Spliceosome-mediated RNA Trans-splicing Research Articles

COL7A1 Editing via RNA Trans-Splicing in RDEB-Derived Skin Equivalents

Mutations in the COL7A1 gene lead to malfunction, reduction or complete absence of type VII collagen (C7) in the skin's basement membrane zone (BMZ), impairing skin integrity. In epidermolysis bullosa (EB), more than 800 mutations in COL7A1 have been reported, leading to the dystrophic form of EB (DEB), a severe and rare skin blistering disease associated with a high risk of developing an aggressive form of squamous cell carcinoma. Here, we leveraged a previously described 3'-RTMS6m repair molecule to develop a non-viral, non-invasive and efficient RNA therapy to correct mutations within COL7A1 via spliceosome-mediated RNA trans-splicing (SMaRT). RTM-S6m, cloned into a non-viral minicircle-GFP vector, is capable of correcting all mutations occurring between exon 65 and exon 118 of COL7A1 via SMaRT. Transfection of the RTM into recessive dystrophic EB (RDEB) keratinocytes resulted in a trans-splicing efficiency of ~1.5% in keratinocytes and ~0.6% in fibroblasts, as confirmed on mRNA level via next-generation sequencing (NGS). Full-length C7 protein expression was primarily confirmed in vitro via immunofluorescence (IF) staining and Western blot analysis of transfected cells. Additionally, we complexed 3'-RTMS6m with a DDC642 liposomal carrier to deliver the RTM topically onto RDEB skin equivalents and were subsequently able to detect an accumulation of restored C7 within the basement membrane zone (BMZ). In summary, we transiently corrected COL7A1 mutations in vitro in RDEB keratinocytes and skin equivalents derived from RDEB keratinocytes and fibroblasts using a non-viral 3'-RTMS6m repair molecule.

5'RNA Trans-Splicing Repair of COL7A1 Mutant Transcripts in Epidermolysis Bullosa.

Mutations within the COL7A1 gene underlie the inherited recessive subtype of the blistering skin disease dystrophic epidermolysis bullosa (RDEB). Although gene replacement approaches for genodermatoses are clinically advanced, their implementation for RDEB is challenging and requires endogenous regulation of transgene expression. Thus, we are using spliceosome-mediated RNA trans-splicing (SMaRT) to repair mutations in COL7A1 at the mRNA level. Here, we demonstrate the capability of a COL7A1-specific RNA trans-splicing molecule (RTM), initially selected using a fluorescence-based screening procedure, to accurately replace COL7A1 exons 1 to 64 in an endogenous setting. Retroviral RTM transduction into patient-derived, immortalized keratinocytes resulted in an increase in wild-type transcript and protein levels, respectively. Furthermore, we revealed accurate deposition of recovered type VII collagen protein within the basement membrane zone of expanded skin equivalents using immunofluorescence staining. In summary, we showed for the first time the potential of endogenous 5′ trans-splicing to correct pathogenic mutations within the COL7A1 gene. Therefore, we consider 5′ RNA trans-splicing a suitable tool to beneficially modulate the RDEB-phenotype, thus targeting an urgent need of this patient population.

Evaluating a Targeted Cancer Therapy Approach Mediated by RNA trans-Splicing In Vitro and in a Xenograft Model for Epidermolysis Bullosa-Associated Skin Cancer.

Conventional anti-cancer therapies based on chemo- and/or radiotherapy represent highly effective means to kill cancer cells but lack tumor specificity and, therefore, result in a wide range of iatrogenic effects. A promising approach to overcome this obstacle is spliceosome-mediated RNA trans-splicing (SMaRT), which can be leveraged to target tumor cells while leaving normal cells unharmed. Notably, a previously established RNA trans-splicing molecule (RTM44) showed efficacy and specificity in exchanging the coding sequence of a cancer target gene (Ct-SLCO1B3) with the suicide gene HSV1-thymidine kinase in a colorectal cancer model, thereby rendering tumor cells sensitive to the prodrug ganciclovir (GCV). In the present work, we expand the application of this approach, using the same RTM44 in aggressive skin cancer arising in the rare genetic skin disease recessive dystrophic epidermolysis bullosa (RDEB). Stable expression of RTM44, but not a splicing-deficient control (NC), in RDEB-SCC cells resulted in expression of the expected fusion product at the mRNA and protein level. Importantly, systemic GCV treatment of mice bearing RTM44-expressing cancer cells resulted in a significant reduction in tumor volume and weight compared with controls. Thus, our results demonstrate the applicability of RTM44-mediated targeting of the cancer gene Ct-SLCO1B3 in a different malignancy.

