Could Charcot-Marie-Tooth (CMT) disease one day be simpler to treat than diabetes? That's the best-case interpretation of results from a small pilot study in which patients with CMT1A who received thrice-weekly injections of neurotrophin-3 improved over the course of six months, while those receiving placebo did not. The study, published in Neurology, is backed up by strong preclinical results as well, and fits well with an understanding of the critical role Schwann cells play in nerve regeneration.
ANIMAL MODELS SUPPORT NT-3 TREATMENT
CMT1A is the most common form of Charcot-Marie-Tooth disease. It is an autosomal dominant disorder caused by a mutation or gene duplication in the PMP22 gene, which is expressed in Schwann cells and encodes the peripheral myelin protein-22. Abnormal amounts of the protein cause a delay of myelination, especially of large-diameter axons, impairing their ability to grow and to regenerate after damage.
But Schwann cells provide more than just myelin to developing axons. They also secrete growth factors, including neurotrophin-3 (NT-3), which promotes axonal regeneration.
According to Zarife Sahenk, MD, PhD, “If you could somehow make the nerve grow more efficiently, you might slow progression of the disease.” Dr. Sahenk, who led the new study, is Professor of Neurology at the Ohio State University College of Medicine.
Supporting these theoretical arguments are animal experiments, many of them done by Dr. Sahenk, which have provided good reasons to think NT-3 might be useful in treating CMT1A patients. “We postulated that, if we supplied neurotrophin-3 exogenously, Schwann cells would survive longer, and allow axons to differentiate properly,” said Dr. Sahenk.
The first test of the hypothesis was performed in “nude” mice, where lack of an immune system allows the animals to accept a small graft of human CMT1A sural nerve. Healthy axons from the mouse have difficulty growing through the mutated transplant with defective human Schwann cells. When treated with NT-3, though, the axons showed “robust growth,” said Dr. Sahenk, with more than 100 percent increase in the density of myelinated fibers.
Neurofilaments in the axonal tip, which become dense in the absence of Schwann cell interaction, also became more normal with NT-3 treatment. These results from an admittedly “quick and dirty” experiment “were exciting,” she said.
She then turned to the Trembler-J mouse model of CMT1A, in which the PMP22 gene has a point mutation similar to that of the human disease. This mutation was discovered in the early 1990s in an inbred line of mice with a defect in myelination.
“This is the only naturally occurring model of the disease,” said Dr. Sahenk. “Pathologically it is not so similar, but it is a genetic match.” In these mice, peripheral nerve regeneration in response to crush injury is severely impaired. NT-3 treatment led to a significant increase in the density of both Schwann cells and myelinated fibers after 12 weeks of treatment.
A PILOT TRIAL
These promising results set the stage for a pilot clinical trial in eight CMT1A patients. The patients in this trial had PMP22 duplications, rather than point mutations. Gene duplications are the most common cause of CMT1A, accounting for over 70 percent of patients. The patients were randomized to receive either placebo or up to 150 micrograms per kilogram NT-3 by subcutaneous injection, three times per week, for 24 weeks. The chosen dose was near the middle of the safe dose range, as established in a large clinical trial of healthy volunteers. Regeneron Pharmaceuticals, the manufacturer of NT-3, supported the study.
Fascicular sural nerve biopsies were obtained before and after treatment, and patients were evaluated with clinical measures that included the Neuropathy Impairment Score, electrophysiologic measurements, manual muscle testing, and pegboard performance.
At the end of the treatment, the Neuropathy Impairment Score of NT-3-treated patients had fallen by about 30 percent, while that of patients receiving placebo increased by 5 percent (p = 0.0041).
The main driver of the improvement was change in sensory and reflex function, with no change in motor performance or muscle strength. Dr. Sahenk said she was not surprised by the lack of improvement in these two areas.
“I think the six-month period is a short time” to see such improvement, she said. On the other hand, the gain in reflex function “was a total surprise.” There was no change in conduction velocity, but this too was not unexpected, since “this is not necessarily a function of recovery,” she said. Pegboard performance worsened in placebo-treated patients, but remained the same in those receiving NT-3.
Examination of the biopsies showed a significant increase in the mean number of myelinated fibers in patients receiving NT-3, but not placebo, and an increase in the mean number of myelinated fibers within each regeneration unit (p < 0.0001). NT-3 treated patients also had a significant increase in Schwann cell density, while placebo-treated patients experienced a decrease.
The most common adverse effects from treatment were diarrhea and injection-site irritation, both of which had been seen previously in a large safety trial in normal controls.
BENEFICIAL EFFECTS AFTER TREATMENT STOPS
Dr. Sahenk noted that there were no data indicating how long the beneficial effects last after stopping treatment, or whether prolonging treatment might lead to further improvement. “We don't know what happens – that's why it is important to launch a larger clinical trial,” she said. “This is really the first pilot trial of a neurotrophic factor in CMT. The results are encouraging, but they have to be substantiated with a larger trial.”
The investigators were interested in the potential of NT-3 because the neurotrophic factor is critical to both the survival and differentiation of Schwann cells.
Other research had also shown the potential of NT-3 for the treatment of peripheral neuropathies. For example, in one animal study, NT-3 seemed to benefit experimental diabetic and acrylamide neuropathies (Hum Gene Ther 2001;12(18):2237-2249).
EXAMINE OTHER ANIMAL MODELS
James Lupski, MD, PhD, agrees that the results are encouraging, despite the small size of the trial. Even though the treatment doesn't address the underlying genetic defect, he said, “a lot of what we do in medicine is to treat the downstream consequences,” and in some instances, “we can have dramatic effects on patients' lives.” Dr. Lupski is Professor of Molecular and Human Genetics at Baylor College of Medicine, and is a leading authority on CMT.
However, he pointed out, testing the same treatment in a different animal model, before further human trials are done, might be more informative. There are several excellent rodent models of gene overexpression. Since so many CMT1A patients have this form of the disease, and there is a good genetic test to identify it, “there is real incentive to deal with that specific form,” he said.
The working hypothesis of CMT1A molecular pathophysiology is that protein accumulates in the endoplasmic reticulum of the Schwann cell, which can't be broken down and which impairs cell function.
Dr. Lupski said that Dr. Sahenk's results point the way to “a lot of good animal studies that could be done,” to determine whether NT-3 is affecting protein accumulation, or acting through some other means to improve Schwann cell function.
ARTICLE IN BRIEF
✓ In a small pilot trial, patients with CMT1A who received thrice-weekly injections of neurotrophin-3 improved over the course of six months, while those receiving placebo did not.