Contagious cancer found in North American catfish

The idea of ​​that Cancer can act like an infectious disease that may seem silly; In humans, cancer cannot be transmitted from one person to another. However, nature is still full of surprises. In exceptional cases, the cancer cells themselves can spread from one individual to another and continue to multiply in a new host. This rare phenomenon has been discovered in animals such as dogs, Tasmanian devils, and some species of mollusks, and it seems we can add another species to this exclusive club: catfish.

An international team of researchers has discovered a transmissible skin cancer in brown catfish (Amurus nebulosus) – The first documented case in fish and also the first recorded case in a freshwater ecosystem. Their findings were published this month in the journal nature.

Impossible pattern

The story began in 2012, when anglers and biologists discovered an unusually large number of catfish infected with black lesions in Lake Memphremagog, a body of water that straddles Vermont and Quebec. Previous studies revealed that these spots were malignant melanomas, and that between 23% and 37% of the samples examined had these tumors, an unusually high percentage for this type.

“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 bottom-dwelling fish get the cancer we associate with exposure to too much sunlight.”

Initially, it was thought the cause might be a toxin or pathogen linked to pollution. One suspicion was that flooding from 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 provides drinking water to more than 175,000 people. However, preliminary genetic analyzes are beginning to point in an unexpected direction.

The researchers then wondered whether the tumors descended from a single original cancer that learned to spread among individual fish, rather than arising independently in each fish.

To test this unusual hypothesis, the team sequenced the entire genomes of tumors and healthy tissues from affected fish, and compared the data with that in healthy samples from different populations.

The results indicated that cancer cells from different fish were more closely related to each other than to the animals in which they developed. In other words, tumors share their own genetic identity, which differs from that of their hosts. Furthermore, hundreds of thousands of genetic variants appeared repeatedly in tumors but were absent from healthy tissue of the same fish.

Such a pattern would be almost impossible if each melanoma had arisen independently. By comparison, the researchers analyzed hundreds of human melanomas and found that almost all of their mutations were unique to each patient. In contrast, most of the mutations found in these fish’s tumors were shared among many individuals, a hallmark of a clonal, transmissible cancer.