The c.3140-26A>G Variant of the CFTR Gene in Homozygous State Causes Mild Cystic Fibrosis – Overview of Longitudinal Clinical Data of the Patient Managed in our CF Center and Review of the Literature

There are over 70.000 patients with cystic fibrosis (CF) in the world and numerous sequence variations in the CFTR gene have been reported but the clinical significance of all of them is still not known. There are currently 195 patients with the c.3140-26A>G (legacy name 3272-26A>G) variant in the CFTR gene listed in the European Cystic Fibrosis Society Patient Registry (ECFSPR) and only 4 are homozygous. We present longitudinal clinical data of one of these patients who is managed in our CF Center at the University Children's Hospital in Ljubljana and compare it with the patient data from the ECFSPR and the CFTR2 database in which additional 3 homozygous patients are described. Moreover, the effect of the detected variant in the described patient was evaluated on the RNA level in nasal epithelial cells. The variant was shown to result in aberrant splicing introducing a frameshift and a premature termination codon while normal transcript was not detected. Alternative spliced mutant transcripts in other tissues or the presence of spliceosome-mediated RNA trans-splicing could explain the mild clinical presentation of patients with this variant in homozygous state.

Gene therapy strategies in the treatment of hypertrophic cardiomyopathy.

Hypertrophic cardiomyopathy (HCM) is an inherited myocardial disease with an estimated prevalence of 1:200 caused by mutations in sarcomeric proteins. It is associated with hypertrophy of the left ventricle, increased interstitial fibrosis, and diastolic dysfunction for heterozygous mutation carriers. Carriers of double heterozygous, compound heterozygous, and homozygous mutations often display more severe forms of cardiomyopathies, ultimately leading to premature death. So far, there is no curative treatment against HCM, as current therapies are focused on symptoms relief by pharmacological intervention and not on the cause of HCM. In the last decade, several strategies have been developed to remove genetic defects, including genome editing, exon skipping, allele-specific silencing, spliceosome-mediated RNA trans-splicing, and gene replacement. Most of these technologies have already been tested for efficacy and efficiency in animal- or human-induced pluripotent stem cell models of HCM with promising results. We will summarize recent technological advances and their implication as gene therapy options in HCM with a special focus on treating MYBPC3 mutations and its potential for being a successful bench to bedside example.

Cancer-type organic anion transporting polypeptide 1B3 is a target for cancer suicide gene therapy using RNA trans-splicing technology

Cancer-type organic anion transporting polypeptide 1B3 (Ct-OATP1B3) has been identified as a cancer-specific transcript in various solid cancers, including colorectal cancer. Given its excellent cancer-specific expression profile, we hypothesized that Ct-OATP1B3 could represent a promising target for cancer-specific expression of the suicide gene, herpes simplex virus 1 thymidine kinase (HSV-tk), via a spliceosome-mediated RNA trans-splicing (SMaRT) approach. SMaRT technology is used to recombine two RNA molecules to generate a chimeric transcript. In this study, we engineered an RNA trans-splicing molecule carrying a translation-defective HSV-tk sequence (RTM44), which was capable of inducing its own trans-splicing to the desired Ct-OATP1B3 pre-mRNA target. RTM44 expression in LS180 cells resulted in generation of Ct-OATP1B3/HSV-tk fusion mRNA. A functional translation start site contributed by the target pre-mRNA restored HSV-tk protein expression, rendering LS180 cells sensitive to ganciclovir treatment in vitro and in xenografted mice. The observed effects are ascribed to accurate and efficient trans-splicing, as they were absent in cells carrying a splicing-deficient mutant of RTM44. Collectively, our data highlights Ct-OATP1B3 as an ideal target for the HSV-tk SMaRT suicide system, which opens up new translational avenues for Ct-OATP1B3-targeted cancer therapy.

