Trans splicing of mRNA precursors in vitro

Counteracting the Common Shwachman-Diamond Syndrome-Causing SBDS c.258+2T>C Mutation by RNA Therapeutics and Base/Prime Editing

Shwachman-Diamond syndrome (SDS) represents one of the most common inherited bone marrow failure syndromes and is mainly caused by SBDS gene mutations. Only supportive treatments are available, with hematopoietic cell transplantation required when marrow failure occurs. Among all causative mutations, the SBDS c.258+2T>C variant at the 5' splice site (ss) of exon 2 is one of the most frequent. Here, we investigated the molecular mechanisms underlying aberrant SBDS splicing and showed that SBDS exon 2 is dense in splicing regulatory elements and cryptic splice sites, complicating proper 5'ss selection. Studies ex vivo and in vitro demonstrated that the mutation alters splicing, but it is also compatible with tiny amounts of correct transcripts, which would explain the survival of SDS patients. Moreover, for the first time for SDS, we explored a panel of correction approaches at the RNA and DNA levels and provided experimental evidence that the mutation effect can be partially counteracted by engineered U1snRNA, trans-splicing, and base/prime editors, ultimately leading to correctly spliced transcripts (from barely detectable to 2.5-5.5%). Among them, we propose DNA editors that, by stably reverting the mutation and potentially conferring positive selection to bone-marrow cells, could lead to the development of an innovative SDS therapy.

Trans-spliced mRNA products produced from circRNA expression vectors

Circular (circ) RNA expression vectors are used as a method of identifying and characterizing RNA sequences that harbor internal ribosome entry site (IRES) activity. During the course of developing a vector series tailored for IRES discovery, we found evidence for the occurrence of trans-spliced mRNAs arising when sequences with promoter activity were embedded between the upstream CTD and downstream NTD exons of the pre-mRNA. These trans-spliced products regenerate the same open reading frame expected from a circRNA and can lead to false-positive signals in screens relying on circRNA expression vectors for IRES discovery. Our results caution against interpretations of IRES activity solely based on results obtained from circRNA expression vectors.

Fusion Genes and RNAs in Cancer Development

Fusion RNAs are a hallmark of some cancers. They result either from chromosomal rearrangements or from splicing mechanisms that are non-chromosomal rearrangements. Chromosomal rearrangements that result in gene fusions are particularly prevalent in sarcomas and hematopoietic malignancies; they are also common in solid tumors. The splicing process can also give rise to more complex RNA patterns in cells. Gene fusions frequently affect tyrosine kinases, chromatin regulators, or transcription factors, and can cause constitutive activation, enhancement of downstream signaling, and tumor development, as major drivers of oncogenesis. In addition, some fusion RNAs have been shown to function as noncoding RNAs and to affect cancer progression. Fusion genes and RNAs will therefore become increasingly important as diagnostic and therapeutic targets for cancer development. Here, we discuss the function, biogenesis, detection, clinical relevance, and therapeutic implications of oncogenic fusion genes and RNAs in cancer development. Further understanding the molecular mechanisms that regulate how fusion RNAs form in cancers is critical to the development of therapeutic strategies against tumorigenesis.

Therapeutic applications of trans-splicing

BACKGROUND

RNA trans-splicing joins exons from different pre-mRNA transcripts to generate a chimeric product. Trans-splicing can also occur at the protein level, with split inteins mediating the ligation of separate gene products to generate a mature protein.

SOURCES OF DATA

Comprehensive literature search of published research papers and reviews using Pubmed.

AREAS OF AGREEMENT

Trans-splicing techniques have been used to target a wide range of diseases in both in vitro and in vivo models, resulting in RNA, protein and functional correction.

AREAS OF CONTROVERSY

Off-target effects can lead to therapeutically undesirable consequences. In vivo efficacy is typically low, and delivery issues remain a challenge.

GROWING POINTS

Trans-splicing provides a promising avenue for developing novel therapeutic approaches. However, much more research needs to be done before developing towards preclinical studies.

AREAS TIMELY FOR DEVELOPING RESEARCH

Increasing trans-splicing efficacy and specificity by rational design, screening and competitive inhibition of endogenous cis-splicing.

RNA structures in alternative splicing and back-splicing

Alternative splicing greatly expands the transcriptomic and proteomic diversities related to physiological and developmental processes in higher eukaryotes. Splicing of long noncoding RNAs, and back- and trans- splicing further expanded the regulatory repertoire of alternative splicing. RNA structures were shown to play an important role in regulating alternative splicing and back-splicing. Application of novel sequencing technologies made it possible to identify genome-wide RNA structures and interaction networks, which might provide new insights into RNA splicing regulation in vitro to in vivo. The emerging transcription-folding-splicing paradigm is changing our understanding of RNA alternative splicing regulation. Here, we review the insights into the roles and mechanisms of RNA structures in alternative splicing and back-splicing, as well as how disruption of these structures affects alternative splicing and then leads to human diseases. This article is categorized under: RNA Processing > Splicing Regulation/Alternative Splicing RNA Structure and Dynamics > Influence of RNA Structure in Biological Systems.

Integrative transcriptome sequencing reveals extensive alternative trans-splicing and cis-backsplicing in human cells

Transcriptionally non-co-linear (NCL) transcripts can originate from trans-splicing (trans-spliced RNA; 'tsRNA') or cis-backsplicing (circular RNA; 'circRNA'). While numerous circRNAs have been detected in various species, tsRNAs remain largely uninvestigated. Here, we utilize integrative transcriptome sequencing of poly(A)- and non-poly(A)-selected RNA-seq data from diverse human cell lines to distinguish between tsRNAs and circRNAs. We identified 24,498 NCL events and found that a considerable proportion (20-35%) of them arise from both tsRNAs and circRNAs, representing extensive alternative trans-splicing and cis-backsplicing in human cells. We show that sequence generalities of exon circularization are also observed in tsRNAs. Recapitulation of NCL RNAs further shows that inverted Alu repeats can simultaneously promote the formation of tsRNAs and circRNAs. However, tsRNAs and circRNAs exhibit quite different, or even opposite, expression patterns, in terms of correlation with the expression of their co-linear counterparts, expression breadth/abundance, transcript stability, and subcellular localization preference. These results indicate that tsRNAs and circRNAs may play different regulatory roles and analysis of NCL events should take the joint effects of different NCL-splicing types and joint effects of multiple NCL events into consideration. This study describes the first transcriptome-wide analysis of trans-splicing and cis-backsplicing, expanding our understanding of the complexity of the human transcriptome.

