Evolving cancer could spare Tassie devils
Devil's disease Evolution of the contagious cancer that is killing off Tasmanian devils could help save the animals from extinction, say scientists.
The comments come from genomics researcher, Dr Kathy Belov, of the University of Sydney, who is co-author of a paper on the genetics of the cancer published today in the Proceedings of the Royal Society B.
"We found that over time the cancer is changing and evolving," says Belov.
"We don't have all the answers yet but we if we can get a handle on how it's changing then that will help management of the species."
The rogue cell line that causes Devil Facial Tumour Disease (DFTD) in Tasmanian devils cancer arose more than 15 years ago in a nerve cell of a female devil, says Belov.
Since then it has been passing from animal with a near 100 per cent fatality rate.
"It's gone through 100,000 animals," says Belov. "It's a horrifying disease. At this stage it will lead to extinction of the Tasmanian devil in the wild."
She and colleagues analysed samples of facial tumour cells from animals across Tasmania that had died of the disease over several years.
In line with previous research, Belov and colleagues showed facial tumour cells were all genetically very similar.
"We don't see much difference at the sequence level of the DNA itself," says Belov.
But, the research has shown the tumour has been evolving at the level of epigenetics, which affects the way genes are regulated.
"What we're seeing is that over time more genes are being switched off in the cancer," says Belov.
Belov says this is occurring due to a demethylation enzyme produced by the tumour, which turns off genes by removing methyl groups from specific DNA sequences.
Belov and colleagues found the tumour is evolving epigenetically in different ways in different areas of Tasmania.
While the researchers don't yet know which genes are being switched off, Belov says even randomly switching off genes allows for selection of epigenetic variants that bring some benefit to the cancer.
"Some cells that have a particular combination of switches will be more successful for a particular reason in a particular situation," says Belov.
"Perhaps those genes may help the tumour hide from the immune system, or they may make the tumour able to metastasise more quickly or divide more quickly."
But while the tumour could evolve in some locations to become more aggressive it is also possible it is evolving in other places to be less aggressive, says Belov.
She says this could explain the low prevalence of the DFTD in the northwest of Tasmania.
"The disease has been there four or five years and we'd expect about 80 per cent of the devils to be sick but only 20 per cent of them are sick," says Belov.
Originally scientists thought this could be because devils there might be resistant to disease, but Belov's research suggests the genetics of the tumour in that area could be different.
"Maybe it's evolving to a point where it's not killing its host," says Belov.
If so, this could be a smart survival strategy, she suggests.
"If the disease keeps going the way it's going it will kill all the devils in five years," says Belov.
"If it runs out of devils the tumour's going to die."
Belov says plans to translocate animals from the northwest to populate other areas of Tasmania could be risky because it could expose these animals to more aggressive strains of the tumour.
Belov says it's instructive to look at how another contagious cancer - canine transmissible venereal tumour in dogs - has evolved to become a really efficient parasite.
When it's transmitted, the cancer grows for three months, during which time the dogs become more sexually active and spread the tumour.
The cancer then becomes visible to the dog's immune system and the dog is able to get rid of the tumour and remain immune for life.
It is very rare for cancers to be transmitted between humans, says Belov, but it can occur during organ transplants or from mothers to children via the placenta.
She says the latest research into the devil tumour will provide insights into how human cancers evolve and change.