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.

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.

Cancer-type organic anion transporting polypeptide 1B3 is a promising target for spliceosome-mediated RNA trans-splicing based suicide gene therapy

Cancer-type organic anion transporting polypeptide 1B3 is a promising target for spliceosome-mediated RNA trans-splicing based suicide gene therapy

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.

Gene Therapy for LMNA-related Congenital Muscular Dystrophy (L-CMD) by Trans-Splicing

LMNA-related Congenital Muscular Dystrophy (L-CMD) is a rare genetic disorder characterized by the onset of selective axial weakness and wasting in the first year of life with limited motor achievements, associated with multiple severe contractures and frequent respiratory failure requiring early ventilatory support. We identified heterozygous de novo mutations in LMNA, encoding lamins A/C, as responsible for this sub-group of CMD in which no therapeutic treatment is available[1]. Lamins A/C are nuclear envelope proteins, ubiquitously expressed in all post mitotic cells, which play essential roles in the nucleus structure and in the regulation of gene expression. We generated the first Knock-In mouse model of L-CMD (KI-LmnadelK32) reproducing a LMNA mutation identified in L-CMD patients. Homozygous mice die within the first 3 weeks of life from striated muscles maturation delay and severe metabolic defects[2]. Heterozygous mice develop an isolated dilated cardiomyopathy and die by one year of age[3]. We aim to assess the possibility of LMNA-mRNA repair by spliceosome-mediated RNA trans-splicing (SMarT) as a potential therapeutic approach for L-CMD. This gene therapy strategy will allow inhibition of mutated LMNA transcript expression for the benefit of corresponding wild type transcripts. We developed 5'-RNA pre-trans-splicing molecules (PTM) capable of repairing the murine LMNA transcripts. Efficiency of these PTM was assessed in vitro in C2C12 cells and in vivo using Adeno-Associated Virus (AAV) transduction in tibialis anterior of WT mice. We will now determine the ability of the best PTM to restore normal muscular phenotype, in vitro in KI myoblasts/myotubes and in vivo after injection of AAV-PTM vectors in new born homozygous and adult heterozygous mice. Histological and metabolic parameters will be monitored to evaluate the degree of phenotype rescue.

Can we be SMaRT-er in our approach to cancer therapy?

Can we be SMaRT-er in our approach to cancer therapy?

Repair of Rhodopsin mRNA by Spliceosome-Mediated RNA Trans-Splicing: A New Approach for Autosomal Dominant Retinitis Pigmentosa

The promising clinical results obtained for ocular gene therapy in recent years have paved the way for gene supplementation to treat recessively inherited forms of retinal degeneration. The situation is more complex for dominant mutations, as the toxic mutant gene product must be removed. We used spliceosome-mediated RNA trans-splicing as a strategy for repairing the transcript of the rhodopsin gene, the gene most frequently mutated in autosomal dominant retinitis pigmentosa. We tested 17 different molecules targeting the pre-mRNA intron 1, by transient transfection of HEK-293T cells, with subsequent trans-splicing quantification at the transcript level. We found that the targeting of some parts of the intron promoted trans-splicing more efficiently than the targeting of other areas, and that trans-splicing rate could be increased by modifying the replacement sequence. We then developed cell lines stably expressing the rhodopsin gene, for the assessment of phenotypic criteria relevant to the pathogenesis of retinitis pigmentosa. Using this model, we showed that trans-splicing restored the correct localization of the protein to the plasma membrane. Finally, we tested our best candidate by AAV gene transfer in a mouse model of retinitis pigmentosa that expresses a mutant allele of the human rhodopsin gene, and demonstrated the feasibility of trans-splicing in vivo. This work paves the way for trans-splicing gene therapy to treat retinitis pigmentosa due to rhodopsin gene mutation and, more generally, for the treatment of genetic diseases with dominant transmission.

