Important note: Information in this article was accurate in 1998. The state of the art may have changed since the publication date.

Reuters NewMedia, Inc.; Friday May 8, 7:56 pm EST
Maggie Fox, Health and Science Correspondent

Scientists at the National Cancer Institute (NCI) think it's a reasonable explanation of why up to 15 percent of white people -- but not blacks or Asians -- have the mutation, and they are gathering evidence to back up their theory. "It just hit me in the face. It wasn't an idea, it was an accumulation of data," said Steve O'Brien, an NCI molecular biologist who headed the team.

O'Brien's group has been researching the mutations that protect some people against HIV, or cause the virus to progress more slowly.

Three have been identified so far. Two involve genes known as CCR2 and CCR5, which control receptors, or chemical doorways to the macrophage immune system cells that HIV attacks.

The third mutation involves the gene controlling production of the SDF1 protein, used by immune cells to communicate.

O'Brien, whose team examined the genetic makeup of thousands of people taking part in different AIDS trials, was looking at the CCR5 mutation.

"It was present in a very high frequency among Caucasians and yet it was completely missing in native Africans and native east Asians," he said in a telephone interview on Friday.

Therefore, the mutation must have arisen after humans moved out of Africa. The mutation is also seen more commonly in the north of Europe.

"It displayed the highest frequency in Sweden, a middle amount in France, Italy and Turkey and less than 1 percent in Saudi Arabia and zero in sub-Saharan Africa," he said.

"That kind of information indicates that something funny happened. We didn't know what it was and we didn't know where it was, but we thought it must be something with a high degree of mortality like the present AIDS epidemic."

They used a genetic "clock" to try to date the mutation -- which they can do by looking at the rate of mutations on neighboring genes.

"The number came out to be 700 years," O'Brien said.

"If you look at the history book and see was there any epidemic that knocked out a lot of Caucasians 700 years ago, in fact there was -- bubonic plague." O'Brien's team needed more evidence and did find that bubonic plague, caused by the Yersinia pestis bacterium, also attacks macrophages. They have published their findings in the American Journal of Human Genetics.

"We still don't have the bingo and the bingo is that people with different CCR5 genotypes respond differently to bubonic plague," he said.

The team is now testing this -- taking cells from people with different versions of the CCR5 gene and testing them against plague in test tubes.

Bubonic plague swept through Europe and parts of Asia in the 14th century killing as much as three-quarters of the population in less than 20 years. It killed 70,000 in the 17th century and still infects about 2,000 a year, although it is usually easy to treat with antibiotics.

Many gene mutations are known to protect against disease. For example, people with two copies of one gene mutation get sickle cell anemia, but having one copy is believed to protect against malaria.

Earlier this week, researchers at Brigham and Women's Hospital in Boston said the gene that causes cystic fibrosis may have given its carriers some protection against typhoid.

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