The myotonic dystrophies: molecular, clinical, and therapeutic challenges

Disease mechanisms and therapeutic approaches in spinal muscular atrophy

Motor neuron diseases are neurological disorders characterized primarily by the degeneration of spinal motor neurons, skeletal muscle atrophy, and debilitating and often fatal motor dysfunction. Spinal muscular atrophy (SMA) is an autosomal-recessive motor neuron disease of high incidence and severity and the most common genetic cause of infant mortality. SMA is caused by homozygous mutations in the survival motor neuron 1 (SMN1) gene and retention of at least one copy of the hypomorphic gene paralog SMN2. Early studies established a loss-of-function disease mechanism involving ubiquitous SMN deficiency and suggested SMN upregulation as a possible therapeutic approach. In recent years, greater knowledge of the central role of SMN in RNA processing combined with deep characterization of animal models of SMA has significantly advanced our understanding of the cellular and molecular basis of the disease. SMA is emerging as an RNA disease not limited to motor neurons, but one that involves dysfunction of motor circuits that comprise multiple neuronal subpopulations and possibly other cell types. Advances in SMA research have also led to the development of several potential therapeutics shown to be effective in animal models of SMA that are now in clinical trials. These agents offer unprecedented promise for the treatment of this still incurable neurodegenerative disease.

[Myotonic dystrophy]

No effective treatment was available for myotonic dystrophy, even in animal model. We have established a new antisense oligonucleotide delivery to skeletal muscle of mice with bubble liposomes, and led to increased expression of chloride channel (CLCN1) protein and the amelioration of myotonia. In other experiments, we also identified small molecule compounds that correct aberrant splicing of Clcn1 gene. Manumycin A corrected aberrant splicing of Clcn1 in mouse model.

Resveratrol enhances splicing of insulin receptor exon 11 in myotonic dystrophy type 1 fibroblasts

INTRODUCTION

Myotonic dystrophy type 1 (DM1) is characterized by splicing abnormalities caused by CUG expansion of the DMPK gene transcript. Splicing of exon 11 of the insulin receptor (IR) gene is deregulated to suppress exon 11 inclusion into mRNA in DM1. Consequently, the exon 11-deleted IR isoform that is less sensitive to insulin is predominantly produced, leading to glucose intolerance in DM1. Upregulation of exon 11 retaining full-length IR mRNA is a potential way to recover insulin sensitivity in DM1.

METHODS

We examined candidate chemicals for their ability to enhance inclusion of exon 11 of the IR gene in cultured cells by reverse transcription-PCR amplification of a fragment extending from exons 10 to 12 of IR mRNA.

RESULTS

We revealed that resveratrol (RES) enhanced the percentage of exon 11-containing IR mRNA among the total IR mRNA in HeLa cells. The RES-mediated enhancement of exon 11 inclusion was cell-specific and highest in fibroblasts. We tested RES on four fibroblast samples from three generations of one DM1 family. In each sample, RES treatment significantly upregulated the percentage of exon 11-containing IR mRNA to levels higher than that of the control, irrespective of the length of the sample's CTG repeat expansion.

DISCUSSION

A natural compound, RES, was shown for the first time to upregulate the full-length IR mRNA in fibroblasts from DM1 cases. Our results provide the justification of RES as a leading compound to improve glucose tolerance in DM1.

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.

Cortical involvement during myotonia in myotonic dystrophy: an fMRI study

OBJECTIVE

Myotonic dystrophy type 1 (DM1) is a common adulthood muscular dystrophy, characterized by muscle wasting, myotonia, and multisystemic manifestations. The phenomenon of involuntary muscle contraction during myotonia offers a unique possibility of investigating brain motor functions. This study explores cortical involvement during grip myotonia in DM1.

MATERIALS AND METHODS

Sixteen DM1 patients were enrolled in the study. Eight patients had apparent grip myotonia, while eight patients did not (control subjects). All patients underwent functional MRI grip task examination twice: prior a warm-up procedure (myotonia was elicited in patients with apparent grip myotonia) and after a warm-up procedure (myotonia was attenuated in patients with apparent grip myotonia). No myotonia was elicited during either examination in patients without apparent grip myotonia. Cerebral blood oxygen level-dependent (BOLD) signals were compared both between groups with and without apparent myotonia, and between pre- and post-warm-up sessions.

RESULTS

Significantly higher BOLD signal was found during myotonia phase in patients with apparent grip myotonia compared to corresponding non-myotonia phase of patients without apparent grip myotonia in the supplementary motor area and in the dorsal anterior cingulate cortex. Significant differences in BOLD signal levels of very similar pattern were detected between prewarm-up session myotonia phase and post-warm-up session myotonia absent phase in the group of patients with apparent grip myotonia.

CONCLUSION

We showed that myotonia is related to cortical function in high-order motor control areas. This cortical involvement is most likely to represent action of inhibitory circuits intending motor termination.

