Cancer is a numbers game. Large, animals of longer duration with more cells should get more tumors than small animals, short. Yet, the mice are more susceptible to cancer than we are. Now, a new study offers an attractive explanation. The genomes of small mammals contain more viruses, which the authors suggest may account for their higher rates of cancer.
Aris Katzourakis, an evolutionary biologist at the University of Oxford in the UK, not went to explain cancer rates in animals. He was interested in why over the last 10 million years of the mouse genome have accumulated 10 times smaller RNA viruses called endogenous retroviruses (ERV) that has the human genome. He teamed up with researchers from the University of Plymouth and Glasgow University in the UK to mine for retrovirus in the genomes of a variety of mammals including shrews, humans, dogs and the Dolphins. The researchers then tested whether differences in how long mammals live and how quickly they mature ERV affects how they support.
At the time the team had identified over 27,000 unique viral sequences in 38 different mammals, he saw a clear pattern emerged: small mammals have more ERV that make the greatest. The mice over 3000, while only 55 dolphins, and humans are somewhere in the middle with 348, researchers report online today in PLoS Pathogens .
Larger animals have many more cells, and therefore should have more of these endogenous retroviruses. They have fewer resources, they had to find effective ways to remove them, said Katzourakis. This suggests VRE can be harmful to their hosts, and this evil is more expensive, in an evolutionary sense for large animals.
How bad VRE their hosts? Katzourakis suspected cause of cancer VRE. Viruses integrate into the genome of an organism and to make copies of themselves, and these duplicates are then separated and insert randomly in different places in the genome. More often than not, these viruses do no harm, but sometimes their reintegration turns a healthy cell into a cancer. Such an event has led to the premature death of the first cloned sheep in the world, Dolly, who died of lung cancer caused by retroviruses Jaagsiekte sheep. Katzourakis offers the highest number of VRE in small-bodied animals may account for their higher rates of cancer.
"It's nice to see the experimental actual results may help explain the large differences in cancer susceptibility per gram of tissue between small, animals short and large animals, long life "says epidemiologist Richard Peto of Oxford University, who was not related to the new study. He first recognized the unexpected differences in cancer susceptibility between animals of different body sizes in years 1970 an observation that has become known as "the paradox of Peto."
from the perspective of evolution, Peto says, it is logical that large animals are better at protecting their genomes of viruses potentially carcinogenic. large animals tend to live longer and reproduce later, it is more important for them to delay the onset of cancer.
Although the results PinPoint an underlying mechanism big difference in cancer rates, they do not explain all cancers, said George Kassiotis, a virologist at the National Institute for medical research in London who studies VRE in humans and mice. Despite some VRE, he explains, humans still get cancer. VRE are therefore likely to be one of many factors contributing to cancer rates. "An important aspect of this new study is that it provides a framework to quantify the ERV's contribution to cancer," he says, "which in turn will inform the contribution of other causes of cancer."
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