Dystrophinopathies and the Limb-Girdle Muscular Dystrophies

Rapid change is occurring in the treatment landscape for genetically based neuromuscular disorders. In this article, we discuss treatments for dystrophinopathies, including Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD), as well as the genetically and phenotypically heterogeneous group of conditions collectively known as limb-girdle muscular dystrophies (LGMD). Until recently, these conditions were managed through palliative and supportive care alone. Now, with the potential introduction of genetically targeted therapeutics, disease-modifying therapies have shown promise to change the clinical landscape of these conditions.

Dystrophinopathies

Dystrophinopathies are X-linked neurodegenerative disorders that range in phenotypic severity from the most severe (DMD) to less severe (BMD) to asymptomatic (hyperCKemia). DMD is the most common muscular dystrophy of childhood. Without treatment, the natural history of DMD is characterized by loss of the ability to walk by age 13 years, progressive neuromuscular scoliosis, cardiomyopathy, and the need for pulmonary interventions during adolescence, with death in the late teens to early 20s.¹ BMD is a less severe but more variable condition wherein individuals remain ambulatory beyond age 16 years.

Pre–Genetic Treatment Era

Before the advent of genetically based therapies, the standard of care for dystrophinopathies was interdisciplinary supportive care, and for DMD, corticosteroid treatment. Daily steroid use was found to slow the progression of disease milestones by 1 to 3 years as well as drastically reduce the need for scoliosis surgery.¹ Many acute and chronic iatrogenic comorbidities are associated with regular systemic steroid use, however, including risk for behavioral issues, linear growth suppression, obesity, osteopenia, and cataracts development.¹ The ideal dosing and schedule are not firmly established with current options, including prednisone, prednisolone, or deflazacort, and commonly used schedules include daily, weekend, or 10 days on and 10 days off.

Stop Codon Readthrough Therapy

Roughly 10% of dystrophin variants that result in DMD are pathogenic, caused by nonsense mutations and the creation of a premature stop codon.² Ataluren (Translarna; PTC Therapeutics, South Plainfield, NJ) is a targeted oral small molecule therapy that assists ribosomal transcription to allow for mRNA readthrough of premature stop codons.³ Studies comparing ataluren plus standard of care versus standard of care alone have shown benefit in delayed loss of ambulation and prolonged strength of forced vital capacity.³ Ataluren is available in several countries, including many in the European Union, as well as South Korea and Israel, but is not approved for use in the United States.²

Exon-Skipping Therapies

Exon-skipping antisense oligonucleotide therapies are designed to target a specific portion of the pre–messenger RNA transcribed by the DMD gene causing it to be skipped (not translated), thus restoring the reading frame for the remainder of the downstream transcription and thereby creating a slightly truncated dystrophin protein; the quality of the truncated dystrophin produced is likely variable depending on the size and location of the skipped region. Eteplirsen (Exondys 51; Sarepta Therapeutics, Cambridge, MA), which targets DMD exon 51, was the first DMD drug to be granted conditional approval in the United States, in 2016. Skipping of exon 51 and restoring the reading frame allows for roughly 14% of people with DMD mutations to be amenable to therapy.⁴ Initial approval was primarily based on a phase 2b clinical trial (Safety Study of Eteplirsen to Treat Early Stage Duchenne Muscular Dystrophy, NCT02420379) including 12 boys, with later long-term follow-up data further suggesting efficacy compared with natural history cohorts.^5,6 There has been some controversy in that approval was primarily related to biomarker data of increased dystrophin expression in muscle biopsy compared with pretreatment baseline, with no clear data on how quantity or quality of dystrophin expression relates to meaningful clinical benefit.⁷

Additional exon-skipping therapies include golidersen (Vyondys 53; Sarepta Therapeutics, Cambridge, MA) and viltolarsen (Viltepso; NS Pharma, Paramus, NJ), targeting DMD exon 53 (8% of DMD cases), and casimersen (Amondys 45; Sarepta Therapeutics, Cambridge, MA), targeting DMD exon 45. Exon-skipping therapy is available for 25% to 30% of people with DMD.

