Is Dementia Genetic? What to Expect

Alzheimer’s disease affects over 6 million Americans. It is the most common type of dementia. Multiple factors influence a person’s risk of developing Alzheimer’s disease. These include genes, lifestyle, and environmental factors.

Please continue reading to learn whether Alzheimer’s disease and other dementias are genetic, who is at increased risk of developing dementia, and whether you should get genetic testing.

How likely are you to get dementia if your parent has it?

People with a first-degree relative (parent or sibling) with Alzheimer’s disease are at a higher risk of developing Alzheimer’s themselves compared to those who do not have a family history of this condition. A person’s risk is even higher if they have more than one relative with Alzheimer’s disease, for example, both parents.

However, you do not have to have a family history of Alzheimer’s disease to develop it. There is a risk of developing Alzheimer’s disease even if no one else in your immediate family has had it. Also, even if immediate family members have dementia, it does not necessarily mean you will develop the condition.

Research has shown that while Alzheimer’s disease and other dementias run in families, in addition to the genetic risk, other risk factors like lifestyle and environmental factors also play a role.

Which parent passes down dementia?

Either parent can pass down dementia. Genetic research has identified two types of genes linked to Alzheimer’s disease, which can be passed down from parents to children.

Risk Genes

Researchers have identified a risk gene for Alzheimer’s disease called APOE-e4. People with this gene are at an increased risk of developing Alzheimer’s disease, but it is not guaranteed. Scientists estimate that 40-65% of people diagnosed with Alzheimer’s disease have this risk gene when they undergo genetic testing. There are other forms of the APOE gene called APOE-e2 and APOE-e3 that are associated with a lower risk. We all inherit one copy of the APOE genes from each parent. People who inherit a copy of APOE-e4 from either their mother or father are at increased risk (approximately 20-30% of people in the US have one copy of APOE-e4). Those who inherit one copy each from both parents are at even higher risk (approximately 2% of the US population have two copies of APOE-e4).

Deterministic Genes

Certain genes guarantee that a person will develop a particular disease. Researchers have identified specific genes called deterministic genes for familial Alzheimer’s disease. However, these faulty genes are rare and are present only in a few hundred extended families globally. Only around 1% of Alzheimer’s patients in the general population have these faulty genes. The presence of a deterministic gene increases the risk of developing early onset Alzheimer’s disease (early symptoms in the 40s and 50s as opposed to after age 65 years when most people develop Alzheimer’s).

Is there a test to see if dementia is hereditary?

Alzheimer’s disease genetics is being investigated by two large international groups in research settings. The National Institute on Aging is funding the Dominantly Inherited Alzheimer Network (DIAN), which is looking at deterministic genes linked to familial Alzheimer’s disease. The Alzheimer’s Prevention Initiative (API) is studying a 5,000-member extended family in Colombia, South America, with the early onset form of the disease. This is the world’s largest group in which a specific gene that causes early-onset Alzheimer disease has been identified and the entire family is being investigated.

Genetic tests, investigations, and clinical trials help us understand the disease better. For example, researchers have found that all the genes linked to Alzheimer’s disease increase the production of an amyloid precursor protein called beta-amyloid which is the main suspect in causing brain cell death. This genetic information has allowed scientists to develop a drug called aducanumab (Aduhelm), which removes amyloid deposits from the brain and can help slow cognitive and functional decline in people living with dementia.

What age does hereditary dementia start?

Most people develop Alzheimer’s disease in their 70s and 80s and live a normal lifespan. Only about 3% of people develop it before age 60. In the small number of people who develop Alzheimer’s at an earlier age (50s or 60s), it is called early-onset Alzheimer’s. Early onset Alzheimer’s is more likely caused by a faulty deterministic gene passed down from a parent to a child. In very rare cases, when Alzheimer’s symptoms start in the 30s or 40s, it is almost always caused by a faulty gene.

I have a family history of Alzheimer’s disease. Should I get genetic testing done?

A genetic diagnosis of Alzheimer’s disease can have many social and economic implications. Tests are available for both APOE-e4 and other genes (rare deterministic genes) that can damage nerve cells and cause Alzheimer’s disease. But experts advise caution before you undergo testing, even if you have a strong family history of Alzheimer’s disease and related dementias.

It’s important to have genetic counseling before taking a genetic test so that you understand the implications and can make an informed decision to give informed consent or decline testing.

It is especially important to have genetic counseling before doing at-home genetic tests for Alzheimer’s disease because you could misunderstand the results or make misinformed decisions regarding your health care. For instance, people with the APOE-e4 gene may be at increased risk of serious side effects from an anti-amyloid drug such as aducanumab.

Can Alzheimer’s be detected prenatally?

In families with known early-onset Alzheimer’s disease risk, couples may wish to ensure that a faulty gene is not passed on to their offspring. In such situations, preimplantation genetic diagnosis can be performed to check the genetic risk by testing an embryo for genetic defects before implanting it. Prenatal testing (after a confirmed pregnancy) is not usually advised because it may not be possible to terminate the pregnancy.

Before considering genetic testing to aid family planning, a faulty mutation must be confirmed in other family members. Extensive genetic counseling is recommended before prenatal diagnosis. An extensive discussion needs to take place about issues such as genetics, the process of genetic testing, the expected age of disease onset, whether the parents wish to discover their own genetic status, the ability to care for a child in case of a positive result, and the ability of the unaffected parent with a negative result to care for the affected parent and the child.

When to see a genetic counselor?

The Alzheimer’s Association recommends against routine genetic testing for Alzheimer’s disease until you have seen a genetic counselor. You should see a genetic counselor both before and after taking an at-home genetic test for Alzheimer’s disease to understand the genetic test results properly.

References: