Clam contagious cancer jumps species barrier
And now to the process by which cancer spreads - but we're not talking about within an individual, we're talking about between individuals - so called "transmissible tumours" - where cells physically move between individuals. And Alicia Bruzos, at University of Santiago de Compostela, explains to Chris Smith, she's discovered a new example of a transmissible cancer, this time among clams. But the twist here is that these tumours haven't just jumped ship from one clam to another, they've also jumped the species barrier. By using genetic sequencing, she's been able to show that cancer cells originating in a species known as striped venus clams are now cropping up in their cousins, the warty venus clams, and she suspects that the cancer may even be stowing away aboard ships, with obvious implications for the ecosystem…
Alicia - Usually cancers start in a person or in an animal. They develop and they are able to travel to other organs or tissues of the body, which is called metastasis. But, whenever that individual or that animal dies, the cancer dies with it. But that is not the case for contagious cancers. There are a few exceptions in nature, like dog cancer, Tasmanian devil cancer, and also some viral cancers that are able not only to spread among all the tissues of the individual, but also to jump to other individuals in the population. So, that's why we call it a contagious cancer.
Chris - Literally, then, a cell leaves one individual and gets into the body of another individual and carries the disease process with it. It becomes a new cancer in that new individual?
Alicia - Right.
Chris - In the case of Tasmanian devils, we are comfortable that when they fight and bite each other, they could perhaps inject cells from one animal into the other, and that could happen in the case of dogs, it's a vaneal tumor that's sexually spread. What's the bivalve (shellfish) equivalent?
Alicia - That's a very good question and it has not yet been answered, but we have some ideas of how it can happen. These contagious cancers in bivalves are a leukaemia-like cancer, a cancer that is found in the circulatory system of these animals. So, what we might be happening is that those cells are probably released to the marine environment, by death or by active breeding of the cells out of the body, and somehow these cancer cells are able to last in the marine water. As these animals are filtering the water, by chance, they take those cancer cells that are floating in the marine environment and, once they are inside the body, they start to divide and to make cancer.
Chris - The scientists that have investigated the Tasmanian devil transmissible tumors were able to show, genetically, that the cancer cells are quite distinct from the host they find themselves in, proving that it is the cancer that's spreading. Have you got evidence that the shellfish have done the same thing? Can you find cancer cells and show that they're genetically completely different to the host they're in? Therefore they must have come part and parcel from outside?
Alicia - Yes. When you look at the cells, they are already different from the healthy cells that other individuals have. But when you go and read the DNA and study the genetics of those cells, you find that the DNA of the cancer cells is not similar to the DNA of the healthy cells of that individual. So, yes, we find it is the same as with the Tasmanian devils.
Chris - Is it from the same species? Because the thing about the Tasmanian devils is it's a Tasmanian devil giving a cancer to another Tasmanian devil. When you look at the shellfish, is it the same species of shellfish giving a cancer to another shellfish of the same group?
Alicia - There are several contagious cancers described in bivalves and in some of them, like for instance in cockles, a cockle gave rise to a cancer which was then found spreading among cockles. But, in the case that we are talking about here, the cancer arose in a different clam, the striped venus clam, and now it is currently spreading among the venus clams and all these striped venus clams that we have screened have no cancer, although this cancer originated in them.
Chris - So, putting that another way, you've got one species of clam that spawned the cancer in the first place, and it seems to have not just jumped out of them, it's jumped ship entirely and got into a completely different species of clam and is causing a headache - or a blood problem - for those clams. And it's the difference in the genetics that tells you it's come from one species and into another?
Alicia - Yes, effectively.
Chris - Are you worried that we seem to have a cancer that can transmit between species?
Alicia - Yeah. That's something that we have to be careful with. I mean, if these cancers are able to spread to closely related species, that poses a threat for the environment of these species,.
Chris - I suppose one important vector for that happening is international trade and shipping because we know that boats pump out water from their builds that could contain marine life. We also know that they have, clinging to their hulls, various shellfish and other marine animals that they take on their journey with them. If those animals are discharged at a remote location, yes, they could become an invasive species, but they could also discharge cancer cells that could then infect the locals?
Alicia - You're right. And in this study, we found the same cancer in the Atlantic coast of Spain and also in the Mediterranean coast, more than a thousand miles away. It's difficult to imagine a cancer cell floating and travelling that far, so our suspicion is that yeah, maybe human activity has had some role.
Chris - What's the take home message from your study, then? You've confirmed that there is a transmissible tumour that can jump the species barrier, but it is in quite a limited repertoire of animals at the moment. Are we sort of reassured or alarmed off the back of your study?
Alicia - We are describing a new cancer and, if we are spreading them, we should be taking some measures to avoid that because this could reduce the quantity of species. It could bring disease to places where it was not originally found. And all these things are not good for the environment.
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