Genetic testing for Huntington’s is more than a simple coin toss

More research, genetic testing could answer lingering questions about the disease

B.J. Viau avatar

by B.J. Viau |

Share this article:

Share article via email
banner image for

The risk of potentially having Huntington’s disease (HD) if you have an inherited gene mutation has often been described as a coin toss — you either have the mutant gene that causes HD or you don’t. Your kids will either be at risk or they won’t. It’s easy to describe HD this way, especially to friends, family, and others who simply want to know what Huntington’s is.

But the more I learn about the genetics of HD, the more I realize that we should stop using this analogy in the HD community. In my opinion, you can still describe the risk of HD as a coin toss, but with a much thicker coin, as there’s a small, but not impossible, chance that it lands on its side.

Recommended Reading
A person's brain is shown in a profile drawing.

Effects on Different Neurons May Explain How Symptoms Evolve

How it works

When you undergo genetic testing for HD, you’re after knowledge about CAG repeats in the huntingtin gene. In the clinical world, it’s said that 40 or more CAG repeats means fully penetrant HD, meaning you will one day get it, and your children are at risk. A range of 36 to 39 CAG repeats means you might get HD one day, and your kids are still at risk. A range of 27 to 35 CAG repeats means you likely won’t get it, but your kids have a small risk of getting it. Fewer than 26 CAG repeats mean you won’t get HD, and your kids are not at risk.

Receiving the news of CAG repeats ranging from 27 to 39 represents the side of the coin. It’s a result that’s unlikely, but not unheard of, and it comes with a bit of the unknown. This range is a gray area, which is one of the reasons that having a licensed genetic counselor who is familiar with HD is so important. Too many people still rely on uneducated healthcare professionals who think of testing as a simple coin toss.

It’s also one of the reasons I was passionate about creating the genetic counseling and testing company HD Genetics.

My genetic results fall in the category of the side of the coin, as I inherited 31 CAG repeats from my mom. This means that I likely won’t get HD, as I tested “negative,” but there is still a small chance of passing HD to my biological children.

Research on the gray area is still limited, and as happy as I am to have low, gray-area CAG repeats, it still causes me anxiety from time to time when I think about the many unknowns. Some questions that occasionally pop into my head are: “How specific is the data to indicate that I am really HD negative?” “What is the true risk percentage of my child?” Unfortunately, I don’t think anyone will be able to answer those questions anytime soon.

We can do more

In 2019, multiple studies revealed newly discovered factors in genetic tracts that can determine whether symptoms appear earlier or later in life. So I wonder, why aren’t we providing additional genetic testing to figure out this important additional information for people?

I also wonder whether people would care to know, and if so, would they pay an additional fee to run another specific genetic test? It’s knowledge that could help make the gray area a little less gray. It could also help people with 40 or more CAG repeats understand more about the timing of their onset of symptoms.

If this column does anything, I hope it provides some education about the gray area in genetic testing for HD. I also hope it prompts folks to start asking more questions.

Note: Huntington’s Disease News is strictly a news and information website about the disease. It does not provide medical advice, diagnosis, or treatment. This content is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read on this website. The opinions expressed in this column are not those of Huntington’s Disease News or its parent company, BioNews, and are intended to spark discussion about issues pertaining to Huntington’s disease.


Leave a comment

Fill in the required fields to post. Your email address will not be published.