Reading Exercise 23
Read the passage and choose the best answer to each question.
Elephants And Cancer
Despite their huge size and having many more cells than humans, cancer among elephants is quite rare, and new research may explain why. It turns out that elephant cells "have 38 additional modified copies (alleles) of a gene that encodes p53, a well-defined tumor suppressor, as compared to humans, who have only two," researchers at the University of Utah said in a study appearing in today's issue of the Journal of the American Medical Association (JAMA).
The report also says elephants also have a "more robust mechanism for killing damaged cells" that could become cancerous. According to the researchers, among isolated elephant cells, damaged and possibly precancerous cells are destroyed at twice the rate of healthy human cells and five times the rate of people who have Li-Fraumeni Syndrome, with only one working copy of p53. People with this syndrome have "more than a 90 percent lifetime cancer risk in children and adults."
Because elephants have more than 100 times the number of cells as people, they would seem to have 100 times more chance of a damaged cell becoming cancerous. But this is not the case.
"By all logical reasoning, elephants should be developing a tremendous amount of cancer, and in fact, should be extinct by now due to such a high risk for cancer," said Joshua Schiffman, paediatric oncologist at the Huntsman Cancer Institute at the University of Utah School of Medicine, and Primary Childrenís Hospital, in a statement. "We think that making more p53 is natureís way of keeping this species alive."In fact, his research indicates that elephants, who live between 50 and 70 years, have a cancer mortality rate of just under five percent, compared to 11 to 25 percent among humans.
To try to figure out p53ís role, Schiffmanís team collaborated with Utahís Hogle Zoo and the Ringling Brothers Center for Elephant Conservation. They took blood from circus elephants that had been retired to the Center and purposely damaged the cell DNA, a step that can lead to cancer.The cells "committed suicide," researchers said. "Itís as if the elephants said, ĎItís so important that we donít get cancer, weíre going to kill this cell and start over fresh,í" says Schiffman. "If you kill the damaged cell, itís gone, and it canít turn into cancer. This may be more effective of an approach to cancer prevention than trying to stop a mutated cell from dividing and not being able to completely repair itself."
Additional studies will be needed to determine whether p53 directly protects elephants from cancer. While the research may, one day, yield new drugs to fight cancer in humans, researchers say it would not come immediately.
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