Scientists have identified a widespread sexually transmitted canine tumor that can be spread from dog to dog. Canine transmissible venereal tumor (CTVT) is likely the oldest known cancer and retains much the same genetic structure it had when it began in a single wolf or dog more than 200 years ago. More broadly, the finding casts doubt on a long-held theory about genetic instability in cancer cells and raises new questions about tumor evolution. CTVT is a common, usually nonlethal, cancer that affects dogs of all breeds around the world. Scientists once thought it was caused by a transmissible virus, much like cervical cancer in humans, but recent studies suggested the tumor cells themselves are spread from dog to dog, perhaps during sex or through oral contact with tumors.
Probing that theory, virologists led by University College London scientist Robin Weiss took tumor and blood samples from 16 dogs from three countries. DNA analysis revealed strong genetic similarities between the tumors, evidence that the tumor cells came from a common ancestral cell. Furthermore, there was no genetic match between the tumors and the dogs, which would be expected if the tumors arose by mutation of a dog’s own cells. After examining the genetic structures of tumors taken from 24 other dogs from five more countries, the team determined that the cancer must have originated in one animal, most likely a gray wolf or an old-breed dog between 200 and 2500 years ago.