Neanderthals and Denisovans may be long gone, but their viruses continue to live on inside our bodies. The geneticists who discovered these ancient viruses aren't sure if they're bad for us, but they could make us more susceptible to certain cancers.
Retroviruses are a special class of viruses that are comprised of RNA rather than DNA. They produce an enzyme, called reverse transcriptase, that enables them to reconstitute their RNA into DNA after entering a cell. From there they makes themselves at home inside the chromosomal DNA of host cells where they're expressed. HIV is a prime example of a retrovirus.
But there are also endogenous retroviruses (ERVs) — viruses that get passed down from generation to generation. Around 8% of our DNA is made up of these viruses, and they're part of the 90% of our DNA with no known function — so-called "junk DNA" that don't code for proteins.
But sometimes, two "junk" viruses can combine to cause retroviral diseases. ERVs are known to cause cancer in immune-suppressed mice when activated by certain bacteria, for example.
In the new study, geneticists from Plymouth University and Oxford University found several ERVs after comparing genetic data from the fossils of Neanderthals and Denisovans (an ancient group of hominids who share a common origin with Neanderthals) with genomic data from modern-day cancer patients. After looking at the data from the cancer patients, researchers Robert Belshaw and Gikkas Magiorkinis found that they contained seven of the viral sequences found in Neanderthals and Denisovans.
These ancient viruses belong to the HML2 family of viruses, which may have links to cancer and HIV. And that's a potential problem.
"How HIV patients respond to HML2 is related to how fast a patient will progress to AIDS, so there is clearly a connection there,"noted Magiorkinis in a statement. "HIV patients are also at much higher risk of developing cancer, for reasons that are poorly-understood. It is possible that some of the risk factors are genetic, and may be shared with HML2. They also become reactivated in cancer and HIV infection, so might prove useful as a therapy target in the future."
The next phase is to determine whether these ancient viruses affect a person's risk of developing diseases such as cancer, and how widespread they may be in the general human population.
It may turn out to be nothing, but one thing's clear: these viruses, which originated in our common ancestors more than half a million years ago, are still a big part of our genetic constitution.
Read the entire study in Current Biology: "Neanderthal and Denisovan retroviruses in modern humans."