Repeat count and symptoms
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Q. Hello there, I hope you can help me I’m 27.
My Grandad has HD and started showing symptoms around 71 years old. He is now 81 & in the later stages of it.
My Mum was tested and has HD, she is now 53 and shows no symptoms. She has exactly the same amount of repeats as my Grandad (i’m unsure of the exact number) these are my questions.
- As my Grandad didn’t develop symptoms until later life and my Mum still has no symptoms if I have the gene does this improve my chances of getting symptoms in later life with the same count? And if so how likely is this?
- If I do have a higher count than them does this mean I will get symptoms earlier than my Mum and Grandad?
- How likely is it I’ll have a higher count?
- From getting the symptoms how long/ many years would it be before they were obvious to people around me and I became fully dependent?
I hope to hear from you soon. I am currently debating being tested and these answers play a nig part in my decision.
A. Dear Nancy,
Thank you very much for your email. From your point of view it is encouraging that your granddad developed symptoms later in life and that your mum is now age 53 and still not developed symptoms.
You’ve raised some very interesting questions about the age of onset of symptoms and the limits of information that a predictive test can provide. We do know that there is a relationship between the CAG repeat size and age of onset, but it does not account for all the variability. Scientists are still looking in to the question of what other factors may determine age at onset. Currently we’re not able to give someone an accurate idea of the age when symptoms will develop. How symptoms present can also vary within families.
Although the number of CAG repeats can sometimes increase when passed on, this is more likely to happen when the HD gene fault has been passed on from a father.
So you are right Nancy in recognising that a predictive test would not necessarily answer all your questions. It’s very useful that you are asking these types of questions so as not to have unrealistic expectations of the information a predictive test will provide. Your questions could be discussed more fully with a genetic counsellor (I’m not sure where you live Nancy, but if you are unsure how to access the nearest genetics centre, please email us again). A genetic counsellor would be able to request a copy of the genetic test results of both your grandfather and mother, provided they have consented to this. It would be important to clarify for example whether your mum and your grandfather had a result in the reduced penetrance range (36-39 repeats) or the full expansion (>39 repeats). There is a very good section in HDYO about CAG repeat size and the possible outcomes of a genetic test result.
Having a genetic counselling appointment does not commit you to any course of any action, and we often get asked questions at the stage when someone might be just thinking of having a test or simply wanting more information.
Your final question asked about the rate of progression and at what stage people would begin to notice symptoms. Whilst a predictive test would not answer this question, in the case of someone like your mum, who you say is not showing symptoms but has been shown to have the HD gene fault, follow up at an HD specialist centre can be particularly useful. A medical examination by a doctor with specialist knowledge of HD (a geneticist or neurologist), can help to answer where things are up to and can provide information about any early signs if an individual wishes to know if these are present. There is also the opportunity to participate in research and your mum would be able to discuss this with her doctor.
Thinking more generally about the pros and cons of predictive testing, if you like to read information on the internet as well as individual genetic counselling, you might find the following website a useful resource. http://predictivetestingforhd.com/
I wish you all the best Nancy, I’m sure it will be useful to talk through your options more fully with a genetic counsellor. Please also do feel free to ask as many questions as you like at any stage in the process. Timing may be an issue and the option of a predictive test is one that will always be open to you: your questions are important and deserve very thorough answers.