Targeting Myotonic Dystrophy Type 1 with Metformin

Bruno 1/CELF regulates splicing and cytoskeleton dynamics to ensure correct sarcomere assembly in Drosophila flight muscles

Muscles undergo developmental transitions in gene expression and alternative splicing that are necessary to refine sarcomere structure and contractility. CUG-BP and ETR-3-like (CELF) family RNA-binding proteins are important regulators of RNA processing during myogenesis that are misregulated in diseases such as Myotonic Dystrophy Type I (DM1). Here, we report a conserved function for Bruno 1 (Bru1, Arrest), a CELF1/2 family homolog in Drosophila, during early muscle myogenesis. Loss of Bru1 in flight muscles results in disorganization of the actin cytoskeleton leading to aberrant myofiber compaction and defects in pre-myofibril formation. Temporally restricted rescue and RNAi knockdown demonstrate that early cytoskeletal defects interfere with subsequent steps in sarcomere growth and maturation. Early defects are distinct from a later requirement for bru1 to regulate sarcomere assembly dynamics during myofiber maturation. We identify an imbalance in growth in sarcomere length and width during later stages of development as the mechanism driving abnormal radial growth, myofibril fusion, and the formation of hollow myofibrils in bru1 mutant muscle. Molecularly, we characterize a genome-wide transition from immature to mature sarcomere gene isoform expression in flight muscle development that is blocked in bru1 mutants. We further demonstrate that temporally restricted Bru1 rescue can partially alleviate hypercontraction in late pupal and adult stages, but it cannot restore myofiber function or correct structural deficits. Our results reveal the conserved nature of CELF function in regulating cytoskeletal dynamics in muscle development and demonstrate that defective RNA processing due to misexpression of CELF proteins causes wide-reaching structural defects and progressive malfunction of affected muscles that cannot be rescued by late-stage gene replacement.

Multi-level profiling unravels mitochondrial dysfunction in myotonic dystrophy type 2

Myotonic dystrophy type 2 (DM2) is an autosomal-dominant multisystemic disease with a core manifestation of proximal muscle weakness, muscle atrophy, myotonia, and myalgia. The disease-causing CCTG tetranucleotide expansion within the CNBP gene on chromosome 3 leads to an RNA-dominated spliceopathy, which is currently untreatable. Research exploring the pathophysiological mechanisms in myotonic dystrophy type 1 has resulted in new insights into disease mechanisms and identified mitochondrial dysfunction as a promising therapeutic target. It remains unclear whether similar mechanisms underlie DM2 and, if so, whether these might also serve as potential therapeutic targets. In this cross-sectional study, we studied DM2 skeletal muscle biopsy specimens on proteomic, molecular, and morphological, including ultrastructural levels in two separate patient cohorts consisting of 8 (explorative cohort) and 40 (confirmatory cohort) patients. Seven muscle biopsy specimens from four female and three male DM2 patients underwent proteomic analysis and respiratory chain enzymology. We performed bulk RNA sequencing, immunoblotting of respiratory chain complexes, mitochondrial DNA copy number determination, and long-range PCR (LR-PCR) to study mitochondrial DNA deletions on six biopsies. Proteomic and transcriptomic analyses revealed a downregulation of essential mitochondrial proteins and their respective RNA transcripts, namely of subunits of respiratory chain complexes I, III, and IV (e.g., mt-CO1, mt-ND1, mt-CYB, NDUFB6) and associated translation factors (TACO1). Light microscopy showed mitochondrial abnormalities (e.g., an age-inappropriate amount of COX-deficient fibers, subsarcolemmal accumulation) in most biopsy specimens. Electron microscopy revealed widespread ultrastructural mitochondrial abnormalities, including dysmorphic mitochondria with paracrystalline inclusions. Immunofluorescence studies with co-localization of autophagy (p62, LC-3) and mitochondrial marker proteins (TOM20, COX-IV), as well as immunohistochemistry for mitophagy marker BNIP3 indicated impaired mitophagic flux. Immunoblotting and LR-PCR did not reveal significant differences between patients and controls. In contrast, mtDNA copy number measurement showed a reduction of mtDNA copy numbers in the patient group compared to controls. This first multi-level study of DM2 unravels thus far undescribed functional and structural mitochondrial abnormalities. However, the molecular link between the tetranucleotide expansion and mitochondrial dysfunction needs to be further elucidated.