The antisense oligonucleotides are administered intravenously on a weekly basis, thus often requiring a central line (or port insertion) if repetitive peripheral access presents a challenge. Practical real-world clinical experience has revealed these therapies to be well-tolerated with minimal side effects. A challenging clinical dilemma is the recognition of how much benefit the therapy provides to the individual, and when it may no longer be beneficial, as an individual’s course of decline cannot be known with and without antisense oligonucleotide intervention. People from populations outside of the range studied in the research trials often request access in the clinic, leading to nuanced conversations about expectations for benefit vs potential risk. In addition, the high cost of these medications is an important barrier to access.⁸

Viral-Mediated Gene Transfer Therapy

The new wave of viral-mediated gene transfer therapy technology was introduced in the clinical setting in 2019 with the approval of onasemnogene abeparvovec-xioi (Zolgensma; Novartis, East Hanover, NJ) for the treatment of spinal muscular atrophy with a single administration of a transgene encapsulated in an adeno-associated virus (AAV-9) vector. A similar strategy is under investigation by a number of pharmaceutical companies for the treatment of DMD. Because of the size of the DMD gene, the use of an AAV vector as a carrier does not allow for the entire gene to be included, so various smaller nucleotide burden cassettes known as microdystrophin (or minidystrophin) have been developed using historically identified people with mild BMD with large-region DMD deletions as a scaffolding. A number of concerns remain, including potential cellular immunity issues to truncated dystrophin; preexisting immunity (ie, seropositivity to AAV precluding dosing); lack of full functionality of truncated dystrophin; durability of therapy, given that the AAV genome does not integrate into the host genome but remains episomal; and lack of potential for redosing. There are also concerns regarding toxicity related to immunogenicity to the transgene as well as the capsid, which may result in severe hepatotoxicity, thrombotic microangiopathy, myocarditis, myositis, and other on- and off-target tissue toxicity. Studies have focused on young, ambulatory individuals and the potential to treat older patients who require higher viral loads remains unclear.

Initial small-scale safety data in 4 patients have begun to provide some clarity to these safety concerns (A Gene Transfer Therapy Study to Evaluate the Safety of Delandistrogene Moxeparvovec [SRP-9001] in Participants With Duchenne Muscular Dystrophy [DMD], NCT 03375164).⁹ To increase appropriate tissue tropism, there is an MHCK7 promoter with a high level of skeletal muscle expression as well an enhancer to increase cardiac expression of the transgene.⁹ The AAVrh74 capsid was chosen because of its isolation from rhesus monkey and the hope to decrease likelihood of preexisting vector immunity and minimize immune response with strong tropism for target tissues of skeletal and cardiac muscle.⁹ Of 53 safety events, 18 were believed to be treatment-related, with the most common adverse event being vomiting (50%), and no serious adverse events were detected on hematologic or chemistry panels.⁹ Although some functional outcomes were reported, such as the North Star Ambulatory Assessment, the age of the patients as well as the small sample and short initial follow-up leave the neuromuscular community in pursuit of additional data.

Gene Editing

CRISPR/Cas9-mediated genome editing of the DMD murine model has shown partial restoration of dystrophin expression in cardiac and skeletal muscle. Different strategies have been outlined, including double or single strand breaks allowing for skipping of an exon and thus restoring the normal reading frame or mediating nonsense mutations.¹⁰ Many of the same strategies as outlined previously could be implemented, but ideally with a more favorable frequency (ie, once vs weekly) or with a drastically safer side effect profile (ie, avoiding immune-mediated risks). One study has been reported—the sole human experience of gene editing for DMD—and ended in early death.¹⁰

Limb-Girdle Muscular Dystrophies

The limb-girdle muscular dystrophies (LGMD) are a genetically and phenotypically heterogeneous group that have been classified together given the overall phenotypic similarities of primarily affecting skeletal muscle, leading to predominantly axial muscle weakness, and causing a loss of muscle fibers. Classification often is associated with the specific gene of interest, and subtypes can be inherited in an autosomal-dominant, X-linked, or recessive pattern. Similar to the dystrophinopathies, supportive and multidisciplinary approaches have been the mainstay for the standard of care. Given the genetic heterogeneity of the LGMD, the application of a uniform genetically targeted treatment approach is not plausible. However, there remains significant hope for treatment advancement given the well-suited foundation for treatment in these established, monogenic conditions.

