Systematic characterization of short intronic splicing-regulatory elements in SMN2 pre-mRNA

Intronic splicing enhancers and silencers (ISEs and ISSs) are two groups of splicing-regulatory elements (SREs) that play critical roles in determining splice-site selection, particularly for alternatively spliced introns or exons. SREs are often short motifs; their mutation or dysregulation of their cognate proteins frequently causes aberrant splicing and results in disease. To date, however, knowledge about SRE sequences and how they regulate splicing remains limited. Here, using an SMN2 minigene, we generated a complete pentamer-sequence library that comprises all possible combinations of 5 nucleotides in intron 7, at a fixed site downstream of the 5′ splice site. We systematically analyzed the effects of all 1023 mutant pentamers on exon 7 splicing, in comparison to the wild-type minigene, in HEK293 cells. Our data show that the majority of pentamers significantly affect exon 7 splicing: 584 of them are stimulatory and 230 are inhibitory. To identify actual SREs, we utilized a motif set enrichment analysis (MSEA), from which we identified groups of stimulatory and inhibitory SRE motifs. We experimentally validated several strong SREs in SMN1/2 and other minigene settings. Our results provide a valuable resource for understanding how short RNA sequences regulate splicing. Many novel SREs can be explored further to elucidate their mechanism of action.     

VERSE: a varying effect regression for splicing elements discovery

Identification of splicing regulatory elements (SREs) deserves special attention because these cis-acting short sequences are vital parts of splicing code. The fact that a variety of other biological signals cooperatively govern the splicing pattern indicates the necessity of developing novel tools to incorporate information from multiple sources to improve splicing factor binding sites prediction. Under this context, we proposed a Varying Effect Regression for Splicing Elements (VERSE) to discover intronic SREs in the proximity of exon junctions by integrating other biological features. As a result, 1562 intronic SREs were identified in 16 human tissues, many of which overlapped with experimentally verified binding motifs for several well-known splicing factors, including FOX-1, PTB, hnRNP A/B, hnRNP F/H, and so on. The discovered tissue, region, and conservation preferences of the putative motifs demonstrate that splice site selection is a complicated process that needs subtle and delicate regulation. VERSE may serve as a powerful tool to not only discover SREs by incorporating additional informative signals but also precisely quantify their varying contribution under different biological contexts.     

Temperature-dependent splicing of beta-globin pre-mRNA

A T→G mutation at nucleotide 705 of human β-globin intron 2 creates an aberrant 5′ splice site and activates a cryptic 3′ splice site upstream. In consequence, the pre-mRNA is spliced via aberrant splice sites, despite the presence of the still functional correct sites. Surprisingly, when IVS2-705 HeLa or K562 cells were cultured at temperatures below 30°C, aberrant splicing was inhibited and correct splicing was restored. Similar temperature effects were seen for another β-globin pre-mRNA, IVS2-745, and in a construct in which a β-globin intron was inserted into a coding sequence of EGFP. Temperature-induced alternative splicing was affected by the nature of the internal aberrant splice sites flanking the correct sites and by exonic sequences. The results indicate that in the context of thalassemic splicing mutations and possibly in other alternatively spliced pre-mRNAs, temperature is one of the parameters that affect splice site selection.     

Intronic mutations outside of Alu-repeat-rich domains of the LDL receptor gene are a cause of familial hypercholesterolemia

