In an unexpected discovery, researchers have found that the genomes of humans and other vertebrates contain ancient genetic sequences from two deadly virus families.
It was known previously that retroviruses—RNA viruses that insert DNA copies of their genetic material into their hosts’ genomes when they replicate—have left behind bits of that material in vertebrate genomes. However, neither of the nonretroviral RNA families in question integrates their genetic material into their host, making the discovery especially surprising. The conservation of the sequences over time, however, suggests that they give the host some selective advantage.
“This was a surprise for us,” says virologist Ann Skalka. “It says that the source of our genetic material is considerably wider than we thought.”
In comparing thousands of viral genes from non-retroviral families to the genomes of 48 vertebrate species, including humans, the research team uncovered 80 viral sequence integrations into 19 species. Nearly all of the sequences come from ancient relatives of the Ebola/Marburg and Borna virus families, which include deadly pathogens that cause hemorrhagic fevers and neurological disease, respectively. The findings were published in July in PLoS Pathogens.
Skalka explains the unexpectedness of the find: “These viruses replicate their RNA and are not known to make any DNA. They have no known mechanism for getting their genetic material integrated into the DNA of the host genome.”
It is remarkable, she adds, that the sequences, some of which may have been integrated into the host genomes more than 40 million years ago, have been largely conserved— in a form that suggests they provide or provided some active benefit, such as protection from infections by related viruses. “One might even think of these integrations as genomic vaccinations,” Skalka says.
The research team included investigators at the Institute for Advanced Study in Princeton, where Skalka was on sabbatical when she initiated the study.
Demonstrating conclusively that the viral sequences have a biological function will take additional research. However, the team noted that expression of some of the viral genetic material has been detected in human tissues, supporting the possibility that they are active in host species.
To read more about Skalka’s research and link to a discussion of her work on NPR’s “Science Friday,” visit www.fccc.edu/topics/fossils.