Expansion of CUG RNA repeats causes stress and inhibition of translation in myotonic dystrophy 1 (DM1) cells

Structural and functional insights into the epigenetic regulator MRG15

MORF4-related gene on chromosome 15 (MRG15), a chromatin remodeller, is evolutionally conserved and ubiquitously expressed in mammalian tissues and cells. MRG15 plays vital regulatory roles in DNA damage repair, cell proliferation and division, cellular senescence and apoptosis by regulating both gene activation and gene repression via associations with specific histone acetyltransferase and histone deacetylase complexes. Recently, MRG15 has also been shown to rhythmically regulate hepatic lipid metabolism and suppress carcinoma progression. The unique N-terminal chromodomain and C-terminal MRG domain in MRG15 synergistically regulate its interaction with different cofactors, affecting its functions in various cell types. Thus, how MRG15 elaborately regulates target gene expression and performs diverse functions in different cellular contexts is worth investigating. In this review, we provide an in-depth discussion of how MRG15 controls multiple physiological and pathological processes.

Curriculum vitae of CUG binding protein 1 (CELF1) in homeostasis and diseases: a systematic review

RNA-binding proteins (RBPs) are kinds of proteins with either singular or multiple RNA-binding domains (RBDs), and they can assembly into ribonucleic acid-protein complexes, which mediate transportation, editing, splicing, stabilization, translational efficiency, or epigenetic modifications of their binding RNA partners, and thereby modulate various physiological and pathological processes. CUG-BP, Elav-like family 1 (CELF1) is a member of the CELF family of RBPs with high affinity to the GU-rich elements in mRNA, and thus exerting control over critical processes including mRNA splicing, translation, and decay. Mounting studies support that CELF1 is correlated with occurrence, genesis and development and represents a potential therapeutical target for these malignant diseases. Herein, we present the structure and function of CELF1, outline its role and regulatory mechanisms in varieties of homeostasis and diseases, summarize the identified CELF1 regulators and their structure-activity relationships, and prospect the current challenges and their solutions during studies on CELF1 functions and corresponding drug discovery, which will facilitate the establishment of a targeted regulatory network for CELF1 in diseases and advance CELF1 as a potential drug target for disease therapy.

Therapeutic Targeting of the GSK3β-CUGBP1 Pathway in Myotonic Dystrophy

Myotonic Dystrophy type 1 (DM1) is a neuromuscular disease associated with toxic RNA containing expanded CUG repeats. The developing therapeutic approaches to DM1 target mutant RNA or correct early toxic events downstream of the mutant RNA. We have previously described the benefits of the correction of the GSK3β-CUGBP1 pathway in DM1 mice (HSALR model) expressing 250 CUG repeats using the GSK3 inhibitor tideglusib (TG). Here, we show that TG treatments corrected the expression of ~17% of genes misregulated in DM1 mice, including genes involved in cell transport, development and differentiation. The expression of chloride channel 1 (Clcn1), the key trigger of myotonia in DM1, was also corrected by TG. We found that correction of the GSK3β-CUGBP1 pathway in mice expressing long CUG repeats (DMSXL model) is beneficial not only at the prenatal and postnatal stages, but also during adulthood. Using a mouse model with dysregulated CUGBP1, which mimics alterations in DM1, we showed that the dysregulated CUGBP1 contributes to the toxicity of expanded CUG repeats by changing gene expression and causing CNS abnormalities. These data show the critical role of the GSK3β-CUGBP1 pathway in DM1 muscle and in CNS pathologies, suggesting the benefits of GSK3 inhibitors in patients with different forms of DM1.

Protein Phosphorylation Alterations in Myotonic Dystrophy Type 1: A Systematic Review

Among the most common muscular dystrophies in adults is Myotonic Dystrophy type 1 (DM1), an autosomal dominant disorder characterized by myotonia, muscle wasting and weakness, and multisystemic dysfunctions. This disorder is caused by an abnormal expansion of the CTG triplet at the DMPK gene that, when transcribed to expanded mRNA, can lead to RNA toxic gain of function, alternative splicing impairments, and dysfunction of different signaling pathways, many regulated by protein phosphorylation. In order to deeply characterize the protein phosphorylation alterations in DM1, a systematic review was conducted through PubMed and Web of Science databases. From a total of 962 articles screened, 41 were included for qualitative analysis, where we retrieved information about total and phosphorylated levels of protein kinases, protein phosphatases, and phosphoproteins in DM1 human samples and animal and cell models. Twenty-nine kinases, 3 phosphatases, and 17 phosphoproteins were reported altered in DM1. Signaling pathways that regulate cell functions such as glucose metabolism, cell cycle, myogenesis, and apoptosis were impaired, as seen by significant alterations to pathways such as AKT/mTOR, MEK/ERK, PKC/CUGBP1, AMPK, and others in DM1 samples. This explains the complexity of DM1 and its different manifestations and symptoms, such as increased insulin resistance and cancer risk. Further studies can be done to complement and explore in detail specific pathways and how their regulation is altered in DM1, to find what key phosphorylation alterations are responsible for these manifestations, and ultimately to find therapeutic targets for future treatments.

Nuclear envelope transmembrane proteins involved in genome organization are misregulated in myotonic dystrophy type 1 muscle

Myotonic dystrophy type 1 is a multisystemic disorder with predominant muscle and neurological involvement. Despite a well described pathomechanism, which is primarily a global missplicing due to sequestration of RNA-binding proteins, there are still many unsolved questions. One such question is the disease etiology in the different affected tissues. We observed alterations at the nuclear envelope in primary muscle cell cultures before. This led us to reanalyze a published RNA-sequencing dataset of DM1 and control muscle biopsies regarding the misregulation of NE proteins. We could identify several muscle NE protein encoding genes to be misregulated depending on the severity of the muscle phenotype. Among these misregulated genes were NE transmembrane proteins (NETs) involved in nuclear-cytoskeletal coupling as well as genome organization. For selected genes, we could confirm that observed gene-misregulation led to protein expression changes. Furthermore, we investigated if genes known to be under expression-regulation by genome organization NETs were also misregulated in DM1 biopsies, which revealed that misregulation of two NETs alone is likely responsible for differential expression of about 10% of all genes being differentially expressed in DM1. Notably, the majority of NETs identified here to be misregulated in DM1 muscle are mutated in Emery-Dreifuss muscular dystrophy or clinical similar muscular dystrophies, suggesting a broader similarity on the molecular level for muscular dystrophies than anticipated. This shows not only the importance of muscle NETs in muscle health and disease, but also highlights the importance of the NE in DM1 disease progression.

Intracellular and intercellular transport of RNA organelles in CXG repeat disorders: The strength of weak ties

RNA is a vital biomolecule, the function of which is tightly spatiotemporally regulated. RNA organelles are biological structures that either membrane-less or surrounded by membrane. They are produced by the all the cells and indulge in vital cellular mechanisms. They include the intracellular RNA granules and the extracellular exosomes. RNA granules play an essential role in intracellular regulation of RNA localization, stability and translation. Aberrant regulation of RNA is connected to disease development. For example, in microsatellite diseases such as CXG repeat expansion disorders, the mutant CXG repeat RNA's localization and function are affected. RNA is not only transported intracellularly but can also be transported between cells via exosomes. The loading of the exosomes is regulated by RNA-protein complexes, and recent studies show that cytosolic RNA granules and exosomes share common content. Intracellular RNA granules and exosome loading may therefore be related. Exosomes can also transfer pathogenic molecules of CXG diseases from cell to cell, thereby driving disease progression. Both intracellular RNA granules and extracellular RNA vesicles may serve as a source for diagnostic and treatment strategies. In therapeutic approaches, pharmaceutical agents may be loaded into exosomes which then transport them to the desired cells/tissues. This is a promising target specific treatment strategy with few side effects. With respect to diagnostics, disease-specific content of exosomes, e.g., RNA-signatures, can serve as attractive biomarker of central nervous system diseases detecting early physiological disturbances, even before symptoms of neurodegeneration appear and irreparable damage to the nervous system occurs. In this review, we summarize the known function of cytoplasmic RNA granules and extracellular vesicles, as well as their role and dysfunction in CXG repeat expansion disorders. We also provide a summary of established protocols for the isolation and characterization of both cytoplasmic and extracellular RNA organelles.

