Did you know that Alzheimer’s disease accounts for 60%-80% of dementia cases? What’s more, did you know that over 6 million Americans are living with Alzheimer’s currently? This number could skyrocket to nearly 13 million by 2025, according to the Alzheimer’s Association®.
With such a staggering number of people with Alzheimer’s disease, it’s important to learn what causes it. This can help us determine what we’re able to do to prevent developing the disease, if anything, or if it’s just within someone’s genetics.
What Is Alzheimer’s Disease?
Alzheimer’s disease is a form of dementia that causes problems with memory, thinking or behavior. Ultimately, daily tasks are interfered with, as symptoms develop slowly and get worse over time.
What Causes Alzheimer’s Disease?
While there’s no precise cause, many experts agree that it’s the result of a range of risk factors and genes.
Modifiable Risk Factors of Alzheimer’s Disease
Whether or not someone is diagnosed with Alzheimer’s disease could come down to a number of risk factors. Fortunately, the likelihood can be minimized by changing your habits, taking control of your health, and more. Modifiable risk factors include:
- Heart disease
- High blood pressure
- High cholesterol
- Obesity
- Stroke
- Diabetes
- Likelihood of head injury
- Smoking
- Excessive drinking
- Low physical activity
- Poor diet and nutrition
- Decreased social activity and mental stimulation
Presence of these risk factors does not ultimately mean you’ll be diagnosed with Alzheimer’s disease or develop dementia; it simply means the chances could be higher. On the flip side, just because none of these risk factors apply to you does not mean you won’t be impacted by memory loss.
Nonmodifiable Risk Factors of Alzheimer’s Disease
These risk factors are unable to be changed and are out of our control.
Age
Those ages 65 and older are more likely to be diagnosed with Alzheimer’s disease. However, some in their 40s or 50s are diagnosed with early-onset Alzheimer’s disease.
Race and Ethnicity
Certain ethnic communities are at a higher risk of developing Alzheimer’s. African Americans and Hispanics are more at risk. The reason for this is not well understood, but higher rates of health conditions may be behind it.
Medical Conditions
There are some medical conditions that can increase the risk of developing dementia. These include Parkinson’s disease, multiple sclerosis, kidney disease and HIV.
Family History and Genetics
Those with a parent or sibling with Alzheimer’s are more likely to develop the disease than those with no family history. Risk increases if more than one member of the family has the illness.
To understand more about how these factors can link to Alzheimer’s disease or an increased risk of dementia, continue reading below.
Genetic Links to Alzheimer’s Disease
Do genetics always mean you’ll have an increased risk of developing Alzheimer’s disease? According to the National Institute on Aging (NIA), it depends. But first, it’s important to find out how genetics works.
An article by the NIA states that each human cell contains instructions that each cell needs to do its job. These instructions, DNA, are packed into structures called chromosomes. Each chromosome has thousands of segments, called genes, that are passed down from a person’s biological parents, according to the article.
These genes carry information that defines traits, like eye and hair color, height, and health. If there are permanent changes in one or more genes, also known as genetic mutations, diseases can be caused. Genetic variants may then increase or decrease the risk of developing the disease. According to the NIA, when a genetic variant increases disease risk, but doesn’t directly cause a disease, it’s called a genetic risk factor.
Over 20 genes have been found that affect a person’s risk of developing Alzheimer’s disease. These are summed up in the below section.
Risk Genes
Risk genes increase the likelihood that someone will develop Alzheimer’s disease. However, this doesn’t mean that Alzheimer’s disease will, without a doubt, develop.
APOE
There is one gene with the greatest known effect on increasing Alzheimer’s disease risk. This gene is called apolipoprotein E (APOE). Researchers aren’t sure why, but they do know this protein is found on chromosome 19 and plays a role in handling dietary fats, including cholesterol.
Everyone has two copies of the APOE gene, with six combinations possible. These combinations affect our risk at different rates. APOE-e2, APOE-e3 and APOE-e4 are the most known combinations.
The APOE-e4 gene is the strongest genetic risk factor. Those who inherit one copy of APOE-e4 have an increased risk. Those who inherit two copies have an even higher risk (about three to five times), but it’s still not a certainty. Those who have the variant APOE-e2 carry a slightly lower risk.
According to the article by the NIA, rare forms of the APOE allele may even provide protection against Alzheimer’s disease.
Deterministic Genes
These genes directly cause Alzheimer’s disease.
APP, PSEN1 & PSEN2
These three single-gene mutations are associated with early-onset Alzheimer’s disease, according to the NIA. Amyloid precursor protein (APP) is present on chromosome 21, Presenilin 1 (PSEN1) is on chromosome 14 and Presenilin 2 (PSEN2) is on chromosome 1.
These mutations result in production of abnormal proteins. Since they each play a role in the breakdown of APP, it creates harmful forms of amyloid plaques, a hallmark of Alzheimer’s disease.
If a person has a faulty gene, then their children have a 50% chance of inheriting it and developing dementia. People with Down syndrome also have a higher risk of early-onset Alzheimer’s disease because of being born with an extra copy of chromosome 21, which carries the APP gene.
Alzheimer’s disease caused by these deterministic variations is called autosomal dominant Alzheimer’s disease (ADAD) or familial Alzheimer’s disease. This leads to many family members in multiple generations being affected, with symptoms usually developing in their 40s or 50s.
True familial Alzheimer’s accounts for approximately 1% of cases and has only been found in a few hundred extended families worldwide.
Genetic Testing
Blood tests can identify which APOE alleles someone has, but it can’t determine who will, or who won’t, develop Alzheimer’s disease.
According to the NIA, this type of testing is used primarily in research to identify study participants who may have an increased risk of developing the disease so that scientists can look for early brain changes and compare effectiveness of possible treatments. It’s sometimes used by physicians to diagnose early-onset Alzheimer’s and to test people with a strong family history.
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