Is there a genetic test for Alzheimer’s?
A curious adult from California asks:
“I have heard about a genetic test for Alzheimer’s. Is there one?”
Editor’s Note (7/3/2021): Since this article was published, we have learned a lot more about the genetics of Alzheimer’s disease, and options for genetic testing have expanded. For more information, please see the Alzheimer’s Disease Genetics Fact Sheet.
Actually, there are several. But how reliable they are depends on the type of Alzheimer’s that your family has.
There are two broad categories of Alzheimer’s, early and late onset. Early onset is when Alzheimer’s happens before age 65, late onset is after 65.
Early onset Alzheimer’s disease
Less than 5% of Alzheimer’s cases are the early onset form of the disease. It tends to run in families, and symptoms usually start to appear in a person's 40’s or 50’s.
Many forms of early onset Alzheimer’s are pretty well understood genetically. Three genes have been identified as being involved and all three can be tested for. If you test positive for a known mutation, you will get the disease.
Of course, nothing entirely cut and dry. Testing can give inconclusive results if there is a mutation that hasn’t been found before in one of these three genes.
We all have lots of differences between our DNA. Luckily for our health, most are harmless. But these differences can muddy the waters for genetic testing.
If you have a mutation in one of these genes that hasn't been found before, is it harmless? Or does it mean that you will get the disease?
One way to check would be to look at the gene of a relative with the disease. If they have the same mutation, then odds are higher that having that mutation might lead to getting the disease. As long as that is the only mutation they have.
If they have more than one mutation and you share only one, then what? As you can see, genetic testing doesn’t always give an easy answer.
Late onset Alzheimer’s disease
And the picture is even less clear with late onset Alzheimer’s. There does seem to be some complex inheritance involved but what is going on is pretty unclear.
The average lifetime risk of getting Alzheimer’s is about 5% by age 65, 10-15% by age 75, and 20-40% by age 85.
People who have a parent, a brother, or a sister with Alzheimer’s have about twice the average risk. And the more relatives with Alzheimer’s, the greater the risk.
This is the hallmark of a disease with many genes involved. We can clearly tell that genetics is involved but we don’t know how many or what genes are involved.
All that we can do is study lots of families and see how often Alzheimer’s pops up. And then report those numbers with little understanding about why we get those numbers.
Without knowing all the genes responsible, you can’t have a useful genetic test. That doesn't mean there aren’t tests available. This is because one gene has been identified with increased risk, the APO-E gene.
There are 3 versions of this gene, APO-E2, APO-E3, and APO-E4. If you have one copy of the APO-E4 version, you have an increased chance of developing Alzheimer’s. And if you have 2 copies, the risk goes up even more.
But if you have 2 copies of the APO-E4 version of the gene, that doesn’t mean you will get the disease. So what does knowing really tell you?
If you have 2 copies, there’s not much you can do to prevent the disease. The knowledge doesn’t change your treatment options. It doesn’t tell you that you will get the disease. All you know is you have an increased risk.
This is why no one is currently recommending that people take this test. The test is available at a number of places, it just isn’t all that useful.
As you can see, genetic tests are not a simple matter. It isn’t like a blood sugar or cholesterol test. You might not get a definitive result with a genetic test. And sometimes, there isn’t much you can do with the results.
Anyone considering a genetic test should probably speak with a genetic counselor. A genetic counselor can help a person think through their situation and arrive at the most informed decision they can.
Author: Dr. Barry Starr
Barry served as The Tech Geneticist from 2002-2018. He founded Ask-a-Geneticist, answered thousands of questions submitted by people from all around the world, and oversaw and edited all articles published during his tenure. AAG is part of the Stanford at The Tech program, which brings Stanford scientists to The Tech to answer questions for this site, as well as to run science activities with visitors at The Tech Interactive in downtown San Jose.