Disruption of splicing regulated by a CUG-binding protein in myotonic dystrophy

Myotonic dystrophies: An update on clinical aspects, genetic, pathology, and molecular pathomechanisms

Myotonic dystrophy (DM) is the most common adult muscular dystrophy, characterized by autosomal dominant progressive myopathy, myotonia and multiorgan involvement. To date two distinct forms caused by similar mutations have been identified. Myotonic dystrophy type 1 (DM1, Steinert's disease) is caused by a (CTG)n expansion in DMPK, while myotonic dystrophy type 2 (DM2) is caused by a (CCTG)n expansion in ZNF9/CNBP. When transcribed into CUG/CCUG-containing RNA, mutant transcripts aggregate as nuclear foci that sequester RNA-binding proteins, resulting in spliceopathy of downstream effector genes. However, it is now clear that additional pathogenic mechanism like changes in gene expression, protein translation and micro-RNA metabolism may also contribute to disease pathology. Despite clinical and genetic similarities, DM1 and DM2 are distinct disorders requiring different diagnostic and management strategies. This review is an update on the recent advances in the understanding of the molecular mechanisms behind myotonic dystrophies. This article is part of a Special Issue entitled: Neuromuscular Diseases: Pathology and Molecular Pathogenesis.

LDB3 splicing abnormalities are specific to skeletal muscles of patients with myotonic dystrophy type 1 and alter its PKC binding affinity

Myotonic dystrophy type 1 (DM1) is caused by transcription of CUG repeat RNA, which causes sequestration of muscleblind-like 1 (MBNL1) and upregulation of CUG triplet repeat RNA-binding protein (CUG-BP1). In DM1, dysregulation of these proteins contributes to many aberrant splicing events, causing various symptoms of the disorder. Here, we demonstrate the occurrence of aberrant splicing of LIM domain binding 3 (LDB3) exon 11 in DM1 skeletal muscle. Exon array surveys, RT-PCR, and western blotting studies demonstrated that exon 11 inclusion was DM1 specific and could be reproduced by transfection of a minigene containing the CTG repeat expansion. Moreover, we found that the LDB3 exon 11-positive isoform had reduced affinity for PKC compared to the exon 11-negative isoform. Since PKC exhibits hyperactivation in DM1 and stabilizes CUG-BP1 by phosphorylation, aberrant splicing of LDB3 may contribute to CUG-BP1 upregulation through changes in its affinity for PKC.

DDX6 regulates sequestered nuclear CUG-expanded DMPK-mRNA in dystrophia myotonica type 1

Myotonic dystrophy type 1 (DM1) is caused by CUG triplet expansions in the 3′ UTR of dystrophia myotonica protein kinase (DMPK) messenger ribonucleic acid (mRNA). The etiology of this multi-systemic disease involves pre-mRNA splicing defects elicited by the ability of the CUG-expanded mRNA to ‘sponge’ splicing factors of the muscleblind family. Although nuclear aggregation of CUG-containing mRNPs in distinct foci is a hallmark of DM1, the mechanisms of their homeostasis have not been completely elucidated. Here we show that a DEAD-box helicase, DDX6, interacts with CUG triplet-repeat mRNA in primary fibroblasts from DM1 patients and with CUG–RNA in vitro. DDX6 overexpression relieves DM1 mis-splicing, and causes a significant reduction in nuclear DMPK-mRNA foci. Conversely, knockdown of endogenous DDX6 leads to a significant increase in DMPK-mRNA foci count and to increased sequestration of MBNL1 in the nucleus. While the level of CUG-expanded mRNA is unaffected by increased DDX6 expression, the mRNA re-localizes to the cytoplasm and its interaction partner MBNL1 becomes dispersed and also partially re-localized to the cytoplasm. Finally, we show that DDX6 unwinds CUG-repeat duplexes in vitro in an adenosinetriphosphate-dependent manner, suggesting that DDX6 can remodel and release nuclear DMPK messenger ribonucleoprotein foci, leading to normalization of pathogenic alternative splicing events.

Targeting toxic RNAs that cause myotonic dystrophy type 1 (DM1) with a bisamidinium inhibitor

A working hypothesis for the pathogenesis of myotonic dystrophy type 1 (DM1) involves the aberrant sequestration of an alternative splicing regulator, MBNL1, by expanded CUG repeats, r(CUG)(exp). It has been suggested that a reversal of the myotonia and potentially other symptoms of the DM1 disease can be achieved by inhibiting the toxic MBNL1-r(CUG)(exp) interaction. Using rational design, we discovered an RNA-groove binding inhibitor (ligand 3) that contains two triaminotriazine units connected by a bisamidinium linker. Ligand 3 binds r(CUG)12 with a low micromolar affinity (K(d) = 8 ± 2 μM) and disrupts the MBNL1-r(CUG)12 interaction in vitro (K(i) = 8 ± 2 μM). In addition, ligand 3 is cell and nucleus permeable, exhibits negligible toxicity to mammalian cells, dissolves MBNL1-r(CUG)(exp) ribonuclear foci, and restores misregulated splicing of IR and cTNT in a DM1 cell culture model. Importantly, suppression of r(CUG)(exp) RNA-induced toxicity in a DM1 Drosophila model was observed after treatment with ligand 3. These results suggest ligand 3 as a lead for the treatment of DM1.

Development of an AP-FRET based analysis for characterizing RNA-protein interactions in myotonic dystrophy (DM1)

Förster Resonance Energy Transfer (FRET) microscopy is a powerful tool used to identify molecular interactions in live or fixed cells using a non-radiative transfer of energy from a donor fluorophore in the excited state to an acceptor fluorophore in close proximity. FRET can be a very sensitive tool to study protein-protein and/or protein-nucleic acids interactions. RNA toxicity is implicated in a number of disorders; especially those associated with expanded repeat sequences, such as myotonic dystrophy. Myotonic dystrophy (DM1) is caused by a (CTG)_n repeat expansion in the 3′ UTR of the DMPK gene which results in nuclear retention of mutant DMPK transcripts in RNA foci. This results in toxic gain-of-function effects mediated through altered functions of RNA-binding proteins (e.g. MBNL1, hnRNPH, CUGBP1). In this study we demonstrate the potential of a new acceptor photobleaching assay to measure FRET (AP-FRET) between RNA and protein. We chose to focus on the interaction between MBNL1 and mutant DMPK mRNA in cells from DM1 patients due to the strong microscopic evidence of their co-localization. Using this technique we have direct evidence of intracellular interaction between MBNL1 and the DMPK RNA. Furthermore using the AP-FRET assay and MBNL1 mutants, we show that all four zinc-finger motifs in MBNL1 are crucial for MBNL1-RNA foci interactions. The data derived using this new assay provides compelling evidence for the interaction between RNA binding proteins and RNA foci, and mechanistic insights into MBNL1-RNA foci interaction demonstrating the power of AP-FRET in examining RNA-Protein interactions in DM1.

