The contagious devil facial tumour disease threatens the Tasmanian devil with extinction, but salvation may have arrived now that the disease’s genome has been sequenced.
As its name suggests, devil facial tumour disease (DFTD) causes grotesque bulbous facial growths, usually leading to death within three to five months of contraction. It is thought that the tumours jump between individuals during confrontations over food or during mating. Both are often highly charged and involve biting, giving the tumours a point of entry to other individuals and a mode of dispersal throughout the population.
The disease is a recent phenomenon sparked by spontaneous genetic mutation, enabling the tumours unusually to survive beyond the lifetime of the original sufferer. New research has revealed the progenitor to be female, and although her clonal tumour has diverged genetically, she has effectively become immortal, however at the potential cost of the whole species. So severe is the impact of DFTD on the Tasmanian devil population, it has had an almost genocidal effect on numbers. Since 1996 it is estimated that the population has decreased by as much 90% in some areas.
It is hoped the newly sequenced transmissible cancer genome will hint at a way of stemming the tide of tumours and save the Tasmanian devil from extinction, possibly through the identification of potential genetic targets for intervention and the development of a vaccine.
At least five mass extinctions have occurred in the Earth’s history and it has been suggested that we are currently in a sixth, human-induced extinction event. It is therefore bittersweet but by no means savoury that a species should be threatened, and indeed has been reclassified on the IUCN’s Red list to ‘endangered’, not because of any human influence but by spontaneous natural causes.
Cynics with a devil may care attitude could argue that we should focus conservation attention on species threatened by our own hand and let ‘nature take its course’ with the rest. The Tasmanian devil has very little chance of adapting to such a quick spreading threat – evolution cannot operate within such a short timeframe. Extinction is likely to occur in the next thirty years without our intervention, and if we possess the potential power to develop a vaccine as a result of the sequenced genome, should we not take our conservation opportunities where we can? To do nothing now would be akin to arriving first on the scene at a road traffic accident, and continuing along our way.
Unnecessary self-righteous rant aside, the transmissible quality of the tumours makes DFTD one of two known naturally occurring contagious cancers, the other being the sexually transmitted canine transmissible venereal tumour (CTVT) afflicting dogs. However whereas dogs can combat the disease with their immune system, the Tasmanian devil has a lower genetic diversity reducing its ability to produce the immune response required to overcome the disease.
Perhaps unnervingly, researchers at the Sanger Institute in Cambridge where the genome was sequenced, found that a “high level of genomic instability has not been required for the cancer to become transmissible.” In other words it did not take a great deal of genetic rewiring for the cancer to become contagious. At the risk of scare-mongering, there is no reason why similar changes couldn’t occur in human cancers, or any other species for that matter. This coupled with the fact that humans, like Tasmanian devils, have relatively low genetic diversity, and so may also be unable to produce the required immune response to combat the cancer, is particularly troubling. As of yet, no known naturally occurring contagious human cancer exists, but there have been rare reports of accidental cancer transmission between humans through certain surgical procedures, so it is by no means impossible.
The emphasis here though is on the word ‘rare’, and a benefit of sequencing the DFTD genome in addition to conservation purposes, is the light that will be shed on the biology of cancer transmission between individuals.
The Tasmanian devil is the world’s largest marsupial carnivore and is only found in the wild within Tasmania. Apart from the obvious conservation arguments advocating an urgent need to save this important species, it would be depressing to think that the animal on which a favourite childhood cartoon character for many, is based, has gone extinct in our lifetime. At least now we know why Taz was always so inarticulate and indignant.