Immortal Devil: The Problem of Transmissible Cancer

The Tasmanian Devil made his first Looney Tunes appearance alongside Bugs Bunny in 1954. Warner Brothers, afraid of Taz's violent image, axed him after only a few episodes. But the grumpy, spinning character was so popular among viewers that producers scrambled to bring him back to the screen.

The real Tasmanian devils are small, feisty marsupials with the most powerful bite of any living mammal. In confrontations, blood rushes to the ears to turn them a glowing red. The sound of a fight is unsettling --shrieks and growls that seem to alternate between throaty screams and deep, gasping breaths. And, of course, there is biting. Lots of it. Unfortunately for the devils, biting is part of the reason that the species could become extinct in the near future. And unlike Taz's situation, no amount of fan mail will bring them back once they're gone.

The real Taz, whose species is now threatened with extinction.
Photo credit: David Boon

In 1996, farmers began to notice a decline in devil numbers throughout Tasmania. At the same time, many devils were popping up with gruesome, face-crushing tumors. Scientists quickly discovered the bad news: Tasmanian devil facial tumor disease (DFTD) turns out to be one of only two transmissible, parasitic cancers known to exist in any species.

Unlike nearly all cancers, DFTD cancer cells can be transferred directly between devils through biting  common in mating and feeding behaviors. Normally, immune systems reject foreign cells without a problem. But for reasons still unknown to scientists, DFTD tumor cells go unnoticed by the immune system (some point to the low genetic diversity in devil populations, but others suggest that mutations in immunity-related genes might also play a role). Tumor cells proliferate for a few months until the infected devil finally starves to death.

A study just released in Cell now reports the sequencing of the full devil genome along with two copies of the cancer genome. It seems that the cancer arose from a single female devil. All current tumor cells are clones of her original tumor (in some ways, this reminded me of the immortal HeLa cell line). The paper also outlines the nearly 17,000 base pair substitutions of the devil cancer genome. This is fewer mutations than researchers expected, and genetic stability seems to be a factor in maintaining DFTD's transmissibility.

Devils are dying fast from the disease, and extinction could happen within 30 years without a way to stop the cancer's spread. For now, the devil claims the title of largest carnivorous marsupial in the world. The previous title-holder, the thylacine, went extinct when the last one died in captivity in 1936. Tasmanian officials are hoping that they will not lose the devils within just 100 years of the thylacine's loss.

"Benjamin," the last thylacine circa 1933.