CHICAGO—Sir William Osler understood that when a tragedy occurs, we’re driven to find the cause, to figure out the mechanism. “This is true of natural disasters such as earthquakes, and manmade disasters,” Kenneth H. Fischbeck, MD, observed. “It’s a natural human need to figure out what causes a disaster to happen. It helps us to avoid such tragedies in the future and to better deal with them should they occur again,” he added.

Neurologic diseases are also tragedies. “They are on a smaller scale, perhaps, but just as devastating to the patients who experience them,” Dr. Fischbeck told the 126th Annual Meeting of the American Neurological Association. “And we have a similar need to know the cause. Osler recognized the principle that forms the basis of much of modern medical research. If we can identify the cause of a disease and understand its mechanism, then that’s the best hope we can offer patients for effective treatment.”

The human genome project, he continued, “has given us a powerful tool for finding the causes of hereditary disease. We now have 95% of the genome sequenced, and half of that sequence has been assembled into continuous tracts.” Dr. Fischbeck proceeded to address “what this means for us as neurologists and for our patients with neurologic disease,” by discussing the three main ways the human genome project brings benefits to neurology: disease gene identification, exploration of disease mechanisms, and development of effective treatments.

IDENTIFYING THE ETIOLOGY OF GENETIC DISEASE

Fifteen years ago, Duchenne muscular dystrophy was one of the first diseases to have its gene identified by the process of positional cloning, a strategy for identifying and finding a gene for hereditary disease based on its chromosomal localization. “Duchenne identified this disease in the hospital wards of Paris 160 years ago, and now, in our time, we know the cause of this disease—a defective gene on the X chromosome that encodes the structural muscle protein dystrophin,” Dr. Fischbeck said. “But we still have to face the challenge of using that knowledge that we’ve gained to develop effective treatments for patients,” he added.

Identification of genetic disease etiologies is the beginning of that challenge. Positional cloning involves gathering DNA samples from patients and employing a series of DNA markers “to determine the chromosome location of the disease gene and point to particular candidates in the region of the chromosome. These candidates are then sequenced to identify the particular gene that is specifically mutated in the disease,” Dr. Fischbeck explained.

The human genome project facilitates this process in at least two important ways. “First, through the availability of DNA markers. There are now over three million such markers, for every part of every chromosome. Second, through the identification of candidate genes. Nearly all of the 30,000 to 35,000 human genes have been identified, and this gives us an extraordinary database for disease gene discovery. Now a process that used to take us months or years can be done, in many cases, in weeks or even days.”

Where does this lead? “In the not too distant future,” Dr. Fischbeck said, “nearly all important neurologic disease genes will be known.” The remaining challenges are rare diseases, “those with only one or two identified families and those with complex inheritance or, more particularly, phenotypes that are difficult to delineate clearly.” The identification of disease genes means that genetic testing will become increasingly available, Dr. Fischbeck noted. “As neurologists, it behooves us to know how to use these tests correctly, not only for the diagnosis of symptomatic patients but also for carrier testing and prenatal diagnosis,” he added.

MAPPING THE MECHANISMS OF MALFUNCTION

Thousands of identified genes have been placed on chips and micro arrays for gene expression analysis, he added, allowing researchers to track the metabolic changes that occur in infected cells and tissues as the disease progresses. “Importantly, the genome project allows us to make connections across diseases and to identify shared underlying mechanisms,” he said.

To provide an example illustrating the utility of the human genome project in this field, Dr. Fischbeck returned to the 1880s, when Charcot, Marie, and Tooth described the clinical manifestations of hereditary motor and sensory neuropathy. In the 1960s doctors found that the same physiologic symptoms—progressive distal weakness, muscle atrophy, loss of sensation and reflexes—described by Charcot, Marie, and Tooth can be produced by two different pathophysiologic mechanisms: demyelination in type 1 disease and axonal degeneration in type 2. “We now know that each of these can be caused by defects in a variety of different genes. About 30 different chromosome locations have been implicated in hereditary neuropathy, and at least eight genes have been identified. Genes for demyelinating neuropathy have been known for eight or nine years. Recently, genes for the axonal form have also been identified,” Dr. Fischbeck said.

