The symptoms and characteristics of juvenile Huntington’s disease.
How are large repeat numbers related to the increased severity of juvenile HD?
Individuals with early-onset HD usually have a number of CAG repeats in a particular segment of the Huntington gene that is much larger than the number of repeats seen in adult-onset HD. The largest number of CAG repeats seen thus far is around 250, but most individuals with juvenile HD have between 80 and 100 repeats.
Many studies have shown a correlation between the number of repeats and the age of onset. Usually, the more repeats, the younger the age of onset. This correlation, however, does not prove that the large repeat numbers actually cause the earlier onset. It merely suggests that the two events - large repeat numbers and early age of onset - usually occur together. (For another example of correlation and causation click here.)
Most individuals with juvenile HD experience an age of onset that is much younger than that of their affected parents. They also often face a much more rapid progression of the disease. This occurrence is described as genetic anticipation, where a disease increases in severity in successive generations, and a parent can produce a child with a more severe form of a disease. In the case of HD, the expanded section of triplet repeats provides a possible (though still unconfirmed) explanation for the pattern of anticipation seen in HD inheritance. As the number of repeats grows between generations, the severity of the disease increases, and individuals experience an earlier age of onset and a more rapid development of the disease.
Last Modified: 6-9-04
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