Genes hint at HIV genesis

Monkey probably original source of AIDS virus.
13 June 2003

HELEN PEARSON

Chimps could have acquired SIV from eating monkeys. © GettyImages

Take two monkey viruses, feed to chimps, incubate for several years - and eat. That's the recipe for creating HIV, a new study suggests.

The predominant strain of the virus that causes AIDS, called HIV-1, is thought to have jumped into humans from African chimpanzees, maybe through people eating infected bushmeat. Where the chimp version came from was unknown.

Now a team led by Paul Sharp of the University of Nottingham, UK, reckons that chimps probably acquired the virus, called simian immunodeficiency virus (SIV), by eating other monkeys in west central Africa¹.

By comparing the viruses' genetic sequences, the researchers worked out that Chimp SIV is a cross between SIV strains found in red-capped mangabeys and greater spot-nosed monkeys. The hybrid probably arose when two monkey viruses swapped genes in chimps, and it subsequently spread between them.

This theory "makes sense", agrees Edward Holmes, who studies HIV evolution at the University of Oxford, UK. But it is not known whether the genetic shuffle also enabled the chimp SIV strain to jump into humans and spread; this might instead have occurred by chance.

The study highlights the possibility, Sharp warns, that other SIVs made from jumbled monkey viruses exist in chimps, and may spawn a next-generation HIV in humans. SIVs infect around 30 species of African monkey; chimpanzees are the only apes known to be naturally infected.

A new HIV remains speculation, counters Holmes. SIV could have jumped into people many times in the past but dwindled rather than spread.

Even so, Holmes advocates the systematic logging of SIV strains in chimp populations to find examples that might be heading our way. "We need to see what's out there", he says.

Sharp's team has started on this task. Next month they publish a report showing that 10-15% of one Tanzanian chimp population carry SIV. This is far lower than its 50% prevalence in monkeys, but similar to the HIV rate in some west central African human populations, where the virus first emerged.

The finding implies that, in some respects, chimp SIV infection is similar to human HIV infection. It contrasts with an earlier survey by the same group that found SIV in less than 2% of chimps, causing scientists to wonder how such rare infections in chimps could have triggered the human epidemic².

Importantly, chimps do not fall sick - perhaps because they have evolved to survive the infection or because the virus has become less potent. "We'd like to know what the difference is", says Sharp.

1. Bailes, E. et al. Hybrid origin of SIV in chimpanzees. Science, 300, 1713, (2003). |Homepage|
2. Santiago, M. L. et al. SIVcpz in wild chimpanzees. Science, 295, 465, (2002). |Homepage|