Trans-splicing correction of tau isoform imbalance in a mouse model of tau mis-splicing

The six brain-specific TAU isoforms and their role in Alzheimer's disease and related neurodegenerative dementia syndromes

INTRODUCTION

Alternative splicing of the human MAPT gene generates six brain-specific TAU isoforms. Imbalances in the TAU isoform ratio can lead to neurodegenerative diseases, underscoring the need for precise control over TAU isoform balance. Tauopathies, characterized by intracellular aggregates of hyperphosphorylated TAU, exhibit extensive neurodegeneration and can be classified by the TAU isoforms present in pathological accumulations.

METHODS

A comprehensive review of TAU and related dementia syndromes literature was conducted using PubMed, Google Scholar, and preprint server.

RESULTS

While TAU is recognized as key driver of neurodegeneration in specific tauopathies, the contribution of the isoforms to neuronal function and disease development remains largely elusive.

DISCUSSION

In this review we describe the role of TAU isoforms in health and disease, and stress the importance of comprehending and studying TAU isoforms in both, physiological and pathological context, in order to develop targeted therapeutic interventions for TAU-associated diseases.

HIGHLIGHTS

MAPT splicing is tightly regulated during neuronal maturation and throughout life. TAU isoform expression is development-, cell-type and brain region specific. The contribution of TAU to neurodegeneration might be isoform-specific. Ineffective TAU-based therapies highlight the need for specific targeting strategies.

Tau reduction with artificial microRNAs modulates neuronal physiology and improves tauopathy phenotypes in mice

Abnormal tau accumulation is the hallmark of several neurodegenerative diseases, named tauopathies. Strategies aimed at reducing tau in the brain are promising therapeutic interventions, yet more precise therapies would require targeting specific nuclei and neuronal subpopulations affected by disease while avoiding global reduction of physiological tau. Here, we developed artificial microRNAs directed against the human MAPT mRNA to dwindle tau protein by engaging the endogenous RNA interference pathway. In human differentiated neurons in culture, microRNA-mediated tau reduction diminished neuronal firing without affecting neuronal morphology or impairing axonal transport. In the htau mouse model of tauopathy, we locally expressed artificial microRNAs in the prefrontal cortex (PFC), an area particularly vulnerable to initiating tau pathology in this model. Tau knockdown prevented the accumulation of insoluble and hyperphosphorylated tau, modulated firing activity of putative pyramidal neurons, and improved glucose uptake in the PFC. Moreover, such tau reduction prevented cognitive decline in aged htau mice. Our results suggest target engagement of designed tau-microRNAs to effectively reduce tau pathology, providing a proof of concept for a potential therapeutic approach based on local tau knockdown to rescue tauopathy-related phenotypes.

Oligomeric, phosphorylated, and truncated tau and spliceosome pathology within the entorhinal-hippocampal connectome across stages of Alzheimer's disease

Neurofibrillary tangles (NFTs) contain abnormally phosphorylated tau proteins, which spread within components of the medial temporal lobe (MTL) memory circuit in Alzheimer's disease (AD). Here, we used quantitative immunohistochemistry to determine the density of posttranslational oligomeric (TOC1 and TNT1), phosphorylated (AT8), and late truncated (TauC3) tau epitopes within the MTL subfields including entorhinal cortex (EC) layer II, subiculum, Cornu Ammonis (CA) subfields, and dentate gyrus (DG) in subjects who died with a clinical diagnosis of no cognitive impairment (NCI), mild cognitive impairment (MCI), and AD. We also examined whether alterations of the nuclear alternative splicing protein, SRSF2, are associated with tau pathology. Although a significant increase in TOC1, TNT1, and AT8 neuron density occurred in the EC in MCI and AD, subicular, DG granule cell, and CA1 and CA3 densities were only significantly higher in AD. TauC3 counts were not different between connectome regions and clinical groups. SRSF2 intensity in AT8-positive cells decreased significantly in all regions independent of the clinical groups examined. CA1 and subicular AT8, TauC3, and oligomeric densities correlated across clinical groups. EC AT8 counts correlated with CA subfields and subicular and DG values across clinical groups. Oligomeric and AT8 CA1, EC, and subicular density correlated with Braak stage. Decreased nuclear SRSF2 in the presence of cytoplasmic phosphorylated tau suggests a dual-hit process in NFT formation within the entorhinal hippocampal connectome during the onset of AD. Although oligomeric and phosphorylated tau follow a stereotypical pattern, clinical disease stage determined density of tau deposition and not anatomic location within the entorhinal-hippocampal connectome.

Differential splicing of neuronal genes in a Trem2*R47H mouse model mimics alterations associated with Alzheimer's disease

BACKGROUND

Molecular characterization of late-onset Alzheimer's disease (LOAD), the leading cause of age-related dementia, has revealed transcripts, proteins, and pathway alterations associated with disease. Assessing these postmortem signatures of LOAD in experimental model systems can further elucidate their relevance to disease origins and progression. Model organisms engineered with human genetic factors further link these signatures to disease-associated variants, especially when studies are designed to leverage homology across species. Here we assess differential gene splicing patterns in aging mouse models carrying humanized APOE4 and/or the Trem2*R47H variant on a C57BL/6J background. We performed a differential expression of gene (DEG) and differential splicing analyses on whole brain transcriptomes at multiple ages. To better understand the difference between differentially expressed and differentially spliced genes, we evaluated enrichment of KEGG pathways and cell-type specific gene signatures of the adult brain from each alteration type. To determine LOAD relevance, we compared differential splicing results from mouse models with multiple human AD splicing studies.

RESULTS

We found that differentially expressed genes in Trem2*R47H mice were significantly enriched in multiple AD-related pathways, including immune response, osteoclast differentiation, and metabolism, whereas differentially spliced genes were enriched for neuronal related functions, including GABAergic synapse and glutamatergic synapse. These results were reinforced by the enrichment of microglial genes in DEGs and neuronal genes in differentially spliced genes in Trem2*R47H mice. We observed significant overlap between differentially spliced genes in Trem2*R47H mice and brains from human AD subjects. These effects were absent in APOE4 mice and suppressed in APOE4.Trem2*R47H double mutant mice relative to Trem2*R47H mice.

