We know of few cancers that are contagious. The ones we’re most familiar with are caused by viruses, such as the human papillomavirus (HPV), which can cause cervical cancer, and hepatitis B, which can cause liver (hepatic) cancer. (To read more about these virus-caused cancers and their vaccines, check out my book Biology Bytes: Digestible Essays on Stem Cells and Modern Medicine.) That’s why it’s so surprising, and fascinating, to find cancers that are contagious and not caused by viruses. One example is canine transmissible venereal tumor (CTVT).
CTVT is a cancer that can be transmitted between dogs when they breed. Instead of being transmitted by the usual infectious agents – such as viruses or bacteria – the tumors are spread by the tumor cells themselves. And, based on a genetics study published just last week in the journal Science, it’s now thought that the cancerous cells in these tumors have survived for around an amazing 11,000 years.
In other words, this is a cell line (cells that are derived from the same original cells) that’s about 11,000 years old, making it the oldest known somatic cell line (somatic cells are ones that aren’t sperm or eggs). Based on the number of DNA mutations in the tumor cells, and how quickly they’re likely to arise over time, it’s thought that the original tumor formed in a dog living around 10,200 to 12,900 years ago. Amazingly, based on the tumor’s genetics and what we know of dog breed genetics today, scientists figured out that the dog was likely medium-to-large in size and may have looked similar to an Alaskan malamute or husky – it was solid black or had black-and-white banded coloring.
CTVT tumors in dogs today have an astounding 1.9 million mutations. This is impressive considering that the tumor has been able to survive and spread for so many years while carrying along this huge number of mutations that could easily make it unstable. (Most human cancers only get about 1,000 to 5,000 mutations, but obviously aren’t anywhere near 11,000 years old.) That said, the tumor’s genetics appear to have stabilized in the last few centuries (or before this) – the researchers compared the genetics of CTVT in dog populations that have been separated for about 500 years and found that their tumors were very similar (about 95% of the same mutations). This stabilization may have been needed for the CTVT tumors to continue to survive – it’s possible that if it gets any more mutations, the tumor will be unstable.
There are only two other known cancers that are transmitted from animal-to-animal by the cancer cells themselves – one is Devil facial tumour disease (DFTD) in Tasmanian devils and the other is contagious reticulum cell sarcoma in Syrian hamsters. DFTD is often quickly fatal, killing the Tasmanian devil that has it within six months, and is devastating Tasmanian devil populations. However, dogs with CTVT usually have their immune system reject the tumors (and chemotherapy can also be successfully used to treat dogs with CTVT).
Overall, this is an eye-opening finding – while we usually only think of “descendants” in terms of an animal’s progeny, CTVT (and its 11,000-year-old tumor cell line) is an example of how that can be a limited view of the world.
For further reading:
- Elizabeth P. Murchison et al.’s article “Transmissible Dog Cancer Genome Reveals the Origin and History of an Ancient Cell Lineage” in Science
- Ed Yong’s article “Contagious Dog Cancer Sequenced”
- James S. Welsh’s article “Contagious Cancer“
- Teisha J. Rowland’s book Biology Bytes: Digestible Essays on Stem Cells and Modern Medicine
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