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Deadly form of leukemia in soft-shell clams is contagious

By Karen Graham     Apr 10, 2015 in Science
For decades, a leukemia-like cancer has ravaged soft-shell clam populations along the east coast of North America. While thought to be caused by a virus, researchers found the cancer cells are actually contagious and surprisingly, easily spread.
Researchers from Howard Hughes Medical Institute and Columbia University have tackled and found the long-sought answer to the question of what has been causing the widespread loss of soft-shell clam populations around the world, and particularly along the eastern coast of North America.
It was first suggested that something in the environment may have triggered the spread of the leukemia-like disease, or perhaps something was making the clams more susceptible to the tumor cells. But surprisingly, it was DNA results that provided the answer.
It was discovered that the cancer cells themselves were contagious, and each one had the same origin. Somewhere, at some ill-fated point in time, a soft-shell clam had come down with the cancer, and it had grown and divided, making its way to other clams from New York to Maine and on up to prince Edward Island, Canada.
Stephen Goff, a Columbia microbiology and biochemistry professor, and one of the authors behind the findings said, “It was really wild. It was not what we were expecting.”
Goff explained the project, saying we typically think of cancer cells as a mutation of an organism's own cells. The cells arise in the body and start spreading out of control. But with the clam cells, it was different right from the start.
When Michael Metzger, a fellow Columbia researchers and study author, began analysing the cancer cells from the three different locations along the Atlantic Coast, he discovered the DNA of the cells was the same. They were clones.
“These tumours did not arise from any cells of the host animal,” Goff said. “They were all distinct from the genotype of the animals they were growing inside.”
What the scientists haven't been able to figure out so far is how the soft-shell clam cancer first arose, or where it arose. They do know the cancer cells can survive in salt water, at least long enough to reach another victim, although they are not quite sure what the mechanism might be that allows this to happen.
Graphic showing normal and leukemic clams.
Graphic showing normal and leukemic clams.
Goff et. al.
This latest research finding is important because it suggests that the transmission of cancer cells in the wild may be more widespread than previously thought. There are only two other known cases of a contagious cancer in the wild, one being Tasmanian devil “facial tumor disease," transmitted through a bite. The other is transmissible canine venereal tumors, thought to be spread by sexual contact.
Looking at the broader picture, this research can also expand our knowledge of how cancer cells metastasize, Goff says, The seeding process is maybe similar. It has the potential to teach us a lot about how cells can colonize in a new site.”
This study was published in the Cell Press journal Cell on April 9, 2015 under the title: "Horizontal Transmission of Clonal Cancer Cells Causes Leukemia in Soft-Shell Clams"
More about softshell clams, Leukemia, Contagious, east coast of north america, transmissible cancer
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