Therapeutic impact of systemic AAV-mediated RNA interference in a mouse model of myotonic dystrophy

RNA interference (RNAi) offers a promising therapeutic approach for dominant genetic disorders that involve gain-of-function mechanisms. One candidate disease for RNAi therapy application is myotonic dystrophy type 1 (DM1), which results from toxicity of a mutant mRNA. DM1 is caused by expansion of a CTG repeat in the 3' UTR of the DMPK gene. The expression of DMPK mRNA containing an expanded CUG repeat (CUG(exp)) leads to defects in RNA biogenesis and turnover. We designed miRNA-based RNAi hairpins to target the CUG(exp) mRNA in the human α-skeletal muscle actin long-repeat (HSA(LR)) mouse model of DM1. RNAi expression cassettes were delivered to HSA(LR) mice using recombinant adeno-associated viral (rAAV) vectors injected intravenously as a route to systemic gene therapy. Vector delivery significantly reduced disease pathology in muscles of the HSA(LR) mice, including a reduction in the CUG(exp) mRNA, a reduction in myotonic discharges, a shift toward adult pre-mRNA splicing patterns, reduced myofiber hypertrophy and a decrease in myonuclear foci containing the CUG(exp) mRNA. Significant reversal of hallmarks of DM1 in the rAAV RNAi-treated HSA(LR) mice indicate that defects characteristic of DM1 can be mitigated with a systemic RNAi approach targeting the nuclei of terminally differentiated myofibers. Efficient rAAV-mediated delivery of RNAi has the potential to provide a long-term therapy for DM1 and other dominant muscular dystrophies.

Myotonic dystrophy: diagnosis, management and new therapies

PURPOSE OF REVIEW

Myotonic dystrophies type 1 and type 2 are progressive multisystem genetic disorders with clinical and genetic features in common. Myotonic dystrophy type 1 is the most prevalent muscular dystrophy in adults and has a wide phenotypic spectrum. The average age of death in myotonic dystrophy type 1 is in the fifth decade. In comparison, myotonic dystrophy type 2 tends to cause a milder phenotype with later onset of symptoms and is less common than myotonic dystrophy type 1. Historically, patients with myotonic dystrophy type 1 have not received the medical and social input they need to maximize their quality and quantity of life. This review describes the improved understanding in the molecular and clinical features of myotonic dystrophy type 1 as well as the screening of clinical complications and their management. We will also discuss new potential genetic treatments.

RECENT FINDINGS

An active approach to screening and management of myotonic dystrophies type 1 and type 2 requires a multidisciplinary medical, rehabilitative and social team. This process will probably improve morbidity and mortality for patients. Genetic treatments have been successfully used in in-vitro and animal models to reverse the physiological, histopathological and transcriptomic features.

SUMMARY

Molecular therapeutics for myotonic dystrophy will probably bridge the translational gap between bench and bedside in the near future. There will still be a requirement for clinical screening of patients with myotonic dystrophy with proactive and systematic management of complications.

Peripheral nerve involvement in myotonic dystrophy type 2 - similar or different than in myotonic dystrophy type 1?

INTRODUCTION

Multisystem manifestations of myotonic dystrophies type 1 (DM1) and 2 (DM2) are well known. Peripheral nerve involvement has been reported in DM1 but not in genetically confirmed DM2. The aim of our study was to assess peripheral nerve involvement in DM2 using nerve conduction studies and to compare these results with findings in DM1.

METHODS

We prospectively studied patients with genetically confirmed DM2 (n=30) and DM1 (n=32). All patients underwent detailed neurological examination and nerve conduction studies.

RESULTS

Abnormalities in electrophysiological studies were found in 26.67% of patients with DM2 and in 28.13% of patients with DM1 but the criteria of polyneuropathy were fulfilled in only 13.33% of patients with DM2 and 12.5% of patients with DM1. The polyneuropathy was subclinical, and no correlation was found between its presence and patient age or disease duration.

CONCLUSIONS

Peripheral nerves are quite frequently involved in DM2, but abnormalities meeting the criteria of polyneuropathy are rarely found. The incidence of peripheral nerve involvement is similar in both types of myotonic dystrophy.

