Rogues and devils

The other day I heard about a most fascinating disease for the first time, though it has been around for a decade now, and you probably all heard about it years ago. The disease is Tasmanian Devil Facial Tumour Disease (DFTD) which is threatening to drive the Tasmanian Devil extinct. I'm sure I must have heard before that disease is driving the Devil extinct, but never realised what a fascinating disease it is. DFTD is a transmissible cancer. Individuals “catch” the disease by biting each other in fights, and from sharing food sources. Over the course of a few months tumours balloon up on the faces, they loose their sight or use of their jaws, and starve. We know, of course, that our risk of developing cancers depends on chemicals in the environment, and that certain biological agents, notably viruses like HPV, can play a role. But this is not caused by a virus or bacterium agent: the tumours are genetically identical in each individual, and do not share any of the rare quirky genetic signatures of individual hosts. This tells us that all of the tumour cells are descended from one individual, rather than multiple cells in multiple individuals being transformed by infectious agents or carcinogens. The literature rather charmingly describes it as a "rogue cell line." This is a no doubt supposed to conjure images of uncontrollable criminals in the wild west, but I can't help thinking "Dennis the Menace."

We never hear about one individual “catching” another individual's cancer: such events are negligible, if not non-existent. The reason that DFTD can work depends upon the mechanism of the immune system. We have a set of genes that vary greatly between individuals and which produce proteins that sit on the cell surface acting as a bar code to identify the cell's origin. Certain white blood cells – the lymphocytes – go around the body scanning these bar codes, destroying undesirable foreign cells, such a pathogenic bacteria, and one's own cells when they go wrong, for example, through infection or DNA damage. We can see how effective this system is by looking at organ transplants: if the recipient isn't given immunosuppressants the organ is “rejected.”

Tasmania became an island at the end of the last ice age. Devils once covered the entire continent of Australia, and a small population became isolated on the island as the Bass Strait formed. Typical of small island populations, the Tasmanian Devils were subject to the Founder Effect: the small population had relatively little genetic diversity, and essentially, they have been inbreeding since they became isolated. It's therefore far more difficult to tell individuals apart just by looking at the genes of their immune system. It's not impossible, though, and it's probable that mutations in the tumour line have helped the tumours evade the immune system. This is, therefore, another example of natural selection acting on the level of the transformed cell line, at the expense of individuals (and perhaps soon at the expense of the whole species).

• Hamish McCallum and Menna Jones (2006). To Lose Both Would Look Like Carelessness: Tasmanian Devil Facial Tumour Disease. PLoS Biol 4(10): e342 DOI.
• Pearse A-M, Swift K (2006). Transmission of devil facial-tumour disease. Nature 439: 549 DOI.
• BBC News: Bites spread fatal 'devil' cancer