Scientists find clue to killer of Tasmanian devils

Lauran Neergaard, Associated Press Yahoo News 31 Dec 09;

WASHINGTON – Fierce as they are, Tasmanian devils can't beat a contagious cancer that threatens to wipe them out. Now scientists think they've found the disease's origin, a step in the race to save Australia's snarling marsupial.
The furry black animals spread a fast-killing cancer when they bite each other's faces. Since the disease's discovery in 1996, their numbers have plummeted by 70 percent. Last spring, Australia listed the devils — made famous by their Looney Tunes cartoon namesake Taz — as an endangered species.

There's no treatment, and little hope of finding one until scientists better understand what's fueling this bizarre "devil facial tumor disease." So an international research team picked apart the cancer's genes, and discovered that it apparently first arose in cells that protect the animals' nerves.

The surprise finding, reported in Friday's edition of the journal Science, has led to development of a test to help diagnose this tumor.

Next, scientists are hunting the mutations that turned these cells rogue, work they hope could one day lead to a vaccine to protect remaining Tasmanian devils, or perhaps treatments.

"The clock's ticking," lead researcher Elizabeth Murchison of the Australian National University said by phone from Tasmania. "It's awful to think there could be no devils here in 50 years because they're dying so quickly."

The devils, known for powerful jaws, fierce screeches and voracious consumption of prey, are the world's largest marsupial carnivores. They don't exist in the wild outside Tasmania, an island south of Australia.

What triggered this cancer, which causes tumors that grow so large on the face and neck that the animals eventually can't eat?

It didn't jump from another species, said Murchison. Tasmanian devils, for unknown reasons, are prone to various types of cancer. This tumor's genetic signature suggests that probably no more than 20 years ago, mutations built up in some animals' Schwann cells — cells that produce the insulation, called myelin, crucial for nerves — until the first devil fell ill with this new type.

Those mutations went far beyond a typical cancer. When one sick animal bites another, it transplants living cancer cells that form a copy of the first animal's tumor. Murchison's team tested 25 tumors gathered from devils in different parts of Tasmania, and found the tumors were essentially identical to one another.

It's one of only two forms of cancer known to spread this way, Murchison said; the other is a sexually transmitted cancer in dogs. (That's quite different than people's transmission of a few cancer-causing viruses, such as the human papillomavirus that causes cervical cancer.)

The researchers created a diagnostic test, based in part on a myelin-related protein called periaxin that was present in all the facial tumors but not in other cancers.

Also, the team compiled a catalog of Tasmanian devil genetic information. Among the next goals is to determine which of those genes most influence the spread and severity of this cancer.

Scientists Discover Origin of a Cancer in Tasmanian Devils
Carl Zimmer, The New York Times 31 Dec 09;

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.

But Dr. Pappenfuss and Dr. Belov agree that it will take a while to transform the new results into a successful vaccine.