23/07/10

Huntington disease: An hereditary disorder with
mental and physical deterioration leading to death. Although
characterized as an "adult-onset" disease, it can affect
children as well.

Huntington disease describes an autosomal dominant pattern of
inheritance with high penetrance (a high proportion of persons with
the gene develop the disease). The characteristic findings of
Huntington disease are caused by loss of neurons (nerve cells) in the
brain.

The disease is due a gene in chromosome band 4p16.3. The
gene, called HD, contains an unstable repeating sequence of 3
nucleotide bases (CAG) in the DNA. Normal people have an average of
19 CAG repeats and at most 34 such repeats while virtually all
patients with Huntington disease have more than 40.

The Huntington disease gene codes for a protein that has been
named (confusingly) huntingtin whose function is unknown. The
elevated numbers of CAG repeats in the Huntington disease gene
lead to the production of an elongated huntingtin protein which
appears to correlate with the loss of neurons in the disease.

Mood disturbance is usually the first symptom seen, with bipolar
disorder-like mood swings that may include mania, depression, extreme
irritability or angry outbursts, and psychosis. Other symptoms
include chorea (restless, wiggling, turning movements), muscle
stiffness and slowness of movement, and difficulties with memory and
other cognitive processes. The HD gene is located on chromosome 4,
and is an autosomal dominant gene. Only one copy need be inherited to
cause the illness. Diagnosis is by genetic testing, and family
members of people with Huntington's disease may also want to know if
they carry the HD gene. At this time, there is no cure for HD,
although medication may be used to control symptoms of the illness,
such as mood swings and chorea.

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