But the gene doesn't seem to be affiliated with higher incidence of Alzheimer's among white populations, the scientists said in the report in the Journal of the American Medical Association.
"These findings suggest that the genetic underpinnings of Alzheimer's disease may vary among different populations -- and so should not be treated homogeneously," said first author Christiane Reitz of Columbia University Medical Center.
African-Americans have much higher rates of late-onset Alzheimer's -- by far the most common form of the disease -- than whites. But "until now, data on the genetics of Alzheimer's in this patient population have been extremely limited," said senior author Richard Mayeux, also of Columbia.
The new study analyzed genetic data from nearly 6,000 African-American participants, the largest genome-wide search for Alzheimer's genes among the population group.
The study helped scientists confirm that the ABCA7 gene variant is linked to a higher incidence of late-onset Alzheimer's among African Americans. The gene is involved in the production of cholesterol and lipids.
Imbalances in these two molecules can lead to vascular disease and strokes -- and, thanks to this new research, may be related to the development of dementia.
This suggests that treatments that fight high cholesterol and vascular disease may also prove effective at warding off Alzheimer's disease among African-Americans.
The research also confirmed that a second gene variant, long known to be a risk factor for Alzheimer's among whites, is also affiliated with higher risk among African-Americans.
"Both genes raise the risk of Alzheimer's in this population twofold," said Reitz, who noted that previous results from smaller studies had been inconsistent.
Likewise, the study confirmed that several other genes that had been linked to a higher risk of the disease among whites also played a role among blacks.
"Because they cross ethnic groups, the likelihood increases that these genes are very important in the development of Alzheimer's," Reitz added.
Separately, researchers have genetically engineered a rat with all the symptoms of Alzheimer's, enabling them to pinpoint some of the key mechanisms behind the disease.
The study, also published Tuesday, in the Journal of Neuroscience, supports the theory that Alzheimer's is caused by increases in a molecule called beta-amyloid, which form the plaques in the brain that are the disease's hallmark.
Researchers said the rats will also be helpful for testing the effectiveness of new drugs to fight the dementia.
Some 5.3 million Americans currently suffer from Alzheimer's disease, the most common form of dementia, according to the Centers for Disease Control and Prevention. And nearly 90 percent of those cases are the late-onset form.
By 2050, the number of people with Alzheimer's is expected to more than double because of the aging population, the CDC has said.
A study earlier this month in the New England Journal of Medicine estimated that dementia will cost US society between $41,000 and $56,000 per person every year.