Is dementia inevitable and should you do the gene test?
Actor Chris Hemsworth’s announcement that he carries two copies of the ApoE4 gene, increasing his risk of developing Alzheimer’s disease, has generated a lot of conversation. But what does this mean and is gene testing a good idea?
Is dementia inevitable?
Dementia is not a normal part of ageing and is not inevitable. We know it depends on a combination of age, genes, health and lifestyle.
- The risk increases with age. In people over the age of 65, dementia affects almost one person in 10.
- In people over the age of 85, dementia affects three people in 10.
- People under the age of 65, including in their 30, 40s and 50s, can develop dementia (called younger onset dementia), but it is less common.
There are more than 100 types of dementia and Alzheimer’s disease is the most common form of dementia. Some forms of dementia can be hereditary, but this is quite rare.
How do you know if Alzheimer’s disease is hereditary in your family?
Most cases of dementia are not caused by an inherited genetic mutation. Dementia is so common that having one or two close relatives with dementia is not evidence of a genetic link.
The cause of most cases of Alzheimer’s disease is unknown and not linked to gene mutations (though forms of childhood dementia, which are rare, do have an underlying genetic marker). Many factors combine to alter a person’s risk of developing Alzheimer’s, including lifestyle and environmental factors. We do know that having a close family member with the condition increases risk – but only by a small amount.
The term familial Alzheimer’s disease is used for the very small number of families who carry a genetic mutation that directly causes the disease. Familial Alzheimer’s disease usually affects younger people (under the age of 65), some as young as their 30s and 40s. On average, half of the children of a person with familial Alzheimer’s disease will inherit the faulty gene. All those who inherit it will develop Alzheimer’s disease. People who do not inherit the faulty gene cannot pass it on to their children.
What is the difference between gene mutation and gene susceptibility?
To understand whether dementia is hereditary we must understand a bit about genes.
A mutation is a permanent change in a gene from its normal form. Some gene mutations cause disease, so if a person inherits a mutated gene for a certain disease they will definitely develop that illness in life. Most cases of dementia are not caused by an inherited genetic mutation.
Other genes we will refer to are called susceptibility or risk genes – they do not inevitably lead to a person developing the illness, but they increase the risk of developing the illness.
What genes increase your risk of Alzheimer’s disease?
About 10 susceptibility genes that increase the risk of Alzheimer’s disease have been identified to date.
The most important of these is the apolipoprotein E (ApoE) gene. It comes in three forms – ApoE2, ApoE3 and ApoE4. We have two copies of the gene, which may be the same form as each other or different.
Having one or two copies of ApoE4 increases the chance of developing Alzheimer’s disease but does not make it certain. Some people with ApoE4 never develop Alzheimer’s disease, and others who develop Alzheimer’s disease do not have ApoE4.
ApoE3 is associated with an average risk, and ApoE2 with a reduced risk. A blood test can identify which forms of ApoE a person has, but it is not possible to predict who will or will not develop Alzheimer’s disease.
ApoE testing is therefore generally not available except as part of a research study.
Other susceptibility genes are also associated with increased risk of Alzheimer's disease. They increase the risk by very small amounts, so don’t play as large a role in the risk of Alzheimer’s disease as ApoE4. Carrying ApoE4 has also been associated with increased risk for cardiovascular disease and vascular dementia.
Are other types of dementia genetic?
Some forms of frontotemporal dementia that are inherited are caused by specific gene mutations and are known as familial frontotemporal dementia. Familial frontotemporal dementia accounts for only around 10-15 per cent of all cases.
On average, half of the children of a person with familial frontotemporal dementia will inherit the faulty gene and develop the disease. For affected families, genetic testing is available. Other rare inherited forms of dementia include:
CADASIL (cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy), which is a rare inherited form of vascular dementia.
Familial Lewy body disease is very rare, but a few gene mutations causing the disease have been identified.
Creutzfeldt-Jakob disease, which causes a rapidly progressing dementia. Mutations of a gene called the PRNP gene account for 5-10% of Creutzfeldt-Jakob disease cases.
Huntington’s disease, which causes motor, psychiatric and cognitive symptoms, and in most cases dementia. It is caused by mutations in the Huntington’s gene.
For all the above familial dementias, children of an affected person have a 50 per cent chance of inheriting the gene and developing the disease.
Should you do genetic testing?
Genetic testing is not something that is routinely offered nor is it relevant for most families.
For a person with symptoms of dementia and a family history suggestive of a genetic cause, genetic testing can help establish the diagnosis. For family members, especially children, of a person with a known genetic mutation, genetic testing can predict the development of the disease in the future.
The decision to undergo genetic testing for familial dementia is very complex and the advantages and disadvantages must be carefully considered. The test can only be completed with the informed consent of the person being tested. No one should ever be pressured to have such a test.
Knowing you carry the gene may help some people plan as it enables consideration of future lifestyle and reproductive choices. However, other people may find knowing they carry a gene can sometimes cause unnecessary anxiety.
To help people consider the issues for themselves and their family, please speak with your GP in the first instance. If further support is needed, there is specialised genetic counselling available through state-based genetics services and your doctor can provide details and referrals. The Centre for Genetics Education provides current genetics information for people affected by genetic conditions. Visit genetics.edu.au
Want to know more about brain health? Check out these articles:
How do our brains work? The brain is one of our most important organs, yet most people understand very little about it. Come with us as we explore the different parts of the brain, how they work and why they are important.
How do we maintain a healthy brain? There is no sure way to prevent dementia but there are things we can do to get our brain healthy and keep it healthy.
Is there a way to track your brain health? We all have times where we don’t feel ourselves, but how can we determine if it is temporary or if there is something more going on?
For more resources about brain health visit our library guide here: https://dementia-org.libguides.com/maintain-your-brain
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