Myotonic dystrophy CTG expansion affects synaptic vesicle proteins, neurotransmission and mouse behaviour

Analysis of splicing abnormalities in the white matter of myotonic dystrophy type 1 brain using RNA sequencing

Myotonic dystrophy type 1 (DM1) is a neuromuscular disorder caused by the genomic expansion of CTG repeats, in which RNA-binding proteins, such as muscleblind-like protein, are sequestered in the nucleus, and abnormal splicing is observed in various genes. Although abnormal splicing occurs in the brains of patients with DM1, its relation to central nervous system symptoms is unknown. Several imaging studies have indicated substantial white matter defects in patients with DM1. Here, we performed RNA sequencing and analysis of CTG repeat lengths in the frontal lobe of patients with DM1, separating the gray matter and white matter, to investigate splicing abnormalities in the DM1 brain, especially in the white matter. Several genes showed similar levels of splicing abnormalities in both gray and white matter, with an observable trend toward an increased number of repeats in the gray matter. These findings suggest that white matter defects in DM1 stem from aberrant RNA splicing in both gray and white matter. Notably, several of the genes displaying abnormal splicing are recognized as being dominantly expressed in astrocytes and oligodendrocytes, leading us to hypothesize that splicing defects in the white matter may be attributed to abnormal RNA splicing in glial cells.

Integrative Proteogenomics for Differential Expression and Splicing Variation in a DM1 Mouse Model

Dysregulated mRNA splicing is involved in the pathogenesis of many diseases including cancer, neurodegenerative diseases, and muscular dystrophies such as myotonic dystrophy type 1 (DM1). Comprehensive assessment of dysregulated splicing on the transcriptome and proteome level has been methodologically challenging, and thus investigations have often been targeting only few genes. Here, we performed a large-scale coordinated transcriptomic and proteomic analysis to characterize a DM1 mouse model (HSALR) in comparison to wild type. Our integrative proteogenomics approach comprised gene- and splicing-level assessments for mRNAs and proteins. It recapitulated many known instances of aberrant mRNA splicing in DM1 and identified new ones. It enabled the design and targeting of splicing-specific peptides and confirmed the translation of known instances of aberrantly spliced disease-related genes (e.g., Atp2a1, Bin1, Ryr1), complemented by novel findings (Flnc and Ywhae). Comparative analysis of large-scale mRNA and protein expression data showed quantitative agreement of differentially expressed genes and splicing patterns between disease and wild type. We hence propose this work as a suitable blueprint for a robust and scalable integrative proteogenomic strategy geared toward advancing our understanding of splicing-based disorders. With such a strategy, splicing-based biomarker candidates emerge as an attractive and accessible option, as they can be efficiently asserted on the mRNA and protein level in coordinated fashion.

Excessive rest time during active phase is reliably detected in a mouse model of myotonic dystrophy type 1 using home cage monitoring

Myotonic dystrophy type 1 (DM1) is a dominantly inherited neuromuscular disease caused by the abnormal expansion of CTG-repeats in the 3'-untranslated region of the Dystrophia Myotonica Protein Kinase (DMPK) gene, characterized by multisystemic symptoms including muscle weakness, myotonia, cardio-respiratory problems, hypersomnia, cognitive dysfunction and behavioral abnormalities. Sleep-related disturbances are among the most reported symptoms that negatively affect the quality of life of patients and that are present in early and adult-onset forms of the disease. DMSXL mice carry a mutated human DMPK transgene containing >1,000 CTGrepeats, modeling an early onset, severe form of DM1. They exhibit a pathologic neuromuscular phenotype and also synaptic dysfunction resulting in neurological and behavioral deficits similar to those observed in patients. Additionally, they are underweight with a very high mortality within the first month after birth presenting several welfare issues. To specifically explore sleep/rest-related behaviors of this frail DM1 mouse model we used an automated home cage-based system that allows 24/7 monitoring of their activity non-invasively. We tested male and female DMSXL mice and their wild-type (WT) littermates in Digital Ventilated Cages (DVCR) assessing activity and rest parameters on day and night for 5 weeks. We demonstrated that DMSXL mice show reduced activity and regularity disruption index (RDI), higher percentage of zero activity per each hour and longer periods of rest during the active phase compared to WT. This novel rest-related phenotype in DMSXL mice, assessed unobtrusively, could be valuable to further explore mechanisms and potential therapeutic interventions to alleviate the very common symptom of excessive daytime sleepiness in DM1 patients.

RNA Foci Formation in a Retinal Glial Model for Spinocerebellar Ataxia Type 7

Spinocerebellar ataxia type 7 (SCA7) is a neurodegenerative disorder characterized by cerebellar ataxia and retinopathy. SCA7 is caused by a CAG expansion in the ATXN7 gene, which results in an extended polyglutamine (polyQ) tract in the encoded protein, the ataxin-7. PolyQ expanded ataxin-7 elicits neurodegeneration in cerebellar Purkinje cells, however, its impact on the SCA7-associated retinopathy remains to be addressed. Since Müller glial cells play an essential role in retinal homeostasis, we generate an inducible model for SCA7, based on the glial Müller MIO-M1 cell line. The SCA7 pathogenesis has been explained by a protein gain-of-function mechanism, however, the contribution of the mutant RNA to the disease cannot be excluded. In this direction, we found nuclear and cytoplasmic foci containing mutant RNA accompanied by subtle alternative splicing defects in MIO-M1 cells. RNA foci were also observed in cells from different lineages, including peripheral mononuclear leukocytes derived from SCA7 patient, suggesting that this molecular mark could be used as a blood biomarker for SCA7. Collectively, our data showed that our glial cell model exhibits the molecular features of SCA7, which makes it a suitable model to study the RNA toxicity mechanisms, as well as to explore therapeutic strategies aiming to alleviate glial dysfunction.

Recent Progress and Challenges in the Development of Antisense Therapies for Myotonic Dystrophy Type 1

Myotonic dystrophy type 1 (DM1) is a dominant genetic disease in which the expansion of long CTG trinucleotides in the 3' UTR of the myotonic dystrophy protein kinase (DMPK) gene results in toxic RNA gain-of-function and gene mis-splicing affecting mainly the muscles, the heart, and the brain. The CUG-expanded transcripts are a suitable target for the development of antisense oligonucleotide (ASO) therapies. Various chemical modifications of the sugar-phosphate backbone have been reported to significantly enhance the affinity of ASOs for RNA and their resistance to nucleases, making it possible to reverse DM1-like symptoms following systemic administration in different transgenic mouse models. However, specific tissue delivery remains to be improved to achieve significant clinical outcomes in humans. Several strategies, including ASO conjugation to cell-penetrating peptides, fatty acids, or monoclonal antibodies, have recently been shown to improve potency in muscle and cardiac tissues in mice. Moreover, intrathecal administration of ASOs may be an advantageous complementary administration route to bypass the blood-brain barrier and correct defects of the central nervous system in DM1. This review describes the evolution of the chemical design of antisense oligonucleotides targeting CUG-expanded mRNAs and how recent advances in the field may be game-changing by forwarding laboratory findings into clinical research and treatments for DM1 and other microsatellite diseases.

