Talking endogenous retroviruses with Dr Ravinder Kanda

Endogenous retroviruses (ERVs) constitute a substantial fraction of vertebrate genomes, but how did they get there and why are the important? To find out, we chat to Dr Ravinder Kanda whose research focuses on the various evolutionary processes that shape genomes 

What are endogenous retroviruses and why are they so interesting? Only 2% of our DNA is used to build our bodies. The rest of it – noncoding DNA – is a mixture of old genes that have lost their function, repetitive strings of DNA whose function is not understood, and other elements. Endogenous retroviruses (ERVs) are a kind of noncoding DNA that make up 8% of our DNA. ERVs are all descended from viruses, very like those that cause disease, like HIV, which managed to insert themselves into our ancestors’ DNA in the distant past.

When and how do we think ERVs got incorporated into our DNA? The way this particular group of viruses, called retroviruses, infect a cell involves inserting themselves into the DNA of the cell. New copies of the retrovirus can then be produced using the cell’s machinery and go on to infect other cells. Occasionally, a retrovirus will infect the germ-line cells. When an infected fertilised egg develops into an adult, every single cell in that individual’s body will contain the viral DNA. It is inherited by all the offspring of that individual. There are around 100,000 copies of these ERVs in our genome. By comparing the DNA of other primates and mammals, we can estimate how long ago these ERVs inserted into the DNA of our ancestors.

How might these ‘DNA invaders’ be good for us? In some instances, we have managed to ‘borrow’ some of the viral genes and use them for our benefit. The most famous example is a gene that is involved in pregnancy, specifically with the formation of the placenta. Without it we would not be able to reproduce as we do. In other species, there are instances where having a particular ERV gives you some protection against infection from other related retroviruses. For example, sheep have a particular ERV that can block the receptors of a cell, preventing entry into the cell and therefore infection by other related viruses.

What can ERVs reveal about the evolution of infections in animals and humans? Many ERVs in our DNA are ancient, indicating that this invasion has been occurring for millions of years. By comparing those viruses that are present in DNA to viruses that currently infect and cause disease, we can see that some of these viruses are very good at making a leap and infecting different species to those in which they were originally found, something called cross-species transmission. For example, we know that HIV was a virus that originally infected primates. The subgroup of viruses to which HIV belongs – lentiviruses – has recently been discovered in the DNA of other species. These discoveries challenge our understanding of how these viruses might change and evolve.

What further research is needed to understand more about ERVs? Lots! We are only just beginning to understand what an influential role viruses may have played in many various aspects of the evolution of a species. One consideration is that some viruses can make the leap to infect other species, such as HIV. A better understanding of cross-species transmission, why or how this occurs, and why some viruses are better at doing this than others, may also help us identify potential ‘hotspots’ of infection. For me personally, I am interested in the role that ERVs play with regards to offering immunity against infection from other viruses. Adapted from an interview featured on the Oxford Science Blog