Familial Alzheimer’s disease is an inherited form of Alzheimer’s disease that runs in families (hence the name, ‘familial’ which distinguishes this condition from young onset Alzheimer’s disease or Alzheimer’s disease diagnosed in later life). Like young onset Alzheimer’s disease, familial Alzheimer’s disease is a rare form of dementia accounting for less than 5% of all cases of Alzheimer’s disease. It also usually occurs much earlier. Most commonly, symptoms first develop when individuals are in their 40s or 50s or sometimes as early as in their 30s.
Familial Alzheimer’s disease is different from young onset Alzheimer’s disease in that it is entirely caused by a genetic fault or mutation that runs within families. Genes are basic units of protein or DNA that are passed from parents to their offspring and determine physical and biological traits, such as hair colour for example. Sometimes genes can change or mutate resulting in faulty, disease-carrying genes that are transmitted across generations. To learn more about genes and dementia, you can visit the Alzheimer’s Research UK website OR Alzheimer Society of Canada website.
Mutations in three genes are known to cause most cases of familial Alzheimer’s disease. These are the presenilin 1 (PSEN1), presenilin 2 (PSEN2) and amyloid precursor protein (APP) genes. A mutation in PSEN1 is the most common of these three to cause familial Alzheimer’s disease. Other forms of Alzheimer’s disease (such as young onset Alzheimer’s disease and Alzheimer’s disease diagnoses in later life) are often described as ‘sporadic’ because they are not familial or inherited from a genetic mutation. As a result, relatives of someone who has these sporadic forms of Alzheimer’s disease have no greater risk of getting the disease than anyone else. Whereas, for a relative of someone who has familial Alzheimer’s disease (such as a child or sibling), the chance or likelihood of developing the disease is much higher, estimated at 50%.
An affected parent could have all children affected or unaffected, or a mixture of both. Familial Alzheimer’s disease affects males and females equally and does not skip generations. However, it may appear to skip a generation if a person with the faulty gene dies of another cause before symptoms develop. Individuals with familial Alzheimer’s disease usually have a strong family history of the illness, which means that, as well as one of their parents, they know of cousins, aunts/uncles and grandparents who were affected at a similar age and who belong to the side of the family of their affected parent. In some cases, an individual may not know whether they have a family history of familial Alzheimer’s disease, for example if they do not know their biological parents or if their parents died young.
As far as we know, if an individual is tested and has inherited one of these faulty genes, then they will almost certainly go on to develop familial Alzheimer’s disease at some point in their life. They are also highly likely to develop symptoms around the same age that their parent developed them. Within a single family, people tend to develop the disease at broadly similar ages. This means if you are a lot older than an affected parent and do not have problems then you are unlikely to have inherited the genetic mutation and your children are not at risk.
The initial symptoms of familial Alzheimer’s disease are usually similar to those of other forms of Alzheimer’s disease – primarily memory loss. The memory problems in Alzheimer’s disease are different from everyday forgetfulness. Importantly, they are ‘progressive’, meaning that they become worse over time. They are also different in that in familial Alzheimer’s disease, other people are usually more aware of the memory problems than the person experiencing them. Over time, the memory problems affect other mental abilities such as planning or performing calculations. Symptoms can be hard to detect, but a clear sign is a distinctive and progressive decline in abilities.
Sometimes familial Alzheimer’s disease is referred to as autosomal dominant Alzheimer’s disease (ADAD). This is a different name to describe the same condition. It means that in each family the disease is caused by a mutation in a single gene and that a single copy of the mutated gene, inherited from one parent, will cause the disease. If someone has a relative with Alzheimer’s disease and is concerned about whether it may be familial, they should gather as much information as they can about their relative’s family history so that they can discuss it with their family doctor (or nurse practitioner), who will also be able to refer them to a specialist genetic counselling service if appropriate.
A genetic test for familial Alzheimer’s disease can be performed on a blood sample and is usually done because several other family members have Alzheimer’s disease. However, it may also be performed when symptoms or brain scans suggest familial Alzheimer’s disease. The results of any genetic test have important implications not only for the person being tested but also for the rest of the family. It is important to discuss these carefully with a specialist who has expertise in genetic counselling before any genetic tests are performed.
For more information about familial Alzheimer’s disease, including genetic testing visit Genetic testing and Alzheimer’s disease | Alzheimer Society of Canada.