Canine transmissible venereal tumor (CTVT) is a sexually transmitted cancer that affects dogs. Its genome has recently been sequenced, revealing that it originated in one dog and has continued to live on for an astonishing 11,000 years; the longest known living cell lineage. The study was led by Elizabeth Murchison of the Wellcome Trust Sanger Institute and was published in Science.
The team was able to find that around 11,000 years ago, CTVT originated in one dog who passed it on during mating. That cell lineage has persisted through all of this time, manifesting in large tumors on the genitals. Throughout its extensive history, the genome has picked up about two million mutations, whereas the majority of human cancers have about 1000-5000. The differences might not be too surprising since human cancers also die with the carrier, but that is not the case for CTVT.
By analyzing the genome, researchers were able to discover that patient zero for CTVT most likely resembled an Alaskan Malamute or Husky, with short dark fur. Though the gender of the dog could not be determined, it appears to have been fairly inbred. It is also unknown exactly where this dog lived.
Researchers are not able to say why this cancer emerged and became so infectious, but they were able to determine how it began to spread. As humans and their canine companions began to travel all over the world about 500 years ago, so did CTVT. Prior to the last few centuries, the disease was very localized. It is now commonly found in dogs all over the world.
Cancer cells are capable of moving from their point of origin, but they almost always stay within the same organism and metastasize different tissues. Aside from CTVT, the only known transmissible cancer is the lethal devil facial tumor disease (DFTD), which Tasmanian devils spread by biting.
By understanding CTVT’s genome, researchers will receive a wealth of pertinent information. Not only will they be able to better treat dogs who acquire the disease, but they have a unique opportunity to study the evolutionary processes that have affected it over the last 11,000 years. Additionally, understanding how cancers become transmissible could prove to be life-saving in the event that a similar disease ever arises in humans.