The transmission of cancer between humans is an anomalous phenomenon, but it is not in the other branches of the evolutionary tree. It is something we have known for a long time, but now a group of experts has set out to study this phenomenon in depth to understand how cancer spreads within the body of the same person. They have just taken an important step: sequencing the cancer genome.
Unstable genetics. After analyzing thousands of cockles (Cerastoderma edule) captured on the coasts of 11 different countries, an international team of biologists has carried out an extensive study of the transmissible cancer that affects these animals. What they observed was immense chromosomal instability throughout the different tumor samples analyzed throughout the study.
Transmissible bivalve neoplasia. The cancer analyzed in these animals is bivalve transmissible neoplasia or BTN, a type of cancer that affects the cells of the immune system of these cockles, similar in some ways to leukemia in humans. Eight types of BTN have been documented, two of which were present in the populations analyzed.
Researchers believe that this cancer comes from a specimen that lived tens or hundreds of thousands of years ago and that has been transmitted through generations of bivalves. Despite this, the team observed notable variability between tumor cells
A reflection of this variability is the number of chromosomes present in these cells. They found tumor cells with 11 chromosomes to cells with 354 chromosomes. For reference, a common cockle cell has 38 chromosomes.
7000 cockles. The researchers analyzed 7,000 specimens of the bivalve from 36 points on the coasts of 11 countries, including Spain, Portugal, Ireland and Morocco. They sequenced the genetics of 61 tumors in which they found two varieties of BTN. Details of the study have been recently published in an article in the journal Nature Cancer.
“We have clarified the existence of two independent transmissible cancers, and we suspect that there are more different types out there. Having a more extensive view of the different types of transmissible cancers can give us insight into the conditions necessary for tumors to evolve and survive in the long term,” Alicia Bruzos explained in a press release.
Millennial. And this form of cancer transmission implies that these tumors, despite their genetic variability, come to maintain the same unit since the tumor was developed for the first time. It would act like an organism that expands in the body of a cockle until one of its cells breaks off and reaches another to expand again.
This phenomenon has been replicated in vitro in human cancers, notably in the HeLa cell line, obtained in 1951 from a patient who would die months later, Henrietta Lacks. This cell line remains alive in dozens of laboratories throughout the planet. Curiously, this cell line served (through an experiment of dubious ethics) to verify the extent to which human tumors are not very contagious.
“We have some estimates that suggest that the age of this tumor is between 100,000 and half a million years, but this requires a more rigorous study,” José Tubío, who heads the group of researchers responsible for the study, explained to El País. “It is probably the oldest known transmissible cancer.”
A long list. Transmissible cancers are (as far as we know, which is little) more the exception than the rule. So far we have detected eleven: in addition to the eight that affect bivalves, we are aware of two that affect the Tasmanian devil (Sarcophilus harrisii) and another that affects dogs.
Why the interest in a cancer that does not affect humans? In medical terms, cockle cancer has no implications for humans, but in economic and ecological terms it can, at least if the disease spreads in places where these animals are fished or if it ends up putting this species at risk.
Beyond that, the work developed by this team can help us better understand the genetic variability in human cancers. Furthermore, it could help us fight against metastasis, the form of “contagion” with which tumors spread in our body and very closely linked to the mortality associated with these diseases.
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