The idea that cancer may behave like an infectious disease, it may seem absurd; in humans, cancer cannot spread from one person to another. However, nature is still full of surprises. In exceptional cases, the cancer cells themselves can pass from one individual to another and continue to multiply in the up-to-date host. This uncommon phenomenon has been detected in animals such as dogs, Tasmanian devils and some species of molluscs, and it seems we can add one more to this exclusive club: catfish.
An international team of scientists has identified an infectious melanoma in brown catfish (Better shadowy) — the first documented case in fish and the first recorded in a freshwater ecosystem. Their research was published this month in the journal Nature.
An impossible pattern
The story began in 2012, when fishermen and biologists detected unusually huge numbers of catfish with black markings in Lake Memphremagog, a body of water that straddles Vermont and Quebec. Previous research has shown that these spots are malignant melanomas and that 23 to 37 percent of samples tested had these tumors, an unusually high rate for this species.
“It was surprising” he said Julie Dragon, a researcher at the University of Vermont and co-author of the study. “We wanted to know how bottom-dwelling fish develop cancer, which we associate with exposure to too much sunlight.”
Initially, it was thought that some toxic substance or pathogen related to the contamination might be the cause. One suspicion was that flooding caused by Tropical Storm Irene in 2011 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, initial genetic analyzes began to point in an unexpected direction.
It was then that researchers wondered whether the tumors originated from a single primary tumor that had learned to spread from one fish to another, rather than arising independently in each fish.
To test this unusual hypothesis, the team sequenced the complete genomes of tumors and fit tissue from diseased fish and compared the data with data from fit specimens from different populations.
The results showed that the cancer cells in the different fish were much more closely related to each other than to the animals in which they developed. In other words, the tumors shared a common genetic identity that was distinct from that of the hosts. Moreover, hundreds of thousands of genetic variants repeatedly appeared in the tumors, but were not present in fit tissues of the same fish.
Such a pattern would be virtually impossible if each melanoma arose independently. For comparison, the researchers analyzed hundreds of human melanomas and found that almost all of their mutations were unique to each patient. However, most of the mutations found in these fish cancers occurred in multiple individuals, which is a hallmark of infectious clonal cancer.