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Spinal muscular atrophy is a neurodegenerative disorder caused by the deletion or mutation of the survival-of-motor-neuron gene, SMN1. An SMN1 paralog, SMN2, differs by a C-->T transition in exon 7 that causes substantial skipping of this exon, such that SMN2 expresses only low levels of functional protein. A better understanding of SMN splicing mechanisms should facilitate the development of drugs that increase survival motor neuron (SMN) protein levels by improving SMN2 exon 7 inclusion. In addition, exonic mutations that cause defective splicing give rise to many genetic diseases, and the SMN1/2 system is a useful paradigm for understanding exon-identity determinants and alternative-splicing mechanisms. Skipping of SMN2 exon 7 was previously attributed either to the loss of an SF2/ASF-dependent exonic splicing enhancer or to the creation of an hnRNP A/B-dependent exonic splicing silencer, as a result of the C-->T transition. We report the extensive testing of the enhancer-loss and silencer-gain models by mutagenesis, RNA interference, overexpression, RNA splicing, and RNA-protein interaction experiments. Our results support the enhancer-loss model but also demonstrate that hnRNP A/B proteins antagonize SF2/ASF-dependent ESE activity and promote exon 7 skipping by a mechanism that is independent of the C-->T transition and is, therefore, common to both SMN1 and SMN2. Our findings explain the basis of defective SMN2 splicing, illustrate the fine balance between positive and negative determinants of exon identity and alternative splicing, and underscore the importance of antagonistic splicing factors and exonic elements in a disease context.
Pubmed ID: 16385450 RIS Download
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A web-based analysis service for identifying exonic splicing enhancers in eukaryotic genes. ESEfinder accept sequences in the FASTA format. A typical mammalian gene is composed of several relatively short exons that are interrupted by much longer introns. To generate correct mature mRNAs, the exons must be identified and joined together precisely and efficiently, in a process that requires the coordinated action of five small nuclear (sn)RNAs (U1, U2, U4, U5 and U6) and more than 60 polypeptides. The inaccurate recognition of exon/intron boundaries or the failure to remove an intron generates aberrant mRNAs that are either unstable or code for defective or deleterious protein isoforms. Exonic enhancers are thought to serve as binding sites for specific serine/arginine-rich (SR) proteins, a family of structurally related and highly conserved splicing factors characterized by one or two RNA-recognition motifs (RRM) and by a distinctive C-terminal domain highly enriched in RS dipeptides (the RS domain). The RRMs mediate sequence-specific binding to the RNA, and so determine substrate specificity, whereas the RS domain appears to be involved mainly in protein-protein interactions. SR proteins bound to ESEs can promote exon definition by directly recruiting the splicing machinery through their RS domain and/or by antagonizing the action of nearby silencer elements. Sponsors: ESEfinder is supported by the Cold Spring Harbor Laboratory.
View all literature mentionsSpecific short oligonucleotide sequences that enhance pre-mRNA splicing when present in exons, termed exonic splicing enhancers (ESEs), play important roles in constitutive and alternative splicing (ESE References). A hybrid computational/experimental method, RESCUE-ESE, was recently developed for identifying sequences with ESE activity. In this approach, specific hexanucleotide sequences are identified as candidate ESEs on the basis that they have both significantly higher frequency of occurrence in exons than in introns and also significantly higher frequency in exons with weak (non-consensus) splice sites than in exons with strong (consensus) splice sites. Representative hexamers from ten different classes of candidate ESEs, together with 6 or 7 bases of flanking sequence context on each side, were introduced into a weak (poorly spliced) exon in a splicing reporter construct. These reporter minigenes were then transfected into cultured cells, where they are transcribed and spliced, and the relative level of inclusion of the test exon was assayed by quantitative (radio-labeled) RT-PCR. Point mutants of these sequences were also analyzed to confirm the precise motifs responsible for ESE activity. The RESCUE-ESE approach identified 238 hexamers as candidate ESEs using a large database of human genes of known exon-intron structure containing over 30,000 nonredudant exons. In more recent analyses by Yeo et al., the RESCUE-ESE approach was utilized to predict hexamers as candidate ESEs in other vertebrate genes, namely, Fugu rubipes, Zebrafish and Mouse. This allows the identification of motifs that are conserved in vertebrates. This web server allows a sequence to be checked for presence of these candidate ESE hexamers.
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