Predictive Testing for Huntington Disease:

Predictive testing for Huntington disease is a process that occurs over several months and involves consultation with different members of the Maritime Medical Genetics Service Predictive Testing team. This may include a genetic counsellor, medical geneticist (doctor) and social worker. Consultations may be in person, by telephone and/or by telehealth (video conference). These sessions are based on international guidelines and include exploring an individual's motives for testing, possible impact of positive and negative test results, coping strategies and insurance issues, among other things. It is hoped that by following this process, that an individual will be prepared as best as possible to receive their results, or, in some cases, realize that the time is not best for them to proceed with the testing.

It is also recommended that anyone deciding to proceed with this process involve a support person, such as a family member or friend, who can participate in these discussions. As well, we hope that the individual will make a health care provider of their choice aware that they are going through predictive testing.

Referrals to Maritime Medical Genetics Service can be made by a family physician, neurologist or other physician involved in an individual's care.

Predictive Testing for Other Conditions:

Maritime Medical Genetics Service is also involved with predictive testing for other adult onset neurological and neuromuscular conditions such as Alzheimer's, Amyotrophic Lateral Sclerosis, Hereditary Sensory and Autonomic Neuropathy, CADASIL, among others.

Please call our office if you, or your physician, have any questions about referrals.

Genetics of Huntington Disease:

The human body consists of billions of cells, much like a building is made of bricks. In the center of every cell is our genetic information (genes). Our genes direct the growth, development and function of the human body. We have over 20,000 genes in our body and each of them comes as a pair. One member of each pair is inherited from our mother and one member of each pair is inherited from our father.

All of our genes are made up of DNA, which is made up of long stretches of four small molecules called bases: A (adenine), T (thymine), G (guanine), C (cytosine). The order of these four letters is called our genetic code and various orders determine the function of different genes. Any changes to the sequence, or order, of these letters in a gene can change the way a gene functions, so much so that it may not function at all.

The gene that is associated with Huntington disease is referred to here as the HD gene. Although we each have two copies of this gene, people with Huntington disease have one of their HD genes that is a bit larger than usual. The gene is bigger in a region where the three letters of the genetic code -C-A-G- are repeated many times. Usually the gene has less than 26 copies of this so-called CAG triplet-repeat. An individual with 27-35 repeats will not go on to develop Huntington disease but in some cases may pass on an expansion to their children who will then be at higher risk for developing the condition. An individual with 36-39 repeats may or may not go on to develop signs of Huntington disease. When there are more than 40 copies of this triplet repeat this means that the HD gene will not be able to function properly and that an individual will go on to develop signs of Huntington disease at some point in their life. It does not necessarily mean that they have signs of the condition already but it predicts that they will have Huntington disease at some point. There is an inverse correlation between the number of -C-A-G- repeats and the age of onset of Huntington disease (i.e. the more repeats, the earlier the age of onset). However we cannot entirely predict the age of onset, severity, type of symptoms or rate of progression in an asymptomatic individual.

The HD gene is passed on in families in a dominant manner. This means that you only need to inherit one copy of the changed (expanded) gene in order to develop Huntington disease. When someone carries such a gene change, there is a 50% chance that the gene will be passed on to each of his or her children. Alternatively, there is also a 50% chance that each child will inherit the normal, working copy of the gene. It is important to remember that no one has any control over which genes they pass on to their children.

Predictive Testing Team at Maritime Medical Genetics Service:

Medical Geneticists:

Dr. Mark Ludman

Dr. Lynette Penney

Genetic Counsellors:

Meghan Ferguson, MSc, CCGC, CGC

M. Jill Beis, MSc, CCGC

Social Work:

TBD

Contact Information:

Referrals can be mailed or faxed to:

Maritime Medical Genetics Service

5850/5980 University Avenue

PO BOX 9700

Halifax, NS, B3K 6R8

Phone: 902-470-8754

Fax: 902-470-8709