Background: From mitochondrial function to organelle contacts

A study published in PNAS reported that delivery of the normal MFN2 gene restored endoplasmic reticulum (ER)–mitochondria contacts and preserved motor function in a mouse model of Charcot–Marie–Tooth disease type 2A (CMT2A). The researchers assessed the therapeutic potential of increased normal MFN2 expression in transgenic mice and induced pluripotent stem cell (iPSC)-derived motor neurons. This was a preclinical proof-of-concept study, not a clinical trial demonstrating efficacy in patients.

CMT2A is the most common axonal form of Charcot–Marie–Tooth disease and is characterized by early onset and severe motor neuropathy. It is primarily caused by dominant variants in the MFN2 gene. Mitofusin 2 (MFN2), encoded by this gene, is a GTPase located on the outer mitochondrial membrane and in the ER; MFN2 variants disrupt mitochondrial dynamics.

In earlier work, the researchers reported that mutant MFN2 also disrupts contact between the ER and mitochondria, leading to axonal degeneration. This study asked whether restoring contact between the two organelles could be a therapeutic strategy. The paper’s abstract states that no treatment for CMT2A was available at the time and that approaches under development focused mainly on restoring mitochondrial function.

Key findings: Normal MFN2 delivery and contact restoration

The researchers increased normal MFN2 expression in neurons. Beneficial effects were reported in CMT2A transgenic mice and CMT2A iPSC-derived motor neurons. However, the provided abstract does not specify characteristics of the cell donors, the types of MFN2 variants, or sample sizes.

In the mouse experiments, an adeno-associated virus serotype 9 (AAV9) vector expressing normal MFN2 was administered intrathecally. The vector was delivered to target motor and sensory neurons, and ER–mitochondria contacts and mitochondrial morphology were restored. The researchers also reported preservation of the structural integrity of neuromuscular junctions, where nerves connect to muscles, and of motor function.

Effects were also observed when the vector was administered after symptoms appeared. This supports the possibility of intervention after symptom onset, but does not mean that existing nerve damage in patients with advanced disease was reversed. The abstract alone does not allow the magnitude or duration of effects at different treatment times to be assessed.

The researchers also tested an ER-targeted isoform of mitofusin 1 (MFN1). This approach, which selectively enhances ER–mitochondria contacts, was also reported to correct CMT2A pathology in vitro and in vivo. This finding supports the therapeutic potential of restoring organelle contacts through an approach distinct from increasing normal MFN2 expression.

Meaning and outlook: The limits of preclinical efficacy and safety

These results provide a basis for considering ER–mitochondria contacts as a target for CMT2A gene therapy. The study reported not only changes at the organelle level but also preservation of neuromuscular junctions and motor function. However, the available information does not establish the relative effects of the two approaches or their superiority over existing treatment candidates.

According to the abstract, AAV administration was well tolerated in the experiments, with no evidence of hepatotoxicity or dorsal root ganglion inflammation. This should not be taken to mean that safety in humans or long-term safety has been established. Dose, control groups, statistical analyses, follow-up duration, and other toxicity measures require review of the full paper and supplementary materials.

Further development will need to establish how long contact restoration and functional preservation last, and under what conditions efficacy and toxicity differ. The current evidence is limited to supporting the potential of a preclinical therapeutic strategy. This account of the research is based on the provided Europe PMC bibliographic record and abstract.