Charles Q. Choi

A virus that invaded the genomes of humanity's ancestors millions of years ago now plays a critical role in the embryonic stem cells from which all cells in the human body derive, new research shows.

The discovery sheds light on the role viruses play in human evolution and could help scientists better understand how to use stem cells in advanced therapies or even how to convert normal cells into stem cells.

Embryonic stem cells are pluripotent, meaning they are capable of becoming any other kind of cell in the body. Scientists around the world hope to use this capability to help patients recover from injury and disease.

Researchers have struggled for decades to figure out how pluripotency works. These new findings reveal that "material from viruses is vital in making human embryonic stem cells what they are," said computational biologist Guillaume Bourque at McGill University in Montreal, a co-author of the study published online March 30 in Nature Structural & Molecular Biology.

Viral Invasion

To make copies of itself, a virus has to get inside a cell and co-opt its machinery. When one type of virus called a retrovirus does this, it slips its own genes into the DNA of its host cell. The cell is then tricked into assembling new copies of the retrovirus. The most infamous retrovirus is HIV, the virus behind AIDS.

In rare cases, retroviruses infect sperm or egg cells. If that sperm or egg becomes part of a person, their cells will contain retrovirus DNA, and they can pass that DNA on to their descendants. Past research suggests that at least 8 percent of the human genome is composed of these so-called endogenous retroviruses-leftovers from retroviral infections our ancestors had millions of years ago.

Scientists long thought that endogenous retroviruses were junk DNA that didn't do anything within the human genome, said study co-author Huck-Hui Ng, a molecular biologist at the Genome Institute of Singapore.

However, recent studies have revealed that might not be true for one class of endogenous retroviruses known as human endogenous retrovirus subfamily H. HERV-H DNA was surprisingly active in human embryonic stem cells but not in other regular types of human cells.

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Ancient Virus DNA Gives Stem Cells the Power to Transform