Contagious cancer: the allograft your dog doesn’t want

Can one creature directly give another creature cancer? It appears so in dogs and that cancer is highly contagious.

When we think about transplants of cells, we think about hospitals and high-tech equipment used to give patients stem cells or other types of cells to help treat them for a disease. Generally cancer patients are excluded from being donors during transplants because there is some concern that a recipient of a transplant from a cancer patient, even in remission, could contain cancer cells. If the donor and recipient are a match, even a few or in theory as little as 1 cancer cell from the donor might not be recognized as foreign by the recipient’s immune system, especially if the patient is ill or immunocompromised. As a result the patient might get cancer from the donor so generally cancer survivors cannot be donors.

While in humans, as far as we know, this kind of event is rare and isolated to the hospital transplant setting, the situation in dogs appears quite different.  There is a particular kind of cancer in dogs, called canine transmissible venereal tumor (CTVT), which is contagious and is transmitted as an allograft during sex. What this means is one dog with cancer can “infect” another by transmission of cancer cells.  Not a virus, but actual cells.

Austin Burt’s group has just published a paper in Science on CTVT that comes to some remarkable conclusions.

Perhaps most amazing is the argument that CTVT is in fact a single immortal living “organism” that has spread through dogs around the world and may be as old as 10,000 years. When the researchers analyzed the DNA of many different CTVT tumors from different dogs, the tumors were not similar to the dogs in which they grew, but rather were more similar to each other.

One exception to this was that the tumor mitochondrial DNA had some similarities to the host, suggesting that the CTVT tumor cells in essence steal mitochondria from their hosts to keep their “power supply” high.

An article in the NY Times on this subject mentioned that scientists have also found a contagious facial cancer in Tasmanian Devils, which appear to transmit the disease during fights via bites to the face.

Fortunately, as far as scientists know, contagious cancer does not exist in humans. Thank goodness. A fascinating question is why?

Just as interesting is the question of how CTVT evades the canine immune systems of those that it colonizes?

In terms of the mitochondria finding, one might ask if in the human transplant setting, if a patient receives a stem cell transplant, could those stem cells also “steal” mitochondria from the recipient?

Also, we know that in mice that transplanted stem cells can fuse with host cells to make abnormal new hybrid cells that sometimes are tetraploid. Will this happen in the human setting?

There is an enormous amount that we do not know about when cells from two individuals are mixed in a living being. As the number of stem and non-stem cell transplants likely explodes in the coming decades, these issues are of critical importance.

More immediately, pets including dogs are already receiving stem cell-based transplants at an increasing rate and it is possible that dogs or other pets might have other contagious cancers that could be transmitted between donor and recipients.