The Inheritance of HD
Part 7

An introductory guide to the genetics of Huntington’s disease.

If I have the gene, what does that mean? Will it definitely express itself, and if so, when will this happen?

Historically, an individual known to have an HD allele almost always developed symptoms of the disease, unless he or she died of some other cause prior to onset of symptoms. Individual cases have varied greatly in severity and in rate of progression for reasons that are still not yet fully understood. As a very general rule, the typical age of onset for adult-onset HD is between the ages of 30 and 50. In most instances, individuals live with the disease for 10 to 25 years. Several studies indicate that the number of CAG codons plays a role in how soon symptoms appear. The general trend appears to be “the greater the number of repeats, the earlier the onset of the disease” although there is considerable variation. There is also evidence to suggest that the average age of onset is later for individuals who inherited the HD allele from their mothers than for individuals who inherited the allele from their fathers. It follows then that, as a general rule, onset occurs earlier when the HD allele is inherited via one’s father.

About 10 percent of all HD cases are classified as the juvenile form, which has an age of onset between infancy and 20 years. The juvenile form generally occurs when the number of CAG codons is especially large (on the order of 55 and above). Individuals with juvenile HD have different symptoms, such as rigidity, seizures, and dementia, than do individuals with adult HD. In addition, the progression of juvenile HD is usually much more rapid. (Click here for more about juvenile Huntington’s disease.)

Figure C-4 shows the correlation between increasing number of CAG repeats, from 39 to 50, and decreasing age of onset. The shaded bars show the median age of onset for individuals with a given number of CAG repeats. The exact numbers used for this graph are shown in Table C-2. Table C-2 also shows, in its third column, a way to represent the range in age of onset that occurs at each number of repeats. The figures in the third column correspond to 85% confidence intervals (C.I.) around the average age of onset. A confidence interval means that for a given number of repeats, we can be 85% sure that the actual age of onset lies within the given age range. This range is represented below in Figure C-4 by yellow bars. Please note that these range figures must be interpreted carefully: they specifically do not imply that an individual who remains symptom-free throughout a given range is exempt from HD. Any particular individual may very well develop symptoms at either an earlier or later age than those shown in the table.

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-A. Hsu, updated 7-1-04

For further reading:

1. "Huntington's Disease: A Brief Explanation" International Huntington Association. Online.
2. "Huntington's Disease" Online Mendelian Inheritance in Man. Online.
3. "GuideLines for Genetic Testing for Huntington's Disease." Huntington's Disease Society of America. Online.

Last Modified: 08/04/2008

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