By David N. Leff

When a researcher detects an effect, but can't quite determine its cause, a convenient stop-gap word is "factor."

In neuroscience, the number of neurotrophic factors is large, and still counting. Among the front-runners: brain-derived neurotrophic factor (BDNF), ciliary neurotrophic factor (CNTF), fibroblast growth factor (FGF) family, glial cell-derived neurotrophic factor, insulin-like growth factor (ILGF), leukemia inhibitory factor (LIF) and neurotrophin-3 (NT-3), -4 and -5.

Nerves need factors like these to be born, grow and stay healthy. So to treat neurons that lose their myelin insulation or die off in neurodegenerative diseases, neuroscientists are experimenting with these neurotrophic factors as restorative agents.

Lou Gehrig's disease * amyotrophic lateral sclerosis (ALS) * is one such candidate for neurotrophic-factor therapy to halt or slow the die-back of motor neurons that marks ALS.

This neuromuscular affliction affects 70,000 people worldwide, half in the U.S. It leads inexorably to death from respiratory failure three to five years after onset of the disorder.

Clinical trials with recombinant CNTF protein, injected systemically into ALS patients, have been disappointing. Because of its short plasma half-life, the factor rapidly degraded and little of it reached its motor neuron target, while conferring severe side effects.

In France, molecular geneticist Axel Kahn and his associates reasoned that delivering the gene expressing a neurotrophic factor to the muscles innervated by the wasting motor neurons of ALS might offer a better shot.

This month's Nature Medicine, dated April 1997, reports their first attempt at this approach. Its title: "Gene therapy of murine motor neuron disease using adenoviral vectors for neurotrophic factors." Kahn chairs the Cochin Institute of Molecular Genetics at INSERM (France's National Institute of Health and Medical Research), in Paris.

As a natural in vivo model of the human disease, Kahn and his co-authors chose the pmn mouse, so named for its inborn progressive motor neuropathy. This is an autosomal recessive genetic disease. The pmn gene, still unknown, maps to murine chromosome 13.

From about two weeks of age, this spontaneously mutated rodent begins to drag its hindquarters; the paralysis then moves upward along the body's musculature to the forelimbs, and kills the mouse five weeks later.

Gene Therapy Vehicle, Payload, Target

Choosing neurotrophin-3 (NT-3) as their first therapeutic factor, the team hitched the complementary DNA sequence that expresses NT-3 to a non-replicating adenovirus vector and injected this plasmid into three muscle sites in neonatal pmn mice.

"The efficacy of in vivo adenoviral gene therapy with neurotrophic factors has never been established in any model of motor neuron degeneration," Kahn's paper observed.

The efficacy they reported took several forms: It increased survival of the treated animals by 50 percent over untreated control animals, enhanced reinnervation of muscle fibers, reduced the loss of myelinated fibers and strengthened neuromuscular function.

Encouraged by these preliminary results, Kahn's group then added the cDNA for a second neurotrophic factor, CNTF, to their plasmid, and reaped synergistic benefits. The dual gene therapy package increased maximum life span beyond that obtained by either NT-3 or CNTF alone. "These findings," they note, "stress the therapeutic potential of coadministering neurotrophic factors belonging to different families."

That's precisely what they are planning next. "Now we are testing different combinations of neurotrophic factors," Kahn told BioWorld Today. "Also, we are comparing the results obtained by intramuscular injection with other ways, such as the intravenous route.

"The other direction in which we are working," he added, "is to change the viral vector, because it's clear that if we are willing to demand a clinical trial, the third-generation viral vectors won't work, because of their intrinsic immunogenicity." Among alternatives his group is "also constructing new adenovirus-associated vectors."

Armed with these new delivery vehicles and gene payloads, they expect "probably this year" to test their gene therapy strategy in a breed of spaniels with hereditary spinal muscular atrophy. In humans, this motor neuron disease strikes one in 6,000 children and adolescents.

"If our improved system works in dogs," Kahn concluded, "then we really think we will be able to demand clinical trials."

Good Start Now Needs Late-Onset Models

Neuroscientist and molecular biologist Jean-Pierre Julien, at McGill University, in Montreal, is a leading ALS researcher and familiar with Kahn's work.

"I think it's fair to say," Julien told BioWorld Today, that his must be the first report of that nature for motor neuron disease. That is probably why Nature Medicine accepted it."

But he raises a question: "Will this approach in pmn mice be useful for treating other mouse models of motor neuron disease, for example the transgenic that expresses a mutant superoxide dismutase [SOD], which we find in ALS patients?"

He explained: "ALS is a late-onset progressive disease, not early-onset like that of the pmn mice. So I think it would be a very nice approach to test their gene therapy on transgenic mice that express human mutants such as SOD, and to work on late-onset rather than early-onset disease."

Julien observed that the canine model Kahn is planning to use next "gets pathology very similar to ALS people. But," he concluded, "it will be a long time, I guess, before they get the answer with dogs." *