Most common late-onset Alzheimer's gene

The most common type of Alzheimer's disease usually begins after age 65 (late-onset Alzheimer's disease). The most common gene associated with late-onset Alzheimer's disease is a risk gene called apolipoprotein E (APOE).

APOE has three common forms:

• APOE e2 — the least common — reduces the risk of Alzheimer's.
• APOE e4 — a little more common — increases the risk of Alzheimer's and is associated with getting the disease at an earlier age. Approximately 15% to 25% of the general population carries an APOE e4 allele.
• APOE e3 — the most common — doesn't seem to affect the risk of Alzheimer's

Genes aren't the only factor

Because you inherit one APOE gene from your mother and another from your father, you have two copies of the APOE gene. Having at least one APOE e4 gene increases your risk of developing Alzheimer's disease two- to threefold. If you have two APOE e4 genes, your risk is even higher, approximately eight- to twelvefold.

But not everyone who has one or even two APOE e4 genes develops Alzheimer's disease. And the disease occurs in many people who don't even have an APOE e4 gene, suggesting that the APOE e4 gene affects risk but is not a cause. Other genetic and environmental factors likely are involved in the development of Alzheimer's disease.

Other late-onset genes

As research on the genetics of Alzheimer's progresses, researchers are uncovering links between late-onset Alzheimer's and a number of other genes. Several examples include:

• ABCA7. The exact role of ABCA7 isn't clear, but the gene seems to be linked to a greater risk of Alzheimer's disease. Researchers suspect that it may have something to do with the gene's role in how the body uses cholesterol.
• CLU. This gene helps regulate the clearance of amyloid-beta from the brain. Research supports the theory that an imbalance in the production and clearance of amyloid-beta is central to the development of Alzheimer's disease.
• CR1. A deficiency of the protein this gene produces may contribute to chronic inflammation in the brain. Inflammation is another possible factor in the development of Alzheimer's disease.
• PICALM. This gene is linked to the process by which brain nerve cells (neurons) communicate with each other. Smooth communication between neurons is important for proper neuron function and memory formation.
• PLD3. Scientists don't know much about the role of PLD3 in the brain. But it's recently been linked to a significantly increased risk of Alzheimer's disease.
• TREM2. This gene is involved in the regulation of the brain's response to inflammation. Rare variants in this gene are associated with an increased risk of Alzheimer's disease.
• SORL1. Some variations of SORL1 on chromosome 11 appear to be associated with Alzheimer's disease.

Researchers are continuing to learn more about the basic mechanisms of Alzheimer's disease, which may potentially lead to new ways to treat and prevent the disease.

As with APOE, these genes are risk factors, not direct causes. In other words, having a variation of one of these genes may increase your risk of Alzheimer's. However, not everyone who has one will develop Alzheimer's disease.