Outbreaks of leukemia that have devastated some populations of soft-shell clams along the east coast of North America for decades can be explained by the spread of cancerous tumor cells from one clam to another. Researchers call the discovery, which was featured as the cover story in the April 9, 2015 issue of Cell, "beyond surprising." The article is titled “Horizontal Transmission of Clonal Cancer Cells Causes Leukemia in Soft-Shell Clam.” "The evidence indicates that the tumor cells themselves are contagious--that the cells can spread from one animal to another in the ocean," said Dr. Stephen Goff of the Howard Hughes Medical Institute and Columbia University. "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, the cancer that has killed so many clams all trace to one incidence of disease. The cancer originated in some unfortunate clam somewhere and has persisted ever since as those cancerous cells divide, break free, and make their way to other clams. Only two other examples of transmissible cancer are known in the wild. These cancers 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 cancer in clams, Dr. Goff and his colleagues found that a particular sequence of DNA (which they named Steamer) was found at incredibly high levels in leukemic versus normal clam cells. While normal cells contain only two to five copies of Steamer, cancerous clam cells can have 150 copies. The researchers at first thought that this difference was the result of a genetic amplification process occurring within each individual clam.
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