Étude du métabolisme des ARNm aberrants du gène codant pour la fumarylacétoacétate hydrolase

La cellule a élaboré des mécanismes de surveillance afin d'assurer la fidélité de l'expression génique. Le nonsense-mediated mRNA decay (NMD) reconnaît et en général, dégrade rapidement les ARNm nonsens, qui contiennent des codons stop prématurés. Le NMD a longtemps été considéré comme un mécanisme de protection, permettant d'éviter la synthèse de protéines tronquées, potentiellement néfastes pour la cellule. Plus récemment, un nouveau rôle du NMD a émergé : il semble réguler l'expression génique, notamment lorsqu'il est couplé à l'épissage alternatif. D'autre part, bien que les mécanismes de reconnaissance et de dégradation des ARNm nonsens soient de mieux en mieux compris, une controverse subsiste quand à la localisation subcellulaire du NMD chez les mammifères. Ces deux aspects du NMD ont été abordés au cours de ma thèse dans le cas de l'étude du métabolisme des ARNm aberrants du gène de la fah, responsable de la tyrosinémie héréditaire de type I. Afin de mieux comprendre les bases moléculaires de la maladie, j'ai caractérisé les ARNm de la FAH contenant des mutations d'épissage (Q279R et V259L) ou nonsens (W262X) dans l'exon 9. Deux transcrits alternatifs du gène de la fah ont ainsi été mis en évidence pour ces trois mutations : del100 a éliminé l'exon 8 et del231 les exons 8 et 9. Del231 est fortement produit lorsque les mutations d'épissage Q279R et V259L sont présentes. L'étude des transcrits chez les patients avec ces deux mutations a permis de proposer un modèle d'épissage pour les exons 8 et 9. Une analyse plus poussée sur del100 et del231 a révélé que ceux-ci étaient en fait produits de façon minoritaire dans des cellules avec un gène normal de la fah, par épissage alternatif des exons 8 et 9. Del100, qui contient de nombreux PTCs suite à l'élimination de l'exon 8, est dégradé par NMD. Cependant, la quantité de transcrit restante semble suffisante pour la synthèse d'une protéine de 31-kDa, détectable dans plusieurs tissus humains. Ce premier transcrit alternatif de la fah est important à deux points de vue : premièrement, il semble illustrer le rôle possible du NMD couplé à l'épissage alternatif dans la régulation génique et deuxièmement, la protéine synthétisée pourrait être importante pour la tyrosinémie héréditaire de type I

A minor alternative transcript of the fumarylacetoacetate hydrolase gene produces a protein despite being likely subjected to nonsense-mediated mRNA decay

BACKGROUND: Coupling of alternative splicing with nonsense-mediated mRNA decay (NMD) may regulate gene expression. We report here the identification of a nonsense alternative transcript of the fumarylacetoacetate hydrolase (fah) gene, which produces a protein despite the fact that it is subject to NMD. RESULTS: During the characterization of the effects of the W262X nonsense mutation on FAH mRNA metabolism, two alternative transcripts (del100 and del231) of the fah gene were identified. Del100 lacks exon 8 and as a consequence, the reading frame is shifted and a premature termination codon appears at the 3'end of exon 10. Exons 8 and 9 are skipped in del231, without any disruption of the reading frame. Specific amplification of these transcripts demonstrate that they are produced through minor alternative splicing pathways, and that they are not caused by the W262X mutation per se. As shown with an antiserum raised against the C-terminal part of the putative DEL100 protein, the del100 transcript produces a protein, expressed at different levels in various human tissues. Interestingly, the del100 transcript seems to be subjected to nonsense-mediated mRNA decay, as its level was stabilized following a cycloheximide treatment. CONCLUSIONS: The del100 and del231 transcripts arise due to minor alternative splicing pathways and del100 is likely subjected to nonsense-mediated mRNA decay. However the remaining amount of transcript seems sufficient to produce a protein in different human tissues. This suggests that NMD has a broader role than simply eliminating aberrant transcripts and when coupled to alternative splicing, may act to modulate gene expression, by allowing the production of low amounts of protein

A Missense Mutation (Q279R) in the Fumarylacetoacetate Hydrolase Gene, Responsible for Hereditary Tyrosinemia, Acts as a Splicing Mutation

