In the 1970s, soft shell mussels began mysteriously dying out in Maine and the Chesapeake Bay. Years later, scientists identified the culprit. It was a strange form of cancer that spread like an epidemic.
When a person develops cancer, it usually occurs when some of their own cells mutate and grow out of control. However, the clams were being invaded by floating cells that came from other clams. The alien cancer cells multiplied within the bodies of new victims, and some of their descendant cells then escaped and attacked other shellfish.
It turns out that other types of shellfish are victims of contagious cancers as well. Now, researchers have discovered that these cancer cell lineages have jumped from one shellfish to the next over centuries, perhaps even millennia, and in the process have revealed an astonishing number of discovered that they had discovered a mutation.
Adrian Báez Ortega, a computational biologist at Britain’s Wellcome Sanger Institute and author of one of two studies on the cancer published Monday, said: “These cancers are It is biologically inconceivable that this phenomenon is occurring.” “Yet they live longer.”
In 2015, researchers sequenced short DNA fragments from cancer cells taken from softshell turtles in Canada, Maine, and New York. Genetic analysis revealed that the cancer cells did not originate from the animal in which they originated. Instead, they were all related to each other and descended from a single ancestral cell.
Until then, researchers were only aware of contagious cancers in two mammalian species: the Tasmanian devil and dogs.
Tasmanian devil cancer forms tumors on the marsupial’s face. When animals bite into each other during fights or mating, cancer cells can become attached. The disease wiped out 90 percent of the entire species.
In contrast, dogs can develop fairly benign cancers that spread during mating. These cells form growths around the genitals, and the dog’s immune system usually destroys the invader within a few weeks.
The discovery of a contagious cancer in soft-shelled mussels has spurred research into other shellfish. So far, scientists have published details about eight more types of contagious cancers, including mussels and cockles.
“There’s going to be a lot more to come. There’s going to be a lot more that we know and probably a lot more that we don’t know,” said Michael Metzger, a biologist at the Pacific Northwest Research Institute in Seattle. .
In recent years, Dr. Metzger and his colleagues have Catalog all mutations This happens because the cancerous cells detach from the shellfish they came from and become contagious. Dr. Baez-Ortega and his colleagues conducted a similar study on: common cocklesfound along the Atlantic coast of Europe.
Instead of sequencing small pieces of DNA taken from cancer cells, the researchers sequenced the entire genome, not just the animal’s genome. Scientists then compared DNA taken from both healthy and diseased cells in the animals and were able to find hundreds of thousands of mutations that occur in the contagious cancer.
Certain cancer cells had common mutations not found in other cancer cells. This pattern reveals how they descended from a common ancestor and their family tree branched out. Dr. Metzger’s team found that the soft-shelled turtle tree has two branches, one linked to cancer cells around Prince Edward Island and the other linked to cancer cells found off the northeast coast of the United States. I discovered that there is.
Dr. Metzger and his colleagues looked at the number of mutations accumulated in different branches to estimate how long ago the original ancestral cancer cell was released. They estimate that it became contagious more than 200 years ago, or perhaps centuries ago.
Dr. Báez-Ortega and colleagues concluded that cockle cancer is similarly old, but estimates were not available. “They are probably thousands of years old,” he said.
In both species, cancer may have started as immune cells that mutated and multiplied. Those cells were then released into the water, taken up by another shellfish, and grew again like a cancer. Eventually, the cancer cells acquired mutations that allowed them to survive in water for several months until they found a new host.
Studies on Tasmanian devils and dogs have revealed that their cancerous DNA remains relatively unchanged. In the case of the Tasmanian devil, this finding is less surprising. The Tasmanian devil probably contracted cancer just 40 years ago.But the dog got cancer 11,000 years ago. And all the while, cancer cells make only small changes to their genomes.
In contrast, cancer cells in both clams and cockles have repeatedly undergone dramatic changes. Some cancer cells ended up with extra chromosomes, sometimes hundreds of chromosomes. Some people have lost long stretches of their DNA. In some cases, the entire genome has been duplicated.
“This level of instability is usually lethal to cancer cells,” Dr. Baez-Ortega said. Neither he nor Dr. Metzger can explain how a contagious cancer has survived for centuries in this state of genetic confusion.
Beata Ujvari, an evolutionary ecologist at Australia’s Deakin University who was not involved in the study, said the large-scale mutations may be explained by the way contagious cancers reproduce. Instead of combining her two sets of DNA from shellfish eggs and sperm, Cancer clones herself.
In that sense, they have become more like bacteria than animals. And, like bacteria, they may be trying to outcompete other cancers by mutating faster, Dr. Ujvari said. He pointed out that new research on cockles reveals that two different contagious cancers can invade the same animal.
Dr. Metzger hopes that by solving this puzzle, he and other scientists may be able to discover hidden rules of cancer that apply not only to shellfish but also to humans.
It may be possible to find new targets for drugs by focusing on parts of the genome that are altered in cancer cells. He is also examining the genomes of shellfish to see if they have evolved new ways to resist invasive cancers.
“Nature has basically been conducting large-scale experiments,” Dr. Metzger says. “If there is a way that animals have evolved resistance to cancer, I would like to know what it is.”