Huntington Disease Dementia (cont.)

Huntington disease is a hereditary disease. This means it is passed down in families from one generation to the next via an abnormal gene.

Genes are made up of DNA (deoxyribonucleic acid). DNA molecules are made up of building blocks called bases. The 4 bases are adenine, thymine, cytosine, and guanine, called A, T, C, and G for short.

• A gene consists of a sequence of bases that together determine a characteristic or trait, such as eye color or body type.
  
  
• Each gene in the body has its unique location somewhere on one of 23 chromosomes.
  
  
• Each person inherits 2 of each chromosome, one from his or her mother and one from the father. Thus, each healthy person has 2 copies of every human gene.

The normal HD gene, which is located on chromosome 4, contains the base sequence CAG. The abnormal repetition of this sequence is known as the HD mutation.

• The number of repeats varies and is key to a person’s risk of developing the disease.
  
  
• People who have 38 or more CAG repeats in the HD gene are genetically programmed to develop the disease.
  
  
• This is determined when the person is conceived.
  
  

Huntington disease is an autosomal dominant disorder. This means that the HD mutation takes precedence over the normal gene.

• Each of us inherits 2 copies of each gene, 1 from each parent. A person who has a parent with Huntington disease has a 50% chance of inheriting the HD mutation.
  
  
• Inheriting the HD mutation does not automatically mean that the person will develop the disease.
  
  
• The number of CAG repeats determines the person’s risk of developing the disease and, to a certain extent, his or her children's risk.
  
  
• Occasionally a person with no family history of HD develops the gene abnormality and thus the disease. Such “sporadic” cases are rare.
  
  

The HD gene directs production of a protein called huntington. This protein is found in neurons throughout the brain, but its normal function is unknown.

• The abnormal huntington protein produced by the abnormal gene is believed to interact with other proteins to cause destruction of neurons.
  
  
• Most of the damage occurs in the basal ganglia, a part of the brain that controls movement and many other important functions.
  
  
• The damage also affects parts of the brain that control cognitive processes such as thinking, memory, and perception.