Small molecule kinase inhibitors alleviate different molecular features of myotonic dystrophy type 1

Expandable (CTG)n repeats in the 3′ UTR of the DMPK gene are a cause of myotonic dystrophy type 1 (DM1), which leads to a toxic RNA gain-of-function disease. Mutant RNAs with expanded CUG repeats are retained in the nucleus and aggregate in discrete inclusions. These foci sequester splicing factors of the MBNL family and trigger upregulation of the CUGBP family of proteins resulting in the mis-splicing of their target transcripts. To date, many efforts to develop novel therapeutic strategies have been focused on disrupting the toxic nuclear foci and correcting aberrant alternative splicing via targeting mutant CUG repeats RNA; however, no effective treatment for DM1 is currently available. Herein, we present results of culturing of human DM1 myoblasts and fibroblasts with two small-molecule ATP-binding site-specific kinase inhibitors, C16 and C51, which resulted in the alleviation of the dominant-negative effects of CUG repeat expansion. Reversal of the DM1 molecular phenotype includes a reduction of the size and number of foci containing expanded CUG repeat transcripts, decreased steady-state levels of CUGBP1 protein, and consequent improvement of the aberrant alternative splicing of several pre-mRNAs misregulated in DM1.

Alternative splicing regulates vesicular trafficking genes in cardiomyocytes during postnatal heart development

During postnatal development the heart undergoes a rapid and dramatic transition to adult function through transcriptional and post-transcriptional mechanisms, including alternative splicing (AS). Here we perform deep RNA-sequencing on RNA from cardiomyocytes and cardiac fibroblasts to conduct a high-resolution analysis of transcriptome changes during postnatal mouse heart development. We reveal extensive changes in gene expression and AS that occur primarily between postnatal days 1 and 28. Cardiomyocytes and cardiac fibroblasts show reciprocal regulation of gene expression reflecting differences in proliferative capacity, cell adhesion functions, and mitochondrial metabolism. We further demonstrate that AS plays a role in vesicular trafficking and membrane organization, These AS transitions are enriched among targets of two RNA-binding proteins, Celf1 and Mbnl1, which undergo developmentally regulated changes in expression. Vesicular trafficking genes affected by AS during normal development (when Celf1 is down-regulated) show a reversion to neonatal splicing patterns after Celf1 re-expression in adults. Short-term Celf1 induction in adult animals results in disrupted transverse tubule organization and calcium handling. These results identify potential roles for AS in multiple aspects of postnatal heart maturation, including vesicular trafficking and intracellular membrane dynamics.

Targeting the r(CGG) repeats that cause FXTAS with modularly assembled small molecules and oligonucleotides

We designed small molecules that bind the structure of the RNA that causes fragile X-associated tremor ataxia syndrome (FXTAS), an incurable neuromuscular disease. FXTAS is caused by an expanded r(CGG) repeat (r(CGG)^exp) that inactivates a protein regulator of alternative pre-mRNA splicing. Our designed compounds modulate r(CGG)^exp toxicity in cellular models of FXTAS, and pull-down experiments confirm that they bind r(CGG)^expin vivo. Importantly, compound binding does not affect translation of the downstream open reading frame (ORF). We compared molecular recognition properties of our optimal compound to oligonucleotides. Studies show that r(CGG)^exp’s self-structure is a significant energetic barrier for oligonucleotide binding. A fully modified 2′-OMethyl phosphorothioate is incapable of completely reversing an FXTAS-associated splicing defect and inhibits translation of the downstream ORF, which could have deleterious effects. Taken together, these studies suggest that a small molecule that recognizes structure may be more well suited for targeting highly structured RNAs that require strand invasion by a complementary oligonucleotide.

Transcriptionally correlated subcellular dynamics of MBNL1 during lens development and their implication for the molecular pathology of myotonic dystrophy type 1

DM1 (myotonic dystrophy type 1) is caused by elongation of a CTG repeat in the DMPK (dystrophia myotonica-protein kinase) gene. mRNA transcripts containing these CUGexp (CUG expansion) repeats form accumulations, or foci, in the nucleus of the cell. The pathogenesis of DM1 is proposed to result from inappropriate patterns of alternative splicing caused by sequestration of the developmentally regulated alternative splicing factor MBNL1 (muscleblind-like 1) by these foci. Since eye lens cataract is a common feature of DM1 we have examined the distribution and dynamics of MBNL1 in lens epithelial cell lines derived from patients with DM1. The results of the present study demonstrate that only a small proportion of nuclear MBNL1 accumulates in CUGexp pre-mRNA foci. MBNL1 is, however, highly mobile and changes localization in response to altered transcription and splicing activity. Moreover, immunolocalization studies in lens sections suggest that a change in MBNL1 distribution is important during lens growth and differentiation. Although these data suggest that the loss of MBNL1 function due to accumulation in foci is an unlikely explanation for DM1 symptoms in the lens, they do demonstrate a strong relationship between the subcellular MBNL1 localization and pathways of cellular differentiation, providing an insight into the sensitivity of the lens to changes in MBNL1 distribution.

Consensus paper: pathological mechanisms underlying neurodegeneration in spinocerebellar ataxias

Intensive scientific research devoted in the recent years to understand the molecular mechanisms or neurodegeneration in spinocerebellar ataxias (SCAs) are identifying new pathways and targets providing new insights and a better understanding of the molecular pathogenesis in these diseases. In this consensus manuscript, the authors discuss their current views on the identified molecular processes causing or modulating the neurodegenerative phenotype in spinocerebellar ataxias with the common opinion of translating the new knowledge acquired into candidate targets for therapy. The following topics are discussed: transcription dysregulation, protein aggregation, autophagy, ion channels, the role of mitochondria, RNA toxicity, modulators of neurodegeneration and current therapeutic approaches. Overall point of consensus includes the common vision of neurodegeneration in SCAs as a multifactorial, progressive and reversible process, at least in early stages. Specific points of consensus include the role of the dysregulation of protein folding, transcription, bioenergetics, calcium handling and eventual cell death with apoptotic features of neurons during SCA disease progression. Unresolved questions include how the dysregulation of these pathways triggers the onset of symptoms and mediates disease progression since this understanding may allow effective treatments of SCAs within the window of reversibility to prevent early neuronal damage. Common opinions also include the need for clinical detection of early neuronal dysfunction, for more basic research to decipher the early neurodegenerative process in SCAs in order to give rise to new concepts for treatment strategies and for the translation of the results to preclinical studies and, thereafter, in clinical practice.

Ultrasound-enhanced delivery of morpholino with Bubble liposomes ameliorates the myotonia of myotonic dystrophy model mice

Phosphorodiamidate morpholino oligonucleotide (PMO)-mediated control of the alternative splicing of the chloride channel 1 (CLCN1) gene is a promising treatment for myotonic dystrophy type 1 (DM1) because the abnormal splicing of this gene causes myotonia in patients with DM1. In this study, we optimised a PMO sequence to correct Clcn1 alternative splicing and successfully remedied the myotonic phenotype of a DM1 mouse model, the HSALR mouse. To enhance the efficiency of delivery of PMO into HSALR mouse muscles, Bubble liposomes, which have been used as a gene delivery tool, were applied with ultrasound exposure. Effective delivery of PMO led to increased expression of Clcn1 protein in skeletal muscle and the amelioration of myotonia. Thus, PMO-mediated control of the alternative splicing of the Clcn1 gene must be important target of antisense therapy of DM1.