CONCLUSIONS

The cross-species observation that alternative splicing observed in LOAD are present in Trem2*R47H mouse models suggests a novel link between this candidate risk gene and molecular signatures of LOAD in neurons and demonstrates how deep molecular analysis of new genetic models links molecular disease outcomes to a human candidate gene.

SMaRT modulation of tau isoforms rescues cognitive and motor impairments in a preclinical model of tauopathy

Tau is a microtubule-associated protein predominantly expressed in neurons, which participates in microtubule polymerization and axonal transport. Abnormal tau metabolism leads to neurodegenerative diseases named tauopathies, such as Alzheimer’s disease and frontotemporal dementia. The alternative splicing of exon 10 (E10) in the primary transcript produces tau protein isoforms with three (3R) or four (4R) microtubule binding repeats, which are found in equal amounts in the normal adult human brain. Several tauopathies are associated with abnormal E10 alternative splicing, leading to an imbalance between 3R and 4R isoforms, which underlies disease. Correction of such imbalance represents a potential disease-modifying therapy for those tauopathies. We have previously optimized a trans-splicing RNA reprogramming strategy to modulate the 3R:4R tau content in a mouse model of tauopathy related to tau mis-splicing (htau mice), and showed that local modulation of E10 inclusion in the prefrontal cortex prevents cognitive decline, neuronal firing impairments and hyperphosphorylated tau accumulation. Furthermore, local shifting of 3R–4R tau into the striatum of htau mice prevented motor coordination deficits. However, a major bottleneck of our previous work is that local splicing regulation was performed in young mice, before the onset of pathological phenotypes. Here we tested whether regulation of tau E10 splicing could rescue tau pathology phenotypes in htau mice, after the onset of cognitive and motor impairments, comparable to early stages of human tauopathies. To determine phenotypic time course and affected brain nuclei, we assessed htau mice using behavioural tests and microPET FDG imaging over time, similarly to diagnosis methods used in patients. Based on these analyses, we performed local delivery of pre-trans splicing molecules to regulate E10 inclusion either into the medial prefrontal cortex (mPFC) or the striatum at 6-month-old once behavioral phenotypes and metabolic changes were detected. Tau isoforms modulation into the mPFC restored cognitive performance in mice that previously showed mild to severe memory impairment while motor coordination deficit was rescued after striatal injection of trans-splicing molecules. Our data suggest that tau regulation could recover pathological phenotypes early after phenotypic onset, raising promising perspectives for the use of RNA based therapies in tauopathies related to MAPT abnormal splicing.

Emerging strategies for the genetic dissection of gene functions, cell types, and neural circuits in the mammalian brain

The mammalian brain is composed of a large number of highly diverse cell types with different molecular, anatomical, and functional features. Distinct cellular identities are generated during development under the regulation of intricate genetic programs and manifested through unique combinations of gene expression. Recent advancements in our understanding of the molecular and cellular mechanisms underlying the assembly, function, and pathology of the brain circuitry depend on the invention and application of genetic strategies that engage intrinsic gene regulatory mechanisms. Here we review the strategies for gene regulation on DNA, RNA, and protein levels and their applications in cell type targeting and neural circuit dissection. We highlight newly emerged strategies and emphasize the importance of combinatorial approaches. We also discuss the potential caveats and pitfalls in current methods and suggest future prospects to improve their comprehensiveness and versatility.

A new non-aggregative splicing isoform of human Tau is decreased in Alzheimer's disease

Tauopathies, including Alzheimer's disease (AD) and frontotemporal lobar degeneration with Tau pathology (FTLD-tau), are a group of neurodegenerative disorders characterized by Tau hyperphosphorylation. Post-translational modifications of Tau such as phosphorylation and truncation have been demonstrated to be an essential step in the molecular pathogenesis of these tauopathies. In this work, we demonstrate the existence of a new, human-specific truncated form of Tau generated by intron 12 retention in human neuroblastoma cells and, to a higher extent, in human RNA brain samples, using qPCR and further confirming the results on a larger database of human RNA-seq samples. Diminished protein levels of this new Tau isoform are found by Westernblotting in Alzheimer's patients' brains (Braak I n = 3; Braak II n = 6, Braak III n = 3, Braak IV n = 1, and Braak V n = 10, Braak VI n = 8) with respect to non-demented control subjects (n = 9), suggesting that the lack of this truncated isoform may play an important role in the pathology. This new Tau isoform exhibits similar post-transcriptional modifications by phosphorylation and affinity for microtubule binding, but more interestingly, is less prone to aggregate than other Tau isoforms. Finally, we present evidence suggesting this new Tau isoform could be linked to the inhibition of GSK3β, which would mediate intron 12 retention by modulating the serine/arginine rich splicing factor 2 (SRSF2). Our results show the existence of an important new isoform of Tau and suggest that further research on this less aggregation-prone Tau may help to develop future therapies for Alzheimer's disease and other tauopathies.

Fyn knockdown prevents levodopa-induced dyskinesia in a mouse model of Parkinson's disease

Dopamine replacement by levodopa is the most widely used therapy for Parkinson's disease (PD), however patients often develop side effects, known as levodopa-induced dyskinesia (LID), that usually need therapeutic intervention. There are no suitable therapeutic options for LID, except for the use of the NMDA receptor antagonist amantadine, which has limited efficacy. The NMDA receptor is indeed the most plausible target to manage LID in PD and recently the kinase Fyn- one of its key regulators- became a new putative molecular target involved in LID. The aim of this work was to reduce Fyn expression to alleviate LID in a mouse model of PD. We performed intra-striatal delivery of a designed micro-RNA against Fyn (miRNA-Fyn) in 6-OHDA-lesioned mice treated with levodopa. The miRNA-Fyn was delivered either before or after levodopa exposure to assess its ability to prevent or revert dyskinesia. Pre-administration of miRNA-Fyn reduced LID with a concomitant reduction of FosB-ΔFosB protein levels -a marker of LID- as well as decreased phosphorylation of the NR2B-NMDA subunit, which is a main target of Fyn. On the other hand, post L-DOPA delivery of miRNA-Fyn was less effective to revert already established dyskinesia, suggesting that early blocking of Fyn activity might be a more efficient therapeutic approach. Together, our results provide proof of concept about Fyn as a plausible therapeutic target to manage LID, and validate RNA silencing as a potential approach to locally reduce striatal Fyn, rising new perspectives for RNA therapy interventions in PD.Significance StatementLevodopa induced dyskinesia (LID) is an incapacitant side effect of treatment in Parkinson's disease (PD). LID is a therapeutic challenge, lacking an effective pharmacological treatment, except for the use of inhibitors of the NMDA receptor, which have limited efficacy and may trigger untoward side effects. The kinase Fyn is a key regulator of NMDA function and a potential therapeutic target to control LID. Here, we show that RNA interference therapy to reduce the amount of Fyn mRNA in the adult brain is effective to prevent LID in a mouse model of PD, setting the grounds for future biomedical interventions to manage LID in PD.