Celf1 regulates cell cycle and is partially responsible for defective myoblast differentiation in myotonic dystrophy RNA toxicity

Myotonic dystrophy is a neuromuscular disease of RNA toxicity. The disease gene DMPK harbors expanded CTG trinucleotide repeats on its 3'-UTR. The transcripts of this mutant DMPK led to misregulation of RNA-binding proteins including MBNL1 and Celf1. In myoblasts, CUG-expansion impaired terminal differentiation. In this study, we formally tested how the abundance of Celf1 regulates normal myocyte differentiation, and how Celf1 expression level mediates CUG-expansion RNA toxicity-triggered impairment of myocyte differentiation. As the results, overexpression of Celf1 largely recapitulated the defects of myocytes with CUG-expansion, by increasing myocyte cycling. Knockdown of endogenous Celf1 level led to precocious myotube formation, supporting a negative connection between Celf1 abundance and myocyte terminal differentiation. Finally, knockdown of Celf1 in myocyte with CUG-expansion led to partial rescue, by promoting cell cycle exit. Our results suggest that Celf1 plays a distinctive and negative role in terminal myocyte differentiation, which partially contribute to DM1 RNA toxicity. Targeting Celf1 may be a valid strategy in correcting DM1 muscle phenotypes, especially for congenital cases.

Truths and misinformation: a qualitative exploration of myotonic dystrophy

BACKGROUND

Myotonic dystrophy (DM1) is an autosomal dominant, progressive, and multisystem condition that impacts affected individuals physically, socially, and emotionally. Understanding individuals' perceptions of their disease is critical to ensuring appropriate information, education, and counseling.

METHODS

We conducted a content analysis of findings from a larger study that used a novel, qualitative research approach called photovoice to explore nine patients' experiences of living with DM1. Participants took pictures that illustrated barriers or facilitators to living with DM1; their photographs then formed the basis of semistructured interviews. Transcripts were analyzed and, among themes, we identified one titled "DM1 truths and misinformation" that described participants' disease knowledge. Analysis revealed four categories within this broader theme: "the physical and emotional cost of DM1," "managing my DM1," "genetics and me" and "patients as advocates and educators."

RESULTS

Findings showed that DM1 participants had good core knowledge with respect to their disease and its implications. However, each participant held as fact fragments of misinformation that shaped decision-making and pointed to a clear need for strategies to mitigate variable interpretation of health information.

CONCLUSIONS

We conclude that there is a need for increased education and awareness about symptoms, genetic information and treatment strategies for patients, their family members, and health care providers.

Report by the Spanish Foundation for the Brain on the social impact of amyotrophic lateral sclerosis and other neuromuscular disorders

INTRODUCTION

A thorough knowledge of the socioeconomic scope of neuromuscular disease is essential for managing resources and raising social awareness.

DEVELOPMENT

Our group reviewed current data on the epidemiology, mortality and dependence rates, and socioeconomic impact of amyotrophic lateral sclerosis and neuromuscular diseases in Spain. We also recorded how neurological care for these patients is organised.

CONCLUSIONS

Neuromuscular disorders are a very heterogeneous group of diseases, and some are very rare. These disorders account for between 2.8% and 18% of the total motives for a neurological consultation. In Spain, prevalence and incidence figures for amyotrophic lateral sclerosis are similar to those in other countries; however, figures for patients with other neuromuscular diseases are not known. Since the diseases are chronic, progressive, and debilitating, they cause considerable disability and dependence, which in turn directly affects healthcare and social costs associated with the disease. The costs generated by one patient with amyotrophic lateral sclerosis or Duchenne disease have been calculated at about 50 000 euros per year. Neuromuscular disease shows aetiological, diagnostic, and prognostic complexity, and it requires multidisciplinary management. Follow-up for these patients should be entrusted to specialised units.

Cell membrane integrity in myotonic dystrophy type 1: implications for therapy

Myotonic Dystrophy type 1 (DM1) is a multisystemic disease caused by toxic RNA from a DMPK gene carrying an expanded (CTG•CAG)n repeat. Promising strategies for treatment of DM1 patients are currently being tested. These include antisense oligonucleotides and drugs for elimination of expanded RNA or prevention of aberrant binding to RNP proteins. A significant hurdle for preclinical development along these lines is efficient systemic delivery of compounds across endothelial and target cell membranes. It has been reported that DM1 patients show elevated levels of markers of muscle damage or loss of sarcolemmal integrity in their serum and that splicing of dystrophin, an essential protein for muscle membrane structure, is abnormal. Therefore, we studied cell membrane integrity in DM1 mouse models commonly used for preclinical testing. We found that membranes in skeletal muscle, heart and brain were impermeable to Evans Blue Dye. Creatine kinase levels in serum were similar to those in wild type mice and expression of dystrophin protein was unaffected. Also in patient muscle biopsies cell surface expression of dystrophin was normal and calcium-positive fibers, indicating elevated intracellular calcium levels, were only rarely seen. Combined, our findings indicate that cells in DM1 tissues do not display compromised membrane integrity. Hence, the cell membrane is a barrier that must be overcome in future work towards effective drug delivery in DM1 therapy.

