Genetic testing and Alzheimer's disease

The genetics of Alzheimer's disease

Genetics is the study of how a specific feature, such as a disease, is passed from one family generation to another. Researchers are working hard trying to learn the role of genetics in Alzheimer's disease.

It's also a major concern for people with Alzheimer's disease and their families. However, note that sometimes people are concerned about a family history of Alzheimer’s disease when, in fact, they have a family history of a different dementia.

So far, researchers have found over 70 genes that may increase the risk of developing Alzheimer’s disease.

Sporadic Alzheimer's disease

Most cases of Alzheimer’s disease are sporadic, meaning they do not run in families. Sporadic Alzheimer’s disease is due to a complex combination of our genes, our environment and our lifestyle.

Most of the genes that researchers have found do not directly cause Alzheimer’s disease, but they do make you more susceptible to developing it in your lifetime. These are called susceptibility genes.

In particular, the presence of the apoE4 gene indicates susceptibility for developing sporadic Alzheimer's disease.

Inherited or familial Alzheimer's disease

Families with inherited or familial Alzheimer’s disease have very strong family histories of Alzheimer’s disease. Familial Alzheimer’s disease is due to changes or alterations in specific genes that can be directly passed on from parent to child.

Only rare instances of Alzheimer’s disease are inherited or familial, accounting for less than five percent of all cases.

Three familial Alzheimer’s disease genes have been discovered so far: two presenilin genes (PSEN1 and PSEN2), and an amyloid precursor protein (APP) gene. These three disease genes together are responsible for about half of familial cases.

If you have an alteration in any one of these genes, you will almost certainly develop young onset familial Alzheimer’s disease (before the age of 65). Alterations in these genes are rarely known to cause Alzheimer’s disease among people 65 years and older.

Familial Alzheimer’s disease is hereditary. If a parent has any of the faulty genes (PSEN1, PSEN2 or APP), their children have a 50% chance of inheriting the disease. If a person does not inherit the disease-causing gene they cannot pass it on to their children.

Should I get genetic testing?

No reliable genetic test exists for the common sporadic form of Alzheimer's disease.

Therefore, for most cases, genetic testing is not recommended because, at best, it can only point to susceptibility. The testing can never predict whether a person will or will not get Alzheimer's disease.

Genetic testing is only an option for families that have young-onset familial Alzheimer's disease.

If you're thinking about genetic testing for Alzheimer’s disease, it's important to get help from a healthcare professional or a certified genetic counsellor.

• They will help you find out whether genetic testing is relevant for you based on your family history.
• They will also help make sure you understand the testing process and all the things you should think about before getting tested.

If there is someone in your family who may have had Alzheimer's disease and is no longer living, a retrospective diagnosis can often be proposed following a careful review of autopsy reports and medical records, if they're available.

What else should I know about genetic testing?

The decision to participate in genetic testing is a personal one.

Prior to consenting to genetic assessment or testing, it's important to consider the psychological, legal, social and ethical implications of genetic testing for yourself.

If you decide that genetic testing is appropriate, here are three points to follow:

• Give informed consent to the testing
• Receive counselling from a trained professional
• Be guaranteed that the test results will remain confidential