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Tuesday, 1 June 2010

10% of Europeans have immunity to HIV (14-16%) amongst Scandinavians

In some parts of Europe today, up to 20% of the population carry at least one copy of the protective allele. However, the populations of Asia and Africa were not exposed to the same epidemics; very few Asians and Africans now carry the allele (see map above). Thus, CCR5 is fairly common in northern Europe but its frequency diminishes as one moves south, and the mutation is rare in the rest of the world. Evolution Berkely

From Science News

About 10 percent of Europeans have a mutation that disables a protein the Human Immunodeficiency Virus (HIV-1) uses to slip into immune system cells. HIV-1 has a harder time infecting people who have a mutation in one of the two genes that code for this receptor protein, and if these people become infected, their disease progresses more slowly. Those with mutations in both copies of the gene are almost completely resistant to the virus

This genetic mutation arose as recently as 700 years ago, and some researchers have suggested that the bubonic plague that devastated Europe periodically over the past 1,000 years may have selected for the mutation by sparing those who lacked one or both copies of the gene.

In a paper appearing this week in the online Early Edition of The Proceedings of the National Academy of Sciences, the two UC Berkeley researchers argue that smallpox, not bubonic plague, is the most likely cause of the spread of this mutation throughout the European population in such a short time.

That is, the same genetic mutation that confers resistance to HIV-1 protects against death from smallpox.

"Our population genetic model finds that genetic selection from plague wouldn't have been sufficient to drive the frequency of this genetic mutation to its current level," said Alison P. Galvani, a Miller Postdoctoral Fellow at UC Berkeley. "It was sufficient for smallpox."

Bubonic plague hasn't been a major source of death in Europe or elsewhere for the last 250 years, while smallpox was only eradicated in 1978, at the same time AIDS (acquired immune deficiency syndrome) appeared. The survival advantage this genetic mutation provided against smallpox has thus been transferred to AIDS, the authors noted.

Following a 1998 paper that linked the gene deletion with bubonic plague, "bubonic plague had been cited as a classical example of a historical selection pressure acting on a clinically important locus," she said. That classic example now changes, with smallpox replacing the plague.

The gene produces a receptor, called CCR5, that is the main entry port for HIV-1 into T cells and macrophages. While most people around the world have two CCR5 genes or alleles, about 10 percent of Europeans, on average, lack one of the alleles. They thus produce fewer CCR5 receptors, which hinders initial infection by HIV-1 and slows spread within the body once an immune cell has been infected. Those lacking both alleles produce no CCR5 receptor. Read more


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