Development of Therapeutic Approaches for Myotonic Dystrophies Type 1 and Type 2

Myotonic Dystrophies type 1 (DM1) and type 2 (DM2) are complex multisystem diseases without disease-based therapies. These disorders are caused by the expansions of unstable CTG (DM1) and CCTG (DM2) repeats outside of the coding regions of the disease genes: DMPK in DM1 and CNBP in DM2. Multiple clinical and molecular studies provided a consensus for DM1 pathogenesis, showing that the molecular pathophysiology of DM1 is associated with the toxicity of RNA CUG repeats, which cause multiple disturbances in RNA metabolism in patients' cells. As a result, splicing, translation, RNA stability and transcription of multiple genes are misregulated in DM1 cells. While mutant CCUG repeats are the main cause of DM2, additional factors might play a role in DM2 pathogenesis. This review describes current progress in the translation of mechanistic knowledge in DM1 and DM2 to clinical trials, with a focus on the development of disease-specific therapies for patients with adult forms of DM1 and congenital DM1 (CDM1).

Pharmacological and exercise-induced activation of AMPK as emerging therapies for myotonic dystrophy type 1 patients

Myotonic dystrophy type 1 (DM1) is a multisystemic disorder with variable clinical features. Currently, there is no cure or effective treatment for DM1. The disease is caused by an expansion of CUG repeats in the 3' UTR of DMPK mRNAs. Mutant DMPK mRNAs accumulate in nuclei as RNA foci and trigger an imbalance in the level and localization of RNA-binding proteins causing the characteristic missplicing events that account for the varied DM1 symptoms, a disease mechanism referred to as RNA toxicity. In recent years, multiple signalling pathways have been identified as being aberrantly regulated in skeletal muscle in response to the CUG expansion, including AMPK, a sensor of energy status, as well as a master regulator of cellular energy homeostasis. Converging lines of evidence highlight the benefits of activating AMPK signalling pharmacologically on RNA toxicity, as well as on muscle histology and function, in preclinical DM1 models. Importantly, a clinical trial with metformin, an activator of AMPK, resulted in functional benefits in DM1 patients. In addition, exercise, a known AMPK activator, has shown promising effects on RNA toxicity and muscle function in DM1 mice. Finally, clinical trials involving moderate-intensity exercise also induced functional benefits for DM1 patients. Taken together, these studies clearly demonstrate the molecular, histological and functional benefits of AMPK activation and exercise-based interventions on the DM1 phenotype. Despite these advances, several key questions remain; in particular, the extent of the true implication of AMPK in the observed beneficial improvements, as well as how, mechanistically, activation of AMPK signalling improves the DM1 pathophysiology.

Condensation properties of stress granules and processing bodies are compromised in Myotonic Dystrophy Type 1

RNA regulation in mammalian cells requires complex physical compartmentalisation, using structures thought to be formed by liquid-liquid phase separation. Disruption of these structures is implicated in numerous degenerative diseases. Myotonic dystrophy type 1 (DM1) is a multi-systemic trinucleotide repeat disorder resulting from an expansion of nucleotides CTG (CTGexp) in the DNA encoding DM1 protein kinase (DMPK). The cellular hallmark of DM1 is the formation of nuclear foci that contain expanded DMPK RNA (CUGexp) (with thymine instead of uracil). We report here the deregulation of stress granules (SGs) and processing bodies (P-bodies), two cytoplasmic structures key for mRNA regulation, in cell culture models of DM1. Alterations to the rates of formation and dispersal of SGs suggest an altered ability of cells to respond to stress associated with DM1, while changes to the structure and dynamics of SGs and P-bodies suggest that a widespread alteration to the biophysical properties of cellular structures is a consequence of the presence of CUGexp RNA.

Summary: Validation of an inducible model of myotonic dystrophy type 1 that shows altered cellular stress responses. These involve phase-separated cellular structures also implicated in other degenerative conditions.

The polyG diseases: a new disease entity

Recently, inspired by the similar clinical and pathological features shared with fragile X-associated tremor/ataxia syndrome (FXTAS), abnormal expansion of CGG repeats in the 5' untranslated region has been found in neuronal intranuclear inclusion disease (NIID), oculopharyngeal myopathy with leukoencephalopathy (OPML), and oculopharyngodistal myopathy (OPDMs). Although the upstream open reading frame has not been elucidated in OPML and OPDMs, polyglycine (polyG) translated by expanded CGG repeats is reported to be as a primary pathogenesis in FXTAS and NIID. Collectively, these findings indicate a new disease entity, the polyG diseases. In this review, we state the common clinical manifestations, pathological features, mechanisms, and potential therapies in these diseases, and provide preliminary opinions about future research in polyG diseases.

Myotonic Dystrophies: A Genetic Overview

Myotonic dystrophies (DM) are the most common muscular dystrophies in adults, which can affect other non-skeletal muscle organs such as the heart, brain and gastrointestinal system. There are two genetically distinct types of myotonic dystrophy: myotonic dystrophy type 1 (DM1) and myotonic dystrophy type 2 (DM2), both dominantly inherited with significant overlap in clinical manifestations. DM1 results from CTG repeat expansions in the 3'-untranslated region (3'UTR) of the DMPK (dystrophia myotonica protein kinase) gene on chromosome 19, while DM2 is caused by CCTG repeat expansions in intron 1 of the CNBP (cellular nucleic acid-binding protein) gene on chromosome 3. Recent advances in genetics and molecular biology, especially in the field of RNA biology, have allowed better understanding of the potential pathomechanisms involved in DM. In this review article, core clinical features and genetics of DM are presented followed by a discussion on the current postulated pathomechanisms and therapeutic approaches used in DM, including the ones currently in human clinical trial phase.

Deciphering the Complex Molecular Pathogenesis of Myotonic Dystrophy Type 1 through Omics Studies

Omics studies are crucial to improve our understanding of myotonic dystrophy type 1 (DM1), the most common muscular dystrophy in adults. Employing tissue samples and cell lines derived from patients and animal models, omics approaches have revealed the myriad alterations in gene and microRNA expression, alternative splicing, 3′ polyadenylation, CpG methylation, and proteins levels, among others, that contribute to this complex multisystem disease. In addition, omics characterization of drug candidate treatment experiments provides crucial insight into the degree of therapeutic rescue and off-target effects that can be achieved. Finally, several innovative technologies such as single-cell sequencing and artificial intelligence will have a significant impact on future DM1 research.

Deciphering the mechanisms underlying brain alterations and cognitive impairment in congenital myotonic dystrophy

Myotonic dystrophy type 1 (DM1) is a multisystemic and heterogeneous disorder caused by the expansion of CTG repeats in the 3' UTR of the myotonic dystrophy protein kinase (DMPK) gene. There is a congenital form (CDM1) of the disease characterized by severe hypotonia, respiratory insufficiency as well as developmental delays and intellectual disabilities. CDM1 infants manifest important brain structure abnormalities present from birth while, in contrast, older patients with adult-onset DM1 often present neurodegenerative features and milder progressive cognitive deficits. Promising therapies targeting central molecular mechanisms contributing to the symptoms of adult-onset DM1 are currently in development, but their relevance for treating cognitive impairment in CDM1, which seems to be a partially distinct neurodevelopmental disorder, remain to be elucidated. Here, we provide an update on the clinical presentation of CDM1 and review recent in vitro and in vivo models that have provided meaningful insights on its consequences in development, with a particular focus on the brain. We discuss how enhanced toxic gain-of-function of the mutated DMPK transcripts with larger CUG repeats and the resulting dysregulation of RNA-binding proteins may affect the developing cortex in utero. Because the methylation of CpG islets flanking the trinucleotide repeats has emerged as a strong biomarker of CDM1, we highlight the need to investigate the tissue-specific impacts of these chromatin modifications in the brain. Finally, we outline promising potential therapeutic treatments for CDM1 and propose future in vitro and in vivo models with great potential to shed light on this disease.