Tau exon 2 responsive elements deregulated in myotonic dystrophy type I are proximal to exon 2 and synergistically regulated by MBNL1 and MBNL2

The splicing of the microtubule-associated protein Tau is regulated during development and is found to be deregulated in a growing number of pathological conditions such as myotonic dystrophy type I (DM1), in which a reduced number of isoforms is expressed in the adult brain. DM1 is caused by a dynamic and unstable CTG repeat expansion in the DMPK gene, resulting in an RNA bearing long CUG repeats (n>50) that accumulates in nuclear foci and sequesters CUG-binding splicing factors of the muscle blind-like (MBNL) family, involved in the splicing of Tau pre-mRNA among others. However, the precise mechanism leading to Tau mis-splicing and the role of MBNL splicing factors in this process are poorly understood. We therefore used new Tau minigenes that we developed for this purpose to determine how MBNL1 and MBNL2 interact to regulate Tau exon 2 splicing. We demonstrate that an intronic region 250 nucleotides downstream of Tau exon 2 contains cis-regulatory splicing enhancers that are sensitive to MBNL and that bind directly to MBNL1. Both MBNL1 and MBNL2 act as enhancers of Tau exon 2 inclusion. Intriguingly, the interaction of MBNL1 and MBNL2 is required to fully reverse the mis-splicing of Tau exon 2 induced by the trans-dominant effect of long CUG repeats, similar to the DM1 condition. In conclusion, both MBNL1 and MBNL2 are involved in the regulation of Tau exon 2 splicing and the mis-splicing of Tau in DM1 is due to the combined inactivation of both.

Brain pathology in myotonic dystrophy: when tauopathy meets spliceopathy and RNAopathy

Myotonic dystrophy (DM) of type 1 and 2 (DM1 and DM2) are inherited autosomal dominant diseases caused by dynamic and unstable expanded microsatellite sequences (CTG and CCTG, respectively) in the non-coding regions of the genes DMPK and ZNF9, respectively. These mutations result in the intranuclear accumulation of mutated transcripts and the mis-splicing of numerous transcripts. This so-called RNA gain of toxic function is the main feature of an emerging group of pathologies known as RNAopathies. Interestingly, in addition to these RNA inclusions, called foci, the presence of neurofibrillary tangles (NFT) in patient brains also distinguishes DM as a tauopathy. Tauopathies are a group of nearly 30 neurodegenerative diseases that are characterized by intraneuronal protein aggregates of the microtubule-associated protein Tau (MAPT) in patient brains. Furthermore, a number of neurodegenerative diseases involve the dysregulation of splicing regulating factors and have been characterized as spliceopathies. Thus, myotonic dystrophies are pathologies resulting from the interplay among RNAopathy, spliceopathy, and tauopathy. This review will describe how these processes contribute to neurodegeneration. We will first focus on the tauopathy associated with DM1, including clinical symptoms, brain histology, and molecular mechanisms. We will also discuss the features of DM1 that are shared by other tauopathies and, consequently, might participate in the development of a tauopathy. Moreover, we will discuss the determinants common to both RNAopathies and spliceopathies that could interfere with tau-related neurodegeneration.

RNA-binding protein misregulation in microsatellite expansion disorders

RNA-binding proteins (RBPs) play pivotal roles in multiple cellular pathways from transcription to RNA turnover by interacting with RNA sequence and/or structural elements to form distinct RNA-protein complexes. Since these complexes are required for the normal regulation of gene expression, mutations that alter RBP functions may result in a cascade of deleterious events that lead to severe disease. Here, we focus on a group of hereditary disorders, the microsatellite expansion diseases, which alter RBP activities and result in abnormal neurological and neuromuscular phenotypes. While many of these diseases are classified as adult-onset disorders, mounting evidence indicates that disruption of normal RNA-protein interaction networks during embryogenesis modifies developmental pathways, which ultimately leads to disease manifestations later in life. Efforts to understand the molecular basis of these disorders has already uncovered novel pathogenic mechanisms, including RNA toxicity and repeat-associated non-ATG (RAN) translation, and current studies suggest that additional surprising insights into cellular regulatory pathways will emerge in the future.

RNA-binding proteins in heart development

RNA-binding proteins (RBPs) are key players of posttranscriptional regulation occurring during normal tissue development. All tissues examined thus far have revealed the importance of RBPs in the regulation of complex networks involved in organ morphogenesis, maturation, and function. They are responsible for controlling tissue-specific gene expression by regulating alternative splicing, mRNA stability, translation, and poly-adenylation. The heart is the first organ form during embryonic development and is also the first to acquire functionality. Numerous remodeling processes take place during late cardiac development since fetal heart first adapts to birth and then undergoes a transition to adult functionality. This physiological remodeling involves transcriptional and posttranscriptional networks that are regulated by RBPs. Disruption of the normal regulatory networks has been shown to cause cardiomyopathy in humans and animal models. Here we review the complexity of late heart development and the current information regarding how RBPs control aspects of postnatal heart development. We also review how activities of RBPs are modulated adding complexity to the regulation of developmental networks.

Induction and reversal of myotonic dystrophy type 1 pre-mRNA splicing defects by small molecules

The ability to control pre-mRNA splicing with small molecules could facilitate the development of therapeutics or cell-based circuits that control gene function. Myotonic dystrophy type 1 (DM1) is caused by the dysregulation of alternative pre-mRNA splicing due to sequestration of muscleblind-like 1 protein (MBNL1) by expanded, non-coding r(CUG) repeats (r(CUG)^exp). Here we report two small molecules that induce or ameliorate alternative splicing dysregulation. The thiophene-containing small molecule (1) inhibits the interaction of MBNL1 with its natural pre-mRNA substrates. Compound (2), a substituted naphthyridine, binds r(CUG)^exp and displaces MBNL1. Structural models show that 1 binds MBNL1 in the Zn-finger domain and that 2 interacts with UU loops in r(CUG)^exp. This study provides a structural framework for small molecules that target MBNL1 by mimicking r(CUG)^exp and shows that targeting MBNL1 causes dysregulation of alternative splicing, suggesting that MBNL1 is thus not a suitable therapeutic target for the treatment of DM1.

Splicing biomarkers of disease severity in myotonic dystrophy

OBJECTIVE

To develop RNA splicing biomarkers of disease severity and therapeutic response in myotonic dystrophy type 1 (DM1) and type 2 (DM2).