“In a recent report, the gene for Charcot-Marie-Tooth disease type 2A was described,” he added. “Now we have both an animal model in mice and a mechanism—defective axonal transport—for the axonal form of demyelinating motor and sensory neuropathy.”

In the other type of repeat expansion disorder, the mutation is in a coding region, or an exon. Researchers investigating Huntington’s disease, an example of this type of disorder, were able to pinpoint the disease to a change on a gene on chromosome 4 and to identify the gene eight years ago. “We now know that Huntington’s is one of at least nine neurodegenerative diseases that are caused by expanded CAG repeats in the coding regions of genes,” said Dr. Fischbeck. The CAG codon encodes the amino acid glutamine, and repeat expansion leads to an expanded polyglutamine tract in the diseased protein. “Thus we refer to these disorders as polyglutamine expansion diseases,” he noted.

Polyglutamine diseases include Huntington’s disease, spinocerebellar ataxia types 1, 2, 3, 6, 7, and 17, and spinal and bulbar muscular atrophy. Although initiated by mutations in different genes that result in distinctive clinical manifestations caused by the degeneration of distinct, but overlapping, populations of nerve cells, “it is likely that these diseases all share the same underlying mechanism: toxicity of expanded polyglutamine-containing protein,” Dr. Fischbeck said.

Each of the polyglutamine expansion diseases shows a correlation between repeat length and age of onset: The longer the repeat the earlier the onset and the more severe the disease manifestations. According to Dr. Fischbeck, the same correlation exists on a much smaller time scale between repeat length and the tendency for mutant protein to aggregate in cell-free systems and cell culture. “It’s the same threshold, about 40 glutamines, for aggregation and disease,” Dr. Fischbeck said. “This indicates that protein aggregation, or at least the tendency of the diseased protein to aggregate, either with itself or with other critical proteins that contain polyglutamine tracts themselves, is likely involved in the disease mechanism.”

Protein toxicity and aggregation classifies the polyglutamine diseases together with other important neurologic diseases: “Alzheimer’s disease with beta-amyloid protein, Parkinson’s disease with alpha synuclein, and Creutzfeldt disease and the other spongiform encephalopathies with prion protein, to name a few,” Dr. Fischbeck observed. “We’ve seen a remarkable convergence of research in these diseases recently. This points to shared mechanisms, and perhaps to common therapeutic targets.”

CHARTING A COURSE TOWARD TREATMENT

The opportunities for pharmacologic treatment are better, he added. “The human genome project gives us new targets for drug development. Consider spinal muscular atrophy, a severe disease with a genetic cause that was identified several years ago. Knowing that cause points the way to developing effective treatment.”

Spinal muscular atrophy is an autosomal recessive neuromuscular disease caused by mutation of the gene for the survival motor neuron protein, which is known to play a role in RNA splicing. Survival motor neuron protein deficiency leads to defective spliceosome assembly and deficient splicing of mRNA, “something that is very important for our gene expression,” Dr. Fischbeck explained.

he National Institute of Neurological Disorders and Stroke (NINDS), where Dr. T Fischbeck is the Neurogenetics Branch Chief, defines its mission as lessening the burden of neurologic disease. “One of the Institute’s strategic aims is to encourage research on those diseases where we can get from third base to home, where concerted efforts might bring about effective treatment,” Dr. Fischbeck said. “The genome project presents us with many such opportunities. It is up to us to take advantage of them—to translate gene discoveries into viable therapies where none are currently available. In the last 15 years, we’ve learned more about the causes of hereditary neurologic disorders than in all the preceding centuries back to the time of Hippocrates. Our challenge now is to use the knowledge we have gained to bring effective treatment to patients with these disorders.”

NR

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