Huntington disease is caused by a genetic defect on chromosome 4. The defect causes a part of DNA, called a CAG repeat, to occur many more times than it is supposed to. Normally, this section of DNA is repeated 10 to 28 times. But in persons with Huntington disease, it is repeated 36 to 120 times.
As the gene is passed down through families, the number of repeats tend to get larger. The larger the number of repeats, the higher your chance of developing symptoms at an earlier age. Therefore, as the disease is passed along in families, symptoms develop at younger and younger ages.
There are two forms of Huntington disease.
- Adult-onset Huntington disease is the most common. Persons with this form usually develop symptoms in their mid 30s and 40s.
- Early-onset Huntington disease affects a small number of cases and begins in childhood or the teens.
If one of your parents has Huntington disease, you have a 50% chance of getting the gene. If you get the gene from both your parents, you will develop the disease at some point in your life. You will also pass it on to your children. If you do not get the gene from your parents, you cannot pass the gene on to your children.