In Our Genes, Old Fossils Take On New Roles
Monday, September 1, 2008
"The past is never dead. It's not even past."
-- William Faulkner
Over the past 15 years, scientists have been comparing the inherited genetic material -- the genomes -- of dozens of organisms, acquiring a life history of life itself. What they're finding would impress even novelist William Faulkner, the great chronicler of how the past never really goes away.
It turns out that about 8 percent of the human genome is made up of viruses that once attacked our ancestors. The viruses lost. What remains are the molecular equivalents of mounted trophies, insects preserved in genomic amber, DNA fossils.
The thousands of human endogenous retroviruses, or HERVs, sketch a history of rough times during the 550 million years of vertebrate evolution. The best-preserved one, HERV-K113, probably arrived less than 200,000 years ago, long after human beings and chimpanzees diverged from a common ancestor.
But these retroviruses are more than just curiosities. They are some of the most important enemies we ever had. They helped mold the immune system that is one of the evolutionary marvels of life on Earth.
In the past two years, a laboratory in France and another in the United States independently reconstructed a functioning HERV-K retrovirus from pieces found in the human genome. This summer, both showed that the gene sequences of some of those viruses bear the characteristic fingerprints of APOBEC3, a human enzyme that mutated them into submission.
"It is fascinating there is this fossil record in the genomes of modern organisms, and that we are able to see it, analyze it and reconstruct it," said Paul D. Bieniasz, a virologist at the Rockefeller University and the Aaron Diamond AIDS Research Center in New York, who leads the American lab.
Retroviruses differ from ordinary viruses in that they stitch themselves into the genes of the animals they infect. They become permanent residents of their hosts. Conventional viruses, such as the ones that cause measles, influenza and colds, don't do that.
At the moment, the world is in the middle of a huge retrovirus epidemic: AIDS. Its virus, HIV, attacks cells of the immune system, principally lymphocytes, and stitches itself into them. Once there, HIV is reborn constantly as the cells grow and divide (which is one reason there is no cure for the infection). The AIDS virus dies only when the host does.