By Edward Edelson
　　HealthSCOUT Reporter

TUESDAY, Sept. 12 (HealthSCOUT) -- Scientists have begun hunting for a hidden recessive gene linked to Alzheimer's disease.

Linking a recessive gene to Alzheimer's would be a first because all genes linked to the disease so far have been dominant, experts say. A single dominant gene can produce an effect, while two recessive genes -- one inherited from each parent -- must be present to create an effect.

The search stems from a discovery that the gene thought to be responsible for most cases of Alzheimer's appears in extremely low numbers among a particular community of Arabs who have an unusually high incidence of the mind-robbing condition.

Roughly 20 percent of the Arab residents of the Israeli farming community of Wadi Ara have Alzheimer's, twice the expected rate. Among those aged 85 and older, more than 60 percent have Alzheimer's, compared with about 40 percent of older people in general.

But few of the residents carry apolipoprotein E-4 (apoE-4), the dominant gene variant identified as a major risk factor for the disease. Genetic testing found only 4 percent of 256 Arab residents of Wadi Ara had the gene -- the lowest level on record, the researchers say. The expected rate in most populations is 15 percent.

The research team, led by Dr. Amos Korczyn, a neurologist at Tel Aviv University, speculates that the absence of apoE-4 hints at the presence of a recessive gene. The evidence so far is tantalizing but not conclusive, the team reports in today's issue of the journal Neurology.

The fact that marriages between close relatives are fairly common among Arabs who live in Israel supports their theory, the researchers say. In the community, 44 percent of marriages are between persons with many genes in common, upping the likelihood of someone inheriting the hidden gene from both parents, they say.

"Identifying the gene would likely have a major impact on our understanding of how and why Alzheimer's occurs," Korczyn says. "We would also then have to look at whether the gene is involved in causing Alzheimer's disease among other populations."

One of the researchers, Dr. Robert Friedland, an associate professor of neurology at Case Western Reserve University, says the team has made progress toward finding the location of the gene on a specific chromosome in the cell nucleus.

Discovery of such a gene "could be of diagnostic value and could also help in development of a treatment, if we knew what the gene does," he says.

Researchers are genetically screening all Wadi Ara residents over age 60, Friedland says.

Bill Thies, medical director of the Alzheimer's Association, says discovery of a recessive gene related to the disease would add one more piece to the Alzheimer's puzzle.

"What has already emerged is a picture of multiple genes that influence one's risk for developing Alzheimer's disease. It looks like a whole bunch of genes -- it could be as many as 20 or 30 -- could be involved. If that is the case, the risk could be determined by all of those genes," he says.

While neurologists and geneticists would like a simple genetic marker that singles out high-risk people, Thies says, "Because so many genes are being linked to Alzheimer's disease, we are not likely to have that kind of fortunate likelihood."

"Finding another gene that predicts risk is important, but it is unlikely to revolutionize the field," Thies says.

(From Yahoo!)