An old wolf's DNA could be living on in a world-wide tumour.Aflo / naturepl.comA contagious form of dog cancer that is transmitted by sex has been traced back to its probable origins: a single wolf or dog that lived in Asia more than 200 years ago.
The disease seems to have been more aggressive in its past, the researchers say. This is unusual — most cancers become worse over time. If we could work out how and why the disease became less deadly, it may help in finding cancer treatments.
Most cancers are formed when an organism's own cells grow out of control. But canine transmissible venereal tumor (CTVT) is spread by tumor cells that move from dog to dog during sex. The disease attacks the face and genitals, but usually clears up within months.
Researchers have suspected that the diseased cells originated in a single animal. The cancer now affects dogs in Japan, the United States, Europe, China, the Far East, the Middle East and parts of Africa.
Claudio Murgia of University College London (UCL) and his colleagues examined the DNA of tumor cells from 16 unrelated dogs being treated for CTVT in Italy, India and Kenya. "We can tell that the tumor didn't belong to the dog because it's genetically different from its host," says team member Robin Weiss, a virologist at UCL.
To find out where the tumours came from, the team analysed more than 400 dogs from 85 different breeds. The cancer's DNA was most similar to modern wolves. There was also a link to Asian dogs such as shitzus.
To calculate when the tumour parted ways from its original host, the researchers counted the genetic differences between wolf and cancer. The tumour turns out to be at least 200 years old. "If it is any older than 250 years than it's the oldest cancer known to mankind," says Weiss. The work is published in Cell1.
Tamed tumour
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Examination of dog cancers from the past 30 years, which were frozen and collected from seven different countries, showed that the tumour was once much more aggressive. If we can work out how this happened, says Murgia, we might be able to cause the same transformation in human tumours.
When the current version of CTVT infects a new dog, it secretes a chemical that inhibits the immune system, so that the host cannot fight it off. But after a few months, the dog's immune system can usually oust the intruder.
Other transmissible cancers are nastier. A contagious facial tumour is now ravaging the Tasmanian devil (Sarcophilus harrisii) population, killing 90% of some populations. Tasmanian devils are very inbred, so the cancer may be more similar to its hosts than CTVT, making it harder for the immune system to recognise the invader.
Contagion does not seem to be a major source of human tumours. People can catch cancer from tumorous organ transplants if their immune system is weak, and, theoretically, a patient with AIDS could catch cancer through sex, although this has never been shown to happen.
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Professor of Virology at the University College of London
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