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A cancer in dogs is spread by the tumor cell itself, according to a study in this week's Cell. All cases of this cancer originated from a single cancerous cell in a dog or wolf ancestor hundreds of years ago, the researchers report...

During the study, the authors sampled canine transmissible venereal tumor (CTVT) cells from dogs all over the world. They found that the tumors are genetically distinct from the host animals, but closely related to each other -- a finding that challenges current thinking that cancer cells are inherently unstable, and gather more mutations over time.  

CTVT is usually transmitted sexually between dogs but can also spread through biting, licking, or sniffing. Healthy dogs who develop the tumor often recover on their own, but not before expressing the tumor for several months and possibly spreading it to other animals...

To pin down CTVT's mode of transmission, University College London researchers led by Claudio Murgia teamed up with University of Chicago scientists to compare normal and tumor tissue from dogs in Kenya, India, and Italy. They analyzed highly polymorphic regions of the dog major histocompatibility complex (MHC) and found that genetic patterns differed between tumors and host animals but were identical among tumors. Genotyping of microsatellites and mitochondrial DNA also confirmed that the tumors were genetically separate from the canine hosts but closely related to each other.  

The researchers also found that all CTVT cells derive from a single cancerous clone, and that this clonal line later split into two distinct genetic clades. This split \"must have happened early in its evolution,\" Weiss said, \"because both of those are widespread across the world...

The authors' microsatellite data show that CTVT likely originated between 200 and 2,500 years ago, Weiss said, which means that its genome has been remarkably stable for hundreds of years.  

\"The current view -- almost a dogma -- in cancer cell biology is that, as a cancer evolves in a person or an animal, it clocks up more and more mutations, more and more abnormal chromosomes, and becomes more virulent,\" Weiss said. The CTVT genome, on the other hand, \"obviously went through a great deal of chromosome rearrangement early in its evolution, but then it became stabilized,\" he said, \"so the idea of progressive instability of cancer cells is challenged by this tumor.'
Source

See also

Murgia et al., "Clonal origin and evolution of a transmissible cancer," Cell, August 11, 2006.
 

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Interesting Betsy! I read in the article about the tumor in the tasmanian devil and just for the hec of it did a search on google image for a picture. It appears from the picture that this one may have the facial tumor mentioned in the article.
 
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