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A photo of a dog.

Scrooble the dog, who overcame cancer, looks through a CT scanner.

PHOTOGRAPH BY DANNY LAWSON, PA WIRE VIA AP

Marc Silver

National Geographic

Published January 25, 2014

Canines are in a rare category when it comes to cancer: They and Tasmanian devils are the only two animals that can transmit it from one individual to another.

A new genetic study reveals that the dog form of the cancer, which causes genital tumors, is 11,000 years old—making it the oldest continuously living cancer.

Canines can also develop cancers that are akin to human cancers, but their transmissible cancer spreads when cells from one dog's tumor rub off during sexual contact and grow into a new tumor on the other animal. The study notes that the cancer originated in an ancient dog closest to the modern-day breeds of Alaskan malamutes and huskies.

We spoke with Elizabeth Murchison, the study's first author. She's a cancer geneticist at the University of Cambridge and the Wellcome Trust Sanger Institute in the U.K.

What made you decide to study canine cancer?

I knew about this disease for some time because I'm actually Tasmanian, so I was very familiar with the Tasmanian devil cancer. In my studies I learned about this dog cancer and found it fascinating. I was keen to use genetics and genomics to find out more—to help us help the devils as well as the dogs.

Is this dog cancer related to the Tasmanian devil cancer?

It's a different disease. The [cancers] arose separately, but they're the only two known naturally occurring transmissible cancers, spread by the transmission of cancer cells themselves. The Tasmanian devil cancer is very aggressive, spread by biting, and it threatens the devils with extinction. The canine cancer is widespread but actually quite treatable; [it] can be cured with chemotherapy.

And there's no human cancer that spreads this way? What about the role of HPV [human papillomavirus] in cervical cancer?

HPV is strongly associated with human cervical cancer, but rather than spreading the cancer itself, it's a virus [that increases] the risk of getting the cancer. The closest thing I can think of with human cancers is the HeLa [line of] cells, which came from one original patient who lived several decades ago. They're dependent on lab scientists to help them to survive. But the concept—of the cancer surviving beyond the death of its original host and taking on life of its own and proliferating and spreading—is similar.

The age of the cancer is getting a lot of attention. But I'm equally intrigued by the idea that the canine—and Tasmanian devil—cancers are transmissible.

These cancers break so many rules we thought we understood. For instance, they're able to spread between hosts, and to exist and live in different hosts, despite the immune system, which is supposed to detect non-self tissues and reject them.

These cancers somehow trick the immune system into not seeing them as foreign—or perhaps not seeing them at all. The study of these cancers may lead us to greater insights into how cancers in general interact with the immune system.

Could that be helpful in treating cancer in humans?

I think we need to be equipped and prepared in case a cancer like this occurs in a different species, or even in humans. It just takes one cancer to become transmissible; then you have a new infectious disease on your hands.

Does the canine cancer pose any risk to humans?

At the beginning of the 20th century, [there were] a lot of studies to see if this tumor could go to other species, including rodents and other canids. It couldn't grow in rodents or cats, so I'm sure it would not grow in humans. But it can grow in other canid species, including coyotes and jackals.

Eleven thousand years is a long time ago. Is this the earliest known cancer as well?

There's actually evidence of cancer stretching back millions of years—of metastatic brain cancers in dinosaurs. Cancer's been around for a long time. But this is the oldest cancer that is continuing to thrive, [that] is still alive today.

This interview has been edited and condensed.

2 comments
Dan Keller
Dan Keller

Update: Upon further reading, the canine transmissible venereal cancer is transmitted by the cancer cells themselves. Killed tumor cells or cell-free filtrates do not transmit the disease. Furthermore, karyotype and DNA analysis indicate that the tumor cells are different from the host, so the tumors are a direct result of proliferation of the cells that were transmitted from one dog to the next.

Dan Keller
Dan Keller

Are these tumors direct descendants of the original ones, which could be determined by genotyping the cells to see if they are of the donor or host (recipient) origin. Or are the tumor cells merely carrying a virus that transforms host cells to become cancerous? Also, can tumor transmission be achieved by using cell-free extracts of the tumors, which would suggest a viral etiology. If one speaks of tumorigenesis caused by cells being passed from one dog to the next and they themselves growing out as new tumors, then this may be an unusual mechanism, with the Tasmanian devil tumor being the only other example.

However, if the tumors can be transmitted by a virus, then there are several other examples: rous sarcoma virus in chickens, avian sarcoma leukosis virus, feline leukemia virus, and HPV, HTLV, HBV and HCV (liver cancer), Epstein-Barr virus (Burkitt's lymphoma), Kaposi's sarcoma-associated herpesvirus, mouse mammary tumor virus, among others, as well as possibly HIV. (Whether HIV is in itself tumorigenic or if it just weakens the immune system so that immune surveillance is ineffective is a question.)

So the question is: Are these canine cancers of today direct descendants of the original tumor cells, or are those cells transmitting a biological agent (most likely a virus) that causes new tumors to form?

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