Large in-frame 5' deletions in DMD associated with mild Duchenne muscular dystrophy: Two case reports and a review of the literature

Increase in cathepsin K gene expression in Duchenne muscular dystrophy skeletal muscle

Dystrophinopathy is caused by alterations in the dystrophin gene. The severe phenotype, Duchenne muscular dystrophy (DMD), is caused by a lack of dystrophin in skeletal muscles, resulting in necrosis and regenerating fibers, inflammatory cells, and muscle fibrosis. Progressive muscle weakness is a characteristic finding of this condition. Here, we encountered a rare case of a 10-year-old patient with asymptomatic dystrophinopathy with no dystrophin expression and investigated the reason for the absence of muscle weakness to obtain therapeutic insights for DMD. Using RNA-seq analysis, gene expression in skeletal muscles was compared among patients with asymptomatic dystrophinopathy, three patients with typical DMD, and two patients without dystrophinopathy who were leading normal daily lives. Cathepsin K (CTSK), myosin heavy chain 3 (MYH3), and nodal modulator 3-like genes exhibited a >8-fold change, whereas crystallin mu gene (CRYM) showed a <1/8-fold change in patients with typical DMD compared with their expression in the patient with asymptomatic dystrophinopathy. Additionally, CTSK and MYH3 expression exhibited a >16-fold change (P < 0.01), whereas CRYM expression showed a <1/16-fold change (P < 0.01) in patients with typical DMD compared with their expression in those without dystrophinopathy. CTSK plays an essential role in skeletal muscle loss, fibrosis, and inflammation in response to muscles injected with cardiotoxin, one of the most common reagents that induce muscle injury. Increased CTSK expression is associated with muscle injury or necrosis in patients with DMD. The lack of muscle weakness in the patient with asymptomatic dystrophinopathy might be attributed to the low CTSK expression in the muscles. To the best of our knowledge, this is the first report to demonstrate that CTSK expression was significantly higher in the skeletal muscles of patients with DMD with a typical phenotype than in those without dystrophinopathy.

The impact of genotype on age at loss of ambulation in individuals with Duchenne muscular dystrophy treated with corticosteroids: A single-center study of 555 patients

INTRODUCTION/AIMS

Studies have demonstrated that certain genotypes in Duchenne muscular dystrophy (DMD) have milder or more severe phenotypes. These studies included individuals treated and not treated with corticosteroids and multiple sites with potentially varying standards of care. We aimed to assess genotype-phenotype correlations for age at loss of ambulation (LoA) in a large cohort of individuals with DMD treated with corticosteroids at one center.

METHODS

In this retrospective review of medical records, encounters were included for individuals diagnosed with DMD if prescribed corticosteroids, defined as daily deflazacort or prednisone or high-dose weekend prednisone, for 12 consecutive months. Encounters were excluded if the participants were taking disease-modifying therapy. Data were analyzed using survival analysis for LoA and Fisher's exact tests to assess the percentage of late ambulatory (>14 years old) individuals for selected genotypes.

RESULTS

Overall, 3948 encounters from 555 individuals were included. Survival analysis showed later age at LoA for exon 44 skip amenable (p = .004), deletion exons 3-7 (p < .001) and duplication exon 2 (p = .043) cohorts and earlier age at LoA for the exon 51 skip amenable cohort (p < .001) when compared with the rest of the cohort. Individuals with deletions of exons 3-7 had significantly more late ambulatory individuals than other cohorts (75%), while those with exon 51 skip amenable deletions had significantly fewer (11.9%) compared with other cohorts.

DISCUSSION

This confirms previous observations of genotype-phenotype correlations in DMD and enhances information for trial design and clinical management.

Duchenne muscular dystrophy caused by a deletion (c.5021del) in exon 35 of the DMD gene: A case report and review of the literature

Duchenne muscular dystrophy (DMD MIM#310200) is a degenerative muscle disease caused by mutations in the dystrophin gene located on Xp21.2. The clinical features encompass muscle weakness and markedly elevated serum creatine kinase levels. An 8-year-old Chinese boy was diagnosed with Duchenne muscular dystrophy (DMD). Whole exome gene sequencing was conducted and the Sanger method was used to validate sequencing. A deletion (c.5021del) in exon 35 of the dystrophin gene was identified, which was predicted to generate a frameshift mutation and create an early termination codon (p.Leu1674CysfsTer47). It has a pathogenic effect against dystrophin in the muscle cell membrane of the patient. As such, prednisone treatment at a dose of 0.75 mg/kg.d was administered. After one month, a notable reduction in fall frequency was observed. Our new finding will expand the pathogenic mutation spectrum causing DMD.