Efficient system for upstream mRNA trans-splicing to generate covalent, head-to-tail, protein multimers

We present a plasmid-based system in which upstream trans-splicing efficiently generates mRNAs that encode head-to-tail protein multimers. In this system, trans-splicing occurs between one of two downstream splice donors in the sequence encoding a C-terminal V5 epitope tag and an upstream splice acceptor in the 5′ region of the pCS2(+) host plasmid. Using deletion and fusion constructs of the DUX4 protein as an example, we found that this system produced trans-spliced mRNAs in which coding regions from independent transcripts were fused in phase such that covalent head-to-tail protein multimers were translated. For a cDNA of ~450 bp, about half of the expressed proteins were multimeric, with the efficiency of trans-splicing and extent of multimer expression decreasing as cDNA length increased. This system generated covalent heterodimeric proteins upon co-transfections of plasmids encoding separate proteins and did not require a long complementary binding domain to position mRNAs for trans-splicing. This plasmid-based trans-splicing system is adaptable to multiple gene delivery systems, and it presents new opportunities for investigating molecular mechanisms of trans-splicing, generating covalent protein multimers with novel functions within cells, and producing mRNAs encoding large proteins from split precursors.

Spliceosome-Mediated Pre-mRNA trans-Splicing Can Repair CEP290 mRNA

Ocular gene therapy with recombinant adeno-associated virus (AAV) has shown vector-mediated gene augmentation to be safe and efficacious in the retina in one set of diseases (retinitis pigmentosa and Leber congenital amaurosis (LCA) caused by RPE65 deficiency), with excellent safety profiles to date and potential for efficacy in several additional diseases. However, size constraints imposed by the packaging capacity of the AAV genome restrict application to diseases with coding sequence lengths that are less than 5,000 nt. The most prevalent retinal diseases with monogenic inheritance are caused by mutations in genes that exceed this capacity. Here, we designed a spliceosome mediated pre-mRNA trans-splicing strategy to rescue expression of CEP290, which is associated with Leber congenital amaurosis type 10 (LCA10) and several syndromic diseases including Joubert syndrome. We used this reagent to demonstrate editing of CEP290 in cell lines in vitro and in vivo in a mini-gene mouse model. This study is the first to show broad editing of CEP290 transcripts and in vivo proof of concept for editing of CEP290 transcripts in photoreceptors and paves the way for future studies evaluating therapeutic effects.

Trans-spliced long non-coding RNA: an emerging regulator of pluripotency

With dual capacities for unlimited self-renewal and pluripotent differentiation, pluripotent stem cells (PSCs) give rise to many cell types in our body and PSC culture systems provide an unparalleled opportunity to study early human development and disease. Accumulating evidence indicates that the molecular mechanisms underlying pluripotency maintenance in PSCs involve many factors. Among these regulators, recent studies have shown that long non-coding RNAs (lncRNAs) can affect the pluripotency circuitry by cooperating with master pluripotency-associated factors. Additionally, trans-spliced RNAs, which are generated by combining two or more pre-mRNA transcripts to produce a chimeric RNA, have been identified as regulators of various biological processes, including human pluripotency. In this review, we summarize and discuss current knowledge about the roles of lncRNAs, including trans-spliced lncRNAs, in controlling pluripotency.

RNA Structure Design Improves Activity and Specificity of trans-Splicing-Triggered Cell Death in a Suicide Gene Therapy Approach

Spliceosome-mediated RNA trans-splicing enables correction or labeling of pre-mRNA, but therapeutic applications are hampered by issues related to the activity and target specificity of trans-splicing RNA (tsRNA). We employed computational RNA structure design to improve both on-target activity and specificity of tsRNA in a herpes simplex virus thymidine kinase/ganciclovir suicide gene therapy approach targeting alpha fetoprotein (AFP), a marker of hepatocellular carcinoma (HCC) or human papillomavirus type 16 (HPV-16) pre-mRNA. While unstructured, mismatched target binding domains significantly improved 3′ exon replacement (3’ER), 5′ exon replacement (5’ER) correlated with the thermodynamic stability of the tsRNA 3′ end. Alternative on-target trans-splicing was found to be a prevalent event. The specificity of trans-splicing with the intended target splice site was improved 10-fold by designing tsRNA that harbors secondary target binding domains shielding alternative on-target and blinding off-target splicing events. Such rationally designed suicide RNAs efficiently triggered death of HPV-16-transduced or hepatoblastoma-derived human tissue culture cells without evidence for off-target cell killing. Highest cell death activities were observed with novel dual-targeting tsRNAs programmed for trans-splicing toward AFP and a second HCC pre-mRNA biomarker. Our observations suggest trans-splicing represents a promising approach to suicide gene therapy.

Circular RNA Splicing

Circular RNAs (circRNAs) are covalently closed single-stranded RNA molecules derived from exons by alternative mRNA splicing. Circularization of single-stranded RNA molecules was already described in 1976 for viroids in plants. Since then several additional types of circular RNAs in many species have been described such as the circular single-stranded RNA genome of the hepatitis delta virus (HDV) or circular RNAs as products or intermediates of tRNA and rRNA maturation in archaea. CircRNAs are generally formed by covalent binding of the 5' site of an upstream exon with the 3' of the same or a downstream exon. Meanwhile, two different models of circRNA biogenesis have been described, the lariat or exon skipping model and the direct backsplicing model. In the lariat model, canonical splicing occurs before backsplicing, whereas in the direct backsplicing model, the circRNA is generated first. In this chapter, we will review the formation of circular RNAs and highlight the derivation of different types of circular RNAs.