Tau mis-splicing correlates with motor impairments and striatal dysfunction in a model of tauopathy

Tauopathies are neurodegenerative diseases caused by the abnormal metabolism of the microtubule associated protein Tau, which is highly expressed in neurons and critically involved in microtubule dynamics. In the adult human brain, the alternative splicing of exon 10 in tau pre-mRNA produces equal amounts of protein isoforms with either three (3 R) or four (4 R) microtubule binding domains. Imbalance in the 3 R : 4 R tau ratio is associated with primary tauopathies that develop atypical parkinsonism, such as Progressive Supranuclear Palsy and Corticobasal Degeneration. Yet, the development of effective therapies for those pathologies is an unmet goal. Here we report motor coordination impairments in the htau mouse model of tauopathy which bear abnormal 3 R : 4 R tau isoforms contents, and contrariwise to TauKO mice, are unresponsive to L-DOPA. Preclinical-PET imaging, array tomography and electrophysiological analyses pointed the dorsal striatum as the candidate structure mediating such phenotypes. Indeed, local modulation of tau isoforms by RNA trans-splicing in the striata of adult htau mice, prevented motor coordination deficits and restored basal neuronal firing. Together, these results constitute readout that abnormal striatal tau-isoforms contents might lead to parkinsonian-like phenotypes and provide proof of concept that modulation of tau mis-splicing could be a plausible disease-modifying therapy for some primary tauopathies.

Alternative splicing in Alzheimer's disease

Alzheimer's disease (AD) is the most frequent neurodegenerative disorder in the elderly, occurring in approximately 20% of people older than 80. The molecular causes of AD are still poorly understood. However, recent studies have shown that Alternative Splicing (AS) is involved in the gene expression reprogramming associated with the functional changes observed in AD patients. In particular, mutations in cis-acting regulatory sequences as well as alterations in the activity and sub-cellular localization of trans-acting splicing factors and components of the spliceosome machinery are associated with splicing abnormalities in AD tissues, which may influence the onset and progression of the disease. In this review, we discuss the current molecular understanding of how alterations in the AS process contribute to AD pathogenesis. Finally, recent therapeutic approaches targeting aberrant AS regulation in AD are also reviewed.

Targeting Alternative Splicing as a Potential Therapy for Episodic Ataxia Type 2

Episodic ataxia type 2 (EA2) is an autosomal dominant neurological disorder characterized by paroxysmal attacks of ataxia, vertigo, and nausea that usually last hours to days. It is caused by loss-of-function mutations in CACNA1A, the gene encoding the pore-forming α1 subunit of P/Q-type voltage-gated Ca2+ channels. Although pharmacological treatments, such as acetazolamide and 4-aminopyridine, exist for EA2, they do not reduce or control the symptoms in all patients. CACNA1A is heavily spliced and some of the identified EA2 mutations are predicted to disrupt selective isoforms of this gene. Modulating splicing of CACNA1A may therefore represent a promising new strategy to develop improved EA2 therapies. Because RNA splicing is dysregulated in many other genetic diseases, several tools, such as antisense oligonucleotides, trans-splicing, and CRISPR-based strategies, have been developed for medical purposes. Here, we review splicing-based strategies used for genetic disorders, including those for Duchenne muscular dystrophy, spinal muscular dystrophy, and frontotemporal dementia with Parkinsonism linked to chromosome 17, and discuss their potential applicability to EA2.

Astrocyte senescence may drive alterations in GFAPα, CDKN2A p14ARF, and TAU3 transcript expression and contribute to cognitive decline

The accumulation of senescent cells in tissues is causally linked to the development of several age-related diseases; the removal of senescent glial cells in animal models prevents Tau accumulation and cognitive decline. Senescent cells can arise through several distinct mechanisms; one such mechanism is dysregulation of alternative splicing. In this study, we characterised the senescent cell phenotype in primary human astrocytes in terms of SA-β-Gal staining and SASP secretion, and then assessed splicing factor expression and candidate gene splicing patterns. Finally, we assessed associations between expression of dysregulated isoforms and premature cognitive decline in 197 samples from the InCHIANTI study of ageing, where expression was present in both blood and brain. We demonstrate here that senescent astrocytes secrete a modified SASP characterised by increased IL8, MMP3, MMP10, and TIMP2 but decreased IL10 levels. We identified significant changes in splicing factor expression for 10/20 splicing factors tested in senescent astrocytes compared with early passage cells, as well as dysregulation of isoform levels for 8/13 brain or senescence genes tested. Finally, associations were identified between peripheral blood GFAPα, TAU3, and CDKN2A (P14^ARF) isoform levels and mild or severe cognitive decline over a 3–7-year period. Our data are suggestive that some of the features of cognitive decline may arise from dysregulated splicing of important genes in senescent brain support cells, and that defects in alternative splicing or splicing regulator expression deserve exploration as points of therapeutic intervention in the future.

RNA Splicing and Disease: Animal Models to Therapies

Alternative splicing of pre-mRNA increases genetic diversity, and recent studies estimate that most human multiexon genes are alternatively spliced. If this process is not highly regulated and accurate, it leads to mis-splicing events, which may result in proteins with altered function. A growing body of work has implicated mis-splicing events in a range of diseases, including cancer, neurodegenerative diseases, and muscular dystrophies. Understanding the mechanisms that cause aberrant splicing events and how this leads to disease is vital for designing effective therapeutic strategies. In this review, we focus on advances in therapies targeting splicing, and highlight the animal models developed to recapitulate disease phenotypes as a model for testing these therapies.

