Magdalyn Snyder

Background: Duchenne muscular dystrophy (DMD) results from a frameshift or nonsense mutation in the DMD gene found on the X chromosome, which encodes dystrophin (a 427-kDa cytoskeletal protein). Dystrophin plays a crucial role in skeletal and cardiac muscle and individuals with DMD experience progressive muscle weakness resulting in motor delays, functional decline, and death usually before 30 despite treatment.^1,2,3 Currently, treatments include physiotherapy with glucocorticoids; however, these do not target the underlying disease cause.⁴ Therefore, emerging techniques for dystrophin recovery, such as targeting stop codons, exon skipping, and gene delivery using viral vectors, are of interest.^1,2 In particular, adeno-associated vectors (AAVs) may be an optimal therapy for single gene disorders as the functional gene can be delivered and expressed in target tissues.⁵

Methods: A PubMed search was conducted utilizing to key terms: “DMD”, “genetic therapies”, and “genetic vectors”. Studies published beyond five years were excluded.

Results: Systemic injection of rAAVrh74.MHCK7.micro-dystrophin in DMD mouse models led to increased expression of dystrophin in cardiac and skeletal muscle, improving muscle function parameters, such as force generation and output.⁶ AAV safety was proven as the highest systemic dose administered displayed minimal toxicity.⁶ Due to success of these safety trials, rAAVrh74.MHCK7.micro-dystrophin underwent phase 1/2a clinical trials. Systemic injection in males aged 4-7, with confirmed DMD mutations, resulted in correctly located dystrophin expression and improvements in function, measured via ambulatory assessments.⁷ During the year-long follow-up, the most common adverse events (AEs) were vomiting and liver enzyme elevation, which quickly resolved with corticosteroids.⁶ No serious AEs were identified from blood analysis and chemical panels, and no immune responses or toxic effects were observed.^5,7 Due to restricted carrying capacity of AAVs (4.7kb) there are limitations of use with larger genes, such as full-length dystrophin. In other disorders, methods to overcome the size restriction have been investigated including the use of a dual AAV system that successfully reconstituted a full-length protein via trans-splicing of smaller fragments.⁸

Conclusion: The use of AAV genetic therapy for DMD has shown safety and efficacy in the clinic. However, AAVs are limited in their carrying capacity, and due to the large size of the dystrophin gene, only portions of micro-dystrophin are able to be delivered.² Adoption of a multiple AAV system to deliver trans-spliced dystrophin has the potential to lead to introduction of fully functional dystrophin to individuals with DMD.

Works Cited:

1. Damon R. Asher, Khampaseuth Thapa, Sachi D. Dharia, Navid Khan, Rachael A. Potter, Louise R. Rodino-Klapac & Jerry R. Mendell (2020) Clinical development on the frontier: gene therapy for duchenne muscular dystrophy, Expert Opinion on Biological Therapy, 20:3, 263-274, DOI: 10.1080/14712598.2020.1725469.
2. Function and Genetics of Dystrophin and Dystrophin-Related Proteins in Muscle Derek J. Blake, Andrew Weir, Sarah E. Newey, and Kay E. Davies Physiological Reviews 2002 82:2, 291-329.
3. Capitanio D., Moriggi M., Torretta E., Barbacini P., De Palma S., Viganò A., Lochmüller H., Muntoni F., Ferlini A., Mora M., and Gelfi C. (2020) Comparative proteomic analyses of Duchenne muscular dystrophy and Becker muscular dystrophy muscles: changes contributing to preserve muscle function in Becker muscular dystrophy patients, Journal of Cachexia, Sarcopenia and Muscle, 11, 547–563. https://doi.org/10.1002/jcsm.12527.
4. Birnkrant DJ, Bushby K, Bann CM, et al. Diagnosis and management of Duchenne muscular dystrophy, part 1: diagnosis, and neuromuscular, rehabilitation, endocrine, and gastrointestinal and nutritional management. Lancet Neurol. 2018;17(3):251-267. doi:10.1016/S1474-4422(18)30024-3.
5. Anita P. Liu, Shailin K. Patel, Tao Xing, Yuetian Yan, Shunhai Wang, Ning Li. Characterization of Adeno-Associated Virus Capsid Proteins Using Hydrophilic Interaction Chromatography Coupled with Mass Spectrometry, Journal of Pharmaceutical and Biomedical Analysis, Volume 189, 2020, 113481, ISSN 0731-7085, https://doi.org/10.1016/j.jpba.2020.113481.
6. Dose-Escalation Study of Systemically Delivered rAAVrh74.MHCK7.micro-dystrophin in the mdx Mouse Model of Duchenne Muscular Dystrophy Rachael A. Potter, Danielle A. Griffin, Kristin N. Heller, Ellyn L. Peterson, Emma K. Clark, Jerry R. Mendell, and Louise R. Rodino-Klapac. Human Gene Therapy, 2021 32:7-8, 375-389.
7. Mendell JR, Sahenk Z, Lehman K, Nease C, Lowes LP, Miller NF, Iammarino MA, Alfano LN, Nicholl A, Al-Zaidy S, Lewis S, Church K, Shell R, Cripe LH, Potter RA, Griffin DA, Pozsgai E, Dugar A, Hogan M, Rodino-Klapac LR. Assessment of Systemic Delivery of rAAVrh74.MHCK7.micro-dystrophin in Children With Duchenne Muscular Dystrophy: A Nonrandomized Controlled Trial. JAMA Neurol. 2020 Sep 1;77(9):1122-1131. doi: 10.1001/jamaneurol.2020.1484. PMID: 32539076; PMCID: PMC7296461.
8. Padula A, Petruzzelli R, Philbert SA, Church SJ, Esposito F, Campione S, Monti M, Capolongo F, Perna C, Nusco E, Schmidt HH, Auricchio A, Cooper GJS, Polishchuk R, Piccolo P. Full-length ATP7B reconstituted through protein trans-splicing corrects Wilson disease in mice. Mol Ther Methods Clin Dev. 2022 Aug 13;26:495-504. doi: 10.1016/j.omtm.2022.08.004. PMID: 36092366; PMCID: PMC9436707.