From Exon Skipping to Ultra-Mini Dystrophin, the Landscape of Muscular Dystrophy Treatment is Changing
Background
Duchenne Muscular Dystrophy (DMD), occurring in approximately 1 in 3,500 to 5,000 male births, is an X-linked recessive rare genetic disease characterized by rapid muscle degeneration from childhood. Patients exhibit symptoms such as Gower's sign (using hands to push up from the floor) and calf pseudohypertrophy during early childhood. By adolescence, most lose the ability to walk, and by their 20s or 30s, they face life-threatening respiratory failure or cardiomyopathy.
The fundamental cause is a mutation in the dystrophin gene located on chromosome Xp21.1. This gene is one of the largest in the human genome, consisting of 79 exons and 2.5 million base pairs. The expressed dystrophin protein connects the cytoskeletal actin to the extracellular matrix via the dystrophin-associated protein complex (DAPC), stabilizing the cell membrane during muscle contraction. Mutations causing protein deficiency lead to membrane ruptures under mechanical stress, triggering a cascade of intracellular calcium dysregulation, nitric oxide signaling defects, oxidative stress, and inflammation. Ultimately, muscle fibers undergo progressive necrosis and are replaced by fibrosis and adipose tissue.
Standard therapy has relied on corticosteroids to extend ambulation and partially improve quality of life. However, long-term administration is accompanied by significant side effects such as osteoporosis, weight gain, and growth impairment, failing to fundamentally alter the disease progression.
Key Findings
Advances in molecular genetics are expanding the treatment paradigm toward strategies that directly restore missing proteins or correct genetic defects. A representative approach is exon skipping using Antisense Oligonucleotides (ASOs). By skipping the mutated exon during pre-mRNA splicing, it restores the reading frame and induces the expression of a truncated dystrophin that retains partial function. Drugs such as eteplirsen (targeting exon 51), golodirsen (exon 53), viltolarsen (exon 53), and casimersen (exon 45) have been developed and introduced into clinical practice.
AAV (Adeno-Associated Virus) vector-based gene therapy has evolved to overcome the large cargo capacity limitations of the vector. The original dystrophin gene, spanning 2.5 million base pairs, far exceeds the limited packaging capacity of AAV (approximately 4.7kb). Researchers have established a method to deliver microdystrophin—a compressed gene containing only the domains essential for muscle contraction support—to muscle cells in vivo.
Various other platforms are proving effective, including research using CRISPR-Cas9 to permanently correct or excise mutant exons, nonsense mutation read-through compounds that bypass premature stop codons, and stem cell therapies to aid muscle regeneration.
Implications and Outlook
As therapeutic approaches expand, a precision medicine approach is becoming possible in the clinical field of DMD, where single-agent control was previously difficult. Combined strategies are being discussed, such as prescribing specific ASOs for patients with particular exon deletions while using AAV vectors to supplement minimal functional proteins throughout systemic muscle tissue.
Clear barriers to overcome remain. The formation of neutralizing antibodies and immune toxicity following high-dose AAV administration limits re-administration. For exon skipping therapies, the patient population that benefits is limited to those with specific mutations, and challenges remain in improving delivery efficiency within tissues and the clinical significance of the resulting protein expression levels. Furthermore, the enormous treatment costs, ranging from hundreds of millions to billions of won per patient, are a barrier to healthcare accessibility. For precise gene therapies with proven efficacy and safety to take root, optimization of vector processes must be closely integrated with multidisciplinary collaborative systems.
Duchenne Muscular Dystrophy (DMD) is a chronic X-linked recessive neuromuscular disorder with a prevalence of 1 in 3,500 to 5,000 live male births worldwide. Alterations in the dystrophin gene, one of the biggest known human genes with 2.5 million base pairs and 79 exons, located at Xp21.1, are responsible for the disease. The dystrophin protein, an essential structural component that links the muscle cell cytoskeleton actin to the extracellular matrix through the dystrophin-associated protein complex, is either lacking or dysfunctional due to these mutations. Dystrophin deficiency compromises muscle membrane stability, leading to gradual deterioration of muscle fibers, fibrosis, and replacement by adipose tissue. Numerous disrupted physiological pathways, such as calcium dysregulation, faulty nitric oxide signaling, altered PI3K-Akt-mTOR growth signaling, chronic inflammatory responses, and oxidative stress, are implicated in the pathophysiology of DMD. Affected males clinically demonstrate early childhood muscular weakness; characteristic signs such as Gower's maneuver and calf hypertrophy; increasing ambulation loss by adolescence; and potentially lethal cardiorespiratory problems that generally manifest in the third decade of life. Corticosteroids remain the primary therapeutic modality, significantly improving ambulation and quality of life; nevertheless, extended use entails severe adverse effects. Recent treatment advancements provide renewed optimism via diverse innovative strategies, namely CRISPR-Cas9 gene editing, antisense oligonucleotide-mediated exon skipping (eteplirsen, golodirsen, viltolarsen, and casimersen), microdystrophin gene therapy using Adeno-Associated Viral (AAV) vectors, stop codon read-through therapy, and stem cell-based treatments. But difficulties with cost, immunogenicity, efficacy, and mutant specificity persist. Through multidisciplinary treatment and continuous scientific progress, individualized precision medicine that integrates
This research presents a personalized treatment pathway based on detailed genetic mutation analysis of DMD patients. For patients with exon 51, 53, or 45 deletions, an algorithm could be implemented in clinical practice to delay the loss of ambulation by administering targeted ASO agents from early diagnosis, while selectively applying microdystrophin gene therapy early to preserve cardiac and respiratory muscle function. In the pharmaceutical industry, this serves as a catalyst to accelerate the development of improved delivery capsids to overcome large gene payload limitations, AAV production processes that reduce hepatotoxicity, and gene editing technologies based on non-viral nanoparticles (LNPs). Combining multidisciplinary rehabilitation management with gene-targeted drugs can improve long-term survival rates and enable patients to maintain independent daily living.