The complex landscape of DMD mutations: moving towards personalized medicine

Duchenne muscular dystrophy (DMD) is a severe genetic disorder characterized by progressive muscle degeneration, with respiratory and cardiac complications, caused by mutations in the DMD gene, encoding the protein dystrophin. Various DMD mutations result in different phenotypes and disease severity. Understanding genotype/phenotype correlations is essential to optimize clinical care, as mutation-specific therapies and innovative therapeutic approaches are becoming available. Disease modifier genes, trans-active variants influencing disease severity and phenotypic expressivity, may modulate the response to therapy, and become new therapeutic targets. Uncovering more disease modifier genes via extensive genomic mapping studies offers the potential to fine-tune prognostic assessments for individuals with DMD. This review provides insights into genotype/phenotype correlations and the influence of modifier genes in DMD.

Efficacy of exon-skipping therapy for DMD cardiomyopathy with mutations in actin binding domain 1

Exon-skipping therapy is a promising treatment strategy for Duchenne muscular dystrophy (DMD), which is caused by loss-of-function mutations in the DMD gene encoding dystrophin, leading to progressive cardiomyopathy. In-frame deletion of exons 3-9 (Δ3-9), manifesting a very mild clinical phenotype, is a potential targeted reading frame for exon-skipping by targeting actin-binding domain 1 (ABD1); however, the efficacy of this approach for DMD cardiomyopathy remains uncertain. In this study, we compared three isogenic human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) expressing Δ3-9, frameshifting Δ3-7, or intact DMD. RNA sequencing revealed a resemblance in the expression patterns of mechano-transduction-related genes between Δ3-9 and wild-type samples. Furthermore, we observed similar electrophysiological properties between Δ3-9 and wild-type hiPSC-CMs; Δ3-7 hiPSC-CMs showed electrophysiological alterations with accelerated CaMKII activation. Consistently, Δ3-9 hiPSC-CMs expressed substantial internally truncated dystrophin protein, resulting in maintaining F-actin binding and desmin retention. Antisense oligonucleotides targeting exon 8 efficiently induced skipping exons 8-9 to restore functional dystrophin and electrophysiological parameters in Δ3-7 hiPSC-CMs, bringing the cell characteristics closer to those of Δ3-9 hiPSC-CMs. Collectively, exon-skipping targeting ABD1 to convert the reading frame to Δ3-9 may become a promising therapy for DMD cardiomyopathy.

Modeling Patient-Specific Muscular Dystrophy Phenotypes and Therapeutic Responses in Reprogrammed Myotubes Engineered on Micromolded Gelatin Hydrogels

In vitro models of patient-derived muscle allow for more efficient development of genetic medicines for the muscular dystrophies, which often present mutation-specific pathologies. One popular strategy to generate patient-specific myotubes involves reprogramming dermal fibroblasts to a muscle lineage through MyoD induction. However, creating physiologically relevant, reproducible tissues exhibiting multinucleated, aligned myotubes with organized striations is dependent on the introduction of physicochemical cues that mimic the native muscle microenvironment. Here, we engineered patient-specific control and dystrophic muscle tissues in vitro by culturing and differentiating MyoD–directly reprogrammed fibroblasts isolated from one healthy control subject, three patients with Duchenne muscular dystrophy (DMD), and two Limb Girdle 2A/R1 (LGMD2A/R1) patients on micromolded gelatin hydrogels. Engineered DMD and LGMD2A/R1 tissues demonstrated varying levels of defects in α-actinin expression and organization relative to control, depending on the mutation. In genetically relevant DMD tissues amenable to mRNA reframing by targeting exon 44 or 45 exclusion, exposure to exon skipping antisense oligonucleotides modestly increased myotube coverage and alignment and rescued dystrophin protein expression. These findings highlight the value of engineered culture substrates in guiding the organization of reprogrammed patient fibroblasts into aligned muscle tissues, thereby extending their value as tools for exploration and dissection of the cellular and molecular basis of genetic muscle defects, rescue, and repair.

