In 1993, scientists discovered exactly where the gene which causes Huntington’s disease is located in our DNA and figured out what it was doing. Because of this people are now able to have what is called a ‘predictive test’. It is known as a predictive test because the test can be taken by people at risk of Huntington’s disease who currently show no symptoms and want to learn whether they will develop the condition in the future. The result of this test will usually be able to tell a person whether they have the expanded gene or not.

It is important to note that if a person already has symptoms but has not been diagnosed, then a different approach would be taken which is known as ‘symptomatic testing’. This would involve that person going through a series of assessments with a doctor to confirm a diagnosis of Huntington’s disease and is quite different to ‘Predictive Testing’, which is what we are focusing on in this section.

The Science Behind Testing

You may hear people say that they ‘have the gene’ when referring to themselves as having Huntington’s disease. But the truth is, all humans have the gene that may cause Huntington’s disease – even those who never develop the condition. So if we all have the gene, why do some develop Huntington’s disease when others do not? The difference is people who develop symptoms of Huntington’s disease have a larger expansion of the gene than normal.

What is This Gene Expansion?

Without getting too scientific, there is a section, in the gene, which repeats itself over and over again. This is known as a ‘CAG repeat’ and each individual has a different amount of repeats. However, people with Huntington’s disease tend to have more repeats than usual, which means that the gene expands more than it should. This expansion can lead to Huntington’s disease.

The reason we are explaining CAG repeats is because of their relevance to predictive testing for Huntington’s disease. When people at-risk for Huntington’s disease have a predictive test, that person’s DNA is sent to a lab, and the clinical scientists will use special machines and lab techniques that will count how many CAG repeats that person has in the gene specific to Huntington’s disease. Once they know how many repeats a person has, they can tell if they will get the disease or not.

How Many CAG Repeats Will Cause Huntington’s Disease?

CAG repeats are split into four categories. If a person has less than 27 CAG repeats then they are what is called, ‘negative’ – meaning they will not get Huntington’s disease and they are not at risk of passing the disease onto any children that person has.

Someone who has 40 CAG repeats or more will definitely develop symptoms of Huntington’s disease in their lifetime (although it is not possible to say when symptoms will start) and the risk to any children is 50%.

If a person has between 27-35 CAG repeats, they will not develop Huntington’s disease, but there will still be a very small risk of that person passing on the disease to any children (the risk is about 5%).

Whereas somebody who has a CAG repeat between 36-39, would be in what is sometimes called the ‘grey area’, because it is uncertain whether they will develop symptoms in their lifetime or not.

Less than 27 CAG repeats

Negative – will not develop Huntington’s disease and children not at risk

27-35 CAG repeats

Negative – will not develop the disease but small risk to children.

36-39 CAG repeats

Positive – but may develop symptoms at an older age. 50% risk to children.

40+ CAG repeats

Positive – will develop symptoms in your lifetime. 50% risk to children.

Technically people in the 36-39 CAG repeat range do have the expanded gene, but people in this range may develop symptoms of Huntington’s disease at an older age. However, the risk of passing the expanded gene onto any children is 50%.

If you have been tested it is always important to be clear about the result of your predictive test. If you have any questions, or do not understand what the result has shown, do go back to the clinician who arranged your test.