CELF Family Proteins in Cancer: Highlights on the RNA-Binding Protein/Noncoding RNA Regulatory Axis

Post-transcriptional modifications to coding and non-coding RNAs are unquestionably a pivotal way in which human mRNA and protein diversity can influence the different phases of a transcript's life cycle. CELF (CUGBP Elav-like family) proteins are RBPs (RNA-binding proteins) with pleiotropic capabilities in RNA processing. Their responsibilities extend from alternative splicing and transcript editing in the nucleus to mRNA stability, and translation into the cytoplasm. In this way, CELF family members have been connected to global alterations in cancer proliferation and invasion, leading to their identification as potential tumor suppressors or even oncogenes. Notably, genetic variants, alternative splicing, phosphorylation, acetylation, subcellular distribution, competition with other RBPs, and ultimately lncRNAs, miRNAs, and circRNAs all impact CELF regulation. Discoveries have emerged about the control of CELF functions, particularly via noncoding RNAs, and CELF proteins have been identified as competing, antagonizing, and regulating agents of noncoding RNA biogenesis. On the other hand, CELFs are an intriguing example through which to broaden our understanding of the RBP/noncoding RNA regulatory axis. Balancing these complex pathways in cancer is undeniably pivotal and deserves further research. This review outlines some mechanisms of CELF protein regulation and their functional consequences in cancer physiology.

High-throughput kinome-RNAi screen identifies protein kinase R activator (PACT) as a novel genetic modifier of CUG foci integrity in myotonic dystrophy type 1 (DM1)

Myotonic Dystrophy Type 1 (DM1) is the most common form of adult muscular dystrophy (~1:8000). In DM1, expansion of CTG trinucleotide repeats in the 3' untranslated region of the dystrophia myotonica protein kinase (DMPK) gene results in DMPK mRNA hairpin structures which aggregate as insoluble ribonuclear foci and sequester several RNA-binding proteins. The resulting sequestration and misregulation of important splicing factors, such as muscleblind-like 1 (MBNL1), causes the aberrant expression of fetal transcripts for several genes that contribute to the disease phenotype. Previous work has shown that antisense oligonucleotide-mediated disaggregation of the intranuclear foci has the potential to reverse downstream anomalies. To explore whether the nuclear foci are, to some extent, controlled by cell signalling pathways, we have performed a screen using a small interfering RNA (siRNA) library targeting 518 protein kinases to look at kinomic modulation of foci integrity. RNA foci were visualized by in situ hybridization of a fluorescent-tagged (CAG)10 probe directed towards the expanded DMPK mRNA and the cross-sectional area and number of foci per nuclei were recorded. From our screen, we have identified PACT (protein kinase R (PKR) activator) as a novel modulator of foci integrity and have shown that PACT knockdown can both increase MBNL1 protein levels; however, these changes are not suffcient for significant correction of downstream spliceopathies.

Transcriptome Analysis in a Primary Human Muscle Cell Differentiation Model for Myotonic Dystrophy Type 1

Myotonic dystrophy type 1 (DM1) is caused by CTG-repeat expansions leading to a complex pathology with a multisystemic phenotype that primarily affects the muscles and brain. Despite a multitude of information, especially on the alternative splicing of several genes involved in the pathology, information about additional factors contributing to the disease development is still lacking. We performed RNAseq and gene expression analyses on proliferating primary human myoblasts and differentiated myotubes. GO-term analysis indicates that in myoblasts and myotubes, different molecular pathologies are involved in the development of the muscular phenotype. Gene set enrichment for splicing reveals the likelihood of whole, differentiation stage specific, splicing complexes that are misregulated in DM1. These data add complexity to the alternative splicing phenotype and we predict that it will be of high importance for therapeutic interventions to target not only mature muscle, but also satellite cells.

A CRISPR-Cas13a Based Strategy That Tracks and Degrades Toxic RNA in Myotonic Dystrophy Type 1

Cas13a, an effector of type VI CRISPR-Cas systems, is an RNA guided RNase with multiplexing and therapeutic potential. This study employs the Leptotrichia shahii (Lsh) Cas13a and a repeat-based CRISPR RNA (crRNA) to track and eliminate toxic RNA aggregates in myotonic dystrophy type 1 (DM1) – a neuromuscular disease caused by CTG expansion in the DMPK gene. We demonstrate that LshCas13a cleaves CUG repeat RNA in biochemical assays and reduces toxic RNA load in patient-derived myoblasts. As a result, LshCas13a reverses the characteristic adult-to-embryonic missplicing events in several key genes that contribute to DM1 phenotype. The deactivated LshCas13a can further be repurposed to track RNA-rich organelles within cells. Our data highlights the reprogrammability of LshCas13a and the possible use of Cas13a to target expanded repeat sequences in microsatellite expansion diseases.

The hallmarks of myotonic dystrophy type 1 muscle dysfunction

Myotonic dystrophy type 1 (DM1) is the most prevalent form of muscular dystrophy in adults and yet there are currently no treatment options. Although this disease causes multisystemic symptoms, it is mainly characterised by myopathy or diseased muscles, which includes muscle weakness, atrophy, and myotonia, severely affecting the lives of patients worldwide. On a molecular level, DM1 is caused by an expansion of CTG repeats in the 3' untranslated region (3'UTR) of the DM1 Protein Kinase (DMPK) gene which become pathogenic when transcribed into RNA forming ribonuclear foci comprised of auto complementary CUG hairpin structures that can bind proteins. This leads to the sequestration of the muscleblind-like (MBNL) family of proteins, depleting them, and the abnormal stabilisation of CUGBP Elav-like family member 1 (CELF1), enhancing it. Traditionally, DM1 research has focused on this RNA toxicity and how it alters MBNL and CELF1 functions as key splicing regulators. However, other proteins are affected by the toxic DMPK RNA and there is strong evidence that supports various signalling cascades playing an important role in DM1 pathogenesis. Specifically, the impairment of protein kinase B (AKT) signalling in DM1 increases autophagy, apoptosis, and ubiquitin-proteasome activity, which may also be affected in DM1 by AMP-activated protein kinase (AMPK) downregulation. AKT also regulates CELF1 directly, by affecting its subcellular localisation, and indirectly as it inhibits glycogen synthase kinase 3 beta (GSK3β), which stabilises the repressive form of CELF1 in DM1. Another kinase that contributes to CELF1 mis-regulation, in this case by hyperphosphorylation, is protein kinase C (PKC). Additionally, it has been demonstrated that fibroblast growth factor-inducible 14 (Fn14) is induced in DM1 and is associated with downstream signalling through the nuclear factor κB (NFκB) pathways, associating inflammation with this disease. Furthermore, MBNL1 and CELF1 play a role in cytoplasmic processes involved in DM1 myopathy, altering proteostasis and sarcomere structure. Finally, there are many other elements that could contribute to the muscular phenotype in DM1 such as alterations to satellite cells, non-coding RNA metabolism, calcium dysregulation, and repeat-associated non-ATG (RAN) translation. This review aims to organise the currently dispersed knowledge on the different pathways affected in DM1 and discusses the unexplored connections that could potentially help in providing new therapeutic targets in DM1 research.

An Overview of Alternative Splicing Defects Implicated in Myotonic Dystrophy Type I

Myotonic dystrophy type I (DM1) is the most common form of adult muscular dystrophy, caused by expansion of a CTG triplet repeat in the 3′ untranslated region (3′UTR) of the myotonic dystrophy protein kinase (DMPK) gene. The pathological CTG repeats result in protein trapping by expanded transcripts, a decreased DMPK translation and the disruption of the chromatin structure, affecting neighboring genes expression. The muscleblind-like (MBNL) and CUG-BP and ETR-3-like factors (CELF) are two families of tissue-specific regulators of developmentally programmed alternative splicing that act as antagonist regulators of several pre-mRNA targets, including troponin 2 (TNNT2), insulin receptor (INSR), chloride channel 1 (CLCN1) and MBNL2. Sequestration of MBNL proteins and up-regulation of CELF1 are key to DM1 pathology, inducing a spliceopathy that leads to a developmental remodelling of the transcriptome due to an adult-to-foetal splicing switch, which results in the loss of cell function and viability. Moreover, recent studies indicate that additional pathogenic mechanisms may also contribute to disease pathology, including a misregulation of cellular mRNA translation, localization and stability. This review focuses on the cause and effects of MBNL and CELF1 deregulation in DM1, describing the molecular mechanisms underlying alternative splicing misregulation for a deeper understanding of DM1 complexity. To contribute to this analysis, we have prepared a comprehensive list of transcript alterations involved in DM1 pathogenesis, as well as other deregulated mRNA processing pathways implications.

