Huntington's Disease Society of America

Genetic Testing

In 1993 scientists discovered

exactly where the gene which causes Huntington’s disease (HD) is located so it is now possible to be tested to see if you carry the changed HD gene. This is called a predictive test and is one you can take before any symptoms of HD develop. N.B. Everyone has a copy of the HD gene on Chromosome 4 but someone who gets Huntington’s disease has a gene which is bigger than normal (see ‘What Causes HD?’ and ‘CAG repeats’).

The decision about whether to be tested can be very difficult. Below you will find some answers to commonly asked questions about the predictive genetic test and genetic counselling.

In this section we have only provided some basic information. If you want to find out more about genetic testing for HD we would encourage you to make an appointment with your local Genetics Service who can give you the most up to date information (see contact details below). You may also find some of the links to further information useful to read.

But a few things first…

At the NYA we advocate that taking the test to find out if you carry the changed HD gene is a very personal decision. Some people absolutely want to know, and others are quite clear that they prefer to live with not knowing. Some people also find that they change their minds from month to month, week to week, or day to day. It is up to each individual to think about what getting tested means for them, and how the knowledge of having or not having the changed gene will affect their life.

We encourage people to find out as much information as they can beforehand and to think through the advantages and disadvantages of knowing, or not knowing. We also advise people to think of the kind of support they might want if they go through the testing process.

And just because you can take the test, doesn’t mean you have to. In fact the majority of people who are at risk for HD choose not to take the test. Most people actually choose to live with the uncertainty of not knowing. At the same time there are those who are sure they want to know. So, there is no right or wrong answer, you must decide to do whatever is best for you and your circumstances. Different people make different decisions, decisions that are right for them.

Reasons for taking or not taking the test

Those who choose to be tested usually do so in order to be able to make informed plans for the future regarding marriage, having children, career, finances and so on. Others may simply crave relief from the anguish of being "at risk." For them, knowing, whatever the outcome, is better than not knowing.

However, experience has shown that while many at risk individuals say they wish to know their gene carrier status, far fewer actually undergo testing. The majority of people find the emotional burden of knowing, or risks to confidentiality e.g. parents demanding to know the results, outweigh the benefits of learning that they carry the changed gene.

It is important to remember that individuals should not be coerced into testing, whether by a spouse, another family member, a doctor, an insurance company or an employer.

Why do I have to be 18 to take the test?

There are International Guidelines for testing for HD which advise that the test should only be carried out on a person who has ‘reached the age of majority’ in their country. The age of majority is the chronological moment when children legally cease to be a minor and assumes control over their persons, actions and decisions, thereby terminating the legal control and legal responsibilities of their parents over and for them. In the United Kingdom the age of majority is 18. So, it is advised that young people do not go for the test until they are at least 18. This should not stop any young person who is under 18 seeking help or advice from a genetics or health/social care professional about HD. Also, these guidelines are just that; they are a set of principles written to guide professionals. In extreme circumstances some individuals have been tested under the age of 18. BUT there are very good reasons for not getting the test until you are at least 18. Here are some of them:

To protect a young person’s autonomy. This means having the right or power to govern oneself i.e. to determine things for yourself. Research has shown that during adolescence young people may be influenced by many things that can change what decisions they make e.g. family, friends, media etc. Of course this is true for adults too, but young people may be strongly influenced by other people’s opinions. In order to avoid this it is advised that young people put off testing until they are older and can be certain that it istheir choice to take the test. Also, recent research shows that the part of our brain that helps us make decisions and judgements only fully matures once we have reached early adulthood. Because of this it is advised that teenagers put off making such an important life decision until their brain has fully matured. Overall, professionals want to make sure that people who take the test for HD are choosing to do it themselves, and that they are certain this is the right choice for them.

To avoid changing the attitudes of a family towards the young person.Research tells us that when a person finds out they are a gene carrier, or not, people’s attitudes can change towards them, including family members. For example if you were found to be a carrier of the changed HD gene when you were young you’re parents may become incredibly overprotective and as a result you don’t grow up living a ‘normal’ childhood. In order to avoid this happening when a child is growing up it is advised that young people do not take the test until they are older.

To avoid other forms of discrimination. Unfortunately we live in a world where people are discriminated against, and people who live in a family with a genetic disorder may experience discrimination e.g. by insurers, employers, peers, in relationships and so on. In order to protect young people who carry the gene from certain types of discrimination it is advised they put off testing until they are at least 18, and are able to think through the impact of possible discrimination.

What is Genetic Counselling? Why do I have to go before I take the test?

The main purpose of genetic counselling is to provide information about HD so that individuals can make informed choices. Genetic counselling should be seen as a process which means the information will be given over a period of time, usually about 3 months. This means you will be able to talk about the pros and cons of taking the test, and about your specific situation. Once an individual has been given their test result it is information which cannot be taken back. Genetic counsellors want to make sure that someone who takes the test has thought through all the possible consequences and is well prepared before they get their result. This is why the genetic counselling process can take so long. Individuals are also encouraged to take along a person they trust to support them though the process.

Why does it take so long?

In the US an individual will usually have one or more pre-test sessions (genetic counselling, a neurological evaluation and sometimes a psychological evaluation). Everyone who goes for the test should receive genetic counselling and be seen by a neurologist (this is to confirm that they are showing no signs of the illness). People should also be seen by a psychologist (this is to confirm that someone is able to cope with the result). Post-test counselling sessions will also be offered.

As deciding to take the test is such a personal decision individuals are encouraged to talk though their feelings with trusted friends, family members and/or health care professionals.

What is the test?

If you decide to go for the test you will need to give some blood because that is what the test is done on. Your blood sample will be sent to a laboratory to test to see if you carry the changed HD gene or not. This usually takes two to three weeks. You will usually find out your result within this time from your Genetic Counsellor. They will ask you to come back to the clinic for this, they will not tell you over the phone. You can discuss how you would like to be told beforehand.

What can the test tell me?

The result will tell you whether or not you carry the changed HD gene. If you do carry the changed gene then you will get HD at some point in your life. The test cannot tell you when the disease will start or what it will be like for you. If you do not carry the changed gene then you will not get HD, nor will you pass on the changed gene to your children.

What are CAG repeats?

The gene which causes HD is bigger in people who get the illness. The gene has a ‘coding element’ called a CAG repeat and if this is longer than 36 repeats a person will develop HD at some point in their life. In peoplewithout HD the CAG is repeated up to 26 times. For further information on CAG repeats see:http://www.stanford.edu/group/hopes/basics/basichd/a2.html

What is the grey area in predictive testing?

The vast majority of people who carry the changed HD gene have 40 or more repeats and will develop HD. A very small minority may be told their CAG repeat is between 27 and 39. These numbers are called the ‘grey area.’

A few people have between 27 and 39 repeats. These people may get HD however research suggest that it will be a late onset or less severe form of HD and there is still a risk to their children.

What about confidentiality?

You might be worried that your details will not remain confidential if you attend a genetics service, but genetic services are as confidential as other medical services. Members of staff will not talk about you to other family members, and vice versa. Some people decide that they do not want their doctor to know they are going for the test, and contact the genetics service directly themselves. You are allowed to do this if you would prefer to.