What is Huntington's disease?

Huntington's disease (HD) is an inherited neurological illness causing involuntary movements, severe emotional disturbance and cognitive decline. In the United States alone, about 30,000 people have HD. In addition, 35,000 people exhibit some symptoms and 75,000 people carry the abnormal gene that will cause them to develop the disease. There is no cure for this fatal disease.

A single abnormal gene produces HD. In 1993, scientists finally isolated the HD gene on chromosome 4. The gene codes for production of a protein called "huntingtin," whose function is still unknown. But the defective version of the gene has excessive repeats of a three-base sequence, "CAG." In the normal huntingtin gene, this sequence is repeated between 11 and 29 times. In the mutant gene, the repeat occurs over and over again, from 40 times to more than 80.

This defect causes the resulting huntingtin protein to be malformed, prone to clumping in the brain and causing the death of nearby nerve cells. Cells of the basal ganglia, a brain area responsible for coordinating movement, and of the cortex, which controls thought, perception and memory, are most often affected.