Overexpression of Staufen1 in DM1 mouse skeletal muscle exacerbates dystrophic and atrophic features

In myotonic dystrophy type 1 (DM1), the CUG expansion (CUGexp) in the 3' untranslated region of the dystrophia myotonica protein kinase messenger ribonucleic acid affects the homeostasis of ribonucleic acid-binding proteins, causing the multiple symptoms of DM1. We have previously reported that Staufen1 is increased in skeletal muscles from DM1 mice and patients and that sustained Staufen1 expression in mature mouse muscle causes a progressive myopathy. Here, we hypothesized that the elevated levels of Staufen1 contributes to the myopathic features of the disease. Interestingly, the classic DM1 mouse model human skeletal actin long repeat (HSALR) lacks overt atrophy while expressing CUGexp transcripts and elevated levels of endogenous Staufen1, suggesting a lower sensitivity to atrophic signaling in this model. We report that further overexpression of Staufen1 in the DM1 mouse model HSALR causes a myopathy via inhibition of protein kinase B signaling through an increase in phosphatase tensin homolog, leading to the expression of atrogenes. Interestingly, we also show that Staufen1 regulates the expression of muscleblind-like splicing regulator 1 and CUG-binding protein elav-like family member 1 in wild-type and DM1 skeletal muscle. Together, data obtained from these new DM1 mouse models provide evidence for the role of Staufen1 as an atrophy-associated gene that impacts progressive muscle wasting in DM1. Accordingly, our findings highlight the potential of Staufen1 as a therapeutic target and biomarker.

TCF4-mediated Fuchs endothelial corneal dystrophy: Insights into a common trinucleotide repeat-associated disease

Fuchs endothelial corneal dystrophy (FECD) is a common cause for heritable visual loss in the elderly. Since the first description of an association between FECD and common polymorphisms situated within the transcription factor 4 (TCF4) gene, genetic and molecular studies have implicated an intronic CTG trinucleotide repeat (CTG18.1) expansion as a causal variant in the majority of FECD patients. To date, several non-mutually exclusive mechanisms have been proposed that drive and/or exacerbate the onset of disease. These mechanisms include (i) TCF4 dysregulation; (ii) toxic gain-of-function from TCF4 repeat-containing RNA; (iii) toxic gain-of-function from repeat-associated non-AUG dependent (RAN) translation; and (iv) somatic instability of CTG18.1. However, the relative contribution of these proposed mechanisms in disease pathogenesis is currently unknown. In this review, we summarise research implicating the repeat expansion in disease pathogenesis, define the phenotype-genotype correlations between FECD and CTG18.1 expansion, and provide an update on research tools that are available to study FECD as a trinucleotide repeat expansion disease. Furthermore, ongoing international research efforts to develop novel CTG18.1 expansion-mediated FECD therapeutics are highlighted and we provide a forward-thinking perspective on key unanswered questions that remain in the field.

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FECD is a common, age-related corneal dystrophy.

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The majority of cases are associated with expansion of a CTG repeat (CTG18.1).

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FECD is the most common trinucleotide repeat expansion disease in humans.

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Evidence supports multiple molecular mechanisms underlying the pathophysiology.

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Novel CTG18.1-targeted therapeutics are in development.

RNA toxicity in non-coding repeat expansion disorders

Several neurodegenerative disorders like amyotrophic lateral sclerosis (ALS) and spinocerebellar ataxia (SCA) are caused by non-coding nucleotide repeat expansions. Different pathogenic mechanisms may underlie these non-coding repeat expansion disorders. While gain-of-function mechanisms, such as toxicity associated with expression of repeat RNA or toxicity associated with repeat-associated non-ATG (RAN) products, are most frequently connected with these disorders, loss-of-function mechanisms have also been implicated. We review the different pathways that have been linked to non-coding repeat expansion disorders such as C9ORF72-linked ALS/frontotemporal dementia (FTD), myotonic dystrophy, fragile X tremor/ataxia syndrome (FXTAS), SCA, and Huntington's disease-like 2. We discuss modes of RNA toxicity focusing on the identity and the interacting partners of the toxic RNA species. Using the C9ORF72 ALS/FTD paradigm, we further explore the efforts and different methods used to disentangle RNA vs. RAN toxicity. Overall, we conclude that there is ample evidence for a role of RNA toxicity in non-coding repeat expansion diseases.

Correction of RNA-Binding Protein CUGBP1 and GSK3β Signaling as Therapeutic Approach for Congenital and Adult Myotonic Dystrophy Type 1

Myotonic dystrophy type 1 (DM1) is a complex genetic disease affecting many tissues. DM1 is caused by an expansion of CTG repeats in the 3′-UTR of the DMPK gene. The mechanistic studies of DM1 suggested that DMPK mRNA, containing expanded CUG repeats, is a major therapeutic target in DM1. Therefore, the removal of the toxic RNA became a primary focus of the therapeutic development in DM1 during the last decade. However, a cure for this devastating disease has not been found. Whereas the degradation of toxic RNA remains a preferential approach for the reduction of DM1 pathology, other approaches targeting early toxic events downstream of the mutant RNA could be also considered. In this review, we discuss the beneficial role of the restoring of the RNA-binding protein, CUGBP1/CELF1, in the correction of DM1 pathology. It has been recently found that the normalization of CUGBP1 activity with the inhibitors of GSK3 has a positive effect on the reduction of skeletal muscle and CNS pathologies in DM1 mouse models. Surprisingly, the inhibitor of GSK3, tideglusib also reduced the toxic CUG-containing RNA. Thus, the development of the therapeutics, based on the correction of the GSK3β-CUGBP1 pathway, is a promising option for this complex disease.

Correction of Glycogen Synthase Kinase 3β in Myotonic Dystrophy 1 Reduces the Mutant RNA and Improves Postnatal Survival of DMSXL Mice

Myotonic dystrophy type 1 (DM1) is a multisystem neuromuscular disease without cure. One of the possible therapeutic approaches for DM1 is correction of the RNA-binding proteins CUGBP1 and MBNL1, misregulated in DM1. CUGBP1 activity is controlled by glycogen synthase kinase 3β (GSK3β), which is elevated in skeletal muscle of patients with DM1, and inhibitors of GSK3 were suggested as therapeutic molecules to correct CUGBP1 activity in DM1. Here, we describe that correction of GSK3β with a small-molecule inhibitor of GSK3, tideglusib (TG), not only normalizes the GSK3β-CUGBP1 pathway but also reduces the mutant DMPK mRNA in myoblasts from patients with adult DM1 and congenital DM1 (CDM1). Correction of GSK3β in a mouse model of DM1 (HSA^LR mice) with TG also reduces the levels of CUG-containing RNA, normalizing a number of CUGBP1- and MBNL1-regulated mRNA targets. We also found that the GSK3β-CUGBP1 pathway is abnormal in skeletal muscle and brain of DMSXL mice, expressing more than 1,000 CUG repeats, and that the correction of this pathway with TG increases postnatal survival and improves growth and neuromotor activity of DMSXL mice. These findings show that the inhibitors of GSK3, such as TG, may correct pathology in DM1 and CDM1 via several pathways.

Mitigating RNA Toxicity in Myotonic Dystrophy using Small Molecules

This review, one in a series on myotonic dystrophy (DM), is focused on the development and potential use of small molecules as therapeutics for DM. The complex mechanisms and pathogenesis of DM are covered in the associated reviews. Here, we examine the various small molecule approaches taken to target the DNA, RNA, and proteins that contribute to disease onset and progression in myotonic dystrophy type 1 (DM1) and 2 (DM2).

