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How predictive is age of onset from previous generations?

May 23, 2017

Huntington's Disease Youth Organization

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How predictive is age of onset from previous generations?

Q. My boyfriend is positive for the HD gene. His grandmother was diagnosed in her 70’s. His mother is also positive for the gene. She is 60 and showing no symptoms yet with a count of 40. How predictive is age of onset from previous generations? Does this mean he may not present until later in life since that seems to be the trend in his family history?

Melissa, Young Adult, USA

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A. Hi Melissa,

Many thanks for your question.

It is definitely good news that your boyfriend’s mother has not had any symptoms at 60. Age at onset from previous generations can be a rough guide – for example it is unusual for the age of onset to be 70’s in one generation and 30’s the next (particularly when the gene expansion has been passed through a female). However, there can still be quite a lot of variability, even within the same family, so it has to be used with caution.

You also mention that your boyfriend’s mother has 40 CAG repeats (only just into the range that definitely causes symptoms). As you may know, some of the variation in age of onset is linked to CAG repeat size, although for an individual it is currently of pretty limited predictive value. There is a lot of research going on into possible other factors that could contribute to age of onset, and it is likely that these include other gene(s) as well as non-genetic factors (possibly things like diet and lifestyle).

To summarise, I would certainly be hopeful that your boyfriend is more likely to present later in life, but unfortunately the variability means there is currently no way of predicting this with any certainty.

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