The genome of this 11,000-year-old cancer carries about two million mutations -- many more mutations than are found in most human cancers, the majority of which have between 1,000 and 5,000 mutations. The team used one type of mutation², known to accumulate steadily³ over time as a "molecular⁴ clock," to estimate that the cancer first arose 11,000 years ago.

 

"The genome of this remarkable⁵ 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," says Dr Elizabeth Murchison, first author from the Wellcome Trust Sanger Institute and the University of Cambridge.

 

The genome of the transmissible dog cancer still harbors the genetic⁶ variants⁷ of the individual dog that first gave rise to the cancer 11,000 years ago. Analysis of these genetic variants revealed that this dog may have resembled an Alaskan Malamute or Husky. It probably had a short, straight coat that was colored either grey/brown or black. Its genetic sequence could not determine if this dog was a male or a female, but did indicate that it was a relatively⁸ inbred（近亲交配的） individual.

 

"We do not know why this particular individual gave rise to a transmissible cancer," says 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 spawned⁹."

 

Transmissible dog cancer is a common disease found in dogs around the world today. The genome sequence has helped scientists to further understand how this disease has spread.

 

"The patterns of genetic variants in tumors from different continents suggested that the cancer existed in one isolated¹⁰ population of dogs for most of its history," says Dr Murchison. "It spread around the world within the last 500 years, possibly carried by dogs accompanying seafarers on their global explorations during the dawn of the age of exploration."

 

Transmissible cancers are extremely rare in nature. Cancers, in humans and animals, arise when a single cell in the body acquires mutations that cause it to produce more copies of itself. Cancer cells often spread to different parts of the body in a process known as metastasis. However, it is very rare for cancer cells to leave the bodies of their original hosts and to spread to other individuals. Apart from the dog transmissible cancer, the only other known naturally occurring transmissible cancer is an aggressive transmissible facial cancer in Tasmanian devils that is spread by biting.

 

"The genome of the transmissible dog cancer will help us to understand the processes that allow cancers to become transmissible," says Professor Sir Mike Stratton, senior author and Director of the Sanger Institute. "Although transmissible cancers are very rare, we should be prepared in case such a disease emerged in humans or other animals. Furthermore, studying the evolution of this ancient cancer can help us to understand factors driving cancer evolution more generally."

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