Altered Behavioral Responses Show GABA Sensitivity in Muscleblind-Like 2-Deficient Mice: Implications for CNS Symptoms in Myotonic Dystrophy

Considerable evidence from mouse models and human postmortem brain suggests loss of Muscleblind-like protein 2 (MBNL2) function in brain is a major driver of CNS symptoms in Myotonic dystrophy type 1 (DM1). Increased hypersomnia, fatigue, and surgical complications associated with general anesthesia suggest possible sensitivity to GABAergic inhibition in DM1. To test the hypothesis that MBNL2 depletion leads to behavioral sensitivity to GABA_A receptor (GABA_A-R) modulation, Mbnl2 knock-out (KO) and wild-type (WT) littermates were treated with the anesthetic sevoflurane, the benzodiazepine diazepam, the imidazopyridine zolpidem, and the benzodiazepine rescue agent, flumazenil (Ro 15-1788), and assessed for various behavioral metrics. Mbnl2 KO mice exhibited delayed recovery following sevoflurane, delayed emergence and recovery from zolpidem, and enhanced sleep time at baseline that was modulated by flumazenil. A significantly higher proportion of Mbnl2 KO mice also loss their righting reflex [loss of righting reflex (LORR)] from a standard diazepam dose. We further examined whether MBNL2 depletion affects total GABA_A-R mRNA subunit levels and validated RNA-sequencing data of mis-spliced Gabrg2, whose isoform ratios are known to regulate GABA sensitivity and associated behaviors. While no other GABA_A-R subunit mRNA levels tested were altered in Mbnl2 KO mouse prefrontal cortex, Gabrg2S/L mRNA ratio levels were significantly altered. Taken together, our findings indicate that loss of MBNL2 function affects GABAergic function in a mouse model of myotonic dystrophy (DM1).

Myotonic dystrophy RNA toxicity alters morphology, adhesion and migration of mouse and human astrocytes

Brain dysfunction in myotonic dystrophy type 1 (DM1), the prototype of toxic RNA disorders, has been mainly attributed to neuronal RNA misprocessing, while little attention has been given to non-neuronal brain cells. Here, using a transgenic mouse model of DM1 that expresses mutant RNA in various brain cell types (neurons, astroglia, and oligodendroglia), we demonstrate that astrocytes exhibit impaired ramification and polarization in vivo and defects in adhesion, spreading, and migration. RNA-dependent toxicity and phenotypes are also found in human transfected glial cells. In line with the cell phenotypes, molecular analyses reveal extensive expression and accumulation of toxic RNA in astrocytes, which result in RNA spliceopathy that is more severe than in neurons. Astrocyte missplicing affects primarily transcripts that regulate cell adhesion, cytoskeleton, and morphogenesis, and it is confirmed in human brain tissue. Our findings demonstrate that DM1 impacts astrocyte cell biology, possibly compromising their support and regulation of synaptic function.

Myotonic dystrophy type 1 (DM1) is characterized by debilitating neurological symptoms. Dinca et al. demonstrate the pronounced impact of DM1 on the morphology and RNA metabolism of astrocytes. Their findings suggest astroglial pathology in DM1 brain dysfunction.

MECP2-related pathways are dysregulated in a cortical organoid model of myotonic dystrophy

Myotonic dystrophy type 1 (DM1) is a multisystem, autosomal-dominant inherited disorder caused by CTG microsatellite repeat expansions (MREs) in the 3' untranslated region of the dystrophia myotonica-protein kinase (DMPK) gene. Despite its prominence as the most common adult-onset muscular dystrophy, patients with congenital to juvenile-onset forms of DM1 can present with debilitating neurocognitive symptoms along the autism spectrum, characteristic of possible in utero cortical defects. However, the molecular mechanism by which CTG MREs lead to these developmental central nervous system (CNS) manifestations is unknown. Here, we showed that CUG foci found early in the maturation of three-dimensional (3D) cortical organoids from DM1 patient-derived induced pluripotent stem cells (iPSCs) cause hyperphosphorylation of CUGBP Elav-like family member 2 (CELF2) protein. Integrative single-cell RNA sequencing and enhanced cross-linking and immunoprecipitation (eCLIP) analysis revealed that reduced CELF2 protein-RNA substrate interactions results in misregulation of genes critical for excitatory synaptic signaling in glutamatergic neurons, including key components of the methyl-CpG binding protein 2 (MECP2) pathway. Comparisons to MECP2(y/-) cortical organoids revealed convergent molecular and cellular defects such as glutamate toxicity and neuronal loss. Our findings provide evidence suggesting that early-onset DM1 might involve neurodevelopmental disorder-associated pathways and identify N-methyl-d-aspartic acid (NMDA) antagonists as potential treatment avenues for neuronal defects in DM1.

Elevated serum Neurofilament Light chain (NfL) as a potential biomarker of neurological involvement in Myotonic Dystrophy type 1 (DM1)

Background

Cognitive and behavioural symptoms due to involvement of the central nervous system (CNS) are among the main clinical manifestations of Myotonic Dystrophy type 1 (DM1). Such symptoms affect patients’ quality of life and disease awareness, impacting on disease prognosis by reducing compliance to medical treatments. Therefore, CNS is a key therapeutic target in DM1. Deeper knowledge of DM1 pathogenesis is prompting development of potential disease-modifying therapies: as DM1 is a rare, multisystem and slowly progressive disease, there is need of sensitive, tissue-specific prognostic and monitoring biomarkers in view of forthcoming clinical trials. Circulating Neurofilament light chain (NfL) levels have been recognized as a sensitive prognostic and monitoring biomarker of neuroaxonal damage in various CNS disorders.

Methods

We performed a cross-sectional study in a cohort of 40 adult DM1 patients, testing if serum NfL might be a potential biomarker of CNS involvement also in DM1. Moreover, we collected cognitive data, brain MRI, and other DM1-related diagnostic findings for correlation studies.

Results

Mean serum NfL levels resulted significantly higher in DM1 (25.32 ± 28.12 pg/ml) vs 22 age-matched healthy controls (6.235 ± 0.4809 pg/ml). Their levels positively correlated with age, and with one cognitive test (Rey’s Auditory Verbal learning task). No correlations were found either with other cognitive data, or diagnostic parameters in the DM1 cohort.

Conclusions

Our findings support serum NfL as a potential biomarker of CNS damage in DM1, which deserves further evaluation on larger cross-sectional and longitudinal studies to test its ability in assessing brain disease severity and/or progression.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00415-022-11165-0.

