The idea that cancer could behave like a contagious disease may seem absurd; in humans, cancer cannot be transmitted from one person to another. Yet nature is still full of surprises. In exceptional cases, cancer cells themselves can pass from one individual to another and continue to multiply in a new host. This rare phenomenon has been detected in animals such as dogs, Tasmanian devils, and some species of mollusks, and it seems we can add one more to that exclusive club: catfish.
An international team of researchers has identified a transmissible melanoma in the brown catfish (Ameiurus nebulosus)—the first documented case in fish and also the first recorded in a freshwater ecosystem. Their findings were reported this month in the journal Nature.
An Impossible Pattern
The story began in 2012, when fishermen and biologists detected an unusually high number of catfish with black lesions in Lake Memphremagog, a body of water straddling Vermont and Quebec. Previous studies revealed that these spots were malignant melanomas, and that between 23 percent and 37 percent of the specimens examined had these tumors—an unusually high incidence for this species.
“That was surprising,” said Julie Dragon, a researcher at the University of Vermont and co-lead author of the study. “We wanted to know how a bottom-dwelling fish was getting a cancer we associate with exposure to too much sunlight.”
Initially, it was thought that the cause might be some toxic substance or a pathogen linked to pollution. One suspicion was that the flooding caused by Tropical Storm Irene in 2011 had degraded water quality by washing environmental pollutants into the lake. There were also concerns about the health of the lake, which supplies drinking water to more than 175,000 people. However, initial genetic analyses began to point in an unexpected direction.
It was then that the researchers wondered whether the tumors were descended from a single original cancer that had learned to spread among individual fish, rather than arising independently in each fish.
To test this unusual hypothesis, the team sequenced the complete genomes of tumors and healthy tissues from affected fish, and compared the data with that from healthy specimens from different populations.
The results indicated that the tumor cells from different fish were much more closely related to one another than to the animals in which they developed. In other words, the tumors shared their own genetic identity, distinct from that of their hosts. Furthermore, hundreds of thousands of genetic variants appeared repeatedly in the tumors but were absent from the healthy tissues of the same fish.
Such a pattern would be virtually impossible if each melanoma had arisen independently. By way of comparison, the researchers analyzed hundreds of human melanomas and found that nearly all of their mutations were unique to each patient. In contrast, most of the mutations present in the tumors of these fish were shared by numerous individuals—a hallmark of a transmissible clonal cancer.