Dementia Therapy Targeting Tau

Tau is a microtubule-associated tau proteins but it has also non-microtubular functions. It aggregates in Alzheimer's disease and many neurodegenerative disorders referred to as tauopathies. Such aggregation may result from mutations on the tau gene, MAPT, dysregulation in alternative splicing, post-translational modifications or truncation. This final chapter addresses some of the various researches on a therapeutic potential around the tau protein and its gene, MAPT. Many therapeutic strategies are ongoing but they are hampered by the lack of knowledge on tau physiological functions.

Targeting Alzheimer's disease with gene and cell therapies

Alzheimer's disease (AD) causes dementia in both young and old people affecting more than 40 million people worldwide. The two neuropathological hallmarks of the disease, amyloid beta (Aβ) plaques and neurofibrillary tangles consisting of protein tau are considered the major contributors to the disease. However, a more complete picture reveals significant neurodegeneration and decreased cell survival, neuroinflammation, changes in protein and energy homeostasis and alterations in lipid and cholesterol metabolism. In addition, gene and cell therapies for severe neurodegenerative disorders have recently improved technically in terms of safety and efficiency and have translated to the clinic showing encouraging results. Here, we review broadly current data within the field for potential targets that could modify AD through gene and cell therapy strategies. We envision that not only Aβ will be targeted in a disease-modifying treatment strategy but rather that a combination of treatments, possibly at different intervention times may prove beneficial in curing this devastating disease. These include decreased tau pathology, neuronal growth factors to support neurons and modulation of neuroinflammation for an appropriate immune response. Furthermore, cell based therapies may represent potential strategies in the future.

Novel monoclonal antibodies targeting the microtubule-binding domain of human tau

Tauopathies including Alzheimer's disease and Progressive Supranuclear Palsy are a diverse group of progressive neurodegenerative disorders pathologically defined by inclusions containing aberrantly aggregated, post-translationally modified tau. The tau pathology burden correlates with neurodegeneration and dementia observed in these diseases. The microtubule binding domain of tau is essential for its physiological functions in promoting neuronal cytoskeletal stability, however it is also required for tau to assemble into an amyloid structure that comprises pathological inclusions. A series of novel monoclonal antibodies were generated which recognize the second and fourth microtubule-binding repeat domain of tau, thus enabling the identification specifically of 4-repeat tau versus 3-/4-repeat tau, respectively. These antibodies are highly specific for tau and recognize pathological tau inclusions in human tauopathies including Alzheimer's disease and Progressive Supranuclear Palsy and in transgenic mouse models of tauopathies. These new antibodies will be useful for identifying and characterizing different tauopathies and as tools to target tau pathology in these diseases.

RNA Trans-Splicing Modulation via Antisense Molecule Interference

In recent years, RNA trans-splicing has emerged as a suitable RNA editing tool for the specific replacement of mutated gene regions at the pre-mRNA level. Although the technology has been successfully applied for the restoration of protein function in various genetic diseases, a higher trans-splicing efficiency is still desired to facilitate its clinical application. Here, we describe a modified, easily applicable, fluorescence-based screening system for the generation and analysis of antisense molecules specifically capable of improving the RNA reprogramming efficiency of a selected KRT14-specific RNA trans-splicing molecule. Using this screening procedure, we identified several antisense RNAs and short rationally designed oligonucleotides, which are able to increase the trans-splicing efficiency. Thus, we assume that besides the RNA trans-splicing molecule, short antisense molecules can act as splicing modulators, thereby increasing the trans-splicing efficiency to a level that may be sufficient to overcome the effects of certain genetic predispositions, particularly those associated with dominantly inherited diseases.

An RNA-targeted therapy for dystrophic epidermolysis bullosa

Functional impairment or complete loss of type VII collagen, caused by mutations within COL7A1, lead to the severe recessive form of the skin blistering disease dystrophic epidermolysis bullosa (RDEB). Here, we successfully demonstrate RNA trans-splicing as an auspicious repair option for mutations located in a wide range of exons by fully converting an RDEB phenotype in an ex vivo pre-clinical mouse model based on xenotransplantation. Via a self-inactivating (SIN) lentiviral vector a 3' RNA trans-splicing molecule, capable of replacing COL7A1 exons 65-118, was delivered into type VII collagen deficient patient keratinocytes, carrying a homozygous mutation in exon 80 (c.6527insC). Following vector integration, protein analysis of an isolated corrected single cell clone showed secretion of the corrected type VII collagen at similar levels compared to normal keratinocytes. To confirm full phenotypic and long-term correction in vivo, patches of skin equivalents expanded from the corrected cell clone were grafted onto immunodeficient mice. Immunolabelling of 12 weeks old skin specimens showed strong expression of human type VII collagen restricted to the basement membrane zone. We demonstrate that the RNA trans-splicing technology combined with a SIN lentiviral vector is suitable for an ex vivo molecular therapy approach and thus adaptable for clinical application.

Alternative Splicing in Genetic Diseases: Improved Diagnosis and Novel Treatment Options

Alternative splicing is an important mechanism to regulate gene expression and to expand the repertoire of gene products in order to accommodate an increase in complexity of multicellular organisms. It needs to be precisely regulated, which is achieved via RNA structure, splicing factors, transcriptional regulation, and chromatin. Changes in any of these factors can lead to disease. These may include the core spliceosome, splicing enhancer/repressor sequences and their interacting proteins, the speed of transcription by RNA polymerase II, and histone modifications. While the basic principle of splicing is well understood, it is still very difficult to predict splicing outcome, due to the multiple levels of regulation. Current molecular diagnostics mainly uses Sanger sequencing of exons, or next-generation sequencing of gene panels or the whole exome. Functional analysis of potential splicing variants is scarce, and intronic variants are often not considered. This likely results in underestimation of the percentage of splicing variants. Understanding how sequence variants may affect splicing is not only crucial for confirmation of diagnosis and for genetic counseling, but also for the development of novel treatment options. These include small molecules, transsplicing, antisense oligonucleotides, and gene therapy. Here we review the current state of molecular mechanisms of splicing regulation and how deregulation can lead to human disease, diagnostics to detect splicing variants, and novel treatment options based on splicing correction.