Molecular Therapies for Myotonic Dystrophy Type 1: From Small Drugs to Gene Editing

Myotonic dystrophy type 1 (DM1) is the most common muscular dystrophy affecting many different body tissues, predominantly skeletal and cardiac muscles and the central nervous system. The expansion of CTG repeats in the DM1 protein-kinase (DMPK) gene is the genetic cause of the disease. The pathogenetic mechanisms are mainly mediated by the production of a toxic expanded CUG transcript from the DMPK gene. With the availability of new knowledge, disease models, and technical tools, much progress has been made in the discovery of altered pathways and in the potential of therapeutic intervention, making the path to the clinic a closer reality. In this review, we describe and discuss the molecular therapeutic strategies for DM1, which are designed to directly target the CTG genomic tract, the expanded CUG transcript or downstream signaling molecules.

Time-controlled and muscle-specific CRISPR/Cas9-mediated deletion of CTG-repeat expansion in the DMPK gene

CRISPR/Cas9-mediated therapeutic gene editing is a promising technology for durable treatment of incurable monogenic diseases such as myotonic dystrophies. Gene-editing approaches have been recently applied to in vitro and in vivo models of myotonic dystrophy type 1 (DM1) to delete the pathogenic CTG-repeat expansion located in the 3′ untranslated region of the DMPK gene. In DM1-patient-derived cells removal of the expanded repeats induced beneficial effects on major hallmarks of the disease with reduction in DMPK transcript-containing ribonuclear foci and reversal of aberrant splicing patterns. Here, we set out to excise the triplet expansion in a time-restricted and cell-specific fashion to minimize the potential occurrence of unintended events in off-target genomic loci and select for the target cell type. To this aim, we employed either a ubiquitous promoter-driven or a muscle-specific promoter-driven Cas9 nuclease and tetracycline repressor-based guide RNAs. A dual-vector approach was used to deliver the CRISPR/Cas9 components into DM1 patient-derived cells and in skeletal muscle of a DM1 mouse model. In this way, we obtained efficient and inducible gene editing both in proliferating cells and differentiated post-mitotic myocytes in vitro as well as in skeletal muscle tissue in vivo.

A comprehensive atlas of fetal splicing patterns in the brain of adult myotonic dystrophy type 1 patients

In patients with myotonic dystrophy type 1 (DM1), dysregulation of RNA-binding proteins like MBNL and CELF1 leads to alternative splicing of exons and is thought to induce a return to fetal splicing patterns in adult tissues, including the central nervous system (CNS). To comprehensively evaluate this, we created an atlas of developmentally regulated splicing patterns in the frontal cortex of healthy individuals and DM1 patients, by combining RNA-seq data from BrainSpan, GTEx and DM1 patients. Thirty-four splice events displayed an inclusion pattern in DM1 patients that is typical for the fetal situation in healthy individuals. The regulation of DM1-relevant splicing patterns could partly be explained by changes in mRNA expression of the splice regulators MBNL1, MBNL2 and CELF1. On the contrary, interindividual differences in splicing patterns between healthy adults could not be explained by differential expression of these splice regulators. Our findings lend transcriptome-wide evidence to the previously noted shift to fetal splicing patterns in the adult DM1 brain as a consequence of an imbalance in antagonistic MBNL and CELF1 activities. Our atlas serves as a solid foundation for further study and understanding of the cognitive phenotype in patients.

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.

DM1 Transgenic Mice Exhibit Abnormal Neurotransmitter Homeostasis and Synaptic Plasticity in Association with RNA Foci and Mis-Splicing in the Hippocampus

Myotonic dystrophy type 1 (DM1) is a severe neuromuscular disease mediated by a toxic gain of function of mutant RNAs. The neuropsychological manifestations affect multiple domains of cognition and behavior, but their etiology remains elusive. Transgenic DMSXL mice carry the DM1 mutation, show behavioral abnormalities, and express low levels of GLT1, a critical regulator of glutamate concentration in the synaptic cleft. However, the impact of glutamate homeostasis on neurotransmission in DM1 remains unknown. We confirmed reduced glutamate uptake in the DMSXL hippocampus. Patch clamp recordings in hippocampal slices revealed increased amplitude of tonic glutamate currents in DMSXL CA1 pyramidal neurons and DG granule cells, likely mediated by higher levels of ambient glutamate. Unexpectedly, extracellular GABA levels and tonic current were also elevated in DMSXL mice. Finally, we found evidence of synaptic dysfunction in DMSXL mice, suggestive of abnormal short-term plasticity, illustrated by an altered LTP time course in DG and in CA1. Synaptic dysfunction was accompanied by RNA foci accumulation in localized areas of the hippocampus and by the mis-splicing of candidate genes with relevant functions in neurotransmission. Molecular and functional changes triggered by toxic RNA may induce synaptic abnormalities in restricted brain areas that favor neuronal dysfunction.

Defects in Mouse Cortical Glutamate Uptake Can Be Unveiled In Vivo by a Two-in-One Quantitative Microdialysis

Extracellular glutamate levels are maintained low by efficient transporters, whose dysfunction can cause neuronal hyperexcitability, excitotoxicity, and neurological disease. While many methods estimate glutamate uptake in vitro/ex vivo, a limited number of techniques address glutamate transport in vivo. Here, we used in vivo microdialysis in a two-in-one approach combining reverse dialysis of isotopic glutamate to measure uptake ability and zero-flow (ZF) methods to quantify extracellular glutamate levels. The complementarity of both techniques is discussed on methodological and anatomical basis. We used a transgenic mouse model of human disease, expressing low levels of the EAAT-2/GLT1 glutamate transporter, to validate our approach in a relevant animal model. As expected, isotopic analysis revealed an overall decrease in glutamate uptake, while the ZF method unveiled higher extracellular glutamate levels in these mice. We propose a sensitive and expedite two-in-one microdialysis approach that is sufficiently robust to reveal significant differences in neurotransmitter uptake and extracellular levels through the analysis of a relatively low number of animals.

Antisense oligonucleotides as a potential treatment for brain deficits observed in myotonic dystrophy type 1

Myotonic dystrophy, or dystrophia myotonica type 1 (DM1), is a multi-systemic disorder and is the most common adult form of muscular dystrophy. It affects not only muscles but also many organs, including the brain. Cerebral impairments include cognitive deficits, daytime sleepiness, and loss of visuospatial and memory functions. The expression of mutated transcripts with CUG repeats results in a gain of toxic mRNA function. The antisense oligonucleotide (ASO) strategy to treat DM1 brain deficits is limited by the fact that ASOs do not cross the blood-brain barrier after systemic administration, indicating that other methods of delivery should be considered. ASO technology has emerged as a powerful tool for developing potential new therapies for a wide variety of human diseases, and its potential has been proven in a recent clinical trial. Targeting DMPK mRNA in neural cells derived from human induced pluripotent stem cells obtained from a DM1 patient with the IONIS 486178 ASO abolished CUG-expanded foci, enabled nuclear redistribution of MBNL1/2, and corrected aberrant splicing. Intracerebroventricular injection of the IONIS 486178 ASO in DMSXL mice decreased the levels of mutant DMPK mRNAs by up to 70% throughout different brain regions. It also reversed behavioral abnormalities following neonatal administration. The present study indicated that the IONIS 486178 ASO targets mutant DMPK mRNAs in the brain and strongly supports the feasibility of a therapy for DM1 patients based on the intrathecal injection of an ASO.

