New study on Huntington's disease reveals pathology and new treatment options

July 30, 2004

Cynthia Lepore, Public Affairs

Belmont, MA - In a groundbreaking study on Huntington's disease, Hyemyung Seo, PhD, Kai Sonntag, MD, PhD and Ole Isacson, Dr Med Sc, of the Neuroregeneration Laboratory at Harvard-affiliated McLean Hospital, describe one new, and several related, cellular problems that are caused by the mutated protein huntingtin.

The study (PubMed), published this week in the pre-publication online version of the Annals of Neurology, investigated the consequences of the mutation on many complex processes in cells. The study revealed that the severe, selective and regional brain pathology is a consequence both of the mutated protein and very special characteristics of the vulnerable brain region, which showed a unique combination of cellular problems. Surprisingly, even skin cells that show no obvious pathology were also impaired, but the study showed how many cells in the body can compensate for the pathogenic protein without dying or becoming severely damaged.

Huntington's disease (HD), for which there currently is no treatment, is exclusively caused by an inherited mutated gene. HD was the first brain disease to have its pathology-provoking gene identified (in 1983) and then eventually genetically decoded (in 1993). The mutation, an extended DNA CAG stutter base repeat in exon 1 of the gene named huntingtin, produces a huntingtin protein with abnormal polyglutamine length (polyQ). Symptoms of HD appear in middle age and lead to a devastating movement and psychiatric disorder. The pathology involves the dysfunction and eventual death of certain specific inhibitory neurons in the brain region called the striatum. This neuronal dysfunction results or participates in the signs of involuntary movements, cognitive disturbances and loss of brain function. Most genetically linked diseases or their idiopathic forms create a highly specific pattern of pathology in select regions of tissues or cell groups. In some such inherited diseases, scientists know the mechanisms or biochemical pathways inside a cell explaining how a mutated protein causes loss of needed function or produces a toxic and pathological effect. However, in many genetic diseases, even when a specific protein change is detected, it is not clear how some tissues or cell groups cope with the mutated new form of a protein, while other cells and tissues degenerate and die.

Curiously, although normal huntingtin or its mutated form in HD carriers is ubiquitously expressed in most cells of the human body, only neurons in some specific brain regions, such as the striatum, seem to be vulnerable and are progressively lost during the disease process. The reason for this relative brain and regional vulnerability is not fully understood. There is a brain region- and time-dependent intracellular aggregation of the expanded polyQ huntingtin in HD, but this only partially correlates with the brain disorder, and not its functional or final pathology. There are also known growth factor and energy metabolic disturbances in many HD nerve cells and these changes may also underlie the cellular dysfunctions.

Using post-mortem samples of HD patient brain and skin fibroblast material, the McLean Hospital study began with the examination of the abnormally shaped HD protein in its effect on general enzymatic protein breakdown in the cell's specialized proteasome particles, which is critical to maintain normal levels of functional proteins inside all cells. The investigations led to fundamental questions of many disturbances caused by mutated huntingtin in both HD skin and brain, and the specific and additive toxic effects the mutated gene seems to have in brain regions where vulnerable neurons eventually dysfunction and die.

In conducting their study, Seo, Sonntag and Isacson first measured proteasome activities and ubiquitin levels in several brain regions of a significant collection of HD post-mortem samples from patients at different progressive disease stages (brain disease grade 0-3). For example, grade 0-1 included carriers that had the HD gene but had not yet expressed significant brain pathology. Grade 3 samples included cases that already had severe brain pathology. To address the possibility of general cellular effects of mutated huntingtin on ubiquitin proteasome system (UPS) function, fibroblasts from skin of many HD patients were studied. An overall reduction of proteasome activity was found at both early and late stages of the disease in several brain regions, including the caudate-putamen, cortex and cerebellum. Remarkably, this was also found to be the case in HD skin fibroblasts. The discovery of these major and general changes in the UPS system led to critical questioning of the rather simplistic assumption that only one type of cell disturbance (such as UPS inhibition) would be sufficient to disable the cells, since clinical HD is not a skin disease.

The researchers compared the novel UPS finding with other known or potential huntingtin related effects on growth factor BDNF expression levels and mitochondrial enzyme complex II/III activities, which are known to be altered in the HD. They found that BDNF levels were decreased in the HD brain at both early and late disease stages. However, a selective increase in ubiquitin levels combined with reductions of mitochondrial metabolic complex II/III activity was observed only in the HD striatum. Finally, in the physiological and functional analyses of the UPS, by creating gene therapy-mediated overexpression of the proteasome activator PA28, proteasome activity was increased in normal, but not in HD fibroblasts, another indication of a generalized UPS problem in HD.

The findings of similarly decreased proteasome activities in skin-showing no clinical pathology in HD-and brain, indicate that such UPS dysfunction is probably not sufficient by itself to seriously damage human cells. Similarly, additional findings from analysis of the growth factor and energy status of tissues, also indicate that interference by the mutated huntingtin on transcription of the trophic factor BDNF is probably not by itself sufficient to cause cell and tissue damage.

Isacson, who directed the study, and his colleagues reason that the tissue and characteristic neurons of the striatum experience most pathology because of multiple interferences by mutated huntingtin on critical cellular function that cause: reduced proteolytic activity derived from proteasome function, causing accumulation of and increased levels of dysfunctional and ubiquitinated huntingtin; reduced BDNF trophic factor expression and levels causing neuronal dysfunction; and impaired metabolic mitochondrial complex II energy production, further compromising cellular function. Critically and over time, these combined problems appear to be most irresolvable for the GABAergic neurons in the striatum of HD patients.

Mark Cookson, PhD, a National Institutes of Health expert on neurodegenerative mechanisms caused by genetically aberrant protein, writes in an upcoming Annals editorial: "The [study] refines our current thinking about the role of proteasome dysfunction in neurodegenerative disease by suggesting that additional damaging events directed at neurons are important in the development of real pathology.

"The [study] also includes an approach to deal with increasing the system's ability to delay or prevent neuronal damage," writes Cookson. "These scientists were able to increase expression of components of the proteasome system using gene therapy approaches."

The study also provides a comprehensive view of combined cellular dysfunctions that may have implications for the understanding of the disease processes involved in other types of adult onset neurodegenerative diseases, such as Parkinson's disease, amyotrophic lateral sclerosis and Alzheimer s disease, which share many analogous mechanisms of degeneration to those seen in HD.

The new findings have led Isacson and his team to new experiments that will try to delineate cell-type specific compensatory mechanisms and pathways, to elucidate cellular defense mechanisms that may yield new therapeutic insights for this and similar disorders. New therapies based on such compensatory mechanisms would delay or prevent the brain dysfunction and degeneration.

McLean Hospital maintains the largest research program of any private, U.S. psychiatric hospital. It is the largest psychiatric affiliate of Harvard Medical School.

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