Gene Therapy via Trans-Splicing for LMNA-Related Congenital Muscular Dystrophy

We assessed the potential of Lmna-mRNA repair by spliceosome-mediated RNA trans-splicing as a therapeutic approach for LMNA-related congenital muscular dystrophy. This gene therapy strategy leads to reduction of mutated transcript expression for the benefit of corresponding wild-type (WT) transcripts. We developed 5′-RNA pre-trans-splicing molecules containing the first five exons of Lmna and targeting intron 5 of Lmna pre-mRNA. Among nine pre-trans-splicing molecules, differing in the targeted sequence in intron 5 and tested in C2C12 myoblasts, three induced trans-splicing events on endogenous Lmna mRNA and confirmed at protein level. Further analyses performed in primary myotubes derived from an LMNA-related congenital muscular dystrophy (L-CMD) mouse model led to a partial rescue of the mutant phenotype. Finally, we tested this approach in vivo using adeno-associated virus (AAV) delivery in newborn mice and showed that trans-splicing events occurred in WT mice 50 days after AAV delivery, although at a low rate. Altogether, while these results provide the first evidence for reprogramming LMNA mRNA in vitro, strategies to improve the rate of trans-splicing events still need to be developed for efficient application of this therapeutic approach in vivo.

Chimeric RNAs and their implications in cancer

Chimeric RNAs have been believed to be solely produced by gene fusions resulting from chromosomal rearrangement, thus unique features of cancer. Detected chimeric RNAs have also been viewed as surrogates for the presence of gene fusions. However, more and more research has demonstrated that chimeric RNAs in general are not a hallmark of cancer, but rather widely present in non-cancerous cells and tissues. At the same time, they may be produced by other mechanisms other than chromosomal rearrangement. The field of non-canonical chimeric RNAs is still in its infancy, with many challenges ahead, including the lack of a unified terminology. However, we believe that these non-canonical chimeric RNAs will have significant impacts in cancer detection and treatment.

Early history of circular RNAs, children of splicing

In this commentary we briefly summarize early work on circular RNAs derived from spliceosome mediated circularization. We highlight how this early work inspired work on the basic mechanisms of nuclear RNA splicing, the possible function of circular RNAs and the potential uses of circular RNAs as tools in biomedicine. Recent developments in the study of circular RNAs, summarized in this volume, have brought these questions back to the foreground.

Toward RNA Repair of Diamond Blackfan Anemia Hematopoietic Stem Cells

Diamond blackfan anemia (DBA) is a well-known inherited bone marrow failure syndrome mostly caused by mutations in ribosomal protein (RP) genes but also rarely in the hematopoietic transcription factor gene, GATA1, or TSR2, a ribosomal protein (Rps26) chaperone gene. About 25% of patients have heterozygous mutations in the RPS19 gene, which leads to haploinsufficiency of Rps19 protein in most cases. However, some RPS19 missense mutations appear to act in a dominant negative fashion. DBA typically leads to a hypoplastic anemia that becomes apparent during the first year of life, and standard treatment includes steroids or red blood cell transfusions, each modality having attendant side effects. The only curative therapy is allogeneic stem-cell transplantation, but this option is limited to patients with a histocompatible donor. DBA-mutant embryonic, induced pluripotent, and hematopoietic stem cells all exhibit growth abnormalities that can be corrected by DNA gene transfer, suggesting the possibility of ex vivo autologous gene therapy. The authors have been interested in the application of spliceosome-mediated mRNA trans-splicing (SMaRT) technology to RNA repair of DBA stem cells. Compared with gene replacement or other RNA re-programming approaches, SMaRT has several potential advantages. First, delivery of the entire normal cDNA is unnecessary, thus minimizing the overall size of the construct for packaging into a viral delivery vector. Second, RNA transcription of the corrected gene relies on the cell's endogenous transcriptional, processing, and regulatory machinery, thereby ensuring faithful and contextual expression. Third, RNA trans-splicing employs the endogenous spliceosome enzymatic machinery present in nearly all cells. Fourth, RNA trans-splicing converts mutant transcripts into therapeutically useful mRNA, and thus may be capable of treating disorders caused by dominant negative mutations. This review critically assesses prospects for both gene and RNA repair in DBA stem cells.

mRNA trans-splicing in gene therapy for genetic diseases

Spliceosome-mediated RNA trans-splicing, or SMaRT, is a promising strategy to design innovative gene therapy solutions for currently intractable genetic diseases. SMaRT relies on the correction of mutations at the post-transcriptional level by modifying the mRNA sequence. To achieve this, an exogenous RNA is introduced into the target cell, usually by means of gene transfer, to induce a splice event in trans between the exogenous RNA and the target endogenous pre-mRNA. This produces a chimeric mRNA composed partly of exons of the latter, and partly of exons of the former, encoding a sequence free of mutations. The principal challenge of SMaRT technology is to achieve a reaction as complete as possible, i.e., resulting in 100% repairing of the endogenous mRNA target. The proof of concept of SMaRT feasibility has already been established in several models of genetic diseases caused by recessive mutations. In such cases, in fact, the repair of only a portion of the mutant mRNA pool may be sufficient to obtain a significant therapeutic effect. However in the case of dominant mutations, the target cell must be freed from the majority of mutant mRNA copies, requiring a highly efficient trans-splicing reaction. This likely explains why only a few examples of SMaRT approaches targeting dominant mutations are reported in the literature. In this review, we explain in details the mechanism of trans-splicing, review the different strategies that are under evaluation to lead to efficient trans-splicing, and discuss the advantages and limitations of SMaRT. WIREs RNA 2016, 7:487-498. doi: 10.1002/wrna.1347 For further resources related to this article, please visit the WIREs website.