Brain Pathogenesis and Potential Therapeutic Strategies in Myotonic Dystrophy Type 1

Myotonic dystrophy type 1 (DM1) is the most common muscular dystrophy that affects multiple systems including the muscle and heart. The mutant CTG expansion at the 3′-UTR of the DMPK gene causes the expression of toxic RNA that aggregate as nuclear foci. The foci then interfere with RNA-binding proteins, affecting hundreds of mis-spliced effector genes, leading to aberrant alternative splicing and loss of effector gene product functions, ultimately resulting in systemic disorders. In recent years, increasing clinical, imaging, and pathological evidence have indicated that DM1, though to a lesser extent, could also be recognized as true brain diseases, with more and more researchers dedicating to develop novel therapeutic tools dealing with it. In this review, we summarize the current advances in the pathogenesis and pathology of central nervous system (CNS) deficits in DM1, intervention measures currently being investigated are also highlighted, aiming to promote novel and cutting-edge therapeutic investigations.

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.

Synaptopathy Mechanisms in ALS Caused by C9orf72 Repeat Expansion

Amyotrophic Lateral Sclerosis (ALS) is a complex neurodegenerative disease caused by degeneration of motor neurons (MNs). ALS pathogenic features include accumulation of misfolded proteins, glutamate excitotoxicity, mitochondrial dysfunction at distal axon terminals, and neuronal cytoskeleton changes. Synergies between loss of C9orf72 functions and gain of function by toxic effects of repeat expansions also contribute to C9orf72-mediated pathogenesis. However, the impact of haploinsufficiency of C9orf72 on neurons and in synaptic functions requires further examination. As the motor neurons degenerate, the disease symptoms will lead to neurotransmission deficiencies in the brain, spinal cord, and neuromuscular junction. Altered neuronal excitability, synaptic morphological changes, and C9orf72 protein and DPR localization at the synapses, suggest a potential involvement of C9orf72 at synapses. In this review article, we provide a conceptual framework for assessing the putative involvement of C9orf72 as a synaptopathy, and we explore the underlying and common disease mechanisms with other neurodegenerative diseases. Finally, we reflect on the major challenges of understanding C9orf72-ALS as a synaptopathy focusing on integrating mitochondrial and neuronal cytoskeleton degeneration as biomarkers and potential targets to treat ALS neurodegeneration.

Integrative Cell Type-Specific Multi-Omics Approaches Reveal Impaired Programs of Glial Cell Differentiation in Mouse Culture Models of DM1

Myotonic dystrophy type 1 (DM1) is a neuromuscular disorder caused by a non-coding CTG repeat expansion in the DMPK gene. This mutation generates a toxic CUG RNA that interferes with the RNA processing of target genes in multiple tissues. Despite debilitating neurological impairment, the pathophysiological cascade of molecular and cellular events in the central nervous system (CNS) has been less extensively characterized than the molecular pathogenesis of muscle/cardiac dysfunction. Particularly, the contribution of different cell types to DM1 brain disease is not clearly understood. We first used transcriptomics to compare the impact of expanded CUG RNA on the transcriptome of primary neurons, astrocytes and oligodendrocytes derived from DMSXL mice, a transgenic model of DM1. RNA sequencing revealed more frequent expression and splicing changes in glia than neuronal cells. In particular, primary DMSXL oligodendrocytes showed the highest number of transcripts differentially expressed, while DMSXL astrocytes displayed the most severe splicing dysregulation. Interestingly, the expression and splicing defects of DMSXL glia recreated molecular signatures suggestive of impaired cell differentiation: while DMSXL oligodendrocytes failed to upregulate a subset of genes that are naturally activated during the oligodendroglia differentiation, a significant proportion of missplicing events in DMSXL oligodendrocytes and astrocytes increased the expression of RNA isoforms typical of precursor cell stages. Together these data suggest that expanded CUG RNA in glial cells affects preferentially differentiation-regulated molecular events. This hypothesis was corroborated by gene ontology (GO) analyses, which revealed an enrichment for biological processes and cellular components with critical roles during cell differentiation. Finally, we combined exon ontology with phosphoproteomics and cell imaging to explore the functional impact of CUG-associated spliceopathy on downstream protein metabolism. Changes in phosphorylation, protein isoform expression and intracellular localization in DMSXL astrocytes demonstrate the far-reaching impact of the DM1 repeat expansion on cell metabolism. Our multi-omics approaches provide insight into the mechanisms of CUG RNA toxicity in the CNS with cell type resolution, and support the priority for future research on non-neuronal mechanisms and proteomic changes in DM1 brain disease.

Transcriptional signatures of synaptic vesicle genes define myotonic dystrophy type I neurodegeneration

Aim

To delineate the neurogenetic profiles of brain degeneration patterns in myotonic dystrophy type I (DM1).

Methods

In two cohorts of DM1 patients, brain maps of volume loss (VL) and neuropsychological deficits (NDs) were intersected to large‐scale transcriptome maps provided by the Allen Human Brain Atlas (AHBA). For validation, neuropathological and RNA analyses were performed in a small series of DM1 brain samples.

Results

Twofold: (1) From a list of preselected hypothesis‐driven genes, confirmatory analyses found that three genes play a major role in brain degeneration: dystrophin (DMD), alpha‐synuclein (SNCA) and the microtubule‐associated protein tau (MAPT). Neuropathological analyses confirmed a highly heterogeneous Tau‐pathology in DM1, different to the one in Alzheimer's disease. (2) Exploratory analyses revealed gene clusters enriched for key biological processes in the central nervous system, such as synaptic vesicle recycling, localization, endocytosis and exocytosis, and the serotonin and dopamine neurotransmitter pathways. RNA analyses confirmed synaptic vesicle dysfunction.

Conclusions

The combination of large‐scale transcriptome interactions with brain imaging and cognitive function sheds light on the neurobiological mechanisms of brain degeneration in DM1 that might help define future therapeutic strategies and research into this condition.

Human brain pathology in myotonic dystrophy type 1: A systematic review

Brain involvement in myotonic dystrophy type 1 (DM1) is characterized by heterogeneous cognitive, behavioral, and affective symptoms and imaging alterations indicative of widespread grey and white matter involvement. The aim of the present study was to systematically review the literature on brain pathology in DM1. We conducted a structured search in EMBASE (index period 1974–2017) and MEDLINE (index period 1887–2017) on December 11, 2017, using free text and index search terms related to myotonic dystrophy type 1 and brain structures or regions. Eligible studies were full‐text studies reporting on microscopic brain pathology of DM1 patients without potentially interfering comorbidity. We discussed the findings based on the anatomical region and the nature of the anomaly. Neuropathological findings in DM1 can be classified as follows: (1) protein and nucleotide deposits; (2) changes in neurons and glial cells; and (3) white matter alterations. Most findings are unspecific to DM1 and may occur with physiological aging, albeit to a lesser degree. There are similarities and contrasts with Alzheimer's disease; both show the appearance of neurofibrillary tangles in the limbic system without plaque occurrence. Likewise, there is myelin loss and gliosis, and there are dilated perivascular spaces in the white matter resemblant of cerebral small vessel disease. However, we did not find evidence of lacunar infarction or microbleeding. The various neuropathological findings in DM1 are reflective of the heterogeneous clinical and neuroimaging features of the disease. The strength of conclusions from this study's findings is bounded by limited numbers of participants in studies, methodological constraints, and lack of assessed associations between histopathology and clinical or neuroimaging findings.

