Genetics: clams open up secrets of cancer in the wild

An outbreak of leukaemia has devastated clam populations along the east coast of North America. Now a genetic study has found that the disease is spread directly by cancer cells moving from one mollusc to the next. This suggests that directly transmissible cancers may be more common in nature than scientists had suspected.

A team from Columbia University and Environment Canada analysed soft-shell clams - a favourite American seafood also known as steamers and little necks - from New York, Maine and Prince Edward Island. They wanted to understand the cause of the fatal leukaemia, first noted in the 1970s, which has hit clam harvests hard.

Many experts had suspected that a virus was transmitting the disease but searches for an infectious agent were fruitless. Now it turns out that cancer cells themselves are contagious, spreading between clams in seawater.

The DNA analysis, published in the journal Cell, found that cancerous cells collected from clams along 1,000km of coastline were genetically almost identical to each other - and quite distinct from the genome of their host molluscs.

"We were astonished to realise that the tumours did not arise from the cells of their diseased host animals but from a rogue clonal cell line spreading over huge geographical distances," says Stephen Goff of Columbia, the project leader. In other words, the leukaemia that has killed so many molluscs over such a wide area originated more than 40 years ago with a mutation in one clam; the cancerous cells then divided, broke free and survived in seawater for long enough to lodge in other clams.

Only two other examples of transmissible cancer are known in the wild: a canine tumour transmitted by sexual contact between dogs, and a facial tumour spread between Tasmanian devil marsupials by biting.

"Natural horizontal transmission of cancer between individuals has been considered rare and restricted to two exceptional cases in mammals," the Cell paper concludes. "Our finding extends the phenomenon to the marine environment and demonstrates that this mechanism is more widespread in nature than previously supposed."

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