Diagnostic Value of Dystrophin Immunostaining in the Diagnosis of Duchenne and Becker Muscular Dystrophy Patients

Background: Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD) are X-linked recessive muscular disorders caused by the absence or reduction of the muscle cytoskeletal protein dystrophin. Standard procedures to detect deletion and duplication of the DMD gene use Multiplex Ligation-Dependent Probe Amplification (MLPA). However, genetic testing, such as MLPA, is not covered by the national insurance scheme in Indonesia. Immunohistochemical (IHC) staining of dystrophin from muscle biopsy in the form of Formalin-Fixed Paraffin-Embedded (FFPE) specimens can be an alternative method to detect dystrophin expression in protein levels to establish the diagnosis of DMD or BMD. Objectives: To determinate sensitivity, specificity and accuracy of IHC analysis of dystrophin in DMD/BMD patient in comparison with the standard genetic testing, MLPA. Methods: Twenty-six patients enrolled in this study were clinically diagnosed as DMD/BMD in Dr. Sardjito Hospital and Universitas Gadjah Mada Academic Hospital. Genomic DNA was isolated from 3 mL of EDTA-peripheral whole blood samples. The deletion and duplication of DMD genes were detected by MLPA. IHC examination was performed using a specific antibody dystrophin (DYS2). Complete loss of dystrophin staining indicated DMD, while partial loss of dystrophin staining indicated BMD. MLPA result was used as the gold standard to determine sensitivity, specificity, and accuracy of IHC technique using a 2x2 table. Results: MLPA results revealed 18 (18/26; 69.3%) patients with deletion and 3 (3/26; 11.5%) patients with duplication. Five (5/26; 19.2%) patients who showed no deletion nor duplication were excluded from the analysis. Among 21 patients with deletion or duplication, 18 (18/21; 85.7%) patients were out-of-frame (DMD) and 3 (3/21; 14.3%) patients were in-frame (BMD). Six patients showed a discrepancy between the IHC and MLPA results with 9.5% (2/21) false positive and 19% (4/21) false negative. The sensitivity of dystrophin IHC was 77.78%, specificity 33.33%, positive predictive value 87.5%, negative predictive value 20%, and accuracy 71.43%. Conclusion: Muscle biopsy followed by IHC can be one of the diagnostic tools to diagnose BMD or DMD, with high sensitivity. The protein-based strategy is probably the most efficient way to approach the diagnosis of Duchenne and Becker muscular dystrophy in limited health care settings.

Genotype-Phenotype Correlations in Duchenne and Becker Muscular Dystrophy Patients from the Canadian Neuromuscular Disease Registry

Duchenne muscular dystrophy (DMD) is a fatal neuromuscular disorder generally caused by out-of-frame mutations in the DMD gene. In contrast, in-frame mutations usually give rise to the milder Becker muscular dystrophy (BMD). However, this reading frame rule does not always hold true. Therefore, an understanding of the relationships between genotype and phenotype is important for informing diagnosis and disease management, as well as the development of genetic therapies. Here, we evaluated genotype-phenotype correlations in DMD and BMD patients enrolled in the Canadian Neuromuscular Disease Registry from 2012 to 2019. Data from 342 DMD and 60 BMD patients with genetic test results were analyzed. The majority of patients had deletions (71%), followed by small mutations (17%) and duplications (10%); 2% had negative results. Two deletion hotspots were identified, exons 3-20 and exons 45-55, harboring 86% of deletions. Exceptions to the reading frame rule were found in 13% of patients with deletions. Surprisingly, C-terminal domain mutations were associated with decreased wheelchair use and increased forced vital capacity. Dp116 and Dp71 mutations were also linked with decreased wheelchair use, while Dp140 mutations significantly predicted cardiomyopathy. Finally, we found that 12.3% and 7% of DMD patients in the registry could be treated with FDA-approved exon 51- and 53-skipping therapies, respectively.