... The most recent datings of pandemic HIV-1 point earlier: 1908 to 1921 are more probable times of origin of the human epidemic. These new datings were
Message 1 of 2
, Jul 31, 2010
VIENNAThousands of years ago, a piece of West Africa separated
from the mainland and formed the island of Bioko. The monkeys that
inhabit the island may be crucial to unraveling the puzzling origins of
the AIDS epidemic in humans, according to a study presented here last
week at the 18th International AIDS Conference.
Scientists have argued about the origin of the AIDS epidemic since it
surfaced in 1981, but this much is widely accepted today: Sometime around
1931, HIV-1, the main virus driving the epidemic, likely entered humans
from chimpanzees, which are infected with a related virus called SIVcpz.
The chimp virus, in turn, is a blend of SIVs from two different monkey
Less clear is when the monkey viruses moved into chimpanzees. Last year,
one prominent investigator in the origin field, evolutionary biologist
Michael Worobey of the University of Arizona in Tucson, found evidence
that the monkey-to-chimp jump occurred sometime between 1266 and 1685.
Worobey and his team used changes in the RNA of these SIVs to calculate
their age. These so-called molecular clocks depend on how they're
calibrated, however, and not everyone was convinced.
Skeptics, including virologist Preston Marx of the Tulane National
Primate Research Center in Covington, Louisiana, suspected the leap from
monkeys to chimps occurred tens of thousands of years earlier. SIVs and
SIVcpz are found everywhere from East to West Africa, and Marx reasoned
that it "was just not possible" for the viruses to have spread
so widely in 500 years. So he came up with a new way to calibrate the
molecular clock that relied on Bioko's known separation date from the
mainland, and he recruited Worobey to help him analyze the data.
Marx and his team collected samples of SIV in dead monkeys on Bioko,
which were killed for
bushmeat. The researchers isolated SIVs from four different species
on the island. One species, the Bioko drill (Mandrillus leucophaeus
poensis), has a mainland counterpart that also harbors SIV. The fact
that the virus was in both drills and that the island separated from the
mainland 12,000 years ago provided a precise way to calibrate the
molecular clock, and comparisons of the SIVs confirmed Marx's suspicion
that the jump into chimpanzees must have occurred much earlier than
Worobey's previous estimates.
As it turned out, the SIV from the drills closely matches SIV from
red-capped mangabeys, one of the two contributors to SIVcpz. So an
ancestor of this drill could have infected chimpanzees. According to
Marx's analysis, a virus related to the Bioko drill's SIV infected
chimpanzees at least 22,000 years ago.
Worobey's earlier studies did not date the origin of SIV itself but
suggested that it was "relatively young." Others have argued
that the SIVs emerged millions of years ago. The new analysis of all four
monkey species suggests that, at a minimum, the SIVs are 76,000 years
oldalthough Marx suspects that they evolved far earlier. This longer
history of primates harboring the viruses may explain why SIVs cause no
harm in the African monkeys they infect, Marx noted: The hosts have had
more time to evolve appropriate immune responses or cellular changes that
make them less vulnerable to the viruses. (Recent reports strongly
indicate that SIVcpz can cause AIDS in chimpanzees.)
"The data are excellent," says Simon Wain-Hobson, a virologist
at the Pasteur Institute in Paris who has been involved in origin studies
since the start of the AIDS epidemic. But he cautions that the SIV on the
island may have been introduced recently, upending Marx and Worobey's
clock calibration. "The jury is out," says Wain-Hobson, noting
that he is not ready to discard the substantial evidence from other
molecular-clock analyses that SIVcpz is younger.
Paul Sharp, an evolutionary biologist at the University of Edinburgh in
the United Kingdom who first described the SIVs that led to SIVcpz, has
more confidence that the new findings will hold up to scrutiny.
"Molecular-clock analyses have suggested that the SIVs arose within
the last few thousand years," he says. "These Bioko viruses are
clear evidence that the SIVs must be much older than that."
As a short summary of my proposal, we have now pandemic HIV because there
was an unprecedented pandemic of syphilis 1 century ago.
The study of SIVs in the Bioko island is wonderful, and it was
communicated to me by Preston Marx himself during a meeting in Paris,
The fact that SIVs, and particularly the chimp and gorilla versions are
much older than previously thought makes even more mysterious the question
of why, during millenia of human exposure to primate bushmeat, didn't
SIVs became epidemic in humans. And why did several strains managed to
become so at the same time in 20th century? I hope I have provided an
answer to these questions.
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