My grandad got HD in his 50s - could my mum and I have it?
Q. Hi. My grandad died of HD in his 50’s. He and my grandmother had 5 children. 2 of their sons (my uncles) developed the disease in their late 20’s and died, and one of their daughters (my auntie) developed it in her 30’s and has also passed away. My mum is now 64 and symptom free. What are the chances that she is a carrier and could develop late-onset Huntington’s? And if she is a carrier and has passed on the faulty gene to me, would it also be a late-onset form of the disease, or is the CAG count random and in no way liked to the family history? Thank you in advance.
Laura, Young Adult, France
A. Hi Laura,
Thank you for your question, and sorry for the delay in reply.
It is certainly reassuring that your mum does not have symptoms of HD into her 60’s, particularly given that other members of the family have had symptoms by a young age. As you may know, because of the way the HD gene is inherited, your mother would have had a 50% chance of inheriting the faulty gene when she was born, but given she has not had symptoms by 64 the chance she carries it now will be quite a bit lower than 50%, and this in turn reduces the likelihood that you will have inherited it. Because the age at which symptoms develop is quite variable, we could never rule it out completely just on the basis of someone having no symptoms, but it is certainly now looking less likely that your mother carries the faulty HD gene.
Your question about CAG counts is a very good one. Whenever the HD gene is passed on from a parent to a child, the CAG repeat size usually stays roughly the same. However, occasionally it can change in size (usually getting bigger, but sometimes smaller) and this appears to be slightly more likely when passed on from a male parent. CAG size is relevant to an extent, as we know that some of the variability in age at onset of HD symptoms is due to differences in CAG repeat size – i.e. often people who develop HD at a younger age than average have a larger CAG size than individuals who develop symptoms later in life. However, CAG repeat size is only half the explanation, and lots of research is going on to try to identify what other genes and/or other factors might influence age at onset. Therefore, at the current time, knowing an individual’s CAG size is of limited use in predicting their age of onset of HD symptoms.
I hope you find this helpful, and of some reassurance. Please do not hesitate to get back to us if you have any queries.