Gene replacement therapy for centronuclear myopathy: A breakthrough in complex genetic muscle disease

Main text

Centronuclear myopathy (CNM) is a congenital myopathy characterized by muscle weakness accompanied by abnormal positioning of cell structures in skeletal muscle fibers. For decades, CNM has been considered essentially untreatable, with pathological mechanisms not fully understood and no effective therapeutic interventions available. In this issue of Molecular Therapy, Ji and colleagues demonstrated that adeno-associated virus (AAV)-mediated bridging integrator 1 (BIN1) gene replacement can prevent and reverse disease phenotypes in a mouse model of BIN1-related CNM.¹ The treatment restored major CNM hallmarks, including defects in motor function, muscle strength, and cellular organization, validating gene replacement as a potential therapeutic option for future trials in humans.

The challenge of CNMs

CNM is a hereditary muscular disorder diagnosed by clinical features of congenital myopathy with pathological characteristics of centralized nuclei in skeletal muscle biopsy.^2,3 Seven causative genes have been identified so far: MTM1, SPEG, BIN1, DNM2, RYR1, TTN, and CCDC78. Among these genes, BIN1 encodes a protein called BIN1 (also known as amphiphysin 2) that senses and creates membrane curvature.⁴

The BIN1 protein serves multiple essential functions in mature skeletal muscle. It acts as a membrane sculptor that generates T-tubules, specialized membrane invaginations critical for excitation-contraction coupling.⁵ BIN1 contains an N-terminal BIN/amphiphysin/Rvs (N-BAR) domain for membrane curvature, a central PI domain that binds phosphoinositide PI(4)P and PI(4,5)P₂ in a muscle-specific isoform (mBIN1), and a C-terminal Src homology 3 (SH3) domain that mediates protein-protein interactions. BIN1 also binds and regulates membrane fission activity of dynamin 2 during T-tubule biogenesis.^6,7 The autosomal recessive and dominant forms of BIN1-related CNM manifest during childhood and adulthood, respectively, and they are associated with progressive muscle weakness.^8,9

The multifunctional nature of BIN1 has made BIN1-CNM a challenging therapeutic target, as successful treatment requires restoration of BIN1’s membrane remodeling, phosphoinositide binding, and protein-interacting functions. Gene replacement therapy represents an attractive strategy despite the technical challenges of delivering complex multifunctional proteins in skeletal muscle. The absence of effective treatments for CNM has left this condition as a therapeutic orphan disease.

Comprehensive disease prevention and reversal through gene replacement

Given these therapeutic challenges, the Ji study used a well-characterized Bin1^mck−/− mouse model that recapitulates the key features of human BIN1-CNM, including muscle weakness, altered fiber morphology, disrupted T-tubule organization, and impaired excitation-contraction coupling.¹⁰ Using this model, the researchers demonstrated that delivery of BIN1 using AAV could achieve remarkable therapeutic efficacy, whether administered preventively or after disease onset.

The scope of functional improvement was particularly impressive. Treated mice showed normalization of motor performance, restoration of muscle force generation, and correction of cellular abnormalities, including centralized nuclei, mitochondrial positioning, and fiber size variability. Immunofluorescence and electron microscopic analyses revealed restoration of proper T-tubule network and architecture. Strikingly, kyphosis in the mouse model was also corrected by intravenous injection of AAV-mBIN1 after disease onset, indicating that gene replacement addressed fundamental cellular defects rather than merely improving muscle contractility.

The study revealed that successful therapy required delivery of the full-length BIN1, especially the muscle-specific BIN1 isoform (mBIN1), rather than isolated protein domains. The isolated N-BAR domain did not rescue any CNM hallmarks, whereas the N-BAR+PI domains ameliorated muscle force and T-tubule network defects, while the isolated SH3 improved myofiber size, muscle mass, and mitochondrial positioning. Only the complete muscle-specific BIN1 provided comprehensive therapeutic benefit. Using in cellulo reconstitution assays for T-tubule-like structures,⁶ the authors demonstrated that BAR+PI domains are essential for membrane tubulation, while the SH3 domain is necessary for recruitment of dynamin 2 and regulation of its membrane fission activity. This finding highlights the importance of maintaining the protein’s integrated functionality for effective disease intervention that challenges conventional approaches targeting individual protein functions.