METHODS

In a discovery cohort, we used microarrays to perform global analysis of alternative splicing in DM1 and DM2. The newly identified splicing changes were combined with previous data to create a panel of 50 putative splicing defects. In a validation cohort of 50 DM1 subjects, we measured the strength of ankle dorsiflexion (ADF) and then obtained a needle biopsy of tibialis anterior (TA) to analyze splice events in muscle RNA. The specificity of DM-associated splicing defects was assessed in disease controls. The CTG expansion size in muscle tissue was determined by Southern blot. The reversibility of splicing defects was assessed in transgenic mice by using antisense oligonucleotides to reduce levels of toxic RNA.

RESULTS

Forty-two splicing defects were confirmed in TA muscle in the validation cohort. Among these, 20 events showed graded changes that correlated with ADF weakness. Five other splice events were strongly affected in DM1 subjects with normal ADF strength. Comparison to disease controls and mouse models indicated that splicing changes were DM-specific, mainly attributable to MBNL1 sequestration, and reversible in mice by targeted knockdown of toxic RNA. Splicing defects and weakness were not correlated with CTG expansion size in muscle tissue.

INTERPRETATION

Alternative splicing changes in skeletal muscle may serve as biomarkers of disease severity and therapeutic response in myotonic dystrophy.

Most expression and splicing changes in myotonic dystrophy type 1 and type 2 skeletal muscle are shared with other muscular dystrophies

The prevailing pathomechanistic paradigm for myotonic dystrophy (DM) is that aberrant expression of embryonic/fetal mRNA/protein isoforms accounts for most aspects of the pleiotropic phenotype. To identify aberrant isoforms in skeletal muscle of DM1 and DM2 patients, we performed exon-array profiling and RT-PCR validation on the largest DM sample set to date, including Duchenne, Becker and tibial muscular dystrophy (NMD) patients as disease controls, and non-disease controls. Strikingly, most expression and splicing changes in DM patients were shared with NMD controls. Comparison between DM and NMD identified almost no significant differences. We conclude that DM1 and DM2 are essentially identical for dysregulation of gene expression, and DM expression changes represent a subset of broader spectrum dystrophic changes. We found no evidence for qualitative splicing differences between DM1 and DM2. While some DM-specific splicing differences exist, most of the DM splicing differences were also seen in NMD controls. SSBP3 exon 6 missplicing was observed in all diseased muscle and led to reduced protein. We conclude there is no widespread DM-specific spliceopathy in skeletal muscle and suggest that missplicing in DM (and NMD) may not be the driving mechanism for the muscle pathology, since the same pathways show expression changes unrelated to splicing.

High-content screening identifies small molecules that remove nuclear foci, affect MBNL distribution and CELF1 protein levels via a PKC-independent pathway in myotonic dystrophy cell lines

Myotonic dystrophy (DM) is a multi-system neuromuscular disorder for which there is no treatment. We have developed a medium throughput phenotypic assay, based on the identification of nuclear foci in DM patient cell lines using in situ hybridization and high-content imaging to screen for potentially useful therapeutic compounds. A series of further assays based on molecular features of DM have also been employed. Two compounds that reduce and/or remove nuclear foci have been identified, Ro 31-8220 and chromomycin A3. Ro 31-8220 is a PKC inhibitor, previously shown to affect the hyperphosphorylation of CELF1 and ameliorate the cardiac phenotype in a DM1 mouse model. We show that the same compound eliminates nuclear foci, reduces MBNL1 protein in the nucleus, affects ATP2A1 alternative splicing and reduces steady-state levels of CELF1 protein. We demonstrate that this effect is independent of PKC activity and conclude that this compound may be acting on alternative kinase targets within DM pathophysiology. Understanding the activity profile for this compound is key for the development of targeted therapeutics in the treatment of DM.

Treatment of type 1 myotonic dystrophy by engineering site-specific RNA endonucleases that target (CUG)(n) repeats

Myotonic dystrophy type 1 (DM1) is caused by the expansion of (CTG)n in the 3' untranslated region of the dystrophia myotonica-protein kinase (DMPK) gene, which is transcribed as (CUG)n repeats that accumulate in the nucleus. The RNA repeats specifically sequester or change the expression levels of several RNA-binding proteins, leading to aberrant splicing of many target genes. In this study, we developed artificial site-specific RNA endonucleases (ASREs) that specifically bind and cleave (CUG)n repeats RNA. We have generated one ASRE that can target the expanded RNA repeats in DM1 patient cells and specifically degrade the pathogenic DMPK messenger RNAs with minimal effect on wild-type alleles. Such ASRE treatment significantly decreased the number of nuclear foci in DM1 patient cells and can reverse the missplicing of many genes affected in DM1 patients. Taken together, the application of ASRE provides a new route of gene therapy for DM1 treatment.

Features of modularly assembled compounds that impart bioactivity against an RNA target

Transcriptomes provide a myriad of potential RNAs that could be the targets of therapeutics or chemical genetic probes of function. Cell-permeable small molecules, however, generally do not exploit these targets, owing to the difficulty in the design of high affinity, specific small molecules targeting RNA. As part of a general program to study RNA function using small molecules, we designed bioactive, modularly assembled small molecules that target the noncoding expanded RNA repeat that causes myotonic dystrophy type 1 (DM1), r(CUG)(exp). Herein, we present a rigorous study to elucidate features in modularly assembled compounds that afford bioactivity. Different modular assembly scaffolds were investigated, including polyamines, α-peptides, β-peptides, and peptide tertiary amides (PTAs). On the basis of activity as assessed by improvement of DM1-associated defects, stability against proteases, cellular permeability, and toxicity, we discovered that constrained backbones, namely, PTAs, are optimal. Notably, we determined that r(CUG)(exp) is the target of the optimal PTA in cellular models and that the optimal PTA improves DM1-associated defects in a mouse model. Biophysical analyses were employed to investigate potential sources of bioactivity. These investigations show that modularly assembled compounds have increased residence times on their targets and faster on rates than the RNA-binding modules from which they were derived. Moreover, they have faster on rates than the protein that binds r(CUG)(exp), the inactivation of which gives rise to DM1-associated defects. These studies provide information about features of small molecules that are programmable for targeting RNA, allowing for the facile optimization of therapeutics or chemical probes against other cellular RNA targets.

Purification, crystallization and preliminary crystallographic studies of C-terminal RNA recognition motif (RRM-3) of human ELAV-type RNA-binding protein 3 (ETR-3)

Human embryonically lethal abnormal vision (ELAV)-type RNA-binding protein 3 (ETR-3) has been implicated in many aspects of RNA-processing events including alternative splicing, stability, editing and translation. RNA recognition motif 3 (RRM-3) is an independent C-terminal RNA-binding domain of ETR-3 that preferentially binds to UG-rich repeats of the nuclear or cytoplasmic pre-mRNA, and along with the other domains mediates the inclusion of cardiac troponin T (c-TNT) exon 5 in embryonic muscle, which is otherwise excluded in the adult. In the present study, RRM-3 was cloned, overexpressed, purified and crystallized by the hanging-drop vapour-diffusion method. The crystals diffracted to 3 Å resolution at the home source and belonged to space group P2₁3, with unit-cell parameters a=b=c=118.5 Å, α=β=γ=90°. There were two molecules of RRM-3 in the asymmetric unit and the calculated Matthews coefficient (VM) was 6.35 Å3 Da(-1), with a solvent content of 80.62%. Initial phases were determined by molecular replacement.