Spectrum of neurodevelopmental disabilities: a cohort study in hungary

The spectrum of neurodevelopmental disabilities was studied in a cohort of patients in Hungary. A search for etiologies and assessment of the degree of intellectual disability were carried out. The study included 241 (131 boys) patients. Disability occurred without any prenatal, perinatal, and/or neonatal adverse events in 167 patients. They were classified into the following subgroups: genetic syndromes with recognized etiology, global developmental delay/intellectual disability in association with dysmorphic features but unknown etiology, global developmental delay/intellectual disability without dysmorphic features and recognized etiology, brain malformations, inborn errors of metabolism, leukoencephalopathies, epileptic syndromes, developmental language impairment, and neuromuscular disorders. Adverse events occurred in 74 children classified into subgroups such as cerebral palsy after delivery preterm or at term, and disabilities without cerebral palsy. The etiology was identified in 66.4%, and genetic diagnosis was found in 19.5%. Classification of neurodevelopmental disorders contribute to etiological diagnosis, proper rehabilitation, and genetic counseling.

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.

Common recessive limb girdle muscular dystrophies differential diagnosis: why and how?

Limb girdle muscular dystrophies are heterogeneous autosomal hereditary neuromuscular disorders. They produce dystrophic changes on muscle biopsy and they are associated with mutations in several genes involved in muscular structure and function. Detailed clinical, laboratorial, imaging, diagnostic flowchart, photographs, tables, and illustrated diagrams are presented for the differential diagnosis of common autosomal recessive limb girdle muscular dystrophy subtypes diagnosed nowadays at one reference center in Brazil. Preoperative image studies guide muscle biopsy site selection. Muscle involvement image pattern differs depending on the limb girdle muscular dystrophy subtype. Muscle involvement is conspicuous at the posterior thigh in calpainopathy and fukutin-related proteinopathy; anterior thigh in sarcoglycanopathy; whole thigh in dysferlinopathy, and telethoninopathy. The precise differential diagnosis of limb girdle muscular dystrophies is important for genetic counseling, prognostic orientation, cardiac and respiratory management. Besides that, it may probably, in the future, provide specific genetic therapies for each subtype.

Rare diseases in clinical endocrinology: a taxonomic classification system

PURPOSE

Rare endocrine-metabolic diseases (REMD) represent an important area in the field of medicine and pharmacology. The rare diseases of interest to endocrinologists involve all fields of endocrinology, including rare diseases of the pituitary, thyroid and adrenal glands, paraganglia, ovary and testis, disorders of bone and mineral metabolism, energy and lipid metabolism, water metabolism, and syndromes with possible involvement of multiple endocrine glands, and neuroendocrine tumors. Taking advantage of the constitution of a study group on REMD within the Italian Society of Endocrinology, consisting of basic and clinical scientists, a document on the taxonomy of REMD has been produced.

METHODS AND RESULTS

This document has been designed to include mainly REMD manifesting or persisting into adulthood. The taxonomy of REMD of the adult comprises a total of 166 main disorders, 338 including all variants and subtypes, described into 11 tables.

CONCLUSIONS

This report provides a complete taxonomy to classify REMD of the adult. In the future, the creation of registries of rare endocrine diseases to collect data on cohorts of patients and the development of common and standardized diagnostic and therapeutic pathways for each rare endocrine disease is advisable. This will help planning and performing intervention studies in larger groups of patients to prove the efficacy, effectiveness, and safety of a specific treatment.

Facial memory deficits in myotonic dystrophy type 1

OBJECTIVES

To evaluate facial memory ability (FMA) in patients with myotonic dystrophy type 1 (DM1). We also explored the relationship between FMA and neuropsychological data, disease-related factors, and CTG repeat expansion size.

MATERIALS AND METHODS

Patients with DM1 (n = 33) and healthy subjects (n = 30) were tested with the faces task of the Rivermead Behavioural Memory Test - Extended version (RBMT-E) and an additional set of neuropsychological tests. Clinical data were collected, and CTG repeat size was quantified in blood lymphocytes.

RESULTS

Low results on the faces task were more common in patients with DM1 compared with healthy subjects (P < 0.05), with 36% of the patients showing a poor/impaired performance. DM1 patients with deficits in FMA performed significantly worse on tests measuring visual-construction ability and memory. Furthermore, these patients more often falsely recognised unknown faces as known. Deficits in FMA were not associated with any disease-related factor, including CTG repeat expansion size.

CONCLUSIONS

These findings revealed deficits in FMA in the DM1 group, which was associated with reduced construction- and visual memory ability.

Triadopathies: an emerging class of skeletal muscle diseases

The triad is a skeletal muscle substructure responsible for the regulation of excitation-contraction coupling. It is formed by the close apposition of the T-tubule and the terminal sarcoplasmic reticulum. A rapidly growing list of skeletal myopathies, here referred to as triadopathies, are caused by gene mutations in components of the triad. These disorders, at their root, are caused by defects in excitation contraction coupling and intracellular calcium homeostasis. Secondary abnormalities in triad structure and/or function are also reported in several muscle diseases, most notably certain muscular dystrophies. This review highlights the current understanding of both primary and secondary triadopathies, and identifies important concepts yet to be fully addressed in the field. The emphasis of the review is both on the pathogenesis of triadopathies and their potential treatment.