Familial hypercholesterolemia (FH), a frequent monogenic condition complicated by premature cardiovascular disease, is characterized by high allelic heterogeneity at the low-density lipoprotein receptor ( LDLR) locus. Despite more than a decade of genetic testing, knowledge about intronic disease-causing mutations has remained limited because of lack of available genomic sequences. Based on the finding from bioinformatic analysis that Alu repeats represent 85% of LDLR intronic sequences outside exon-intron junctions, we designed a strategy to improve the exploration of genomic regions in the vicinity of exons in 110 FH subjects from an admixed population. In the first group of 42 patients of negative mutation carriers, as previously established by former screening strategies (denaturing gradient gel electrophoresis, DNA sequencing with former primers overlapping splice-sites, Southern Blotting), about half ( n=22) were found to be carriers of at least one heterozygous mutation. Among a second group of 68 newly recruited patients, 27% of mutation carriers ( n=37) had a splicing regulatory mutation. Overall, out of the 54 mutations identified, 13 were intronic, and 18 were novel, out of which nearly half were intronic. Two novel intronic mutations (IVS8-10G-->A within the polypyrimidine tract and IVS7+10G-->A downstream of donor site) might create potential aberrant splice sites according to neural-network computed estimation, contrary to 31 common single nucleotide variations also identified at exon-intron junctions. This new strategy of detecting the most likely disease-causing LDLR mutations outside of Alu-rich genomic regions reveals that intronic mutations may have a greater impact than previously reported on the molecular basis of FH.     

A novel point mutation (G-1 to T) in a 5' splice donor site of intron 13 of the dystrophin gene results in exon skipping and is responsible for Becker muscular dystrophy.

The mutations in one-third of Duchenne and Becker muscular dystrophy patients remain unknown, as they do not involve gross rearrangements of the dystrophin gene. We now report a defect in the splicing of precursor mRNA (pre-mRNA), resulting from a maternally inherited mutation of the dystrophin gene in a patient with Becker muscular dystrophy. This defect results from a G-to-T transversion at the terminal nucleotide of exon 13, within the 5' splice site of intron 13, and causes complete skipping of exon 13 during processing of dystrophin pre-mRNA. The predicted polypeptide encoded by the aberrant mRNA is a truncated dystrophin lacking 40 amino acids from the amino-proximal end of the rod domain. This is the first report of an intraexon point mutation that completely inactivates a 5' splice donor site in dystrophin pre-mRNA. Analysis of the genomic context of the G-1-to-T mutation at the 5' splice site supports the exon-definition model of pre-mRNA splicing and contributes to the understanding of splice-site selection.     

Nature and frequency of mutations in the argininosuccinate synthetase gene that cause classical citrullinemia

Citrullinemia is an autosomal recessive disorder caused by a genetic deficiency of argininosuccinate synthetase (ASS). So far 20 mutations in ASS mRNA have been identified in human classical citrullinemia, including 14 single base changes causing missense mutations in the coding sequence of the enzyme, 4 mutations associated with an absence of exons 5, 6, 7, or 13 in mRNA, 1 mutation with a deletion of the first 7 bases in exon 16 (which is caused by abnormal splicing), and 1 mutation with an insertion of 37 bases between the exon 15 and 16 regions in mRNA. In order to identify the abnormality in the ASS gene causing the exon 7 and 13 deletion mutations and the 37-base insertion mutation between exons 15 and 16 in mRNA, and to establish a DNA diagnostic test, we isolated and sequenced the genomic DNA surrounding each exon. The absence of exon 7 or 13 in ASS mRNA resulted from abnormal splicing caused by a single base change in the intron region: IVS-6^–2 (a transition of A to G at the second nucleotide position within the 3 splice cleavage site of intron 6) and IVS-13⁺⁵ (a transition of G to A at the fifth nucleotide position within the 5 splice cleavage site of intron 13), respectively. The IVS-6^–2 mutation resulted in the creation of an MspI restriction site. DNA diagnostic analysis of 33 Japanese alleles with classical citrullinemia showed that 19 alleles had the IVS-6^–2 mutation (over 50% of the mutated alleles in Japanese patients). It was thus confirmed that one mutation is predominant in Japan. This differs from the situation in the USA where there is far greater heterogeneity. The insertion mutation in mRNA on the other hand resulted from abnormal splicing caused by a 13-bp deletion at the splice-junction between exon 15 and intron 15. The deletion had a short direct repeat (CTCAGG) at the breakpoint junction and presumably resulted from slipped mispairing.     