Tau Isoforms Imbalance Impairs the Axonal Transport of the Amyloid Precursor Protein in Human Neurons

Tau, as a microtubule (MT)-associated protein, participates in key neuronal functions such as the regulation of MT dynamics, axonal transport, and neurite outgrowth. Alternative splicing of exon 10 in the tau primary transcript gives rise to protein isoforms with three (3R) or four (4R) MT binding repeats. Although tau isoforms are balanced in the normal adult human brain, imbalances in 3R:4R ratio have been tightly associated with the pathogenesis of several neurodegenerative disorders, yet the underlying molecular mechanisms remain elusive. Several studies exploiting tau overexpression and/or mutations suggested that perturbations in tau metabolism impair axonal transport. Nevertheless, no physiological model has yet demonstrated the consequences of altering the endogenous relative content of tau isoforms over axonal transport regulation. Here, we addressed this issue using a trans-splicing strategy that allows modulating tau exon 10 inclusion/exclusion in differentiated human-derived neurons. Upon changes in 3R:4R tau relative content, neurons showed no morphological changes, but live imaging studies revealed that the dynamics of the amyloid precursor protein (APP) were significantly impaired. Single trajectory analyses of the moving vesicles showed that predominance of 3R tau favored the anterograde movement of APP vesicles, increasing anterograde run lengths and reducing retrograde runs and segmental velocities. Conversely, the imbalance toward the 4R isoform promoted a retrograde bias by a significant reduction of anterograde velocities. These findings suggest that changes in 3R:4R tau ratio has an impact on the regulation of axonal transport and specifically in APP dynamics, which might link tau isoform imbalances with APP abnormal metabolism in neurodegenerative processes.

SIGNIFICANCE STATEMENT

The tau protein has a relevant role in the transport of cargos throughout neurons. Dysfunction in tau metabolism underlies several neurological disorders leading to dementia. In the adult human brain, two tau isoforms are found in equal amounts, whereas changes in such equilibrium have been associated with neurodegenerative diseases. We investigated the role of tau in human neurons in culture and found that perturbations in the endogenous balance of tau isoforms were sufficient to impair the transport of the Alzheimer's disease-related amyloid precursor protein (APP), although neuronal morphology was normal. Our results provide evidence of a direct relationship between tau isoform imbalance and defects in axonal transport, which induce an abnormal APP metabolism with important implications in neurodegeneration.

Targeting Splicing in the Treatment of Human Disease

The tightly regulated process of precursor messenger RNA (pre-mRNA) alternative splicing (AS) is a key mechanism in the regulation of gene expression. Defects in this regulatory process affect cellular functions and are the cause of many human diseases. Recent advances in our understanding of splicing regulation have led to the development of new tools for manipulating splicing for therapeutic purposes. Several tools, including antisense oligonucleotides and trans-splicing, have been developed to target and alter splicing to correct misregulated gene expression or to modulate transcript isoform levels. At present, deregulated AS is recognized as an important area for therapeutic intervention. Here, we summarize the major hallmarks of the splicing process, the clinical implications that arise from alterations in this process, and the current tools that can be used to deliver, target, and correct deficiencies of this key pre-mRNA processing event.

RNA-based therapies for genodermatoses

Genetic disorders affecting the skin, genodermatoses, constitute a large and heterogeneous group of diseases, for which treatment is generally limited to management of symptoms. RNA-based therapies are emerging as a powerful tool to treat genodermatoses. In this review, we discuss in detail RNA splicing modulation by antisense oligonucleotides and RNA trans-splicing, transcript replacement and genome editing by in vitro-transcribed mRNAs, and gene knockdown by small interfering RNA and antisense oligonucleotides. We present the current state of these therapeutic approaches and critically discuss their opportunities, limitations and the challenges that remain to be solved. The aim of this review was to set the stage for the development of new and better therapies to improve the lives of patients and families affected by a genodermatosis.

Designing Efficient Double RNA trans-Splicing Molecules for Targeted RNA Repair

RNA trans-splicing is a promising tool for mRNA modification in a diversity of genetic disorders. In particular, the substitution of internal exons of a gene by combining 3' and 5' RNA trans-splicing seems to be an elegant way to modify especially large pre-mRNAs. Here we discuss a robust method for designing double RNA trans-splicing molecules (dRTM). We demonstrate how the technique can be implemented in an endogenous setting, using COL7A1, the gene encoding type VII collagen, as a target. An RTM screening system was developed with the aim of testing the replacement of two internal COL7A1 exons, harbouring a homozygous mutation, with the wild-type version. The most efficient RTMs from a pool of randomly generated variants were selected via our fluorescence-based screening system and adapted for use in an in vitro disease model system. Transduction of type VII collagen-deficient keratinocytes with the selected dRTM led to accurate replacement of two internal COL7A1 exons resulting in a restored wild-type RNA sequence. This is the first study demonstrating specific exon replacement by double RNA trans-splicing within an endogenous transcript in cultured cells, corroborating the utility of this technology for mRNA repair in a variety of genetic disorders.

Reprogramming the Dynamin 2 mRNA by Spliceosome-mediated RNA Trans-splicing

Dynamin 2 (DNM2) is a large GTPase, ubiquitously expressed, involved in membrane trafficking and regulation of actin and microtubule cytoskeletons. DNM2 mutations cause autosomal dominant centronuclear myopathy which is a rare congenital myopathy characterized by skeletal muscle weakness and histopathological features including nuclear centralization in absence of regeneration. No curative treatment is currently available for the DNM2-related autosomal dominant centronuclear myopathy. In order to develop therapeutic strategy, we evaluated here the potential of Spliceosome-Mediated RNA Trans-splicing technology to reprogram the Dnm2-mRNA in vitro and in vivo in mice. We show that classical 3′-trans-splicing strategy cannot be considered as accurate therapeutic strategy regarding toxicity of the pre-trans-splicing molecules leading to low rate of trans-splicing in vivo. Thus, we tested alternative strategies devoted to prevent this toxicity and enhance frequency of trans-splicing events. We succeeded to overcome the toxicity through a 5′-trans-splicing strategy which also allows detection of trans-splicing events at mRNA and protein levels in vitro and in vivo. These results suggest that the Spliceosome-Mediated RNA Trans-splicing strategy may be used to reprogram mutated Dnm2-mRNA but highlight the potential toxicity linked to the molecular tools which have to be carefully investigated during preclinical development.

