Ever wonder why you have blue eyes, like your mother, or are going bald, like your grandpa? It’s all about your genes, those tiny bits of biological material your parents handed down to you. Genes are the reason some of us have brown hair and others have red; why some of us are tall and others are short. They are also why some of us develop Alzheimer’s.
Although scientists believe that many things influence Alzheimer’s, they are increasingly convinced that genes have a starring role in how the disease begins and how it progresses. That’s because every cell in our body contains instructions that it needs to do its job. These instructions are made up of DNA, short for deoxyribonucleic acid. DNA is a spiral-shaped molecule that acts like a safe-deposit box stuffed with the information that determines a person’s physical characteristics. At times, however, nature can throw a monkey wrench into our DNA. When that happens, genes can permanently mutate, sometimes causing a specific disease, such as sickle cell anemia or cystic fibrosis. Genetic mutations can also cause Alzheimer’s.
Genes and Early Onset Alzheimer’s
In fact, scientists are fairly sure that a genetic mutation of one particular gene is the main culprit behind early-onset familial Alzheimer’s disease, or eFAD. By definition, eFAD occurs in people in their 30s, 40s, 50s, and early 60s. This type of AD strikes less than 5 percent of all people with Alzheimer’s. One of the things that makes eFAD so menacing is that only one parent has to have a copy of the mutant gene for it to be passed on to his or her offspring. As a result, a child has a 50-50 chance of inheriting the changeling gene, which is located on three different chromosomes, the structure that contains DNA. A person inheriting the gene from either parent is thus in the high-risk category of having Alzheimer’s symptoms before the age of 65.
Why is this mutant gene such bad news? The gene causes the production of an excessive amount of a protein called amyloid-beta peptide. This peptide (a peptide consists of two or more amino acids) builds up in the brain and forms fragments of amyloid plaque, the hallmark of Alzheimer’s disease. These fragments build up between nerve cells to form a protein called tau, the tangles in the brain associated with the types of abnormal brain functions that are typically common with Alzheimer’s. Some people who have eFAD do not have the mutant gene, however, suggesting that other genetic abnormalities or other factors might be involved.
“As we get older, our risk increases,” says Daniel Potts, M.D., a neurologist at the US Department of Veterans Affairs. “It is easier to diagnose when a person gets Alzheimer’s at a younger age than when a person gets it when they are older. Genetics don’t play much of a role, we don’t think, in the late onset of the disease as opposed to the early onset.”
Late-onset Alzheimer’s, the most common form of AD, usually begins after age 65. Researchers have found a gene, called apolipoprotein E, or APOE, that they believe plays a role in late-onset AD. APOE has three common forms. The least common is APOE e2, which scientists believe reduces the risk of Alzheimer’s. Another form of the gene, APOE e4, is a bit more common and appears to increase a person’s chances of late-onset AD. And the most common is APOE e3, which doesn’t seem to affect the risk of Alzheimer’s.
People inherit one APOE gene from their mother and one APOE gene from their father. That means each of us has two copies of the gene. Having one APOE e4 gene increases the chances of getting Alzheimer’s. And having both makes the chances even greater.
Just because you have APOE e4, however, doesn’t mean you’ll necessarily get Alzheimer’s. While many people with the gene develop the disease, many don’t. Conversely, some people who do not have APOE e4 do develop Alzheimer’s. But APOE e4 is not the only gene associated with late-onset AD. Over the past few years, scientists have uncovered a number of other genes that might be related to the disease.
This gene’s job is to regulate cholesterol and lipids. Scientists believe this gene puts a person in the high-risk category of developing Alzheimer’s.
The job of this gene is to clear away amyloid-beta from the brain. If there is an imbalance in the production and clearance of amyloid-beta, the risk of developing Alzheimer’s is higher.
This gene produces protein. If the gene mutates, it could inflame the brain. Inflammation, scientists have determined, is another possible factor in Alzheimer’s
Scientists do not know a lot about this gene, but they have linked it to an increased risk of Alzheimer’s. It’s important to remember that these genes, including APOE e4, do not directly cause Alzheimer’s. However, their presence does put a person at a greater risk of developing the disease. In other words, these genes are so-called genetic variants. A genetic variant is a change within a gene that can increase the odds of a person developing a specific disease or another malady while not directly causing the ailment.
Should You Get Tested?
“Should I get tested for Alzheimer’s?” It’s a question you might ask, especially if you have a parent or sibling with the disease. Keep in mind that genetic tests provide only bare-bones information about increased Alzheimer’s risk, which is one of the reasons doctors seldom recommend testing for people, especially with late-onset Alzheimer’s. If you need to cross that bridge, there are people like Susan Hahn who can guide you.
Hahn is a member of the National Society of Genetic Counselors and specializes in assessing a patient’s risk for Alzheimer’s disease. But while information is power, you should think long and hard before getting tested, says Hahn, especially if you plan on using one of the commercially available genetic-testing companies. For one thing, what you learn is permanent and there’s no going back. “You have to think about what you want to do with this information and why do you want to know it,” Hahn says. “You can be saddled with something that you have no control over.” Moreover, there are other people who can be affected as well. “If I learn something about myself, it applies a risk to my kids; it applies a risk to my siblings,” she explains. “It says something about my parents that they may not want to know.”
Make Sure You’re Prepared for Anything
Before you or a loved one get tested, you need to prepare yourself for the emotional and economic roller coaster that might follow. You need to brace yourself if the tests come back positive. The results might also affect your eligibility for certain forms of insurance, including disability and life insurance.
Hahn says that knowing you are genetically at risk could lead to anxiety and depression. It can also impact your family relationships. “For some people, when they find out, they turn lemons into lemonade,” Hahn says. “They begin living more healthy lifestyles. Everybody is different. Everybody needs different things to feel in control. Some people do okay, some people don’t. It’s a Pandora’s box.”
A version of this article appeared in our partner magazine Alzheimer’s: New Hope for a Cure, in 2020.
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