Transcriptome Analysis Reveals Altered Inflammatory Pathway in an Inducible Glial Cell Model of Myotonic Dystrophy Type 1

Myotonic dystrophy type 1 (DM1), the most frequent inherited muscular dystrophy in adults, is caused by the CTG repeat expansion in the 3′UTR of the DMPK gene. Mutant DMPK RNA accumulates in nuclear foci altering diverse cellular functions including alternative splicing regulation. DM1 is a multisystemic condition, with debilitating central nervous system alterations. Although a defective neuroglia communication has been described as a contributor of the brain pathology in DM1, the specific cellular and molecular events potentially affected in glia cells have not been totally recognized. Thus, to study the effects of DM1 mutation on glial physiology, in this work, we have established an inducible DM1 model derived from the MIO-M1 cell line expressing 648 CUG repeats. This new model recreated the molecular hallmarks of DM1 elicited by a toxic RNA gain-of-function mechanism: accumulation of RNA foci colocalized with MBNL proteins and dysregulation of alternative splicing. By applying a microarray whole-transcriptome approach, we identified several gene changes associated with DM1 mutation in MIO-M1 cells, including the immune mediators CXCL10, CCL5, CXCL8, TNFAIP3, and TNFRSF9, as well as the microRNAs miR-222, miR-448, among others, as potential regulators. A gene ontology enrichment analyses revealed that inflammation and immune response emerged as major cellular deregulated processes in the MIO-M1 DM1 cells. Our findings indicate the involvement of an altered immune response in glia cells, opening new windows for the study of glia as potential contributor of the CNS symptoms in DM1.

Transcriptome alterations in myotonic dystrophy frontal cortex

Myotonic dystrophy (DM) is caused by expanded CTG/CCTG repeats, causing symptoms in skeletal muscle, heart, and central nervous system (CNS). CNS issues are debilitating and include hypersomnolence, executive dysfunction, white matter atrophy, and neurofibrillary tangles. Here, we generate RNA-seq transcriptomes from DM and unaffected frontal cortex and identify 130 high-confidence splicing changes, most occurring only in cortex, not skeletal muscle or heart. Mis-spliced exons occur in neurotransmitter receptors, ion channels, and synaptic scaffolds, and GRIP1 mis-splicing modulates kinesin association. Optical mapping of expanded CTG repeats reveals extreme mosaicism, with some alleles showing >1,000 CTGs. Mis-splicing severity correlates with CTG repeat length across individuals. Upregulated genes tend to be microglial and endothelial, suggesting neuroinflammation, and downregulated genes tend to be neuronal. Many gene expression changes strongly correlate with mis-splicing, suggesting candidate biomarkers of disease. These findings provide a framework for mechanistic and therapeutic studies of the DM CNS.

Otero et al. characterize DM1 frontal cortex transcriptomes and observe extreme CTG repeat lengths even in individuals showing modest mis-splicing. Mis-splicing is enriched in ion channels, neurotransmitter receptors, and synaptic scaffolds, in some cases potentially altering synaptic trafficking. Inferred expression signatures from microglia and endothelial cells suggest neuroinflammation.

Central Nervous System Involvement as Outcome Measure for Clinical Trials Efficacy in Myotonic Dystrophy Type 1

Increasing evidences indicate that in Myotonic Dystrophy type 1 (DM1 or Steinert disease), an autosomal dominant multisystem disorder caused by a (CTG)n expansion in DMPK gene on chromosome 19q13. 3, is the most common form of inherited muscular dystrophy in adult patients with a global prevalence of 1/8000, and involvement of the central nervous system can be included within the core clinical manifestations of the disease. Variable in its severity and progression rate over time, likely due to the underlying causative molecular mechanisms; this component of the clinical picture presents with high heterogeneity involving cognitive and behavioral alterations, but also sensory-motor neural integration, and in any case, significantly contributing to the disease burden projected to either specific functional neuropsychological domains or quality of life as a whole. Principle manifestations include alterations of the frontal lobe function, which is more prominent in patients with an early onset, such as in congenital and childhood onset forms, here associated with severe intellectual disabilities, speech and language delay and reduced IQ-values, while in adult onset DM1 cognitive and neuropsychological findings are usually not so severe. Different methods to assess central nervous system involvement in DM1 have then recently been developed, these ranging from more classical psychometric and cognitive functional instruments to sophisticated psycophysic, neurophysiologic and especially computerized neuroimaging techniques, in order to better characterize this disease component, at the same time underlining the opportunity to consider it a suitable marker on which measuring putative effectiveness of therapeutic interventions. This is the reason why, as outlined in the conclusive section of this review, the Authors are lead to wonder, perhaps in a provocative and even paradoxical way to arise the question, whether or not the myologist, by now the popular figure in charge to care of a patient with the DM1, needs to remain himself a neurologist to better appreciate, evaluate and speculate on this important aspect of Steinert disease.

Protective effects of mirtazapine in mice lacking the Mbnl2 gene in forebrain glutamatergic neurons: Relevance for myotonic dystrophy 1

Myotonic dystrophy type 1 (DM1) is a multisystemic disorder characterized by muscle weakness and wasting and by important central nervous system-related symptoms including impairments in executive functions, spatial abilities and increased anxiety and depression. The Mbnl2 gene has been implicated in several phenotypes consistent with DM1 neuropathology. In this study, we developed a tissue-specific knockout mouse model lacking the Mbnl2 gene in forebrain glutamatergic neurons to examine its specific contribution to the neurobiological perturbations related to DM1. We found that these mice exhibit long-term cognitive deficits and a depressive-like state associated with neuronal loss, increased microglia and decreased neurogenesis, specifically in the dentate gyrus (DG). Chronic treatment with the atypical antidepressant mirtazapine (3 and 10 mg/kg) for 21 days rescued these behavioral alterations, reduced inflammatory microglial overexpression, and reversed neuronal loss in the DG. We also show that mirtazapine re-established 5-HT_1A and histaminergic H₁ receptor gene expression in the hippocampus. Finally, metabolomics studies indicated that mirtazapine increased serotonin, noradrenaline, gamma-aminobutyric acid and adenosine production. These data suggest that loss of Mbnl2 gene in the glutamatergic neurons of hippocampus and cortex may underlie the most relevant DM1 neurobiological and behavioral features, and provide evidence that mirtazapine could be a novel potential candidate to alleviate these debilitating symptoms in DM1 patients.

