Long-awaited SMA Mouse Model Points to Possible Therapy

Aurthur Burghes

In a major step forward in SMA research, three groups of scientists announced simultaneously in February the creation of lines of mice with the genetic defects that cause spinal muscular atrophy (SMA) in humans. Disease onset and symptoms in all of the new mouse lines are similar to those in the human forms of the disease.

Even better, early experimental results with mice from the MDA-funded group led by Arthur Burghes of Ohio State University in Columbus demonstrate that a particular genetic manipulation can prevent the disease in mice. Researchers hope to further develop this method to prevent or arrest SMA in humans.

The work is reported in the February 2000 issue of Human Molecular Genetics.

In SMA, the spinal cord nerve cells that control muscle movement (motor neurons) are lost, leading to muscle weakness and paralysis. Forms of the disease vary in severity and time of onset. Although science has perfected techniques to insert or delete genes in mice to mimic human diseases, creating mice with the same genetic defects that cause human SMA proved more challenging. The difficulty occurs because mice, unlike humans, have only one copy of the gene that's defective in SMA, the survival motor neuron, or SMN, gene. When scientists initially attempted to remove or impair the single mouse SMN gene, the mice didn't survive to birth.

Researchers now suspect that humans with SMA are able to survive longer because humans have a "backup" gene for the one that's defective. Although this backup gene, SMN-C, isn't as good at making the SMN protein as the first gene, it does make enough of the missing protein to allow a human baby to develop. Unfortunately, it doesn't make enough of the SMN protein to maintain healthy spinal motor neurons throughout life.

To recreate the same genetic situation in mice that occurs in humans with SMA, Burghes generated mice that lack the mouse SMN gene, but have one or more copies of the human SMN-C gene. Mice that got only one copy of the backup gene had symptoms that most closely imitated those of humans with the most severe form of SMA. These mice were born and appeared normal until shortly after birth, when the nerve cells in their spinal cords began to degenerate. They all died in early infancy. In contrast, mice that got eight copies of the backup gene were born and matured normally, with no evidence of nerve cell degeneration.

"What this means," Burghes says, "is that the SMN-C gene can compensate for the loss of the first SMN gene, if it's present in enough copies."

Although we can't genetically manipulate humans in the same way that these mice were manipulated, this observation does suggest that increasing the activity of a person's backup SMN gene or genes (some people may naturally have more than one backup gene) should be enough to prevent or halt SMA. Burghes has already begun screening the first of 40,000 small-molecule compounds in the search for a drug that will increase the activity of the human SMN-C gene.

In another finding, Burghes clears up a long-standing question in SMA research: Does the shortage of the SMN protein cause mature spinal motor neurons to die after birth, or does it simply prevent them from developing in the first place?

His group examined and counted the nerve cells in the spinal cords of the most severely affected mice, those that received only one backup gene, and determined that neural development occurred normally until shortly after birth.

"This is a really important point from the perspective of a preventative therapy," Burghes says. "The fact that the motor neurons are not lost until later in the disease process may give us a window of opportunity to begin a potential treatment before any nerve cells are irrevocably lost."

If a drug could be developed to treat the disease, Burghes envisions a genetic neonatal screening process to detect infants who are at risk of developing SMA, so that treatment could be started before spinal motor neurons begin to degenerate.

A Zinc Link in ALS?

A new study from researchers at the University of Alabama in Birmingham suggests that the loss of zinc from a key enzyme could be the common trigger for familial (inherited) and sporadic ALS. The enzyme in question, superoxide dismutase 1 (SOD1), has long been linked to a subset of cases of the inherited form of ALS, but how mutations in this enzyme lead to the disease isn't completely understood. Meanwhile, even less is known about the cause of the disease in the 98 percent of people with either inherited or sporadic ALS who don't have mutations in SOD1.

Still, many researchers think the ability of mutated SOD1 to cause symptoms indistinguishable from those seen in the other forms of the disease provides an important clue to processes that all forms may have in common.

