Ancient viruses lurking in our DNA: study

Our DNA is actually less human than previously thought, according to a new study that found 19 new pieces of non-human DNA left by viruses in our ancestors hundreds of thousands of years ago.
One stretch of newfound DNA, in about 50 of the 2,500 people studied, contains an intact, full genetic recipe for an entire virus, said researchers from University of Michigan and Tufts University.
Researchers do not know yet whether it can replicate or reproduce. But other studies of ancient virus DNA have shown it can affect the humans who carry it, they said.
In addition to finding these new stretches, researchers also confirmed 17 other pieces of virus DNA found in human genomes by other scientists in recent years.

The study looked at the entire span of DNA, or genome, from people from around the world, including a large number from Africa - where the ancestors of modern humans originated before migrating around the world.
Researchers used sophisticated techniques to compare key areas of each person's genome to the "reference" human genome.

The findings add to what science already knows about human endogenous retroviruses (HERVs). HERVs is the name for the ancient infectious viruses that inserted a DNA-based copy of their own RNA genetic material into our ancestors' genomes.
They are part of the same type of virus that includes the modern human immunodeficiency virus, which causes AIDS.

Over generations, the virus-generated DNA kept getting copied and handed down when humans reproduced. That is how it ended up in our DNA today, researchers said.
About 8 per cent of what we think of as our "human" DNA actually came from viruses. In some cases, HERV sequences have been adopted by the human body to serve a useful purpose, such as one that helps pregnant women's bodies build a cell layer around a developing foetus to protect it from toxins in the mother's blood, they said.
The new HERVs are part of the family called HERV-K. The intact whole viral genome, or provirus, just found was on the X chromosome; it has been dubbed Xq21. It is only the second intact provirus found to be hiding in human DNA, researchers said.

"This one looks like it is capable of making infectious virus, which would be very exciting if true, as it would allow us to study a viral epidemic that took place long ago," said John Coffin from Tufts University.
"This research provides important information necessary for understanding how retroviruses and humans have evolved together in relatively recent times," Coffin said.
The findings were published in the journal PNAS.