The odd case of transmissible tumour
What do Tasmanian devils, hamsters, dogs and marine bivalves have in common? All four of them are the only animals where clonally transmissible cancers are known to occur. And for Tasmania devil this is a big issue, as it is threatening the survival of the species. How can cancer be a contagious disease? Let’s find out!
We do not normally consider cancer as a transmissible disease: in fact, it arises from a combination of genetic and environmental factors that are not “contagious”. Nonetheless, there are some exceptions. The first and most famous is that of virally induced tumours, like cervical cancer, whose principal aetiological agent is HPV virus. However, some tumours are defined “transmissible” with no viruses responsible for the transmission: the cancer cells themselves become the “infectious agents” or “parasites” spreading from one individual to another.
The first to be identified in 1996 was the facial tumour of Tasmanian devils (DFT); in 2006 and 2015 two other transmissible tumours were discovered in dogs (canine transmissible venereal tumour) and marine bivalves (clam leukaemia), respectively. This is a recent discovery and scientists think that such contagious tumours might be more common than we thought and that others might exist that we have not identified yet.
Should we be concerned that such pathologies might arise in humans as well? Up to now, there are no cases of tumours spreading through a population in humans. Our immune system protects us, rejecting any tissues coming from another individual of our (or other) species, including cancerous cells.
This does not mean that we should underestimate the problem, since it has become a serious threat for biodiversity, putting Tasmanian devils at risk of extinction. The contagion occurs mostly through bites, which are very frequent during mating or fights over food. Unfortunately, the disease is lethal for the animal, which becomes unable to feed as tumour masses grow around its eyes and mouth.
Why does the immune system of Tasmanian devils (and of the other animals suffering from communicable tumours) not reject the foreign cells? Some findings suggest that it might be due to the lack of MHC (Major Histocompatibility Complex) molecules on the surface of tumour cells. Normally, the cells of an organism present the same MHC, so the immune system can recognize and destroy cells expressing MHC molecules that do not match with the “self”: however, it is not rare that cancer cells reduce the expression of such molecules so as to be no longer visible and get away with it. The immune system becomes “blind” and does not notice the presence of invaders.
There is good news for Tasmanian devils, though. First, some specimens within the population seem to be immune to contagion. Maybe, we are assisting at the birth of a “resistance” to the disease, which means that casual mutations in genes controlling immune functions are conferring to some individuals the ability not to get sick when in contact with cancerous cells. Second, recently published studies suggest the possibility to treat DFT with inhibitors of an enzyme called receptor tyrosine kinase, which regulates cells’ proliferation and differentiation, or with immunotherapy, through a vaccine that will trigger the Tasmanian devil’s immune system to recognize and destroy the established tumours. Research in this field might lead to important applications in both evolutionary biology and clinics and hopefully to the survival of the endangered Tasmanian devil.
We all root for him, hoping that he can win his personal battle against extinction!