The spread of cancer's reach

The ocean contains a vast number of living things, including many pathogens — from bacteria that thrive on coral to fungi that infect lobsters. A drop of seawater can hold 10 million viruses. Recently, a team of scientists revealed a frightening member of this menagerie: free-floating cancer cells that cause contagious tumours in shellfish. Last year, they found one such cancer in a species of clam. Recently, they reported that three more species were plagued with contagious cancers. The cancers are specific to shellfish and do not appear to pose a danger to humans.

But until now, infectious cancer was considered something of a fluke in the natural world, initially observed only in dogs and Tasmanian devils. The latest research has made scientists wonder whether infectious tumours are actually more widespread. “We were always thinking there would be more contagious cancer out there, but we didn’t know where they would be discovered,” said Elizabeth P Murchison, a cancer biologist at the University of Cambridge in England who was not involved in the new study.

Two exceptions
In the traditional view of cancer, mutations strike a cell. These mutations have several causes, including toxins and viruses. However they arise, they drive a cell to multiply uncontrollably until the cancer is wiped out by the immune system or kills its victim. Either way, the cancer stays inside the body where it started. A decade ago, scientists discovered two exceptions.

In the 1990s, Tasmanian devils in Australia began developing deadly face tumours. Tasmanian facial tumour disease, as it was eventually called, appears to have had its start in a single Tasmanian devil that lived in the 1980s. Transmitted by bites, the cancer spread to other Tasmanian devils. Dogs, too, can get a type of contagious cancer called canine transmissible venereal tumour, which jumps from host to host during sex.

For years, Tasmanian devils and dogs were the only species known to contract contagious cancer. But last year, Stephen P Goff, a molecular biologist at Columbia University, and his colleagues found contagious cancer in soft-shell clams. From New York to Prince Edward Island, these clams have suffered from aggressive tumours since the 1970s. Carol Reinisch, a marine biologist at Environment Canada, found that the cancer clustered in populations, as if it was caused by an outbreak of some sort.

A single common ancestor
She suspected a cancer-causing virus moving from host to host. For help, Carol turned to Stephen. The two researchers found no evidence of a virus in the soft-shell clams. But they did discover that DNA in the tumour cells carried a genetic sequence not found in healthy cells in the clams. After examining the DNA, they confirmed that the cancer cells in different clams all came from a single common ancestor. “Somehow, this cancer has been spreading from clam to clam up the coast,” Stephen said.

He and his colleagues began to wonder if other species of clams or related animals had contagious cancers of their own, and, if so, why. They chose to study cancers in mussels, cockles and golden carpet shell clams. In every case, the researchers reported in the journal Nature, the cancers in the animals were contagious. As it turned out, the cancer in cockles comprises two separate strains.

Even stranger, the cancer cells in the golden carpet shell clams did not develop from the species’s own cells. Instead, the scientists matched the cancer’s DNA to pullet shell clams, which live in the same intertidal beds off the coast of Spain. But Stephen and his colleagues could not locate any pullet shell clams with this disease. They concluded that this strain of contagious cancer must have started in pullet shell clams and then jumped species, infecting golden carpet shell clams. It killed all the vulnerable pullet shell clams, leaving only resistant ones behind.

The new study, Elizabeth said, shows that contagious cancer can indeed cross the species barrier. So should people worry about an outbreak of infectious cancer? “I don’t think we should be starting to panic,” Elizabeth said. There have been rare reports of people transmitting cancer. An estimated 0.04% of organ transplant recipients contract cancer from the donor organ, for example. But in these cases, the cancer does not spread like a true parasite from host to host. Yet it’s not inconceivable that a human cancer might gain that power. Stephen and his colleagues are now trying to turn the contagious cancer in shellfish into a model for human cancers. They hope to find clues to metastasis. The evolution of a contagious cancer in some ways may mirror the evolution of a single tumour in the body. In shellfish, Elizabeth said, “we can see the effect of evolution on their genomes in a more pronounced way.”