Compound loss of muscleblind-like function in myotonic dystrophy

Myotonic dystrophy (DM) is a multi-systemic disease that impacts cardiac and skeletal muscle as well as the central nervous system (CNS). DM is unusual because it is an RNA-mediated disorder due to the expression of toxic microsatellite expansion RNAs that alter the activities of RNA processing factors, including the muscleblind-like (MBNL) proteins. While these mutant RNAs inhibit MBNL1 splicing activity in heart and skeletal muscles, Mbnl1 knockout mice fail to recapitulate the full-range of DM symptoms in these tissues. Here, we generate mouse Mbnl compound knockouts to test the hypothesis that Mbnl2 functionally compensates for Mbnl1 loss. Although Mbnl1^−/−; Mbnl2^−/− double knockouts (DKOs) are embryonic lethal, Mbnl1^−/−; Mbnl2^+/− mice are viable but develop cardinal features of DM muscle disease including reduced lifespan, heart conduction block, severe myotonia and progressive skeletal muscle weakness. Mbnl2 protein levels are elevated in Mbnl1^−/− knockouts where Mbnl2 targets Mbnl1-regulated exons. These findings support the hypothesis that compound loss of MBNL function is a critical event in DM pathogenesis and provide novel mouse models to investigate additional pathways disrupted in this RNA-mediated disease.

Transcriptional changes and developmental abnormalities in a zebrafish model of myotonic dystrophy type 1

Myotonic dystrophy type I (DM1) is a multi-system, autosomal dominant disorder caused by expansion of a CTG repeat sequence in the 3′UTR of the DMPK gene. The size of the repeat sequence correlates with age at onset and disease severity, with large repeats leading to congenital forms of DM1 associated with hypotonia and intellectual disability. In models of adult DM1, expanded CUG repeats lead to an RNA toxic gain of function, mediated at least in part by sequestering specific RNA splicing proteins, most notably muscleblind-related (MBNL) proteins. However, the impact of CUG RNA repeat expression on early developmental processes is not well understood. To better understand early developmental processes in DM1, we utilized the zebrafish, Danio rerio, as a model system. Direct injection of (CUG)₉₁ repeat-containing mRNA into single-cell embryos induces toxicity in the nervous system and muscle during early development. These effects manifest as abnormal morphology, behavioral abnormalities and broad transcriptional changes, as shown by cDNA microarray analysis. Co-injection of zebrafish mbnl2 RNA suppresses (CUG)₉₁ RNA toxicity and reverses the associated behavioral and transcriptional abnormalities. Taken together, these findings suggest that early expression of exogenously transcribed CUG repeat RNA can disrupt normal muscle and nervous system development and provides a new model for DM1 research that is amenable to small-molecule therapeutic development.

GSK3β is a new therapeutic target for myotonic dystrophy type 1

Myotonic dystrophy type 1 (DM1), an incurable, neuromuscular disease, is caused by the expansion of CTG repeats within the 3′ UTR of DMPK on chromosome 19q. In DM1 patients, mutant DMPK transcripts deregulate RNA metabolism by altering CUG RNA-binding proteins. Several approaches have been proposed for DM1 therapy focused on specific degradation of the mutant CUG repeats or on correction of RNA-binding proteins, affected by CUG repeats. One such protein is CUG RNA-binding protein (CUGBP1). The ability of CUGBP1 to increase or inhibit translation depends on phosphorylation at Ser302, which is mediated by cyclin D3-CDK4. The mutant CUG repeats increase the levels of CUGBP1 protein and inhibit Ser302 phosphorylation, leading to the accumulation of CUGBP1 isoforms that repress translation (i.e., CUGBP1^REP). Elevation of CUGBP1^REP in DM1 is caused by increased GSK3β kinase, which reduces the cyclin D3-CDK4 pathway and subsequent phosphorylation of CUGBP1 at Ser302. In this review, we discuss our recent discovery showing that correction of GSK3β activity in the DM1 mouse model (i.e., HSA^LR mice) reduces DM1 muscle pathology. These findings demonstrate that GSK3β is a novel therapeutic target for treating DM1.

Posttranscriptional regulation of insulin family ligands and receptors

Insulin system including ligands (insulin and IGFs) and their shared receptors (IR and IGFR) are critical regulators of insulin signaling and glucose homeostasis. Altered insulin system is associated with major pathological conditions like diabetes and cancer. The mRNAs encoding for these ligands and their receptors are posttranscriptionally controlled by three major groups of regulators; (i) alternative splicing regulatory factors; (ii) turnover and translation regulator RNA-binding proteins (TTR-RBPs); and (iii) non-coding RNAs including miRNAs and long non-coding RNAs (lncRNAs). In this review, we discuss the influence of these regulators on alternative splicing, mRNA stability and translation. Due to the pathological impacts of insulin system, we also discussed the possibilities of discovering new potential regulators which will improve understanding of insulin system and associated diseases.

Age of onset of RNA toxicity influences phenotypic severity: evidence from an inducible mouse model of myotonic dystrophy (DM1)

Myotonic dystrophy type 1 (DM1) is the most common muscular dystrophy in adults. It is caused by an expanded (CTG)n tract in the 3′ UTR of the Dystrophia Myotonica Protein Kinase (DMPK) gene. This causes nuclear retention of the mutant mRNA into ribonuclear foci and sequestration of interacting RNA-binding proteins (such as muscleblind-like 1 (MBNL1)). More severe congenital and childhood-onset forms of the disease exist but are less understood than the adult disease, due in part to the lack of adequate animal models. To address this, we utilized transgenic mice over-expressing the DMPK 3′ UTR as part of an inducible RNA transcript to model early-onset myotonic dystrophy. In mice in which transgene expression was induced during embryogenesis, we found that by two weeks after birth, mice reproduced cardinal features of myotonic dystrophy, including myotonia, cardiac conduction abnormalities, muscle weakness, histopathology and mRNA splicing defects. Notably, these defects were more severe than in adult mice induced for an equivalent period of exposure to RNA toxicity. Additionally, the utility of the model was tested by over-expressing MBNL1, a key therapeutic strategy being actively pursued for treating the disease phenotypes associated with DM1. Significantly, increased MBNL1 in skeletal muscle partially corrected myotonia and splicing defects present in these mice, demonstrating the responsiveness of the model to relevant therapeutic interventions. Furthermore, these results also represent the first murine model for early-onset DM1 and provide a tool to investigate the effects of RNA toxicity at various stages of development.