Pharmacological and physiological activation of AMPK improves the spliceopathy in DM1 mouse muscles

Myotonic dystrophy type 1 (DM1) is a debilitating multisystemic disorder caused by a triplet repeat expansion in the 3' untranslated region of dystrophia myotonica protein kinase mRNAs. Mutant mRNAs accumulate in the nucleus of affected cells and misregulate RNA-binding proteins, thereby promoting characteristic missplicing events. However, little is known about the signaling pathways that may be affected in DM1. Here, we investigated the status of activated protein kinase (AMPK) signaling in DM1 skeletal muscle and found that the AMPK pathway is markedly repressed in a DM1 mouse model (human skeletal actin-long repeat, HSALR) and patient-derived DM1 myoblasts. Chronic pharmacological activation of AMPK signaling in DM1 mice with 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside (AICAR) has multiple beneficial effects on the DM1 phenotype. Indeed, a 6-week AICAR treatment of DM1 mice promoted expression of a slower, more oxidative phenotype, improved muscle histology and corrected several events associated with RNA toxicity. Importantly, AICAR also had a dose-dependent positive effect on the spliceopathy in patient-derived DM1 myoblasts. In separate experiments, we also show that chronic treatment of DM1 mice with resveratrol as well as voluntary wheel running also rescued missplicing events in muscle. Collectively, our findings demonstrate the therapeutic potential of chronic AMPK stimulation both physiologically and pharmacologically for DM1 patients.

Repeat-associated non-AUG (RAN) translation and other molecular mechanisms in Fragile X Tremor Ataxia Syndrome

Fragile X-associated tremor/ataxia syndrome (FXTAS) is a late-onset inherited neurodegenerative disorder characterized by progressive intention tremor, gait ataxia and dementia associated with mild brain atrophy. The cause of FXTAS is a premutation expansion, of 55 to 200 CGG repeats localized within the 5'UTR of FMR1. These repeats are transcribed in the sense and antisense directions into mutants RNAs, which have increased expression in FXTAS. Furthermore, CGG sense and CCG antisense expanded repeats are translated into novel proteins despite their localization in putatively non-coding regions of the transcript. Here we focus on two proposed disease mechanisms for FXTAS: 1) RNA gain-of-function, whereby the mutant RNAs bind specific proteins and preclude their normal functions, and 2) repeat-associated non-AUG (RAN) translation, whereby translation through the CGG or CCG repeats leads to the production of toxic homopolypeptides, which in turn interfere with a variety of cellular functions. Here, we analyze the data generated to date on both of these potential molecular mechanisms and lay out a path forward for determining which factors drive FXTAS pathogenicity.

Modeling of Myotonic Dystrophy Cardiac Phenotypes in Drosophila

After respiratory distress, cardiac dysfunction is the second most common cause of fatality associated with the myotonic dystrophy (DM) disease. Despite the prevalance of heart failure in DM, physiopathological studies on heart symptoms have been relatively scarce because few murine models faithfully reproduce the cardiac disease. Consequently, only a small number of candidate compounds have been evaluated in this specific phenotype. To help cover this gap Drosophila combines the amenability of its invertebrate genetics with the possibility of quickly acquiring physiological parameters suitable for meaningful comparisons with vertebrate animal models and humans. Here we review available descriptions of cardiac disease in DM type 1 and type 2, and three recent papers reporting the cardiac toxicity of non-coding CUG (DM1) and CCUG (DM2) repeat RNA in flies. Notably, flies expressing CUG or CCUG RNA in their hearts developed strong arrhythmias and had reduced fractional shortening, which correlates with similar phenotypes in DM patients. Overexpression of Muscleblind, which is abnormally sequestered by CUG and CCUG repeat RNA, managed to strongly suppress arrhythmias and fractional shortening, thus demonstrating that Muscleblind depletion causes cardiac phenotypes in flies. Importantly, small molecules pentamidine and daunorubicin were able to rescue cardiac phenotypes by releasing Muscleblind from sequestration. Taken together, fly heart models have the potential to make important contributions to the understanding of the molecular causes of cardiac dysfunction in DM and in the quick assessment of candidate therapeutics.

Of Mice and Men: Advances in the Understanding of Neuromuscular Aspects of Myotonic Dystrophy

Intensive effort has been directed toward the modeling of myotonic dystrophy (DM) in mice, in order to reproduce human disease and to provide useful tools to investigate molecular and cellular pathogenesis and test efficient therapies. Mouse models have contributed to dissect the multifaceted impact of the DM mutation in various tissues, cell types and in a pleiotropy of pathways, through the expression of toxic RNA transcripts. Changes in alternative splicing, transcription, translation, intracellular RNA localization, polyadenylation, miRNA metabolism and phosphorylation of disease intermediates have been described in different tissues. Some of these events have been directly associated with specific disease symptoms in the skeletal muscle and heart of mice, offering the molecular explanation for individual disease phenotypes. In the central nervous system (CNS), however, the situation is more complex. We still do not know how the molecular abnormalities described translate into CNS dysfunction, nor do we know if the correction of individual molecular events will provide significant therapeutic benefits. The variability in model design and phenotypes described so far requires a thorough and critical analysis. In this review we discuss the recent contributions of mouse models to the understanding of neuromuscular aspects of disease, therapy development, and we provide a reflective assessment of our current limitations and pressing questions that remain unanswered.

Myoediting: Toward Prevention of Muscular Dystrophy by Therapeutic Genome Editing

Muscular dystrophies represent a large group of genetic disorders that significantly impair quality of life and often progress to premature death. There is no effective treatment for these debilitating diseases. Most therapies, developed to date, focus on alleviating the symptoms or targeting the secondary effects, while the underlying gene mutation is still present in the human genome. The discovery and application of programmable nucleases for site-specific DNA double-stranded breaks provides a powerful tool for precise genome engineering. In particular, the CRISPR/Cas system has revolutionized the genome editing field and is providing a new path for disease treatment by targeting the disease-causing genetic mutations. In this review, we provide a historical overview of genome-editing technologies, summarize the most recent advances, and discuss potential strategies and challenges for permanently correcting genetic mutations that cause muscular dystrophies.

miR-23b and miR-218 silencing increase Muscleblind-like expression and alleviate myotonic dystrophy phenotypes in mammalian models

Functional depletion of the alternative splicing factors Muscleblind-like (MBNL 1 and 2) is at the basis of the neuromuscular disease myotonic dystrophy type 1 (DM1). We previously showed the efficacy of miRNA downregulation in Drosophila DM1 model. Here, we screen for miRNAs that regulate MBNL1 and MBNL2 in HeLa cells. We thus identify miR-23b and miR-218, and confirm that they downregulate MBNL proteins in this cell line. Antagonists of miR-23b and miR-218 miRNAs enhance MBNL protein levels and rescue pathogenic missplicing events in DM1 myoblasts. Systemic delivery of these "antagomiRs" similarly boost MBNL expression and improve DM1-like phenotypes, including splicing alterations, histopathology, and myotonia in the HSA^LR DM1 model mice. These mammalian data provide evidence for therapeutic blocking of the miRNAs that control Muscleblind-like protein expression in myotonic dystrophy.

Cells of Matter-In Vitro Models for Myotonic Dystrophy

Myotonic dystrophy type 1 (DM1 also known as Steinert disease) is a multisystemic disorder mainly characterized by myotonia, progressive muscle weakness and wasting, cognitive impairments, and cardiac defects. This autosomal dominant disease is caused by the expression of nuclear retained RNAs containing pathologic expanded CUG repeats that alter the function of RNA-binding proteins in a tissue-specific manner, leading ultimately to neuromuscular dysfunction and clinical symptoms. Although considerable knowledge has been gathered on myotonic dystrophy since its first description, the development of novel relevant disease models remains of high importance to investigate pathophysiologic mechanisms and to assess new therapeutic approaches. In addition to animal models, in vitro cell cultures provide a unique resource for both fundamental and translational research. This review discusses how cellular models broke ground to decipher molecular basis of DM1 and describes currently available cell models, ranging from exogenous expression of the CTG tracts to variable patients' derived cells.