NCLscan: accurate identification of non-co-linear transcripts (fusion, trans-splicing and circular RNA) with a good balance between sensitivity and precision

Analysis of RNA-seq data often detects numerous ‘non-co-linear’ (NCL) transcripts, which comprised sequence segments that are topologically inconsistent with their corresponding DNA sequences in the reference genome. However, detection of NCL transcripts involves two major challenges: removal of false positives arising from alignment artifacts and discrimination between different types of NCL transcripts (trans-spliced, circular or fusion transcripts). Here, we developed a new NCL-transcript-detecting method (‘NCLscan’), which utilized a stepwise alignment strategy to almost completely eliminate false calls (>98% precision) without sacrificing true positives, enabling NCLscan outperform 18 other publicly-available tools (including fusion- and circular-RNA-detecting tools) in terms of sensitivity and precision, regardless of the generation strategy of simulated dataset, type of intragenic or intergenic NCL event, read depth of coverage, read length or expression level of NCL transcript. With the high accuracy, NCLscan was applied to distinguishing between trans-spliced, circular and fusion transcripts on the basis of poly(A)- and nonpoly(A)-selected RNA-seq data. We showed that circular RNAs were expressed more ubiquitously, more abundantly and less cell type-specifically than trans-spliced and fusion transcripts. Our study thus describes a robust pipeline for the discovery of NCL transcripts, and sheds light on the fundamental biology of these non-canonical RNA events in human transcriptome.

A conserved intronic U1 snRNP-binding sequence promotes trans-splicing in Drosophila

Gao et al. investigate mod(mdg4), a classic trans-spliced gene in Drosophila, and report that two critical RNA sequences in the middle of the last 5′ intron, TSA and TSB, promote trans-splicing of mod(mdg4). In TSA, a 13-nt core motif is conserved across Drosophila species and is essential and sufficient for trans-splicing, which binds U1 snRNP through strong base-pairing with U1 snRNA. In TSB, a conserved secondary structure acts as an enhancer. Deletions of TSA and TSB result in developmental defects in flies.

Unlike typical cis-splicing, trans-splicing joins exons from two separate transcripts to produce chimeric mRNA and has been detected in most eukaryotes. Trans-splicing in trypanosomes and nematodes has been characterized as a spliced leader RNA-facilitated reaction; in contrast, its mechanism in higher eukaryotes remains unclear. Here we investigate mod(mdg4), a classic trans-spliced gene in Drosophila, and report that two critical RNA sequences in the middle of the last 5′ intron, TSA and TSB, promote trans-splicing of mod(mdg4). In TSA, a 13-nucleotide (nt) core motif is conserved across Drosophila species and is essential and sufficient for trans-splicing, which binds U1 small nuclear RNP (snRNP) through strong base-pairing with U1 snRNA. In TSB, a conserved secondary structure acts as an enhancer. Deletions of TSA and TSB using the CRISPR/Cas9 system result in developmental defects in flies. Although it is not clear how the 5′ intron finds the 3′ introns, compensatory changes in U1 snRNA rescue trans-splicing of TSA mutants, demonstrating that U1 recruitment is critical to promote trans-splicing in vivo. Furthermore, TSA core-like motifs are found in many other trans-spliced Drosophila genes, including lola. These findings represent a novel mechanism of trans-splicing, in which RNA motifs in the 5′ intron are sufficient to bring separate transcripts into close proximity to promote trans-splicing.

Modular protein expression by RNA trans-splicing enables flexible expression of antibody formats in mammalian cells from a dual-host phage display vector

A recently described dual-host phage display vector that allows expression of immunoglobulin G (IgG) in mammalian cells bypasses the need for subcloning of phage display clone inserts to mammalian vectors for IgG expression in large antibody discovery and optimization campaigns. However, antibody discovery and optimization campaigns usually need different antibody formats for screening, requiring reformatting of the clones in the dual-host phage display vector to an alternative vector. We developed a modular protein expression system mediated by RNA trans-splicing to enable the expression of different antibody formats from the same phage display vector. The heavy-chain region encoded by the phage display vector is directly and precisely fused to different downstream heavy-chain sequences encoded by complementing plasmids simply by joining exons in different pre-mRNAs by trans-splicing. The modular expression system can be used to efficiently express structurally correct IgG and Fab fragments or other antibody formats from the same phage display clone in mammalian cells without clone reformatting.

Is an observed non-co-linear RNA product spliced in trans, in cis or just in vitro?

Global transcriptome investigations often result in the detection of an enormous number of transcripts composed of non-co-linear sequence fragments. Such ‘aberrant’ transcript products may arise from post-transcriptional events or genetic rearrangements, or may otherwise be false positives (sequencing/alignment errors or in vitro artifacts). Moreover, post-transcriptionally non-co-linear (‘PtNcl’) transcripts can arise from trans-splicing or back-splicing in cis (to generate so-called ‘circular RNA’). Here, we collected previously-predicted human non-co-linear RNA candidates, and designed a validation procedure integrating in silico filters with multiple experimental validation steps to examine their authenticity. We showed that >50% of the tested candidates were in vitro artifacts, even though some had been previously validated by RT-PCR. After excluding the possibility of genetic rearrangements, we distinguished between trans-spliced and circular RNAs, and confirmed that these two splicing forms can share the same non-co-linear junction. Importantly, the experimentally-confirmed PtNcl RNA events and their corresponding PtNcl splicing types (i.e. trans-splicing, circular RNA, or both sharing the same junction) were all expressed in rhesus macaque, and some were even expressed in mouse. Our study thus describes an essential procedure for confirming PtNcl transcripts, and provides further insight into the evolutionary role of PtNcl RNA events, opening up this important, but understudied, class of post-transcriptional events for comprehensive characterization.

Homologous SV40 RNA trans-splicing: Special case or prime example of viral RNA trans-splicing?

To date the Simian Virus 40 (SV40) is the only proven example of a virus that recruits the mechanism of RNA trans-splicing to diversify its sequences and gene products. Thereby, two identical viral transcripts are efficiently joined by homologous trans-splicing triggering the formation of a highly transforming 100 kDa super T antigen. Sequences of other viruses including HIV-1 and the human adenovirus type 5 were reported to be involved in heterologous trans-splicing towards cellular or viral sequences but the meaning of these events remains unclear. We computationally and experimentally investigated molecular features associated with viral RNA trans-splicing and identified a common pattern: Viral RNA trans-splicing occurs between strong cryptic or regular viral splice sites and strong regular or cryptic splice sites of the trans-splice partner sequences. The majority of these splice sites are supported by exonic splice enhancers. Splice sites that could compete with the trans-splicing sites for cis-splice reactions are weaker or inexistent. Finally, all but one of the trans-splice reactions seem to be facilitated by one or more complementary binding domains of 11 to 16 nucleotides in length which, however occur with a statistical probability close to one for the given length of the involved sequences. The chimeric RNAs generated via heterologous viral RNA trans-splicing either did not lead to fusion proteins or led to proteins of unknown function. Our data suggest that distinct viral RNAs are highly susceptible to trans-splicing and that heterologous viral trans-splicing, unlike homologous SV40 trans-splicing, represents a chance event.

