One thing I really enjoy about my job is the opportunity to constantly be learning something new. For example, I hadn't realized that about eight percent of human DNA is actually left-behind detritus from ancient viral infections. I knew they were there, but eight percent? That's a lot of genetic baggage.
These sequences are often inactive in mature cells, but recent research has shown they can become activated in some tumor cells or in human embryonic stem cells. Now developmental biologist Joanna Wysocka, PhD, and graduate student Edward Grow, have shown that some of these viral bits and pieces spring back to life in early human embryos and may even affect their development.
Retroviruses are a class of virus that insert their DNA into the genome of the host cell for later reactivation. In this stealth mode, the virus bides its time, taking advantage of cellular DNA replication to spread to each of an infected cell's progeny every time the cell divides. HIV is one well-known example of a retrovirus that infects humans.
When a retrovirus infects a germ cell, which makes sperm and eggs, or infects a very early-stage embryo before the germ cells have arisen, the viral DNA is passed along to future generations. Over evolutionary time, however, these viral genomes often become mutated and inactivated. About 8 percent of the human genome is made up of viral sequences left behind during past infections. One retrovirus, HERVK, however, infected humans repeatedly relatively recently — within about 200,000 years. Much of HERVK's genome is still snuggled, intact, in each of our cells.
Wysocka and Grow found that human embryonic cells begin making viral proteins from these HERVK sequences within just a few days after conception. What's more, the non-human proteins have a noticeable effect on the cells, increasing the expression of a cell surface protein that makes them less susceptible to subsequent viral infection and also modulating human gene expression.
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But it's not clear whether this sequence of events is the result of thousands of years of co-existence, a kind of evolutionary symbiosis, or if it represents an ongoing battle between humans and viruses.
"Does the virus selfishly benefit by switching itself on in these early embryonic cells?" said Grow. "Or is the embryo instead commandeering the viral proteins to protect itself? Can they both benefit? That's possible, but we don't really know."
Wysocka describes the findings as "fascinating, but a little creepy." I agree. But I can't wait to hear what they discover next.
Previously: Viruses can cause warts on your DNA, Stanford researcher wins Vilcek Prize for Creative Promise in Biomedical Science and Species-specific differences among placentas due to long-ago viral infection, say Stanford researchers
Photo of Joanna Wysocka by Steve Fisch