Bruno-3 regulates sarcomere component expression and contributes to muscle phenotypes of myotonic dystrophy type 1

Steinert disease, or myotonic dystrophy type 1 (DM1), is a multisystemic disorder caused by toxic noncoding CUG repeat transcripts, leading to altered levels of two RNA binding factors, MBNL1 and CELF1. The contribution of CELF1 to DM1 phenotypes is controversial. Here, we show that the Drosophila CELF1 family member, Bru-3, contributes to pathogenic muscle defects observed in a Drosophila model of DM1. Bru-3 displays predominantly cytoplasmic expression in muscles and its muscle-specific overexpression causes a range of phenotypes also observed in the fly DM1 model, including affected motility, fiber splitting, reduced myofiber length and altered myoblast fusion. Interestingly, comparative genome-wide transcriptomic analyses revealed that Bru-3 negatively regulates levels of mRNAs encoding a set of sarcomere components, including Actn transcripts. Conversely, it acts as a positive regulator of Actn translation. As CELF1 displays predominantly cytoplasmic expression in differentiating C2C12 myotubes and binds to Actn mRNA, we hypothesize that it might exert analogous functions in vertebrate muscles. Altogether, we propose that cytoplasmic Bru-3 contributes to DM1 pathogenesis in a Drosophila model by regulating sarcomeric transcripts and protein levels.

Correction of GSK3β at young age prevents muscle pathology in mice with myotonic dystrophy type 1

Myotonic dystrophy type 1 (DM1) is a progressive neuromuscular disease caused by expanded CUG repeats, which misregulate RNA metabolism through several RNA-binding proteins, including CUG-binding protein/CUGBP1 elav-like factor 1 (CUGBP1/CELF1) and muscleblind 1 protein. Mutant CUG repeats elevate CUGBP1 and alter CUGBP1 activity via a glycogen synthase kinase 3β (GSK3β)-cyclin D3-cyclin D-dependent kinase 4 (CDK4) signaling pathway. Inhibition of GSK3β corrects abnormal activity of CUGBP1 in DM1 mice [human skeletal actin mRNA, containing long repeats ( HSA^LR) model]. Here, we show that the inhibition of GSK3β in young HSA^LR mice prevents development of DM1 muscle pathology. Skeletal muscle in 1-yr-old HSA^LR mice, treated at 1.5 mo for 6 wk with the inhibitors of GSK3, exhibits high fiber density, corrected atrophy, normal fiber size, with reduced central nuclei and normalized grip strength. Because CUG-GSK3β-cyclin D3-CDK4 converts the active form of CUGBP1 into a form of translational repressor, we examined the contribution of CUGBP1 in myogenesis using Celf1 knockout mice. We found that a loss of CUGBP1 disrupts myogenesis, affecting genes that regulate differentiation and the extracellular matrix. Proteins of those pathways are also misregulated in young HSA^LR mice and in muscle biopsies of patients with congenital DM1. These findings suggest that the correction of GSK3β-CUGBP1 pathway in young HSA^LR mice might have a positive effect on the myogenesis over time.-Wei, C., Stock, L., Valanejad, L., Zalewski, Z. A., Karns, R., Puymirat, J., Nelson, D., Witte, D., Woodgett, J., Timchenko, N. A., Timchenko, L. Correction of GSK3β at young age prevents muscle pathology in mice with myotonic dystrophy type 1.

Misregulation of calcium-handling proteins promotes hyperactivation of calcineurin-NFAT signaling in skeletal muscle of DM1 mice

Myotonic Dystrophy type 1 (DM1) is caused by an expansion of CUG repeats in DMPK mRNAs. This mutation affects alternative splicing through misregulation of RNA-binding proteins. Amongst pre-mRNAs that are mis-spliced, several code for proteins involved in calcium homeostasis suggesting that calcium-handling and signaling are perturbed in DM1. Here, we analyzed expression of such proteins in DM1 mouse muscle. We found that the levels of several sarcoplasmic reticulum proteins (SERCA1, sarcolipin and calsequestrin) are altered, likely contributing to an imbalance in calcium homeostasis. We also observed that calcineurin (CnA) signaling is hyperactivated in DM1 muscle. Indeed, CnA expression and phosphatase activity are both markedly increased in DM1 muscle. Coherent with this, we found that activators of the CnA pathway (MLP, FHL1) are also elevated. Consequently, NFATc1 expression is increased in DM1 muscle and becomes relocalized to myonuclei, together with an up-regulation of its transcriptional targets (RCAN1.4 and myoglobin). Accordingly, DM1 mouse muscles display an increase in oxidative metabolism and fiber hypertrophy. To determine the functional consequences of this CnA hyperactivation, we administered cyclosporine A, an inhibitor of CnA, to DM1 mice. Muscles of treated DM1 mice showed an increase in CUGBP1 levels, and an exacerbation of key alternative splicing events associated with DM1. Finally, inhibition of CnA in cultured human DM1 myoblasts also resulted in a splicing exacerbation of the insulin receptor. Together, these findings show for the first time that calcium-CnA signaling is hyperactivated in DM1 muscle and that such hyperactivation represents a beneficial compensatory adaptation to the disease.

Myotonic dystrophy: candidate small molecule therapeutics

Myotonic dystrophy type 1 (DM1) is a rare multisystemic neuromuscular disorder caused by expansion of CTG trinucleotide repeats in the noncoding region of the DMPK gene. Mutant DMPK transcripts are toxic and alter gene expression at several levels. Chiefly, the secondary structure formed by CUGs has a strong propensity to capture and retain proteins, like those of the muscleblind-like (MBNL) family. Sequestered MBNL proteins cannot then fulfill their normal functions. Many therapeutic approaches have been explored to reverse these pathological consequences. Here, we review the myriad of small molecules that have been proposed for DM1, including examples obtained from computational rational design, HTS, drug repurposing, and therapeutic gene modulation.

(CCUG)n RNA toxicity in a Drosophila model of myotonic dystrophy type 2 (DM2) activates apoptosis

The myotonic dystrophies are prototypic toxic RNA gain-of-function diseases. Myotonic dystrophy type 1 (DM1) and type 2 (DM2) are caused by different unstable, noncoding microsatellite repeat expansions – (CTG)_DM1 in DMPK and (CCTG)_DM2 in CNBP. Although transcription of mutant repeats into (CUG)_DM1 or (CCUG)_DM2 appears to be necessary and sufficient to cause disease, their pathomechanisms remain incompletely understood. To study the mechanisms of (CCUG)_DM2 toxicity and develop a convenient model for drug screening, we generated a transgenic DM2 model in the fruit fly Drosophila melanogaster with (CCUG)_n repeats of variable length (n=16 and 106). Expression of noncoding (CCUG)₁₀₆, but not (CCUG)₁₆, in muscle and retinal cells led to the formation of ribonuclear foci and mis-splicing of genes implicated in DM pathology. Mis-splicing could be rescued by co-expression of human MBNL1, but not by CUGBP1 (CELF1) complementation. Flies with (CCUG)₁₀₆ displayed strong disruption of external eye morphology and of the underlying retina. Furthermore, expression of (CCUG)₁₀₆ in developing retinae caused a strong apoptotic response. Inhibition of apoptosis rescued the retinal disruption in (CCUG)₁₀₆ flies. Finally, we tested two chemical compounds that have shown therapeutic potential in DM1 models. Whereas treatment of (CCUG)₁₀₆ flies with pentamidine had no effect, treatment with a PKR inhibitor blocked both the formation of RNA foci and apoptosis in retinae of (CCUG)₁₀₆ flies. Our data indicate that expression of expanded (CCUG)_DM2 repeats is toxic, causing inappropriate cell death in affected fly eyes. Our Drosophila DM2 model might provide a convenient tool for in vivo drug screening.

Summary: A Drosophila model of myotonic dystrophy type 2 (DM2) recapitulates several features of the human disease, identifies apoptosis as a contributing factor to DM2, and is likely to provide a convenient tool for drug screening.

Disease Phenotypes in a Mouse Model of RNA Toxicity Are Independent of Protein Kinase Cα and Protein Kinase Cβ

Myotonic dystrophy type 1(DM1) is the prototype for diseases caused by RNA toxicity. RNAs from the mutant allele contain an expanded (CUG)_n tract within the 3' untranslated region of the dystrophia myotonica protein kinase (DMPK) gene. The toxic RNAs affect the function of RNA binding proteins leading to sequestration of muscleblind-like (MBNL) proteins and increased levels of CELF1 (CUGBP, Elav-like family member 1). The mechanism for increased CELF1 is not very clear. One favored proposition is hyper-phosphorylation of CELF1 by Protein Kinase C alpha (PKCα) leading to increased CELF1 stability. However, most of the evidence supporting a role for PKC-α relies on pharmacological inhibition of PKC. To further investigate the role of PKCs in the pathogenesis of RNA toxicity, we generated transgenic mice with RNA toxicity that lacked both the PKCα and PKCβ isoforms. We find that these mice show similar disease progression as mice wildtype for the PKC isoforms. Additionally, the expression of CELF1 is also not affected by deficiency of PKCα and PKCβ in these RNA toxicity mice. These data suggest that disease phenotypes of these RNA toxicity mice are independent of PKCα and PKCβ.

