Familial frontotemporal dementia (fFTD) is an inherited form of frontotemporal dementia (FTD), a group of dementias which mainly affects personality and behaviour or language and speech, depending on which areas of the brain are affected. In most people (50-70%), frontotemporal dementia (FTD) is a sporadic condition. This means that it occurs in an individual person by chance, without anyone else in the family having been affected. In most cases, the relatives of someone who has been diagnosed with FTD do not have an increased chance of also getting this condition.
In some cases, FTD appears to run in families and there is likely to be a genetic cause. Three genes cause the majority of genetic FTD cases – progranulin, tau and C9orf72. At least six other genes can cause familial FTD, and more may be discovered. In around 30-40% of people diagnosed with FTD, the condition is known to be caused by an abnormality in a single gene. Of the different forms of FTD, behavioural variant FTD (bvFTD) is the one that is inherited most often. Primary progressive aphasia, particularly progressive nonfluent aphasia (PNFA), can also have a genetic cause and run in families, though it is much rarer.
If someone has one of the abnormal genes that cause FTD, then each of their children has a 50% change of inheriting the faulty gene. Each sibling of someone with the faulty gene also has a 50% chance of inheriting it. If someone is tested and has inherited one of these gene abnormalities, then they are highly likely to develop FTD. Only a very small minority of people who have inherited a faulty gene live without developing FTD in their lifetime.
The age at which symptoms of fFTD begin is very variable. Some people develop fFTD around the same age that their parent developed it. However, for other people, there can be up to 20 years’ difference in the age that fFTD develops.
In families where a gene abnormality is known or suspected, expert genetic advice is important. 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 and is familiar with the family before any genetic tests are performed. A genetic test for FTD can be performed on a blood sample and is usually done because several other family members have FTD or an associated disorder. However, it may also be performed when symptoms or brain scans suggest familial FTD. For more detailed information on genetic testing and counselling for FTD, please see the Association for Frontotemporal Degeneration website or Genetic testing and Alzheimer’s disease | Alzheimer Society of Canada.