Analysis: viruses have been with us every step of the evolutionary way, sometimes harming us and sometimes helping us
A souvenir could be a photograph from a great trip or a fridge magnet or some tacky ornament made of seashells. Or it could be some DNA from a viral infection our ancestors got millions of years ago.
Our DNA is not only human and up to 8% of it came from viruses. These little infectious agents are so small that they don’t even contain sufficient tools to reproduce. They get around this problem by infecting cells and using their machinery to make lots of copies of themselves, which then go on to infect more cells and so on. The damage they cause to cells when they’re doing this is what makes us sick.
There are many different types of viruses, but the retroviruses are special. These ones have the ability not just to use cells as virus factories, but to insert their own DNA, containing instructions to build the virus, into the cell’s DNA, where it becomes a part of it.
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The HIV virus is probably the best known retrovirus out there, but there are plenty more and it seems they have been with us for a very long time. For instance, we share most of the viral bits in our DNA with non-human primates such as chimpanzees. This means that our common ancestor got infected with retroviruses millions of years ago and the viral DNA that got inserted into eggs or sperm was then transferred to all descendants. Some of the DNA bits of viral origin that we carry are even older, seeing as we share them with mice, while some are a lot more recent.
We have all heard about the theory of evolution and how natural selection eliminates the genes that we don’t need and keeps the ones we do. So why have viral genes been kept inside our DNA? Why would we need viral genes if we’re not going to go around infecting other people?
One of the viral DNA genes in our bodies has instructions to make a protein called syncytin. This protein causes a virus-infected cell to fuse with another and thus helps to propagate infection in an easy way. Syncytin also appears to be able to tone down the response of our immune system, presumably to protect viruses from being destroyed. Syncytin is actually produced by human cells, specifically by placental cells called syncytiotrophoblasts, which fuse to one another forming a boundary between maternal and fetal tissue during pregnancy.
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This boundary plays an important role in the exchange of food between mother and fetus, and it also protects the fetus from the maternal immune system, which might attack it thinking it’s an invader. So the properties of syncytin are actually quite useful for mammals, including us humans. It is possible that acquiring the ability to produce this viral protein was what allowed mammals to develop placentas in the first place, since animals that lay eggs don’t have the gene. This would mean that it was viruses that gave rise to mammalian reproduction, and possibly to mammals themselves, by helping shape the way we reproduce.
This may not be the only way that viruses have changed us for the better. When the viral DNA inserts itself in the middle of our genes, it can affect the way our own proteins are produced. For example, viral DNA stimulates the production of a protein in saliva that helps break up starch, and this may have helped our ancestors – who mostly ate meat and fruit – to incorporate bread and potatoes to their diet.
It sounds like the viral genes were good for us, but are they dangerous at all since they came from a virus? It certainly doesn’t look like our cells are getting infected. That is probably because almost all the viruses in our DNA are incomplete, missing some of the bits they’d need to become infectious.
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However, there are studies in mice showing that viruses inserted in the DNA can reactivate and even cause cancer when their immune system doesn’t work properly. This has not been shown in humans, but many proteins of viral origin are found in human tumours, suggesting that our viral DNA may play a role in cancer. It is not yet known whether these proteins play a role in causing cancer or whether the changes that occur in cancer cells are responsible for producing the proteins, so more research is needed.
The most interesting effect of viral DNA and the proteins it produces is observed in multiple sclerosis (MS), a disease in which the immune system attacks the protective cover of the nerves resulting in numbness, fatigue and weakness. As it happens in cancer, viral proteins are found in the brain of MS patients, but there are also signs that their immune system is fighting these proteins, something that doesn’t happen in healthy people.
Viruses have made us what we are today by leaving souvenirs of their visit all over our DNA
When an MS patient suffering from HIV was treated with antiretroviral medication, the symptoms of MS disappeared and the patient remained disease-free for several years. This finding was confirmed by two studies showing that antiretroviral treatment reduces the chances of suffering from MS and prompted clinical trials to study the effectiveness of this type of treatment. Establishing retroviruses buried in our DNA as new targets for therapy could revolutionise the treatment of MS patients.
We’ve walked a long way along the evolutionary road, and viruses have been with us every step of the way, sometimes harming us, sometimes helping us, but always close. So close indeed that viruses made us what we are today, by leaving souvenirs of their visit all over our DNA.
The views expressed here are those of the author and do not represent or reflect the views of RTÉ