Staufen1 impairs stress granule formation in skeletal muscle cells from myotonic dystrophy type 1 patients

Myotonic dystrophy (DM1) is caused by an expansion of CUG repeats (CUG(exp)) in the DMPK mRNA 3'UTR. CUG(exp)-containing mRNAs become toxic to cells by misregulating RNA-binding proteins. Here we investigated the consequence of this RNA toxicity on the cellular stress response. We report that cell stress efficiently triggers formation of stress granules (SGs) in proliferating, quiescent, and differentiated muscle cells, as shown by the appearance of distinct cytoplasmic TIA-1- and DDX3-containing foci. We show that Staufen1 is also dynamically recruited into these granules. Moreover, we discovered that DM1 myoblasts fail to properly form SGs in response to arsenite. This blockage was not observed in DM1 fibroblasts, demonstrating a cell type-specific defect. DM1 myoblasts display increased expression and sequestration of toxic CUG(exp) mRNAs compared with fibroblasts. Of importance, down-regulation of Staufen1 in DM1 myoblasts rescues SG formation. Together our data show that Staufen1 participates in the inhibition of SG formation in DM1 myoblasts. These results reveal that DM1 muscle cells fail to properly respond to stress, thereby likely contributing to the complex pathogenesis of DM1.

Feedback Regulation of Kinase Signaling Pathways by AREs and GREs

In response to environmental signals, kinases phosphorylate numerous proteins, including RNA-binding proteins such as the AU-rich element (ARE) binding proteins, and the GU-rich element (GRE) binding proteins. Posttranslational modifications of these proteins lead to a significant changes in the abundance of target mRNAs, and affect gene expression during cellular activation, proliferation, and stress responses. In this review, we summarize the effect of phosphorylation on the function of ARE-binding proteins ZFP36 and ELAVL1 and the GRE-binding protein CELF1. The networks of target mRNAs that these proteins bind and regulate include transcripts encoding kinases and kinase signaling pathways (KSP) components. Thus, kinase signaling pathways are involved in feedback regulation, whereby kinases regulate RNA-binding proteins that subsequently regulate mRNA stability of ARE- or GRE-containing transcripts that encode components of KSP.

Watson-Crick-like pairs in CCUG repeats: evidence for tautomeric shifts or protonation

RNA transcripts that include expanded CCUG repeats are associated with myotonic dystrophy type 2. Crystal structures of two CCUG-containing oligomers show that the RNA strands associate into slipped duplexes that contain noncanonical C-U pairs that have apparently undergone tautomeric transition or protonation resulting in an unusual Watson-Crick-like pairing. The overhanging ends of the duplexes interact forming U-U pairs, which also show tautomerism. Duplexes consisting of CCUG repeats are thermodynamically less stable than the trinucleotide repeats involved in the TRED genetic disorders, but introducing LNA residues increases their stability and raises the melting temperature of the studied oligomers by ∼10°C, allowing detailed crystallographic studies. Quantum mechanical calculations were performed to test the possibility of the tautomeric transitions or protonation within the noncanonical pairs. The results indicate that tautomeric or ionic shifts of nucleobases can manifest themselves in biological systems, supplementing the canonical "rules of engagement."

RNA FISH for detecting expanded repeats in human diseases

RNA fluorescence in situ hybridization (FISH) is a widely used technique for detecting transcripts in fixed cells and tissues. Many variants of RNA FISH have been proposed to increase signal strength, resolution and target specificity. The current variants of this technique facilitate the detection of the subcellular localization of transcripts at a single molecule level. Among the applications of RNA FISH are studies on nuclear RNA foci in diseases resulting from the expansion of tri-, tetra-, penta- and hexanucleotide repeats present in different single genes. The partial or complete retention of mutant transcripts forming RNA aggregates within the nucleoplasm has been shown in multiple cellular disease models and in the tissues of patients affected with these atypical mutations. Relevant diseases include, among others, myotonic dystrophy type 1 (DM1) with CUG repeats, Huntington's disease (HD) and spinocerebellar ataxia type 3 (SCA3) with CAG repeats, fragile X-associated tremor/ataxia syndrome (FXTAS) with CGG repeats, myotonic dystrophy type 2 (DM2) with CCUG repeats, amyotrophic lateral sclerosis/frontotemporal dementia (ALS/FTD) with GGGGCC repeats and spinocerebellar ataxia type 32 (SCA32) with GGCCUG. In this article, we summarize the results obtained with FISH to examine RNA nuclear inclusions. We provide a detailed protocol for detecting RNAs containing expanded CAG and CUG repeats in different cellular models, including fibroblasts, lymphoblasts, induced pluripotent stem cells and murine and human neuronal progenitors. We also present the results of the first single-molecule FISH application in a cellular model of polyglutamine disease.

Congenital and childhood myotonic dystrophy: Current aspects of disease and future directions

Myotonic dystrophy type 1 (DM1) is multisystem disease arising from mutant CTG expansion in the non-translating region of the dystrophia myotonica protein kinase gene. While DM1 is the most common adult muscular dystrophy, with a worldwide prevalence of one in eight thousand, age of onset varies from before birth to adulthood. There is a broad spectrum of clinical severity, ranging from mild to severe, which correlates with number of DNA repeats. Importantly, the early clinical manifestations and management in congenital and childhood DM1 differ from classic adult DM1. In neonates and children, DM1 predominantly affects muscle strength, cognition, respiratory, central nervous and gastrointestinal systems. Sleep disorders are often under recognised yet a significant morbidity. No effective disease modifying treatment is currently available and neonates and children with DM1 may experience severe physical and intellectual disability, which may be life limiting in the most severe forms. Management is currently supportive, incorporating regular surveillance and treatment of manifestations. Novel therapies, which target the gene and the pathogenic mechanism of abnormal splicing are emerging. Genetic counselling is critical in this autosomal dominant genetic disease with variable penetrance and potential maternal anticipation, as is assisting with family planning and undertaking cascade testing to instigate health surveillance in affected family members. This review incorporates discussion of the clinical manifestations and management of congenital and childhood DM1, with a particular focus on hypersomnolence and sleep disorders. In addition, the molecular genetics, mechanisms of disease pathogenesis and development of novel treatment strategies in DM1 will be summarised.

CUG-BP1 regulates RyR1 ASI alternative splicing in skeletal muscle atrophy

RNA binding protein is identified as an important mediator of aberrant alternative splicing in muscle atrophy. The altered splicing of calcium channels, such as ryanodine receptors (RyRs), plays an important role in impaired excitation-contraction (E-C) coupling in muscle atrophy; however, the regulatory mechanisms of ryanodine receptor 1 (RyR1) alternative splicing leading to skeletal muscle atrophy remains to be investigated. In this study we demonstrated that CUG binding protein 1 (CUG-BP1) was up-regulated and the alternative splicing of RyR1 ASI (exon70) was aberrant during the process of neurogenic muscle atrophy both in human patients and mouse models. The gain and loss of function experiments in vivo demonstrated that altered splicing pattern of RyR1 ASI was directly mediated by an up-regulated CUG-BP1 function. Furthermore, we found that CUG-BP1 affected the calcium release activity in single myofibers and the extent of atrophy was significantly reduced upon gene silencing of CUG-BP1 in atrophic muscle. These findings improve our understanding of calcium signaling related biological function of CUG-BP1 in muscle atrophy. Thus, we provide an intriguing perspective of involvement of mis-regulated RyR1 splicing in muscular disease.

