Infrequent translation of a nonsense codon is sufficient to decrease mRNA level.
(1/1486)
In many organisms nonsense mutations decrease the level of mRNA. In the case of mammalian cells, it is still controversial whether translation is required for this nonsense-mediated RNA decrease (NMD). Although previous analyzes have shown that conditions that impede translation termination at nonsense codons also prevent NMD, the residual level of termination was unknown in these experiments. Moreover, the conditions used to impede termination might also have interfered with NMD in other ways. Because of these uncertainties, we have tested the effects of limiting translation of a nonsense codon in a different way, using two mutations in the immunoglobulin mu heavy chain gene. For this purpose we exploited an exceptional nonsense mutation at codon 3, which efficiently terminates translation but nonetheless maintains a high level of mu mRNA. We have shown 1) that translation of Ter462 in the double mutant occurs at only approximately 4% the normal frequency, and 2) that Ter462 in cis with Ter3 can induce NMD. That is, translation of Ter462 at this low (4%) frequency is sufficient to induce NMD. (+info)
Genetic heterogeneity in propionic acidemia patients with alpha-subunit defects. Identification of five novel mutations, one of them causing instability of the protein.
(2/1486)
The inherited metabolic disease propionic acidemia (PA) can result from mutations in either of the genes PCCA or PCCB, which encode the alpha and beta subunits, respectively, of the mitochondrial enzyme propionyl CoA-carboxylase. In this work we have analyzed the molecular basis of PCCA gene defects, studying mRNA levels and identifying putative disease causing mutations. A total of 10 different mutations, none predominant, are present in a sample of 24 mutant alleles studied. Five novel mutations are reported here for the first time. A neutral polymorphism and a variant allele present in the general population were also detected. To examine the effect of a point mutation (M348K) involving a highly conserved residue, we have carried out in vitro expression of normal and mutant PCCA cDNA and analyzed the mitochondrial import and stability of the resulting proteins. Both wild-type and mutant proteins were imported into mitochondria and processed into the mature form with similar efficiency, but the mature mutant M348K protein decayed more rapidly than did the wild-type, indicating a reduced stability, which is probably the disease-causing mechanism. (+info)
Facile characterization of translation initiation via nonsense codon suppression.
(3/1486)
A new strategy for studying the mechanism of translation initiation in eukaryotes has been developed. The strategy involves the use of an in vitro translation system to incorporate a non-natural fluorescent amino acid into a protein from a suppressor tRNAPheCUA misacylated with that amino acid. It is thereby possible to monitor translation initiation efficiency at an AUG codon in different contexts; this is illustrated for three constructs encoding Escherichia coli dihydrofolate reductase mRNA with different translation initiation regions. Fluorescence measurements after in vitro translation of the mRNAs in rabbit reticulocyte lysate reflected differences in the position and efficiency of translation initiation and, therefore, can be used for characterization of the translation initiation process. (+info)
A newly identified patient with clinical xeroderma pigmentosum phenotype has a non-sense mutation in the DDB2 gene and incomplete repair in (6-4) photoproducts.
(4/1486)
We report here a patient (Ops1) with clinical photosensitivity, including pigmented or depigmented macules and patches, and multiple skin neoplasias (malignant melanomas, basal cell carcinomas, and squamous cell carcinomas in situ) in sun-exposed areas. These clinical features are reminiscent of xeroderma pigmentosum. As cells from Ops1 showed normal levels in DNA repair synthesis in vivo (unscheduled DNA synthesis and recovery of RNA synthesis after ultraviolet irradiation), we performed a postreplication repair assay and recovery of replicative DNA synthesis after ultraviolet irradiation to investigate if Ops1 cells belonged to a xeroderma pigmentosum variant pattern. Ops1 cells were normal, but there was an incomplete pattern repair in (6-4) photoproducts in contrast to a normal pattern repair in cis-syn cyclobutane pyrimidine dimers by repair kinetics using the enzyme-linked immunosorbent assay. Moreover, Ops1 cells were defective in a damage-specific DNA binding protein and carried a non-sense mutation in the DDB2 gene. These results suggest that (i) the DDB2 gene is somewhat related to skin carcinogenesis, photoaging skin, and the removal of (6-4) photoproducts; (ii) although it is believed that cyclobutane pyrimidine dimers are the principal mutagenic lesion and (6-4) photoproducts are less likely to contribute to ultraviolet-induced mutations in mammals, Ops1 is one of the ultraviolet-induced mutagenic models induced by (6-4) photoproducts. (+info)
Nonsense-mediated mRNA decay in health and disease.
