The New York Times
, January 1, 2010Link
The Tasmanian devil, the spaniel-size marsupial found on the Australian island of Tasmania, has been hurtling toward extinction in recent years, the victim of a bizarre and mysterious facial cancer that spreads like a plague.
Now Australian scientists say they have discovered how the cancer originated. The finding, being reported Friday in the journal Science, sheds light on how cancer cells can sometimes liberate themselves from the hosts where they first emerged. On a more practical level, it also opens the door to devising vaccines that could save the Tasmanian devils.
“It’s a great paper,” said Katherine Belov, a geneticist at the University of Sydney who was not involved in the study. “Previously, we were stumbling in the dark.”
The cancer, devil’s facial tumor disease, is transmitted when the animals bite one another’s faces during fights. It grows rapidly, choking off the animal’s mouth and spreading to other organs. The disease has wiped out 60 percent of all Tasmanian devils since it was first observed in 1996, and some ecologists predict that it could obliterate the entire wild population within 35 years.
When the tumor disease was discovered, many scientists assumed that it was caused by a rapidly spreading virus. Viruses cause 15 percent of all cancers in humans and are also widespread in animals.
But subsequent studies failed to turn up a virus. Instead, Anne-Maree Pearse and Kate Swift, of the Department of Primary Industries, Water and Environment in Tasmania, discovered something strange about the tumor cells. The chromosomes looked less like those in the animal’s normal cells and more like those in the tumors growing in other Tasmanian devils.
In 2007, Dr. Belov and her colleagues compared DNA from 26 sick and healthy Tasmanian devils with DNA from the tumors. They found that cancer cells from different animals shared distinctive genetic markers not found in the animals themselves.
A team of Australian and American scientists has now followed up on Dr. Belov’s study, using more powerful gene-sequencing technology to take a closer look at a larger number of Tasmanian devils. To trace the origin of the tumors, the scientists looked at individual cancer cells, recording which genes were active. They found a set of genes normally active only in a type of nerve cell known as Schwann cells. They argue that a single Schwann cell in a single animal was the progenitor of all the devil facial tumor disease cells.
“The lack of genetic variation suggests that the tumors are young,” said a co-author of the study, Tony Pappenfuss, a bioinformatician at the Walter and Eliza Hall Institute of Medical Research in Australia.
Scientists have found only one other case in which cancer cells naturally spread like parasites, a disease in dogs known as canine transmissible venereal tumor. Comparisons of tumors collected from dogs around the world indicate that they descend from a single ancestral cell that existed several thousand years ago. Ever since, the tumor cells have evolved to move among hosts and avoid their immune systems.
Infectious cancer poses a puzzle for biologists. “It is somehow a new organism,” Dr. Pappenfuss said. “I think of it as a parasite.”
Dr. Pappenfuss and his colleagues are now studying how the tumor cells have evolved from Schwann cells into such successful parasites. Their research may help in the development of a vaccine that could prime Tasmanian devils to fight invading cancer cells.
Copyright 2010 The New York Times Company. Reprinted with permission