Evaluating the effects of CELF1 deficiency in a mouse model of RNA toxicity

Myotonic dystrophy type 1 (DM1), the most common form of adult-onset muscular dystrophy, is caused by an expanded (CTG)n repeat in the 3' untranslated region of the DM protein kinase (DMPK) gene. The toxic RNA transcripts produced from the mutant allele alter the function of RNA-binding proteins leading to the functional depletion of muscleblind-like (MBNL) proteins and an increase in steady state levels of CUG-BP1 (CUGBP-ETR-3 like factor 1, CELF1). The role of increased CELF1 in DM1 pathogenesis is well studied using genetically engineered mouse models. Also, as a potential therapeutic strategy, the benefits of increasing MBNL1 expression have recently been reported. However, the effect of reduction of CELF1 is not yet clear. In this study, we generated CELF1 knockout mice, which also carry an inducible toxic RNA transgene to test the effects of CELF1 reduction in RNA toxicity. We found that the absence of CELF1 did not correct splicing defects. It did however mitigate the increase in translational targets of CELF1 (MEF2A and C/EBPβ). Notably, we found that loss of CELF1 prevented deterioration of muscle function by the toxic RNA, and resulted in better muscle histopathology. These data suggest that while reduction of CELF1 may be of limited benefit with respect to DM1-associated spliceopathy, it may be beneficial to the muscular dystrophy associated with RNA toxicity.

MBNL142 and MBNL143 gene isoforms, overexpressed in DM1-patient muscle, encode for nuclear proteins interacting with Src family kinases

Myotonic dystrophy type-1 (DM1) is the most prevalent form of muscular dystrophy in adults. This disorder is an RNA-dominant disease, caused by expansion of a CTG repeat in the DMPK gene that leads to a misregulation in the alternative splicing of pre-mRNAs. The longer muscleblind-like-1 (MBNL1) transcripts containing exon 5 and the respective protein isoforms (MBNL142-43) were found to be overexpressed in DM1 muscle and localized exclusively in the nuclei. In vitro assays showed that MBNL142-43 bind the Src-homology 3 domain of Src family kinases (SFKs) via their proline-rich motifs, enhancing the SFK activity. Notably, this association was also confirmed in DM1 muscle and myotubes. The recovery, mediated by an siRNA target to Ex5-MBNL142-43, succeeded in reducing the nuclear localization of both Lyn and MBNL142-43 proteins and in decreasing the level of tyrosine phosphorylated proteins. Our results suggest an additional molecular mechanism in the DM1 pathogenesis, based on an altered phosphotyrosine signalling pathway.

Rational design of chemical genetic probes of RNA function and lead therapeutics targeting repeating transcripts

RNA is an important yet vastly underexploited target for small molecule chemical probes or lead therapeutics. Small molecules have been used successfully to modulate the function of the bacterial ribosome, viral RNAs and riboswitches. These RNAs are either highly expressed or can be targeted using substrate mimicry, a mainstay in the design of enzyme inhibitors. However, most cellular RNAs are neither highly expressed nor have a lead small molecule inhibitor, a significant challenge for drug discovery efforts. Herein, I describe the design of small molecules targeting expanded repeating transcripts that cause myotonic muscular dystrophy (DM). These test cases illustrate the challenges of designing small molecules that target RNA and the advantages of targeting repeating transcripts. Lastly, I discuss how small molecules might be more advantageous than oligonucleotides for targeting RNA.

A defective Krab-domain zinc-finger transcription factor contributes to altered myogenesis in myotonic dystrophy type 1

Myotonic dystrophy type 1 (DM1) is an RNA-mediated disorder caused by a non-coding CTG repeat expansion that, in particular, provokes functional alteration of CUG-binding proteins. As a consequence, several genes with misregulated alternative splicing have been linked to clinical symptoms. In our search for additional molecular mechanisms that would trigger functional defects in DM1, we took advantage of mutant gene-carrying human embryonic stem cell lines to identify differentially expressed genes. Among the different genes found to be misregulated by DM1 mutation, one strongly downregulated gene encodes a transcription factor, ZNF37A. In this paper, we show that this defect in expression, which derives from a loss of RNA stability, is controlled by the RNA-binding protein, CUGBP1, and is associated with impaired myogenesis-a functional defect reminiscent of that observed in DM1. Loss of the ZNF37A protein results in changes in the expression of the subunit α1 of the receptor for the interleukin 13. This suggests that the pathological molecular mechanisms linking ZNF37A and myogenesis may involve the signaling pathway that is known to promote myoblast recruitment during development and regeneration.

Emerging complexity of the HuD/ELAVl4 gene; implications for neuronal development, function, and dysfunction

Precise control of messenger RNA (mRNA) processing and abundance are increasingly being recognized as critical for proper spatiotemporal gene expression, particularly in neurons. These regulatory events are governed by a large number of trans-acting factors found in neurons, most notably RNA-binding proteins (RBPs) and micro-RNAs (miRs), which bind to specific cis-acting elements or structures within mRNAs. Through this binding mechanism, trans-acting factors, particularly RBPs, control all aspects of mRNA metabolism, ranging from altering the transcription rate to mediating mRNA degradation. In this context the best-characterized neuronal RBP, the Hu/ELAVl family member HuD, is emerging as a key component in multiple regulatory processes--including pre-mRNA processing, mRNA stability, and translation--governing the fate of a substantial amount of neuronal mRNAs. Through its ability to regulate mRNA metabolism of diverse groups of functionally similar genes, HuD plays important roles in neuronal development and function. Furthermore, compelling evidence indicates supplementary roles for HuD in neuronal plasticity, in particular, recovery from axonal injury, learning and memory, and multiple neurological diseases. The purpose of this review is to provide a detailed overview of the current knowledge surrounding the expression and roles of HuD in the nervous system. Additionally, we outline the present understanding of the molecular mechanisms presiding over the localization, abundance, and function of HuD in neurons.

The cell biology of disease: cellular and molecular mechanisms underlying muscular dystrophy

The muscular dystrophies are a group of heterogeneous genetic diseases characterized by progressive degeneration and weakness of skeletal muscle. Since the discovery of the first muscular dystrophy gene encoding dystrophin, a large number of genes have been identified that are involved in various muscle-wasting and neuromuscular disorders. Human genetic studies complemented by animal model systems have substantially contributed to our understanding of the molecular pathomechanisms underlying muscle degeneration. Moreover, these studies have revealed distinct molecular and cellular mechanisms that link genetic mutations to diverse muscle wasting phenotypes.

