What makes a cancer contagious?

Question

Most human cancers are not (very?) contagious (perhaps besides a couple of incidents). But the Tasmanian devil seems to have a form of cancer which is contagious. Now what makes the difference between a contagious cancer and a non contagious cancer in the DNA mutations of a cell?

Answer 1

Tasmanian devils can spread a facial cancer by biting and fighting other individuals that have the cancer. This cancer takes hold and can evade detection by the host immune system. Some cancers are caused by viruses but there is no virus involved in the Tasmanian devil facial cancer. Right now scientists are not sure why this cancer is so contagious, as general studies show that grafting cancer from one individual to another, in multiple types of animals, results in rejection of the foreign cancer, and Tasmanian devils have a normal immune response to other foreign invaders. This cancer has some specific mutations that help it to evade the immune system very effectively, but we don't have a solid grasp of what those mutations are.

Answer 2

You are correct that cancers in humans are not contagious unless the causal factor is a virus wherein if the virus is transmitted from person to person it will increase the risk of developing cancer (not a definitive, but increased risk--this requires other hits possibly to the genome). There are two known contagious cancers in Tasmanian devils DFT1 and DFT2 and spread occurs when devils come into contact to allow for the transfer of tumor cells.

Note: if you want to create a parallel scenario in humans think about metastasis. Cancer cells from lung cancer for example can access the blood stream and travel to other organs and start multiplying in bone or liver etc. Its not actually a bone cancer but it has the nutrients and resources to grow there.

Now back to the devils: generally when foreign cells (or any material really) is introduced into a new system (e.g. transfer from one animal to another via a bite) the immune system should be able to tag it as foreign and destroy it. Currently research suggests the reason why this doesn't happen in the case of tasmanian devils has to do with downregulation of immune cell surface molecules called MHC complexes.

In terms of whether the actual mutations are different between contagious and non-contagious cancers, the knowledge is incomplete. The actual process and types of mutation are not, but their downstream effects like evading the immune system might be.