My latest Mind and Matter column for the Wall Street Journal is on the strange phenomenon of contagious cancer in dogs and Tasmanian devils, and whether it could happen to us. Elizabeth Murchison is speaking about this at the TED Global meeting in Edinburgh next week.
The human body is a teeming city of cells, each working selflessly for the whole. But as the body grows older, the chances increase that one cell might suddenly breach the social contract and begin selfishly to grow and divide at the expense of its fellow cells. This is cancer.
Mercifully, cancer is self-destructive: It dies when the body dies. Although the cause of cancer can be contagious-some viruses cause it-the tumor itself cannot be caught from somebody else. Or can it?
There are now two epidemics raging in the world caused not by microbes but by immortal lines of cancer cells themselves, setting up home in new victims before old ones die. One is found in dogs and is transmitted sexually, causing cancers of the genitals. The other began to spread 15 years ago among Tasmanian devils-marsupial carnivores that once lived throughout Australia but have been confined to Tasmania for the past four centuries.
Devil Facial Tumor Disease, which has spread from east to west across the island, causes tumors to grow on the faces of the devils, killing them within six months. It's now threatening the survival of the species in the wild. Yet the tumors are not derived from the cells of the infected animals. Tell-tale genetic markers reveal that they all descend from a tumor that appeared about a decade and a half ago in a long-dead devil.
In effect, this cancer has found a way to survive the death of its host. Because Tasmanian devils bite each other on the face when fighting over food and mating, the cancer spreads from face to face, growing into the body of the bitten individual and commandeering its blood supply, like Barbary pirates preying upon a Mediterranean city.
How does the tumor evade the immune system of a new host? One theory, advanced by Stephan Schuster of Pennsylvania State University, is that the devils are so inbred, having colonized Tasmania in small numbers more than 10,000 years ago, that they cannot reject each other's tissue.
But this may not be the whole story. In the case of the dog disease, it seems that the tumor has also evolved a way to suppress the activity of MHC genes, which signal individual identity and resist foreign tissue. There is also some evidence that the cancer originated in a wolf and can affect many breeds of dog and even related species. Perhaps the MHC system-the bane of the organ-transplant field-evolved in the first place as a defense against invasion by contagious cancers.
Elizabeth Murchison of the Wellcome Trust Sanger Institute in Cambridge, England, reckons that this kind of contagious cancer could affect other species. It requires a combination of genetic mutations in the tumor, to evade immune rejection, with a violent habit on the part of the host species that injects tissues into other individuals.
A physiological peculiarity of sex in dogs-that pairs cannot disengage for some time after intercourse-frequently results in genital wounds, especially to the male. Hence the opportunity for a genital cancer to spread.
Dr. Murchison raises the disturbing possibility that the tumors may even alter the behavior of the dogs to make them more interested in sex, or the behavior of the devils to make them keener to bite. Viruses can certainly do this, rabies being a prime example.
Luckily we human beings do not regularly wound each other with our teeth. If we did, we might be vulnerable to contagious cancers.
There have been rare cases in the past where organ donors have given cancers to recipients, or surgeons have cut themselves while operating on cancer victims and have then grown the patients' tumors within their own bodies. But the outbred genetic diversity of our species and the absence of habits or rituals of biting prevent such cases from starting an epidemic.