New Features about Tau Function and Dysfunction

Tau is a brain microtubule-associated protein that directly binds to a microtubule and dynamically regulates its structure and function. Under pathological conditions, tau self-assembles into filamentous structures that end up forming neurofibrillary tangles. Prominent tau neurofibrillary pathology is a common feature in a number of neurodegenerative disorders, collectively referred to as tauopathies, the most common of which is Alzheimer’s disease (AD). Beyond its classical role as a microtubule-associated protein, recent advances in our understanding of tau cellular functions have revealed novel insights into their important role during pathogenesis and provided potential novel therapeutic targets. Regulation of tau behavior and function under physiological and pathological conditions is mainly achieved through post-translational modifications, including phosphorylation, glycosylation, acetylation, and truncation, among others, indicating the complexity and variability of factors influencing regulation of tau toxicity, all of which have significant implications for the development of novel therapeutic approaches in various neurodegenerative disorders. A more comprehensive understanding of the molecular mechanisms regulating tau function and dysfunction will provide us with a better outline of tau cellular networking and, hopefully, offer new clues for designing more efficient approaches to tackle tauopathies in the near future.

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.

Intracellular and extracellular microtubule associated protein tau as a therapeutic target in Alzheimer disease and other tauopathies

INTRODUCTION

Microtubule associated protein tau, a protein mainly expressed in neurons, plays an important role in several diseases related to dementia, named tauopathies. Alzheimer disease is the most relevant tauopathy. The role of tau protein in dementia is now a topic under discussion, and is the focus of this review.

AREAS COVERED

We have covered two major areas: tau pathology and tau as a therapeutic target. Tau pathology is mainly related to a gain of toxic function due to an abnormal accumulation, aberrant modifications (such as hyperphosphorylation and truncation, among others) and self-aggregation of tau into oligomers or larger structures. Also, tau can be found extracellularly in a toxic form. Tau-based therapy is mainly centered on avoiding the gain of these toxic functions of tau.

EXPERT OPINION

Tau therapies are focused on lowering tau levels, mainly of modified tau species that could be toxic for neurons (phosphorylated, truncated or aggregated tau), in intracellular or extracellular form. Decreasing the levels of those toxic species is a possible therapeutic strategy.

CD22ΔE12 as a molecular target for corrective repair using RNA trans-splicing: anti-leukemic activity of a rationally designed RNA trans-splicing molecule

Our recent studies have demonstrated that the CD22 exon 12 deletion (CD22ΔE12) is a characteristic genetic defect of therapy-refractory clones in pediatric B-precursor acute lymphoblastic leukemia (BPL) and implicated the CD22ΔE12 genetic defect in the aggressive biology of relapsed or therapy-refractory pediatric BPL. The purpose of the present study was to further evaluate the biologic significance of the CD22ΔE12 molecular lesion and determine if it could serve as a molecular target for corrective repair using RNA trans-splicing therapy. We show that both pediatric and adult B-lineage lymphoid malignancies are characterized by a very high incidence of the CD22ΔE12 genetic defect. We provide experimental evidence that the correction of the CD22ΔE12 genetic defect in human CD22ΔE12(+) BPL cells using a rationally designed CD22 RNA trans-splicing molecule (RTM) caused a pronounced reduction of their clonogenicity. The RTM-mediated correction replaced the downstream mutation-rich segment of Intron 12 and remaining segments of the mutant CD22 pre-mRNA with wildtype CD22 exons 10-14, thereby preventing the generation of the cis-spliced aberrant CD22ΔE12 product. The anti-leukemic activity of this RTM against BPL xenograft clones derived from CD22ΔE12(+) leukemia patients provides the preclinical proof-of-concept that correcting the CD22ΔE12 defect with rationally designed CD22 RTMs may provide the foundation for therapeutic innovations that are needed for successful treatment of high-risk and relapsed BPL patients.

Invited review: Drug development for tauopathies

Many different approaches to treating tauopathies are currently being explored, with a few compounds already in clinical development (including small molecules such as anti-aggregation compound LMTX and active vaccines AADvac1 and ACI-35). This review aims to summarize the status of the clinical candidates and to highlight the emerging areas of research that hold promise for drug development. Tau is post-translationally modified in several different ways (phosphorylated, acetylated, glycosylated and truncated). The extent of these modifications can be manipulated to influence tau aggregation state and pathogenesis and the enzymes involved provide tractable targets for drug intervention. In addition, modulation of tau expression levels is an attractive therapeutic approach. Finally, the recently described prion-like spreading of tau between cells opens up novel avenues from the tau drug development perspective. The review compares the merits of small-molecule and antibody-based therapies and emphasizes the need for amenable clinical biomarkers for drug development, particularly PET imaging.

Dysferlin rescue by spliceosome-mediated pre-mRNA trans-splicing targeting introns harbouring weakly defined 3' splice sites

The modification of the pre-mRNA cis-splicing process employing a pre-mRNA trans-splicing molecule (PTM) is an attractive strategy for the in situ correction of genes whose careful transcription regulation and full-length expression is determinative for protein function, as it is the case for the dysferlin (DYSF, Dysf) gene. Loss-of-function mutations of DYSF result in different types of muscular dystrophy mainly manifesting as limb girdle muscular dystrophy 2B (LGMD2B) and Miyoshi muscular dystrophy 1 (MMD1). We established a 3' replacement strategy for mutated DYSF pre-mRNAs induced by spliceosome-mediated pre-mRNA trans-splicing (SmaRT) by the use of a PTM. In contrast to previously established SmaRT strategies, we particularly focused on the identification of a suitable pre-mRNA target intron other than the optimization of the PTM design. By targeting DYSF pre-mRNA introns harbouring differentially defined 3' splice sites (3' SS), we found that target introns encoding weakly defined 3' SSs were trans-spliced successfully in vitro in human LGMD2B myoblasts as well as in vivo in skeletal muscle of wild-type and Dysf(-/-) mice. For the first time, we demonstrate rescue of Dysf protein by SmaRT in vivo. Moreover, we identified concordant qualities among the successfully targeted Dysf introns and targeted endogenous introns in previously reported SmaRT approaches that might facilitate a selective choice of target introns in future SmaRT strategies.

