The higher-risk variant was seen in 7 percent of the study participants, Reitz said.
As for what it all means, she said, it's too early to tell.
"This finding needs to be replicated, and we need to learn more about the biological mechanisms," Reitz said.
Like ApoE, the ABCA7 gene is involved in cholesterol metabolism, Reitz said. High cholesterol, heart disease and stroke are considered risk factors for Alzheimer's, and black Americans have higher rates of those cardiovascular problems than whites.
But it's not clear if cardiovascular disease explains why the genes are linked to Alzheimer's -- or why ABCA7 is more important in blacks' risk.
"It's all up in the air," editorial author Nussbaum said. "Gene studies like this are good for finding connections between genes and [disease], but they can't tell us what the mechanisms are."
He said his "gut feeling" is that ApoE and ABCA7 are linked to Alzheimer's independent of their effects on cholesterol and heart disease.
Research has shown that ABCA7 also is involved in transporting amyloid precursor proteins, which help feed the "plaques" that build up in the brains of people with Alzheimer's. That is another potential reason that the gene is linked to Alzheimer's, Reitz said.
But, Nussbaum said, much more research is needed to understand the biology of Alzheimer's. For example, he said, after years of study, researchers are still divided over whether the amyloid plaques in the brains of people with Alzheimer's actually cause the disease.
Reitz said it's too early for people to seek testing for which ABCA7 variant they carry.
You would not be able to run to your doctors' office for such a test anyway -- although there are private companies that offer ApoE testing,
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