(5/1486)
All eukaryotes possess the ability to detect and degrade transcripts harboring premature signals for the termination of translation. Despite the ubiquitous nature of nonsense-mediated mRNA decay (NMD) and its demonstrated role in the modulation of phenotypes resulting from selected nonsense alleles, very little is known regarding its basic mechanism or the selective pressure for complete evolutionary conservation of this function. This review will present the current models of NMD that have been generated during the study of model organisms and mammalian cells. The physiological burden of nonsense transcripts and the emerging view that NMD plays a broad and critical role in the regulation of gene expression will also be discussed. Such issues are relevant to the proposal that pharmacological manipulation of NMD will find therapeutic application. (+info)
Clinical and molecular genetic analysis of 19 Wolfram syndrome kindreds demonstrating a wide spectrum of mutations in WFS1.
(6/1486)
Wolfram syndrome is an autosomal recessive neurodegenerative disorder characterized by juvenile-onset diabetes mellitus and progressive optic atrophy. mtDNA deletions have been described, and a gene (WFS1) recently has been identified, on chromosome 4p16, encoding a predicted 890 amino acid transmembrane protein. Direct DNA sequencing was done to screen the entire coding region of the WFS1 gene in 30 patients from 19 British kindreds with Wolfram syndrome. DNA was also screened for structural rearrangements (deletions and duplications) and point mutations in mtDNA. No pathogenic mtDNA mutations were found in our cohort. We identified 24 mutations in the WFS1 gene: 8 nonsense mutations, 8 missense mutations, 3 in-frame deletions, 1 in-frame insertion, and 4 frameshift mutations. Of these, 23 were novel mutations, and most occurred in exon 8. The majority of patients were compound heterozygotes for two mutations, and there was no common founder mutation. The data were also analyzed for genotype-phenotype relationships. Although some interesting cases were noted, consideration of the small sample size and frequency of each mutation indicated no clear-cut correlations between any of the observed mutations and disease severity. There were no obvious mutation hot spots or clusters. Hence, molecular screening for Wolfram syndrome in affected families and for Wolfram syndrome-carrier status in subjects with psychiatric disorders or diabetes mellitus will require complete analysis of exon 8 and upstream exons. (+info)
Mutations in VPS16 and MRT1 stabilize mRNAs by activating an inhibitor of the decapping enzyme.
(7/1486)
Decapping is a rate-limiting step in the decay of many yeast mRNAs; the activity of the decapping enzyme therefore plays a significant role in determining RNA stability. Using an in vitro decapping assay, we have identified a factor, Vps16p, that regulates the activity of the yeast decapping enzyme, Dcp1p. Mutations in the VPS16 gene result in a reduction of decapping activity in vitro and in the stabilization of both wild-type and nonsense-codon-containing mRNAs in vivo. The mrt1-3 allele, previously shown to affect the turnover of wild-type mRNAs, results in a similar in vitro phenotype. Extracts from both vps16 and mrt1 mutant strains inhibit the activity of purified Flag-Dcp1p. We have identified a 70-kDa protein which copurifies with Flag-Dcp1p as the abundant Hsp70 family member Ssa1p/2p. Intriguingly, the interaction with Ssa1p/2p is enhanced in strains with mutations in vps16 or mrt1. We propose that Hsp70s may be involved in the regulation of mRNA decapping. (+info)
An internal open reading frame triggers nonsense-mediated decay of the yeast SPT10 mRNA.
(8/1486)
Yeast cells containing a temperature-sensitive mutation in the PRT1 gene were found to selectively stabilize mRNAs harboring early nonsense codons. The similarities between the mRNA decay phenotypes of prt1-1 cells and those lacking the nonsense-mediated mRNA decay (NMD) factor Upf1p led us to determine whether both types of mutations cause the accumulation of the same mRNAs. Differential display analysis and mRNA half-life measurements demonstrated that the HHF2 mRNA increased in abundance in prt1-1 and upf1Delta cells, but did not manifest a change in decay rate. In both mutant strains this increase was attributable to stabilization of the SPT10 transcript, an mRNA encoding a transcriptional regulator of HHF2. Analyses of chimeric mRNAs used to identify the cis-acting basis for NMD of the SPT10 mRNA indicated that ribosomes scan beyond its initiator AUG and initiate at the next downstream AUG, resulting in premature translation termination. By searching a yeast database for transcripts with sequence features similar to those of the SPT10 mRNA, other transcripts that decay by the NMD pathway were identified. Our results demonstrate that mRNAs undergoing leaky scanning are a new class of endogenous NMD substrate, and suggest the existence of a novel cellular regulatory circuit. (+info)