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24 October 2012
Image: The blue colouring show numerous retrovirus particles found in a lymphoma that developed in one of the mice used in the study© George Kassiotis
A large part of our DNA is made up of genes from ancient viruses that have accumulated in our genome during evolution but can no longer replicate. Researchers have now shown that in mice with compromised immune systems, exposure to bacteria (including those that naturally live in our bodies) may enable these ancient viruses to reactivate, causing infections and eventually cancer.
"This work suggests that the part of our genome which originates from these ancient viruses - called endogenous retroviruses (ERVs) - harbours elements that may pose a threat to human health," said George Kassiotis of the MRC's National Institute for Medical Research (NIMR).
Dr Kassiotis and Jonathan Stoye's teams at NIMR worked with colleagues at the National Cancer Institute in Frederick, USA, and the Centre d'Immunologie de Marseille-Luminy in France. They studied mice bred with immunodeficiencies that affected their production of antibodies and monitored pathogenic viruses that emerged from ERVs in the mouse DNA. Antibodies allow healthy immune systems to identify and fight infections, meaning that this capability was missing in the study mice.
The mice spontaneously developed infections with a virus called ecotropic murine leukaemia virus (eMLV), which then triggered the development of lymphoma. eMLV is one of the viruses that has become incorporated into the mouse genome over time but under normal circumstances is unable to replicate itself. Interestingly, pathogenic retroviruses were not found in mice that were bred without bacteria in their gut, showing that these microbes played a role in reactivating the ERVs.
Dr Kassiotis explained: "ERVs are generally unable to replicate in our cells, partly due to mutations that have made them inactive. Their potential effects on health or disease were not known.
"Our results show that, when immune systems are compromised, increased exposure to other microorganisms - such as gut bacteria - drives ERVs to repair their replication defects and give rise to fully infectious viruses that cause cancer."
The paper, ' Resurrection of endogenous retroviruses in antibody-deficient mice', is published in Nature.
- Ancient viruses that have become part of our DNA may be able to reactivate and cause infections and eventually cancer.
- The scientists studied mice bred with compromised immune systems (so they didn't produce infection-fighting antibodies). They found a link between these ancient viruses in the mouse DNA, exposure to bacteria in their intestines and infection and cancer.
- The ancient viruses, known as endogenous retroviruses (ERVs), are found in the part of our genome that isn't thought to code for anything. They usually can't replicate and their potential health effects are not known.
- A study published earlier this year showed that ERVs were widespread in the genomes of 38 mammals including humans. Astonishingly, some of the viruses could be traced back to common ancestors that existed up to 100 million years ago. Although they had lost the ability to replicate, many ERVs were so rampant throughout the mammal genomes studied that they could be likened to a disease epidemic.