You'd think the larger animals are more prone to tumors, but mice are more susceptible to cancer than us. A study offers an explanation; genomes smaller mammals contain more viruses.
Cancer is a numbers game. large animals, long life should suffer more tumors than small animals short-lived. However, mice are more susceptible to cancer than us. Now, a new study offers a tempting explanation: the genomes of smaller mammals contain more viruses, so it is suggested can be given higher rates of cancer.
Aris Katzourakis, an evolutionary biologist at the University of Oxford in the UK, did not intend to explain cancer rates in animals, he was interested in knowing why in the last 10 million years in the genomes of mice have accumulated 10 times more small RNA virus, Retrovirus called Endogenous (VRE), which has the human genome. He teamed up with researchers at the University of Plymouth and the University of Glasgow in the UK by the Retrovirus in the genome of a variety of mammals; including shrews, humans, dogs, and dolphins. The researchers then tested how much differences in living mammals and how quickly they mature ERV house.
So far the team had identified more than 27,000 unique viral sequences through 38 different mammals, and was a clear pattern: small mammals have more ERV than larger. Mice have more than 3,000 while the Dolphins have only 55, and humans are in half to 348, the researchers report in PLoS Pathogens.
Small mammals are more ERV than larger. Mice have more than 3,000 while the Dolphins have only 55, and humans are in half with 348.
Larger animals have many more cells, and therefore, should have more of these Endogenous retroviruses. They have fewer means that they must have found effective ways to eliminate them, says Katzourakis. That suggests that ERV may be more harmful to them, and this damage is more expensive, in an evolutionary sense in large animals.
How ERV harms your hosts? Katzourakis suspect that some ERV causes cancer. Viruses are inserted into the genome of an organism and make copies of themselves, and these duplicate then separated and reinserted at random in different locations in the genome. In many cases these viruses are harmless, but sometimes their reintegration transforms a healthy cell into a cancerous one. One of these events led to the premature death of Dolly, the first cloned sheep in the world, who died of lung cancer caused by sheep retrovirus Jaagsiekte. Katzourakis proposes that the largest number of VRE in animals of a small body may account for their higher rates of cancer.
"It's nice to see real results in experiments that may help explain the large differences in susceptibility to cancer, taking into account grams in the tissue between small animals, short-lived and large long-lived animals," says epidemiologist Richard Peto, University of Oxford. He was the first to recognize the unexpected differences in cancer susceptibility between animals of different sizes back in the 1970s, an observation that became known as "The paradox of Peto".
From an evolutionary perspective, Peto explains; it makes sense that larger animals are best to protect their genomes of potentially cancer-causing viruses. Since they tend to live longer and reproduce later, so it is more important to them postpone the onset of cancer.
From an evolutionary perspective, it makes sense that larger animals are best to protect their genomes of potentially cancer-causing viruses.
- Richard Peto
While the findings suggest a mechanism underlying the difference in cancer rates do not explain all types of cancer, says George Kassiotis, a virologist at the National Institute for Medical Research in London who studies VRE in humans and mice. Despite having few ERV explains, in humans still remain cancer. VRE is, therefore, one of the many factors contributing to cancer rates. "An important aspect of this new study is that it provides a framework to quantify the contribution of ERV with cancer," he says, "which in turn inform the contribution of the other causes of cancer."