Background: Tyrosinemia type I, the most severe disease of the tyrosine catabolic pathway is caused by a deficiency in fumarylacetoacetate hydrolase (FAH). A patient showing few of the symptoms associated with the disease, was found to be a compound heterozygote for a splice mutation, IVS6-1g->t, and a putative missense mutation, Q279R. Analysis of FAH expression in liver sections obtained after resection for hepatocellular carcinoma revealed a mosaic pattern of expression. No FAH was found in tumor regions while a healthy region contained enzymeexpressing nodules. Results: Analysis of DNA from a FAH expressing region showed that the expression of the protein was due to correction of the Q279R mutation. RT-PCR was used to assess if Q279R RNA was produced in the liver cells and in fibroblasts from the patient. Normal mRNA was found in the liver region where the mutation had reverted while splicing intermediates were found in nonexpressing regions suggesting that the Q279R mutation acted as a splicing mutation in vivo. Sequence of transcripts showed skipping of exon 8 alone or together with exon 9. Using minigenes in transfection assays, the Q279R mutation was shown to induce skipping of exon 9 when placed in a constitutive splicing environment. Conclusion: These data suggest that the putative missense mutation Q279R in the FAH gene acts as a splicing mutation in vivo. Moreover FAH expression can be partially restored in certain liver cells as a result of a reversion of the Q279R mutation and expansion of the corrected cells

Interaction between methionine synthase isoforms and MMACHC: characterization in cblG-variant, cblG and cblC inherited causes of megaloblastic anaemia

The cblG and cblC disorders of cobalamin (Cbl) metabolism are two inherited causes of megaloblastic anaemia. In cblG, mutations in methionine synthase (MTR) decrease conversion of hydroxocobalamin (HOCbl) to methylcobalamin, while in cblC, mutations in MMACHC disrupt formation of cob(II)alamin (detected as HOCbl). Cases with undetectable methionine synthase (MS) activity are extremely rare and classified as ‘cblG-variant'. In four ‘cblG-variant' cases, we observed a decreased conversion of cyanocobalamin to HOCbl that is also seen in cblC cases. To explore this observation, we studied the gene defects, splicing products and expression of MS, as well as MS/MMACHC protein interactions in cblG-variant, cblG, cblC and control fibroblasts. We observed a full-size MS encoded by MTR-001 and a 124 kDa truncated MS encoded by MTR-201 in cblG, cblC, control fibroblasts and HEK cells, but only the MTR-201 transcript and inactive truncated MS in cblG-variant cells. Co-immunoprecipitation and proximity ligation assay showed interaction between truncated MS and MMACHC in cblG-variant cells. This interaction decreased 2.2, 1.5 and 5.0-fold in the proximity ligation assay of cblC cells with p.R161Q and p.R206W mutations, and HEK cells with knock down expression of MS by siRNA, respectively, when compared with control cells. In 3D modelling and docking analysis, both truncated and full-size MS provide a loop anchored to MMACHC, which makes contacts with R-161 and R-206 residues. Our data suggest that the interaction of MS with MMACHC may play a role in the regulation of the cellular processing of Cbls that is required for Cbl cofactor synthesi

Structural and functional analysis of the Rous Sarcoma virus negative regulator of splicing and demonstration of its activation by the 9G8 SR protein

Retroviruses require both spliced and unspliced RNAs for replication. Accumulation of Rous Sarcoma virus (RSV) unspliced RNA depends upon the negative regulator of splicing (NRS). Its 5′-part is considered as an ESE binding SR proteins. Its 3′-part contains a decoy 5′-splice site (ss), which inhibits splicing at the bona fide 5′-ss. Only the 3D structure of a small NRS fragment had been experimentally studied. Here, by chemical and enzymatic probing, we determine the 2D structure of the entire RSV NRS. Structural analysis of other avian NRSs and comparison with all sequenced avian NRSs is in favour of a phylogenetic conservation of the NRS 2D structure. By combination of approaches: (i) in vitro and in cellulo splicing assays, (ii) footprinting assays and (iii) purification and analysis of reconstituted RNP complex, we define a small NRS element retaining splicing inhibitory property. We also demonstrate the capability of the SR protein 9G8 to increase NRS activity in vitro and in cellulo. Altogether these data bring new insights on how NRS fine tune splicing activity

