The largest study to date looking for genetic causes of Alzheimer's in African Americans may offer new clues about why blacks in the United States are twice as likely as whites to develop the deadly, brain-wasting disease.

The findings, published in the Journal of the American Medical Association on Tuesday, show that mutations in two genes that play a role in whites also contribute to Alzheimer's risk in blacks. One of those, known as ABCA7, may double the risk in blacks who have the mutation versus those who don't.

Although many genes have been found to raise the risk of Alzheimer's, most studies have been conducted in largely white populations, and few studies have looked specifically at genes that drive Alzheimer's in blacks. Part of that is because very few African Americans take part in gene studies looking at Alzheimer's risk.

The latest findings will need to be confirmed by other research teams, and critics say the study is incomplete until that work is done.

To get enough participants for the newly published study, researchers combined genetic information from 18 different Alzheimer's Disease Centers funded by the U.S. National Institutes of Health. They gathered information on 6,000 African Americans, 2,000 of whom had late-onset Alzheimer's disease, the most common form that occurs in older people.

The team then looked for genes that were most strongly associated with Alzheimer's. The strongest link was with a variant of a gene called apolipoprotein E or APOE, a gene that contains instructions for making a protein that carries cholesterol and is well-known risk factor for Alzheimer's.

The team found that a variant of this gene called APOE-e4 doubled the risk of Alzheimer's in blacks, in much the same way it does in whites.

But the study also turned up another gene that has only been weakly associated with Alzheimer's in whites. This gene, called ABCA7, which also plays a role in the production of cholesterol and fats, appears to have a much stronger effect in blacks.

"In whites, it increases risk by 10 to 20 percent, but in African Americans, it increases risk by about 70 to 80 percent. It has a way larger effect size in African Americans," said Dr. Christiane Reitz of Columbia University Medical Center, who conducted the genetic analyses on the study.

ABCA7 is also involved in cholesterol metabolism, as are several of the genes which have been found in the past five years or so to be linked with Alzheimer's in whites.

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Study finds gene that may raise Alzheimer's risk in blacks

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