[OANNES Foro] Humans have spread a contagious form of cancer afflicting mussels around the world

[Mario Cabrejos]

A cancer afflicting mussels originated off the Pacific coast of Canada, but
then crossed into other species in Europe and South America. 

Humans Shipped an Awful Cargo Across the Seas: Cancer

By  <https://www.nytimes.com/by/carl-zimmer> Carl Zimmer

Nov. 6, 2019.

https://www.nytimes.com/2019/11/06/science/contagious-cancer-mussels.html?ut
m_source=Nature+Briefing&utm_campaign=afb6bb4ab4-briefing-dy-20191111&utm_me
dium=email&utm_term=0_c9dfd39373-afb6bb4ab4-42095803

 

Humans have spread a contagious form of cancer around the world.

Researchers reported on Tuesday that the cancer, which invades mussels, has
spread across the Equator. Originating in one species in the Northern
Hemisphere, the cancer  <https://doi.org/10.7554/eLife.47788> has
established itself in another species in the Southern Hemisphere.

“There’s no natural explanation for how that happened without human help,”
said Michael Metzger, a biologist at the Pacific Northwest Research
Institute in Seattle and a co-author of the report, published in the journal
eLife.

In its familiar form, cancer arises when cells gain new mutations and begin
to multiply aggressively. But these lineages of cancer are short-lived. Some
of them are killed by our immune systems or drugs used to combat them. Other
cancers die when they kill their hosts.

In the 1990s, researchers discovered an exception: Tasmanian devils suddenly
began developing tumors on their faces. The DNA in the tumors did not match
that of the afflicted animals, and scientists deduced that the cancers were
coming from other devils. 

As it turned out, when Tasmanian devils bite one another, they may swallow
tumor cells that make their way to the faces of their new hosts.

Researchers later discovered that dogs also have a transmissible form of
cancer, which spreads when dogs mate. In a study published in August,
Elizabeth Murchison of the University of Cambridge in Britain and her
colleagues analyzed the genomes of dog cancer cells from around the world.

They concluded that the cancer  <http://dx.doi.org/10.1126/science.aau9923>
arose in a dog that lived several thousand years ago in Asia. The Tasmanian
devil facial tumors, by contrast, appeared within just the past few decades,
on two separate occasions.

While working as a postdoctoral researcher at Columbia University, Dr.
Metzger discovered that four species of shellfish — including soft-shell
clams and bay mussels — had transmissible cancers of their own. It was the
first time anyone
<https://www.nytimes.com/2016/06/28/science/contagious-cancer-clams.html>
had discovered contagious cancer in aquatic animals.

Dr. Metzger and his colleagues suspect that sick shellfish release cancer
cells. The cells float along the currents until they are sucked up by
healthy animals as they filter seawater for food. In one case, the
researchers found, the cancer cells had moved from one species of shellfish
into another.

Nicolas Bierne, a geneticist at the University of Montpellier in France who
studies blue mussels, realized that this finding perhaps held the solution
to a mystery that he had been struggling with.

Analyzing the DNA from blue mussels in France, he and his colleagues had
discovered some genetic markers that looked as if they belonged to another
species: bay mussels. He might have expected such a result if these species
interbred — but bay mussels don’t even live in French waters.

“It was a puzzle for me,” said Dr. Bierne. “Why do I sometimes find what
looks like a hybrid, but I never find one of the parent species?”

Cancer might be the answer. Dr. Metzger had found a contagious form of
cancer in bay mussels on the Pacific coast of Canada. Perhaps, Dr. Bierne
thought, his blue mussels had been attacked by bay mussel cancer cells.

He and his colleagues isolated cancer cells from French blue mussels and
analyzed their DNA. It turned out to be profoundly different from the DNA of
the healthy cells, more closely related to that of bay mussels.

Meanwhile, a team of South American researchers got in touch with Dr.
Metzger with another potential case of contagious cancer. On the coasts of
both Argentina and Chile, Nuria Vázquez of the Institute of the Biology of
Marine Organisms in Argentina and her colleagues found beds of mussels in
which as many as 13 percent of the animals were sick.

And when they looked at the cancers under a microscope, the cells had an
unusual rounded shape that Dr. Metzger had found in contagious cancers.
Analyzing the DNA, the researchers determined that the Chilean mussels had
contagious cancer, too. Once again, bay mussels were the source.

The biggest surprise of the research came when the scientists combined their
research on all three species of mussels from all three continents. The
cancer in the French blue mussels and the cancer in the Chilean mussels
turned out to be practically identical.

The scientists have put together a scenario to explain all their findings.

In the Pacific, a bay mussel developed cancer. The cancer then spread to
other bay mussels along the coast. Dr. Metzger and his colleagues have named
this lineage of cancer BTN1.

Independently, a second bay mussel developed contagious cancer of its own.
The scientists named this second strain BTN2.

BTN2’s genes show that it came from a bay mussel. But no one has found the
cancer in a bay mussel. It’s possible that bay mussels became resistant to
BTN2. But the cancer lives on, because it has managed to jump into both blue
mussels and Chilean mussels.

The exact route of transmission is still unknown, but the journey must have
been lengthy, since the Chilean mussels and blue mussels are separated by
several thousand miles. Dr. Metzger and his colleagues argue that the cancer
cells couldn’t have made the trip on their own. 

For one thing, ocean currents would have prevented the cells from crossing
the Equator. Instead, Dr. Metzger thinks, humans gave cancer cells a ride.
Mussels and other shellfish readily grow on the sides of ships. In some
cases, the ships have transported them to new ranges where they become
invasive species.

One of these voyages may have delivered cancer to a new home as well. “All
you’d need was one diseased animal,” Dr. Metzger said.

Dr. Murchison has also found signs of human influence in her study of dog
cancer. Canine transmissible venereal tumor, as the condition is known, was
limited at first to a small group of dogs somewhere in Asia.

But about 2,000 years ago, the cancer spread along the Silk Route. About 500
years ago, it popped up in the Americas, and in the 19th century it spread
to most of the world, including remote islands.

“It seems very likely it’s humans taking dogs on boats,” Dr. Murchison said.

Some scientists are confident that there are many more contagious cancers to
find in shellfish — and not just in shellfish.

“I’m starting to believe that transmissible cancers are much more common
than we have previously thought, particularly in aquatic ecosystems,” said
Beata Ujvari, an evolutionary biologist at Deakin University in Australia
who was not involved in the new study.

In fact, scientists may already have the evidence without realizing it.

Geneticists who find odd DNA sequences tend to dismiss them as contamination
or hybridization. The researchers may actually be looking at more cancers
that have broken free from their hosts.

“Perhaps we have to open our imaginations a bit more,” Dr. Murchison said.

 

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