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Will I get earlier onset than my dad?

November 30, 2016

Huntington's Disease Youth Organization

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Will I get earlier onset than my dad?

Q. My dad was recently diagnosed with late onset hd. He is pre-symptomatic and 60. I am aware that the disease often gets worse passed from father to children. Is there any data regarding how often anticipation occurs? Would a longer cag repeat length than his tell me whether I would be earlier onset than him? Thanks!

Julianna, Young Adult, USA

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A. Hi Julianna

Many thanks for contacting HDYO, you raise some very good questions and I will do my best to answer them. To take your last question about CAG repeats, the short answer is that we are not currently able to use CAG repeat size to make any meaningful prediction about age of onset in an individual. This is because the CAG repeat size is only relevant to an extent – while it is true there is a general association between larger repeat sizes and earlier age at onset, it only explains about half of the variability we see in age at onset of symptoms. Therefore, at the current time, knowing an individual’s CAG size is of fairly limited use – as an example, two individuals with exactly the same CAG repeat length can have very different ages at onset of symptoms. Lots of research is going on to try to identify what other genes and/or other factors might influence the age of onset, so it may be that in the future knowing the CAG size will be more helpful.

In terms of your question about anticipation, it is known that the age of onset can sometimes get earlier as the gene is passed down the generations. This is partly because when the gene is passed on by a male parent, the CAG repeats can be a bit more unstable and can sometimes get bigger. However, this is still relatively unusual. Exact figures are quite hard to find, but one research article I found reported that only around 6% of children who had inherited HD from their father had a significantly younger age of onset.

I hope this helps answer your questions, and please get back to us if you have any queries.

You may also find it helpful to see a Genetic Counsellor at some stage, as they could go over your family history in more detail and would then be able to give you more tailored information. I notice you live in the USA Julianna, so you may find this website helpful to identify a local Genetics service if you wish.

Best wishes,