The current clinical perception of myotonic dystrophy type 2

PURPOSE OF REVIEW

Myotonic dystrophy type 2 (DM2) is a genetic disorder belonging to the spectrum of myotonic dystrophies. DM2 is characterized by progressive muscle weakness, wasting and muscle pain (myalgia), but can also affect many other organ systems. In this review, we provide an updated overview on the research literature on DM2 with a focus on the management of multisystemic involvement and atypical clinical phenotypes.

RECENT FINDINGS

Recent studies have focused on different aspects of multisystemic involvement. Early and severe cardiac involvement can occur in DM2 and needs to be managed appropriately. Diabetes has been shown to be more common in DM2 than in DM1, while a combination of symptoms (cataracts, myotonia, tremor) can be used to raise clinical suspicion and initiate genetic testing for DM2. Autoimmune disease has been shown to occur in up to one-third of DM2 patients, possibly due to altered immune pathways. New evidence also suggests a childhood-onset phenotype presenting with foot deformities.

SUMMARY

The multisystemic aspects of the disease require a multidisciplinary approach for some patients, most likely even including state-of-the-art cardiac and brain imaging to detect and treat complications earlier. Of note, our concept of DM2 as an adult-onset disease is somewhat challenged by evidence suggesting a few pediatric DM2 patients and possibly anticipation, at least in some DM2 families. More studies, including larger cohorts, are needed to better understand this possible early-onset DM2 phenotype variant.

Male infertility with muscle weakness: a point of view

Introduction and importance

The most common causes of infertility are idiopathic spermatogenetic disorders, occurring in multiple reproductive or systemic diseases. The underlying genetic disorders influence the treatment and transmission of the disease to the offspring.

Case presentation

A 32-year-old Syrian male, married for 6 years, presented with primary infertility. The patient had a history of muscle dystrophy for 12 years. He had no previous medical or drug addiction or family history. He had gynecomastia. Semen analysis revealed oligospermia in the patient. Follicle-stimulating hormone was elevated. Gene analysis could not be done due to funding issues. The percutaneous testicular biopsy revealed hypospermatogenesis, atrophy, and marked hyalinization of the seminiferous tubules. Electromyography of the upper extremities demonstrated myotonic discharges, with a waxing-waning frequency, amplitude, and a characteristic 'engine revving' sound.

Clinical discussion

Myotonic dystrophy (MD) is an autosomal dominant inheritance disease with adult onset. Muscle weakness is the predominant presenting feature, with early involvement of the distal limbs and neck muscles and a characteristic facial appearance.Systemic clinical manifestations may include cardiac conduction defects, cataracts, insulin resistance and diabetes, testicular atrophy with impaired spermatogenesis, and others. Testicular biopsy findings are specific. To our knowledge, this is the first case of male infertility associated with MD in Syria. However, there are no data on the prevalence of myotonic dystrophy type 1 (MD1) in Syria.

Conclusion

The practicing physician should keep in mind the frequent association between MD and infertility.

Myotonic dystrophy type 1 in the COVID-19 era

INTRODUCTION

Myotonic dystrophy type 1 (DM1) is the most prevalent muscular dystrophy in adults. People with DM1 might represent a high-risk population for respiratory infections, including COVID-19. Our aim was to evaluate the characteristics of COVID-19 infection and vaccination rate in DM1 patients.

METHODS

This cross-sectional cohort study included 89 patients from the Serbian registry for myotonic dystrophies. Mean age at testing was 48.4 ± 10.4 years with 41 (46.1%) male patients. Mean duration of the disease was 24.0 ± 10.3 years.

RESULTS

COVID-19 infection was reported by 36 (40.4%) DM1 patients. Around 14% of patients had a more severe form of COVID-19 requiring hospitalization. The severity of COVID-19 was in accordance with the duration of DM1. A severe form of COVID-19 was reported in 20.8% of patients who were not vaccinated against SARS-CoV-2 and in none of the vaccinated ones. The majority of 89 tested patients (66.3%) were vaccinated against SARS-CoV-2. About half of them (54.2%) received three doses and 35.6% two doses of vaccine. Mild adverse events after vaccination were recorded in 20.3% of patients.

CONCLUSIONS

The percentage of DM1 patients who suffered from COVID-19 was like in general population, but with more severe forms in DM1, especially in patients with longer DM1 duration. The study indicated an overall favorable safety profile of COVID-19 vaccines among individuals with DM1 and its ability to protect them from severe COVID-19.

