Genetics of Huntington's Disease: An Introduction
Huntington's disease (also known as HD or Huntington disease) is a condition that is passed from parent to child through a defect (mutation) of a gene located on chromosome number 4. This gene is known as the Huntington disease gene or HD gene.
Understanding the HD Gene and Huntington's Disease
The Huntington disease gene makes a protein called huntingtin. The function of this protein is unknown, but it most likely plays an important role in nerve cells. When a defect occurs in the Huntington disease gene, it leads to the body creating an abnormal huntingtin protein. This abnormal protein disrupts the normal function of certain nerve cells and ultimately leads to their premature death. The dysfunction and loss of nerve cells cause the signs and
symptoms of Huntington disease.
Genetics of Huntington's Disease: Testing for HD
The mutation in the Huntington disease gene is a short DNA sequence called a CAG repeat that is abnormally repeated many times. The expanded repeat leads to the production of a huntingtin protein that contains a long stretch of the amino acid
glutamine (amino acids are the building blocks of proteins).
The Huntington disease genetic test involves counting the number these repeats in the Huntington disease gene region. Individuals who do not have HD usually have 28 or fewer CAG repeats. Individuals with HD usually have 40 or more repeats. A small percentage of individuals, however, have a number of repeats that fall within a borderline region (see Table 1).
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