Missense,nonsense, and neutral mutations define juxtaposed regulatory elements of splicing in cystic fibrosis transmembrane regulator exon 9

Exonic sequence variations may induce exon inclusion or exclusion from the mature mRNA by disrupting exonic regulatory elements and/or by affecting a nuclear reading frame scanning mechanism. We have carried out a systematic study of the effect on cystic fibrosis transmembrane regulator exon 9 splicing of natural and site-directed sequence mutations. We have observed that changes in the splicing pattern were not related to the creation of premature termination codons, a fact that indicates the lack of a significant nuclear check of the reading frame in this system. In addition, the splice pattern could not be predicted by available Ser/Arg protein matrices score analysis. An extensive site-directed mutagenesis of the 3' portion of the exon has identified two juxtaposed splicing enhancer and silencer elements. The study of double mutants at these regulatory elements showed a complex regulatory activity. For example, one natural mutation (146C) enhances exon inclusion and overrides all of the downstream silencing mutations except for a C to G transversion (155G). This unusual effect is explained by the creation of a specific binding site for the inhibitory splicing factor hnRNPH. In fact, on the double mutant 146C-155G, the silencing effect is dominant. These results indicate a strict dependence between the two juxtaposed enhancer and silencer sequences and show that many point mutations in these elements cause changes in splicing efficiency by different mechanisms.     

Silencer elements as possible inhibitors of pseudoexon splicing

Human pre-mRNAs contain a definite number of exons and several pseudoexons which are located within intronic regions. We applied a computational approach to address the question of how pseudoexons are neglected in favor of exons and to possibly identify sequence elements preventing pseudoexon splicing. A search for possible splicing silencers was carried out on a pseudoexon selection that resembled exons in terms of splice site strength and exon splicing enhancer (ESE) representation; three motifs were retrieved through hexamer composition comparisons. One of these functions as a powerful silencer in transfection-based splicing assays and matches a previously identified silencer sequence with hnRNP H binding ability. The other two motifs are novel and failed to induce skipping of a constitutive exon, indicating that they might act as weak repressors or in synergy with other unidentified elements. All three motifs are enriched in pseudoexons compared with intronic regions and display higher frequencies in intronless gene-coding sequences compared with exons. We consider that a subpopulation of pseudoexons might rely on negative regulators for splicing repression; this hypothesis, if experimentally verified, might improve our understanding of exonic splicing regulatory sequences and provide the identification of a novel mutation target for human genetic diseases.     

Characterization of exon skipping mutants of the COP1 gene from Arabidopsis

The removal of introns from pre-mRNA requires accurate recognition and selection of the intron splice sites. Mutations which alter splice site selection and which lead to skipping of specific exons are indicative of intron/exon recognition mechanisms involving an exon definition process. In this paper, three independent mutants to the COP1 gene in Arabidopsis which show exon skipping were identified and the mutations which alter the normal splicing pattern were characterized. The mutation in cop1–1 was a G→A change 4 nt upstream from the 3′ splice site of intron 5, while the mutation in cop1–2 was a G→A at the first nucleotide of intron 6, abolishing the conserved G within the 5′ splice site consensus. The effect of these mutations was skipping of exon 6. The mutation in cop1–8 was G→A in the final nucleotide of intron 10 abolishing the conserved G within the 3′ splice site consensus and leading to skipping of exon 11. The splicing patterns surrounding exons 6 and 11 of COP1 in these three mutant lines of Arabidopsis provide evidence for exon definition mechanisms operating in plant splicing.     

Human Splicing Finder: an online bioinformatics tool to predict splicing signals

Thousands of mutations are identified yearly. Although many directly affect protein expression, an increasing proportion of mutations is now believed to influence mRNA splicing. They mostly affect existing splice sites, but synonymous, non-synonymous or nonsense mutations can also create or disrupt splice sites or auxiliary cis-splicing sequences. To facilitate the analysis of the different mutations, we designed Human Splicing Finder (HSF), a tool to predict the effects of mutations on splicing signals or to identify splicing motifs in any human sequence. It contains all available matrices for auxiliary sequence prediction as well as new ones for binding sites of the 9G8 and Tra2-β Serine-Arginine proteins and the hnRNP A1 ribonucleoprotein. We also developed new Position Weight Matrices to assess the strength of 5′ and 3′ splice sites and branch points. We evaluated HSF efficiency using a set of 83 intronic and 35 exonic mutations known to result in splicing defects. We showed that the mutation effect was correctly predicted in almost all cases. HSF could thus represent a valuable resource for research, diagnostic and therapeutic (e.g. therapeutic exon skipping) purposes as well as for global studies, such as the GEN2PHEN European Project or the Human Variome Project.     

