Scientists discovered the gene for Huntington's disease in 1993, but they couldn't explain how the gene leads to the death of a small patch of nerve cells in a key part of the brain.
Now studies from two laboratories at Johns Hopkins suggest precisely what goes awry in the brain cells marked for destruction: A mutant protein "hijacks" a key molecule in a cell's survival system. Using what they've learned, the researchers also have been able to fully reverse impending cell death in laboratory cultures of human cells containing the mutant HD gene.
An account of the study appears this week in the journal Science.
"Obviously, our goal has been to understand HD's mechanism so we can interfere with it early on with drugs," says lead researcher Christopher A. Ross. "But this is also a broader advance," says neuroscientist Ted M. Dawson. "It shows us a new way in which genetic errors could cause disease."
Huntington's disease is a fatal hereditary disorder, marked by death of nerve cells chiefly in the corpus striatum, a part of the brain that helps control movement and thought. Patients need inherit only a single mutant gene to get HD. Symptoms typically begin in middle age, usually as uncontrollable movement followed by progressive dementia and death.
"We've long known that the abnormal gene produces a flawed form of a protein called huntingtin," says Ross. Like a train with too many dining cars, the irregular molecule has too many repeats of glutamine, one of its amino acid subunits. Brain cells of patients with HD show characteristic clumping of the flawed huntingtin.
But the clumped molecule itself apparently isn't harmful. "The real problem is that the abnormally shaped protein attracts and becomes entangled with a smaller, critical protein in the cell nucleus," says doctoral student Frederick C. Nucifora Jr. The smaller protein--a regulatory molecule called CBP--gets "pulled away" from its place of action alongside DNA and then becomes entangled and useless, Ross says.
"Without CBP," he continues, "a pathway crucial for cell survival never gets turned on."
To prove CBP gets hijacked, the researchers attached different colored fluorescent markers to DNA, huntingtin and CBP and watched what happened inside cells to which they'd added mutant HD genes. They could see the CBP get sequestered out of the nucleus. They also showed this "hijacking" in live mice carrying the human HD gene and in postmortem brains from human HD patients.
Assays of gene activity in the nerve cells showed that, under these conditions, CBP's normal gene-regulating activity--turning on genes for survival pathways--wasn't happening.
But most striking, the researchers say, was being able to reverse the process in the test tube, turning around the cells' slide into death.
In earlier studies, when researchers in Ross' lab inserted mutant HD genes into nerve cells in culture, the cells died in a way identical to brain cells of HD patients. But this time, when the scientists introduced mutant HD genes into cultured cells, they also added a bogus version of CBP with the molecular areas normally attracted to mutant huntingtin snipped out.
Now, unable to be hijacked, the engineered CBP could perform its survival task. "Instead of degenerating," Ross says, "cells in these cultures remained healthy. We were able to rescue them completely."
The researchers have not yet demonstrated the turnaround in a live mouse model, according to Ross. That's a critical step, both in proving the principle and taking a future road to human therapy. The researchers anticipate technical details will complicate this work. "Our research so far, however, offers a needed target for developing and testing new drugs." The results of the study also apply to a growing family of neurological genetic diseases that operate on a similar principle, the researchers say. They include the spinocerebellar ataxias, a set of rare but debilitating diseases of movement and gait.
The research was funded by grants from the Huntington's Disease Society of America, the Hereditary Disease Foundation and the National Institute of Neurological Disorders and Stroke.
Other researchers from Hopkins on the team are Masayuki Sasaki, Mathew F. Peters, Hui Huang, Jillian K. Cooper, Juan Troncoso and Valina Dawson. Hitoshi Takahashi, Mitsunori Yamada and Shoji Tsuji from Niigata University in Japan also participated.