Ancient Viruses Lurk in Human DNA

Scientists have discovered 19 new pieces of non-human DNA – left by viruses that infected our ancestors hundreds of thousands of years ago – in the genomes of modern humans.

Scientists analyzed the genomes of some 2,500 people from around the world, and one stretch of new-found DNA, found in about 50 of the 2,500 people, contains a full genetic recipe for an entire virus. Whether or not it can replicate is not yet known, but other studies of ancient virus DNA have proved it can affect the humans who carry it.

The study investigated the entire span of DNA from people from around the world, including a large number from Africa, where our ancestors originated before migration.

Human endogenous retroviruses, or HERVs, are ancient viruses that succeeded in depositing DNA-based copies of their RNA genetic material into the genomes of early humans. Over generations, the virus-generated DNA kept getting copied and passed on when humans reproduced, which is how it ended up in our DNA today. In fact, about 8 percent 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 valuable purpose, such as one that helps pregnant women’s bodies build a cell layer around a developing fetus to protect it from toxins in the mother’s blood.

The new HERVs are part of the family called HERV-K. The intact whole viral genome, just found was on the X chromosome and it’s been dubbed Xq21. “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 senior author of the study and virologist John Coffin, Ph.D. of the Tufts University School of Medicine.

“Many studies have tried to link these endogenous viral elements to cancer and other diseases, but a major difficulty has been that we haven’t actually found all of them yet,” said co-author Zachary H. Williams.

The new discovery will open up many doors to research, according to co-first author Julia Wildschutte. “What’s more, we have confirmed in this paper that we can use genomic data from multiple individuals compared to the reference human genome to detect new HERVs. But this has also shown us that some people carry insertions that we can’t map back to the reference.”

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