The symptoms and characteristics of juvenile Huntington’s disease.
How is juvenile HD inherited?
Juvenile HD is caused by the same gene that causes adult-onset HD. The version of the Huntington allele causing early-onset HD usually has, however, a greater number of CAG repeats. Because the early-onset and late-onset forms depend upon the same gene, early-onset HD is inherited in the same manner as adult-onset HD. (To read about how the HD allele is inherited, click here.)
An individual with juvenile HD inherited the HD allele from one of his or her parents. In most cases, this allele seems to be paternally inherited, following the “paternal triplet expansion hypothesis” discussed previously. Usually, it is not maternally inherited unless the mother herself had juvenile HD (since in this case the mother would already have a number of repeats on the order of those seen in juvenile HD).
Due to the rapid progression of the disease, most individuals with juvenile HD do not survive to bear children of their own. For those who do, however, their children have the same 50% risk of inheriting the HD allele as the children of individuals with adult-onset HD. The number of CAG repeats in the Huntington gene of an individual with juvenile HD is normally very high (even compared to that of individuals with adult-onset HD). Since repeat numbers tend to increase rather than decrease in successive generations, it is likely that the child of such an individual will have a similar or larger number of repeats if he or she inherits the altered allele. Given the correlation between repeat number and age of onset (discussed in the previous section), it is very likely that the child will also develop juvenile HD. In short, the child of an individual with juvenile HD has a 50% chance of inheriting the HD allele. If the child does inherit the altered allele, he or she is very likely to develop juvenile HD.
Last Modified: 6-9-04
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