Homologous SV40 RNA trans-splicing: a new mechanism for diversification of viral sequences and phenotypes

Simian Virus 40 (SV40) is a polyomavirus found in both monkeys and humans, which causes cancer in some animal models. In humans, SV40 has been reported to be associated with cancers but causality has not been proven yet. The transforming activity of SV40 is mainly due to its 94-kD large T antigen, which binds to the retinoblastoma (pRb) and p53 tumor suppressor proteins, and thereby perturbs their functions. Here we describe a 100 kD super T antigen harboring a duplication of the pRB binding domain that was associated with unusual high cell transformation activity and that was generated by a novel mechanism involving homologous RNA trans-splicing of SV40 early transcripts in transformed rodent cells. Enhanced trans-splice activity was observed in clones carrying a single point mutation in the large T antigen 5' donor splice site (ss). This mutation impaired cis-splicing in favor of an alternative trans-splice reaction via a cryptic 5'ss within a second cis-spliced SV40 pre-mRNA molecule and enabled detectable gene expression. Next to the cryptic 5'ss we identified additional trans-splice helper functions, including putative dimerization domains and a splice enhancer sequence. Our findings suggest RNA trans-splicing as a SV40-intrinsic mechanism that supports the diversification of viral RNA and phenotypes.

Detection of novel paraja ring finger 2-fer tyrosine kinase mRNA chimeras is associated with poor postoperative prognosis in non-small cell lung cancer

Previously, we reported that the overexpression of fer tyrosine kinase (FER), a non-receptor tyrosine kinase, is correlated with poor postoperative prognosis and cancer-cell survival in non-small cell lung cancer (NSCLC). In the present study, we further analyzed FER-overexpressed NSCLC cases and identified various patterns of chimeric mRNAs, composed of paraja ring finger 2 (PJA2) and FER. We detected no genomic rearrangements between PJA2 and FER and attributed these chimeric mRNAs to alterations at the transcriptome level: i.e., trans-splicing. Several chimeric patterns were detected concurrently in each patient, and the pattern sets varied among patients, although the pattern in which PJA2 exon 1 was fused to FER exon 3 (designated as Pe1-Fe3 mRNA) was detected constantly. Therefore, in a wide screening for PJA2-FER mRNAs in NSCLC, we focused on this chimeric pattern as a representative chimera. In analyses of 167 NSCLC samples, Pe1-Fe3 mRNA was identified in about 10% of the patients, and the presence of chimeric mRNA was significantly correlated with a high expression level of parental FER mRNA. Furthermore, we found that the detection of Pe1-Fe3 mRNA was correlated with poor postoperative survival periods in NSCLC, consistent with a previous finding in which FER overexpression was correlated with poor postoperative prognosis in NSCLC. This report is the first to suggest a correlation between chimeric mRNA and the expression level of parental mRNA. Furthermore, our findings may be clinically beneficial, suggesting that PJA2-FER mRNAs might serve as a novel prognostic biomarker in NSCLC.

Novel female-specific trans-spliced and alternative splice forms of dsx in the silkworm Bombyx mori

The Bombyx mori doublesex gene (Bmdsx) plays an important role in somatic sexual development. Its pre-mRNA splices in a sex-specific manner to generate two female-specific and one male-specific splice forms. The present study investigated six novel dsx variants generated by trans-splicing between female dsx transcripts and two additional novel genes, dsr1 and dsr2. Expression analysis indicated that Bmdsx-dsr1 represented splicing noise, whereas dsr2, which trans-spliced with dsx to generate five variants, regulated the expression of the female-specific B. mori dsx transcript Bmdsx(F)s. We unexpectedly found a novel exon 2n insertion during Bmdsx transcription, which did not influence the validity of the novel protein, BmDSX(F3). Ectopic expression of BmDSX(F3) repressed the pheromone-binding protein gene and the testis-specific gene A2 in males, and activated of the storage protein 1 gene. Our findings suggest that trans-splicing is a novel regulatory function of Bmdsx, which participates in female sexual development by regulating the expression of three BmDSX(F) proteins.

Intronic features that determine the selection of the 3' splice site

Most eukaryotic primary transcripts include segments, or introns, that will be accurately removed during RNA biogenesis. This process, known as pre-messenger RNA splicing, is catalyzed by the spliceosome, accurately selecting a set of intronic marks from others apparently equivalent. This identification is critical, as incorrectly spliced RNAs can be toxic for the organism. One of these marks, the dinucleotide AG, signals the intronic 3' end, or 3' splice site (ss). In this review we will focus on those intronic features that have an impact on 3' ss selection. These include the location and type of neighboring sequences, and their distance to the 3' end. We will see that their interplay is needed to select the right intronic end, and that this can be modulated by additional intronic elements that contribute to alternative splicing, whereby diverse RNAs can be generated from identical precursors. This complexity, still poorly understood, is fundamental for the accuracy of gene expression. In addition, a clear knowledge of 3' ss selection is needed to fully decipher the coding potential of genomes.

Trans-splicing in Higher Eukaryotes: Implications for Cancer Development?

Trans-splicing, the possibility of exons from distinct pre-mRNAs to join together, is still a concept in gene expression that is generally regarded of limited significance. However, recent work has provided evidence that in human tumors trans-splicing events may precede chromosomal rearrangements. In fact, it has been suggested that the trans-spliced molecules could act as “guides” that facilitate the genomic translocation. This perspective highlights the development of the ideas of trans-splicing in higher eukaryotes during the last 25 years, from a bizarre phenomenon to a biological event that is attaining stronger recognition.