Neurologic disease with pregnancy and considerations for the obstetric anesthesiologist

Women with neurologic conditions present a challenge during pregnancy and in the peripartum period. Given the low prevalence of these diseases during pregnancy, most management decisions are guided by retrospective reviews and case reports. This article reviews current literature for some of the more common or complex neurologic conditions affecting pregnancy with special consideration for anesthetic management. In particular, epilepsy; multiple sclerosis; primary intracranial hypertension; secondary intracranial hypertension-Arnold-Chiari malformations and intracranial neoplasms; spinal cord injury; neuromuscular junction disorders-myasthenia gravis; and hereditary neuromuscular disorders-myotonic dystrophy and spinal muscular atrophy will be discussed. By increasing understanding of anesthetic issues for parturients with neurologic disease, providers may more effectively anticipate anesthetic considerations, thereby optimizing care plans.

Borderlines between Sarcopenia and Mild Late-Onset Muscle Disease

Numerous natural or disease-related alterations occur in different tissues of the body with advancing age. Sarcopenia is defined as age-related decrease of muscle mass and strength beginning in mid-adulthood and accelerating in people older than 60 years. Pathophysiology of sarcopenia involves both neural and muscle dependent mechanisms and is enhanced by multiple factors. Aged muscles show loss in fiber number, fiber atrophy, and gradual increase in the number of ragged red fibers and cytochrome c oxidase-negative fibers. Generalized loss of muscle tissue and increased amount of intramuscular fat are seen on muscle imaging. However, the degree of these changes varies greatly between individuals, and the distinction between normal age-related weakening of muscle strength and clinically significant muscle disease is not always obvious. Because some of the genetic myopathies can present at a very old age and be mild in severity, the correct diagnosis is easily missed. We highlight this difficult borderline zone between sarcopenia and muscle disease by two examples: LGMD1D and myotonic dystrophy type 2. Muscle magnetic resonance imaging (MRI) is a useful tool to help differentiate myopathies from sarcopenia and to reach the correct diagnosis also in the elderly.

Development of a Drosophila melanogaster spliceosensor system for in vivo high-throughput screening in myotonic dystrophy type 1

Alternative splicing of pre-mRNAs is an important mechanism that regulates cellular function in higher eukaryotes. A growing number of human genetic diseases involve splicing defects that are directly connected to their pathology. In myotonic dystrophy type 1 (DM1), several clinical manifestations have been proposed to be the consequence of tissue-specific missplicing of numerous genes. These events are triggered by an RNA gain-of-function and resultant deregulation of specific RNA-binding factors, such as the nuclear sequestration of muscleblind-like family factors (MBNL1–MBNL3). Thus, the identification of chemical modulators of splicing events could lead to the development of the first valid therapy for DM1 patients. To this end, we have generated and validated transgenic flies that contain a luciferase-reporter-based system that is coupled to the expression of MBNL1-reliant splicing (spliceosensor flies), to assess events that are deregulated in DM1 patients in a relevant disease tissue. We then developed an innovative 96-well plate screening platform to carry out in vivo high-throughput pharmacological screening (HTS) with the spliceosensor model. After a large-scale evaluation (>16,000 chemical entities), several reliable splicing modulators (hits) were identified. Hit validation steps recognized separate DM1-linked therapeutic traits for some of the hits, which corroborated the feasibility of the approach described herein to reveal promising drug candidates to correct missplicing in DM1. This powerful Drosophila-based screening tool might also be applied in other disease models displaying abnormal alternative splicing, thus offering myriad uses in drug discovery.

MBNL proteins and their target RNAs, interaction and splicing regulation

Muscleblind-like (MBNL) proteins are key regulators of precursor and mature mRNA metabolism in mammals. Based on published and novel data, we explore models of tissue-specific MBNL interaction with RNA. We portray MBNL domains critical for RNA binding and splicing regulation, and the structure of MBNL's normal and pathogenic RNA targets, particularly in the context of myotonic dystrophy (DM), in which expanded CUG or CCUG repeat transcripts sequester several nuclear proteins including MBNLs. We also review the properties of MBNL/RNA complex, including recent data obtained from UV cross-linking and immunoprecipitation (CLIP-Seq), and discuss how this interaction shapes normal MBNL-dependent alternative splicing regulation. Finally, we review how this acquired knowledge about the pathogenic RNA structure and nature of MBNL sequestration can be translated into the design of therapeutic strategies against DM.

