All About Mutations
Part 2

What it means to have a mutation and what role mutations play in Huntington’s disease.

If an expansion occurs, what is the effect on the child?

For a person to either develop HD himself or remain asymptomatic but have children with HD, the absolute minimum number of CAGs in the repeat region of the Huntington gene is generally recognized by researchers as 36. (See Table A-1.) The consequences of expansions of the CAG repeat region (in the transmission from parent to child) depend on the number of repeats in the parents. We consider the three possibilities. The most common situation involves asymptomatic parents who both have fewer than 36 CAGs in the repeat region. Only very rarely does an expansion occur in this situation, and when one does, the children typically remain asymptomatic because the expansion is unlikely to be large enough to push the children over the 40 CAG mark. It is incredibly rare for a person to develop HD when his or her parents did not have at least 36 copies of CAG in the Huntington gene.

The second scenario involves people with between 36 and 40 copies of CAG. As mentioned in The Basics of HD, these people may or may not have HD themselves. However, regardless of whether they have HD or not, they are at risk of having children with HD because expansion will likely result in the children having more than 40 CAGs, and more than 40 CAGs almost always means that the person will develop HD.

The third scenario involves people who themselves have HD. If expansion occurs in this situation, not only will the child likely have HD, but also the increased number of CAGs means that the child's symptoms will likely start showing at an earlier age than the parent's symptoms did. (Click here for more about early onset in children with HD.) This happens because generally the higher the number of CAGs in the repeat region of the Huntington gene, the earlier the age of onset. (To read about more about juvenile HD, click here.)

Last Modified: 1-28-04

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