Fighting Ancient Virus May Have Left Humans Vulnerable to HIV

New study helps explain why other primates aren't sickened by AIDS.

By E.J. Mundell
HealthDay Reporter
Thursday, June 21, 2007; 12:00 AM

Copyright © 2007 ScoutNews, LLC. All rights reserved.

THURSDAY, June 21 (HealthDay News) -- Humans' distant ancestors beat back a now-extinct virus 4 million years ago, but that victory left the species vulnerable to HIV today, scientists believe.

The finding may also explain why chimpanzees, gorillas and other primates resist infection with the virus that causes AIDS, while humans are much more easily infected.

The difference lies in a human gene called TRIM5alpha, which, in lab experiments, effectively blocked infection from a reconstructed portion of the ancient retrovirus, called PtERV1. That virus probably flourished 4 million or 5 million years ago but is extinct today.

Writing in the June 22 issue of Science, the researchers say PtERV1 left behind telltale traces of its DNA inside the genomes of many primates -- with the notable exception of humans, suggesting it never gained a foothold there.

In fact, when geneticists first compared the genomes of humans against those of chimps, "the biggest difference is the presence [in nonhuman primates] of this virus, PtERV1," said study senior author Michael Emerman, a member of the Human Biology and Basic Sciences divisions at the Fred Hutchinson Cancer Research Center, in Seattle.

"There are about 130 copies in the genomes of chimps and gorillas, and maybe other primates, but it's not present at all in the genome of humans," Emerman said.

That could mean one of two things, he said: Either the ancestors of Homo sapiens already carried the TRIM5alpha gene and couldn't be infected by PtERV1, or PtERV1 wiped out all but a few individuals who were lucky enough to carry the gene. Those individuals might then have passed the gene on through their progeny, right up to the humans of today.

However, that early antiviral victory may have a darker legacy.

The experts found that various primate species carry slightly different versions of TRIM5alpha, one of a class of infection-blocking genes called "cellular restriction factors." In the case of Homo sapiens, the gene's ability to block infection against PtERV1 does not seem to extend to another dangerous retrovirus, HIV.