Understanding What Causes Contagious Cancers

Elizabeth Murchison is a cancer researcher who grew up in Tasmania, surrounded by wilderness and wildlife. Tasmania is perhaps best known as the home of the Tasmanian devil, made famous by Looney Tunes. However, in the last few decades this iconic animal has been in the spotlight for a different reason. It’s one of only two mammals (the other being dogs) that are susceptible to contagious cancer.

The cancer, called Devil Facial Tumor Disorder (DFTD), grows on the devils’ faces or inside their mouths. It’s spread by biting, which these feisty creatures do a lot of. Unlike transmissible cancer in dogs which is almost “magically” responsive to chemotherapy treatment, chemotherapy doesn’t work very well or only has a temporary effect on DFTD. The cancer often kills the devils just as they reach breeding age, and as a result, the Tasmanian devil population has massively declined. Since the cancer was first observed in 1996, it has spread to devils throughout Tasmania. By 2008, a large proportion of the devil population had been killed by DFTD and there is concern that DFTD could cause the species to go extinct.

Elizabeth is a Reader (Associate Professor) in the Department of Veterinary Medicine at the University of Cambridge. She researches transmissible cancers in dogs and Tasmanian devils. Elizabeth’s research group uses genetics to understand how cancers become transmissible and spread through populations. To save the Tasmanian devil from extinction, scientists have to understand how the cancer first emerged and spread. Cancers arise when the body’s cells acquire mutations and start to rapidly divide. They are usually limited by the life-spans of their hosts. However, transmissible cancers survive beyond their original hosts by transmitting living cancer cells to other hosts.

In 2012, Elizabeth led a team of researchers who sequenced the Tasmanian devil genome and mapped the mutations in the species’ transmissible cancer. Researchers took the devil DNA, sheared it into small pieces, and fed it into a DNA sequencing machine. She and her collaborators put the DNA pieces back together computationally into a genome assembly of the Tasmanian devil genome. A team of gene annotators then mined this sequence to find genes. After they had identified specific genes, Elizabeth collected DNA from the devils’ transmissible cancer and sequenced it. She then searched for the mutations that make the devil cancer different to the normal devil genome. “Looking for these mutations is similar to looking for a needle in a haystack,” explains Elizabeth. “Once we found them, we needed to further assess which are likely to have been important in triggering the emergence of the devil cancer.”

This assessment step is challenging as researchers do not have DNA from the original devil that first gave rise to the transmissible cancer. As a result, they are constantly guessing about whether the approximately 20,000 mutations they have found are important for the cancer, or if they are simply “normal” mutations that were present in the original animal that first spawned the transmissible cancer cell line.

In addition to aiding devil conservation efforts, understanding the Tasmanian devil genome opens the door for researchers to learn more about this species and its cancer, as well as the long-term evolution of cancer more generally. Most cancers are very short-lived and survive only as long as their hosts do. By understanding the evolutionary processes behind transmissible cancers like DFTD, we can learn more about the biology and immunology of human cancers. “I am inspired by the knowledge that I may help save the Tasmanian devil one of the most unique and iconic species in the world,” says Elizabeth.