Contagious cancer in humans: a medical rarity

So far, cases of contagious cancers have only been documented in these three animals: tasmanian devils, dogs and certain bivalves. But we can still learn a lot about cancer biology in general from these cases and question whether these could arise elsewhere. Elizabeth Murchison from the University of Cambridge explains to Chris Smith…

Elizabeth - One thing about contagious cancers is that they frequently live much longer than cancers that just arise and remain within a single individual. That means that we can study cancers' long term evolution, and that can tell us how cancers can continue to adapt to their hosts. This can give us insights into the optimal evolutionary roots that cancers can take, but also they can tell us about how cancers can adapt to escape the immune system under fairly extreme circumstances. Though cancers often adapt to escape the immune system, even when they're only non-transmissible cancers, just arising and staying within their host. But these transmissible cancers have to escape what we call an allogeneic immune system; an immune system of a different individual, a foreign graft in essence. Insights into this very extreme setting can tell us more about how cancers escape the immune system, even when they stay in the same host.

Chris - In recent years, we've discovered that cancers also team up with the microbes in the body. In fact, just this week, there was a paper out that looked at the presence of microorganisms inside cancer cells, endowing those cancer cells with additional abilities such as greater resilience, so they can spread around the body better. Has anyone looked at these transmissible tumours and asked if there are bacterial passengers in them that might be aiding and abetting their spread?

Elizabeth - That's a fascinating question. That's something that we are currently looking at, although we don't have anything conclusive to say yet. One thing that is really interesting in the dog cancer however, is that it sometimes takes up little pieces of DNA called mitochondrial DNA from its hosts. As it spreads along, it's sometimes stolen mitochondrial DNA from its host, which helps it to adapt better to being a cancer. This might also be a mechanism which might occur from time to time in human cancers.

Chris - And speaking of human cancers, has this actually happened? I know I reassured people by saying that there's no defined equivalent to these cancers that's spreading among humans all the time at the moment, but are there isolated examples of this happening?

Elizabeth - Yes. There are quite a number of examples of cancers spreading between two people or between small groups of people. These usually occur in 3 different types of settings. First of all, there are accidents, such as surgical accidents for instance. There's one very terrifying case of a surgeon who was operating on a cancer patient and accidentally cut himself on the hand. Several months later he discovered a cancer in his hand at the surgical injury site and it turned out to be the cells of the patient, although he was cured. Then there are organ transplants. When organ transplants are performed, there are very stringent screening processes in place to prevent accidental transfer of cancer cells, but unfortunately this has happened from time to time and cancers have then manifested in the organ recipient. The final case is during pregnancy. This is usually a cancer from a pregnant mother that can spread to the foetus. This is extraordinarily rare and usually involves leukaemia or lymphoma. These are all extremely tragic and sad, rare cases, but they can happen from time to time.

Chris - I wanted to touch on that point you raised about pregnancy because of course that is the body tolerating the invasive growth of tissue, which is genetically incompatible to it. The baby is genetically different from mum. So is there evidence that these transmissible cancers in the animals that get them are actually exploiting that mechanism that is a necessity of the way we reproduce?

Elizabeth - It could well be that transmissible cancers are making use of some of the adaptations that are there in our cells, which allow pregnancy to occur. Which as you said, is a circumstance when two genetically distinct cell types coexist within the same body. There is some preliminary evidence suggesting that that could well be the case that some of these pregnancy mechanisms are being used.