At first glance, the fact that up to 8% of our DNA originates from viruses can be a frightening thought. But what does this really mean? Where did this viral DNA come from? Can it have negative impacts on modern-day human beings? Or could it be that there are beneficial reasons why this foreign DNA was kept in our DNA for generation after generation?
Endogenous Retrovirus or ERV
Let’s start by getting a better understanding of what this viral DNA is. The scientific name for this type of DNA segment is Endogenous Retrovirus or ERV. Endogenous means that it is inside the genome (the sum of all your DNA) and retroviruses are a family of viruses that are composed of RNA. RNA is a similar molecule to DNA and is naturally found in all living organisms. Retroviruses (the most well-known being HIV) survive by sneaking their genetic material into the host’s and having the host replicate it for them. Many of them are associated with cancers and immune diseases. This phenomenon is by no means unique to humans, they are particularly common in jawed vertebrates (mammals, birds, reptiles, boney fish, etc.). When studying this DNA in humans it is referred to as Human Endogenous Retrovirus or HERV. It is also important to note that there are many different types of HERVs, like there are many types of viruses, and they will thus have different origins, histories and impacts on health.
Retroviruses survive by sneaking into our DNA and tricking our cells into working for it. Most of the time when a retrovirus infects a cell, it’s a somatic or non-reproductive cell but it can sometimes infect a germline or reproductive cell (sperm and eggs). If an infected reproductive cell turns into a new individual, that person will now have the viral DNA, that was found in one of their parent’s reproductive cells, as an integral part of their DNA that can be passed on to their children. Some of the ERVs in our DNA are much older than our species!
Is it dangerous?
Most of the viral DNA that was originally passed on by our ancestors has long since been deactivated by genetic mutations but some of it still seems to be activated in the right circumstances. Since many retroviruses are associated with different types of cancer and autoimmune disease it should come as no surprise that many of the potentially detrimental effects of ERVs are also associated with these types of problems as well. Some ERVs can produce proteins, called oncoproteins, that will encourage the development of different types of cancer. They can also facilitate the production of autoantibodies. Autoantibodies are proteins created by one’s own immune system that will attack other parts of that individual’s body and are associated with numerous autoimmune diseases. They can also destabilize the host’s genetic material leading to chromosomal instability which can also be associated with tumoral development. They have also been associated with neurological diseases such as multiple sclerosis.
Fortunately, even if everybody has ERVs, it’s only a small fraction of people that will develop problems associated with them and the factors that make that happen are being actively researched worldwide.
Is it good?
We have seen that ERVs can be associated with numerous diseases, but evolution isn’t in the habit of leaving things that kill us in our DNA for no reason. Could there have been advantages for our ancestors to preserve and pass on this viral DNA? First, they seem to offer extra protection from similar retroviral infections. In other words, having viral DNA in one’s genome seems to help the immune system fight off infections from other similar viruses. They also seem to play a role in cell differentiation, the process by which our cell gains its specific type and function, and fetal development. With them being more active in the earlier stages of a fetus’s growth and gradually becoming less so as the fetus and eventually the child becomes older. It has also been suggested that they could have a role in the creation of the placenta. Some authors have also claimed that they could help protect our brains from neurological diseases, but this is still controversial.
We started off with the question: Is having ancient viruses in our DNA a bad thing? The short answer is that it can be in the right circumstances, but it also seems to be a good thing in other cases. The long answer will probably still take decades of scientific research and hundreds of research papers. There is still much to learn concerning their evolution, expression and the impacts that ERVs have on our health. If you are interested in having a more in-depth analysis, a few scientific articles concerning ERVs can be found in the reference section at the end of the text.
References
Garcia-Montojo, M.; Doucet-O’Hare, T.; Henderson, L. & Nath, A. (2018) Human endogenous retrovirus-K (HML-2): a comprehensive review, Critical Reviews in Microbiology, 44:6, 715-738, https://www.tandfonline.com/doi/full/10.1080/1040841X.2018.1501345
Giménez-Orenga, K.; Oltra, E. Human Endogenous Retrovirus as Therapeutic Targets in Neurologic Disease. Pharmaceuticals 2021, 14, 495. https://doi.org/10.3390/ph14060495
Grandi, N. & Tramontano, E. (2018) Human Endogenous Retroviruses Are Ancient Acquired Elements Still Shaping Innate Immune Responses Front. Immunol. https://www.frontiersin.org/articles/10.3389/fimmu.2018.02039/full
Nelson PN, Carnegie PR, Martin J, et al. Demystified. Human endogenous retroviruses. Mol Pathol. 2003;56(1):11-18. https://mp.bmj.com/content/56/1/11
Xue, B. ; Sechi, L. A. & Kelvin, D. J. (2020) Front. Microbiol https://www.frontiersin.org/articles/10.3389/fmicb.2020.01690/full