Cancer Paradox

Cancer comparison between human and other species

Hello everyone,

Cancer has always been a challenge. The chances of getting cancer increase with an increasing number of cells and the longer lifespan of an organism. Cancer is a multicellular disease and its chances are more where the number of cells is large. When we correlate the number of cells, and the height in the same species for example human beings, they follow the rule correctly. But when we try to apply this relation to different species, the relationship does not stand up for longer.

Mutated DNA

Cancer occurs through mutations. If a living species has a larger number of cells, it means the chances of mutations are maximum leading to cancer. In a similar manner if a species has a longer lifespan. its cells get enough time to get mutated and thus leading to cell proliferation. It is a well-known fact that larger bodies have a larger lifetime than smaller ones. It means they have more chances that their cells get mutations thus leading to cancer. But in fact, there is no correlation between body size and cancer. The absence of this relationship is known as Peto's Paradox which is named after the epidemiologist Richard Peto. This paradox suggests that large, long-lived animals, such as the blue whale have evolved mechanisms capable of suppressing cancer 1000 times better than those in humans.

In general, if we say that whales being larger in size than humans are more prone to cancer due to a large number of cells, then it is also true that they would probably die before they were able to reproduce and this species would extinct. But this is not the case because their existence suggests that it is possible to suppress cancer many-fold better than is done in humans.

If we consider the example of small-sized mice and a human being, we can see that mice have a small number of cells present in their body and humans have a larger number of cells according to their size. The size of the cells is the same in all species, it is only the number factor that matters.

Now, mice should be less prone to cancer than human beings but it is found that both have the same chances to develop cancer. Even blue whales with about 3,000 times more cells than humans do not seem to get cancer at all.

Mice and humans with different numbers of cells have the same chances to get cancer.

INTERESTING FACT: The chance of cancer increases with size within the same species.

The point to consider is that for larger bodies like whales, the probability of cancer is less but an individual like human beings with a size more than average has the maximum chance of getting cancer. This divergent trend is called SIMPSON’s PARADOX.

HYPOTHESIS to solve the Peto’s Paradox: Hypertumours

This concept possesses a possible reason for Peto’s Paradox. Larger organisms may suffer more cancer, but tumors in larger organisms are more likely to evolve hypertumors ( a tumor that invades and kills part of already existing tumors), which makes a negative correlation between case fatality rate and host body size. Tumors in bigger animals take a longer time to grow and are susceptible to cheater cells which can take the advantage of tumor angiogenic properties and lower the fitness of the whole tumor. For example, Malignant tumors in baleen whales would rarely kill their hosts because a tumor, either primary or metastatic, may experience one or more hypertumors that maintain the tumor at a sublethal size. In a pika ( a rat species), on the other hand, which weighs only about 150 g, most feasible tumors reach lethal size before hypertumors have time to arise. Therefore, large mammals may suffer more malignant neoplasia (uncontrolled and abnormal growth of cells in the body) than small mammals, but the hypertumor concept makes it less susceptible to cancer.

Hypertumors stop following the instructions of tumors and start their own journey which leads to the cancer of the former cancer cells.

Another hypothesis to explain the Peto’s paradox is shorter telomers which will try to inhibit the activity of cell division thus reducing the risk of cancer. However, this solution has not yet been discovered in large animals and more research in this area is needed.

There is another factor that involves the formation of tumor protein P53 from the gene TP53, which acts as a tumor suppressor by regulating the cell division.

Cancer and Dinosaur: Birds evolved from the lineage of theropod dinosaurs are prone to cancer giving us the confirmation that dinosaurs did get cancer.

Looking into Peto’s Paradox can help to prevent cancer.

References:

1.Caulin, A.F; Maley,C.C. Trends Ecol. Evol.; 2011. 26(4), 0–182.

2. Nagy, J. D.; Victor, E. M.; Cropper, J. H. (2007). 47(2), 317–328.