Genetics and dementia

What role do your genes play in your risk of developing dementia?

Can you inherit dementia from your biological parents?

Can you pass it on to your kids or grandkids?

If there’s dementia in your family history, or you’ve been diagnosed with dementia, you probably have questions about genes. Let’s start with what genes are, and how they influence your health.

Understanding genes

In every cell in your body, there are tiny, twisted coils of DNA: the basic instructions for making you. Your genes are made of DNA, and they play a role in things like:

your eye and hair colour
your height
whether you have dimples or not.

You inherit your genes from your biological parents. You might also inherit a mutation, which is a change in the genes you inherit. Most mutations are harmless.

Your genes also play a role in how likely you are to develop some medical conditions.

Almost all of the known genes linked to dementia are risk-factor genes, which means if you have that gene in your DNA, you’re more likely to develop dementia.

Different genes affect your risk of developing different conditions that lead to dementia. Here are some of the most common.

Genetics and Alzheimer’s disease

Having a risk-factor gene for Alzheimer’s disease only makes you slightly more likely to develop it than someone who doesn’t.

The most important known gene affecting your risk of developing Alzheimer’s disease is called Apolipoprotein E. It has three types: type 2, type 3 and type 4.

Every person in the world has two Apolipoprotein genes. You can have two the same (two type 3s, say) or two different (a type 3 and a type 4, say). If you have at least one type 4, and especially if you have two type 4s, you are at higher risk of developing Alzheimer’s disease than people with other types.

But the risk isn’t that much higher: half of all people aged 85 who have two copies of type 4 don’t have symptoms of Alzheimer’s disease.

Type 4 has also been associated with increased risk for cardiovascular disease and vascular dementia.

People with type 2, and especially two type 2s, appear to be protected against developing Alzheimer’s disease, until much later in life. Researchers are working to find out why.

Again, these genes don’t mean you definitely will or won’t develop Alzheimer’s disease. They just change your risk slightly.

Familial Alzheimer’s disease (FAD)

One rare form of Alzheimer’s disease is passed from generation to generation: Familial Alzheimer’s disease (FAD).

There are three genes which everyone has, called:

presenilin 1
presenilin 2
amyloid precursor protein gene (APP).

These genes are usually harmless. But they can mutate in a specific way that causes FAD.

If you have one of these mutated genes, you will eventually develop Alzheimer’s disease, usually in your 40s or 50s. Your biological children will have a fifty per cent chance of inheriting that gene. If they don’t inherit the mutated gene, they can’t pass any risk of FAD on to their children.

Familial Alzheimer’s disease is very rare. At any given time in Australia, no more than 100 people are likely to be affected by FAD.

Three genes have been identified which, if mutated in certain ways, will cause FAD. These are called on chromosome 21.

Genetic testing can identify the presence of gene mutations that cause familial Alzheimer’s disease. This test can tell if a person has familial Alzheimer’s and if a child has inherited the changed gene from a parent and will develop the disease in the future. It cannot determine when the symptoms will begin. Usually though, the age of onset is similar to that of the parent.

Genetics and frontotemporal dementia

There is a rare form of frontotemporal dementia that is inherited through your genes. It’s called familial frontotemporal dementia.

Familial frontotemporal dementia makes up only about ten to fifteen per cent of frontotemporal dementia cases. About half of all familial frontotemporal dementia cases are caused by mutations in two genes:

the MAPT or tau gene
the progranulin gene.

Several other, rarer gene mutations can also cause frontotemporal dementia.

If you have one of these mutated genes, you will eventually develop frontotemporal dementia. Your biological children will have a fifty per cent chance of inheriting that gene. If they don’t inherit the mutated gene, they can’t pass any risk of FAD on to their children.

Other rare forms of inherited dementia

Some other, very rare forms of dementia can also be inherited. These include:

some forms of Frontotemporal Lobar Degeneration, where behaviour change occurs before any change in memory
CADASIL (cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy) is a rare inherited form of vascular dementia. It is caused by mutations in the NOTCH3 gene. CADASIL is characterised by recurrent strokes, lesions in the deep white matter of the brain, migraines, psychiatric symptoms and progressive dementia.
Familial Lewy body disease is very rare, but a few gene mutations causing the disease have been identified. Mutations in the SCNA gene cause Lewy body dementias, which may manifest as dementia with Lewy bodies or Parkinson’s disease dementia.
familial forms of Creutzfeldt-Jakob disease, which causes a rapidly progressing dementia. Mutations of the PRNP gene account for five to ten per cent of Creutzfeldt-Jakob disease cases.
Huntington’s disease: an inherited condition causing motor, psychiatric and cognitive symptoms, and in most cases dementia. It is caused by mutations in the Huntington’s gene.

Biological children of someone with any of these conditions have a fifty per cent chance of inheriting the gene and developing the disease.

Genetic testing

Genetic testing usually isn’t very useful for most families affected by dementia, because inherited forms of dementia are very rare.

If your family is affected by a familial dementia, genetic testing is available. If you have symptoms of dementia and a family history that suggests a genetic cause, genetic testing can help establish the diagnosis.

Children of a person with one of these genetic mutations can get genetic testing to work out their own risk.

Deciding whether to get genetic testing

The tests for familial Alzheimer’s disease, familial frontotemporal dementia and the rarer conditions listed above won’t tell you if your risk is slightly higher. If the test finds you have the mutated gene, you will develop that form of dementia.

For this reason, some people choose to get genetic testing for familial dementia, but others don’t.

Knowing that you have the gene might help you plan for the future, especially choosing whether to have children. If you’re concerned about passing on a faulty gene to future children, in some cases prenatal genetic testing or IVF procedures are available that give you more choice. Talk to your doctor.

However, given that there is no known cure for dementia yet, it’s your choice to decide whether you want to know if you have one of these genes. Some people do, some don’t.

The most important thing is that no one should take any test without their informed consent. No one should ever be pressured to have genetic testing.

Getting support

If you need support or counselling around the question of genetic testing, talk to your doctor. They can refer you to genetic counselling through genetics services in your state.