A Planet of Viruses - Carl Zimmer [19]
Scientists recognized that this new virus was in a class of its own. They called it an endogenous retrovirus—endogenous meaning generated within. They soon found endogenous retroviruses in other animals. In fact, the viruses lurk in the genomes of just about every major group of vertebrates, from fish to reptiles to mammals. Some of the new endogenous retroviruses turned out to cause cancer like avian leukosis virus, but many did not. Some seemed to be effectively muzzled by their host. Certain endogenous retroviruses carried by mice cannot infect mice cells, for example, but they can readily spread among rat cells.
Other endogenous retroviruses turned out to be crippled, carrying mutations that robbed them of the ability to make full-fledged viruses. They could still make new copies of their genes, however, which were then reinserted back into their host’s genome. And scientists also discovered some endogenous retroviruses that were so riddled with mutations that they could no longer do anything at all. They had become nothing more than baggage in their host’s genome.
Endogenous retroviruses can linger in their hosts for millions of years. In 2009, Aris Katzourakis, an evolutionary biologist at the University of Oxford, discovered hundreds of copies of endogenous retroviruses in the genome of the three-toed sloth. Their genes closely matched those of foamy viruses, free-living pathogens that infect primates and other mammals. Katzourakis concluded that foamy viruses infected the common ancestor of three-toed sloths and primates, which lived a hundred million years ago. In primates, they’ve remained free-living. In the sloth lineage, however, they became trapped in their host’s DNA and have remained there ever since.
As scientists discovered endogenous retroviruses in other species, they naturally wondered about our own DNA. After all, we suffer infections from many retroviruses. Virologists tried coaxing endogenous retroviruses out of human cells without any luck. But when they scanned the human genome, they found many segments of DNA that bore a striking resemblance to retroviruses. Many of those segments resembled retrovirus-like segments in apes and monkeys, suggesting that they had infected our ancestors thirty million years ago or more. But some of the retrovirus-like segments in the human genome had no counterparts in any other species. It was possible that the segments unique to humans started out as retroviruses that infected our ancestors a million years ago.
To test this idea, Thierry Heidmann, a researcher at the Gustave Roussy Institute in Villejuif, France, tried to bring a human endogenous retrovirus back to life. Searching through the genomes of different people, he and his colleagues found slightly different versions of one retrovirus-like segment. These differences presumably arose after a retrovirus became trapped in the genomes of ancient humans. In their descendants, mutations struck different parts of the virus’s DNA.
Heidmann and his colleagues compared the variants of the virus-like sequence. It was as if they found four copies of a play by Shakespeare, each transcribed by a slightly careless clerk. Each clerk might make his own set of mistakes. Each copy might have a different version of the same word—say, wheregore, sherefore, whorefore, wherefrom. By comparing all four versions, an historian could figure out that the original word was wherefore.
Using this method, Heidmann and his fellow scientists were able to use the mutated versions in living humans to determine the original sequence of the DNA. They then synthesized a piece of DNA with a matching sequence and insert it into human cells they reared in a culture dish. Some of the