Scientists have unraveled one of the mysteries of the AIDS epidemic - why some escape infection despite thousands of risky sexual encounters.
The answer, for many, appears to be inborn. They have genes that make it extremely difficult to catch HIV, the AIDS virus, according to findings published Friday in the journal Cell.The discovery opens new possibilities for treating and preventing the disease. If scientists can find a way to mimic the effects of this inborn genetic shield, they believe it may be possible to create a pill that will keep people from becoming infected with the AIDS virus.
Scientists made the discovery with the help of two gay men who volunteered at the Aaron Diamond AIDS Research Center in New York City.
The men, who said they had watched 40 or 50 of their friends and lovers die of AIDS, wondered why their own high exposure to the virus had not also led to infection.
The researchers discovered that both men carried a pair of genes that ward off HIV. They were even more surprised to learn how common these genes are.
"We originally thought people with this would be one in a million. Actually, it's more like one in 100" white people, said Dr. Nathan R. Landau, one of the Diamond re-search-ers.
Only whites carry the genes for reasons that are not yet clear.
The key to warding off HIV is a mutation in the genes that direct the body's defenses against disease. In this case, a gene called CCR5 is missing a chunk of information, so the body fails to produce a particular protein. This protein is one of the docking points the AIDS virus needs to invade cells.
Those who are born with two copies of the mutant gene - one from each parent - appear to be highly resistant to AIDS infection, although experts are not sure if their protection is absolute.
A surprising 20 percent or so of whites were found to have one copy. The effects of having one copy are still unclear. But the researchers believe it makes people somewhat less likely to get infected and may help them survive much longer once infection occurs.
Drug companies are already working on drugs to block CCR5 as a way of keeping HIV out of cells, Landau said.
"All over the world, wherever people look, there is a small fraction of people who seem not to get infected," despite multiple exposure, said Dr. Robert W. Doms of the University of Pennsylvania Medical Center.
Already, some experts worry that people will throw away their condoms if they learn they have two copies of the mutant gene.
"That would be folly. We are just on the threshold of understanding all of this," said Dr. Anthony Fauci, head of the National Institute of Allergy and Infectious Diseases.
While only whites have the gene, Doms and others suspect that people of other ethnic origins will turn out to have other protective genes.