In the United States, 5.4 million adults -- one in eight older Americans -- has Alzheimer's disease, according to the Alzheimer's Association.
Alzheimer's disease is a common type of dementia, a degenerative neurological disorder in which patients suffer decline in memory, thinking and language.
No clear cause of Alzheimer's disease has been found, though research has noted several risk factors, including genetics.
The first gene identified as a risk factor for Alzheimer's disease is APOE-e4, one of the three common types of the APOE gene. The other two forms of the APOE gene are APOE-e2 and APOE-e3.
The Alzheimer's Association explained that everyone inherits a copy of APOE (e2, e3 or e4). If an individual inherits one copy of the APOE- e4, her risk for developing Alzheimer's disease increases.
Having two copies of the APOE-e4 gene increases the risk even more, but the Alzheimer's Association noted that it does not mean it is certain the individual will develop a disease.
New research has possibly identified another gene that raises an individual's risk for developing Alzheimer's disease. Research was conducted at two separate labs that came to the same conclusion.
A mutation to the gene TREM2 interferes in the brain preventing plaque buildup. Plaque buildups are one of the hallmark changes to the brain with Alzheimer's disease.
The mutation of the TREM2 gene is rare. The New York Times reported that it is present in about 2 percent of patients with Alzheimer's disease.
In comparison, it is estimated that the APOE e-4 gene causes between 20 to 25 percent of Alzheimer's disease cases, according to the Alzheimer's Association.
In studies with mice, the mutation to the TREM2 gene affected the mice's white blood cells, causing them to be less effective in preventing beta amyloid from building up.
Two different studies were conducted that found the link between increased risk for Alzheimer's disease and the mutated TREM2 gene. One study included 2, 261 Icelanders, in which the researchers looked at their genomes, focusing on TREM2.
They found that Alzheimer's disease patients have the mutation more often.