Restoring the interplay between the endoplasmic reticulum and mitochondria by gene therapy improves Charcot-Marie-Tooth type 2A disease.

Charcot-Marie-Tooth disease type 2A (CMT2A) is the most common axonal CMT and is associated with an early onset and severe motor neuropathy. CMT2A is mainly caused by dominant mutations in the MFN2 gene, encoding mitofusin-2, a GTPase located in the outer membrane of the mitochondria and endoplasmic reticulum (ER). Mutations in MFN2 affect mitochondrial dynamics. We previously demonstrated that mutated MFN2 further disrupts contacts between the ER and the mitochondria, leading to axonal degeneration. There are no treatments for CMT2A, and those currently under development primarily focus on restoring mitochondrial function. Here, we provide proof of concept that neuronal overexpression of wild-type MFN2 (MFN2WT) provides therapeutic benefit in transgenic CMT2A mice as well as in CMT2A-motor neurons derived from induced pluripotent stem cells. Intrathecal delivery of an AAV9 vector expressing MFN2WT effectively targets motor and sensory neurons, restoring ER-mitochondria contacts and mitochondrial morphology, thereby preserving both neuromuscular junction integrity and motor function. Strikingly, therapeutic efficacy is also achieved by administering the vector after the onset of symptoms. Importantly, AAV administration was well tolerated, with no evidence of hepatotoxicity or dorsal root ganglion inflammation. We further show that CMT2A pathology can be corrected in vitro and in vivo using an ER-targeting MFN1 isoform that selectively enhances ER-mitochondria contacts. These results establish that restoring contacts between the ER and mitochondria using gene therapy is a promising therapeutic avenue for CMT2A.