Novel Drosophila model of myotonic dystrophy type 1: phenotypic characterization and genome-wide view of altered gene expression

Myotonic dystrophy type 1 (DM1) is a multisystemic RNA-dominant disorder characterized by myotonia and muscle degeneration. In DM1 patients, the mutant DMPK transcripts containing expanded CUG repeats form nuclear foci and sequester the Muscleblind-like 1 splicing factor, resulting in mis-splicing of its targets. However, several pathological defects observed in DM1 and their link with disease progression remain poorly understood. In an attempt to fill this gap, we generated inducible transgenic Drosophila lines with increasing number of CTG repeats. Targeting the expression of these repeats to the larval muscles recapitulated in a repeat-size-dependent manner the major DM1 symptoms such as muscle hypercontraction, splitting of muscle fibers, reduced fiber size or myoblast fusion defects. Comparative transcriptional profiling performed on the generated DM1 lines and on the muscleblind (mbl)-RNAi line revealed that nuclear accumulation of toxic CUG repeats can affect gene expression independently of splicing or Mbl sequestration. Also, in mblRNAi contexts, the largest portion of deregulated genes corresponded to single-transcript genes, revealing an unexpected impact of the indirect influence of mbl on gene expression. Among the single-transcript Mbl targets is Muscle protein 20 involved in myoblast fusion and causing the reduced number of nuclei in muscles of mblRNAi larvae. Finally, by combining in silico prediction of Mbl targets with mblRNAi microarray data, we found the calcium pump dSERCA as a Mbl splice target and show that the membrane dSERCA isoform is sufficient to rescue a DM1-induced hypercontraction phenotype in a Drosophila model.

Molecular mechanisms of muscle atrophy in myotonic dystrophies

Myotonic dystrophy type 1 (DM1) and myotonic dystrophy type 2 (DM2) are multisystemic diseases that primarily affect skeletal muscle, causing myotonia, muscle atrophy, and muscle weakness. DM1 and DM2 pathologies are caused by expansion of CTG and CCTG repeats in non-coding regions of the genes encoding myotonic dystrophy protein kinase (DMPK) and zinc finger protein 9 (ZNF9) respectively. These expansions cause DM pathologies through accumulation of mutant RNAs that alter RNA metabolism in patients' tissues by targeting RNA-binding proteins such as CUG-binding protein 1 (CUGBP1) and Muscle blind-like protein 1 (MBNL1). Despite overwhelming evidence showing the critical role of RNA-binding proteins in DM1 and DM2 pathologies, the downstream pathways by which these RNA-binding proteins cause muscle wasting and muscle weakness are not well understood. This review discusses the molecular pathways by which DM1 and DM2 mutations might cause muscle atrophy and describes progress toward the development of therapeutic interventions for muscle wasting and weakness in DM1 and DM2. This article is part of a Directed Issue entitled: Molecular basis of muscle wasting.

The alternative heart: impact of alternative splicing in heart disease

Alternative splicing is the main driver of protein diversity and allows the production of different proteins from each gene in the genome. Changes in exon exclusion, intron retention or the use of alternative splice sites can alter protein structure, localisation, regulation and function. In the heart, alternative splicing of sarcomeric genes, ion channels and cell signalling proteins can lead to cardiomyopathies, arrhythmias and other pathologies. Also, a number of inherited conditions and heart-related diseases develop as a result of mutations affecting splicing. Here, we review the impact that changes in alternative splicing have on individual genes and on whole biological processes associated with heart disease. We also discuss promising therapeutic tools based on the manipulation of alternative splicing.

RNA toxicity in polyglutamine disorders: concepts, models, and progress of research

In Huntington's disease and other polyglutamine (polyQ) disorders, mutant proteins containing a long polyQ stretch are well documented as the trigger of numerous aberrant cellular processes that primarily lead to degeneration and, ultimately, the death of neuronal cells. However, mutant transcripts containing expanded CAG repeats may also be toxic and contribute to cellular dysfunction. The exact nature and importance of RNA toxicity in polyQ diseases are only beginning to be recognized, and the first insights have mainly resulted from studies using simple model systems. In this review, we briefly present the basic mechanisms of protein toxicity in polyQ disorders and RNA toxicity in myotonic dystrophy type 1 and discuss recent results suggesting that the pathogenesis of polyQ diseases may also be mediated by mutant transcripts. This review is focused on the experimental systems used thus far to demonstrate RNA toxicity in polyQ disorders and the design of new systems that will be more relevant to the human disease situation and capable of separating RNA toxicity from protein toxicity.

Bidirectional transcription of trinucleotide repeats: roles for excision repair

Genomic instability at repetitive DNA regions in cells of the nervous system leads to a number of neurodegenerative and neuromuscular diseases, including those with an expanded trinucleotide repeat (TNR) tract at or nearby an expressed gene. Expansion causes disease when a particular base sequence is repeated beyond the normal range, interfering with the expression or properties of a gene product. Disease severity and onset depend on the number of repeats. As the length of the repeat tract grows, so does the size of the successive expansions and the likelihood of another unstable event. In fragile X syndrome, for example, CGG repeat instability and pathogenesis are not typically observed below tracts of roughly 50 repeats, but occur frequently at or above 55 repeats, and are virtually certain above 100-300 repeats. Recent evidence points to bidirectional transcription as a new aspect of TNR instability and pathophysiology. Bidirectional transcription of TNR genes produces novel proteins and/or regulatory RNAs that influence both toxicity and epigenetic changes in TNR promoters. Bidirectional transcription of the TNR tract appears to influence aspects of its stability, gene processing, splicing, gene silencing, and chemical modification of DNAs. Paradoxically, however, some of the same effects are observed on both the expanded TNR gene and on its normal gene counterpart. In this review, we discuss the possible normal and abnormal effects of bidirectional transcription on trinucleotide repeat instability, the role of DNA repair in causing, preventing, or maintaining methylation, and chromatin environment of TNR genes.

Alternative splicing of human insulin receptor gene (INSR) in type I and type II skeletal muscle fibers of patients with myotonic dystrophy type 1 and type 2

INSR, one of those genes aberrantly expressed in myotonic dystrophy type 1 (DM1) and type 2 (DM2) due to a toxic RNA effect, encodes for the insulin receptor (IR). Its expression is regulated by alternative splicing generating two isoforms: IR-A, which predominates in embryonic tissue, and IR-B, which is highly expressed in adult, insulin-responsive tissues (skeletal muscle, liver, and adipose tissue). The aberrant INSR expression detected in DM1 and DM2 muscles tissues, characterized by a relative increase of IR-A versus IR-B, was pathogenically related to the insulin resistance occurring in DM patients. To assess if differences in the aberrant splicing of INSR could underlie the distinct fiber type involvement observed in DM1 and DM2 muscle tissues, we have used laser capture microdissection (LCM) and RT-PCR, comparing the alternative splicing of INSR in type I and type II muscle fibers isolated from muscle biopsies of DM1, DM2 patients and controls. In the controls, the relative amounts of IR-A and IR-B showed no obvious differences between type I and type II fibers, as in the whole muscle tissue. In DM1 and DM2 patients, both fiber types showed a similar, relative increase of IR-A versus IR-B, as also evident in the whole muscle tissue. Our data suggest that the distinct fiber type involvement in DM1 and DM2 muscle tissues would not be related to qualitative differences in the expression of INSR. LCM can represent a powerful tool to give a better understanding of the pathogenesis of myotonic dystrophies, as well as other myopathies.