Global analysis of trans-splicing in Drosophila

Precursor mRNA (pre-mRNA) splicing can join exons contained on either a single pre-mRNA (cis) or on separate pre-mRNAs (trans). It is exceedingly rare to have trans-splicing between protein-coding exons and has been demonstrated for only two Drosophila genes: mod(mdg4) and lola. It has also been suggested that trans-splicing is a mechanism for the generation of chimeric RNA products containing sequence from multiple distant genomic sites. Because most high-throughput approaches cannot distinguish cis- and trans-splicing events, the extent to which trans-splicing occurs between protein-coding exons in any organism is unknown. Here, we used paired-end deep sequencing of mRNA to identify genes that undergo trans-splicing in Drosophila interspecies hybrids. We did not observe credible evidence for the existence of chimeric RNAs generated by trans-splicing of RNAs transcribed from distant genomic loci. Rather, our data suggest that experimental artifacts are the source of most, if not all, apparent chimeric RNA products. We did, however, identify 80 genes that appear to undergo trans-splicing between homologous alleles and can be classified into three categories based on their organization: (i) genes with multiple 3' terminal exons, (ii) genes with multiple first exons, and (iii) genes with very large introns, often containing other genes. Our results suggest that trans-splicing between homologous alleles occurs more commonly in Drosophila than previously believed and may facilitate expression of architecturally complex genes.

Proximity-dependent and proximity-independent trans-splicing in mammalian cells

Most human pre-mRNAs are cis-spliced, removing introns and joining flanking exons of the same RNA molecule. However, splicing of exons present on separate pre-mRNA molecules can also occur. This trans-splicing reaction can be exploited by pre-trans-splicing molecules (PTMs), which are incapable of cis-splicing. PTM-mediated trans-splicing has been utilized to repair mutant RNAs as a novel approach to gene therapy. Herein we explore how the site of PTM expression influences trans-splicing activity. We stably inserted a PTM expression cassette into the genome of HEK293 cells, generating clonal lines with single, unique insertion sites. We analyzed trans-splicing to the gene where the PTM was integrated, as well as genes neighboring these loci. We observed some pre-mRNAs only serve as substrates for trans-splicing when they are expressed in immediate proximity to the PTM expression site. The need for PTMs to be in close proximity with pre-mRNAs to trans-splice with them is consistent with the observation that pre-mRNA cis-splicing occurs cotranscriptionally. Interestingly, we identified several cellular pre-mRNAs in one localized area that serve as trans-splicing substrates irrespective of the PTM expression site. Thus, we find multiple cellular pre-mRNAs require PTM expression in close proximity to trans-splice while others do not.

Putative alternative trans-splicing of leukocyte adhesion-GPCR pre-mRNAs generates functional chimeric receptors

The EGF-TM7 receptors, a subfamily of adhesion-GPCRs mostly restricted to leukocytes, are known to express multiple functional protein isoforms through extensive alternative cis-splicing. Here, we demonstrate that EGF-TM7 pre-mRNAs also undergo the rare trans-splicing, leading to the generation of functional chimeric receptors. RT-PCR and in silico analyses of EMR2 transcripts identified unique fragments containing the EGF-like motif 3 of a closely related EGF-TM7 gene, CD97, in addition to the alternative cis-spliced products. The sequence swapping is restricted to the EGF-3 exon, generating unique EMR2(1-2-3*-5) and EMR2(1-2-3*-4-5) molecules, which are functional in ligand-binding as the wild-type EMR2(1-2-3-4-5) and CD97(1-2-3-4-5) receptors. Our results suggest that human leukocytes employ trans-splicing as well as cis-splicing to increase the repertoire of functional adhesion-GPCRs.

Complementary intron sequence motifs associated with human exon repetition: a role for intragenic, inter-transcript interactions in gene expression

MOTIVATION

Exon repetition describes the presence of tandemly repeated exons in mRNA in the absence of duplications in the genome. The regulation of this process is not fully understood. We therefore investigated the entire flanking intronic sequences of exons involved in exon repetition for common sequence elements.

RESULTS

A computational analysis of 48 human single exon repetition events identified two common sequence motifs. One of these motifs is pyrimidine-rich and is more common in the upstream intron, whilst the other motif is highly enriched in purines and is more common in the downstream intron. As the two motifs are complementary to each other, they support a model by which exon repetition occurs as a result of trans-splicing between separate pre-mRNA transcripts from the same gene that are brought together during transcription by complementary intronic sequences. The majority of the motif instances overlap with the locations of mobile elements such as Alu elements. We explore the potential importance of complementary intron sequences in a rat gene that undertakes natural exon repetition in a strain specific manner. The possibility that distant complementary sequences can stimulate inter-transcript splicing during transcription suggests an unsuspected new role for potential secondary structures in endogenous genes.

Alternative trans-splicing: a novel mode of pre-mRNA processing

Alternative splicing is an important process contributing to proteome diversity without involving an increase in the number of genes. In some cases, alternative splicing is carried out under 'trans-mode', called alternative trans-splicing, in which exons located on separate pre-mRNA molecules are selectively joined to produce mature mRNAs encoding proteins with distinct structures and functions. However, it is not known how widespread or how frequently trans-splicing occurs in vivo. Recently, trans-allelic trans-splicing has been unambiguously demonstrated in Drosophila using a SNP (single nucleotide polymorphism) as a marker. In this review, we provide an overview of alternative trans-splicing in Drosophila and mammals, and discuss its mechanisms.

Spliceosome-mediated RNA trans-splicing

RNA repair or reprogramming is a new avenue for human gene therapy. Unlike conventional gene therapy, in which exogenous cDNAs are introduced into cells, RNA repair approaches, which are based on spliceosome-mediated pre-mRNA trans-splicing, trans-splicing ribozymes, and tRNA-splicing endonuclease, allow the correction of endogenous RNA species. Recently published accounts that in vivo phenotypic correction of a variety of inherited diseases can be achieved by RNA repair are encouraging. Nevertheless, the science of RNA repair for treatment of human diseases is just beginning and faces several scientific and technical challenges that must be addressed and surmounted. In this review, we summarize recent advances in spliceosome-mediated pre-mRNA trans-splicing. We also provide an update on the progress of this emerging technology toward the development of molecular therapy and diagnosis for human diseases and discuss the outstanding issues and challenges confronting RNA therapeutics.

