Unhappy as a clam: contagious cancer is widespread in bivalves

Metzger M.J., Villalba A., Carballal M.J., Iglesias D., Sherry J., Reinisch C., Muttray A.F., Baldwin S.A., and Goff S.P. Widespread transmission of independent cancer lineages within multiple bivalve species. Nature. 22 June 2016. doi:10.1038/nature18599.

What are the implications of contagious cancers in marine environments?

The conventional wisdom on cancer is that, unlike the common cold, it is not contagious—that is, one person can’t give another person their cancer. In contrast to viral or bacterial infections that arise from environmental exposure, cancer originates when the cells of a given individual go rogue due to reprogramming of the host’s genetic code. At first a cancerous tumor is contained to single organ of the body, but at later stages the cancer cells may spread through the circulatory system to other organs in a process known as metastasis. So, what if cancer cells could “metastasize” through the environment?

Be happy you’re not a clam. A recent study by a multinational team of researchers lead by Michael J. Metzger has shown that environmental transmission of cancer cells is a commonplace malady within species of marine bivalves like mussels and clams. What’s more they showed that certain cancers could even spread between species, analogous to a dog giving cancer to a human.

The researchers examined cases of hematic neoplasty, a cancer of the blood akin to leukemia, in individuals from three species of bivalve, one species of mussel from the West Vancouver shore, and a clam and cockle species apiece from the Galician Coast. The DNA of cancer cells derived from within an individual should differ only minutely from the host cell, while cancer derived from another individual (i.e., transmissible cancer), would be expected to differ substantially from the host “genotype”. Hence, comparison of the genotypes of cancerous blood cells to healthy cells derived elsewhere in the host could provide a clue on the origin of the cancer cells.

The patients. Bivalves examined by this study.
The patients. Bivalves examined by this study (images courtesy Metzger et al.Nature).

As expected, blood cells in healthy hosts possessed an identical genotype to their healthy counterparts from elsewhere in the host. However, all cases of cancer detected among the three species of bivalve, the genotype of the cancer cells differed substantially from that of the host—the holy grail of transmissible cancer!

In the Canadian mussels, the cancer derived from one forlorn individual accounted for all sick individuals. Meanwhile, separate lineages of cancer that traced back to two individuals lead to transmissible cancers in the Spaniard cockles. Lastly, and most surprisingly of all, the genotype of the cancer found among golden carpet shell clams matched with an entirely different species of clam, the pullet shell clam, also found in the same habitat! Remarkably, no cases of cancer could be found among the pullet shell population, suggesting that susceptible individual may have been killed off, leaving behind a cancer-resistant super-clam.

Taken together, the results of this study suggest that transmissible cancer is a widespread phenomenon in the marine environment. Not only were cases of transmissible cancer found repeatedly across species of bivalve, but in the case of the cockles, were found to arise on two separate occasions from distinct individuals. Widespread transmissible cancer in bivalves joins the cadre of examples of transmissible cancers described in dogs, and Tasmanian devils, while the cross-species transmission of cancer in clams finds analogy in the horrific case of an AIDS patient who contracted cancerous cells from a tapeworm. All in all, this study paints an emerging picture of cancers as a self-inventing contagions. Leave it to the marine environment to throw conventional wisdom for a loop.

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