495. In Vivo Evidence of trans-Splicing in a Humanized Mouse Model of Autosomal Dominant Retinitis Pigmentosa Induced By Mutation of the Rhodopsin Gene

The most frequent cause of retinitis pigmentosa (RP), a group of hereditary retinal dystrophies leading to blindness, is the occurrence of point mutations in the Rhodopsin (RHO) gene. Most of these mutations lead to gain of functions or dominant negative effects deleterious for photoreceptors, thus resulting in a dominant mode of transmission. Moreover, it has been shown that variations in RHO expression level are also deleterious for the retina. Gene transfer strategies modifying RHO mutations should therefore lead both to suppression of mutant protein expression and restoration of the normal one at a physiological level. Spliceosome-mediated RNA trans-splicing should in theory respect these constraints by repairing mutations at the pre-mRNA level. Using this technology, the expression level of the repaired mRNA will indeed depend solely on that of the gene endogenous promoter. To achieve this goal, this approach consists of introducing, by gene transfer, an exogenous RNA – called PTM, for Pre-Trans-splicing Molecule – able to bind the pre-mRNA and promote splicing in trans, leading to replacement of the mutant part of the RHO pre-mRNA.We engineered fourteen different RHO-PTMs able to repair any mutation in RHO exons 2, 3, 4 and 5, which differ only on their binding sequence to RHO pre-mRNA. To determine the efficiency of each PTM, we transiently co-transfected HEK293T cells with a plasmid encoding a PTM and another encoding the wild-type or mutant RHO. The maximum trans-splicing efficiency observed at the mRNA level was about 25%. We improved this efficiency to 40% by refinement of the PTM sequence, through reintroduction of an endogenous RHO intron in the cDNA part of the PTM. We then tested PTM at the protein level in a cellular model expressing RHO stably. While wild-type RHO was localized to the plasma membrane, mutant RHO was sequestrated in the cytoplasmic compartment. Using a flow-imaging methodology (ImageStreamX), we were able to precisely quantify this phenotype and showed that trans-splicing partially restored a correct localization of RHO in this cellular model. Thus, we constructed an AAV vector containing our best PTM to assess its activity in vivo in a humanized mouse model of rhodopsin mutation following subretinal injection. Using co-injection with an AAV-GFP and micro-dissection of the transduced part of the retina, we demonstrated that trans-splicing reached 30% at the RNA level, while it was not detectable in the untransduced part, or in animals that received a control PTM. This study demonstrates that trans-splicing may provide a therapeutic benefit in case of dominant mutations of the rhodopsin gene.

Trans-splicing repair of mutant p53 suppresses the growth of hepatocellular carcinoma cells in vitro and in vivo.

Reactivation of wild-type p53 (wt-p53) function is an attractive therapeutic approach to p53-defective cancers. An ideal p53-based gene therapy should restore wt-p53 production and reduces mutant p53 transcripts simultaneously. In this study, we described an alternative strategy named as trans-splicing that repaired mutant p53 transcripts in hepatocellular carcinoma (HCC) cells. The plasmids which encoded a pre-trans-splicing molecule (PTM) targeting intron 6 of p53 were constructed and then transfected into HCC cells carrying p53 mutation. Phenotypic changes of HCC cells induced by p53-PTM were analyzed through cell cycle, cell apoptosis and the expression of p53 downstream target genes. Spliceosome mediated RNA trans-splicing (SMaRT) reduced mutant p53 transcripts and produced functional wt-p53 protein after the delivery of p53-PTM plasmids, which resulted in phenotype correction of HCC cells. In tumor xenografts established by p53-mutated HCC cells, adenovirus encoding p53-PTM induced cell cycle arrest and apoptosis and then blocked the growth of tumors in mice. Collectively, our results demonstrated for the first time that mutant p53 transcripts were functionally corrected in p53-defective HCC cells and xenografts using trans-splicing, which indicated the feasibility of using trans-splicing to repair p53 mutation in p53-defective cancers.

Trans-splicing improvement by the combined application of antisense strategies.