Abnormal Cortical Thickness Is Associated With Deficits in Social Cognition in Patients With Myotonic Dystrophy Type 1

Aim: To investigate the cortical thickness in myotonic dystrophy type 1 (DM1) and its potential association with patients' genetic triplet expansion and social cognition deficits.

Methods: Thirty patients with DM1 underwent the Social Cognition Battery Test and magnetic resonance imaging (MRI) scanning at 3 T. Twenty-five healthy subjects (HSs) were enrolled in the study to serve as a control group for structural MRI data. To assess changes in cortical thickness in DM1 patients, they were compared to HSs using a t-test model. Correlations were used to assess potential associations between genetic and clinical characteristics and social cognition performances in the patient group. Additionally, multiple regression models were used to explore associations between cortical thickness, CTG triplet expansion size, and scores obtained by DM1 patients on the Social Cognition Battery.

Results: DM1 patients showed low performances in several subtests of the Social Cognition Battery. Specifically, they obtained pathological scores at Emotion Attribution Test (i.e., Sadness, Embarrassment, Happiness, and Anger) and at the Social Situations Test (i.e., recognition of normal situation, recognition of aberrant behavior). Significant negative correlations were found between CTG triplet expansion size and Embarrassment, and Severity of Aberrant Behavior. Similarly, a negative correlation was found between patients' MIRS scores and Sadness. DM1 patients compared to HSs showed reduced thickness in the right premotor cortex, angular gyrus, precuneus, and inferior parietal lobule. Significant associations were found between patients' CTG triplet expansion size and thickness in left postcentral gyrus and in the left primary somatosensory cortex, in the posterior cingulate cortex bilaterally, and in the right lingual gyrus. Finally, significant associations were found between cortical thickness and sadness in the superior temporal gyrus, the right precentral gyrus, the right angular gyrus, and the left medial frontal gyrus bilaterally. DM1 patients showed a negative correlation between cortical thickness in the bilateral precuneus and in the left lateral occipital cortex and performance at the Social Situations Test. Finally, DM1 patients showed a negative correlation between cortical thickness in the left precuneus and in the superior frontal gyrus and scores at the Moral Distinction Test.

Discussion: The present study shows both cortical thickness changes in DM1 patients compared to controls and significant associations between cortical thickness and patients' social cognition performances. These data confirm the presence of widespread brain damages associated with cognitive impairment in DM1 patients.

Brain Structural Features of Myotonic Dystrophy Type 1 and their Relationship with CTG Repeats

Background

Few adequately-powered studies have systematically evaluated brain morphology in adult-onset myotonic dystrophy type 1 (DM1).

Objective

The goal of the present study was to determine structural brain differences between individuals with and without adult-onset DM1 in a multi-site, case-controlled cohort. We also explored correlations between brain structure and CTG repeat length.

Methods

Neuroimaging data was acquired in 58 unaffected individuals (29 women) and 79 individuals with DM1 (50 women). CTG repeat length, expressed as estimated progenitor allele length (ePAL), was determined by small pool PCR. Statistical models were adjusted for age, sex, site, and intracranial volume (ICV).

Results

ICV was reduced in DM1 subjects compared with controls. Accounting for the difference in ICV, the DM1 group exhibited smaller volume in frontal grey and white matter, parietal grey matter as well as smaller volume of the corpus callosum, thalamus, putamen, and accumbens. In contrast, volumes of the hippocampus and amygdala were significantly larger in DM1. Greater ePAL was associated with lower volumes of the putamen, occipital grey matter, and thalamus. A positive ePAL association was observed for amygdala volume and cerebellar white matter.

Conclusions

Smaller ICV may be a marker of aberrant neurodevelopment in adult-onset DM1. Volumetric analysis revealed morphological differences, some associated with CTG repeat length, in structures with plausible links to key DM1 symptoms including cognitive deficits and excessive daytime somnolence. These data offer further insights into the basis of CNS disease in DM1, and highlight avenues for further work to identify therapeutic targets and imaging biomarkers.

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.

Deprivation of Muscleblind-Like Proteins Causes Deficits in Cortical Neuron Distribution and Morphological Changes in Dendritic Spines and Postsynaptic Densities

Myotonic dystrophy (Dystrophia Myotonica; DM) is the most common adult-onset muscular dystrophy and its brain symptoms seriously affect patients’ quality of life. It is caused by extended (CTG)_n expansions at 3′-UTR of DMPK gene (DM type 1, DM1) or (CCTG)_n repeats in the intron 1 of CNBP gene (DM type 2, DM2) and the sequestration of Muscleblind-like (MBNL) family proteins by transcribed (CUG)_n RNA hairpin is the main pathogenic mechanism for DM. The MBNL proteins are splicing factors regulating posttranscriptional RNA during development. Previously, Mbnl knockout (KO) mouse lines showed molecular and phenotypic evidence that recapitulate DM brains, however, detailed morphological study has not yet been accomplished. In our studies, control (Mbnl1^+/+; Mbnl2^cond/cond; Nestin-Cre^−/−), Mbnl2 conditional KO (2KO, Mbnl1^+/+; Mbnl2^cond/cond; Nestin-Cre^+/−) and Mbnl1/2 double KO (DKO, Mbnl1^ΔE3/ΔE3; Mbnl2^cond/cond; Nestin-Cre^+/−) mice were generated by crossing three individual lines. Immunohistochemistry for evaluating density and distribution of cortical neurons; Golgi staining for depicting the dendrites/dendritic spines; and electron microscopy for analyzing postsynaptic ultrastructure were performed. We found distributional defects in cortical neurons, reduction in dendritic complexity, immature dendritic spines and alterations of postsynaptic densities (PSDs) in the mutants. In conclusion, loss of function of Mbnl1/2 caused fundamental defects affecting neuronal distribution, dendritic morphology and postsynaptic architectures that are reminiscent of predominantly immature and fetal phenotypes in DM patients.

Myotonic Dystrophy: an RNA Toxic Gain of Function Tauopathy?

Myotonic dystrophies (DM) are rare inherited neuromuscular disorders linked to microsatellite unstable expansions in non-coding regions of ubiquitously expressed genes. The DMPK and ZNF9/CNBP genes which mutations are responsible for DM1 and DM2 respectively. DM are multisystemic disorders with brain affection and cognitive deficits. Brain lesions consisting of neurofibrillary tangles are often observed in DM1 and DM2 brain. Neurofibrillary tangles (NFT) made of aggregates of hyper and abnormally phosphorylated isoforms of Tau proteins are neuropathological lesions common to more than 20 neurological disorders globally referred to as Tauopathies. Although NFT are observed in DM1 and DM2 brain, the question of whether DM1 and DM2 are Tauopathies remains a matter of debate. In the present review, several pathophysiological processes including, missplicing, nucleocytoplasmic transport disruption, RAN translation which are common mechanisms implicated in neurodegenerative diseases will be described. Together, these processes including the missplicing of Tau are providing evidence that DM1 and DM2 are not solely muscular diseases but that their brain affection component share many similarities with Tauopathies and other neurodegenerative diseases. Understanding DM1 and DM2 pathophysiology is therefore valuable to more globally understand other neurodegenerative diseases such as Tauopathies but also frontotemporal lobar neurodegeneration and amyotrophic lateral sclerosis.

