"The bad Huntington's gene creates a faulty protein that's found in all cells. Yet the only cells that die are certain neurons, mostly those in a movement-controlling brain region called the corpus striatum that by the time patients die is so ravaged that it's tissue-paper thin.
Why? A second protein is the culprit, Snyder's team discovered. It's a little-known molecule named Rhes that is found almost exclusively in the striatum. When Rhes mixes with the mutated Huntington's protein it sparks a chemical reaction, the researchers reported.
First came a simple experiment: They used human embryonic cells and brain cells taken from mice. To each, they mixed in different combinations of the mutated Huntington's protein, its normal version, and Rhes. Only when both the mutant protein and Rhes were in the same cells did those cells start dying.
Then the researchers teased out just what made the chemical reaction, named sumoylation, so toxic. It seems that cells may try to deal with the mutated protein by clumping it out of the way, almost like creating a garbage heap. Adding Rhes led to less clumping along with cell death, suggesting that it's the soluble form of the faulty protein that's dangerous.
And that's the connection to other brain-destroying diseases like Alzheimer's. Most are distinguished by clumps of some type of faulty protein, and there's a raging debate among scientists about whether the clumps, also called "aggregates," are the cause of brain destruction or a frantic attempt by the brain to save itself.
"The answers in one disease may have implications for another," noted Koroshetz of NIH's National Institute of Neurological Disorders and Stroke. "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." "