Tasmanian Devils Fight Cancer
The Tasmanian Devil is today’s largest known marsupial mammal; which is not saying much, barely two feet long aside from tail. Hunted to extinction, these tiny fierce predators remain today on the island of Tasmania. They form extended social networks, fighting, mating, and eating entire carcasses down to the bone–a tidy habit that actually endears them to Tasmanian farmers. Considered an iconic symbol and attractor of tourists to Tasmania, the devil now enjoys the benefits of the Australian government’s Save the Tasmanian Devil Program.
Like most charismatic mammals, the devil’s main enemy for survival has long been considered the humans who hunted them to near extinction. But in recent years, devils face an even more horrible foe: transmissible facial tumors, known as devil facial tumor disease (DFTD). First seen in 1996, the tumors disfigure the face in horrible ways that I prefer not to show here, but you can find them all over the internet. What is unusual about these tumors is that the cancerous tissue actually transmits like a pathogen to other animals, typically during fighting, which the little carnivores spend a lot of time doing. Why does the cancer spread? Apparently because the devil populations have such low diversity that their immune systems fail to recognize the tumor cells as foreign. The tumor spreads so fast that most devils get infected and eventually die of organ failure or starvation. Overall the population has declined by 90%.
Will the devil go extinct? Surprisingly, the answer appears to be no. A new study by an international research team, including Brendan Epstein and Andrew Storfer from Washington State U, focuses on a few isolated populations of devils that seem to be holding their own (pink disks below). This map (Figure 1) shows the steady advance of the disease across Tasmania, and the sites of surviving populations.
The authors of the study reasoned that any devil populations having survived the near-100% fatality rate of DFTD must have undergone selection for specific gene mutations that confer resistance to the tumors. Sure enough, the animals’ sequenced genomes reveal fascinating mutations.
In the genomes, seven DNA regions were found that showed high frequency for particular alleles (variant versions of a gene). These regions are “known to science” because they share similarity with the DNA of all mammals, who share a common ancestor. Five of these seven regions encode proteins that are associated with cancer prevention or the immune system, as studies in other mammals. For example, protein CD146 is known as “melanoma adhesion molecule.” This protein functions in normal cell cycle and adhesion (attachment to other cells) as well as regulation of the immune system that prevents cancer. Another protein is known as a “proto-oncogene,” that is, a gene that can mutate to cancer–but ordinarily protects from cancer. Presumably, the mutant version of these proteins somehow protects the individual devils from the tumors that came close to exterminating them.
The authors title their work, “Rapid Evolutionary Response to a Transmissible Cancer in Tasmanian Devils.” The case shows an unusually sped up version of the evolutionary arms race that faces all living things; how to out-live that which threatens survival and reproduction. The recent finding is good news for devils–and may also shed light on cancer prevention in related species, including our own.