Cell-targeted Gene Therapy May Reverse Nerve Damage in CMTX1

A single dose of a gene therapy delivering a working copy of GJB1 — the gene mutated in X-linked Charcot-Marie-Tooth type 1 disease (CMTX1) — to myelin-producing cells prevented or even reversed myelin loss and motor function impairment in a mouse model of CMTX1.

Notably, these benefits were observed not only when the therapy was given before symptom onset, but also at later stages of the disease.

According to researchers, these results highlight the therapeutic potential of this approach for CMTX1 patients in preventing or reversing the loss of myelin, the protective coating around nerve cells.

However, further studies in larger animal models are needed to confirm these findings before moving to clinical trials, they noted.

The study, “AAV9-mediated Schwann cell-targeted gene therapy rescues a model of demyelinating neuropathy,” was published in the journal Gene Therapy.

CMT affects the peripheral nervous system or PNS, the network of nerves that transmit signals between the central nervous system — composed of the brain and spinal cord — and the rest of the body.

Accounting for nearly 10% of all CMT cases, CMTX1 is caused by the absence of a working connexin-32 (Cx32) protein due to mutations in the GJB1 gene. Cx32 is essential for the function of Schwann cells, which are involved in the production and maintenance of the myelin sheath around nerve cell fibers of the PNS.

This protective layer, which is known to aid in nerve signal transmission, is progressively lost in the peripheral nervous system of people with CMT, affecting nerve cell communication and causing muscle weakness and shrinkage.

By delivering a working copy of a mutated, disease-causing gene to cells, gene therapy has emerged as a potential CMT treatment.

Now, researchers in Cyprus and the U.K. evaluated the therapeutic benefits of a Schwann cell-targeted gene therapy, called AAV9-Mpz.GJB1, in a mouse model of CMTX1 before and after symptom onset.

The single-dose therapy uses a modified and harmless adeno-associated virus (AAV9) to deliver a healthy copy of GJB1 to cells, ensuring that only Schwann cells are able to produce the Cx32 protein.

Given that these mice typically start developing disease features around the age of 3 months, AAV9-Mpz.GJB1 was given directly into their spinal canal either at 2 months (prior to disease onset) or at 6 months (later disease stage). The spinal canal contains cerebrospinal fluid, the liquid that surrounds the brain and spinal cord.

All treated mice were monitored for four months after treatment.

Results showed that mice given AAV9-Mpz.GJB1 had a widespread distribution of the virus in the peripheral nervous system. The animals also showed high levels of Cx32 specifically in myelin-producing Schwann cells (about 60% of them) around peripheral nerves.

“This study provides the proof of principle that [spinal canal] injection of AAV9, a well-established vector for clinical applications, provides widespread biodistribution in the PNS and efficient [targeting] of myelinating Schwann cells,” the researchers wrote.

In addition, motor function was continuously and significantly improved over time in both pre- and post-onset treatment groups, when compared with untreated mice.

At four months post-treatment, mice from both groups had reached motor performances comparable to that of age-matched healthy mice. However, signal transmission along nerve fibers was completely rescued only in mice treated before symptom onset.

Further analyses revealed a significant drop in the number of abnormally myelinated fibers and of pro-inflammatory immune cells — both found at high levels in the untreated mouse model.

Also, the blood levels of two biomarkers of nerve cell damage — neurofilament light chain (NfL) protein and NCAM-1 — were significantly reduced in mice treated at later stages of the disease, approaching those of healthy mice.

These findings highlighted the therapy’s potential to prevent and even reverse disease features by reducing myelin loss and nerve cell damage. While AAV9-Mpz.GJB1 was effective even after disease onset, “early intervention may provide a bigger therapeutic benefit,” the researchers wrote.

Notably, “restoring Cx32 function in at least 50% of myelinating Schwann cells in PNS tissues may be sufficient to provide significant functional and morphological improvement in this model and potentially also in CMT1X patients,” they added.

The team also noted that the results suggested that this AAV9-based gene therapy was more effective than a similar approach that they had previously developed and tested based on another type of virus, called lentivirus.

The data also support the use of NfL and NCAM-1 as biomarkers to measure treatment response in CMTX1 patients. However, since this was the first study showing high NCAM-1 levels in this mouse model, further studies in CMTX1 patients are needed, the team noted.

“Evaluation of this approach in larger animal models would be valuable in order to confirm adequate biodistribution [distribution in the body] and safety facilitating the path towards clinical translation for treating CMT1X patients,” the researchers wrote.