Viral-Mediated Gene Therapy

LGMD2A/R1, also known as calpainopathy, results from CAPN3 gene variants causing a loss of function. Preclinical murine studies have shown improved functional outcomes as well as a switch to fatigue-resistant oxidative fibers on histopathology in female mice treated with an rAAVrh75.tMCK.CAPN3 vector.¹¹ Whether sex differences carry forward into human application will be monitored as clinical trials are prepared.

Preclinical animal studies in gene transfer for LGMD2B/R2, or dysferlinopathies, have evidence of efficacy. The DYSF gene, at 6.9 kb, exceeds the AAV packaging capacity, so a 2-vector system with an overlapping region of homology was developed and tested in mice as well as nonhuman primates.¹¹ Dysferlin expression and muscle magnetic resonance imaging results were improved significantly, showing proof of concept for dual-vector therapy.¹¹

LGMD2D/R3 was the first condition to receive muscle gene transfer in human trials. The rAAV1 vector was injected into the extensor digitorum brevis muscle of 6 participants with LGMD2D, with the contralateral side acting as control in a blinded fashion (A Gene Transfer Therapy Study to Evaluate the Safety of SRP-9004 [Patidistrogene Bexoparvovec] in Participants With Limb-Girdle Muscular Dystrophy, Type 2D [LGMD2D], NCT01976091).¹² Gene expression in all but 1 patient was increased significantly at the 3 time points of biopsy (6 weeks, 12 weeks, and 6 months).¹²

LGMD2E/R4 carries the highest incidence of cardiac involvement of all sarcoglycanopathies, which is an important consideration when pursuing treatment delivery and tissue tropism strategies. Preclinical murine data exist with both intramuscular injection and intravenous administration, showing encouraging proof-of-concept data in histopathology, creatine kinase levels, diaphragm force production, and kyphoscoliosis reduction.¹²

Dystroglycan Glycosylation

LGMD2I/R9 results from variants in the FKRP gene, encoding the fukutin-related protein FKRP. FKRP glycosylates a-dystroglycan (matriglycan) and is one of the most common LGMD, with significant phenotypic variation, including mild LGMD, severe congenital muscular dystrophy, Walker-Warburg syndrome, and muscle-eye-brain disease. Hypoglycosylation of a-dystroglycan—which interacts with extracellular matrix proteins, thus stabilizing the muscle cell membrane—leads to reduced muscle integrity and thus muscle weakness. FKRP uses CDP-ribitol as a substrate to add ribitol-5-phosphate to a-dystroglycan. ISPD protein synthesizes CDP-ribitol, and treating ISPD-deficient cells with ribitol can increase CDP-ribitol levels and partially correct matriglycan synthesis.¹³ Considering this biochemical pathway, FKRP mouse models have shown increased matriglycan expression and improved muscle function with ribitol treatment by ingestion with water.¹⁴ Organization of a phase 3 clinical trial is underway. Additional therapeutic strategies under development are summarized in the Table.

Conclusion

Treatment for muscular dystrophies has evolved rapidly over the past decade as an increasing number of targeted disease-modifying therapies has entered the clinical arena, with many more therapies on the horizon. People with inherited muscular conditions need guidance through the diagnostic process to a genetic diagnosis. Supportive, multidisciplinary care is critical. Prevention and minimization of comorbidities, such as contractures, osteopenia, fractures, respiratory failure, and cardiomyopathy, remains paramount to allow for the maximum achievement of daily function and optimal outcomes as disease burden is lessened. Documentation of the natural history of rare conditions will continue to require attention as promising innovations require careful evaluation in well-designed studies. Better outcomes and improved function are emerging for people with muscular dystrophy.