Three novel serum biomarkers, miR-1, miR-133a, and miR-206 for Limb-girdle muscular dystrophy, Facioscapulohumeral muscular dystrophy, and Becker muscular dystrophy

OBJECTIVES

Muscular dystrophies are a clinically and genetically heterogeneous group of inherited myogenic disorders. In clinical tests for these diseases, creatine kinase (CK) is generally used as diagnostic blood-based biomarker. However, because CK levels can be altered by various other factors, such as vigorous exercise, etc., false positive is observed. Therefore, three microRNAs (miRNAs), miR-1, miR-133a, and miR-206, were previously reported as alternative biomarkers for duchenne muscular dystrophy (DMD). However, no alternative biomarkers have been established for the other muscular dystrophies.

METHODS

We, therefore, evaluated whether these miR-1, miR-133a, and miR-206 can be used as powerful biomarkers using the serum from muscular dystrophy patients including DMD, myotonic dystrophy 1 (DM1), limb-girdle muscular dystrophy (LGMD), facioscapulohumeral muscular dystrophy (FSHD), becker muscular dystrophy (BMD), and distal myopathy with rimmed vacuoles (DMRV) by qualitative polymerase chain reaction (PCR) amplification assay.

RESULTS

Statistical analysis indicated that all these miRNA levels in serum represented no significant differences between all muscle disorders examined in this study and controls by Bonferroni correction. However, some of these indicated significant differences without correction for testing multiple diseases (P < 0.05). The median values of miR-1 levels in the serum of patients with LGMD, FSHD, and BMD were approximately 5.5, 3.3 and 1.7 compared to that in controls, 0.68, respectively. Similarly, those of miR-133a and miR-206 levels in the serum of BMD patients were about 2.5 and 2.1 compared to those in controls, 1.03 and 1.32, respectively.

CONCLUSIONS

Taken together, our data demonstrate that levels of miR-1, miR-133a, and miR-206 in serum of BMD and miR-1 in sera of LGMD and FSHD patients showed no significant differences compared with those of controls by Bonferroni correction. However, the results might need increase in sample sizes to evaluate these three miRNAs as variable biomarkers.

Pathological stress granules in Alzheimer's disease

A feature of neurodegenerative disease is the accumulation of insoluble protein aggregates in the brain. In some conditions, including Amyotrophic Lateral Sclerosis and Frontotemporal lobar degeneration, the primary aggregating entities are RNA binding proteins. Through regulated prion-like assembly, RNA binding proteins serve many functions in RNA metabolism that are essential for the healthy maintenance of cells of the central nervous system. Those RNA binding proteins that are the core nucleating factors of stress granules (SGs), including TIA-1, TIAR, TTP and G3BP1, are also found in the pathological lesions of other neurological conditions, such as Alzheimer's disease, where the hallmark aggregating protein is not an RNA binding protein. This discovery suggests that the regulated cellular pathway, which utilizes assembly of RNA binding proteins to package and silence mRNAs during stress, may be integral in the aberrant pathological protein aggregation that occurs in numerous neurodegenerative conditions.

CGG repeats in RNA modulate expression of TDP-43 in mouse and fly models of fragile X tremor ataxia syndrome

Determining the molecular mechanism(s) leading to Purkinje neuron loss in the neurodegenerative disorder fragile X-associated tremor/ataxia syndrome (FXTAS) is limited by the complex morphology of this cell type. Purkinje neurons are notoriously difficult to isolate and maintain in culture presenting considerable difficultly to identify molecular changes in response to expanded CGG repeat (rCGG)-containing mRNA that induces neurotoxicity in FXTAS. Several studies have uncovered a number of RNA-binding proteins involved in translation that aberrantly interact with the CGG-containing RNA; however, whether these interactions alter the translational profile of cells has not been investigated. Here we employ bacTRAP translational profiling to demonstrate that Purkinje neurons ectopically expressing 90 CGG repeats exhibit a dramatic change in their translational profile even prior to the onset of rCGG-induced phenotypes. This approach identified ∼500 transcripts that are differentially associated with ribosomes in r(CGG)₉₀-expressing mice. Functional annotation cluster analysis revealed broad ontologies enriched in the r(CGG)₉₀ list, including RNA binding and response to stress. Intriguingly, a transcript for the Tardbp gene, implicated in a number of other neurodegenerative disorders, exhibits altered association with ribosomes in the presence of r(CGG)₉₀ repeats. We therefore tested and showed that reduced association of Tardbp mRNA with the ribosomes results in a loss of TDP-43 protein expression in r(CGG)₉₀-expressing Purkinje neurons. Furthermore, we showed that TDP-43 could modulate the rCGG repeat-mediated toxicity in a Drosophila model that we developed previously. These findings together suggest that translational dysregulation may be an underlying mechanism of rCGG-induced neurotoxicity in FXTAS.

