Our ancestors wiped out HIV-type virus 11 million years ago

Retroviruses can leave lasting traces of their existence if they infect cells of germ line

Our ancestors may have eliminated a primordial HIV type virus around 11 million years ago, according to a new study that examined fossil DNA to find out when ancient viruses were at their peak.

Researchers from The Rockefeller University in the US suggest that our primate ancestors evolved a defence mechanism that involved manipulating the function of one of the virus's genes, turning the virus against itself.

"Analysing viral fossils can provide a wealth of insight into events that occurred in the distant past," said Paul Bieniasz, from the Howard Hughes Medical Institute in the US.

"This study is an example of how viruses themselves can provide the genetic material that animals use to combat them, sometimes leading to viral extinction," said Bieniasz.

Retroviruses, a class of viruses that include the human immunodeficiency virus (HIV), are abundant in nature and can leave lasting traces of their existence if they infect cells of the germ line.

Unlike other viruses, they include a step in their life cycle where their genetic material is integrated into the genome of their host - creating a genetic fossil record that can be preserved in the genomes of the host and its evolutionary descendants.

To examine how extinct viral lineages could once have been eliminated, researchers analysed retroviral fossils left by human endogenous retrovirus T (HERV-T), which replicated in our primate ancestors for about 25 million years before it was eradicated about 11 million years ago.

Working with Robert Gifford from the University of Glasgow in the UK, the team first compiled a near-complete catalogue of HERV-T fossils in old-world monkey and ape genomes.

They then reconstructed the HERV-T retrovirus' outer envelope protein - a molecule that allows a virus particle to bind to cells and begin the viral replication cycle.

"Our analyses suggested that HERV-T likely used a cell- surface protein called MCT-1 to bind to cells and infect ancient old-world primates," said Daniel Blanco-Melo, a former graduate student in the Bieniasz lab.

The researches also identified a fossilised HERV-T gene in the genomes of contemporary humans that was absent in more distantly-related primate genomes.

They found that this gene encodes a well-preserved envelope protein that can block retroviral infection by depleting MCT-1 from cell surfaces.

"It appears this gene was integrated into the ancestral primate genome around 13 to 19 million years ago, and we believe it was around this time that the function of this gene switched," said Blanco-Melo.

"Ancestral hominids evolved a defence mechanism against HERV-T, using the virus's own gene against itself, eventually leading to its extinction," Blanco-Melo added.