Huntington's disease

Huntington's disease is an inherited brain disorder that slowly destroys a cluster of cells deep in the brain, robbing a person of smooth movement, clear thinking, and steady mood over many years. Its most striking sign is chorea — restless, dance-like twitches and writhing that the person cannot hold still (the name comes from the Greek word for dance). The disease usually shows itself in midlife, around the thirties or forties, and gradually worsens. It is passed down in families: if a parent carries the faulty gene, each child has a one-in-two chance of inheriting it, and almost everyone who inherits it will eventually develop the illness.

The cause is a single tiny stutter in one gene, called HTT. Inside that gene a short sequence of three DNA letters — C, A, G — is meant to repeat a modest number of times, but in Huntington's disease it repeats far too often, like a key on a keyboard that gets stuck and types the same thing over and over. Roughly 36 or more of these CAG repeats produce a misshapen, sticky version of a protein that slowly poisons neurons, hitting hardest in a region called the striatum that helps plan and smooth out movement. As those cells die, control of movement, memory, judgment, and emotion all unravel together.

There is no cure yet, and the disease is fatal over time, usually ten to twenty years after symptoms begin. Treatment focuses on easing the burden: medicines can quiet the worst of the chorea, lift depression, or steady mood, while physiotherapy and family support help people stay independent for as long as possible. Because the exact gene is known, a simple blood test can tell whether someone carries it — a heavy choice many at-risk families weigh carefully, since the result reaches into the whole family's future.