Antagonistic factors control the unproductive splicing of SC35 terminal intron

Alternative splicing is regulated in part by variations in the relative concentrations of a variety of factors, including serine/arginine-rich (SR) proteins. The SR protein SC35 self-regulates its expression by stimulating unproductive splicing events in the 3′ untranslated region of its own pre-mRNA. Using various minigene constructs containing the terminal retained intron and flanking exons, we identified in the highly conserved last exon a number of exonic splicing enhancer elements responding specifically to SC35, and showed an inverse correlation between affinity of SC35 and enhancer strength. The enhancer region, which is included in a long stem loop, also contains repressor elements, and is recognized by other RNA-binding proteins, notably hnRNP H protein and TAR DNA binding protein (TDP-43). Finally, in vitro and in cellulo experiments indicated that hnRNP H and TDP-43 antagonize the binding of SC35 to the terminal exon and specifically repress the use of SC35 terminal 3′ splice site. Our study provides new information about the molecular mechanisms of SC35-mediated splicing activation. It also highlights the existence of a complex network of self- and cross-regulatory mechanisms between splicing regulators, which controls their homeostasis and offers many ways of modulating their concentration in response to the cellular environment

The Germ Cell Nuclear Proteins hnRNP G-T and RBMY Activate a Testis-Specific Exon

The human testis has almost as high a frequency of alternative splicing events as brain. While not as extensively studied as brain, a few candidate testis-specific splicing regulator proteins have been identified, including the nuclear RNA binding proteins RBMY and hnRNP G-T, which are germ cell-specific versions of the somatically expressed hnRNP G protein and are highly conserved in mammals. The splicing activator protein Tra2β is also highly expressed in the testis and physically interacts with these hnRNP G family proteins. In this study, we identified a novel testis-specific cassette exon TLE4-T within intron 6 of the human transducing-like enhancer of split 4 (TLE4) gene which makes a more transcriptionally repressive TLE4 protein isoform. TLE4-T splicing is normally repressed in somatic cells because of a weak 5′ splice site and surrounding splicing-repressive intronic regions. TLE4-T RNA pulls down Tra2β and hnRNP G proteins which activate its inclusion. The germ cell-specific RBMY and hnRNP G-T proteins were more efficient in stimulating TLE4-T incorporation than somatically expressed hnRNP G protein. Tra2b bound moderately to TLE4-T RNA, but more strongly to upstream sites to potently activate an alternative 3′ splice site normally weakly selected in the testis. Co-expression of Tra2β with either hnRNP G-T or RBMY re-established the normal testis physiological splicing pattern of this exon. Although they can directly bind pre-mRNA sequences around the TLE4-T exon, RBMY and hnRNP G-T function as efficient germ cell-specific splicing co-activators of TLE4-T. Our study indicates a delicate balance between the activity of positive and negative splicing regulators combinatorially controls physiological splicing inclusion of exon TLE4-T and leads to modulation of signalling pathways in the testis. In addition, we identified a high-affinity binding site for hnRNP G-T protein, showing it is also a sequence-specific RNA binding protein

Rapid screening of yeast mutants with reporters identifies new splicing phenotypes

Nuclear precursor mRNA splicing requires the stepwise assembly of a large complex, the spliceosome. Recent large-scale analyses, including purification of splicing complexes, high-throughput genetic screens and interactomic studies, have linked numerous factors to this dynamic process, including a well-defined core conserved from yeast to human. Intriguingly, despite extensive studies, no splicing defects were reported for some of the corresponding yeast mutants. To resolve this paradox, we screened a collection of viable yeast strains carrying mutations in splicing-related factors with a set of reporters including artificial constructs carrying competing splice sites. Previous analyses have indeed demonstrated that this strategy identifies yeast factors able to regulate alternative splicing and whose properties are conserved in human cells. The method, sensitive to subtle defects, revealed new splicing phenotypes for most analyzed factors such as the Urn1 protein. Interestingly, a mutant of PRP8 specifically lacking an N-terminal proline-rich region stimulated the splicing of a reporter containing competing branchpoint/3' splice site regions. Thus, using appropriate reporters, yeast can be used to quickly delineate the effect of various factors on splicing and identify those with the propensity to regulate alternative splicing events

Nutrigenomics and RNA methylation: Role of micronutrients

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