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Viral relics: what can they tell us?

By James Aitchison

Can a viral infection, from millions of years ago, affect you today?  Ancient viruses, it seems, are preserved in the genomes of 38 mammal species — including humans.  Some of our ancient viruses may be protecting us from disease; some may be “full-on parasites”, raising our risks of cancer and other illnesses.

These ancient viruses make up 8% of our DNA.  Once regarded as “junk DNA”, our viral relics have co-evolved with us over millions of years.  More than being evolutionary dross, they could be linked to a range of diseases from multiple sclerosis, haemophilia, certain types of dementia and cancer.  

The human genome contains billions of pieces of information and around 22,000 genes.  In the 1940s, long before the decoding of the human genome, Nobel Prize-winning geneticist Barbara McClintock realised that some stretches of our DNA behaved like infectious invaders.  These DNA “chunks” could move through the genome, “copying and pasting themselves” wherever they saw fit.  She dubbed them “jumping genes”.   

Our viral DNA comes from endogenous retroviruses (ERVs).  When they are inserted into the genome and stay there, there are like a “fossil footprint”.

A retrovirus works by invading a host cell and inserting its genes into that cell’s DNA.  These viral genes co-opt the cell’s machinery, using it to make new viruses that escape to infect more cells.  Many researchers believe that retrovirus sequences have been recycled during the evolutionary process to defend against other retroviruses.  Because, over many generations, much of this viral DNA mutates, ERVs can eventually lose their ability to infect new cells.  

Some ERVs are unique to humans, while others are found in a variety of species.  One retrovirus, common in mammals, is also present in cod and tuna, having invaded our marine creatures 450 million years ago.

According to Oxford University’s Department of Zoology research, viral relics are preserved in every cell of an animal.  In theory, because larger animals have many more cells, they should have more ERVs, but they don’t; it appears that larger animals have more effective anti-viral genes and resources than smaller ones, and “edited out” these potential cancer-causing relics.

The report also showed that taller people have a higher risk of some cancers, yet no evidence was found that viral relics might have casual links with cancer in humans, even though they cause cancers in other animals such as mice.  The distribution of ERVs across a range of mammals, said the report, is like looking at the “footprint cancer has left on our evolution”.

The question is, what more can our viral leftovers tell us about the human condition?  Only more time will tell.

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