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Huntington's Disease

CAG Genetic Coding and Huntington's Disease

03/20/2008

Question:

What is CAG?

Answer:

Huntington's disease is a genetic disorder. Huntington's disease is a type of genetic disorder referred to as a "trinucleotide repeat disorder." Trinucleotides are the three nucleotide codes in the DNA. Each sequence of three nucleotides codes for one amino acid that will be formed into a protein based on the genetic code.

CAG refers to one set of the codes, a trinucleotide. It codes for the protein glutamine. In Huntington's disease, the gene for the protein called Huntington has an increased number of glutamines in a row within the protein, making it bigger than normal and causing the disease.

Everyone has a slightly different number of CAG repeats in their Huntington gene. Most people have between 15 and 25 CAG repeats on their genes. The number of repeats can vary from generation to generation but tends to increase more often than it decreases. Once the repeat number gets bigger than 36 or 37, the individual will develop Huntington's disease.

There is "rough" correlation between CAG repeat number and age of disease onset. However, while this holds for the population as a whole, the number is not really very helpful in predicting an individual's likely age of onset. For example, one day in clinic my first patient had a repeat size of 42 and was already showing symptoms of the disease even though he was 25 years old. My last patient that day also had a CAG repeat size of 42, but he was 65 years and only beginning to show the first signs of the disease. Most people with a repeat size of 42 can be expected to show disease onset at around age 40-50, but the age range when the disease really starts is over 40 years from the 20s to the 60s or more.

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Response by:

Sandra  K Kostyk, MD, PhD Sandra K Kostyk, MD, PhD
Assistant Professor of Neuroscience
Clinical Assistant Professor of Neurology
College of Medicine
The Ohio State University