An exonic splicing enhancer offsets the atypical GU-rich 3' splice site of human apolipoprotein A-II exon 3

Human apolipoprotein A-II (apoA-II) intron 2/exon 3 junction shows a peculiar tract of alternating pyrimidines and purines (GU tract) that makes the acceptor site deviate significantly from the consensus. However, apoA-II exon 3 is constitutively included in mRNA. We have studied this unusual exon definition by creating a construct with the genomic fragment encompassing the whole gene from apoA-II and its regulatory regions. Transient transfections in Hep3B cells have shown that deletion or replacement of the GU repeats at the 3' splice site resulted in a decrease of apoA-II exon 3 inclusion, indicating a possible role of the GU tract in splicing. However, a 3' splice site composed of the GU tract in heterologous context, such as the extra domain A of human fibronectin or cystic fibrosis transmembrane conductance regulator exon 9, resulted in total skipping of the exons. Next, we identified the exonic cis-acting elements that may affect the splicing efficiency of apoA-II exon 3 and found that the region spanning from nucleotide 87 to 113 of human apoA-II exon 3 is essential for its inclusion in the mRNA. Overlapping deletions and point mutations (between nucleotides 91 and 102) precisely defined an exonic splicing enhancer (ESEwt). UV cross-linking assays followed by immunoprecipitation with anti-SR protein monoclonal antibodies showed that ESEwt, but not mutated ESE RNA, was able to bind both alternative splicing factor/splicing factor 2 and SC35. Furthermore, overexpression of both splicing factors enhanced exon 3 inclusion. These results show that this protein-ESE interaction is able to promote the incorporation of exon 3 in mRNA and suggest that they can rescue the splicing despite the noncanonical 3' splice site.     

Gag-processing defect of human immunodeficiency virus type 1 integrase E246 and G247 mutants is caused by activation of an overlapping 5' splice site

We have previously described several human immunodeficiency virus type 1 (HIV-1) mutants that are characterized by an excessive-RNA-splicing phenotype and reduced virus particle production. In one of these mutants (NLD2up), the sequence of 5′ splice site D2 was changed to a consensus splice donor site. This splice site overlaps the HIV-1 integrase reading frame, and thus, the NLD2up mutant also bears a G-to-W change at amino acid 247 of the integrase. A previously described E-to-K mutant at position 246 of the C-terminal domain of the integrase, which resulted in a G-to-A mutation at the +3 position of overlapping splice donor D2 (NLD2A3), was also shown to affect virus particle production and Gag protein processing. By using second-site mutations to revert the excessive-splicing phenotype, we show that the effects on Gag protein processing and virus particle production of both the NLD2up and NLD2A3 mutants are caused by excessive viral RNA splicing due to the activation of the overlapping 5′ splice site and not to the changes in the integrase protein. Both integrase protein mutations, however, are lethal for virus infectivity. These studies suggest that changes in the usage of overlapping splice sites may be a possible alternative explanation for a defective virus phenotype resulting from changes in protein-coding sequences or in the nucleotide sequence during codon optimization.     

5' cleavage site in eukaryotic pre-mRNA splicing is determined by the overall 5' splice region, not by the conserved 5' GU

We have generated all possible single point mutations of the invariant 5' GT of the large beta-globin intron and determined their effect on splicing in vitro. None of the mutants prevented cleavage in the 5' splice region, but many reduced or abolished exon joining. The mutations GT----TT and GT----CT resulted in a shift of the 5' cleavage site on nucleotide upstream; in the case of the mutation GT----TT, this shift was reverted by a second site mutation within the 5' splice region. Our results suggest that the 5' cleavage site is determined not by the conserved GU sequence but by the 5' splice region as a whole, most probably via base-pairing to the 5' end of the U1 snRNA.