Is Alzheimer's Genetic?
In some cases, people may carry a gene that increases their risk of developing Alzheimer’s disease. In rare cases, they may carry a gene that directly causes the disease.
This article will further define Alzheimer’s disease. It will also discuss whether it is genetic, as well as other risk factors for Alzheimer’s disease.
Alzheimer’s disease is the most common form of dementia, accounting for 60–80% of all dementia cases. It affects memory, thinking, and behavior, and it is sometimes severe enough to interfere with daily tasks. Alzheimer’s disease is not a normal part of aging. The disease is progressive and tends to worsen over time. There is no cure for Alzheimer’s disease, but there are medications that can help slow the progress of cognitive decline.
In Alzheimer’s disease, the brain’s nerve cells become damaged. Researchers believe two abnormal structures, called plaques and tangles, are responsible for damaging the nerve cells. Plaques consist of deposits of protein fragments called beta-amyloid that build up in the spaces between the nerve cells. Tangles are twisted fibers of a different protein called tau that builds up inside the nerve cells.
The changes in Alzheimer’s disease typically occur in the part of the brain that affects learning. The earliest symptom of Alzheimer’s disease is having trouble remembering newly learned information.
As it advances, it leads to increased symptoms, such as:
- mood changes
- suspicions about friends, family, or caregivers
- more serious memory loss
- difficulty speaking
- difficulty swallowing
- difficulty walking
Some people with memory loss or early symptoms of Alzheimer’s disease may not recognize they have a problem. Symptoms are often more obvious to family members or friends.
If you or someone you know is exhibiting symptoms of Alzheimer’s disease, contact your doctor right away.
Genes are the segments of chromosomes that pass down information from one generation to the next. This information can define physical traits, such as eye or hair color. They can also determine the health of the body’s cells. In some cases, when there is a problem with a gene, it can cause disease, such as Alzheimer’s disease.
Permanent changes in one or more genes are called genetic mutations, and these can cause disease. If you inherit a genetic mutation that causes a specific disease, you will more than likely get that disease. However, other changes in genes, called genetic variants, only increase your risk of getting an inherited disease.
Identifying genetic mutations and variants can help researchers find the best way of preventing and treating a disease such as Alzheimer’s. The expression of your genes, whether they will switch on or off, is also positively or negatively affected by lifestyle factors, such as diet, exercise, and smoking.
Early onset vs. late onset
There are two types of Alzheimer’s disease — early onset and late onset. Both have a genetic component.
Fewer than 10% of all people with Alzheimer’s disease have the early onset form, where symptoms first appear before a person’s mid-60s.
Early onset Alzheimer’s disease is associated with three single-gene mutations:
- presenilin 1 (PSEN1) on chromosome 14
- presenilin 2 (PSEN2) on chromosome 1
- amyloid precursor protein (APP) on chromosome 21
Each of these mutations plays a role in the breakdown of amyloid precursor protein, which is part of the process that causes harmful forms of amyloid plaques. A person has a 50% chance of developing this genetic mutation if their biological mother or father carries one of the three genes.
Late onset Alzheimer’s disease is more common and usually affects people starting in their mid-60s. Having a genetic variant of the apolipoprotein E (APOE) gene on chromosome 19 increases a person’s risk of developing late onset Alzheimer’s disease. The APOE gene is responsible for making a protein that carries cholesterol and other fat to the bloodstream.
APOE comes in different forms, called alleles, which may increase or decrease a person’s risk of developing Alzheimer’s disease, depending on the allele.
- APOE ε2: This type of allele is rare but may provide some protection against the disease. If someone with this allele develops the disease, it is usually later in life.
- APOE ε3: This is the most common allele and is relatively neutral. It neither increases nor decreases risk.
- APOE ε4: This type of allele increases the risk of developing Alzheimer’s disease and is also associated with an earlier age of onset. However, inheriting this allele does not mean that you will definitely develop the disease.
Recommendations for genetic testing
You can receive a blood test to check which APOE alleles you have through a consumer genetic test or one through a genetic counselor. However, it cannot be used to determine your risk of developing Alzheimer’s disease. The test will only tell you which genes you have. Testing for APOE alleles is sometimes used in research studies. A doctor may order a genetic test to help diagnose early onset Alzheimer’s disease or if you have a strong family history.
The Alzheimer’s Association does not recommend genetic testing for the general population. It is best to talk to a healthcare professional if you are having memory loss or other symptoms of Alzheimer’s disease. If you choose to take an at-home genetic test, talk with a genetic counselor before and after so they can help you interpret the results. This will reduce the likelihood of misunderstanding the results and making poorly informed choices about your health.
Risk genes and deterministic genes
Risk genes and deterministic genes are two categories of genes that influence whether a person will develop Alzheimer’s disease.
Risk genes increase the likelihood of developing Alzheimer’s disease but do not guarantee it. APOE ε4 is an example of a risk gene. It is estimated that 40–65% of people with Alzheimer’s disease have APOE ε4.
People who inherit one copy from their biological mother or father have an increased risk of developing the disease, while people who inherit two copies have an even higher risk. In the United States, 20–30% of individuals have one to two copies of APOE ε4, while 2% of the U.S. population has two copies.
Deterministic genes guarantee that the person will develop the disease if they carry that gene. These genes account for only around 1% of all cases of Alzheimer’s disease and tend to cause the familial early onset forms.
Deterministic genes affect the production of beta-amyloid, the protein fragment responsible for plaque formation and nerve cell death. People with deterministic genes continue to be studied to help gain further insight into Alzheimer’s disease.
Alzheimer’s disease is most commonly caused by a combination of genetic, lifestyle, and environmental factors. There is no single cause.
Other risk factors for Alzheimer’s disease besides the genetic component include:
- Age: After the age of 65, the risk of developing Alzheimer’s disease doubles, and after age 85, close to 1 in 3 people have it.
- Family history: People who have a biological parent or sibling with Alzheimer’s disease are more likely to get it themselves.
- Head injury: Studies have shown a link between head injuries and future risk of dementia. You can protect yourself by wearing a helmet in sports, wearing your seat belt, and making your living environment fall-proof.
- Heart disease: Many conditions that damage the heart and blood vessels also increase the risk of developing Alzheimer’s disease. This includes high blood pressure, diabetes, high cholesterol, stroke, and heart disease. By keeping your heart healthy, you are keeping your brain healthy too.
Alzheimer’s disease typically affects adults over the age of 65, causing memory loss and cognitive decline. The cause of Alzheimer’s disease is a combination of genetics, family history, lifestyle, and environmental factors.
In rare cases, people have a gene that causes Alzheimer’s disease. More commonly, a person with Alzheimer’s disease may carry a gene that only increases their likelihood of developing the disease. There is no way to prevent Alzheimer’s disease, but living a healthy lifestyle can help.