Muscular dystrophies and other genetic myopathies

With advances in the genetics of muscle disease, the term, muscular dystrophy, has expanded to include mutations in an increasing large list of genes. This review discusses the genetics, pathophysiology, and potential treatments of the most common forms of muscular dystrophy: Duchenne muscular dystrophy, Becker muscular dystrophy, facioscapulohumeral muscular dystrophy, and myotonic dystrophy. Other forms of muscular dystrophy and other genetic muscle disorders are also discussed to provide an overview of this complex clinical problem.

Abnormal splicing of NEDD4 in myotonic dystrophy type 2: possible link to statin adverse reactions

Myotonic dystrophy type 2 (DM2) is a multisystemic disorder caused by a (CCTG)n repeat expansion in intron 1 of CNBP. Transcription of the repeats causes a toxic RNA gain of function involving their accumulation in ribonuclear foci. This leads to sequestration of splicing factors and alters pre-mRNA splicing in a range of downstream effector genes, which is thought to contribute to the diverse DM2 clinical features. Hyperlipidemia is frequent in DM2 patients, but the treatment is problematic because of an increased risk of statin-induced adverse reactions. Hypothesizing that shared pathways lead to the increased risk, we compared the skeletal muscle expression profiles of DM2 patients and controls with patients with hyperlipidemia on statin therapy. Neural precursor cell expressed, developmentally downregulated-4 (NEDD4), an ubiquitin ligase, was one of the dysregulated genes identified in DM2 patients and patients with statin-treated hyperlipidemia. In DM2 muscle, NEDD4 mRNA was abnormally spliced, leading to aberrant NEDD4 proteins. NEDD4 was down-regulated in persons taking statins, and simvastatin treatment of C2C12 cells suppressed NEDD4 transcription. Phosphatase and tensin homologue (PTEN), an established NEDD4 target, was increased and accumulated in highly atrophic DM2 muscle fibers. PTEN ubiquitination was reduced in DM2 myofibers, suggesting that the NEDD4-PTEN pathway is dysregulated in DM2 skeletal muscle. Thus, this pathway may contribute to the increased risk of statin-adverse reactions in patients with DM2.

Donor splice-site mutation in CUL4B is likely cause of X-linked intellectual disability

X-linked intellectual disability is the most common form of cognitive disability in males. Syndromic intellectual disability encompasses cognitive deficits with other medical and behavioral manifestations. Recently, a large family with a novel form of syndromic X-linked intellectual disability was characterized. Eight of 24 members of the family are male and had cognitive dysfunction, short stature, aphasia, skeletal abnormalities, and minor anomalies. To identify the causative gene(s), we performed exome sequencing in three affected boys, both parents, and an unaffected sister. We identified a haplotype consisting of eight variants located in cis within the linkage region that segregated with affected members in the family. Of these variants, two were novel. The first was at the splice-donor site of intron 7 (c.974+1G>T) in the cullin-RING ubiquitin ligase (E3) gene, CUL4B. This variant is predicted to result in failure to splice and remove intron 7 from the primary transcript. The second variant mapped to the 3'-UTR region of the KAISO gene (c.1127T>G). Sanger sequencing validated the variants in these relatives as well as in three affected males and five carriers. The KAISO gene variant was predicted to create a binding site for the microRNAs miR-4999 and miR-4774; however, luciferase expression assays failed to validate increased targeting of these miRNAs to the variant 3'-UTR. This SNP may affect 3'-UTR structure leading to decreased mRNA stability. Our results suggest that the intellectual disability phenotype in this family is caused by aberrant splicing and removal of intron 7 from CUL4B gene primary transcript.

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.

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.

Genome wide identification of aberrant alternative splicing events in myotonic dystrophy type 2

Myotonic dystrophy type 2 (DM2) is a genetic, autosomal dominant disease due to expansion of tetraplet (CCTG) repetitions in the first intron of the ZNF9/CNBP gene. DM2 is a multisystemic disorder affecting the skeletal muscle, the heart, the eye and the endocrine system. According to the proposed pathological mechanism, the expanded tetraplets have an RNA toxic effect, disrupting the splicing of many mRNAs. Thus, the identification of aberrantly spliced transcripts is instrumental for our understanding of the molecular mechanisms underpinning the disease. The aim of this study was the identification of new aberrant alternative splicing events in DM2 patients. By genome wide analysis of 10 DM2 patients and 10 controls (CTR), we identified 273 alternative spliced exons in 218 genes. While many aberrant splicing events were already identified in the past, most were new. A subset of these events was validated by qPCR assays in 19 DM2 and 15 CTR subjects. To gain insight into the molecular pathways involving the identified aberrantly spliced genes, we performed a bioinformatics analysis with Ingenuity system. This analysis indicated a deregulation of development, cell survival, metabolism, calcium signaling and contractility. In conclusion, our genome wide analysis provided a database of aberrant splicing events in the skeletal muscle of DM2 patients. The affected genes are involved in numerous pathways and networks important for muscle physio-pathology, suggesting that the identified variants may contribute to DM2 pathogenesis.

