DEAR DOCTOR K:
I heard on the radio about a new research finding that elephants don’t get cancer. If we could figure out why, could we protect ourselves against cancer?
A recent study found that elephants (and many animals) do, in fact, get cancer, but have lower rates of death from it than humans do. The study also found one possible explanation that, as you suggest, might someday help us deal with cancer.
First, some basics. In a phrase coined by cancer biologist Robert Weinberg, every cancer begins with “one renegade cell.” One cell starts to multiply uncontrollably. Remarkable research over the past 40 years has unraveled at least some of the forces that cause cancers to start.
Some genes, called “oncogenes,” cause a cell to divide, again and again. One cell becomes two, two become four, and pretty soon you have a mass of thousands of cells — a tumor. Other genes, called “tumor suppressor genes,” keep a cell from dividing uncontrollably. Many human cancers involve the activation of oncogenes, or the inactivation of tumor suppressor genes, or both.
One tumor suppressor gene is called TP53. Each of us has two copies of the gene, one from each parent. In many human cancers, at least one copy has been knocked out by a mutation. There is a rare disease called Li-Fraumeni syndrome. One parent passes a defective version of the gene to a child, leaving the child with only one healthy copy from birth. That child has a 90 percent chance of developing cancer. Often, the child will develop several different cancers during his or her lifetime.
Now, back to elephants. The fact that elephants are less likely to die of cancer than humans is striking, for two reasons. First, the more cells an animal has, the more likely it should be that one would turn cancerous. And, as you’d expect from their huge size, elephants have a lot more cells than we do. Second, the longer a cell has to turn cancerous, the more likely it is that cancer will start. Elephants live as long as humans do, so elephants should be much more likely than humans to die of cancer, because they’ve got so many more potentially cancerous cells than we do. But, in fact, they’re less likely. That’s puzzling.
The research team found one possible answer. Elephants have 40 copies of the TP53 tumor suppressor gene, whereas we humans have just two. TP53 kicks into action when a cell turns cancerous. The scientists demonstrated this by exposing the elephants’ cells (placed in a laboratory dish) to radiation.
Radiation causes mutations that activate an oncogene, or inactivate a tumor suppressor gene, and turn a cell cancerous. When this happens to an elephant’s cells, the TP53 gene causes the cancerous cell to “commit suicide”: It dies before it has a chance to divide uncontrollably.
What scientists have learned about the forces that cause cancer to start, and the forces that try to protect against a growing cancer, has already led to improved diagnosis and treatment of some cancers. If scientists are encouraged and funded to pursue curious questions — such as why elephants are less likely to die of cancer than we are — we’ll learn even more.