The researchers also demonstrated dose-response relationships and compared two AAV vectors, finding that novel myotropic AAV serotype MyoAAV4A showed superior efficacy and reduced off-target effects compared to AAV9, which is more commonly used in clinical trials for neuromuscular diseases. This vector optimization represents important progress given recent safety concerns, such as hepatotoxicity with AAV-based therapies in clinical trials.

Significance and clinical translation challenges

These remarkable preclinical results raise important questions about broader applications and the path to human trials. The demonstration that a complex, multifunctional protein can be effectively replaced through gene therapy challenges previous assumptions about treating diseases involving proteins with diverse cellular roles. The normalization of molecular pathways, including autophagy markers and excitation-contraction coupling proteins (DHPR and RYR1), suggests that BIN1 replacement addresses root pathological mechanisms rather than providing symptomatic relief.

However, several considerations will be important for clinical translation. The study focused exclusively on complete BIN1 deficiency, while human patients present with a spectrum of mutations, including missense variants that may produce dominant-negative effects or a partial loss of function. The therapeutic efficacy in these more complex genetic backgrounds requires additional preclinical validation. Furthermore, vector selection and dosing optimization represent challenges for human application. While MyoAAV4A showed advantages over AAV9 in the mouse CNM model, human muscle targeting may require different considerations due to species differences. Recent hepatotoxicity concerns with high-dose AAV therapies emphasize the importance of comprehensive safety evaluation.

Patient selection will also require careful consideration. The timing of intervention may be critical, as advanced muscle damage could limit responsiveness to therapy. Development of appropriate biomarkers for monitoring treatment response and establishing efficacy endpoints will be essential for clinical design.

Broader implications for rare disease therapeutics

The success of BIN1 gene replacement therapy may enable similar approaches for CNM caused by MTM1, DNM2, RYR1, and other genes, as well as diseases, including various cancers, late-onset Alzheimer’s disease, and cardiac arrhythmias, where BIN1 dysfunction is implicated. The methodological advances demonstrated in this study, including the systematic comparison of protein isoforms and optimization of muscle-targeting vectors, provide a roadmap for tackling other complex genetic diseases where complete protein replacement is required.

More broadly, this work contributes to evolving models in rare disease therapeutics that emphasize precision approaches designed to specific genetic mechanisms rather than symptomatic treatments. The demonstration that comprehensive disease reversal is achievable through gene replacement therapy, even for proteins with multiple cellular functions, expands the therapeutic target space and opens possibilities for genetic diseases previously considered intractable.

The study also highlights the importance of rigorous preclinical validation using disease models that accurately recapitulate human pathology. The Bin1^mck−/− model’s fidelity to human BIN1-CNM strengthens confidence in the translational relevance and sets a high standard for rare disease therapeutic development.

Conclusion

This work provides convincing proof of concept for gene therapy approaches to complex genetic muscle diseases and represents a potential paradigm shift in treating multifunctional protein disorders. Important questions remain regarding optimal vector selection, clinical stratification, and the applicability of gene replacement strategies for the spectrum of human BIN1 mutations. Nevertheless, the study represents a significant advance and offers hope for patients with these devastating conditions.

As the field moves toward clinical testing, the comprehensive preclinical foundation established by Ji and colleagues provides a strong platform for advancing BIN1-CNM gene therapy while serving as a proof of concept for similar approaches to genetic muscle disorders. Although the path from preclinical results to clinical benefit remains challenging, this work demonstrates that complex genetic diseases involving multifunctional proteins may be treatable through innovative approaches.

Declaration of interests

The author declares no competing interests.