Mechanism of alternative splicing and its regulation

Alternative splicing of precursor mRNA is an essential mechanism to increase the complexity of gene expression, and it plays an important role in cellular differentiation and organism development. Regulation of alternative splicing is a complicated process in which numerous interacting components are at work, including cis-acting elements and trans-acting factors, and is further guided by the functional coupling between transcription and splicing. Additional molecular features, such as chromatin structure, RNA structure and alternative transcription initiation or alternative transcription termination, collaborate with these basic components to generate the protein diversity due to alternative splicing. All these factors contributing to this one fundamental biological process add up to a mechanism that is critical to the proper functioning of cells. Any corruption of the process may lead to disruption of normal cellular function and the eventuality of disease. Cancer is one of those diseases, where alternative splicing may be the basis for the identification of novel diagnostic and prognostic biomarkers, as well as new strategies for therapy. Thus, an in-depth understanding of alternative splicing regulation has the potential not only to elucidate fundamental biological principles, but to provide solutions for various diseases.

Mitochondrial complex 1 inhibition increases 4-repeat isoform tau by SRSF2 upregulation

Progressive Supranuclear Palsy (PSP) is a neurodegenerative disorder characterised by intracellular aggregation of the microtubule-associated protein tau. The tau protein exists in 6 predominant isoforms. Depending on alternative splicing of exon 10, three of these isoforms have four microtubule-binding repeat domains (4R), whilst the others only have three (3R). In PSP there is an excess of the 4R tau isoforms, which are thought to contribute significantly to the pathological process. The cause of this 4R increase is so far unknown. Several lines of evidence link mitochondrial complex I inhibition to the pathogenesis of PSP. We demonstrate here for the first time that annonacin and MPP⁺, two prototypical mitochondrial complex I inhibitors, increase the 4R isoforms of tau in human neurons. We show that the splicing factor SRSF2 is necessary to increase 4R tau with complex I inhibition. We also found SRSF2, as well as another tau splicing factor, TRA2B, to be increased in brains of PSP patients. Thereby, we provide new evidence that mitochondrial complex I inhibition may contribute as an upstream event to the pathogenesis of PSP and suggest that splicing factors may represent an attractive therapeutic target to intervene in the disease process.

Linking traumatic brain injury to chronic traumatic encephalopathy: identification of potential mechanisms leading to neurofibrillary tangle development

Significant attention has recently been drawn to the potential link between head trauma and the development of neurodegenerative disease, namely chronic traumatic encephalopathy (CTE). The acute neurotrauma associated with sports-related concussions in athletes and blast-induced traumatic brain injury in soldiers elevates the risk for future development of chronic neurodegenerative diseases such as CTE. CTE is a progressive disease distinguished by characteristic tau neurofibrillary tangles (NFTs) and, occasionally, transactive response DNA binding protein 43 (TDP43) oligomers, both of which have a predilection for perivascular and subcortical areas near reactive astrocytes and microglia. The disease is currently only diagnosed postmortem by neuropathological identification of NFTs. A recent workshop sponsored by National Institute of Neurological Disorders and Stroke emphasized the need for premortem diagnosis, to better understand disease pathophysiology and to develop targeted treatments. In order to accomplish this objective, it is necessary to discover the mechanistic link between acute neurotrauma and the development of chronic neurodegenerative and neuropsychiatric disorders such as CTE. In this review, we briefly summarize what is currently known about CTE development and pathophysiology, and subsequently discuss injury-induced pathways that warrant further investigation. Understanding the mechanistic link between acute brain injury and chronic neurodegeneration will facilitate the development of appropriate diagnostic and therapeutic options for CTE and other related disorders.

Cis-splicing and translation of the pre-trans-splicing molecule combine with efficiency in spliceosome-mediated RNA trans-splicing

Muscular dystrophies are a group of genetically distinct diseases for which no treatment exists. While gene transfer approach is being tested for several of these diseases, such strategies can be hampered when the size of the corresponding complementary DNA (cDNA) exceeds the packaging capacity of adeno-associated virus vectors. This issue concerns, in particular, dysferlinopathies and titinopathies that are due to mutations in the dysferlin (DYSF) and titin (TTN) genes. We investigated the efficacy of RNA trans-splicing as a mode of RNA therapy for these two types of diseases. Results obtained with RNA trans-splicing molecules designed to target the 3' end of mouse titin and human dysferlin pre-mRNA transcripts indicated that trans-splicing of pre-mRNA generated from minigene constructs or from the endogenous genes was achieved. Collectively, these results provide the first demonstration of DYSF and TTN trans-splicing reprogramming in vitro and in vivo. However, in addition to these positive results, we uncovered a possible issue of the technique in the form of undesirable translation of RNA pre-trans-splicing molecules, directly from open reading frames present on the molecule or associated with internal alternative cis-splicing. These events may hamper the efficiency of the trans-splicing process and/or lead to toxicity.

Brahma regulates a specific trans-splicing event at the mod(mdg4) locus of Drosophila melanogaster

The mod(mdg4) locus of Drosophila melanogaster contains several transcription units encoded on both DNA strands. The mod(mdg4) pre-mRNAs are alternatively spliced, and a very significant fraction of the mature mod(mdg4) mRNAs are formed by trans-splicing. We have studied the transcripts derived from one of the anti-sense regions within the mod(mdg4) locus in order to shed light on the expression of this complex locus. We have characterized the expression of anti-sense mod(mdg4) transcripts in S2 cells, mapped their transcription start sites and cleavage sites, identified and quantified alternatively spliced transcripts, and obtained insight into the regulation of the mod(mdg4) trans-splicing. In a previous study, we had shown that the alternative splicing of some mod(mdg4) transcripts was regulated by Brahma (BRM), the ATPase subunit of the SWI/SNF chromatin-remodeling complex. Here we show, using RNA interference and overexpression of recombinant BRM proteins, that the levels of BRM affect specifically the abundance of a trans-spliced mod(mdg4) mRNA isoform in both S2 cells and larvae. This specific effect on trans-splicing is accompanied by a local increase in the density of RNA polymerase II and by a change in the phosphorylation state of the C-terminal domain of the large subunit of RNA polymerase II. Interestingly, the regulation of the mod(mdg4) splicing by BRM is independent of the ATPase activity of BRM, which suggests that the mechanism by which BRM modulates trans-splicing is independent of its chromatin-remodeling activity.

