Ancient viral battle left people vulnerable to HIV

WASHINGTON, June 22 (Reuters) A battle won by human ancestors against a virus that infected chimpanzees and other primates millions of years ago may have left people today more vulnerable to the AIDS virus, scientists said.

That ancient battle helped humans evolve and rely on a gene that may not protect so well against a modern retrovirus, the human immunodeficiency virus or HIV, the researchers said yesterday.

They focused on an ancient virus, known as Pan troglodytes endogenous retrovirus, or PtERV1, to find clues as to why HIV has exacted such a high toll on humanity.

''Events that happened millions of years ago have shaped human evolution, in particular susceptibility to modern human infectious diseases,'' Michael Emerman of the Fred Hutchinson Cancer Research Center in Seattle, Washington, who led the study, said in a telephone interview.

There is genetic evidence that the PtERV1 virus infected chimpanzees, gorillas and old-world monkeys about 4 million years ago, but no evidence it infected humans. The virus is believed to have gone extinct perhaps 2 million years ago.

Emerman's team was able to resurrect a small portion of the PtERV1 by using DNA remnants of the virus lodged in the genome of chimpanzees.

Working with cells in a laboratory, they found that an old virus-fighting gene present in people, known as TRIM5a, succeeded in neutralizing PtERV1.

The gene makes a protein that binds to and destroys the virus before it can replicate within the body, they report in the journal Science.

''However, while TRIM5a may have served humans well millions of years ago, the antiviral protein does not seem to be good at defending against any of the retroviruses that currently infect humans, such as HIV-1,'' Emerman said in a statement.

VIRAL INFILTRATION Small mutations may account for this. TRIM5a taken from baboons and African green monkeys carried mutations that helped it reduce HIV in lab dishes but it was almost useless against PtERV1.

When TRIM5a worked well against PtERV1, as in humans, it was ineffective against HIV.

''In the end, this drove human evolution to be more susceptible to HIV,'' Emerman said.

Retroviruses have been infiltrating the genomes of human ancestors and other animals for millions of years. These viruses go into the chromosomes and DNA of the cells they infect and can get passed on from generation to generation.

In fact, such vestiges of primitive infections comprise 8 percent of the human genome, Emerman said.

Other researchers found remnants of the PtERV1 retrovirus in the genomes of chimpanzees, gorillas and some other primates in Africa, but not humans or another great ape, the orangutan.

When the chimpanzee genome was mapped, a major difference compared to the human genome was the presence of about 130 copies of PtERV1 in chimpanzees and zero in people, the researchers. The retrovirus is so mutated in living chimps that it is inactivated, the researchers said.

While the study sheds light on the historical human susceptibility to pathogens, Emerman said, it does not appear to provide immediate clues as to how to beat HIV.

REUTERS KK VC0835