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New Approaches to Lou Gehrig’s Disease

By Nancy Touchette

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When baseball slugger Lou Gehrig died in 1939, two years after playing in his last game, little was known about the disease that killed him. Although more than 90 mutations in a gene important for cell maintenance have since been linked to the disease that now bears his name, there is still no treatment or cure.

Knowing the structures of mutant proteins could lead to new drugs.
But new research on proteins and their structures suggests promising ways to think about therapy.

Researchers have determined the structures of mutant forms of a protein associated with Lou Gehrig’s disease, also known as amyotrophic lateral sclerosis, or ALS. They have discovered what may cause the protein to clump up and kill neurons throughout the body. And they have an idea how to prevent this from happening, at least for some forms of the disease.

Two new studies focus on superoxide dismutase-1, or SOD1, a protein that breaks down superoxide radical molecules, which are toxic to the cell. Left unchecked, superoxides can wreak havoc by damaging DNA and other molecules in the cell.

The new studies show how certain mutations can weaken the protein structure, causing it to fold abnormally. It is the aberrant form of SOD1 that is thought to damage the cell. The researchers have also found a way to stabilize and prevent misfolding of SOD1, a finding they hope will lead to new treatments for the disease.

“SOD1 is part of a very finely tuned system,” says Samar Hasnain, of the Daresbury Laboratory in Cheshire, United Kingdom, who led one of the recent studies. “Almost any perturbation will take it out of the system and convert it to a pathogenic form.”

Hasnain belongs to the International Consortium on Superoxide Dismutase and Amyotrophic Lateral Sclerosis, a collaborative effort by six groups of researchers in the United States and the United Kingdom. The goal of the consortium is to determine the detailed structure of mutant forms of SOD1, an approach known as structural genomics.

Comparing forms of the normal protein with those known to cause disease may be the best way to design drugs to prevent formation of abnormal protein structures, the researchers believe. So far the group has determined the structures of four mutant forms of the protein, as well as the normal form of the protein.

In its normal, active form, SOD1 exists as a “dimer,” in which two protein molecules are bound to each other.

The normal form of superoxide dismutase-1 (green) is more stable as a “dimer”, but a mutant (red) forms “monomers” that clump up and cause disease.
In their latest study, reported in Proceedings of the National Academy of Sciences, Hasnain and his collaborators used X-ray analysis to determine the structure of two mutant forms of SOD1 at the atomic level. Both mutations occur in the area of the protein where the two molecules come together.

“If we look at the two individual molecules, we see that they don’t have the correct orientation,” says Hasnain. “It’s a corkscrew type of action, so they don’t align with each other correctly.”

The result is that the proteins are less stable and easily fall apart. They clump into fibers known as amyloid that accumulate in the nerve cells and trigger cell death.

In a second study, reported in Biochemistry, Soumya S. Ray, Peter T. Lansbury, and their colleagues at Harvard Medical School in Boston, Massachusetts, used a high-powered microscope to examine the clumps formed by one of the same mutants studied by Hasnain.

The researchers found that the mutant protein was much more likely than the normal form to clump up, or aggregate.

However, when they engineered a chemical bridge that locked together the two molecules of the mutant protein, the protein failed to clump up.

“This tells you that mutations that make the dimer fall apart could be bad and trying to keep the dimer together might be good,” says Lansbury. “It gives us a target for drug development—something to shoot at.”

Lansbury and Ray are now screening a library of over 2.5 million small molecules in search of those that stabilize the dimer form of SOD1 mutants. So far, they have identified 6,000 candidates from computer modeling that have the right size and shape to hold the dimer together.

A similar approach was recently used by researchers at the Scripps Research Institute in La Jolla, California, to identify drug candidates to treat familial amyloidotic polyneuropathy (FAP). In this disease, a protein called transthyretin clumps up and forms protein deposits in the peripheral nervous system.

Jeffery Kelly and his colleagues at Scripps noted that the normal form of transthyretin contains four protein molecules bound together, but mutant forms of the protein cause the molecules to come apart and clump together, forming a different kind of structure. The researchers looked for small chemical compounds that could keep the protein together without forming clumps.

Last month, Kelly and his colleagues reported in Proceedings of the Academy of Science that two small chemical compounds, resveratrol and chlorinated biaryl amine, fit the bill. They prevented protein clumping in the test tube. The researchers plan to test these and other compounds soon to treat FAP in humans.

Hasnain says that using small compounds to bridge protein molecules together is a promising strategy for developing treatments for Lou Gehrig’s disease, but cautions that it may only work for patients with certain mutations.

“It is important to catalogue a reasonable number of mutant protein structures,” says Hasnain. “This could give us an idea of how to design better drugs.”

Most of the mutations identified so far have only been shown to cause disease in inherited forms of ALS. The vast majority of cases have no known genetic cause. Nevertheless, researchers hope that understanding the inherited forms of disease will also help determine the cause of sporadic cases.

Ray, S.S. and Lansbury, P.T., Jr. A Possible therapeutic target for Lou Gehrig’s disease. Proc. Natl. Acad. Sci. 101, 5701-5702 (April 20, 2004).
Hough, M.A. Dimer destabilization in superoxide dismutase may result in disease-causing properties: Structures of motor neuron disease mutants. Proc. Natl. Acad. Sci. 101, 5976-5981 (April 20, 2004).
Ray, S. S. et al. An intersubunit disulfide bond prevents in vitro aggregation of a superoxide dismutase-1 mutant linked to familial amyotrophic lateral sclerosis. Biochemistry 43, 4899-4905 (May 4, 2004).
Reixach, N. et al. Tissue damage in the amyloidoses: Transthyretin monomers and nonnative oligomers are the major cytotoxic species in tissue culture. Proc. Natl. Acad. Sci. 101 2817-2822 (March 2, 2004).

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