Alzheimer disease therapeutics: focus on the disease and not just plaques and tangles

The bulk of AD research during the last 25 years has been Aβ-centric based on a strong faith in the Amyloid Cascade Hypothesis which is not supported by the data on humans. To date, Aβ-based therapeutic clinical trials on sporadic cases of AD have been negative. Although most likely the major reason for the failure is that Aβ is not an effective therapeutic target for sporadic AD, initiation of the treatment at mild to moderate stages of the disease is blamed as too late to be effective. Clinical trials on presymptomatic familial AD cases have been initiated with the logic that Aβ is a trigger of the disease and hence initiation of the Aβ immunotherapies several years before any clinical symptoms would be effective. There is an urgent need to explore targets other than Aβ. There is now increasing interest in inhibiting tau pathology, which does have a far more compelling rationale than Aβ. AD is multifactorial and over 99% of the cases are the sporadic form of the disease. Understanding of the various etiopathogenic mechanisms of sporadic AD and generation of the disease-relevant animal models are required to develop rational therapeutic targets and therapies. Treatment of AD will require both inhibition of neurodegeneration and regeneration of the brain.

MYBPC3 in hypertrophic cardiomyopathy: from mutation identification to RNA-based correction

Mutations in MYBPC3 gene, encoding cardiac myosin-binding protein C (cMyBP-C), frequently cause hypertrophic cardiomyopathy (HCM), which affects 0.2 % of the general population. This myocardial autosomal-dominant disorder is the leading cause of sudden cardiac death particularly in young athletes. The current pharmacological and surgical treatments of HCM focus on symptoms relief, but do not address the cause of the disease. With the development of novel strategies targeting the endogenous mutation, causal HCM therapy is now possible. This review will discuss the current knowledge on HCM from the identification of MYBPC3 gene mutations to potential RNA-based correction.

Correction of the Middle Eastern M712T mutation causing GNE myopathy by trans-splicing

GNE myopathy is a rare neuromuscular autosomal recessive disease, resulting from mutations in the gene UDP N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE). The most frequent mutation is the single homozygous missense mutation, M712T-the Middle Eastern mutation-located ten amino acids before the end of the protein. We have used an adeno-associated virus (AAV)-based trans-splicing (TS) vector as a gene therapy tool to overcome this mutation by replacing the mutated last exon of GNE by the wild-type exon while preserving the natural endogenous regulatory machinery. We have designed relevant plasmids directed either to mouse or to human GNE. Following transfection of C2C12 murine muscle cells with the mouse TS vectors, we have been able to detect by nested RT-PCR trans-spliced molecules carrying the wild-type exon 12 of GNE. Similarly, transfection of HEK293 human cells with the human-directed TS vectors resulted in the generation of trans-spliced human GNE RNA molecules. Furthermore, infection of primary muscle cells from a GNE myopathy patient carrying the homozygous M712T mutation, with an AAV8-based viral vector carrying a human-directed TS construct, resulted in the generation of wild-type GNE transcripts in addition to the mutated ones. These studies provide a proof of concept that the TS approach could be used to partially correct the Middle Eastern mutation in GNE myopathy patients. These results provide the basis for in vivo research in animal models using the AAV platform with TS plasmids as a potential genetic therapy for GNE myopathy.

The design and optimization of RNA trans-splicing molecules for skin cancer therapy

Targeting tumor marker genes by RNA trans-splicing is a promising means to induce tumor cell-specific death. Using a screening system we designed RNA trans-splicing molecules (RTM) specifically binding the pre-mRNA of SLCO1B3, a marker gene in epidermolysis bullosa associated squamous cell carcinoma (EB-SCC). Specific trans-splicing, results in the fusion of the endogenous target mRNA of SLCO1B3 and the coding sequence of the suicide gene, provided by the RTM. SLCO1B3-specific RTMs containing HSV-tk were analyzed regarding their trans-splicing potential in a heterologous context using a SLCO1B3 expressing minigene (SLCO1B3-MG). Expression of the chimeric SLCO1B3-tk was detected by semi-quantitative RT-PCR and Western blot analysis. Cell viability and apoptosis assays confirmed that the RTMs induced suicide gene-mediated apoptosis in SLCO1B3-MG expressing cells. The lead RTM also showed its potential to facilitate a trans-splicing reaction into the endogenous SLCO1B3 pre-mRNA in EB-SCC cells resulting in tk-mediated apoptosis. We assume that the pre-selection of RTMs by our inducible cell-death system accelerates the design of optimal RTMs capable to induce tumor specific cell death in skin cancer cells.

Repair of Mybpc3 mRNA by 5'-trans-splicing in a Mouse Model of Hypertrophic Cardiomyopathy

RNA trans-splicing has been explored as a therapeutic option for a variety of genetic diseases, but not for cardiac genetic disease. Hypertrophic cardiomyopathy (HCM) is an autosomal-dominant disease, characterized by left ventricular hypertrophy (LVH) and diastolic dysfunction. MYBPC3, encoding cardiac myosin-binding protein C (cMyBP-C) is frequently mutated. We evaluated the 5'-trans-splicing strategy in a mouse model of HCM carrying a Mybpc3 mutation. 5'-trans-splicing was induced between two independently transcribed molecules, the mutant endogenous Mypbc3 pre-mRNA and an engineered pre-trans-splicing molecule (PTM) carrying a FLAG-tagged wild-type (WT) Mybpc3 cDNA sequence. PTMs were packaged into adeno-associated virus (AAV) for transduction of cultured cardiac myocytes and the heart in vivo. Full-length repaired Mybpc3 mRNA represented up to 66% of total Mybpc3 transcripts in cardiac myocytes and 0.14% in the heart. Repaired cMyBP-C protein was detected by immunoprecipitation in cells and in vivo and exhibited correct incorporation into the sarcomere in cardiac myocytes. This study provides (i) the first evidence of successful 5'-trans-splicing in vivo and (ii) proof-of-concept of mRNA repair in the most prevalent cardiac genetic disease. Since current therapeutic options for HCM only alleviate symptoms, these findings open new horizons for causal therapy of the severe forms of the disease.Molecular Therapy-Nucleic Acids (2013) 2, e102; doi:10.1038/mtna.2013.31; published online 2 July 2013.