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.

Downregulation of the Glial GLT1 Glutamate Transporter and Purkinje Cell Dysfunction in a Mouse Model of Myotonic Dystrophy

Brain function is compromised in myotonic dystrophy type 1 (DM1), but the underlying mechanisms are not fully understood. To gain insight into the cellular and molecular pathways primarily affected, we studied a mouse model of DM1 and brains of adult patients. We found pronounced RNA toxicity in the Bergmann glia of the cerebellum, in association with abnormal Purkinje cell firing and fine motor incoordination in DM1 mice. A global proteomics approach revealed downregulation of the GLT1 glutamate transporter in DM1 mice and human patients, which we found to be the result of MBNL1 inactivation. GLT1 downregulation in DM1 astrocytes increases glutamate neurotoxicity and is detrimental to neurons. Finally, we demonstrated that the upregulation of GLT1 corrected Purkinje cell firing and motor incoordination in DM1 mice. Our findings show that glial defects are critical in DM1 brain pathophysiology and open promising therapeutic perspectives through the modulation of glutamate levels.

Neural dysfunction in myotonic dystrophy is not fully understood. Using a transgenic mouse model of the disease, Sicot et al. find electrophysiological and motor evidence for cerebellar dysfunction in association with pronounced signs of RNA toxicity in Bergmann glia. Upregulation of a defective glial-specific glutamate transporter corrects cerebellum phenotypes.

Myotonic Dystrophy and Developmental Regulation of RNA Processing

Myotonic dystrophy (DM) is a multisystemic disorder caused by microsatellite expansion mutations in two unrelated genes leading to similar, yet distinct, diseases. DM disease presentation is highly variable and distinguished by differences in age-of-onset and symptom severity. In the most severe form, DM presents with congenital onset and profound developmental defects. At the molecular level, DM pathogenesis is characterized by a toxic RNA gain-of-function mechanism that involves the transcription of noncoding microsatellite expansions. These mutant RNAs disrupt key cellular pathways, including RNA processing, localization, and translation. In DM, these toxic RNA effects are predominantly mediated through the modulation of the muscleblind-like and CUGBP and ETR-3-like factor families of RNA binding proteins (RBPs). Dysfunction of these RBPs results in widespread RNA processing defects culminating in the expression of developmentally inappropriate protein isoforms in adult tissues. The tissue that is the focus of this review, skeletal muscle, is particularly sensitive to mutant RNA-responsive perturbations, as patients display a variety of developmental, structural, and functional defects in muscle. Here, we provide a comprehensive overview of DM1 and DM2 clinical presentation and pathology as well as the underlying cellular and molecular defects associated with DM disease onset and progression. Additionally, fundamental aspects of skeletal muscle development altered in DM are highlighted together with ongoing and potential therapeutic avenues to treat this muscular dystrophy. © 2018 American Physiological Society. Compr Physiol 8:509-553, 2018.

Altered levels of the splicing factor muscleblind modifies cerebral cortical function in mouse models of myotonic dystrophy

Myotonic dystrophy (DM) is a progressive, multisystem disorder affecting skeletal muscle, heart, and central nervous system. In both DM1 and DM2, microsatellite expansions of CUG and CCUG RNA repeats, respectively, accumulate and disrupt functions of alternative splicing factors, including muscleblind (MBNL) proteins. Grey matter loss and white matter changes, including the corpus callosum, likely underlie cognitive and executive function deficits in DM patients. However, little is known how cerebral cortical circuitry changes in DM. Here, flavoprotein optical imaging was used to assess local and contralateral responses to intracortical motor cortex stimulation in DM-related mouse models. In control mice, brief train stimulation generated ipsilateral and contralateral homotopic fluorescence increases, the latter mediated by the corpus callosum. Single pulse stimulation produced an excitatory response with an inhibitory-like surround response mediated by GABA_A receptors. In a mouse model of DM2 (Mbnl2 KO), we observed prolonged and increased responsiveness to train stimulation and loss of the inhibition from single pulse stimulation. Conversely, mice overexpressing human MBNL1 (MBNL1-OE) exhibited decreased contralateral response to train stimulation and reduction of inhibitory-like surround to single pulse stimulation. Therefore, altering levels of two key DM-associated splicing factors modifies functions of local cortical circuits and contralateral responses mediated through the corpus callosum.

Reduced cytoplasmic MBNL1 is an early event in a brain-specific mouse model of myotonic dystrophy

Myotonic dystrophy type 1 (DM1) is caused by an expansion of CTG repeats in the 3' untranslated region (UTR) of the dystrophia myotonia protein kinase (DMPK) gene. Cognitive impairment associated with structural change in the brain is prevalent in DM1. How this histopathological abnormality during disease progression develops remains elusive. Nuclear accumulation of mutant DMPK mRNA containing expanded CUG RNA disrupting the cytoplasmic and nuclear activities of muscleblind-like (MBNL) protein has been implicated in DM1 neural pathogenesis. The association between MBNL dysfunction and morphological changes has not been investigated. We generated a mouse model for postnatal expression of expanded CUG RNA in the brain that recapitulates the features of the DM1 brain, including the formation of nuclear RNA and MBNL foci, learning disability, brain atrophy and misregulated alternative splicing. Characterization of the pathological abnormalities by a time-course study revealed that hippocampus-related learning and synaptic potentiation were impaired before structural changes in the brain, followed by brain atrophy associated with progressive reduction of axon and dendrite integrity. Moreover, cytoplasmic MBNL1 distribution on dendrites decreased before dendrite degeneration, whereas reduced MBNL2 expression and altered MBNL-regulated alternative splicing was evident after degeneration. These results suggest that the expression of expanded CUG RNA in the DM1 brain results in neurodegenerative processes, with reduced cytoplasmic MBNL1 as an early event response to expanded CUG RNA.

Targeting DMPK with Antisense Oligonucleotide Improves Muscle Strength in Myotonic Dystrophy Type 1 Mice

Myotonic dystrophy type 1 (DM1), a dominant hereditary muscular dystrophy, is caused by an abnormal expansion of a (CTG)_n trinucleotide repeat in the 3′ UTR of the human dystrophia myotonica protein kinase (DMPK) gene. As a consequence, mutant transcripts containing expanded CUG repeats are retained in nuclear foci and alter the function of splicing regulatory factors members of the MBNL and CELF families, resulting in alternative splicing misregulation of specific transcripts in affected DM1 tissues. In the present study, we treated DMSXL mice systemically with a 2′-4′-constrained, ethyl-modified (ISIS 486178) antisense oligonucleotide (ASO) targeted to the 3′ UTR of the DMPK gene, which led to a 70% reduction in CUG^exp RNA abundance and foci in different skeletal muscles and a 30% reduction in the heart. Furthermore, treatment with ISIS 486178 ASO improved body weight, muscle strength, and muscle histology, whereas no overt toxicity was detected. This is evidence that the reduction of CUG^exp RNA improves muscle strength in DM1, suggesting that muscle weakness in DM1 patients may be improved following elimination of toxic RNAs.

