"The significance is that any cell that can be saved by treatment is likely to function normally, since that cell isn't sick."
A new view of the underlying mechanisms of Parkinson's, Huntington's and other neurodegenerative diseases may result in novel treatments for them, researchers report.
Such disorders result from the premature death of nerve cells that appear to function normally to the end and not degenerate progressively, as has been thought, the investigators said.
"The significance of this work is that it completely reorients our understanding of neurological degenerations," said senior study author Roderick McInnes, professor of molecular and medical genetics and pediatrics at the University of Toronto.
The findings may influence how scientists conduct experiments and interpret their results in the field and hold "important implications for the treatment as well," McInnes said.
Researchers have long known that the death of neurons, the nerve cells that carry messages to and from the brain, is behind Alzheimer's, Parkinson's and the sudden loss of motor function in patients suffering from amyotrophic lateral sclerosis, or ALS, better known as Lou Gehrig's disease.
While this common thread of cell fatalities has been shown to run through these and other neurodegenerative disorders, more detailed attempts to understand the factors that unite them have stumbled over such obstacles as their varying times of onset and clinical progression.
Study Indicates Cell Death Not Gradual, But Random
Might the neuronal loss in all of these diseases follow a common course? Or is the demise in each as individualized as some of the symptoms? The experimental evidence from the animal and cell culture studies of Parkinson's, Huntington's and retinal degeneration support a common, "one-hit" model of cell death in inherited neurodegenerative diseases.
The findings argue against the hypothesis that cell damage accumulates until it reaches lethal proportions, indicating instead that the time of death of any neuron is random.
"Our findings demonstrate that, for many neurological degenerations, the basic premise underlying the study of these diseases was incorrect," McInnes said. "That premise, designated 'the cumulative damage' model, is that the mutant neurons (in those neuronal degenerations which are genetic in origin) are initially in good health, but that they accumulate damage as the years go by. Eventually a critical amount of damage accumulates, and the cell dies."
Instead, "we found that they are at a constant increased risk of death, as soon as they are formed," McInnes said, adding that the treatment goal would be to return the risk to normal, i.e., zero, or at least to reduce it.
"The obvious implication for treatment is that, if the affected neurons are alive, they can be saved by any effective treatment that would function well for the life of the patient," McInnes said. "In the cumulative damage model, the situation is quite different. In that case, you might apply treatment to a cell, but if its degeneration has progressed too far, it may never function normally again."
"These results are certain to stimulate much debate and experimentation, aimed both at identifying common mechanisms of neurodegeneration and at developing common ways of intervening in these tragic diseases," said Nathaniel Heintz of the Howard Hughes Medical Institute at the Rockefeller University in New York, who wrote an accompanying article in the British journal Nature.
While convinced of the strength of their data, the study authors conceded their model "might still be incorrect."
Unless damaged, normal neurons live as long as the body they inhabit. In neurodegenerative diseases, one region of the nervous system suffers a progressive loss of these irreplaceable cells. As the fatality count rises, patients begin to experience symptoms, such as the tremors characteristic of Parkinson's.
"The one-hit model proposes that the mutant neurons in inherited neurological degenerations are in general good health and function well, except for one thing: They are at an increased risk of dying compared to normal adult neurons (which generally don't die)," McInnes said.
"The increase in risk of death is constant throughout the life of the individual, and the time at which death occurs is totally random. This model therefore predicts that cells in the affected parts of the brains of patients with these diseases will function well until the cells die. In fact, in those situations where function has been studied, as in retinal degenerations, that is exactly what has been found: If the cell is alive, it is well."
Geoff Clarke of the University of Toronto, lead author of the study published in Nature, generated mathematical equations predicting the speed of neuronal death caused by accumulated damage.
"These equations allowed me to study neuronal death that had been observed by other investigators," he said. "I found that their data weren't consistent with the idea of increasing amounts of cellular damage. Instead, our analysis demonstrated that neuronal cell death in neurodegenerative diseases occurs randomly during the life of the patient."
Taking their findings into account, McInnes, Clarke and colleagues from the Universities of Toronto and British Columbia devised the "mutant steady state model" to explain nerve cell death in inherited neurodegenerative diseases.
Mutant Gene Scenario
In this scenario, mutant genes confer a small but definite increase in risk of sudden programmed cell death in a perfectly normal, healthy cell, they said.
The picture of mutant neurons in neurological degenerations resembles that of very high cholesterol levels in a healthy-looking, athletic adult, the scientists said.
"He looks and feels well and to all appearance is fine. But he is at an increased risk of random death, if his coronary artery suddenly obstructs," McInnes said.
"Our work indicates that the neurons that are still alive are functioning well for years or decades and are not seriously damaged, but they are at increased risk of suddenly dying," he said. "The significance is that any cell that can be saved by treatment is likely to function normally, since that cell isn't sick."
If the new theory holds up, researchers could target whatever mechanism increases the risk of neuronal death caused by mutant genes.
"If we can identify what critical reactions in the neurons lead to the increased risk of programmed cell death, then we can try to push them back towards normal. This will be tough, but there are some candidate molecules that scientists have been investigating," McInnes said. "And while we did not specifically study all neurodegenerative diseases, we suspect that these findings may also apply to others like ALS and Alzheimer's disease."
The scientists now want to determine whether the model applies to other neurological degenerations and to identify the critical biochemical abnormalities that put the neuron at risk of death.
"The ultimate aim is to identify the critical biochemical abnormalities in each disease and to learn how to reverse these abnormalities, thus taking the cell out of harm's way," McInnes said.
This may take years, he said.
"However, this model is a solid first step in a new direction, hopefully the right direction," McInnes said. "Maybe we will be lucky and be able to identify one or more of the critical reactions soon, and maybe some of the critical reactions will be the same in different diseases."
The work was funded by the Foundation Fighting Blindness, The Macular Vision Research Foundation, The RP Eye Research Foundation of Canada, the Medical Research Council of Canada, the Canadian Genetic Disease Network and the Huntington's Disease Society of America.
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(This story was posted on 18 Jul 2000)