I have 42 repeats, my dad had 40 - what does that mean?
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Q. Hello I have not long found out that my father has inherited hd from my grandmother, my father is now 50 and he’s repeat on the scale is 40. Once I found this out I got myself tested and it came back positive my score on the scale was 42. So does this mean I will get symptoms earlier and maybe more severe than what he has? I do not really understand the scale side of things.
Thank you for taking your time to read my question
Tanya, Young Adult, UK
A. Dear Tanya,
Many thanks for your question.
The fact that you have 42 ‘CAG’ repeats does not mean that you will definitely get symptoms any earlier or any more severe compared to your father with 40 CAG repeats. As you probably know, the age at which symptoms of HD begin can be very variable, even within the same family. CAG repeat size is only relevant to an extent – while there is an association between repeat size and age at onset, this only explains about half of the variability we see in age at onset of symptoms, and lots of research is going on to try to identify what other genes and/or other factors might influence this. Therefore, at the current time, knowing an individual’s CAG size is actually of fairly limited use in predicting the age of onset of their HD symptoms. So, while it is possible that you will develop symptoms at an earlier age than your father, it is also entirely possible that symptoms will develop at a later age than his.
It is also important to know that the difficult laboratory techniques involved in HD testing means that there is a margin of error of at least one CAG repeat when the size of the gene is measured. Therefore, it is not impossible that you and your father actually have the same size gene (for example, 41 repeats).
You can find some more information about the science behind CAG repeats in this HDYO article
I hope this is helpful, and please do not hesitate to contact me if you have any further questions or concerns. I’m sure the Genetics team who arranged your test would also be happy to arrange an appointment if you would like to discuss any of this in person.