A retrovirus called KoRV-A has been spreading through the koala population of Australia, leaving the animals vulnerable to infections and cancer. The virus also invades germline cells, so some newborn koalas end up with the pathogen in the DNA of every cell in their bodies.
Now, researchers from the University of Massachusetts Medical School and the University of Queensland are studying KoRV-A to gain new insights into how retroviruses change the evolution of DNA. And they’ve discovered that the genome has its own immune system—one that determines how the DNA of a species changes over time. They published their findings in the journal Cell.
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Retroviruses (HIV among them) don’t usually infect germ cells, but a select few have. Sometimes it’s a good thing: The formation of the placenta in all mammals, including people, was made possible by a gene that originated from a virus, according to a statement from Cell.
What the new study uncovered is the process by which the genome decides what it wants to incorporate and what it wants to throw away. The key lies in spacers called introns. In genes, introns are spliced to make proteins. But retroviruses also have introns, some of which are unspliced, allowing the infection to replicate and spread.
The researchers discovered that germ cells are able to recognize unspliced RNA from viral invaders, and then convert these RNAs to “sense piRNAs,” which block the virus.
“The cells recognize the unspliced sequence as a virus, and not a gene, and mount an initial defense,” said senior author William E. Theurkauf, Ph.D., professor of molecular medicine at UMass Medical School, in a statement from UMass. It’s akin to when “your body mounts an initial innate immune response to viruses and bacteria.”
The next step for the researchers will be to try to figure out how the KoRV-A gets into the germline. Theurkauf and colleagues are planning additional experiments to determine the exact components of cells that are able to recognize viral RNA and then change it so it is no longer pathogenic.
“We think we can sort that out by looking at koalas,” Theurkauf said.56