Getting tested for the Huntington’s disease gene is a difficult decision with far-reaching implications.

Although predictive genetic testing has an important role to play – giving a person the chance to accurately determine whether they will get this serious disorder or not – only 15 – 25% of first-degree relatives of people with Huntington’s disease (HD) opt for the test. The reality is that it has no benefit in terms of treatment.

We take a look at what the process of predictive genetic testing entails.

A few facts

Huntington’s disease is:

A relatively rare neurodegenerative condition. In the western world, the prevalence of HD is estimated to be in the range of 4 – 10 per 100,000 people. In Australia, about 1,200 people have HD, while approximately 6,000 are at risk.
A single-gene disorder. This means that the condition is the result of a single mutated gene – an expanded CAG repeat in the HTT gene, which is located in the short arm of chromosome 4. As a result, accurate genetic testing can be done to determine a person’s risk.
An autosomal dominant disorder. This means that, if a parent has the HD gene, all offspring will have a 50% chance of inheriting the mutated gene.

What is predictive genetic testing?

Predictive genetic testing is done to predict future risk of developing a genetic condition in a person who does not display any disease symptoms. In the case of HD, it gives the person a chance to find out whether he or she has the specific gene mutation linked to the condition.

Predictive genetic testing cannot determine exactly when someone will become symptomatic. However, if a high level of CAG repeats of the HD gene is detected during testing, it might mean that a person will get the disease earlier, but this is not an exact science – the environment and other modifying genes could play a role.

Some people will develop HD in the prime of life; others may only become symptomatic in their senior years. However, the mean age of onset is 40, with death occurring 15 – 20 years from onset.

A difficult decision

Getting tested for HD is a major life decision that should be made with the assistance of a skilled genetic counselling team.

What complicates the decision is the fact that there is no known treatment or cure for HD. On the other hand, the knowledge that the gene wasn’t inherited could give a person a new lease on life. A positive test result will mean that the person will get this progressive, fatal disorder at some point in their life. A negative test result gives a person the all-clear, but often goes hand in hand with significant psychosocial distress and feelings of regret and guilt as other family members may be affected.

Predictive genetic testing is not recommended for people under the age of 18.

Pre-test counselling: what to expect

Internationally, individual sessions with a genetic counsellor, a psychologist, a psychiatrist and/or a neurologist are recommended before predictive genetic testing is done. The protocol differs from country to country, and clinic to clinic, but sessions will usually involve:

A risk-assessment session with a genetic counsellor, during which the person’s family history will be taken into consideration and inheritance patterns will be explained.
A physical examination and brain-imaging tests by a neurologist to see if the person is already symptomatic; if symptoms are present, it is important to rule out other conditions that may mimic Huntington’s disease (e.g. Alzheimer’s disease, Parkinson’s disease, Lewy body disease).
A pre-test counselling session with a psychologist and/or a psychiatrist in preparation for the predictive test. This helps the person to make an informed decision about getting tested or not. It’s important to discuss the pros and cons of testing, and to look at coping skills and strategies.
A results session with a genetic counsellor and/or a psychiatrist/psychologist.

These sessions should be spaced out to give the person enough time to process the information. A month between sessions is advisable, and the person should be encouraged to bring a trusted friend or family member along to each session for support.

Involving a family physician is also advised. The GP acts as the facilitator for the at-risk patient’s overall health and will be able to follow up on their physical and emotional wellbeing. This is important, even if the genetic test indicates that the person is not at risk for HD.

The at-risk person should realise that they can halt this process at any point. “Many patients start the process of pre-test counselling, but then decide not to go ahead with testing,” says genetic counsellor Frieda Loubser. “Some even go for testing, but then decide not to find out what the results were.”

The genetic test

Predictive genetic testing for HD entails a relatively inexpensive, simple blood test. After the blood sample was sent to the testing lab, it might take a couple of weeks before the results are available. During this time, it may be necessary to continue counselling.

A positive result means the person will get HD, with all its associated neurologic and psychiatric symptoms, at some point in their life; a negative result means he or she will not be affected. If the person inherited the “good” gene, they will also not pass the condition on to their children.

Importantly, the test might not show a 100% clear result – an uninterpretable result occurs in 1% of people. Testing is, however, becoming more and more accurate and newer tests exclude large gene expansions, providing better results.

Testing should be done at a time when there are few other stresses in the person’s life. If the test result is positive, several important life decisions will have to be made. Should the person tell his or her employer and risk discrimination? Should they tell family members, some of whom may not want to know their HD risk? Which financial, life and health insurance measures should be taken? How does the test result affect family planning?

In terms of health insurance, it is advisable for concerned parties to disclose their family history of Huntington’s disease before testing, but to ensure that they choose an insurance plan that will cover them sufficiently if they do test positive.

Note that test results are treated as strictly confidential.

Sources:
– How common is Hungtington’s disease? Huntington’s New South Wales, http://www.huntingtonsnsw.org.au/information/hd-facts/how-common
– Predictive Testing Protocol for Huntington’s Disease, University of Cape Town’s  Division of Human Genetics
– Huntington’s disease, Johns Hopkins Department of Pathology: Division of Neuropathology, http://pathology.jhu.edu/department/divisions/neuropathology/research-huntingtons.cfm
– Muthane, U. (July 2011) Predictive genetic testing in Huntington’s disease. Annals of Indian Academy of Neurology, 14(Suppl1): S29-S30. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3152165/

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