Satellite cells deficiency and defective regeneration in dynamin 2-related centronuclear myopathy

[The dynamin-2-gene related centronuclear myopathy]

Autosomal dominant centronuclear myopathy (AD-CNM) is a rare congenital myopathy characterized by muscle weakness and centrally located nuclei in muscle fibers in the absence of any regeneration. AD-CNM is due to mutations in the DNM2 gene encoding dynamin 2 (DNM2), a large GTPase involved in intracellular membrane trafficking and a regulator of actin and microtubule cytoskeletons. DNM2 mutations are associated with a broad clinical spectrum ranging from severe neonatal to less severe late-onset forms. The histopathological signature includes nuclear centralization, predominance and atrophy of type 1 myofibers and radiating sarcoplasmic strands. To explain the muscle dysfunction, several pathophysiological mechanisms affecting key mechanisms of muscle homeostasis have been identified. They include defects in excitation-contraction coupling, muscle regeneration, mitochondria or autophagy. Several therapeutic approaches are under development by modulating the expression of DNM2 in a pan-allelic manner or by allele-specific silencing targeting only the mutated allele, which open the era of clinical trials for this pathology.

Development of versatile allele-specific siRNAs able to silence all the dominant dynamin 2 mutations

Dominant centronuclear myopathy (CNM) is a rare form of congenital myopathy associated with a wide clinical spectrum, from severe neonatal to milder adult forms. There is no available treatment for this disease due to heterozygous mutations in the DNM2 gene encoding Dynamin 2 (DNM2). Dominant DNM2 mutations also cause rare forms of Charcot-Marie-Tooth disease and hereditary spastic paraplegia, and deleterious DNM2 overexpression was noticed in several diseases. The proof of concept for therapy by allele-specific RNA interference devoted to silence the mutated mRNA without affecting the normal allele was previously achieved in a mouse model and patient-derived cells, both expressing the most frequent DNM2 mutation in CNM. In order to have versatile small interfering RNAs (siRNAs) usable regardless of the mutation, we have developed allele-specific siRNAs against two non-pathogenic single-nucleotide polymorphisms (SNPs) frequently heterozygous in the population. In addition, allele-specific siRNAs against the p.S619L DNM2 mutation, a mutation frequently associated with severe neonatal cases, were developed. The beneficial effects of these new siRNAs are reported for a panel of defects occurring in patient-derived cell lines. The development of these new molecules allows targeting the large majority of the patients harboring DNM2 mutations or overexpression by only a few siRNAs.

Centronuclear Myopathy Caused by Defective Membrane Remodelling of Dynamin 2 and BIN1 Variants

Centronuclear myopathy (CNM) is a congenital myopathy characterised by centralised nuclei in skeletal myofibers. T-tubules, sarcolemmal invaginations required for excitation-contraction coupling, are disorganised in the skeletal muscles of CNM patients. Previous studies showed that various endocytic proteins are involved in T-tubule biogenesis and their dysfunction is tightly associated with CNM pathogenesis. DNM2 and BIN1 are two causative genes for CNM that encode essential membrane remodelling proteins in endocytosis, dynamin 2 and BIN1, respectively. In this review, we overview the functions of dynamin 2 and BIN1 in T-tubule biogenesis and discuss how their dysfunction in membrane remodelling leads to CNM pathogenesis.

Benefits of therapy by dynamin-2-mutant-specific silencing are maintained with time in a mouse model of dominant centronuclear myopathy

Dominant dynamin 2 (DNM2) mutations are responsible for the autosomal dominant centronuclear myopathy (AD-CNM), a rare progressive neuromuscular disorder ranging from severe neonatal to mild adult forms. We previously demonstrated that mutant-specific RNA interference is an efficient therapeutic strategy to rescue the muscle phenotype at the onset of the symptoms in the AD-CNM knockin-Dnm2 ^R465W/+ mouse model. Our objective was to evaluate the long-term benefit of the treatment along with the disease time course. We demonstrate here that the complete rescue of the muscle phenotype is maintained for at least 1 year after a single injection of adeno-associated virus expressing the mutant-specific short hairpin RNA (shRNA). This was achieved by a maintained reduction of the mutant Dnm2 transcript. Moreover, this long-term study uncovers a pathological accumulation of DNM2 protein occurring with age in the mouse model and prevented by the treatment. Conversely, a physiological DNM2 protein decrease with age was observed in muscles from wild-type mice. Therefore, this study highlights a new potential pathophysiological mechanism linked to mutant protein accumulation and underlines the importance of DNM2 protein expression level for proper muscle function. Overall, these results strengthen the allele-specific silencing approach as a robust, safe, and efficient therapy for AD-CNM.

Common Pathogenic Mechanisms in Centronuclear and Myotubular Myopathies and Latest Treatment Advances

Centronuclear myopathies (CNM) are rare congenital disorders characterized by muscle weakness and structural defects including fiber hypotrophy and organelle mispositioning. The main CNM forms are caused by mutations in: the MTM1 gene encoding the phosphoinositide phosphatase myotubularin (myotubular myopathy), the DNM2 gene encoding the mechanoenzyme dynamin 2, the BIN1 gene encoding the membrane curvature sensing amphiphysin 2, and the RYR1 gene encoding the skeletal muscle calcium release channel/ryanodine receptor. MTM1, BIN1, and DNM2 proteins are involved in membrane remodeling and trafficking, while RyR1 directly regulates excitation-contraction coupling (ECC). Several CNM animal models have been generated or identified, which confirm shared pathological anomalies in T-tubule remodeling, ECC, organelle mispositioning, protein homeostasis, neuromuscular junction, and muscle regeneration. Dynamin 2 plays a crucial role in CNM physiopathology and has been validated as a common therapeutic target for three CNM forms. Indeed, the promising results in preclinical models set up the basis for ongoing clinical trials. Another two clinical trials to treat myotubular myopathy by MTM1 gene therapy or tamoxifen repurposing are also ongoing. Here, we review the contribution of the different CNM models to understanding physiopathology and therapy development with a focus on the commonly dysregulated pathways and current therapeutic targets.