminates mRNAs containing premature termination codons and thus helps limit the synthesis of abnormal proteins. New results uncover a broader role of NMD as a pathway that also affects the expression of wild-type genes and alternative-splice products. Because the mechanisms by which NMD operates have received much attention, we discuss here the emerging awareness of the impact of NMD on the manifestation of human genetic diseases. We explore how an understanding of NMD accounts for phenotypic differences in diseases caused by premature termination codons. Specifically, we consider how the protective function of NMD sometimes benefits heterozygous carriers and, in contrast, sometimes contributes to a clinical picture of protein deficiency by inhibiting expression of partially functional proteins. Potential 'NMD therapeutics' will therefore need to strike a balance between the general physiological benefits of NMD and its detrimental effects in cases of specific genetic mutations. [ABSTRACT FROM AUTHOR]

Messenger RNAs with premature translation termination codons (PTCs) are degraded by nonsense-mediated mRNA decay (NMD). In mammals, PTCs are discriminated from physiological stop codons by a process thought to involve the splicing-dependent deposition of an exon junction complex (EJC), EJC-mediated recruitment of Upf3, and Upf2 binding to the N terminus of Upf3. Here, we identify a conserved domain of hUpf3b that mediates an interaction with the EJC protein Y14. Tethered function analysis shows that the Y14/hUpf3b interaction is essential for NMD, while surprisingly the interaction between hUpf3b and hUpf2 is not. Nonetheless, hUpf2 is necessary for NMD mediated by tethered Y14. RNAi-induced knockdown and Y14 repletion of siRNA-treated cells implicates Y14 in the degradation of β-globin NS39 mRNA and demonstrates that Y14 is required for NMD induced by tethered hUpf3b. These results uncover a direct role of Y14 in NMD and suggest an unexpected hierarchy in the assembly of NMD complexes. [Copyright &y& Elsevier]

The G→A mutation at position 20210 of the prothrombin or coagulation factor II gene (F2) represents a common genetic risk factor for the occurrence of thromboembolic events. This mutation affects the 3′-terminal nucleotide of the 3′ untranslated region (UTR) of the mRNA and causes elevated prothrombin plasma concentrations by an unknown mechanism. Here, we show that the mutation does not affect the amount of pre-mRNA, the site of 3′ end cleavage or the length of the poly(A) tail of the mature mRNA. Rather, we demonstrate that the physiological F2 3′ end cleavage signal is inefficient and that F2 20210 G→A represents a gain-of-function mutation, causing increased cleavage site recognition, increased 3′ end processing and increased mRNA accumulation and protein synthesis. Enhanced mRNA 3′ end formation efficiency emerges as a novel principle causing a genetic disorder and explains the role of the F2 20210 G→A mutation in the pathogenesis of thrombophilia. This work also illustrates the pathophysiologic importance of quantitatively minor aberrations of RNA metabolism. [ABSTRACT FROM AUTHOR]

Premature translation termination codons are common causes of genetic disorders. mRNAs with such mutations are degraded by a surveillance mechanism termed nonsense-mediated decay (NMD), which represents a phylogenetically widely conserved post-transcriptional mechanism for the quality control of gene expression. How NMD-competent mRNAs are formed and specified remains a central question. Here, we have used human β-globin mRNA as a model system to address the role of splicing and polyadenylation for human NMD. We show that (i) splicing is an indispensable component of the human β-globin NMD pathway, which cannot be compensated for by exonic β-globin 'failsafe' sequences; (ii) the spatial requirements of human β-globin NMD, as signified by the maximal distance of the nonsense mutation to the final exon-exon junction, are less constrained than in yeast; and (iii) non-polyadenylated mRNAs with a histone 3' end are NMD competent. Thus, the formation of NMD-competent mRNA particles critically depends on splicing but does not require the presence of a poly(A) tail. [ABSTRACT FROM AUTHOR]

Premature translation termination codons resulting from nonsense or frameshift mutations are common causes of genetic disorders. Complications arising from the synthesis of C-terminally truncated polypeptides can be avoided by 'nonsense-mediated decay' of the mutant mRNAs. Premature termination codons in the β-globin mRNA cause the common recessive form of β-thalassemia when the affected mRNA is degraded, but the more severe dominant form when the mRNA escapes nonsense-mediated decay.We demonstrate that cells distinguish a premature termination codon within the β-globin mRNA from the physiological translation termination codon by a two-step specification mechanism. According to the binary specification model proposed here, the positions of splice junctions are first tagged during splicing in the nucleus, defining a stop codon operationally as a premature termination codon by the presence of a 3′ splicing tag. In the second step, cytoplasmic translation is required to validate the 3′ splicing tag for decay of the mRNA. This model explains nonsense-mediated decay on the basis of conventional molecular mechanisms and allows us to propose a common principle for nonsense-mediated decay from yeast to man. [ABSTRACT FROM AUTHOR]

Summary: Messenger RNAs (mRNAs) bearing premature translation termination codons (PTCs) are degraded by nonsense-mediated mRNA decay (NMD). For mammalian NMD, current models propose a linear pathway that involves the splicing-dependent deposition of exon-junction complexes (EJCs) and the sequential action of the NMD factors UPF3, UPF2, and UPF1. We show here that different EJC proteins serve as entry points for the formation of distinguishable NMD-activating mRNPs. Specifically, Y14, MAGOH, and eIF4A3 can activate NMD in an UPF2-independent manner, whereas RNPS1-induced NMD requires UPF2. We identify the relevant regions of RNPS1, eIF4A3, Y14, and MAGOH, which are essential for NMD and provide insights into the formation of complexes, that classify alternative NMD pathways. These results are integrated into a nonlinear model for mammalian NMD involving alternative routes of entry that converge at a common requirement of UPF1. [Copyright &y& Elsevier]