Pre-mRNA trans-splicing: from kinetoplastids to mammals, an easy language for life diversity

Since the discovery that genes are split into intron and exons, the studies of the mechanisms involved in splicing pointed to presence of consensus signals in an attempt to generalize the process for all living cells. However, as discussed in the present review, splicing is a theme full of variations. The trans-splicing of pre-mRNAs, the joining of exons from distinct transcripts, is one of these variations with broad distribution in the phylogenetic tree. The biological meaning of this phenomenon is discussed encompassing reactions resembling a possible noise to mechanisms of gene expression regulation. All of them however, can contribute to the generation of life diversity.

Nanoarchaeum equitans creates functional tRNAs from separate genes for their 5'- and 3'-halves

Analysis of the genome sequence of the small hyperthermophilic archaeal parasite Nanoarchaeum equitans has not revealed genes encoding the glutamate, histidine, tryptophan and initiator methionine transfer RNA species. Here we develop a computational approach to genome analysis that searches for widely separated genes encoding tRNA halves that, on the basis of structural prediction, could form intact tRNA molecules. A search of the N. equitans genome reveals nine genes that encode tRNA halves; together they account for the missing tRNA genes. The tRNA sequences are split after the anticodon-adjacent position 37, the normal location of tRNA introns. The terminal sequences can be accommodated in an intervening sequence that includes a 12-14-nucleotide GC-rich RNA duplex between the end of the 5' tRNA half and the beginning of the 3' tRNA half. Reverse transcriptase polymerase chain reaction and aminoacylation experiments of N. equitans tRNA demonstrated maturation to full-size tRNA and acceptor activity of the tRNA(His) and tRNA(Glu) species predicted in silico. As the joining mechanism possibly involves tRNA trans-splicing, the presence of an intron might have been required for early tRNA synthesis.

In-cell generation of antibody single-chain Fv transcripts by targeted RNA trans-splicing

The humoral immune response propels the production of a diversified pool of antibodies with high affinity and selectivity for the eliciting antigen. Their isolation entails either B-cell cloning or the linking of authentic pairs of variable region genes encoding them. We hypothesized that targeted RNA trans-splicing (TS) inside the B-cell nucleus could be harnessed as a novel means to link both variable region genes and reconstitute genuine immune B-cell specificities. This could be accomplished by a special targeting gene harboring a peptide linker exon flanked by sequences capable of targeting both heavy (HC) and light chain (LC) transcripts. Following sequential trans-splicing reactions, the resulting RNA in each cell would encode the two variable regions, joined by the peptide linker. In this study, we examined genetic components and configurations required for the separate trans-splicing steps and for the combined two-step reactions. Using a model antibody, we show that in transiently transfected cells, we can target variable region exons through both their acceptor and donor splice sites, precisely joining an exon encoding a synthetic linker and the complementary variable region so as to form a single-chain Fv. We also demonstrate the accurate formation of single-chain Fv transcript as a result of trans-splicing of RNA synthesized from two chromosomal genes expressed by a stably transfected B-cell hybridoma. Our attempts to link the two variable region genes via a synthetic linker exon through sequential trans-splicing events were only successful with regard to both ends of the linker and to the 3' end of the light chain, but repeatedly resulted in a deletion at the 5' end of the joined heavy chain transcript. The implications of our findings on the potential application of trans-splicing for the isolation of useful antibodies are discussed.

Messenger RNA reprogramming by spliceosome-mediated RNA trans-splicing

In the human genome, the majority of protein-encoding genes are interrupted by introns, which are removed from primary transcripts by a macromolecular enzyme known as the spliceosome. Spliceosomes can constitutively remove all the introns in a primary transcript to yield a fully spliced mRNA or alternatively splice primary transcripts leading to the production of many different mRNAs from one gene. This review examines how spliceosomes can recombine two primary transcripts in trans to reprogram messenger RNAs.

Detection and analysis of spliced chimeric mRNAs in sequence databanks

We have developed a databank screening procedure, the In Silico Trans-splicing Retrieval System (ISTReS), to identify heterologous, spliced mRNAs with potential origin from chromosomal translocations, mRNA trans-splicing and multi-locus transcription. A parsing algorithm to screen cDNA versus genome Blast outputs was implemented. Key filtering criteria were Blast scores of > or =300, match lengths of > or =95% of the query sequences, junction of the two partners at exon-exon borders and concordant 'sense/sense' reading orientation. ISTReS was validated by the successful identification of bona fide chromosomal translocation-derived fusion transcripts in the HGI and RefSeq databanks. The performance of ISTReS was verified against recently identified chimeric antisense transcripts, where it revealed essentially no independent proof of antisense transcription and absence of exon-exon borders at the chimeric join, consistent with an artefactual origin. Analysis of the UNIGENE database revealed 21 742 chimeric sequences overall that correspond to approximately 1% of the database transcripts. Novel FOP-Rho GAP and methionyl tRNA synthetase-advillin chimeric mRNAs with the canonical features of heterologous-genes spliced-transcripts were identified among 246 chimeras from the RefSeq databank. This suggests a frequency of canonically-spliced chimeras of approximately 1% of all the hybrid sequences in current databanks. These findings demonstrate the efficiency of ISTReS and the overall feasibility of sequence/structure-based strategies to search for chimeric mRNAs candidate to derive from the splicing of heterologous transcripts.

The trans-spliced variants of Sp1 mRNA in rat

trans-Splicing is the biological reaction that generates a mature mRNA from separate strands of pre-mRNAs. Previously, we reported that the trans-splicing between the two Sp1 pre-mRNA strands produced an mRNA with the exon 3-2-3 alignment in human HepG2 cells. Here we describe the rat counterpart as well as a newly identified variant with the exon 3-3 alignment in cultured rat cells. A qualitative evaluation of such alignments in poly(A)(+) RNA-rich preparation showed that both alignments arose from trans-splicing rather than circularization of a single strand. The identification of the trans-spliced products in both rat and human raises the possibility that trans-splicing on Sp1 pre-mRNA is rather common to mammals. It was observed that the level of the trans-spliced variants varies in different rat organs.

