Cancer is one of the most feared diseases in humans. However, it can also affect animals. In fact, studying cancer in animals has revealed an unexpected paradox: Contrary to what we might expect, cancer doesn’t affect larger animals more than smaller ones.
A matter of numbers. Cancer arises from cells that malfunction and start reproducing uncontrollably. Simply put, the more cells there are, the higher the likelihood of developing cancer. This likelihood also increases over time because the chances of a cell malfunctioning rise as the years go by..
To explain this, Simon Spiro, a veterinary pathologist at the Zoological Society of London, likened it to "lottery tickets," as reported by The Guardian. With this analogy, an elephant, with its many cells, should have a much shorter lifespan than a human. However, this isn’t the case.
The situation with whales is even more remarkable. If we use the probability of human cell “failure” and apply it to the number of cells in a whale, their lifespan should be less than a year. Yet, these marine mammals can live up to 200 years.
The Peto paradox. This phenomenon is named after Richard Peto, a professor of medical statistics and epidemiology at the University of Oxford. In the 1970s, he pointed out a discrepancy between theory and practice in this context. Peto observed an inverse relationship: The larger the size of an animal, the lower the probability of suffering from the disease.
A hidden variable. The likelihood of developing cancer isn’t solely determined by time and the sheer number of mutations. Cancer is strongly influenced by genetic factors. In fact, scientists have discovered that the solution to this apparent contradiction lies within our genes.
Spiro and his team conducted a study on deceased animals at the London Zoo to determine how frequently their cells underwent mutations, or changes in their DNA. They found a precise inverse relationship between lifespan and the frequency of these mutations: Species with more mutations per year tend to have a shorter lifespan.
Remarkably, different species had similar numbers of mutations over their lifetimes. For instance, a mouse could accumulate around 3,200 mutations over its four-year lifespan at a rate of 400 mutations per year. In comparison, a human would accumulate roughly 3,760 mutations over an 80-year lifespan, at a rate of 47 per year.
P53. Two recent studies–one by the team at Institute of Biotechnology and Biomedicine of the Autonomous University of Barcelona in Spain, and another one by experts at the Universities of Chicago and Utah–have highlighted the significance of a single gene in elephants. Officially known as p53, it’s not surprising that this gene is often called the “guardian of the genome.”
Researchers believe that this gene may be the reason why an elephant's DNA has ability to regenerate automatically. As such, this gene could prevent cellular mutations and the onset of cancer. While humans have one copy of this gene, elephants have about 20, potentially contributing to their lower cancer rates.
A reasonable concern. Cancer has received a lot of attention in the 20th century. This isn’t a coincidence. With the significant decrease in mortality from communicable diseases, people have been facing the challenge of dealing with cancer as a limit to their longevity.
This is why understanding cancer in animals is crucial: The more we learn about it it, the easier it’ll be to find a cure, or at least therapies that reduce the associated risks.
Image | Bering Land Bridge National Preserve [CC BY-SA 2.0]
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