There is a strange contradiction in nature known as Peto’s Paradox. Large, long-lived animals should be much more likely to get cancer than little critters that live fast and die young because they have more cells to mutate and more time for things to go wrong. But they aren’t. Our own cancer risk is 11-25 percent. An elephant’s, on the other hand, is just 5 percent.
So what about the biggest creatures to ever grace our planet?
Despite their immense size, whales are much better at avoiding cancer than us puny humans. A new study, published in the journal Molecular Biology and Evolution, helps us understand why.
Northern Arizona University’s Marc Tollis and his team turned to the humpback whale genome to unravel why whales don’t suffer from cancer as much as they theoretically should. Tumors form when cells divide and mutate. Mutations are common and normally don’t have any ill effects, however, very occasionally, the body fails to right harmful mutations and this can lead to cancer. These mutations can occur randomly or be influenced by environmental factors.
In humans, age and body size are risk factors for cancer. That’s why the disease is more likely to appear in later life or affect those who are overweight.
“[It] is driven by somatic evolution – genetic changes that occur when body cells copy their genomes, divide and produce daughter cells,” said Tollis. “The longer you live, the more cell divisions you have and the higher chance that a cancer-causing mutation will occur in the genome of the descendent cells. Similarly, larger individuals are made of more cells, which also increases the chance of cancer-causing mutations.”
Therefore whales, with their immense bodies, high fat levels, and life spans of up to 90 years, should really be rather susceptible. They can still get cancer, and various forms of the disease have been found in species such as sei, blue, and fin whales, but it isn’t very common.
The researchers sequenced the genome of a humpback whale called Salt, who was first cataloged by researchers back in the 1970s off the coast of Massachusetts. As her life history is well documented, she made the perfect study subject. The team took skin samples and extracted Salt’s DNA before sequencing her entire genome of about 2.7 billion base pairs (we, for comparison, have 3.1 billion).
Comparing Salt’s genome to that of 10 other cetaceans, including blue, fin, bowhead, and sperm whales, the researchers found that parts of the whale genome have evolved surprisingly fast compared to other mammals. Specifically, these sections of the genome include genes linked to controlling the cell cycle, DNA repair, and cell proliferation, all important factors for the normal functioning of healthy cells. When humans get cancer, it is often these genes that mutate.
The researchers also found that the whale genome has evolved many duplications of tumor suppressor genes, which work to prevent cell proliferation and tumor development. Whales also appear to have slower mutation rates than other kinds of mammals, which may reduce how many cancer-causing mutations appear.
Next, the researchers hope to work out exactly which of these genetic features suppresses cancer, and see if they can one day apply their findings to preventing cancer in humans. Tollis hopes whales’ potential contribution to cancer research might even encourage people to conserve them.
“In our current sixth mass extinction, we need all the reasons for conservation that we can get,” he said.
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