Molecular mechanisms governing Pcdh-gamma gene expression: evidence for a multiple promoter and cis-alternative splicing model

The genomic architecture of protocadherin (Pcdh) gene clusters is remarkably similar to that of the immunoglobulin and T cell receptor gene clusters, and can potentially provide significant molecular diversity. Pcdh genes are abundantly expressed in the central nervous system. These molecules are primary candidates for establishing specific neuronal connectivity. Despite the extensive analyses of the genomic structure of both human and mouse Pcdh gene clusters, the definitive molecular mechanisms that control Pcdh gene expression are still unknown. Four theories have been proposed, including (1) DNA recombination followed by cis-splicing, (2) single promoter and cis-alternative splicing, (3) multiple promoters and cis-alternative splicing, and (4) multiple promoters and trans-splicing. Using a combination of molecular and genetic analyses, we evaluated the four models at the Pcdh-gamma locus. Our analysis provides evidence that the transcription of individual Pcdh-gamma genes is under the control of a distinct but related promoter upstream of each Pcdh-gamma variable exon, and posttranscriptional processing of each Pcdh-gamma transcript is predominantly mediated through cis-alternative splicing.

Intergenic mRNA molecules resulting from trans-splicing

Accumulated recent evidence is indicating that alternative splicing represents a generalized process that increases the complexity of human gene expression. Here we show that mRNA production may not necessarily be limited to single genes, as human liver also has the potential to produce a variety of hybrid cytochrome P450 3A mRNA molecules. The four known cytochrome P450 3A genes in humans, CYP3A4, CYP3A5, CYP3A7, and CYP3A43, share a high degree of similarity, consist of 13 exons with conserved exon-intron boundaries, and form a cluster on chromosome 7. The chimeric CYP3A mRNA molecules described herein are characterized by CYP3A43 exon 1 joined at canonical splice sites to distinct sets of CYP3A4 or CYP3A5 exons. Because the CYP3A43 gene is in a head-to-head orientation with the CYP3A4 and CYP3A5 genes, bypassing transcriptional termination can not account for the formation of hybrid CYP3A mRNAs. Thus, the mechanism generating these molecules has to be an RNA processing event that joins exons of independent pre-mRNA molecules, i.e. trans-splicing. Using quantitative real-time polymerase chain reaction, the ratio of one CYP3A43/3A4 intergenic combination was estimated to be approximately 0.15% that of the CYP3A43 mRNAs. Moreover, trans-splicing has been found not to interfere with polyadenylation. Heterologous expression of the chimeric species composed of CYP3A43 exon 1 joined to exons 2-13 of CYP3A4 revealed catalytic activity toward testosterone.

Heterologous HIV-nef mRNA trans-splicing: a new principle how mammalian cells generate hybrid mRNA and protein molecules

Heterologous trans-splicing is a messenger RNA (mRNA) processing mechanism, that joins RNA segments from separate transcripts to generate functional mRNA molecules. We present here for the first time experimental evidence that the proximal segment of the HIV-nef RNA segment can be trans-spliced to both viral (e.g. SV40 T-antigen) and cellular transcripts. Following either microinjection of in vitro synthesized HIV-nef and SV40 T-antigen pre-mRNA or transfection of the HIV-nef DNA into T-antigen positive cells (CV1-B3; Cos7), it was found that recipient cells synthesized HIV-nef/T-antigen hybrid mRNA and protein molecules. To generate the hybrid mRNA, the cells utilized the 5' cryptic splice sites of the HIV-nef (5'cry 66 and 5'cry 74) and the SV40 T/t-antigen 3' splice site. To demonstrate that heterologous trans-splicing also occurs between the HIV-nef RNA and cellular transcripts, a cDNA library was established from HIV-nef positive CV1-B3 cells (CV1-B3/13 cells) and screened for hybrid mRNA molecules. Reverse transcription-PCR and Northern blot analysis revealed that a significant portion of the HIV-nef transcript is involved in heterologous trans-splicing. To date, eight independent HIV-nef/cellular hybrid mRNA molecules have been identified. Five of these isolates contain segments from known cellular genes (KIAA1454, PTPkappa, Alu and transposon gene families), while three hybrid segments contain sequences of not yet known cellular genes (genes 1-3).

Promiscuity of pre-mRNA spliceosome-mediated trans splicing: a problem for gene therapy?

Trans splicing of messenger RNA has been used in experimental settings to replace mutant RNA sequences. We investigated the feasibility of utilizing trans splicing to replace a mutant RET protooncogene sequence known to inappropriately activate this tyrosine kinase receptor. We constructed a pre-trans-splicing molecule (PTM) consisting of a binding domain complementary to the target intron, the 3' splicing signal sequence (3'ss), derived from adenovirus major late transcript intron 1 and a molecular tag sequence. Accurately targeted trans splicing between the human RET exons and the PTM was demonstrated in NIH 3T3 cells cotransfected with the human RET minigene and the PTM. The efficiency of specific trans splicing was estimated to be no more than 15% in the cotransfection experiment. However, in addition to the targeted trans splicing, nontargeted trans splicing to RET exons was observed. Furthermore, the rapid amplification of 5' cDNA ends (5' RACE) analysis demonstrated that nontargeted trans splicing occurred with endogenously expressed pre-mRNAs in TT cells and that specific trans splicing to RET was a rare event. Attempts to reduce nonspecificity by the addition of a stem-loop to the trans-splicing construct designed to suppress nonspecific splicing failed to have the desired effect. These observations suggest that overexpression of a trans-splicing construct containing a 3'ss results in promiscuous trans splicing and raise significant questions about the specificity and usefulness of currently used trans-splicing approaches. In addition, these findings raise the possibility that nonspecific spliced products may be produced by a variety of gene therapy constructs.