While researchers initially drew the analogy between an animal's size and their risk of getting cancer — larger size being more prone to the disease, massive animals like whales and elephants never develop cancer.
It is possibly due to a less understood phenomenon - Peto's Paradox.
Cancer terrorises almost every living creature on earth. From our furry friends to our loved ones, anyone can develop cancer anytime. The worst fact is that every person on earth has some risk of developing cancer — it’s never zero, cancer being an uncontrolled growth of cells leading to tumour formation.
But, it turns out, some animals are almost immune to cancer. With a tiny minority of their population developing it, large animals, almost ironically, are way more resistant to the deadly disease than the rest of us. However, before we understand why these animals rarely get cancer and how these findings help researchers find a cure for cancer, we need to know how cancer develops.
Cancer Formation In Bodies
This is how cancer develops- it goes undetected by our cell system. The cell cycle is a vital process which a cell undergoes to reproduce two daughter cells. The process is divided into four phases:
This is the phase where the cells initiate the process of cell division. It is also the most extended phase in the cycle. Also known as gap phase, the cells usually grow larger and start collecting proteins, energy and the building blocks of DNA necessary for the cell division. Cell organelles such as ribosomes are duplicated.
This is the second gap phase. Similar to G1, the cells are soon preparing for the final stage. More organelles are duplicated. The cells then start organising the organelles within it, and the cells may enlarge. The cell is now ready for the M phase.
This phase is where the cells now replicate a copy of DNA for the daughter cells; hence the S stands for Synthesis. The duplication of centrosomes is also seen. Centrosomes are organelles that will help in the separation of DNA in chromosomes during the M phase.
This is the phase where the cells divide through mitosis or meiosis. But we will only focus on mitosis for now. Here the cells start separating their chromosomes (condensed DNA) into two daughter cells. Once the process is complete, the cell rests.
Now all of these phases have ‘checkpoints’ which are proteins to verify if the cell is appropriate enough to reproduce or not.
What Goes On Further?
Sometimes, DNA gets damaged or corrupt or during the G1 phase. This G1 phase checkpoint ensures that such cells with damaged DNA do not proceed further as they can turn cancerous and are sent into a particular phase known as the G₀ phase, where cells don’t divide.
The G₀ phase cells will continue with their functions but won’t go into the cell division. Neurons are an example of cells in G₀ phase. The protein then checks if the damaged cell can be repaired and if not, then the cell is subjected to apoptosis or programmed cell death. Sometimes, such damaged DNA can go undetected by the G1 checkpoint and then malignant DNA is formed.
However, a single corrupt DNA strand won’t be enough to induce cancer. After a certain amount of damaged cells go undetected, these cells now begin to divide and grow abnormally. This is now called cancer.
Researchers previously believed that since larger animals have more cells, the incidence of getting cancer is increased. Smaller dogs are at less risk of getting cancer as compared to larger dogs. A taller person has a slightly higher risk of getting cancer than a shorter person. But the incidence of cancer in humans is a little less than that of a rat. While giant animals rarely get cancer. Why is that?
Peto's Paradox Reveals a Genetic Advantage in Large Animals
Animals like whales, elephants, cows and horses rarely get cancer. Peto’s Paradox, stated by Richard Peto, says that animals’ mass does not co-relate to them getting cancer. How is that possible? More number of cells, more the risk of producing damaged DNA, right? These animals did evolve some genetic advantages that help reduce the chances of getting cancer to almost zero.
This Paradox confuses researchers to date, with some even wondering if this Paradox exists at all? The mortality rates from cancer for an elephant are astonishingly below 5 per cent, compared to the 11 to 25 per cent mortality rate for a person. However, a study in 2015 has found one of the reasons for the large animals’ strength.
The tumour suppressor gene, also known as the P53 or TP53, is an anti-cancer gene that helps repair damaged DNA. This is usually one of the proteins and other anti-cancer proteins sent to a cell to improve it. Most mammals typically have two copies of this gene within them. Elephants have twenty copies of this same gene. This enables the animal’s body to be more prepared and quicker to detect cancerous cells and repair them or send them for apoptosis.
As further research goes on into this field, researchers assume that these large animals’ disadvantage has low fertility rates. Nevertheless, this Paradox has opened up several researchers’ minds and may one day lead them to understand how the human body can strengthen itself from developing cancer. Till then all we can say is that it’s nature’s gift for these considerable animals to build such a severe disease seldom, but soon humans will be able to join the club as well.