By Carol Beuchat PhD
There is a most unusual cancer that occurs only in dogs. Not only is it contagious, it is also the oldest cancer in the world.
By oldest, I don't mean we've known about it the longest. When a dog has this form of cancer, the tumor cells themselves are 11,000 years old!
By oldest, I don't mean we've known about it the longest. When a dog has this form of cancer, the tumor cells themselves are 11,000 years old!
The cancer is canine transmissible venereal tumor (CTVT), and it was first described by a Russian veterinarian named Mstislav Novinski in the 1870s. It is called a transmissible cancer because it is contagious, having the unusual ability to be spread from dog to dog when cancer cells are transferred during mating or other contact. The original cancerous cells occurred in a single dog about 11,000 years ago and, instead of dying when the dog died, the cancer spread from dog to dog contagiously. It is now can be found in dog populations around the world.
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It took a bit of sleuthing to figure this out. The tipoff was in the mitochondria in the tumor cells. Mitochondria are inside the cells and are the power plants of life. They have their own DNA, and occasionally the mitochondria in the cancer cells can steal DNA from the normal cells of the dog. When this happens, it makes it possible to trace the lineage of cancer cells because of these unique bits of acquired DNA.
By examining the tumors from hundreds of dogs from around the world, researchers were able to determine that transfer of mitochondrial DNA (mtDNA) has occurred at least five times. This left the tumor DNA with a unique genetic signature from each DNA swap that allowed these specific lines of cancer cells (called "clades") to be traced around the world.
The data suggest that that the oldest clade of cancer cells cancer spread from China or Russia about 1,000 years ago, probably via dogs that were traveling with humans. The cancer appeared in America within the last 500 years, probably arriving with the conquistadors, and reached Australia in about 1900 traveling in dogs brought by European settlers.
As if the story of this ancient, contagious cancer wasn't unusual enough, researchers found dogs in which the canine and tumor mtDNA had actually recombined - something that has never been observed before.
Transmissible cancers like the venereal cancer that occurs in dogs are rare in nature. There are two transmissible cancers that can infect the Tasmanian Devil in Tasmania one found in shellfish, and one that occurs in captive Syrian hamsters.
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The five lineages, or "clades", resulting from individual events of stealing mtDNA from the cells of the host dog.
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Geographic locations of the 5 clades of mtDNA from the canine transmissible venereal tumor.
Strakova A, MN Leathlobhair, G-D Wang, and many others. 2016. Mitochondrial genetic diversity, selection and recombination in a canine transmissible cancer. eLife 2016;5:e14552.DOI: http://dx.doi.org/10.7554/eLife.14552
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