The myotonic dystrophy type 1 drug development pipeline: 2022 edition

The beginning of the 20th decade has witnessed an increase in drug development programs for myotonic dystrophy type 1 (DM1). We have collected nearly 20 candidate drugs with accomplished preclinical and clinical phases, updating our previous drug development pipeline review with new entries and relevant milestones for pre-existing candidates. Three interventional first-in-human clinical trials got underway with distinct drug classes, namely AOC 1001 and DYNE-101 nucleic acid-based therapies, and the small molecule pitolisant, which joins the race toward market authorization with other repurposed drugs, including tideglusib, metformin, or mexiletine, already in clinical evaluation. Furthermore, newly disclosed promising preclinical data for several additional nucleic-acid therapeutic candidates and a CRISPR-based approach, as well as the advent into the pipeline of novel therapeutic programs, increase the plausibility of success in the demanding task of providing valid treatments to patients with DM1.

Automatic Text-Mining Approach to Identify Molecular Target Candidates Associated with Metabolic Processes for Myotonic Dystrophy Type 1

Myotonic dystrophy type 1 (DM1) is an autosomal dominant hereditary disease caused by abnormal expansion of unstable CTG repeats in the 3' untranslated region of the myotonic dystrophy protein kinase (DMPK) gene. This disease mainly affects skeletal muscle, resulting in myotonia, progressive distal muscle weakness, and atrophy, but also affects other tissues and systems, such as the heart and central nervous system. Despite some studies reporting therapeutic strategies for DM1, many issues remain unsolved, such as the contribution of metabolic and mitochondrial dysfunctions to DM1 pathogenesis. Therefore, it is crucial to identify molecular target candidates associated with metabolic processes for DM1. In this study, resorting to a bibliometric analysis, articles combining DM1, and metabolic/metabolism terms were identified and further analyzed using an unbiased strategy of automatic text mining with VOSviewer software. A list of candidate molecular targets for DM1 associated with metabolic/metabolism was generated and compared with genes previously associated with DM1 in the DisGeNET database. Furthermore, g:Profiler was used to perform a functional enrichment analysis using the Gene Ontology (GO) and REAC databases. Enriched signaling pathways were identified using integrated bioinformatics enrichment analyses. The results revealed that only 15 of the genes identified in the bibliometric analysis were previously associated with DM1 in the DisGeNET database. Of note, we identified 71 genes not previously associated with DM1, which are of particular interest and should be further explored. The functional enrichment analysis of these genes revealed that regulation of cellular metabolic and metabolic processes were the most associated biological processes. Additionally, a number of signaling pathways were found to be enriched, e.g., signaling by receptor tyrosine kinases, signaling by NRTK1 (TRKA), TRKA activation by NGF, PI3K-AKT activation, prolonged ERK activation events, and axon guidance. Overall, several valuable target candidates related to metabolic processes for DM1 were identified, such as NGF, NTRK1, RhoA, ROCK1, ROCK2, DAG, ACTA, ID1, ID2 MYOD, and MYOG. Therefore, our study strengthens the hypothesis that metabolic dysfunctions contribute to DM1 pathogenesis, and the exploitation of metabolic dysfunction targets is crucial for the development of future therapeutic interventions for DM1.

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

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

Myotonic dystrophy type 1 presenting with dyspnea: A case report

BACKGROUND

Myotonic dystrophy type 1 (DM1) is a genetic neuromuscular disease involving multiple systems, especially the cardiopulmonary system. The clinical phenotype of DM1 patients is highly variable, which limits early diagnosis and treatment. In the present study, we reported a 35-year-old female DM1 patient with dyspnea as the primary onset clinical manifestation, analyzed her family's medical history, and reviewed related literature.

CASE SUMMARY

A 35-year-old woman was admitted to the hospital with dyspnea of 1 mo duration, and sleep apnea for 3 d. Her respiratory pattern and effort were normal, but limb muscle tension was low. Investigation into the patient's medical history revealed that she might have hereditary neuromuscular disease. Electromyography showed that her myotonia potentials were visible in the resting state of the examined muscles, with decreased motor unit potential time limit and amplitude. Genetic testing for DM1 revealed that the cytosine-thymine-guanine (CTG) repeat number of the DMPK gene exceeded 50, while cytosine-CTG expansion in intron 1 of ZNF9 gene was < 30 repeats. The patient was diagnosed with DM1.

CONCLUSION

DM1 is a genetic neuromuscular disease involving multiple systems, and the clinical phenotype in DM1 is extremely variable. Some patients with DM1 may be presented at the respiratory department because of dyspnea, which should be cautioned by the pulmonologists. There may be no obvious or specific symptoms in the early stage of disease, and clinicians should improve their understanding of DM1 and make an early diagnosis, which will improve patients’ quality of life.