Dr. Andy

Reflections on medicine and biology among other things

Tuesday, May 31, 2005

A cool disease with a cool name

Devil facial tumor disease (DFTD)is, unfortunately, devastating the Tasmanian devil population.

Devil facial tumor disease is grotesque; the mother of Rosie and her brothers died when grotesque tumors ballooned out of her face and neck, choking off her ability to eat. It is also an extraordinary puzzle. Scientists do not understand its cause, mode of transmission, time from infection until the tumors appear, or potential to infect others.


Here is the cool part (from a scientific perspective, not cool if you are a Tasmanian devil): DFTD is apparently a tumor which is passed from devil to devil.

While I wouldnt go as far as the article and say:

Their current best guess breaks all the rules of modern biology.

the idea of a transmissable cancer is pretty odd.

To consider why, let's step back and think about why cancer isn't generally transmissable in humans. The reason is that immune markers on cells vary from person to person, particularly one specialized kind of marker that is used to present antigens to T cells. These are called Major Histiocompatability Complex (MHC) molecules. They come in 2 basic flavors, I and II, and are expressed on every cell in the body. When cells with foreign MHC enter the body, they are recognized as intruders by the immune system and killed off. This is one of the reason organ transplantation is so problematic. Depending on the organ, MHC matching is often helpful and patients require profound immune suppression to prevent rejection of the organs (or bone marrow). So any cancer cells that got transmitted from one person to another would almost certainly be killed off by the recipient's immune system

There are rare exceptions to this such as twins who both develop the same leukemia while in utero and this case in which a surgeon operating on a patient with an unusual tumor injured his hand and then developed a cancer at the site of injury, which was excised with apparent cure. Investigation showed the tumors from the patient and surgeon were genetically identical. The MHC alleles varied between surgeon and patient so it isn't clear why the surgeon's immune system didn't kill off the tumor.

It seems that something similar is happening in Tasmanian devils. The devil life style is, shall we say, conducive to spread of tumors, particularly those on the face:

Devil sex turns up the volume. In March and April, males engage in vicious, blood-soaked combat, said Dr. Menna Jones, a wildlife biologist who also works in the environment department. Females select "big butch dudes," Dr. Jones said, and allow themselves to be dragged by the scruff of the neck into a burrow. There they scream and fight for several days, mating many times for hours at a time. At the end of such bouts, the male thrusts his sperm into the female every two minutes.


So we can see how the tumor might be spread, particularly since it affects the face.

It is a bit hard to tell from the article, but it sounds like they have done fairly sophisticated analysis of tumor cells from various affected devils and they are all genetically identical (tumor cells often have rearrangements of the DNA that lead to increased growth) which is pretty good evidence for the transmission of the tumor itself.

The article goes on to speculate that Tasmanian devils have only small differences in genes, perhaps due to a recent population "bottleneck" when there were only a few living devils that then expanded into a larger population. Since all devils descended from those few devils, they lack genetic diversity, meaning they may not recognize the tumor cell as foreign. In addition, tumor cells often develop strategies to avoid detection by the immune system (more formally, mutations that prevent immune recognition are favored by increased survival in the mini-evolution of the tumor) and this tumor may have been particularly succesful at such invasion.

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At 1:04 AM, Blogger xenobiologista said...

About your post on Devil Facial Tumour Disease, I'm taking a course on cell biology =P and this was briefly discussed in class, as well as another transmissible cancer. Apparently the idea of the cancer cell itself being the aetiological agent isn't that weird - there's been something called Canine Transmissible Venereal Tumour - ewww - known for ages (since the 19th century!) Search PubMed, there are heaps of papers on it.

In the case of the canine tumour, Murgia et al. found that MHC class I expression is much lower in tumour cells and MHC class II is absent, that's why newly infected dogs take a while to reject them as nonself. Dogs aren't an endangered species so the low genetic diversity model doesn't apply to them =D

By the way, you should turn on Blogger's option to make commentors verify that they're human, as you seem to be getting an awful lot of comment spam.

 
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