What a tumor that is transmitted between cockles across the sea teaches us about cancer
There is a type of cancer that, like a parasite, is transmitted between cockles across the Atlantic Ocean.
It is a disease similar to leukemia. In it, the tumor cells are able to escape from the animal that houses them and, through the sea, reach a new specimen and cause a new tumor.
A team of researchers from the CiMUS of the University of Santiago de Compostela led by José Tubío, has managed to genetically sequence for the first time these two classes of tumors transmissible between cockles that “represent a paradigm in the world of cancer” and whose study “could reveal relevant questions about how transmissible cancers in particular and cancer in general evolve,” as Tubío explains.
The work, the details of which are published in the latest issue of Nature Cancer, is part of the ERC Starting Grant SCUBA CANCERS project, which started in 2017 to study the genetic causes of this cancer capable of being transmitted through the sea between bivalves.
“In 2017 we sampled the entire European coast, from Morocco to Russia, in search of these tumors. We found infected animals in Spain, Portugal, France, England and Ireland, while the most northern countries of the European continent seem, in principle, to be free of this disease.. In total, almost 7,000 cockles were studied, and about 400 individuals infected with this parasitic tumor were identified,” says Tubío.
With the infected animals in hand, the researchers proceeded to study their genomes with the aim of “studying the genetic causes of the disease and investigating questions about its origin.”
To do this, first, scientists had to obtain the reference genome of the cockle, “a brutal effort” that was not completed until 2020.
The analysis of the genetic sequences of the infected animals first revealed that the disease they had observed in cockles in different parts of the Atlantic were actually two types of cancer “that had arisen independently throughout history.” .
They are not transmitted to humans
“We have not been able to determine the exact longevity of these tumors, but the data lead us to conclude that, without a doubt, they are ancient tumors,” one of the oldest known cancers, which arose thousands of years ago and carry all that time being transmitted through the sea, says Tubío, who emphasizes that this type of tumors cannot be transmitted to humans and only spread among susceptible cockles.
The most surprising finding for the team of scientists was that these cockle tumors have a very unstable and unstructured genome.. Thus, they found that cancer cells within a single tumor contain very different numbers of chromosomes, something that is not observed in other contagious cancers.. According to Tubío, contagious cancers were recently discovered thanks to advances in the field of genetics.. Currently, contagious cancers are only known in dogs, Tasmanian devils, and several marine species.
In this case, the tumor cells had a great variability of chromosomes and a very altered structure.
The researcher José Tubío, in the laboratory.
For example, according to the work, some cells contained only 11 chromosomes and others up to 354, while the number of chromosomes in healthy cells of a normal cockle is always 38 (16 pairs of chromosomes).
This is surprising, the Galician team indicates in a press release “since human cancer cells cannot survive high levels of chromosomal instability, although moderate levels often make tumors more likely to spread to other organs and become resistant to treatment”.
“We believe that these structural mutations are the secondary events that have caused the tumor to evolve.”. This last question represents one of the most important points of the work: how is it possible that a tumor that has such a damaged genome could survive for thousands of years? This question has no answer yet and will be the objective of future research,” comments the CiMus researcher.
The work also suggests that these contagious cancers, originating in the hemolymph tissue, the 'blood' of cockles, have slowly spread through European cockle populations, accumulating diverse mutations and occasionally capturing mitochondria from host cells as replacements. of their own when damaged.
“When I considered the project, I considered studying these contagious metastases in water, because at the end of the day they behave like metastases, which can help us better understand in general how cancer works and how metastasis works,” says Tubío.
“Once the project is concluded, I believe that one of the most interesting questions to continue investigating is genomic instability,” the researcher highlights.
“How is it possible that a tumor with such a tremendously degenerated genome can survive for thousands of years?” he asks.
“In the short term, the clearest impact of the research is in veterinary science, in merchandise control and monitoring of these parasitic tumors on the coasts,” but it opens many avenues to continue investigating the survival and evolution capacity of tumors, he concludes.