A question often asked about cancer is: “Can you catch it from someone else?” Answer: “No you can’t.” But as so often in cancer the true picture requires a more detailed response — something that may make scientists unpopular but it’s not our fault! As Einstein more or less said “make it as simple as possible but no simpler.”
No … but …
So we have to note that some human cancers arise from infection — most notably by human immunodeficiency viruses (HIV) that can cause acquired immunodeficiency syndrome (AIDS) and lead to cancer and by human papillomavirus infection (HPV) that can give rise to lesions that are the precursors of cervical cancer. But in these human cases it is a causative agent (i.e. virus) that is transmitted, not tumour cells.
However, there are three known examples in mammals of transmissible cancers in which tumour cells are spread between individuals: the facial tumours that afflict Tasmanian devils, a venereal tumour in dogs and a sarcoma in Syrian hamsters.
Not to be outdone, the invertebrates have recently joined this select club and we caught up with this extraordinary story in Cockles and Mussels, Alive, Alive-O! It’s a tale of clams and mussels and various other members of the huge family of bivalve molluscs — (over 15,000 species) — that began 50 years ago when some, living along the east and west coasts of North America and the west coast of Ireland, started to die in large numbers. It turned out that the cause was a type of cancer in which some blood cells reproduce in an uncontrolled way. It’s a form of leukemia: the blood turns milky and the animals die, in effect, from asphyxiation. In soft-shell clams the disease had spread over 1,500 km from Chesapeake Bay to Prince Edward Island but the really staggering fact came from applying the power of DNA sequencing to these little beach dwellers. Like all cancers the cause was genetic damage — in this case the insertion of a chunk of extra DNA into the clam genome. But amazingly this event had only happened once: the cancer had spread from a single ‘founder’ clam throughout the population. The resemblance to the way the cancer spreads in Tasmanian devils is striking.
Join the club
In 2016 four more examples of transmissible cancer in bivalves were discovered — in mussels from British Columbia, in golden carpet shell clams from the Spanish coast and in two forms in cockles. As with the soft-shell clams, DNA analysis showed that the disease had been transmitted by living cancer cells, descended from a single common ancestor, passing directly from one animal to another. In a truly remarkable twist it emerged that cancer cells in golden carpet shell clams come from a different species — the pullet shell clam — a species that, by and large, doesn’t get cancer. So they seem to have come up with a way of resisting a cancer that arose in them, whilst at the same time being able to pass live tumour cells on to another species!!
Map of the spread of cancer in mussels. This afflicts the Mytilus group of bivalve molluscs (i.e. they have a shell of two, hinged parts). BTN = bivalve transmissible neoplasias (i.e. cancers). BTN 1 & BTN2 indicates that two separate genetics events have occurred, each causing a similar leukemia. The species involved are Mytilus trossulus (the bay mussel), Mytilus chilensis (the Chilean blue mussel) and Mytilus edulis (the edible blue mussel). The map shows how cancer cells have spread from Northern to Southern Hemispheres and across the Atlantic Ocean. From Yonemitsu et al. (2019).
In the latest instalment Marisa Yonemitsu, Michael Metzger and colleagues have looked at two other species of mussel, one found in South America, the other in Europe. DNA analysis showed that the cancers in the South American and European mussels were almost genetically identical and that they came from a single, Northern hemisphere trossulus mussel. However, this cancer lineage is different from the one previously identified in mussels on the southern coast of British Columbia.
It seems very likely that some of these gastronomic delights have hitched a ride on vessels plying the high seas so that carriers of the cancer have travelled the oceans. Whilst one would not wish to deny them the chance of a holiday, this is serious news because of the commercial value of seafood.
It’s another example of how mankind’s advances, in this case being able to build things like container ships with attractive bottoms, for molluscs at least, can lead to unforeseen problems.
This really bizarre story has only come light because of the depletion of populations of clams and mussels in certain areas but it certainly carries the implication that transmissible cancers may be relatively common in marine invertebrates.