By Rick Weiss
Washington Post Staff Writer
Friday, August 11, 2006
Scientists in England have gathered definitive evidence that a kind of cancer in dogs is contagious -- a peculiar exception to the age-old medical wisdom that you can't "catch" cancer.
Although no human cancer is known to spread naturally from person to person, the finding of such a disease in dogs -- and emerging evidence that a different contagious cancer is spreading among marsupials in Tasmania -- is a reminder, scientists said, that under the rules of evolution, DNA will try anything to perpetuate itself.
A cancer cell is usually an animal's or person's own cell that -- because of exposure to a virus or other environmental agent -- has broken free of normal growth controls. Cancer-causing viruses may spread from person to person, but the cancer does not.
But the dog cancer, known as Sticker's sarcoma, is spread by tumor cells getting passed from dog to dog through sex or from animals biting or licking each other.
Because Sticker's sarcoma is usually not fatal -- and because some of the tumor cells reside in the dogs' genital tracts, where it's a small leap from one animal to another during sex -- today's worldwide distribution of Sticker's tumors represents a single colony of cancer cells, the new research concludes.
Indeed, scientists suspect that the colony, distributed among countless dogs, may be the longest in the world.
"I rather thought we might disprove this, but it came out the other way around," said Robin Weiss, of University College London, who led the study appearing in today's issue of the journal Cell. "It is clearly a dog tumor cell behaving absolutely like a parasite." Weiss called the tumor transmission trick "a curiosity of nature."
Scientists have suspected for decades that Sticker's was being passed directly from dog to dog, but doubts persisted because no other naturally transmitted cancers were known. Rarely, recipients of human organ transplants have "caught" cancer from tumor cells hiding in the organs they received.
Weiss and his colleagues did genetic studies on the tumor cells from 40 dogs with Sticker's sarcoma, collected from five continents. The researchers showed that the cells are not genetically related to the dogs they are in -- proof that they did not arise from the dogs' own cells.
They also showed that all the tumor cells, no matter where they were collected, are clones of each other. That is, they are all progeny of the same parent cell.
Further genetic studies by Weiss's team suggested that the parent cell probably arose in a domesticated dog of Asian origin -- perhaps a husky -- hundreds of years ago, and perhaps more than 1,000 years ago. Since then, the cancer has perpetuated itself by jumping from one dog to another.
Studies suggest that, unlike most tumor cells, which contribute to their own demise by becoming increasingly genetically fragile, Sticker's tumor cells are remarkably genetically stable, perhaps explaining in part their evolutionary success.
Robert A. Weinberg, a pioneer in the genetic underpinnings of cancer at the Whitehead Institute for Biomedical Research in Cambridge, Mass., said he was not surprised to learn that genetic studies had confirmed that Sticker's is a transmissible cancer, given the strength of earlier clues. But he agreed that the phenomenon raises difficult questions about why more cancers do not spread this way.
"We really don't understand the ecological and evolutionary dynamics that would lead to this sort of thing and its transmission from one individual to another," Weinberg said.
Both Weinberg and Weiss expressed concern about the recently reported discovery of a similarly transmissible cancer spreading through populations of Tasmanian devils, the notoriously bad-tempered carnivorous marsupial.
"They fight a lot and have been spreading these facial tumors through bites," Weiss said. "The cancer cells clog up the jaw, and the poor animals die of starvation."
Some experts believe that the epidemic could threaten the devil with extinction.