Joseph Beckman, John Crow and Alvaro EstŽvez say they suspect the common answer may lie in the way SOD1 behaves when bound to different metal ions.

Normally, SOD1 that's bound to both copper and zinc destroys harmful free radicals in our cells. Recently, however, the group found that the normal SOD1 enzyme can become toxic to nerve cells when it loses its zinc but retains its copper. The same proved true for a mutant version of SOD1 that's linked to familial ALS.

ZINC-SOD1 INTERACTION

NORMAL The active SOD1 enzyme binds to both zinc and copper. > Free radicals are removed. > Nerve cells are safe.

SOD1-linked Familial ALS The mutated version of SOD1 is no longer able to bind to zinc effectively. > Free radicals are produced. > Nerve cells are lost.

Sporadic ALS The normal SOD1 can bind to zinc effectively but there is no zinc available because it's bound to other cell proteins. > Free radicals are produced. > Nerve cells are lost.

The researchers now speculate that SOD1 destroys nerve cells in both familial and sporadic ALS because it loses its zinc, but that the zinc is actually lost for different reasons in these two forms of the disease (see illustration).

Although this theory provides a plausible role for abnormal SOD1 in both familial and sporadic ALS, the group hasn't yet shown that the SOD1 in people with either form of the disease actually does lose its zinc. They're trying to show this now in mice with ALS. Even so, the most obvious question is, if the loss of zinc from SOD1 may be the common trigger for ALS, could a person take zinc supplements to boost zinc in the cells?

Unfortunately, Beckman says, zinc isn't very permeable to the brain and he estimates that the amount of zinc needed to be therapeutic would probably approach levels that are toxic in humans. But there are other options.

Because the harmful effects of losing zinc from SOD1 depend on the presence of copper, it may be possible to deactivate harmful zinc-deficient SOD1 by removing its copper molecule. Beckman says copper chelators, compounds that bind strongly to copper, have been tested in the past to treat mice and humans with ALS, with modest benefit.

But, he adds, "Our results indicate that only certain types of copper chelators can pull the copper out of SOD1. We are trying to synthesize better derivatives and are testing existing compounds in mice. However, the existing compounds are too toxic, at present, to try in patients."

Beckman is also looking at specific antioxidants that are best able to counteract the effects of losing zinc from SOD1.

The work appears in the Dec. 24 issue of Science.

Company Supplying Drug to LEMS Patients

A drug known as 3,4-diaminopyridine (DAP for short) has been found safe and effective for people with Lambert-Eaton myasthenic syndrome (LEMS), a form of myasthenia (fluctuating weakness) that involves loss of the chemical signal that normally flows from nerve to muscle cells.

This signal, carried by the chemical acetylcholine, is disturbed in LEMS because the body's immune system mistakenly attacks a part of the nerve cell that's involved in releasing acetylcholine and sending it to the muscle cell.

DAP, which increases the release of acetylcholine by nerve cells, has been used for people with LEMS and other disorders of neuromuscular transmission for many years, but few formal studies have been done.

In a report in the Feb. 8 issue of Neurology, experienced clinicians are quoted as saying: "Among 53 LEMS patients who we treated under this and previous protocols in the last 10 years, 80 percent had moderate or marked improvement from DAP, taken alone or in combination with [the drug] pyridostigmine. No other treatment provided similar benefit to these patients."

In a study of 26 people, 12 of whom took DAP and 14 of whom took a placebo (inert substance), those who received DAP had a significantly greater improvement in their muscle function and in the electrical activity recorded from their muscles. There were no major side effects. MDA clinic co-directors Donald Sanders and Janice Massey of Duke University Medical Center in Durham, N.C., were among the authors of the report.

DAP is made by Jacobus Pharmaceutical Company of Princeton, N.J., which supplied the drug for this trial. The company is now providing the drug free of charge to those with LEMS under an agreement with the U.S. Food and Drug Administration.

If you have LEMS and would like to receive DAP, you or your neurologist can call Laura Jacobus at (609) 921-7447 or fax her at (609) 799-1176.

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