By: Helen Beilinson
Australia has an incredibly unique list of animal inhabitants. From massive pythons to flying foxes (the largest bat species in the world) to ridiculous spiders and centipedes to some of the largest, smallest, and most poisonous jellyfish, Australians definitely have more interesting backyard fauna than I do here in New Haven (although the black squirrels are pretty cute).
Aside from its slightly more terrifying creatures, Australia is home to a huge amount of marsupials. Marsupials are mammals, meaning they feed their young with milk, like humans do. Unlike humans, however, mother marsupials do not carry their young in their uteri until birth. Instead, after a certain time of developing in utero (meaning, in the uterus), marsupial young will climb into a special pouch on their mothers’ belly to continue developing. These pouches contain the mother’s nipples, to feed the young, and offer protection while the baby marsupials continue growing. Some of the best known marsupials are kangaroos, koalas, and the happiest animal on Earth, quokkas. One marsupial that’s predominantly known more for its cartoon depiction is the Tasmanian Devil, which is currently the largest carnivorous marsupial. Unfortunately, their population is at high risk of extinction. Extinction of species is nothing new; it happens all of the time. Extinction can be caused by high predation, changes in food or climate, high rates of disease or infection, or a slew of others reasons. The pathway to extinction of devils is particularly interesting because their population is threatened by a rare type of disease—a transmissible cancer.
That’s right, the devils are transmitting cancer to each other, like humans can spread a cold.
Before the 1400s, Tasmanian Devils populated the entirety of Australia. However, due to heavy predation by dingoes and indigenous people, the devils were isolated to the Australian island of Tasmania. Since then, major population crashes have continued to affect the devil population. From 1830 to 1930, locals made efforts to exterminate the devils because they preyed on their livestock. In 1909 and 1950, there were smaller epidemics of infectious disease that hurt the devil population. In 1941, however, laws were enacted to protect the devil population because half a decade prior, another carnivorous marsupial, the Tasmanian Tiger, went extinct. These laws aided the devil population drastically, until about half a century later.
In 1996, the first case of Devil Facial Tumor Disease (DFTD) was documented in the Tasmanian Devils. This cancer, as the name suggests, causes large facial tumors on the devils. These facial tumors eventually cause the devils to die of thirst and starvation, as they are unable to eat, drink, or see. The fascinating thing about these tumors, as I previously alluded to, is that they are passed from devil to devil through biting. This transmissibility is incredibly rare in cancer. Usually, cancerous cells develop within one individual and cannot be passed from one person to another. Curiously, the same phenomenon helps to explain why cancer cannot be spread in most species and why it is spread through the devil population.
Each of our cells contains markers (these are basically proteins that cover the exterior of the cell such that other cells can “see” them) on its surface to tell other cells that they are part of the same organism. They mark our cells as “self” as opposed to other cells, that either have “nonself” markers or have no “self” markers. One self marker is the Major Histocompatibility Complex (MHC) molecule. These molecules are critical to our immune responses because they will hold motifs (kind of a protein pattern) that allow other immune cells to recognize what is infecting the body. Because they need to bind to so many different kinds of proteins (they have to be able to display features of all the bacteria, viruses, fungi, etc. that invade our bodies), you can imagine that have different forms is a good thing for your immune system, because you can bind various forms of such proteins, instead of a smaller subset, which would allow you to react to a greater variety of things. Mammalian immune systems have this taken into account. MHC genes are some of the most diverse, or polymorphic, genes out there. This means that there are many, many forms of it throughout the population (don’t worry, they still work great!). Because you have two copies of DNA (one from your mother, one from your father), you get two copies of these MHC genes. This means that the more different your mom and dad’s MHC genes are, the greater variety of foreign proteins you can display on your MHC. This positive aspect of MHC genetic variety can be more greatly appreciated when you see a population where this variety doesn’t exist. This loops us back to the Tasmanian Devils.
Due to the massive population downsizing and isolation of a small population of the devils, they have a very limited variety of these MHC molecules. This observation is one of the major reasons why DFTD is so rampant. As I mentioned before, MHC molecules are self markers. The variety that we see in these molecules allows for a greater system of defining what “self” is compared to nonself. For example, if you have a signal of two letters for variety, there’s only 262 (676) options. So out of the 7.1 billion people on earth, your body will see 10.5 MILLION people’s cells are your cells. If you have a signal for ten letters of variety, there’s 2610 (140,000,000,000,000), so your body will only recognize your body’s cells are your cells.
When rejection occurs after an organ transplant, this is due to the acceptor’s body recognizing the MHC on the donor’s organ, thereby seeing it as nonself and attacking it. Although this isn’t good for a transplant, this keeps a lot of problems at bay. Unfortunately, due to their lack of variety of MHC genes, Tasmanian Devils do not have this ability. The cancer cells of the DFTD can be found on teeth and on lesions on infected devils’ faces. When they bite another devil, these cells are transferred into the wounds of the uninfected devil. If there were enough variety in the MHC genes of the devils, the newly infected devil would recognize the cancerous cells as nonself and would eliminate them, preventing the development of a facial tumor. However, because there is so little variety in the devil’s genes, the newly infected organisms do not recognize the cancerous cells as nonself, and instead see them as self. Self cells are not attacked by the system normally, so the cancerous cells stay, and develop into tumors. This perpetuates the cycle, leading to the cancer spreading throughout the population.
There are currently two other known transmissible cancers. One is a venereal tumor that affects dogs that has been spreading around the world for the past 11,000 years. The second was recently confirmed as a transmissible cancer—it is a soft-shell clam leukemia that has spread throughout the east coast of North America.
The extinction of the Tasmanian Devil is being driven by this transmissible cancer that the devils are unable to eliminate. However, without the initial downsizing of their population due to human predation, its highly probable that the population would have enough diversity of their MHC genes that this cancer would not have been able to even come about.