Spliceosome-mediated RNA trans-splicing has become an emergent tool for the repair of mutated pre-mRNAs in the treatment of genetic diseases. RNA trans-splicing molecules (RTMs) are designed to induce a specific trans-splicing reaction via a binding domain for a respective target pre-mRNA region. A previously established reporter-based screening system allows us to analyze the impact of various factors on the RTM trans-splicing efficiency in vitro. Using this system, we are further able to investigate the potential of antisense RNAs (AS RNAs), presuming to improve the trans-splicing efficiency of a selected RTM, specific for intron 102 of COL7A1. Mutations in the COL7A1 gene underlie the dystrophic subtype of the skin blistering disease epidermolysis bullosa (DEB). We have shown that co-transfections of the RTM and a selected AS RNA, interfering with competitive splicing elements on a COL7A1-minigene (COL7A1-MG), lead to a significant increase of the RNA trans-splicing efficiency. Thereby, accurate trans-splicing between the RTM and the COL7A1-MG is represented by the restoration of full-length green fluorescent protein GFP on mRNA and protein level. This mechanism can be crucial for the improvement of an RTM-mediated correction, especially in cases where a high trans-splicing efficiency is required.

Spliceosome Mediated RNA Trans-Splicing for Targeting Kappa+ B-Cell Neoplasms

IntroductionB-cells express either kappa or lambda light chains but not both. B-cell directed therapies such as anti-CD20 antibodies deplete healthy and malignant CD20+ B-cells but spare CD20- plasma and myeloma cells. Furthermore anti-CD20 antibody mediated B-cell depletion can lead to severe hypoimmunoglobulinemia and thereby predispose for infections. Light-chain specific targeting would allow a more specific therapy of lymphomas and myeloma, with collateral damage limited to only half of healthy B-cells. Due to the heterogeneity of immunoglobulines, lymphoma specific therapy so far required customized solutions for every single patient. Despite having 40 variable and 5 joining regions, the kappa light chain has only 1 constant region (IGKC). Therefore the 5’ IGKC intron is conserved in kappa light chain pre-mRNA, regardless of any recombination or somatic hypermutation in the VJ-region.Spliceosome-mediated RNA trans-splicing allows endogenous pre-mRNA to be converted into a new gene product via exon replacement. This can be achieved by the introduction of an RNA trans-splicing molecule (RTMRTM) that binds to endogenous target RNA and induces trans-splicing between the target gene and the RTM. RTMs contain a binding domain (BD), which defines the target specificity, splicing elements for efficient trans-splicing, and the desired coding sequence.This proof-of-principle study was aimed at demonstrating the feasibility of light-chain specific targeting through spliceosome-mediated RNA trans-splicing.MethodsThis study was approved by the local ethics committee of the provincial government of Salzburg. Potential RTMs for trans-splicing were identified using a fluorescence screening procedure as previously described (Gruber C et al. Mol Cancer Ther. 2011 Feb;10(2):233-41.). Primary lymphoma and myeloma samples were collected during routine diagnostic testing. Total RNA isolation was performed with RNeasy® isolation kit (QIAGEN) and reverse transcription of RNA using the i-script cDNASynthesis Kit (Bio-Rad). Quantitative real-time polymerase chain reaction was performed using GoTaq® qPCR Master Mix (Promega). For western blotting, cell lysates were separated using 10% sodium dodecyl sulfate/polyacrylamide gel electrophoresis. After transferring the proteins onto a nitrocellulose membrane, immune detection of specific proteins was performed using anti-Green Fluorescent Protein (GFP) antibodies and enhanced chemiluminescence detection.ResultsBD screening identified RTM 55 as the most efficient trans-splicing molecule and was therefore used for further experiments. Since the light chain locus undergoes substantial physiologic genetic changes in the process of VJ rearrangement and somatic hypermutation, we analysed whether these changes extend to the target sequence of the BD of RTM 55. Sequencing of the complete 5’ IGKC intron in primary kappa+ CLL (N=4), DLBCL (N=3) and myeloma cells (N=3) only showed occasional point mutations. In the following, effective endogenous trans-splicing was confirmed in 2 different kappa+ cell lines, the DLBCL cell line SUDHL4 and the CLL cell line MEC2. We could detect successful trans-splicing on the mRNA level and sequencing of PCR-products confirmed accurate trans-splicing with the last base of the J-region adjoining the first base of the GFP gene. Stable retroviral transfection of SUDHL4 cells also allowed the successful detection of the kappa light-chain-GFP fusion protein by western blotting. Surprisingly, the lambda+ myeloma cell line U266 as well as FACS-sorted healthy lambda+ B-cells showed high expression levels of kappa light chain mRNA. This leaky mRNA expression of the non-dominant light chain could not be seen in kappa+ cells, which may be explained by the programmed sequential activation of the kappa and lambda loci during B-cell development.ConclusionsWe could show that the kappa light chain can be specifically targeted by spliceosome-mediated RNA trans-splicing. The possibility to reprogram the light chain pre-mRNA offers numerous possible applications, such as suicide gene therapy for lymphoma and myeloma. The unexpectedly high expression of kappa mRNA in lambda+ cells but not vice-versa, raises the question whether the lambda light chain locus would offer an even better, since more specific target DisclosuresNo relevant conflicts of interest to declare.

