( Human resistance to a retrovirus that ...)Ancient retrovirus sheds light on modern pandemic

Human resistance to a retrovirus that infected chimpanzees and other nonhuman primates 4 million years ago ironically may be at least partially responsible for the susceptibility of humans to HIV infection today.

These findings, reported by a team of researchers at Fred Hutchinson Cancer Research Center in the June 22 issue of Science, provide a better understanding of this modern pandemic infection through the study of an ancient virus called Pan troglodytes endogenous retrovirus, or PtERV1.

"This ancient virus is a battle that humans have already won. Humans are not susceptible to it and have probably been resistant throughout millennia," said senior author Michael Emerman, Ph.D., a member of the Human Biology and Basic Sciences divisions at the Hutchinson Center. "However, we found that during primate evolution, this innate immunity to one virus may have made us more vulnerable to HIV."

Evidence of human immunity to this ancient retrovirus first emerged with the sequencing of the chimpanzee genome. "When the chimp genome was sequenced, a team of scientists at the University of Washington led by Evan Eichler found the largest difference overall between the chimp and human genomes was the presence or absence of PtERV1," Emerman said. "Chimps have 130 copies of PtERV1 and humans have none."

It is believed that retroviruses have been entering the genome for many millions of years, and so humans share many retroviral DNA fragments with their primate cousins. Such vestiges of primitive infection, rendered inactive by eons of genetic mutation, make up about 8 percent of the human genome.

Innate protection against PtERV1 in humans could be credited, the researchers believe, to the presence of an ancient, rapidly evolving antiviral defense gene called TRIM5a, which produces a protein that binds to and destroys the virus before it can replicate within the body.

"We know that PtERV1 infected chimps, gorillas and old-w
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Source:Fred Hutchinson Cancer Research Center