Photo by Michael J. Metzger
Results of a new study indicate that leukemia may be contagious—at least in clams.
Researchers found that outbreaks of leukemia in soft-shell clams along the east coast of North America can be explained by the spread of cancerous cells from one clam to another.
“The evidence indicates that the tumor cells themselves are contagious—that the cells can spread from one animal to another in the ocean,” said Stephen Goff, PhD, of Columbia University in New York, New York.
“We know this must be true because the genotypes of the tumor cells do not match those of the host animals that acquire the disease, but instead all derive from a single lineage of tumor cells.”
In other words, a leukemia that has killed many clams can be traced to one incidence of disease. The cancer originated in some unfortunate clam and has persisted ever since, as those cancerous cells divide, break free, and make their way to other clams.
Dr Goff and his colleagues described this discovery in Cell.
The researchers noted that only 2 other examples of transmissible cancer are known in the wild. These include the canine transmissible venereal tumor, transmitted by sexual contact, and the Tasmanian devil facial tumor disease, transmitted through biting.
In early studies of the leukemia in clams, Dr Goff and his colleagues found high levels of a particular sequence of DNA they named “Steamer.” While normal cells contain only 2 to 5 copies of Steamer, leukemic cells can have 150 copies. The researchers initially thought this difference was the result of a genetic amplification process occurring within each individual clam.
But when the team analyzed the genomes of leukemia cells collected in New York, Maine, and Prince Edward Island, they discovered something else entirely. The cancerous cells they had collected from clams living at different locations were nearly identical to one another at the genetic level.
“We were astonished to realize that the tumors did not arise from the cells of their diseased host animals, but rather from a rogue clonal cell line spreading over huge geographical distances,” Dr Goff said.
The results showed the cells can survive in seawater long enough to reach and sicken a new host. It is not yet known whether this leukemia can spread to other molluscs or whether there are mechanisms that recognize the malignant cells as foreign invaders and attack them.
Dr Goff said there is plenty the researchers don’t know about this leukemia, including when it first arose and how it spreads from one clam to another. They don’t know what role Steamer played in the cancer’s origin, if any. And they don’t know how often these sorts of cancers might arise in molluscs or other marine animals.
But the findings do suggest that transmissible cancers are more common than anyone suspected.
