WISE sees its first stars
In the new study, researchers led by Keizo Tomonaga of Osaka University found that two human genes are similar to the bornavirus N gene. These two genes, now called EBLN-1 and EBLN-2 for endogenous Borna-like N, are molecular fossils of an ancient bornavirus.
Retroviruses make up about 8 percent of the human genome. When these viruses insert into the genome, the result is usually bad for the host. But not always: Some retrovirus proteins can help fight off infection with other retroviruses. And at least twice in primate evolution retrovirus insertions have added genes to the host genome that aid in making the placenta. Now those proteins are essential for placenta development, says Cédric Feschotte, an evolutionary geneticist at the University of Texas at Arlington. It is not clear what role, if any, the EBLN-1 and EBLN-2 genes play in humans.
Other mammals, including chimpanzees, gorillas, orangutans, macaques, mouse lemur, African elephant, Cape hyrax and 13-lined ground squirrels all carry N gene insertions in their genomes, Tomonaga and colleagues found. The insertion in the ground squirrel genome was probably a relatively recent event, occurring not more than 10 million years ago, the researchers report.
But bornavirus insertion isn’t all ancient history. In laboratory experiments, Tomonaga’s team found that modern bornavirus can integrate into the DNA of human, monkey, rat and dog cells. And mice with bornavirus infections were shown to have new insertions in brain cell DNA.
Modern bornaviruses are known to infect nerve cells, but the new study shows that the viruses are capable of infecting and inserting genes into many other types of cells. For the inserted virus sequences to pass to the next generation, the ancient infection must have happened in tissues that give rise to eggs or sperm. Scientists call these tissues the germ line.
“The fact that these viruses were able to get into the germ line, which requires many chance events, implies that they may insert at some appreciable frequency,” Gifford says.
Feschotte agrees. “That, to me, is a revelation,” he says.
He speculates that bornavirus could be another source of human mutations, especially in neurons. Some studies have linked infection with the virus to psychiatric disorders such as schizophrenia. Feschotte thinks modern bornaviruses may worm their way into human DNA in the neurons, creating mutations in genes that could lead to schizophrenia.