RNA-protein interactions in unstable microsatellite diseases

A novel RNA-mediated disease mechanism has emerged from studies on dominantly inherited neurological disorders caused by unstable microsatellite expansions in non-coding regions of the genome. These non-coding tandem repeat expansions trigger the production of unusual RNAs that gain a toxic function, which involves the formation of RNA repeat structures that interact with, and alter the activities of, various factors required for normal RNA processing as well as additional cellular functions. In this review, we explore the deleterious effects of toxic RNA expression and discuss the various model systems currently available for studying RNA gain-of-function in neurologic diseases. Common themes, including bidirectional transcription and repeat-associated non-ATG (RAN) translation, have recently emerged from expansion disease studies. These and other discoveries have highlighted the need for further investigations designed to provide the additional mechanistic insights essential for future therapeutic development.

Noncoding RNAs: emerging players in muscular dystrophies

The fascinating world of noncoding RNAs has recently come to light, thanks to the development of powerful sequencing technologies, revealing a variety of RNA molecules playing important regulatory functions in most, if not all, cellular processes. Many noncoding RNAs have been implicated in regulatory networks that are determinant for skeletal muscle differentiation and disease. In this review, we outline the noncoding RNAs involved in physiological mechanisms of myogenesis and those that appear dysregulated in muscle dystrophies, also discussing their potential use as disease biomarkers and therapeutic targets.

Neonatal neuroimaging findings in congenital myotonic dystrophy

We report on a preterm neonate of 30 weeks gestational age who presented with marked muscular hypotonia and severe respiratory failure at birth and was diagnosed with congenital myotonic dystrophy. Neuroimaging at 36 gestational weeks demonstrated diffuse T2-hyperintense signal of the supratentorial white matter and a simplified gyration and sulcation pattern. Follow-up imaging showed progressive myelination, brain maturation and decrease in T2-signal of the white matter. We discuss possible pathomechanisms for white matter signal abnormalities in this neonate.

Risk of fracture in patients with muscular dystrophies

The aim of the study was to determine fracture risk in incident muscular dystrophy (MD) patients. Patients with MD are at a 1.4-fold increased risk of fracture as compared with population-based control patients. Risk further increased among elderly and female patients and among patients exposed to oral glucocorticoids.

INTRODUCTION

Muscular dystrophies (MDs) are inherited diseases causing muscle weakness and thereby increase the risk of falling and detrimental effects on bone. Both are recognised risk factors for fracture. Therefore, the aim of this study was to determine the hazard ratio of fracture in patients with MD.

METHODS

We conducted a retrospective cohort study using the UK General Practice Research Database (1987-2012). Each patient with MD was matched by year of birth, sex and practice to up to six patients without a history of MD. Outcome measure was all fractures.

RESULTS

As compared with control patients, risk of any fracture was statistically significantly increased in MD patients (adjusted hazard ratio [AHR], 1.40; 95 % confidence interval [CI], 1.14-1.71). An increased risk of fracture was observed among MD patients with female gender (AHR, 1.78; 95 % CI, 1.33-2.40) and an increasing age as compared with control patients. Stratification to Duchenne MD showed no association with fracture, whereas risk of fracture was increased twofold among patients with myotonic dystrophy (AHR, 2.34; 95 % CI, 1.56-3.51). MD patients had an almost tripled risk of fracture when they used oral glucocorticoids in the previous 6 months as compared to non-users with MD.

CONCLUSION

Patients with MD are at a 1.4-fold increased risk of fracture as compared with population-based control patients. Especially in older age groups and female gender, the fracture risk of MD versus non-MD patients is increased, whereas exposure to glucocorticoids further increased fracture risk among MD patients.

The Mef2 transcription network is disrupted in myotonic dystrophy heart tissue, dramatically altering miRNA and mRNA expression

Cardiac dysfunction is the second leading cause of death in myotonic dystrophy type 1 (DM1), primarily because of arrhythmias and cardiac conduction defects. A screen of more than 500 microRNAs (miRNAs) in a DM1 mouse model identified 54 miRNAs that were differentially expressed in heart. More than 80% exhibited downregulation toward the embryonic expression pattern and showed a DM1-specific response. A total of 20 of 22 miRNAs tested were also significantly downregulated in human DM1 heart tissue. We demonstrate that many of these miRNAs are direct MEF2 transcriptional targets, including miRNAs for which depletion is associated with arrhythmias or fibrosis. MEF2 protein is significantly reduced in both DM1 and mouse model heart samples, and exogenous MEF2C restores normal levels of MEF2 target miRNAs and mRNAs in a DM1 cardiac cell culture model. We conclude that loss of MEF2 in DM1 heart causes pathogenic features through aberrant expression of both miRNA and mRNA targets.