A bizarre facial cancer threatening to wipe out the Tasmanian devil probably evolved from a single female about 16 years ago, new scans of the cancer reveal. The scans are also helping to identify gene mutations found in the cancer but not healthy tissue, which might provide targets for a vaccine to rescue the endangered species.
Devil facial tumour disease is unusual in that the cancer cells themselves act as infectious agents. The cells spread between animals through biting during fights or mating. A vaccine could prime uninfected animals against the cancer if they are subsequently bitten.
“Now we know which genes are mutated, we can begin assessing which ones might be good antigens for a vaccine,” says Elizabeth Murchison of the Wellcome Trust Sanger Institute in Hinxton, UK, who led the team.
After analysing DNA from 104 tumours in 69 devils, Murchison found that all of them trace back to a single female – dubbed the “immortal devil”. This animal must have been the first to develop the cancer, around 16 years ago.
She passed it on by biting other devils, and the cancer then spread through the entire population, which has seen declines of 80 per cent in affected areas.
“It’s one cancer, in one devil, and has now spread through the population, like it has metastasised,” says Murchison. “That devil lived on the east coast of Tasmania, and although she’s dead, her cancer lives on in thousands of devils in Tasmania today, which is why I call her the immortal devil.”
As well as revealing possible leads for vaccines, the study revealed how the original cancer has evolved. In the Forestier peninsula in the south-east of Tasmania, for example, there’s evidence that one variety of the cancer disappeared and another became dominant in its place.
Tracing how the cancer is evolving might help predict how it will spread when it reaches unaffected areas, says Murchison.
Murchison and her colleagues also found evidence of mutations in immunity genes within the cancer, which could help explain why the immune systems of newly infected animals don’t recognise tumours as “foreign” and kill them off.
Other researchers have suggested that extensive inbreeding might explain how the cancers evade destruction, because they are mistaken for part of the animal’s normal tissue. However, Murchison said that a study by researchers at the University of Tasmania appeared to rule out the inbreeding hypothesis by showing that skin grafts between devils were rapidly attacked and eliminated.
“Having the genome should also allow us to identify cancer-causing genes, and hopefully find drugs that may help to control the disease,” says Katherine Belov, an authority on Tasmanian devils at the University of Sydney, who was not involved in the study.
“There is some early evidence that the disease is behaving differently in devils in north-west Tasmanian populations,” says Belov. “The disease there is affecting fewer devils and taking longer to kill them off, and at this stage we don’t know whether that’s because the devils there are more resistant, or the tumour strains are different, or both,” she says. “Genomics will help us to solve this.”
Murchison says that although the research is focused on the devils, the knowledge gained may be valuable if a similar disease ever emerges in humans. The only other known cancer infectious in this way is a venereal disease in dogs.