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Animals and transmissible cancers

How often is cancer transmitted between animals? In the past few years researchers have discovered more transmissible cancers in nature. Initially thought to be contained within a respective host species, new research shows that sometimes even cross-species cancer transmission can occur.

With transmissible cancer, instead of remaining in the singular organism or host, the cancer transfers between animals. “The actual infectious agent is the cancer cell itself,” said Elizabeth Murchison, Ph.D., reader in comparative oncology and genetics in the department of veterinary medicine at the UK’s Cambridge Veterinary School.

Researchers have been studying transmissible cancers in dogs and Tasmanian devils since mid-1990s. This June, a letter published in Nature looked at related findings in several bivalves—a class of thousands of species that includes oysters, clams, and mussels—building off a 2015 report on soft-shell clams.

Researchers explored disseminated neoplasia, a “leukemia-like” disease, after Carol Reinisch, ScD, a marine biologist at Environment Canada in Ontario, approached researchers at Columbia University. “We actually got started on this work looking for a retrovirus as a potential cause for these outbreaks of cancer within bivalves,” said lead author Michael Metzger, Ph.D., a postdoctoral research scientist in the lab of senior author Stephen Goff, Ph.D., Higgins Professor of Biochemistry and Molecular Biophysics and professor of microbiology.

Instead, “the cancers themselves seemed to spreading from one animal to the next,” Metzger said. Researchers initially found transmission among soft-shell clams from New York up to Maine and Canada and eventually traced a single original cancer that was spreading, “almost like an asexual parasite,” Metzger said, within the species. The finding was published in 2015 in Cell.

The researchers wondered whether transmission might also be occurring among mussels, cockles, and golden carpet-shell clams—species that also have a high rate of disseminated neoplasia. Collaborating with researchers in Canada and Spain, the group collected 278 mussels from West Vancouver and Vancouver Island. Researchers also gathered cockles and golden carpet-shell clams from the Galician Coast.

For mussels and cockles, the researchers traced cancers distinct from their hosts, but identical within each species, suggesting a single-origin cancer spreading throughout each population. In cockles, scientists found two cancer lineages of independent origin.

For the golden carpet-shell clam, results deviated. The cancer was markedly different from that in the host species; the genotype of the neoplasia matched that of the cancer found in the pullet-shell clam, a relatively close relation. The results suggested cross-species transmission; yet the pullet-shell clam itself is not susceptible to the cancer it passes on. That finding suggests that there “may actually be resistance developing in the original population,” Metzger said.

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Tasmanian Devil relaxing by Wayne McLean (jgritz). CC BY 2.0 via Wikimedia Commons.

Transmissible cancers have affected the Tasmanian devil population aggressively. Indeed, devil facial tumor disease (DFTD)—the transmissible cancer in devils—has “triggered massive population declines, estimated around 80% or so” in most affected areas, Murchison said.

“Cancer is not rare in nature. In animals and in humans it’s fairly common, and so the steps from being a cancer to being a transmissible cancer must happen from time to time.”

Other research has revealed transmissible cancer in dogs. Canine transmissible venereal cancer appears to be significantly older than the cancers among the Tasmanian population. It has been around “we think for a few thousand years,” Murchison said, and is present in dog populations around the world. But such diseases are much less common in dogs than in Tasmanian devils. She suggested several reasons for the difference: Dog populations might be less susceptible, or it might be “a less infectious disease altogether.”

“Having worked on the dogs and the devils for quite a long time, I had got used to the idea that transmissible cancer might be fairly rare in nature,” Murchison said. But the bivalve findings suggest otherwise. Although surprising, in some ways, it follows: “Cancer is not rare in nature. In animals and in humans we know it’s fairly common, and so the steps from being a cancer to being a transmissible cancer must happen from time to time,” Murchison said. “It only has to happen once” for it to move throughout the species.

The appearance of a case of cross-species transmission in this research piqued the interest of several researchers, including Roxanna Smolowitz, D.V.M., director of the Aquatic Diagnostic Laboratory and assistant professor of biology at Roger Williams University in Bristol, R.I., whom the data swayed. “You would think that different species would be able to reject cells from a foreign source,” Murchison said, “but in these bivalves that doesn’t seem to be the case.”

Murchison suggested that cross-species transmission occurred in bivalves because of their limited ability to recognize foreign bodies. Metzger agreed, suggesting that because invertebrates lack an adaptive immune system, “transmissible cancers may be more likely to occur and spread.”

The marine environment also may be especially conducive to such transmission, Smolowitz said. She said that these bivalves release cells when they die and, as filter feeders, may be more susceptible to transmission—a point Metzger echoed. “It looks like cancer cells can float around and survive in the environment in a way that just doesn’t happen on land.”

Metzger said that Goff’s lab plans to look further into how retrotransposons—the amplified elements of the genome that led to these findings—affect how cancer evolves. The lab also plans to further explore why the cancers are engrafting between some species but not others.

But we still don’t understand why we’ve seen transmissible cancers only in dogs and Tasmanian devils. Rachel Owen is studying the origins of DFTD and contagious cancers under Hannah Siddle, Ph.D., lecturer in molecular biology at the University of Southampton in England. Owen noted that within the devils, DFTD is transmitted through biting, which is “pretty much the staple of their social interactions with each other.”

Owen said the findings show “how much more we have to learn about how our immune system really handles cancer.” Smolowitz said researchers need to do more work to understand why these cells occur in any species and how transmissible cancer starts.

“This is much more widespread than anyone really thought,” and there are “some tantalizing findings,” Metzger said. The findings reveal aspects of “how cancers can evolve and interact with their hosts in nature” and how much more there is to know.

A version of this article was originally published in the Journal of the National Cancer Institute.

Featured image credit: Common Mussels by Republica. CC0 public domain via Pixabay.

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