Calming the Perfect Storm- how evolution is acting against DFTD

Tasmanian devils (Sarcophilus harrisii) are under threat. Over the past 20 years, their small populations found across Tasmania have been devastated by a transmissable form of cancer, which almost always proves fatal. Devil facial tumour disease (DFTD) is transmitted by bites to the face, and has eradicated 80% of the Tasmanian devil population- at the current rate, it is predicted to drive the remaining individuals to extinction.

What is a transmissible cancer?

DFTD is a very unusual disease. Transmissible cancers have only been identified in Tasmanian devils, dogs, and most recently, a species of mollusc. The cancer originated in a single female individual over 20 years ago, and persists as a transmissible parasitic cell line through the hosts that it infects. Low genetic variability within Tasmanian devils means they are more susceptible to diseases, and DFTD itself down-regulates MHC molecules, suppressing the immune system and preventing removal of the cancerous cells. This creates a ‘perfect storm’ for transmissible cancers to become prevalent within the population, and is a storm which has been increasing in severity over the past two decades.

Are the Tasmanian devils doomed?

However, a recently published article by Epstein et al in Nature Communications outlines how some Tasmanian devils have developed some genetic resistance to DFTD.

The researchers sequenced parts of the genome of a number of Tasmanian devils from regions around Tasmania, before and after disease contraction. It revealed that two genes have been rapidly selected for over ~5 generations- CD146 and THY1. This selection acted on pre-existing alleles of the genes, because it is unlikely that new genetic mutations arose and became prevalent in the population over such a short period of time. Related genes in humans are linked to immune regulators, suggesting that these genes may help the TDs’ immune systems recognise that the cancer contains foreign tumour cells, and so assist with their removal.

DFTD puts the Tasmanian devils under a huge amount of selective pressure, which may ultimately lead to its downfall- could DFTD be selecting for devils that are resistant to its own infectivity?

Why is this important? 

Research is currently underway to test the effects of up-regulation of these genes on cells infected with DFTD. Potentially, these genes could be used to generate a vaccine or a cure for DFTD. Tasmanian devils are an endangered marsupial that is endemic to Tasmania, and are key to Tasmania’s ecosystem- it is therefore important that they do not go extinct.

The paper: Epstein et al. Rapid evolutionary response to a transmissible cancer in Tasmanian Devils. 2016. Nature Communications.


Picture: Wikimedia commons


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