Hypogonadism Associated with Cyp19a1 (Aromatase) Posttranscriptional Upregulation in Celf1 Knockout Mice

CELF1 is a multifunctional RNA-binding protein that controls several aspects of RNA fate. The targeted disruption of the Celf1 gene in mice causes male infertility due to impaired spermiogenesis, the postmeiotic differentiation of male gametes. Here, we investigated the molecular reasons that underlie this testicular phenotype. By measuring sex hormone levels, we detected low concentrations of testosterone in Celf1-null mice. We investigated the effect of Celf1 disruption on the expression levels of steroidogenic enzyme genes, and we observed that Cyp19a1 was upregulated. Cyp19a1 encodes aromatase, which transforms testosterone into estradiol. Administration of testosterone or the aromatase inhibitor letrozole partly rescued the spermiogenesis defects, indicating that a lack of testosterone associated with excessive aromatase contributes to the testicular phenotype. In vivo and in vitro interaction assays demonstrated that CELF1 binds to Cyp19a1 mRNA, and reporter assays supported the conclusion that CELF1 directly represses Cyp19a1 translation. We conclude that CELF1 downregulates Cyp19a1 (Aromatase) posttranscriptionally to achieve high concentrations of testosterone compatible with spermiogenesis completion. We discuss the implications of these findings with respect to reproductive defects in men, including patients suffering from isolated hypogonadotropic hypogonadism and myotonic dystrophy type I.

Muscle wasting in myotonic dystrophies: a model of premature aging

Myotonic dystrophy type 1 (DM1 or Steinert’s disease) and type 2 (DM2) are multisystem disorders of genetic origin. Progressive muscular weakness, atrophy and myotonia are the most prominent neuromuscular features of these diseases, while other clinical manifestations such as cardiomyopathy, insulin resistance and cataracts are also common. From a clinical perspective, most DM symptoms are interpreted as a result of an accelerated aging (cataracts, muscular weakness and atrophy, cognitive decline, metabolic dysfunction, etc.), including an increased risk of developing tumors. From this point of view, DM1 could be described as a progeroid syndrome since a notable age-dependent dysfunction of all systems occurs. The underlying molecular disorder in DM1 consists of the existence of a pathological (CTG) triplet expansion in the 3′ untranslated region (UTR) of the Dystrophia Myotonica Protein Kinase (DMPK) gene, whereas (CCTG)n repeats in the first intron of the Cellular Nucleic acid Binding Protein/Zinc Finger Protein 9(CNBP/ZNF9) gene cause DM2. The expansions are transcribed into (CUG)n and (CCUG)n-containing RNA, respectively, which form secondary structures and sequester RNA-binding proteins, such as the splicing factor muscleblind-like protein (MBNL), forming nuclear aggregates known as foci. Other splicing factors, such as CUGBP, are also disrupted, leading to a spliceopathy of a large number of downstream genes linked to the clinical features of these diseases. Skeletal muscle regeneration relies on muscle progenitor cells, known as satellite cells, which are activated after muscle damage, and which proliferate and differentiate to muscle cells, thus regenerating the damaged tissue. Satellite cell dysfunction seems to be a common feature of both age-dependent muscle degeneration (sarcopenia) and muscle wasting in DM and other muscle degenerative diseases. This review aims to describe the cellular, molecular and macrostructural processes involved in the muscular degeneration seen in DM patients, highlighting the similarities found with muscle aging.

p300 Regulates Liver Functions by Controlling p53 and C/EBP Family Proteins through Multiple Signaling Pathways

The histone acetyltransferase p300 has been implicated in the regulation of liver biology; however, molecular mechanisms of this regulation are not known. In this paper, we examined these mechanisms using transgenic mice expressing a dominant negative p300 molecule (dnp300). While dnp300 mice did not show abnormal growth within 1 year, these mice have many alterations in liver biology and liver functions. We found that the inhibition of p300 leads to the accumulation of heterochromatin foci in the liver of 2-month-old mice. Transcriptome sequencing (RNA-Seq) analysis showed that this inhibition of p300 also causes alterations of gene expression in many signaling pathways, including chromatin remodeling, apoptosis, DNA damage, translation, and activation of the cell cycle. Livers of dnp300 mice have a high rate of proliferation and a much higher rate of proliferation after partial hepatectomy. We found that livers of dnp300 mice are resistant to CCl4-mediated injury and have reduced apoptosis but have increased proliferation after injury. Underlying mechanisms of resistance to liver injury and increased proliferation in dnp300 mice include ubiquitin-proteasome-mediated degradation of C/EBPα and translational repression of the p53 protein by the CUGBP1-eukaryotic initiation factor 2 (eIF2) repressor complex. Our data demonstrate that p300 regulates a number of critical signaling pathways that control liver functions.

The role of the immune system in triplet repeat expansion diseases

Trinucleotide repeat expansion disorders (TREDs) are a group of dominantly inherited neurological diseases caused by the expansion of unstable repeats in specific regions of the associated genes. Expansion of CAG repeat tracts in translated regions of the respective genes results in polyglutamine- (polyQ-) rich proteins that form intracellular aggregates that affect numerous cellular activities. Recent evidence suggests the involvement of an RNA toxicity component in polyQ expansion disorders, thus increasing the complexity of the pathogenic processes. Neurodegeneration, accompanied by reactive gliosis and astrocytosis is the common feature of most TREDs, which may suggest involvement of inflammation in pathogenesis. Indeed, a number of immune response markers have been observed in the blood and CNS of patients and mouse models, and the activation of these markers was even observed in the premanifest stage of the disease. Although inflammation is not an initiating factor of TREDs, growing evidence indicates that inflammatory responses involving astrocytes, microglia, and the peripheral immune system may contribute to disease progression. Herein, we review the involvement of the immune system in the pathogenesis of triplet repeat expansion diseases, with particular emphasis on polyglutamine disorders. We also present various therapeutic approaches targeting the dysregulated inflammation pathways in these diseases.

Molecular mechanisms in DM1 - a focus on foci

Myotonic dystrophy type 1 is caused by abnormal expansion of a CTG-trinucleotide repeat in the gene encoding Dystrophia Myotonica Protein Kinase (DMPK), which in turn leads to global deregulation of gene expression in affected individuals. The transcribed mRNA contains a massive CUG-expansion in the 3' untranslated region (3'UTR) facilitating nucleation of several regulatory RNA-binding proteins, which are thus unable to perform their normal cellular function. These CUG-expanded mRNA-protein aggregates form distinct, primarily nuclear foci. In differentiated muscle cells, most of the CUG-expanded RNA remains in the nuclear compartment, while in dividing cells such as fibroblasts a considerable fraction of the mutant RNA reaches the cytoplasm, consistent with findings that both nuclear and cytoplasmic events are mis-regulated in DM1. Recent evidence suggests that the nuclear aggregates, or ribonuclear foci, are more dynamic than previously anticipated and regulated by several proteins, including RNA helicases. In this review, we focus on the homeostasis of DMPK mRNA foci and discuss how their dynamic regulation may affect disease-causing mechanisms in DM1.

RAN translation and frameshifting as translational challenges at simple repeats of human neurodegenerative disorders

Repeat-associated disorders caused by expansions of short sequences have been classified as coding and noncoding and are thought to be caused by protein gain-of-function and RNA gain-of-function mechanisms, respectively. The boundary between such classifications has recently been blurred by the discovery of repeat-associated non-AUG (RAN) translation reported in spinocerebellar ataxia type 8, myotonic dystrophy type 1, fragile X tremor/ataxia syndrome and C9ORF72 amyotrophic lateral sclerosis and frontotemporal dementia. This noncanonical translation requires no AUG start codon and can initiate in multiple frames of CAG, CGG and GGGGCC repeats of the sense and antisense strands of disease-relevant transcripts. RNA structures formed by the repeats have been suggested as possible triggers; however, the precise mechanism of the translation initiation remains elusive. Templates containing expansions of microsatellites have also been shown to challenge translation elongation, as frameshifting has been recognized across CAG repeats in spinocerebellar ataxia type 3 and Huntington's disease. Determining the critical requirements for RAN translation and frameshifting is essential to decipher the mechanisms that govern these processes. The contribution of unusual translation products to pathogenesis needs to be better understood. In this review, we present current knowledge regarding RAN translation and frameshifting and discuss the proposed mechanisms of translational challenges imposed by simple repeat expansions.