G.P.45: Therapy of the dynamin 2-linked centronuclear myopathy by mRNA repair using Spliceosome-Mediated RNA Trans-splicing (SMarT) strategy

The autosomal dominant centronuclear myopathy (AD-CNM) is a rare congenital myopathy defined by skeletal muscle weakness and characteristic histopathological changes. Heterozygous mutations in the DNM2 gene are associated with entire clinical spectrum of AD-CNM. DNM2 gene encodes dynamin 2 (DNM2), a large GTPase ubiquitously expressed and involved in membrane trafficking. A Knock-In mouse model (KI-Dnm2R465W) expressing the most frequent mutation found in patients has been recently developed in the laboratory. The heterozygous mice progressively develop a muscle phenotype which recapitulates many aspects of the human condition. The purpose of this project is to evaluate the therapeutic potential of DNM2-mRNA repair by Spliceosome-Mediated RNA Trans-splicing (SMarT) technology able to reprogram the 5′, 3′ or internal coding sequence of endogenous mRNA. We have shown that classical 3′ strategy cannot be considered as accurate therapeutic strategy regarding the toxic effect of the Pre-Transplicing Molecules (PTMs) leading to low rate of trans-splicing in vivo. Thus, we tested several alternative strategies in order to prevent this toxicity and enhance frequency of trans-splicing events. We finally succeeded to overcome the toxicity using a 5′ trans-splicing strategy and obtained hopeful results. Indeed, we detected trans-splicing events at mRNA and protein levels in vitro and in vivo.

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.

Considerations for a Successful RNA Trans-splicing Repair of Genetic Disorders

To the Editor: Spliceosome-mediated RNA trans-splicing (SMaRT) has emerged as a novel technology for the repair of mutations in monogenic disorders. SMaRT can be used to replace 5′, 3′, or internal sequences in an exon-wise manner. For this purpose, RNA trans-splicing molecules (RTMs) are engineered, which comprise (i) the wild-type coding region to be replaced, (ii) a splicing domain that includes essential splicing elements such as branch point and polypyrimidine tract, (iii) a binding domain (BD) hybridizing to a selected target region to bring RTM and target transcript into close proximity, and (iv) a spacer sequence for steric reasons. The replacement of exonic portions provides several advantages relative to other gene therapeutic approaches, such as complementary DNA (cDNA) therapy. First, for very large genes, the sequence to be replaced can be reduced to a size that facilitates viral packaging and transduction. Second, the increase of wild-type alleles and the concomitant decrease of mutated alleles leads to an enhanced shift in their quantitative proportions, making this technology interesting for the correction of dominant mutations. Third, as trans-splicing (TS) takes place at the messenger RNA (mRNA) level, target gene expression remains under the control of the respective endogenous promoter, and problems arising from transgene overexpression can be excluded.