July 19, 2013
One of the first broad studies of genetics in African Americans has found that blacks and whites of European ancestry share similar genes when it comes to the risk of developing Alzheimer’s disease, though there are slight differences. The research is important because it expands the genetic study of Alzheimer’s to a wider and more diverse population, helping researchers better understand the genetic basis of a disease that is expected to strike 30 million people worldwide in coming decades.
Blacks, in general, have a slightly higher risk for developing Alzheimer’s disease in old age than whites living in the same community. But researchers have been uncertain how great a role genetics plays.
“Late-onset Alzheimer disease is the most common cause of dementia, increasing in frequency from 1 percent at age 65 years to more than 30 percent for people older than 80 years,” the authors of the current study write. “Genetic variants associated with susceptibility to late-onset Alzheimer disease are known for individuals of European ancestry, but whether the same or different variants account for the genetic risk of Alzheimer’s disease in African American individuals is unknown. Identification of disease-associated variants helps identify targets for genetic testing, prevention, and treatment.”
For the current study, published in Journal of the American Medical Association (JAMA), researchers at Columbia University in New York gathered genetic data from 5,896 African Americans aged 60 and older from 18 medical centers across the country. All were part of a large genetic study called the Alzheimer Disease Genetics Consortium, and data was collected from 1989 to 2011. About 2,000 of them had Alzheimer’s disease.
The researchers found that carrying a gene called APOE-E4 increased the risk of Alzheimer’s in blacks. The same gene has also long been known to raise the risk of Alzheimer’s in older white people as well, from two- to four-fold (200 to 400 percent) or more.
Blacks who carried another gene called ABCA7 were also at increased Alzheimer’s risk. The same gene raises the risks in those of European ancestry, though the risk was stronger in blacks. Blacks who carried the ABCA7 were about 80 percent more likely to develop Alzheimer’s than those who didn’t have the gene; about 9 of every 100 African Americans with Alzheimer’s had the gene, compared to 6 out of 100 without the disease. Whites who carried the gene, by comparison, had a 10 to 20 percent increased risk. Risk numbers below 200 percent are generally considered modest.
Findings these genes in both groups further strengthens the idea that genetics plays an important role in Alzheimer’s onset. Both the APOE-E4 and ABCA7 genes are involved in the way the body moves cholesterol in and out of cells. ABCA7 is also involved in the transport of beta-amyloid, the toxic protein that builds up in the brains of those with Alzheimer’s, and is suspected as a risk factor for heart disease as well.
More research is needed to verify the findings. But “if validated by future replication and functional studies, identification of ABCA7 as a risk gene in late-onset Alzheimer’s disease among African Americans not only may help elucidate the disease etiology but also may have major implications for developing targets for genetic testing, prevention, and treatment,” the authors write.
Sources: Robert L. Nussbaum, MD: “Genome-Wide Association Studies, Alzheimer Disease, and Understudied Populations.” JAMA Vol. 309. No. 14, April 10, 2013. Christiane Reitz, MD, PhD, Gyungah Jun, PhD, Adam Naj, PhD, et al: “Variants in the ATP-Binding Cassette Transporter (ABCA7), Apolipoprotein E e4, and the Risk of Late-Onset Alzheimer Disease in African Americans.” JAMA, April 10, 2013, pages 1483-1492.