Study sheds light on Huntington disease with implications for other neurodegenerative disorders
A common theme among neurodegenerative diseases is that there is a some kind of “nucleating protein” which aggregates within specific areas of the brain. There is debate as to whether the aggregates cause disease, or are simply an attempt by brain cells to sequester bad proteins. If the former is true, then therapy should be aimed at disaggregating the bad protein. If the latter is true, then therapy should be focused on helping the brain to inactivate the free-floating bad proteins. A recent advance in the understanding of Huntington disease (HD) suggests that the free-floating form of the protein may be the culprit, lending support to the idea that other neurodegenerative diseases like Alzheimer’s and Parkinson’s may also be caused by soluble proteins rather than the aggregates that we neuropathologists focus on under the microscope.
Scientists have known for some time that HD is associated with a trinucleotide repeat mutation in the protein huntingtin on chromosome 4. But, since huntingtin is present throughout the brain, why does neurodegeneration in HD take place predominantly in the striatum (caudate and putamen)? Solomon H. Snyder and his team at Johns Hopkins University, in the June 5, 2009 issue of the journal Science, show that cytotoxicity in HD takes place because of the interaction of mutant huntingtin with a second protein, known as Rhes. It turns out that Rhes is a striatal specific protein, thus explaining the anatomic specificity of neurodegeneration in HD. Snyder and colleagues go on to show that cells in culture tend to sequester mutant huntingtin into an aggregate. But in the presence of Rhes, mutatant huntingtin cannot aggregate, suggesting that the soluble form of the bad protein is what causes damage.
An Associated Press article about the discovery posted on Forbes.com quotes Walter J. Koroshetz of NIH’s National Institute of Neurological Disorders and Stroke as follows regarding the implications of this new study: “The answers in one disease may have implications for another… There’s been people on both sides of the fence. This story plays to the role of the aggregates as not being the major problem but the soluble protein as being the major problem.”
Reference:
Subramaniam S, Sixt KM, Barrow R, Snyder SH. Rhes, a Striatal Specific Protein, Mediates Mutant-Huntingtin Cytotoxicity. Science 5 June 2009:
Vol. 324. no. 5932, pp. 1327 – 1330.
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