It has long been thought that cancer is not contagious. Recent discoveries, however, have suggested otherwise. A venereal disease in dogs as well as facial tumors in Tasmanian devils have previously been identified as transmittable cancers, and this week, researchers at Columbia University found that a third cancer belonged to this group: hemic neoplasia. This cancer was first noticed in the 1970s in soft-shell clams, which were dying from a quickly growing numbers of immune cells in their circulatory fluid (1).
Studies into soft-shelled clams have shed light onto the phenomenon of transmittable cancers.
A few years ago, Carol Reinisch, a researcher at the Marine Biological Laboratory in Woods Hole,Massachusetts, first noticed this phenomenon, and contacted Stephen Goff at Columbia University to understand if a virus was involved in the death of the species. Together, they tested dead diseased clams for reverse transcriptase, an enzyme related to retroviruses that cause cancer, and found that the samples were incredibly positive for the enzyme. The researchers realized, however, that this abundant reverse transcriptase had not come from a virus but instead had originated in the cancer cells’ genome. In the genome, the transcriptase is encoded by a retrotransposon, which is a particular fragment of DNA that is able to replicate and insert itself into multiple different genomic locations (1).
The researchers also found that while the typical healthy clam has fewer than ten copies of the retrotransposon, these diseased clams had approximately 150 copies per cell. Additionally, they discoveredthat the copies of the retrotransposon had not developed individually but instead were incredibly similar from one clam to the next, suggesting that it had spread between the clams. Further supporting this claim, Columbia University’s Michael Metzger confirmed that all of the clams’ cancer cells originated from one mutated cell that subsequently spread between clams, implicating that this type of cancer can be transmitted and is contagious.
Bruce Barber, a visiting invertebrate pathologist at Eckerd College in St. Petersburg, Florida, hypothesized two processes for the transmission of the cancer that most likely worked in tandem. The first hypothesis is that the cancer spread slowly between different populations of clams along the coastline from Newfoundland to Chesapeake Bay, while the second supposition is that individuals put healthy clams near infected, cancerous one, thus increasing the rate of transmission. (1).
Certain mysteries still remain, however, such as how the disease is transmitted when the disease originated. It is possible that the cancer spreads by diseased clams releasing the toxic cells while spawning, when injured, or after death; it is also feasible that the cancerous cells travel from the organism’s digestive tract to their circulatory system as they eat. As scientists search for these answers, they are also checking to see if other shellfish diseases are a result of transmittable cancer, as there is a strong possibility that contagious cancer is not as uncommon as was once thought (1)
References:
- Stokstad, Erik. “Infectious Cancer Found in Clams.” Science 348.6231 (2015): 170. Science. Web. 12 Apr. 2015.