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.
