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According to everything we know about cancer, elephants should be hit hard by the disease.
Cancer is a disease of aging, and elephants can live up to 70 years. Over the course of a life that long, elephants grow a lot – burg
eoning from 200-pound babies to 12,000-pound giants. All that growth involves cell division, a process that provides opportunities for potentially lethal genetic mistakes.
Yet cancer is relatively rare in elephants. Fewer than 5% of elephant deaths in captivity are related to cancer.
A new study suggests a possible reason why: Elephants have 20 times as many copies of a key cancer-fighting gene as humans.
Humans typically have just two copies of a tumor-blocking gene called TP53, inheriting one from their mother and one from their father, said Joshua Schiffman, co-author of the study published Thursday in JAMA.
In contrast, elephants have 40 copies, said Schiffman, a pediatric oncologist at the Huntsman Cancer Institute at the University of Utah.
TP53 plays a vital role in preventing cancer, said Schiffman, who describes it as the “guardian of the genome,” scanning cells for genetic mistakes and destroying ones that can’t be fixed.
Scientist Robert Weinberg, whose lab discovered the first tumor suppressor gene, said the new study provides a clue to an evolutionary puzzle.
“Every time a cell divides is a potential disaster,” said Weinberg, a member of the Whitehead Institute for Biomedical Research and Professor of Biology at the Massachusetts Institute of Technology in Cambridge, Mass., who wasn’t involved in the new study. “As we evolved from small, short-lived animals into larger, long-lived animals, there has been the co-evolution of anti-cancer mechanisms in our tissue, so we don’t have an ever-increasing risk of cancer as we get older.”
Elephants aren’t the only large animals to have evolved ways to defeat cancer.
Bowhead whales are even larger and live longer than elephants, surviving up to 200 years.
Cancer death rates vary widely by species, the new study says, ranging from 1% in the rock hyrax, a small African mammal related to elephants, to 8% of deaths in African wild dogs and more than 20% of deaths in cheetahs.
Cancer causes 11% to 25% of deaths in humans; many of those deaths due to lifestyle issues such as smoking, the study said.
One species that’s been hit particularly hard by cancer in recent years is the Tasmanian devil, a species at risk of becoming extinct over the next decade because of lethal cancers spread by a parasite.
The myth that sharks don’t get cancer has encouraged people to kill them and sell their cartilage for nutritional supplements. Clinical trials of shark cartilage supplements have found no benefit for cancer patients.
Schiffman said he hopes his study will lead to better treatments for cancer, especially for children with a rare condition called Li-Fraumeni syndrome, in which they have only one working copy of TP53. These children have up to a 90% risk of cancer.
That sort of drug, however, is so far into the future that cancer researcher Bert Vogelstein, co-director of the Ludwig Center at the Johns Hopkins Kimmel Cancer Center, calls it “science fiction.” Vogelstein was not involved in the new study.
Rather than offering short-term hope for cancer patients, Vogelstein said, the new study is “just a fascinating story about elephants.”