Huntington's disease, which killed folk singer Woody Guthrie, seems to put into overdrive the main chemical that turns on brain cells, ultimately leading to their death.
The normal function of the neurotransmitter glutamate, the chemical overproduced in Huntington's, is also intimately involved with learning.
Researchers from Ruhr University and the University of Dortmund in Germany have been intrigued by the question of whether the neurodegeneration initiated by glutamate in this genetic disorder is all bad. Is it simply burning out brain circuits? Or might an excess of the chemical also help presymptomatic carriers of the Huntington's gene or even patients with the disease itself, learn some things faster or better?
"Neurotransmission causes cell death but we know from the vast amount of literature that learning processes very much depend on glutamate neurotransmission; so there may be two effects of one and the same process," says Christian Beste of Ruhr University. "On the one hand this process may lead to neurodegeneration. But on the other hand, it may augment a cognitive process that depends on glutamate transmission."
Beste is the lead author on a paper published this month in Current Biology that found that those who have the genetic mutation for Huntington's but who have yet to develop inevitable symptoms of the disease perform better on a learning task than a control group that lacks the mutation. The 29 Huntington's gene carriers learned to detect twice as fast as the 45 controls a change in brightness of a small bar as its orientation on a computer screen altered. In fact, the Huntington's carriers with the most pronounced mutations—the number of repetitions of a short DNA segment determines how early disease onset occurs—logged the best performance.
But doesn't the cognitive benefit disappear as the disease develops and patients develop symptoms such as jerking movements, dementia and hallucinations? Actually, maybe not. In a 2008 study published in the Journal of Neuroscience, the two university teams found that 13 Huntington's patients actually performed better in detecting whether a tone was long or short than 25 healthy controls, half of whom had the mutation but had yet to experience any symptoms. The paper forwarded a challenge to "the view that late-stage neurodegeneration is necessarily related to a decline in cognitive abilities in Huntington's disease. In contrast, selectively enhanced cognitive functioning can emerge with otherwise impaired cognitive functioning."
These findings will probably not help patients with a diagnosis cope better with the disease, Beste says, but they may provide a way to determine whether a drug for Huntington's is effective: performance on a psychological test or electroencephalograph may actually decline as the patient gets better. "I don't think this will be implemented in a rehabilitation program," he comments "But it may be useful as a readout for clinical trials where we are examining drug effects targeting the excitotoxicity problem in Huntington's disease. A few trials targeting this excitotoxicity mechanism have not been very successful. In part this lack of success may depend on lack of a well-tuned readout measure; if we have a better readout measure, it may be possible to detect therapeutic effects." Apart from any potential benefits in testing, the set of experiments provide new perspective on what happens in the brain as disease takes over and cells start to die.
Image Source: Library of Congress