Understanding How Some Dementia Is Hereditary

When someone close to you has been diagnosed with a chronic, life-altering condition like dementia, you might wonder about the likelihood that you’ll someday develop the same or similar conditions.

Indeed, family history plays an important role in many areas of healthcare like heart health or cancer — but is all dementia hereditary? Or are only some kinds of dementia passed down from one generation to the next?

Neuropsychologist Aaron Bonner-Jackson, PhD, explains why some types of dementia are hereditary and what it means for your chance of developing these neurological complications.

Does dementia run in families?

Most cases of dementia aren’t inherited by children or grandchildren. Most often, dementia is influenced by a combination of factors like aging, genetics, lifestyle behaviors and environmental factors. So, dementia can be hereditary — but it’s rare.

While there may be a strong genetic link for rare and early-onset types of dementia, those cases are minimal compared to the number of other cases where people develop dementia by other means.

“In the vast majority of cases, we don’t think that simply because you have a family member who has one of these conditions that you will necessarily get it,” clarifies Dr. Bonner-Jackson. “In a lot of cases, we think of these as risk factors. So, having that gene or family history may increase your likelihood of getting it — but it’s not definitive.”

How does dementia get passed down?

For neurodegenerative diseases like dementia to be hereditary, there often needs to be some sort of mutation or error built into specific genes you receive from your parents (which they received from their parents, and so on).

Sometimes, simply having too many copies of a single gene or too many replicants of a specific genetic sequence can cause these conditions to develop later in life, as is the case with Huntington’s disease.

But just because you have a specific gene doesn’t mean you’re always guaranteed to develop a specific condition, especially when it comes to dementia.

For example, we know that all humans inherit one copy of the apolipoprotein-E (APOE) gene from each of their biological parents. Having at least one copy of a specific form of this gene known as APOE-ε4 increases your risk for developing late-onset Alzheimer’s disease (that is, after age 65). And those who have two copies (one from each of their parents) are at even higher risk of developing Alzheimer’s.

But some people who have this form of the APOE gene never develop Alzheimer’s. In fact, you can develop Alzheimer’s by some other means even if you don’t have this specific form of the APOE gene.

“Depending on which copies of the APOE gene you get, you may be more likely to develop Alzheimer’s disease,” says Dr. Bonner-Jackson. “You can get genetic testing for APOE to know which copies of the gene you have, and that would help you know a little bit about your risk for it.”

Which type of dementia is hereditary?

Familial Alzheimer’s disease is a form of hereditary dementia that makes up only 5% of all Alzheimer’s disease cases. Your risk for developing familial Alzheimer’s disease increases with each generation that has the APOE-ε4 gene or a mutation in these three genes:

• Amyloid precursor protein (APP) on chromosome 21.
• Presenilin-1 (PSEN1) on chromosome 14.
• Presenilin-2 (PSEN2) on chromosome 1.

According to the U.S. National Institute on Aging, a child whose biological parent carries one of these genetic variants has a 50/50 chance of inheriting that altered version of the gene, leading to the onset of Alzheimer’s much earlier in life, sometimes, in their 40s or 50s. Early-onset Alzheimer’s makes up less than 10% of all Alzheimer’s cases. Of those, 10% to 15% can be attributed to changes in these three genes.

“These are very rare genes that are quite uncommon,” notes Dr. Bonner-Jackson. “Those tend to be clustered in families, as you’d expect, and they’re highly unusual.”

Other forms of hereditary dementia include:

• Familial prion disease.
• Frontotemporal dementia.
• Lewy body dementia.
• Creutzfeldt-Jakob disease.

What is your risk for dementia if it runs in your family?

The possibility of you developing dementia increases for every generation that’s had it and for every immediate family member (like a parent or sibling) who’s had it. Similarly, your risk for developing dementia increases if you have a family history of heart disease, diabetes or stroke — all things that directly impact how your brain functions.

“How likely you are to get dementia is largely dependent on how close genetically you are related to someone,” explains Dr. Bonner-Jackson. “If it’s a family where many people have it, that would probably be an increased risk for you. If you have a twin who has it, then your risk is going to be fairly high for also getting it. If it’s a much more distant relative like a cousin or second cousin, then the risk is lower.”

That said, again, keep in mind that just because these conditions run in your family or you have the specific genes that increase your risk for dementia doesn’t always mean you’ll develop dementia at some point. It’s kind of like riding a bike — just because you get on the bike doesn’t guarantee you’ll fall. But there is always that possibility.

If you’re worried dementia is hereditary in your family, consider taking some genetic tests to determine your level of risk for dementia.

“Genetic tests for dementia can’t definitively tell you whether or not you will develop these diseases, but they can inform you about your potential risk for it,” explains Dr. Bonner-Jackson.

Dementia may not be 100% preventable, but you can certainly reduce the likelihood you’ll develop it by taking care of your mental, emotional and physical health as early as possible. Some of the most helpful actions you can do include:

• Eat healthy foods.
• Make time for exercise.
• Challenge your brain to learn new things or pick up new hobbies.

“We can’t do anything about our family history or our genetics, but we can do something about a lot of these lifestyle factors and try to maintain good habits in those areas,” he encourages. “Doing so could either reduce our risk for dementia or delay the onset of any symptoms for years.”