An international team of researchers found that a contagious cancer that affects dogs around the world, may have originated somewhere in central or north Asia some 6,000 years ago, but had spread to different continents. This happened when European seafarers set out to conquer other parts of world about 500 years ago.

What is CTVT?

Canine transmissible venereal tumour (CTVT) is a genital cancer that transmits from one dog to another. It is unique because unlike other diseases, where pathogens spread from one organism to another, CTVT transmits through the transfer of living cancer cells. The tumour has survived thousands of years, mutating and evolving over time, said the scientists led by Elizabeth Murchison, a Cambridge University geneticist, who has been studying such infectious cancers in animals. The study, which includes the contributions by a veterinary scientist from Sikkim, was published in the prestigious Science journal on Thursday.

What is peculiar about CTVT is also that these “tumour cells” have nothing to do with cells of host (carrier) dog, but are those of the original dog in which the cancer arose, some 6,000 years ago. In other words, the cancer lineage continues to survive, thousands of years after the founder dog had long dead and gone.

"This is really exciting - we've never seen anything like the pattern caused by this carcinogen before.It looks like the tumour was exposed to something thousands of years ago that caused changes to its DNA for some length of time and then disappeared. It's a mystery what the carcinogen could be. Perhaps it was something present in the environment where the cancer first arose," said Murchison.

Such transmissible cancers are rare. Apart from dogs, they are found in Tasmanian devils and marine bivalves such as clams, mussels, cockles.

“The history of CTVT is closely tied to the history of dogs and humans, and its current spread is largely a consequence of the development of intercontinental travel starting in the 15th century, mostly led by the European colonial economies,” said Adrian Baez-Ortega, first author of the Science paper and doctoral student in the Transmissible Cancer Group, part of Cambridge's Department of Veterinary Medicine.

Details of the study

For the study, the scientists compared differences in tumours taken from 546 dogs worldwide to try to understand how the disease arose and how it managed to spread around the world. "Changes to its DNA tell a story of where it has been and when, almost like a historical travel journal," said Baez-Ortega.

According to him, some CTVT tumours found in India are among the most interesting ones. This is because they seem to have been “split” from the rest of the lineage at an early stage. “Those tumours settled in India at least 2000 years ago, while the rest of tumours went towards Europe and then America. Because they separated from the rest of the lineage so early, this Indian tumours look very different from the rest, genetically speaking,” Baez-Ortega told BusinessLine.

However, there are other CTVT variants present in India too. Geneticists believe they would have arrived recently, probably due to the maritime trade routes between India, Europe and America.

CTVT in humans

The research also helped the scientists to discover that CTVT is evolving in a different way than human cancers. “I imagine that CTVT might have been quite more aggressive at the beginning of its evolution than it is now, but we cannot answer this from the data we have. It does seem that CTVT has evolved to become a relatively mild form of cancer, and to behave more like a parasite than like an aggressive tumour,” the Baez-Ortega said.

Thinlay N Bhutia, director of animal husbandry in Sikkim, who also contributed to the paper, said CTVT is normally found in street dogs, not in pets. The primary mortality due to the tumour is low, but the animals tend to die of other reasons like starvation as they are hounded by people.

Mapping CTVT cell evolution and its spread throughout the world has many parallels to mapping how mutant cells evolve and spread within a human body. Although the scales are different, the genomes that are increasingly being sequenced from human cells can also record what those cells do, how they spread, and what they were exposed to, said Carlo C Maley of Arizona Cancer Evolution Centre at Arizona State University and Darryl Shibata of the University of Southern California in a commerntary piece in the same Science issue. "This is valuable because it is difficult to observe the life and death of most human cells over time. Understanding how mammalian cancers can evolve over long periods of time will likely be important in future attempts to manage that evolution to prevent mortality and morbidity due to cancer," they said.