Myotonic Dystrophies: State of the Art of New Therapeutic Developments for the CNS

Myotonic dystrophies are multisystemic diseases characterized not only by muscle and heart dysfunction but also by CNS alteration. They are now recognized as brain diseases affecting newborns and children for myotonic dystrophy type 1 and adults for both myotonic dystrophy type 1 and type 2. In the past two decades, much progress has been made in understanding the mechanisms underlying the DM symptoms allowing development of new molecular therapeutic tools with the ultimate aim of curing the disease. This review describes the state of the art for the characterization of CNS related symptoms, the development of molecular strategies to target the CNS as well as the available tools for screening and testing new possible treatments.

Understanding the molecular consequences of inherited muscular dystrophies: advancements through proteomic experimentation

INTRODUCTION

Proteomic techniques offer insights into the molecular perturbations occurring in muscular-dystrophies (MD). Revisiting published datasets can highlight conserved downstream molecular alterations, which may be worth re-assessing to determine whether their experimental manipulation is capable of modulating disease severity.

AREAS COVERED

Here, we review the MD literature, highlighting conserved molecular insights warranting mechanistic investigation for therapeutic potential. We also describe a workflow currently proving effective for efficient identification of biomarkers & therapeutic targets in other neurodegenerative conditions, upon which future MD proteomic investigations could be modelled. Expert commentary: Studying disease models can be useful for identifying biomarkers and model specific degenerative cascades, but rarely offer translatable mechanistic insights into disease pathology. Conversely, direct analysis of human samples undergoing degeneration presents challenges derived from complex chronic degenerative molecular processes. This requires a carefully planed & reproducible experimental paradigm accounting for patient selection through to grouping by disease severity and ending with proteomic data filtering and processing.

Sense and Antisense DMPK RNA Foci Accumulate in DM1 Tissues during Development

Myotonic dystrophy type 1 (DM1) is caused by an unstable expanded CTG repeat located within the DMPK gene 3’UTR. The nature, severity and age at onset of DM1 symptoms are very variable in patients. Different forms of the disease are described, among which the congenital form (CDM) is the most severe. Molecular mechanisms of DM1 are well characterized for the adult form and involve accumulation of mutant DMPK RNA forming foci in the nucleus. These RNA foci sequester proteins from the MBNL family and deregulate CELF proteins. These proteins are involved in many cellular mechanisms such as alternative splicing, transcriptional, translational and post-translational regulation miRNA regulation as well as mRNA polyadenylation and localization. All these mechanisms can be impaired in DM1 because of the deregulation of CELF and MBNL functions. The mechanisms involved in CDM are not clearly described. In order to get insight into the mechanisms underlying CDM, we investigated if expanded RNA nuclear foci, one of the molecular hallmarks of DM1, could be detected in human DM1 fetal tissues, as well as in embryonic and neonatal tissues from transgenic mice carrying the human DMPK gene with an expanded CTG repeat. We observed very abundant RNA foci formed by sense DMPK RNA and, to a lesser extent, antisense DMPK RNA foci. Sense DMPK RNA foci clearly co-localized with MBNL1 and MBNL2 proteins. In addition, we studied DMPK sense and antisense expression during development in the transgenic mice. We found that DMPK sense and antisense transcripts are expressed from embryonic and fetal stages in heart, muscle and brain and are regulated during development. These results suggest that mechanisms underlying DM1 and CDM involved common players including toxic expanded RNA forming numerous nuclear foci at early stages during development.

Tractography reveals diffuse white matter abnormalities in Myotonic Dystrophy Type 1

Cerebral involvement in Myotonic Dystrophy Type 1 (DM1) is well-established but not well characterized. This study applied new Diffusion Tensor Imaging (DTI) tractography to characterize white matter disturbance in adults with DM1. Forty-five participants with DM1 and 44 control participants had MRIs on a Siemens 3T TIM Trio scanner. Data were processed with TRActs Constrained by UnderLying Anatomy (TRACULA) and 7 tracts were evaluated. Bilateral disturbances in white matter integrity were seen in all tracts in participants with DM1 compared to controls. There were no right-left hemisphere differences. The resulting DTI metrics were correlated with cognitive functioning, particularly working memory and processing speed. Motor speed was not significantly correlated with white matter microstructural integrity and, thus, was not the core explanation for the working memory and processing speed findings. White matter integrity was correlated with important clinical variables including the muscular impairment rating scale (MIRS). CTG repeat length was moderately associated with white matter status in corticospinal tract and cingulum. Sleepiness (Epworth Sleepiness Scale) was moderately associated with white matter status in the superior longitudinal fasciculus and cingulum. Overall, the results add to an emerging literature showing widespread white matter disturbances in both early-onset and adult-onset DM1. Results suggest that further investigation of white matter pathology is warranted in DM1 and that non-invasive measures such as DTI have a potentially important clinical value in characterizing the status of individuals with DM1.

Consensus brain-derived protein, extraction protocol for the study of human and murine brain proteome using both 2D-DIGE and mini 2DE immunoblotting

Two-dimensional gel electrophoresis (2DE) is a powerful tool to uncover proteome modifications potentially related to different physiological or pathological conditions. Basically, this technique is based on the separation of proteins according to their isoelectric point in a first step, and secondly according to their molecular weights by SDS polyacrylamide gel electrophoresis (SDS-PAGE). In this report an optimized sample preparation protocol for little amount of human post-mortem and mouse brain tissue is described. This method enables to perform both two-dimensional fluorescence difference gel electrophoresis (2D-DIGE) and mini 2DE immunoblotting. The combination of these approaches allows one to not only find new proteins and/or protein modifications in their expression thanks to its compatibility with mass spectrometry detection, but also a new insight into markers validation. Thus, mini-2DE coupled to western blotting permits to identify and validate post-translational modifications, proteins catabolism and provides a qualitative comparison among different conditions and/or treatments. Herein, we provide a method to study components of protein aggregates found in AD and Lewy body dementia such as the amyloid-beta peptide and the alpha-synuclein. Our method can thus be adapted for the analysis of the proteome and insoluble proteins extract from human brain tissue and mice models too. In parallel, it may provide useful information for the study of molecular and cellular pathways involved in neurodegenerative diseases as well as potential novel biomarkers and therapeutic targets.

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.

RAB GTPases and RAB-interacting proteins and their role in the control of cognitive functions

A RAS-related class of small monomeric G proteins, the RAB GTPases, is emerging as of key biological importance in compartment specific directional control of vesicles formation, transport and fusion. Thanks to human genetic observation and to the consequent dedicated biochemical work, substantial progress has been made on the understanding of the role played by RAB GTPases and their effector proteins on neuronal development and the shaping of cognitive functions. This review is highlighting these initial elements to broaden the current scope of research on developmental cognitive deficits and take the point of view of RAB GTPases control on membrane transport in neurons and astrocytes.