An inbred dog that lived about 11,000 years ago was ground zero for a form of canine cancer that spread via mating and is afflicting dogs around the world to this day, according to new research in the journal Science.
The cancer-canine transmissible venereal tumor (CTVT) is one of only two known transmissible cancers. The other is found among afflicted Tasmanian Devils. CTVT causes genital tumors and can be fatal, particularly in older dogs or those suffering from an immune system deficiency.
The cancer spread, in part, because it wasn’t always fatal. If afflicted dogs died before mating, the disease wouldn’t spread across generations.
To track down the cancer’s source, Elizabeth Murchison, from the Wellcome Trust Sanger Institute and the University of Cambridge, and her colleagues sequenced the genome for CTVT. It is now believed to be the world’s oldest continuously surviving cancer.
“The genome of this remarkable long-lived cancer has demonstrated that, given the right conditions, cancers can continue to survive for more than 10,000 years despite the accumulation of millions of mutations,” Murchison said in a press release.
Remarkably, the genome of the cancer carries at least 2 million mutations. This is many more than in most human cancers. (Most of those have anywhere from 1,000 to 5,000 mutations.) For the study, the researchers used one type of mutation, known to accumulate steadily over time as a “molecular clock,” to estimate that the cancer first arose 11,000 years ago.
The genome also contains clues about what the prehistoric, ground-zero dog looked like. The researchers think it resembled an Alaskan malamute or husky. It likely had a short, straight coat that was grey/born or black.
The genetic sequence could not determine if the dog was male or female, but the scientists could determine that this dog was inbred. That might have led to the cancer arising in the first place, but the first appearance of the disease is still a mystery at this point.
“We do not know why this particular individual gave rise to a transmissible cancer,” said Dr Murchison. “But it is fascinating to look back in time and reconstruct the identity of this ancient dog whose genome is still alive today in the cells of the cancer that it spawned.”
She thinks the cancer first existed in an isolated population, before rapidly spreading around the world in recent centuries.
“It spread around the world within the last 500 years,” she explained, “possibly carried by dogs accompanying seafarers on their global explorations during the dawn of the age of exploration.”
“The genome of the transmissible dog cancer will help us to understand the processes that allow cancers to become transmissible,” said Sir Mike Stratton, senior author of the paper and director of the Sanger Institute. “We should be prepared in case such a disease emerged in humans or other animals.”
Photo: Alaskan husky. Credit: Randy Hausken, Wikimedia Commons