No one knows the exact cause of Alzheimer's disease, but scientists do know that genes are involved. For many years, researchers have looked for specific genes that affect how likely you are to get Alzheimer's disease. There have been some intriguing clues, but more research will help doctors fully understand the genetic links in the disease.
How Do Genes Work?
Genes are the basic building blocks that direct almost every aspect of how you’re built and how you work. They’re the blueprint that tells your body what color your eyes should be or if you’re likely to get some kinds of diseases.
You get your genes from your parents. They come grouped in long strands of DNA called chromosomes. Every healthy person is born with 46 chromosomes in 23 pairs. Usually, you get one chromosome in each pair from each parent.
Gene Links to Alzheimer’s
Scientists have found evidence of a link between Alzheimer's disease and genes on four chromosomes, labeled as 1, 14, 19, and 21.
One connection lies between a gene on chromosome 19, called the APOE gene, and late-onset Alzheimer's. That’s the most common form of the disease that affects people over age 65. Dozens of studies around the world have shown that when a person has one type of the APOE gene, called APOE4, it increases their odds of getting Alzheimer's at some point in their lives.
But the link isn’t completely clear-cut. Some people who have APOE4 don’t get Alzheimer’s. And others have the disease even though they don’t have APOE4 in their DNA. In other words, though the APOE gene clearly influences Alzheimer's risk, it’s not a consistent sign that someone will have the disease. Scientists need to learn more about the connection.
Familial Alzheimer's Disease
Alzheimer's disease strikes early and fairly often in some families -- often enough that experts single it out as a separate form of the disease. It’s called early-onset familial Alzheimer's disease.
By studying the DNA of these families, researchers have found that many of them have flaws in related genes on chromosomes 1 and 14. A few of the families share a difference in one gene on chromosome 21.
The chromosome 21 gene is an interesting Alzheimer's clue because of its role in Down syndrome. People with Down syndrome have an extra copy of chromosome 21. As they grow older, they often get Alzheimer’s symptoms, though at a younger age than others who get the disease. Their brain cells also show the same damage that happens to brains affected by Alzheimer’s. Scientists are still trying to fully understand the link between these two conditions.
Few researchers think that the search for Alzheimer's genes is over. But they also know genes aren’t the only cause of the disease. More research will show how DNA, lifestyle habits, and things in the environment play roles in making people more likely to get the condition.