Metabolic Myopathies in the Era of Next-Generation Sequencing

The utility of electrodiagnostic testing in unprovoked rhabdomyolysis in the era of next-generation sequencing

INTRODUCTION/AIMS

Rhabdomyolysis is an etiologically heterogeneous, acute necrosis of myofibers characterized by transient marked creatine kinase (CK) elevation associated with myalgia, muscle edema, and/or weakness. The study aimed to determine the role of electrodiagnostic (EDX) testing relative to genetic testing and muscle biopsy in patients with unprovoked rhabdomyolysis in identifying an underlying myopathy.

METHODS

EDX database was reviewed to identify unprovoked rhabdomyolysis patients who underwent EDX testing between January 2012 and January 2022. Each patient's clinical profile, EDX findings, muscle pathology, laboratory, and genetic testing results were analyzed.

RESULTS

Of 66 patients identified, 32 had myopathic electromyography (EMG). Muscle biopsy and genetic testing were performed in 41 and 37 patients, respectively. A definitive diagnosis was achieved in 15 patients (11 myopathic EMG and 4 nonmyopathic EMG; p = .04) based on abnormal muscle biopsy (4/11 patients) or genetic testing (12/12 patients, encompassing 5 patients with normal muscle biopsy and 3 patients with nonmyopathic EMG). These included seven metabolic and eight nonmetabolic myopathies (five muscular dystrophies and three ryanodine receptor 1 [RYR1]-myopathies). Patients were more likely to have baseline weakness (p < .01), elevated baseline CK (p < .01), and nonmetabolic myopathies (p = .03) when myopathic EMG was identified.

DISCUSSION

Myopathic EMG occurred in approximately half of patients with unprovoked rhabdomyolysis, more likely in patients with weakness and elevated CK at baseline. Although patients with myopathic EMG were more likely to have nonmetabolic myopathies, nonmyopathic EMG did not exclude myopathy, and genetic testing was primarily helpful to identify an underlying myopathy. Genetic testing should likely be first-tier diagnostic testing following unprovoked rhabdomyolysis.

Metabolic aspects of glycogenolysis with special attention to McArdle disease

The physiological function of muscle glycogen is to meet the energy demands of muscle contraction. The breakdown of glycogen occurs through two distinct pathways, primarily cytosolic and partially lysosomal. To obtain the necessary energy for their function, skeletal muscles utilise also fatty acids in the β-oxidation. Ketogenesis is an alternative metabolic pathway for fatty acids, which provides an energy source during fasting and starvation. Diseases arising from impaired glycogenolysis lead to muscle weakness and dysfunction. Here, we focused on the lack of muscle glycogen phosphorylase (PYGM), a rate-limiting enzyme for glycogenolysis in skeletal muscles, which leads to McArdle disease. Metabolic myopathies represent a group of genetic disorders characterised by the limited ability of skeletal muscles to generate energy. Here, we discuss the metabolic aspects of glycogenosis with a focus on McArdle disease, offering insights into its pathophysiology. Glycogen accumulation may influence the muscle metabolic dynamics in different ways. We emphasize that a proper treatment approach for such diseases requires addressing three important and interrelated aspects, which include: symptom relief therapy, elimination of the cause of the disease (lack of a functional enzyme) and effective and early diagnosis.

Gene therapy for primary myopathies: literature review and prospects

Gene therapy has emerged as a promising frontier in the pursuit of effective treatments for primary myopathies. This scientific review explores the application of viral vectors and more specifically of recombinant adeno-associated virus (rAAV) vectors as a potent gene delivery tool in the context of primary myopathies, highlighting its transformative potential. Focusing on primary myopathies, including Duchenne muscular dystrophy (DMD), limb-girdle muscular dystrophies (LGMDs), X-linked myotubular myopathy (XLMTM), and Pompe disease, we review the ongoing pre-clinical and clinical trials that underscore the therapeutic promise of rAAV-based gene therapies. Recent developments in gene therapy have unveiled innovative gene transfer approaches, particularly with rAAV vectors. These vectors offer a well-tolerated and efficient means of delivering corrective genetic material to diseased muscles, thereby addressing the root causes of primary myopathies. Encouraging data from pre-clinical studies and early clinical trials have demonstrated the potential to ameliorate muscle function, reduce pathological manifestations, and enhance the quality of life for patients afflicted with these devastating diseases. However, the transition from bench to bedside is not without challenges. This review emphasizes the critical need for a comprehensive risk management strategy to better handle potential side effects and immune responses associated with gene therapy. As the field of gene therapy for primary myopathies is advancing, it is imperative to refine and optimize safety measures, ensuring that the transformative potential of these therapies is realized while the risks are minimized. © 2023 Published by Elsevier Masson SAS on behalf of French Society of Pediatrics.