My family seems to develop HD later, if I test positive will I?
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Q. My great nan had huntingtons disease (did not pass away until early 80’s) my nan currently has it and my my mum currently has it. I have not had the test. My nan is 67 and still lives alone with a little help from my mum but copes day to day still. My mum is 50 and although I am starting to see behavioural changes she too is doing very well. As my line of family seem to develop the disease a lot later does this mean if I tested positive I would be more likely to develop the disease at a later age?
Charlotte, Young Adult, UK
A. Hi Charlotte,
Thank you for contacting HDYO. I am sorry to hear that your mum and nan have HD, although it sounds like you are all coping very well in the circumstances, particularly when your nan is still living alone.
In terms of age of onset, it is certainly reassuring that no-one in your family has developed symptoms at a young age. While this would make a later age at onset more likely if you have inherited it, unfortunately it does not completely guarantee it. This is because age at onset can be quite variable even within a family, and appears to be affected by a variety of different factors, some of which we know about but others are (as yet) unknown.
You may be aware that some of the variability is linked with the size of the HD gene (the ‘CAG’ repeat size – see this HDYO article http://en.hdyo.org/eve/articles/50 for more information about CAG repeats). We know that, in general, people who have a young age at onset tend to have larger CAG repeats. If you have inherited it, we would expect your CAG size to be around the same size as your mum’s. BUT the repeat size is only half the explanation – so at the moment knowing the exact CAG repeat size is not really very helpful in predicting the age of onset of symptoms for an individual (i.e. two people with exactly the same CAG size can still have very different ages at onset). There is lots of research going on to investigate what other genetic factors, as well as potential lifestyle/environmental factors, could modify the age of symptoms. This may mean that in the future it may become possible to predict the age of onset more accurately.
I hope this helps answer your question but please do not hesitate to get back to us if you have any further queries.