The Paper: Metzger, M. J., Reinisch, C., Sherry, J., Goff, S. P., Metzger, M. J., Reinisch, C., … Goff, S. P. (2015). Horizontal Transmission of Clonal Cancer Cells Causes Leukemia in Soft-Shell Clams Article Horizontal Transmission of Clonal Cancer Cells Causes Leukemia in Soft-Shell Clams. Cell, 161(2), 255–263. doi:10.1016/j.cell.2015.02.042
Cancer has many causes: genetics, environmental factors, viruses…the list goes on and on. Regardless of the type of cancer, the formula is always the same. The genes of a healthy cell mutate, causing cells to grow uncontrollably. These unchecked cells continue to grow, multiply and invade healthy tissues in the body, possibly even spreading and infecting other tissues. While there is something unsettling about your own body staging a mutiny against yourself, there is some comfort in the fact that cancer is not contagious. Unlike the common cold, at least you can’t catch your cancer from that snotty kid on the bus licking all the handrails.
However, there are two types of cancer known to break this rule and spread though a contagious cell lineage- one that affects dogs and is transmitted via sexual contact (canine transmissible venereal tumor) and one that is spread by skin to skin contact in Tasmanian devils (Tasmanian devil facial tumor disease). Unfortunately for soft-shelled clams (Mya arenaria; Fig 1.), researchers have identified a third contagious cancer lineage affecting marine bivalves.
Since the 1970’s, soft-shell clam populations along coasts of the North Eastern coast of North America have suffered large mortalities from outbreaks of a type of cancer called disseminated neoplasia. This is a leukemia-like cancer, which affects the hemolymph (essentially the bivalve blood- so instead of affecting white blood cells like leukemia in humans, this cancer affects the infection fighting, immune cells of hemolymph) of clams, mussels, cockles and oysters. In affected clams, the hemolymph immune cells proliferate and lose their functionality (i.e. lose the immune system’s ability to “stick” to invaders, as displayed by losing the capacity to cling to jars in which the cells are placed; Fig. 2). The fatal cancer has resulted in depleted clam populations in affected areas, sending researchers in search of a cause of the widespread cancer.
Scientists examined the DNA of soft-shelled clams that were affected by the cancer and healthy individuals from four different locations along the North Eastern coast of North America (Fig 3): Prince Edward Island, Canada, Larrabee Cove, Maine, St. George, Maine, and Port Jefferson, New York. Scientists found a highly abundant reverse transcriptase (basically a piece of DNA that can insert itself into another piece of DNA, altering the genetic code) in cancer cells that were only found in small quantities in normal cells. They named this reverse transcriptase Steamer and further analyzed the Steamer integration sites, and tiny pieces of clam DNA, or single nucleotide polymorphisms (SNPs for short) of different tissues within healthy and cancerous clams.
Results and Discussion:
The cancerous cells in clams had a high number of Steamer copies, in comparison to normal clam cells- and by a high number. We are talking 150-300 copies per cell compared to normal cells having fewer than 10 copies. Furthermore, the number of Steamer copies was significantly higher in the hemolymph (the target tissue type of this cancer!) than in any other tissue in infected individuals (Fig. 4). What was more shocking than the large number of Steamer copies, was that the copies were consistently found in the same locations within the genomes of different infected individuals. This is pretty crazy because normally these inserted fragments of DNA are found at random locations in different individuals.
Crazier still, when the SNPs from infected tissue were analyzed, researchers found that the genome of the cancer cells did not match the genome of the host cells. Cancer that we are used to is derived from host cells, and therefore the genomes of the cancer cells match the host’s genome (of course with minor alterations…). According to normal cancer rules, the genome of these clam cancer cells has no business having a separate genome! This would mean that the cancer cells were actually derived from another individual entirely—Um…WHAT??
And the story gets even more twisted. Analysis of the SNPs revealed that the cancer cell genomes of different individuals were almost identical, even between the different sample locations. The implication of this is that all of these cancer cells were derived from a single individual and the cancerous cells have been transmitted to other individuals throughout several different populations. Yes, you read correctly- this is a contagious cancer originating from a single infected individual (what a jerk of a clam to spread its cancer!).
Researchers suspect that this cancerous cell lineage may be over 40 years old based on the first reports of the disease. The question now is: how is this cancer being transmitted across hundreds of miles of ocean to different clam populations? Unlike the other two forms of contagious cancer where direct contact with an infected individual is necessary to spread the cancer, clams do not need to come in direct contact with one another to catch the cancer. It is possible that infected cells are released into the environment naturally during the disease, during spawning events, or after death. These cancerous cells may then be absorbed by uninfected clams as they filter feed (sucking in large volumes of water containing these freely floating cancerous cells), resulting in infection.
Okay- to ease the minds of all of you clam-eaters: No. You cannot catch this cancer from eating infected clams. There have been no negative effects observed from eating infected clams.
The major take-away from this finding is that contagious cancers are more abundant than we formerly thought. Because similar cancers affect other bivalves, researchers are looking into the possibility that these cancers may also be contagious forms of the disease.
Let me know what you think of contagious cancers and the effect the cancer could have on clams in the comment section!