Biology Behind: The contagious devil cancer

Greetings everybody,

Happy Friday!! As always. Today is a lovely Biology Behind on a not so lovely disease. Everybody has a similar reaction when they hear the C word, that word being cancer, at least we can say that it is not contagious... or is it. Within this post I am going to enlighten or further consolidate your pre-existing knowledge on the Tasmanian devils contagious face cancer! Or in scientific words the Devils Facial Tumor Disease (DFTD).

First of, what is a Tasmanian Devil? Interestingly, it is the largest carnivorous marsupial mammal and is endemic to the Australian Island of Tasmania. Great but what do they look like? Well, get that image of Taz from Looney Tunes out of your minds, because they look very different to that. So different that you will wonder where Taz came from in the first place, take a look.

Just to let you guys know there are a few disturbing pictures in this post

The rise of a contagious cancer:

In 1996 a photographer came across a Tasmanian Devil with a large facial tumour, however, no significant action was taken - after all cancer has been recorded in many animals. Interestingly, that photographer may have seen patient zero for a disease that has now blown through 90% of Tasmania's devil population. 
Devil Facial Tumour Disease is fast approaching to be the first disease to threaten an entire species with extinction. So, how does a cancer such as DFTD spread and kill 90% of a population? Originally, scientists believed this cancer was being triggered through viral infection similar to how the Human Papilloma Virus causes the majority of cervical cancers, this could explain how it spread through the population so fast. As always, nothing is ever so simple and it turned out that the success behind this cancer was far worse.  
Turns out DFTD is one of two confirmed contagious cancers - you read that right contagious! This disease is transmitted through physical transfer of cancerous cells from an infected individual. Devilish behaviour is a key part to this diseases's success, when two Devils meet it is often a violent interaction resulting in a bite. Interestingly, tumours are localised around a Devils face, neck and inside of their mouth- places that receive the most amount of bites. Once bitten, cancerous cells invades the body of its new host, takes root and does what cancer does- proliferate, exponentially. 
After this proliferation period, death soon follows whether it be starvation due to multiple mouth tumours or the tumour diverting nutrient flow to benefit its own growth. 

Passing of cancerous cells from one devil to another fascinated scientists, how can these foreign cells grow inside an unrelated host, why is this not rejected? Tasmanian Devils have a major glitch within their immune systems- well, the part that deals with detecting foreign bodies. Histocompatibility complexes are the most genetically varied structures found in all jawed vertebrate immune systems, the Devil population all have very low levels of genetic variation- as such their ability to recognise foreign bodies is greatly hindered. DFTD took advantage of this glitch and based their entire transmission route around it. 

Darwinian theory and cancer:

It is easy for us to forget that every living cell is seeking out their own optimal growth, their own evolutionary path- so is cancer. DFTD is a contagious cancer, being contagious has allowed this cancer to achieve an evolutionary win. Cancer as we know it, arises from the hosts own cells, proliferates quickly,and evades our bodies immune system because we have trained our immune system to not destroy our cells. Cancer is self destructive and fairly short sighted, by securing its own growth it further kills its host, and there lie every cancers limitation- it is an infinite cell living in a finite host.  Then comes along DFTD an infectious cancer, a cancer able to spread between individuals within a population, allowing them to outlive their original hosts.
Was this an inevitability for cancer? Infinite cancer cells that were only restricted by their finite hosts have cleaved their bonds. Scary! 
In this case it is the basic laws of natural selection that we all know and secretly love for simplicity. 

Evolution 101:
  1. Within a bunch of cancerous cells, some contain a mutation. This mutation allows them to be more virulent, divide faster, acquirer sources better or be passed on to others. 
  2. This new mutation allows these select cells to survive and reproduce
  3. Eventually, cancerous cells with this mutation are now dominating the population. 
There you have it cancer evolves. Necessity is the mother of invention. 

Devils genes and the search for patient 0:

Alright guys, we are going to go diving into a bit of genetics so stick it out with me. In a bid to understand what is going on in this disease scientist took samples from the tumours of several Tasmanian devils as well as a sample from the devils non cancerous tissue. What they found was crazy! 
Alright, so what we know about cancers in humans is that it arises from our own cells right? Yes. But not for the devils cancer, it turned out that the tumours found on the faces of the devils were as genetically different from their host as I am to you guys! 

In summary, the cancers are genetically different from their hosts tissue but are the cancers genetically different from one another?  The answer is- there not. All the cancers that have been sampled from the Devil population are genetically identical! This means the the devils cancer arose from one individual- patient 0. At some point, some of the cells within patient 0 had mutated, these cancer cells contained a majority of Patient 0's DNA, this devil bit another devil and passed on the cancer and eventually it lead to the epidemic we have today. 

Possible ethical debate?

Recently, many species are being threatened with extinction and a lot of these are down to human activities. However, this one is actually not our fault, this one is completely caused by mother nature herself. The devils population has fallen from 140,000 to 20,000 if this trend continues within the next 25-30 years the Devil will be extinct- without intervention. 
Breeding programmes, and recovery populations have been set up for the Devils as there is no practical way of helping those individuals within the wild. A question raised to me by my dad was, should we intervene? 
Here is my train of thought. This is not a human induced extinction. Extinction is a natural phenomenon. History has taught us that when species go extinct this allows for a speciation event to occur, and thus the process continues to go on. Conservation is a massive issue at the moment, but here is the question should we conserve the species (the Tasmanian Devil) or conserve the natural processes (extinction and speciation)? 

Well that was massive question to start of your weekend, let me know what you think. 

Over and out

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