What are the chances of getting JHD?
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Q. Everyone at risk of HD has a 50% chance of getting it but how do I know what my chances of getting JHD? Is it something you only get if your family had the JHD gene? Or is it something that any one at risk of normal HD can get irrelevant to when their parent got the disease?
Alexandra, 19, Australia
A. Dear Alexandra,
This is a very important question. The answer is likely to long, so apologies in advance.
Juvenile HD (JHD) occurs in children & teenagers and is traditionally defined as HD with an age of onset of symptoms less than 20. However, there is a continuum with HD occurring in adults in their 20s. That is, there is no hard and fast cut off point between JHD and HD in young adults.
The usual situation in JHD/young adult onset HD is that the young person has inherited the gene from their father and that the CAG repeat length (the number in the gene test report) is larger than usual for HD. However, JHD/young adult onset HD can much less commonly occur when there is inheritance form the mother.
Most people with HD have a CAG repeat number in the low 40s and get symptoms between 35 & 55. People with JHD/young adult onset HD usually have a CAG repeat length of over 50. In general, the bigger the number the younger the age of onset. However, the exact size of the CAG repeat length does not allow a doctor to accurately predict the likely age of onset.
If a parent has HD then each child is at a 50% (1 in 2) risk of inheriting the HD gene. However, only a few percent of people with HD have juvenile onset disease. JHD is rare
The reason JHD/young adult onset HD most often occurs when the gene is inherited from the father is that at conception, when egg meets sperm & the genes of both parents combine to make the new child, the CAG repeat length can increase. Why this is commoner when the gene is passed from father to child rather than from mother to child is unknown.
The process of increased size of the CAG repeat length and therefore an earlier age of onset is called anticipation because the age of onset in the next generation is earlier (i.e. anticipates) the age of onset in the previous generation. However, anticipation is rare. That is, most times if the child inherits the HD gene from the father the CAG repeat length is the same.
It is very difficult to predict the age of onset from the size of the CAG repeat length. For that reason I don’t use the age at which a parent first got symptoms to predict the age at which a child who inherits the gene will get HD. That is, I find the CAG repeat length to be more useful than the age of onset of HD in the parent. However, even the CAG repeat length is a poor guide to age of onset.
I hope this answers your question,