Scientists discover ancient cancer that can SPREAD like a virus among shellfish – as they

A transmissible cancer that spreads like a virus may sound like part of the plot of an apocalyptic movie – but it is a reality in the animal kingdom.

A study published last week found two strains of an ancient form of leukemia-like cancer that has been spreading silently among shellfish for centuries.

The transmissible tumor cells float freely in the water like microscopic bacteria before being taken up by clams, multiplying and replicating in their host before escaping to attack others.

This mode of spread is similar among Tasmanian devils, which pick up cancer cells when they bite and fight each other, and in dogs that pass it from one another through mating.

Contagious cancer has been considered a fluke in the natural world, occurring in a limited number of animal species. But the latest discovery in shellfish raises the possibility that there are more forms of cancer like it out there that pose a potential threat to humans.

As far as scientists know, only in extremely rare circumstances can cancers be passed on in humans. For example, there have been a handful of cases of mothers passing cancer on to their babies during pregnancy. 

Researchers from the UK, Spain, France, Poland, Belgium, Portugal, Norway, and Korea sampled nearly 7,000 cockles at 36 locations from 11 countries and genetically sequenced 61 cockle tumors. They were then able to show two different strains of a transmissible leukemia-type cancer

Researchers sequenced genetic data for cockles, a clam-like mollusk, and tumor cells inside them that had permeated their gills and circulated through their bloodstream, before killing their hosts and traveling through the water looking for their next victims

The latest research sequenced tumor genes sampled across 7,000 cockles, edible mollusks similar to clams, at three dozen locations across 11 countries including Spain, Portugal, the UK, Ireland, and Morocco between 2016 and 2021.

The type of cancer specifically is bivalve transmissible neoplasia (BTN), which enters through the gills and spreads throughout the mollusk’s circulatory system.

British and European researchers led by Dr Adrian Baez-Ortega, a biologist at the UK’s Wellcome Sanger Institute mapped out the DNA of entire cancer cells as well as the animals harboring them in order to pinpoint the hundreds of thousands of genetic mutations that have occurred over hundreds of years.

Through tracking the cancerous tumor cells’ genetic transformation, the researchers discovered wild inconsistencies that suggest this type of cancer in mollusks is unlike any other transmissible cancer in animals.

Some tumor cells had more or fewer chromosomes than others, the result of generations of cell division and other abnormal changes in the genetics of the cancer cells. 

While a normal cockle cell has 38 chromosomes, they found tumor cells with as few as 11 chromosomes and as many as 354.

This type of chromosomal instability is believed to have helped the BTN lineage, which researchers think originated from a single mutant cell within one individual bivalve, survive centuries of genetic chaos.

Dr Alicia Bruzos, co-author and molecular researcher at the Universidade de Santiago de Compostela in Spain, said: ‘We clarified the existence of two independent transmissible cancers, and we suspect that there are many more different types out there.

‘Having a wider view of the different types of transmissible cancers can give us more insight into the conditions necessary for tumors to evolve and survive long-term.’

Their finding – that an ancient cancer can thrive in an animal population through years of genetic overhaul – runs counter to the general understanding that cancer cells can only replicate, mutate, and spread within a larger organism that has a genetic makeup that has undergone relatively few DNA changes over time.

Too many changes to the environment in which the cancer lives would typically prove lethal to the cancer cell, according to Dr Baez-Ortega.

Apart from mollusks, the only other animals that have been found to be affected by transmissible cancer are dogs, which transmit it through mating and biting, and Tasmanian devils, which pass along tumor cells growing on their faces when other devils bite them.

But in those species, their complete sets of chromosomes have remained virtually unchanged over time – even the transmissible cancer in dogs which cropped up for the first time 11,000 years ago.

Dr Daniel Garcia-Souto, another co-author and researcher at the Universidade de Santiago de Compostela, said: ‘Our study showed that the cells in these cockle tumours contain highly variable amounts of genetic material, which is very unusual compared to other types of cancer.

Tasmanian devils are one of two other species, in addition to domesticated dogs, known to be susceptible to a contagious cancer. The tumors grow on their otherwise adorable faces. When two devils fight, one of them will bite down on the tumor on one of their faces. The tumor cells then enter the other devil through wounds on its body

‘These cancers have been undergoing extreme chromosomal changes and continuous genetic reorganisation, probably for hundreds or thousands of years, which challenges the theory that cancers require stable genomes to survive long-term.’

The fact that the transmissible neoplasia spreads like a bacteria through water to infect the gills of mollusks suggests that there might be undiscovered rules about the way cancer invades the human body.

Dogs and Tasmanian devils have relatively small gene pools, which allows cancers to take a stronger hold because the hosts remain largely unchanged. But human DNA is far more varied thanks to a long history of human migration and evolution that has endowed humans with cancer-fighting immune systems.

But the latest findings from Wellcome Sanger Institute suggest that even genetically diverse species such as humans could one day see some versions of a transmissible leukemia-like cancer currently felling mollusks.

There have been some instances of transmissible cancers in humans, but only in medical or research settings when the skin’s protection was breached. For instance, a surgeon removing a tumor from a patient accidentally cut his hand and, five months later, there was a tumor growing there genetically linked to his patient.

The team’s findings were published in the journal Nature Cancer.