The source of a cancer that affects dogs around the world has been traced by scientists and vets at UCL (University College London) to a single wolf or dog, which probably lived in China or Siberia more than 250 years ago. In canine transmissible venereal tumour (CTVT), the cells of the tumour itself are transmitted between dogs during sex. No equivalent form of contagious cancer exists in humans, but the new findings challenge current thinking about the nature of cancer.
Some human cancers such as cervical cancer may be considered to be 'catchable', as they are initiated by viruses transmitted between people – in the case of cervical cancer, by certain types of papilloma virus. What is unusual about CTVT in dogs, however, is that no virus is involved – the cancer itself is effectively passed on.
In a paper published in the journal Cell, veterinarian researcher Dr Claudio Murgia conducted forensic DNA tests on tumour tissues from 16 dogs affected by CTVT. The dogs were being treated for the cancer by vets in Italy, India and Kenya who provided the biopsies. He found that in all cases, the tumours were genetically different from the affected dog – in other words, the cancer had come from a different dog. A further analysis of 40 tumours archived in vet labs in five continents showed that the tumours were genetically almost identical and demonstrated that CTVT originally came from a single source and has since spread across the globe.
To trace this source, the UCL team worked with geneticists and computer experts in Chicago and compared the DNA in the tumours to that in specific dog breeds. They found that the cancer most likely first arose in either a wolf or an 'old' Asian dog breed such as a Husky or Shih Tzu. The number of mutations accumulated in the DNA also enabled the researchers to obtain a rough estimate the age of the disease, which came out at around 1,000 years and not less than 250 years old.
CTVT is a serious but seldom fatal disease. Unless the dogs are already in poor condition, the tumours usually regress three to nine months after their appearance, leaving the dogs immune to re-infection. But that leaves enough time for the dogs and bitches to pass the tumour on through further sexual encounters. The tumour is rarely seen in pedigree dogs because they are not allowed casual sex, but it is relatively common in strays. It does not occur in the UK, because quarantine regulations (for rabies) effectively screened it out.
Professor Robin Weiss of the UCL Division of Infection and Immunity, who led the research team, says: "It appears that man's best friend can be its own worst enemy. Our study shows that CTVT has become a parasite that has long outlived its original host.
"Our discovery is of much broader significance than simply a disease in dogs. Firstly, CTVT represents the longest-lived cancer 'clone' known to science. It contradicts the current view that cancer cells generate more and more mutations and inevitably become more aggressive if untreated.
"Secondly, recent research in Australia has revealed the existence of a newly emerged tumour in Tasmanian Devils that also appears to be caused by transmissible cancer cells, in this case by biting. Devils are an endangered marsupial species and there are fears that the new tumour might finally kill them off altogether. The methods used at UCL for dogs could help to determine whether the Devil tumour is also a 'parasitic' cancer.
"Thirdly, our findings also show that cancer cells can evade immune responses and CTVT is particularly smart in this regard. On rare occasions cancer cells have been transmitted from one human to another by hiding in organ transplants. Because the recipient is treated with immunosuppressants in order to prevent rejection, the transferred cancer cells can then grow into tumours just like CTVT. That is why people who have suffered from cancer should not become organ donors."
Journal
Cell