Dogs are vulnerable to a sexually transmissible cancer. Terrible as that is, the cancer itself is fascinating from a biological perspective, and treatments have proven surprisingly effective. Indeed, they have worked so well that their success could point towards ways to tackle tumors in humans or other cancer types in our best animal friends.
Normally, when people talk about transmissible cancers, it's a sloppy way of referring to something like cervical cancer, where an infectious disease, in that case the human papillomavirus, can trigger cell changes that eventually lead to cancer. However, eight types of tumor cells are known to be infectious themselves. Fortunately, none of these infect humans. Besides dogs, five occur in marine bivalves and two in Tasmanian devils.
Anything so rare has to be at least a little interesting, and these cancers have drawn attention from scientists for reasons besides the need to save the devil from extinction. The fact that the canine transmissible venereal tumors (CTVT) sometimes spontaneously regress is intriguing scientists even more since no other transmissible cancer does this.
"We found that activation of the innate immune system and production of certain molecules called chemokines by the host tissue around the tumor is critical to attract immune cells within the tumor and trigger a chain reaction that leads to the rejection of the cancer and its elimination," said Professor Ariberto Fassati of University College London in a statement.
Fassati is senior author of a paper in Cancer Cell investigating these regressions, which also often are triggered by chemotherapy. After collecting biopsies from CTVTs before treatment with the chemotherapy drug vincristine, and six and 14 days afterward, Fassati compared gene activity in regressing tumors with those that were expanding.
The paper reveals an inflammatory response after vincristine treatment, with the chemokine CCL5 being released to attract immune cells to attack the tumor. There is an accompanying production of skin cells that may serve to contain the cancer, leading Fassati to conclude that the reaction of the host cells is more important than what goes on within the tumor.
Stimulating the body's defenses against cancers has been a dream for a while, and early-stage trials against ovarian cancer are promising, but the example of a situation where it happens naturally could refine our approaches. Fassati hopes that by focusing on the healthy tissue around tumors, and stimulating signaling proteins to attract immune cells, it may be possible to produce treatments for human cancers that are more powerful and lower on side-effects than existing options.
CTVT has been known since the 1800s and was recently shown to have existed for 11,000 years, with genes from host dogs incorporated at least five times in that period. Although described as a venereal disease, it can be transmitted by dogs licking or biting each other, as well as through sex.