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Mum's CAG repeat is 30 - how are her children affected?

September 27, 2016

Huntington's Disease Youth Organization

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Mum's CAG repeat is 30 - how are her children affected?

Q. Hello! My aunt passed from huntington’s several years ago even though it appeared that neither grandparent (gma passed at 75 and gpa passed at 86 of natural causes and heart disease) had it.

We thought she was a complete mutation. When my sister spoke with her ob/gyn about having another baby she mentioned that HD was in our family. They said my mom should get tested as that would give us all a negative if she had a negative result..

Her results came back yesterday and she has a CAG repeat of 30. I understand that she will never show symptoms in her lifetime. What I am unclear about is how that affects me and my siblings. Is there a chance that if we received the mutated gene that it could expand into either the reduced or full penetrance range? What about her grandchildren?

I appreciate any thoughts you could give me on this as I am very afraid!

Thank you.


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A. A. Hi Sarah,

Thank you for contacting us, and I am sorry to hear this is causing you a lot of worry.

You are absolutely correct that a CAG repeat size of 30 would not be expected to cause HD for your mom as it is only in the ‘intermediate’ range (of 27-35 repeats). Individuals with an intermediate CAG repeat size are not considered to be at risk of HD themselves, but there can be implications for future generations, which brings me to your main question.

I can completely understand your concerns about whether you could have a risk of a larger CAG expansion, but there are two reasons why the situation might not be as bad as you fear:

1) Even in the intermediate range, when the gene is passed on to the next generation, the CAG repeat size usually stays more or less the same. To put it another way, the majority of children will have exactly the same number of CAG repeats as their parent.

2) Although occasionally an expansion (or contraction) of an intermediate gene between generations does happen, this is almost always only when it has been passed on by a male parent.

So, in answer to your question, I would be keen to reassure you that it is very unlikely indeed that you (or your siblings) would have inherited a reduced penetrance or full expansion HD gene from your mom. As you probably know there will be a 50% chance you inherited the intermediate size HD gene from her, but even if you have, it would almost certainly be also an intermediate size. Therefore the same would apply to any children you (or your sisters) may have. So your mom’s result only really affects any male descendants she may have, as if they inherited the intermediate gene there may be a chance (albeit still quite a small chance) that they could have a child with a larger HD gene.

I hope this makes sense and helps answer your question. Please feel free to get back to me if you have any further queries. Depending on where in the world you live, you may also be able to arrange an appointment in a Genetics clinic to clarify things further, should you wish.

Best wishes