Leukemia-like diseases known as disseminated neoplasia (DN) were reported in many bivalve species in the twentieth century even though the contagious nature of some of these cancers was not established until 2015. DN has been recently reported to be contagious in seven species of the American, Asian, and European coasts.

Contagious cancers occur when cancer cells spread between individuals. Currently, contagious cancers are known to naturally-occur in dogs, Tasmanian devils, and various species of mussels, clams, and cockles. A recent study of clams in Spain shows how these cancer cells can infect individuals of other clam species in two seas in southern Europe.

“Genomic studies in recent decades have transformed cancer biology because it is now possible to determine in which individual a cancer cell originated. “

Cancer occurs when a cell in the body suffer genetic changes that cause inappropriate cell proliferation. Once started, cancer evolves and some of these cells spread throughout the host’s body, serving as a seed for new tumors. This process is known as metastasis.

However, sometimes cancer cells do not travel to other parts of the host’s body but can go further by infecting another individual by transferring these cancer cells from the organism that originated them to a new host. These are clonal transmissible cancers, also known as contagious cancers, and they are sort of “large-scale metastasis”.

Most of the known contagious cancers can only spread between individuals of the same species; however, some contagious cancers that affect marine bivalves can sometimes infect a different species than the one that caused the cancer.

A new case of contagious cancer

Recently, warty venus clams (Venus verrucosa) from six countries on the southern coast of Europe were collected and screened (Figure 1).

Cancer contagion on clams

A cancer called disseminated neoplasia (DN) was detected in the hemolymph of clams from two Spanish locations: Mahon in the Balearic Islands of the Mediterranean Sea and Ribeira on the Galician coast of the Atlantic Ocean.

A genetic study confirmed that it is the same contagious cancer that is affecting warty venus clams on the two locations, that is, cancer cells found in warty venus clams of the Atlantic and Mediterranean Seas have the same origin.

“These locations are separated by more than 1,069 nautical miles. How is it possible that a contagion of cancer occur?”

Two hypotheses are considered: either transmissible marine cancers move using ocean currents to colonize new clams from other regions; or they can be inadvertently introduced by human action

Researchers have not found genetic variation between cancer cells from the Atlantic and the Mediterranean Sea what means the contagion occurred in the recent times.

Crossing the species barrier

An unexpected finding of this study was that the DNA extracted from warty venus clams showed genetic matches with the DNA of two species: the expected one, the warty venus clam, and a second unexpected one, the striped venus clam (Chamelea gallina) (Figure 2).

Cancer contagion on clams

The analysis of mitochondrial DNA and several nuclear genes revealed that the DNA of the cancer cells belongs to a striped venus clam in which this cancer originated, which is now parasitizing specimens of the warty venus clam.

However, striped venus clams, which share habitat with warty venus clams in the Mediterranean, have not yet been diagnosed with disseminated neoplasia. Perhaps the striped venus clam has adapted to resist infection by the contagious cancer being the first evidence in a member of its own species; despite this, the cancer has survived by grafting onto a new host species.

A potential ecological threat

The risk of cancers that do not respect individual or even species barriers highlights the importance of identifying and monitoring these contagious diseases to prevent a possible ecological disaster for these species overall when humans move individuals from one place to another.

This is a summarized version developed by the editorial team of Aquaculture Magazine based on the review article titled “MITOCHONDRIAL GENOME SEQUENCING OF MARINE LEUKEMIAS REVEALS CANCER CONTAGION BETWEEN CLAM SPECIES IN THE SEAS OF SOUTHERN EUROPE” developed by: GARCÍASOUTO, D.-Universidade de Santiago de Compostela y Wellcome Sanger Institute, BRUZOS, A.L.- Universidade de Santiago de Compostela; DÍAZ-COSTAS, S.- Universidade de Santiago de Compostela; ROCHA, S.-Universidade de Vigo; PEQUEÑO, A.- Universidade de Santiago de Compostela; ROMAN-LEWIS, C.- Universidade de Vigo; ALONSO, J.-Universidade de Vigo; RODRIGUEZ, R.-Universidade de Vigo; COSTAS, D.-Universidade de Vigo; RODRIGUEZ-CASTRO, J.- Universidade de Santiago de Compostela; VILLANUEVA, A.-Universidade de Vigo; SILVA, L.-Instituto Español de Oceanografía; VALENCIA, J.- Laboratori d’Investigacions Marines i Aqüicultura e Instituto de Investigaciones Agroambientales y de Economía del Agua; ANNONA, G.- Stazione Zoologica Anton Dohrn; TARALLO, A.- Stazione Zoologica Anton Dohrn; RICARDO, F.-University of Aveiro; BRATOS CETINIC, A.- University of Dubrovnik; POSADA, D.-Universidade de Vigo; PASANTES, J.J.- Universidade de Vigo; TUBÍO, J.M.C.-Universidade de Santiago de Compostela.
The original article was published on JANUARY, 2022 through eLife under the use of a creative commons open access license.
The full version can be accessed freely online through this link: https://doi.org/10.7554/eLife.66946

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