cancer as an infectious disease

The New York Times just posted a very interesting piece (by Erica Rex) on cancer in the population of Tasmanian Devils (yes they are real) in Australia. The Devils have recently been added to Australia's endangered species list because of a widespread cancer that is devastating the population.

Although seeing a new species added to the endangered species list is, unfortunately, sort of a routine and utterly not-fantastic event today, this particular species is decline is especially fascinating (and frightening) from a biological perspective. This is because the cancer devastating the Devils is working as an infectious disease, spreading throughout the population.

In humans, cancer is a product of a person's own cells dividing out of control. Typically, cells in your body are stopped from dividing ad infinitum by a series of cell cycle regulating molecules, and typically cancer is a result of malfunctions in any of a variety of these regulatory molecules. From this perspective, then, cancer has really very little to do with a person's immune system, and cancers in humans are not known to be at all infectious, but are rather largely genetic.

The cancer taking out Tasmanian Devils is different, however. As a brief review of the immune system, your body uses a set of molecules called the MHC (for major histocompatibility complex). These are little markers on the outside of all cells which work like a language to figure out which cells are supposed to be in our body and which aren't, and then other cells in your immunse system digest the stuff that's not supposed to be there.

It's a great system, until something learns to work around it. It is the misfortune of the Devils to be a sort of experiment in evolution, where certain cancerous cells have evolved to not have any of the cellular markers that tell the body that it is foreign, or from another organism. This means that this cancer can begin to act as an infectious disease, spreading between individuals.

Typically, if cancerous cells from another human were to invade your body, your immune system would quickly recognize these cells as foreign, mark and digest them, and the cancer would not be able to proliferate within your body. In the Devils however, this is no longer true, and a single cancer has begun to spread throughout the population. Amazingly, this entire cancer is probably the product of a mutation in one cell in one tasmanian devil, and this cell has now begun to divide out of control and spread throughout an entire population. Frightening, but very cool.