How Viruses Infiltrated Our DNA and Supercharged Our Immune System!:
“Evolution, it turns out, is really good at irony.”
By Ed Yong
Hundreds of millions of years ago, prehistoric viruses inserted their genes into the genomes of our ancestors. They found their way into eggs or sperm, and then into embryos.
As they passed down from one generation to the next, they picked up mutations that disabled their ability to infect new cells. Eventually, they became permanent fixtures of our genomes, as much a part of our DNA as our own genes.
Today, these ‘endogenous retroviruses’ or ERVs make up 8 percent of our genome. They are genetic fossils—remnants of our viral ancestors, and records of epidemics past.
ERVs are not passive stowaways. They provided raw material for evolution, in the form of viral genes that our ancestors could tinker with and put to new use. For example, syncytin, a gene that’s essential for creating the placenta, came from a virus; in the words of Carl Zimmer, “If not for a virus, none of us would ever be born.”
ERVs also contain sequences that dictate when and where genes should be switched on. So when they first infiltrate a genome and hop around, they can rapidly and radically re-wire entire networks of genes. Edward Chuong, Nels Elde, and Cédric Feschotte from University of Utah School of Medicine have now found the most dramatic example of this re-wiring. They have shown that some ERVs that have shaped the evolution of our immune system, are now controlling the activity of critical immunity genes.
Which means that we defend ourselves from some viral infections with the repurposed remains of ancient viruses.
When we suffer from infections, two proteins called IRF and STAT coordinate our immune responses. They hover over our DNA, looking for particular sequences that they can stick to. When they find these docking sites, they land and activate nearby genes. And when Chuong looked at these docking sites, he found that many of them came from ERVs. By hopping around our genomes, these viruses created new ports for IRF and STAT, and changed the portfolio of immune genes that they turn on.
