A new study reveals for the first time how gene mutations lead to the inherited form of amyotrophic lateral sclerosis (ALS), or Lou Gehrig's disease. The study suggests that the two most prominent theories of how familial ALS (FALS) and other related diseases develop are both right in part.
"No one has ever demonstrated at the molecular level how ALS mutations might lead to disease," says study author John Hart, Ph.D., director of the University of Texas Health Science Center X-ray Crystallographic Core Laboratory in San Antonio. "Using a technique commonly used in structural biology, we could see the intimate details of how toxic familial ALS proteins interact. And we found out that the proteins are interacting in a way they shouldn't be." The study was funded by the National Institute of Neurological Disorders and Stroke and appears in the June 2003 issue of Nature Structural Biology.
ALS is a progressive, fatal neurological disease that usually strikes in mid-life. It causes muscle weakness, leads to paralysis, and usually ends in death within 2 to 5 years of diagnosis. Affecting as many as 20,000 Americans, ALS occurs when specific nerve cells in the brain and spinal cord that control voluntary movement gradually degenerate.
About 10 percent of ALS cases are familial ALS. Only one parent needs to have FALS to pass it on to his or her children, although men are about one-and-a-half times more likely to develop the disease than women. Studies that reveal how FALS develops may give researchers new clues about the other 90 percent of ALS cases – known as sporadic ALS – and other neurodegenerative diseases, such as Alzheimer's, Parkinson's, and Huntington's diseases.
Scientists studying FALS patients have identified more than 90 mutations in the gene that directs the production of the protein copper-zinc superoxide dismutase (SOD1). In FALS, proteins accumulate in a way they shouldn't to form large protein complexes. Scientists believe these complexes interfere with nerve cell transport, cellular waste management, and other cellular activities that prevent cell death. Similar large protein complexes have been implicated in other neurodegenerative diseases.
Using a 3-dimensional imaging technique called x-ray crystallography, Dr. Hart and his colleagues compared the interactions among proteins in the FALS mutant protein complexes to interactions among normal proteins.
Normally, proteins protect themselves from sticking to one another by covering their edges with loop-shaped ends. The researchers found that in the mutant proteins, the loops were in the wrong position. This loss of protection appears to lead to the toxic accumulation of proteins in FALS. The finding reveals a new mechanism for researchers to exploit in their efforts to find ways to prevent or treat neurodegenerative diseases.
For years scientists have speculated about the disease mechanisms in ALS. Researchers initially thought that the FALS mutation in SOD1 led to a decrease in SOD1 activity and subsequent oxidative damage to cells. But a recent study disproved the idea, showing that mice completely lacking SOD1 lived to adulthood without developing movement disorders. Mice with the human FALS -SOD1 mutation, however, became paralyzed despite normal SOD1 levels.
Scientists now have two primary theories for why the mere presence of the mutant SOD1 protein seems to cause FALS without interfering with SOD1 activity.
The new oxidative damage theory holds that mutant SOD1 proteins produce chemicals called oxidants that damage and kill cells. In a nutshell, the SOD1 protein needs to bind to a reactive metal in order to form loops to protect its edges. The oxidants, however, often damage the mutant SOD1 protein itself, interfering with metal binding and leaving the protein unprotected.
The aggregation theory, on the other hand, maintains that mutant SOD1 proteins fold improperly, causing them to stick together and form large toxic protein complexes. Researchers believe that those protein complexes interfere specifically with transport machinery within the nerve cells that control voluntary movement.
A recently substantiated addition to the aggregation theory, suggested by studies in Parkinson's and Alzheimer's Diseases, is that pore-like precursors of the protein aggregates – not the aggregates themselves – may be killing the nerve cells. In this study, Dr. Hart and Dr. Samar Hasnain saw those helical, pore-like precursors using x-ray crystallography, providing striking evidence implicating the aggregation theory in ALS.
"Our study provides a model for how protein aggregation in FALS occurs," says Dr. Hart. "But it also suggests that deadly oxidative chemistry can lead to metal loss which in turn can lead to aggregation. These are very exciting findings, because we have 3-D pictures that support two separate hypotheses."
These findings offer a unique contribution to the enormous effort to understand not only the causes of, but also the possible ways to treat or prevent FALS and other neurodegenerative disorders.
"If we can understand what is going on at the molecular level, we may eventually be able to develop a drug to prevent the defect that leads to disease," says study co-author Jennifer Stine Elam, a graduate student in Dr. Hart's laboratory.
The NINDS is a component of the National Institutes of Health within the Department of Health and Human Services and is the nation's primary supporter of biomedical research on the brain and nervous system.
NIH/National Institute Of Neurological Disorders And Stroke. May 2003.