Progressive impairment of muscle regeneration in muscleblind-like 3 isoform knockout mice

The muscleblind-like (MBNL) genes encode alternative splicing factors that are essential for the postnatal development of multiple tissues, and the inhibition of MBNL activity by toxic C(C)UG repeat RNAs is a major pathogenic feature of the neuromuscular disease myotonic dystrophy. While MBNL1 controls fetal-to-adult splicing transitions in muscle and MBNL2 serves a similar role in the brain, the function of MBNL3 in vivo is unknown. Here, we report that mouse Mbnl3, which encodes protein isoforms that differ in the number of tandem zinc-finger RNA-binding motifs and subcellular localization, is expressed primarily during embryonic development but also transiently during injury-induced adult skeletal muscle regeneration. Mbnl3 expression is required for normal C2C12 myogenic differentiation and high-throughput sequencing combined with cross-linking/immunoprecipitation analysis indicates that Mbnl3 binds preferentially to the 3' untranslated regions of genes implicated in cell growth and proliferation. In addition, Mbnl3ΔE2 isoform knockout mice, which fail to express the major Mbnl3 nuclear isoform, show age-dependent delays in injury-induced muscle regeneration and impaired muscle function. These results suggest that Mbnl3 inhibition by toxic RNA expression may be a contributing factor to the progressive skeletal muscle weakness and wasting characteristic of myotonic dystrophy.

Smaug/SAMD4A restores translational activity of CUGBP1 and suppresses CUG-induced myopathy

We report the identification and characterization of a previously unknown suppressor of myopathy caused by expansion of CUG repeats, the mutation that triggers Myotonic Dystrophy Type 1 (DM1). We screened a collection of genes encoding RNA-binding proteins as candidates to modify DM1 pathogenesis using a well established Drosophila model of the disease. The screen revealed smaug as a powerful modulator of CUG-induced toxicity. Increasing smaug levels prevents muscle wasting and restores muscle function, while reducing its function exacerbates CUG-induced phenotypes. Using human myoblasts, we show physical interactions between human Smaug (SMAUG1/SMAD4A) and CUGBP1. Increased levels of SMAUG1 correct the abnormally high nuclear accumulation of CUGBP1 in myoblasts from DM1 patients. In addition, augmenting SMAUG1 levels leads to a reduction of inactive CUGBP1-eIF2α translational complexes and to a correction of translation of MRG15, a downstream target of CUGBP1. Therefore, Smaug suppresses CUG-mediated muscle wasting at least in part via restoration of translational activity of CUGBP1.

RNA toxicity in human disease and animal models: from the uncovering of a new mechanism to the development of promising therapies

Mutant ribonucleic acid (RNA) molecules can be toxic to the cell, causing human disease through trans-acting dominant mechanisms. RNA toxicity was first described in myotonic dystrophy type 1, a multisystemic disorder caused by the abnormal expansion of a non-coding trinucleotide repeat sequence. The development of multiple and complementary animal models of disease has greatly contributed to clarifying the complex disease pathways mediated by toxic RNA molecules. RNA toxicity is not limited to myotonic dystrophy and spreads to an increasing number of human conditions, which share some unifying pathogenic events mediated by toxic RNA accumulation and disruption of RNA-binding proteins. The remarkable progress in the dissection of disease pathobiology resulted in the rational design of molecular therapies, which have been successfully tested in animal models. Toxic RNA diseases, and in particular myotonic dystrophy, clearly illustrate the critical contribution of animal models of disease in translational research: from gene mutation to disease mechanisms, and ultimately to therapy development. This article is part of a Special Issue entitled: Animal Models of Disease.

The changing scene of amyotrophic lateral sclerosis

Several recent breakthroughs have provided notable insights into the pathogenesis of amyotrophic lateral sclerosis (ALS), with some even shifting our thinking about this neurodegenerative disease and raising the question as to whether this disorder is a proteinopathy, a ribonucleopathy or both. In addition, these breakthroughs have revealed mechanistic links between ALS and frontotemporal dementia, as well as between ALS and other neurodegenerative diseases, such as the cerebellar atrophies, myotonic dystrophy and inclusion body myositis. Here, we summarize the new findings in ALS research, discuss what they have taught us about this disease and examine issues that are still outstanding.

Next-generation sequencing: from understanding biology to personalized medicine

Within just a few years, the new methods for high-throughput next-generation sequencing have generated completely novel insights into the heritability and pathophysiology of human disease. In this review, we wish to highlight the benefits of the current state-of-the-art sequencing technologies for genetic and epigenetic research. We illustrate how these technologies help to constantly improve our understanding of genetic mechanisms in biological systems and summarize the progress made so far. This can be exemplified by the case of heritable heart muscle diseases, so-called cardiomyopathies. Here, next-generation sequencing is able to identify novel disease genes, and first clinical applications demonstrate the successful translation of this technology into personalized patient care.

Antisense oligonucleotides: rising stars in eliminating RNA toxicity in myotonic dystrophy

Myotonic dystrophy (DM) is a dominantly inherited, multisystemic disease caused by expanded CTG (type 1, DM1) or CCTG (type 2, DM2) repeats in untranslated regions of the mutated genes. Pathogenesis results from expression of RNAs from the mutated alleles that are toxic because of the expanded CUG or CCUG repeats. Increased understanding of the repeat-containing RNA (C/CUG(exp) RNA)-induced toxicity has led to the development of multiple strategies targeting the toxic RNA. Among these approaches, antisense oligonucleotides (ASOs) have demonstrated high potency in reversing the RNA toxicity in both cultured DM1 cells and DM1 animal models, thus offering great promise for the potential treatment of DM1. ASO targeting approaches will also provide avenues for the treatment of other repeat RNA-mediated diseases.

Myotonic dystrophy: is a narrow focus obscuring the rest of the field?

PURPOSE OF REVIEW

The myotonic dystrophies (DM1 and DM2) are the paradigm for RNA toxicity in disease pathogenesis. The emphasis of this review will be on recent developments and issues in understanding the pathogenesis of DM1 and how this is driving the accelerated pace of translational and therapeutic developments.

RECENT FINDINGS

RNA toxicity in myotonic dystrophy is now associated with bi-directional antisense transcription, dysregulation of microRNAs and potentially non-ATG-mediated translation of homopolymeric toxic proteins. The role of other RNA-binding proteins beyond MBNL1 and CUGBP1, such as Staufen 1 and DDX5, are being identified and studied with respect to their role in myotonic dystrophy. New functions for MBNL1 in miR-1 biogenesis might have a clinically relevant role in myotonic dystrophy cardiac conduction defects and pathology. Advances are being made in identifying and characterizing small molecules with the potential to disrupt CUG-MBNL1 interactions.

SUMMARY

Mechanisms of RNA toxicity are moving beyond a simplistic 'foci-centric' view of DM1 pathogenesis as a spliceopathy due to MBNL1 sequestration. Therapeutic development for myotonic dystrophy is moving rapidly with the development of antisense and small molecule therapies. Clinically, significant emphasis is being placed on biomarker discovery and outcome measures as an essential prelude to clinical trials.