Duchenne Muscular Dystrophy (DMD)

Muscular dystrophy is a genetic, progressive disorder of the muscles. It can be classified genetically, wherein susceptible patients inherit mutated dystrophin genes. Most commonly, the mutation occurs as Duchenne muscular dystrophy (DMD). Other dystrophinopathies include Becker muscular dystrophy (BMD), isolated quadriceps myopathy, and rhabdomyolysis. Clinical manifestations of DMD are seen in boys between the ages of 3 and 5 years where they find it difficult to jump and run. They suffer from delayed motor milestones with multiple falls. Examination of their calves will reveal hypertrophy with lordotic posture, waddling gait, and poor hip excursion during running.¹

Patients with muscular dystrophies may develop cardiac involvement, such as dilated cardiomyopathy or cardiac fibrosis, which may lead to atrial and ventricular arrhythmias. Physicians treating patients with DMD recommend cardioprotective agents such as angiotensin converting enzymes (ACE) inhibitors and beta blockers to reduce progression and improve ventricular function. Patients suffering from DMD require a multidisciplinary approach in order to mitigate cardiac, pulmonary, and orthopedic issues. Susceptible patients with DMD are prescribed a corticosteroid regimen involving prednisone (0.75 mg/kg/d), deflazacort (0.9 mg/kg/d), prednisolone (0.75 mg/kg/d) with prednisone (2.5 mg/kg to 5 mg/kg), vitamin D supplements, and dietary consideration.²

Experimental Therapy for DMD: Cell Transplantation

Dystrophin serves as a biochemical marker. The rationale of the therapy involves replacing dystrophin in the muscle fibers. In a lower body treatment (LBT) trial, the lower body muscles of boys suffering with DMD were strengthened. LBT assesses the feasibility of culturing 5 billion myoblasts from 1g of donor muscle biopsy. The donor myoblasts were derived from cell cultures of young males aged 9-14 years old. The study involved 32 boys suffering from DMD, aged 6-14 years old. The physical examination involved dystrophin deficiency analysis, electromyography, muscle biopsy, and lab results for serum creatinine kinase (CK), blood urea nitrogen (BUN), alanine aminotransferase (ALT), and aspartate aminotransferase (AST). Donor myoblasts were not extracted from female family members as they can act as a carrier for DMD.

The study revealed proliferation of the myoblasts. Ten billion myoblasts were cultured from a 1g muscle biopsy. A total of five billion (5 x 10⁹) cultured myoblasts in 90 mL were injected into the major muscles of the lower limbs through 48 injections. The sites for the injections were as follows: quadriceps femoris, hamstrings, gluteals, ankle extensors, and ankle plantar flexors. The study showed that billions of myoblasts can be injected without tumor formation. Cyclosporine immunosuppression allowed myoblasts to survive and develop. Myoblast transfer therapy (MTT) is more effective for younger subjects than older ones. The knee extensors showed increased strength in both limbs. It also showed that the dosage was more relevant for ankle plantar flexors. Overall, MTT improved muscle function for boys aged 6 to 14 years.³

Gene Therapy for Muscular Dystrophy

There are various strategies to replace the mutated gene. This involves gene delivery to post-mitotic muscle fibers. Adeno-associated virus (AAV) vectors are currently the vector of choice as they have reduced size and immunogenicity, and they can accommodate smaller genes like sarcoglycans. They cannot accommodate full-size dystrophin complementary DNA. Phase I and Phase II clinical trials for limb-girdle muscular dystrophy 2C involved intramuscular injections of gamma sarcoglycan into the radial muscle. It was expressed with an AAV vector. Clinical experimentation was conducted with exon skipping where a type 1 viral vector produced small nuclear U7 RNA targeting exons 6–8, which led to “quasi” normal translated dystrophin protein within the in-frame transcript.

Systemic delivery was achieved through high-pressure intravenous delivery with transient immune suppression, scaling up of AAV production, and expression of dystrophin variants of reduced size but within the in-frame transcript. In an alternate therapy, minigenes were constructed where amino and carboxyl domains were preserved. The minigene replaced the wild-type gene as per the study conducted in the multidimensional expression (MDX) mice model, resulting in microdystrophin. It was found that they were less effective in maintaining the integrity of muscle fibers in larger animals. Moreover, it failed to maintain the force of contraction despite widespread microdystrophin expression. Overall, gene therapy with AAV vectors is efficient in terms of exon skipping and translating “quasi” normal dystrophin protein.⁴

Stem Cell Therapy as a Potential Treatment for DMD

Stem cell therapy for DMD needs to fulfill certain criteria in terms of its capability to express dystrophin, proliferate, and migrate to the host muscle. Phase I and II clinical trials are being planned to assess the safety of stem cell therapy for treating DMD. The clinical trial (NCT01610440)⁵ aims to study the muscle strength and motor functions of boys aged 5-12 years suffering from DMD. The study involves mesenchymal stem cell transplantation of the human umbilical cord and its effects. Current challenges involve maximizing engraftment of donor cells, finding an optimal mode of delivery, and isolating a sufficient stem cell population for therapeutic transplantation.⁶

References

1. Wicklund MP. The muscular dystrophies. Continuum (Minneap Minn). 2013;19(6 Muscle Disease):1535-1570. doi:10.1212/01.CON.0000440659.41675.8b
2. Falzarano MS, Scotton C, Passarelli C, Ferlini A. Duchenne Muscular Dystrophy: From Diagnosis to Therapy. Molecules. 2015 Oct 7;20(10):18168-84. doi: 10.3390/molecules201018168. PMID: 26457695; PMCID: PMC6332113.
3. Law PK, Goodwin TG, Fang Q, et al. Cell transplantation as an experimental treatment for Duchenne muscular dystrophy. Cell Transplant. 1993;2(6):485-505. doi:10.1177/096368979300200607
4. Cossu G, Sampaolesi M. New therapies for Duchenne muscular dystrophy: challenges, prospects and clinical trials. Trends Mol Med. 2007;13(12):520-526. doi:10.1016/j.molmed.2007.10.003
5. Safety and efficacy of umbilical cord mesenchymal stem cell therapy for patients with Duchenne muscular dystrophy. ClinicalTrials.gov. June 4, 2012. Updated November 30, 2012. Accessed July 8, 2021.
6. Liew WKM, Kang PB. Recent developments in the treatment of Duchenne muscular dystrophy and spinal muscular atrophy. Ther Adv Neurol Disord. 2013;6(3):147-160. doi:10.1177/1756285612472386

Reviewed by Harshi Dhingra, MD, on 7/1/2021.

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Debjyoti Talukdar, MD

Dr. Deb Talukdar is a medical doctor from New Delhi, India. His research interest includes cancer therapeutics, Parkinson’s Disease, inflammatory and immunosuppressive drugs, COVID-19 predictive modeling and vaccination program, public health research associated with DHS and rare diseases such Pulmonary arterial hypertension (PAH). Previously, he was involved in AI research at Yale University. Currently, he is affiliated with All Saints University School of Medicine in Dominica.