This is a complex question because different types of dementia have different genetic involvement.
The vast majority of dementia is not strongly inherited and the biggest risk factor for dementia is age. Our ageing population and the higher rate of dementia in the elderly means that many people have a history of dementia in their family.
Some genes are what is called “autosomal dominant”. This means they are highly expressed. If a person has an autosomal dominant gene there is a 50 % chance of transmission of that gene to their children. We may suspect the presence of these genes in our patients if there is a strong family history of younger onset dementia (<65) in multiple family members.
Autosomal dominant genes occur very rarely in Alzheimer’s disease – less than 1 % of all cases. The frequency of these genes increases in very young onset Alzheimer’s disease cases (especially when age of onset is less than 50).
Autosomal dominant genes are more common in frontotemporal dementia (FTD), occurring in up to 30 % of cases. One of these genes, C9orf72, is linked to both FTD and motor neuron disease (MND). Family members can be affected by FTD or MND or even both in the same person.
In patients where assessments raise suggestion of these autosomal dominant genes, the option of gene testing may be discussed. Gene testing is available via specialised neurogenetic services, and involves in-depth genetics counselling. Genetic counselling is important so that the person is fully informed about the implications of genetic testing on themselves and their children, and is well supported in their decision.
Another group of genes are known as “susceptibility genes” that interact with certain environmental factors during a person’s life to increase risk of dementia.
In Alzheimer’s disease the most relevant gene is known as APOE-E4. This can impact the ageing brain’s ability to clear the abnormal protein involved in this condition called amyloid.
We do not routinely test for these susceptibility genes because there still remains a very strong chance that a person will not develop Alzheimer’s disease if they are a carrier, and testing may well cause unnecessary worry instead of much benefit.