Contagious canine cancers
Tasmanian devils are one mammal that experiences these transmissible tumours. But another animal, probably one that's even more familiar to you, is one we all know and love, it's our best friend and that's dogs. They're also victims of a contagious cancer. Elizabeth Murchison from the Transmissible Cancers Group at the University of Cambridge told Chris Smith about what this transmissible tumour in dogs is…
Elizabeth - Dog transmissible cancer is quite different to the one Tasmanian devils. It's also spread between animals by the transfer of living cancer cells themselves. But in this case, the cells are spread during mating and this results in genital tumors. It's actually very, very common, although most people in the UK probably haven't heard of it. And that's because it's main reservoir is free roaming, sexually active, uncontrolled dog populations. And as we know, they don't occur in the UK anymore, but actually interestingly, this disease was first described in the scientific literature in the UK 200 years ago in London. So we know it was common here once, but it disappeared in the 20th century due to the control of dogs.
Chris - And what fraction of dogs worldwide have got it?
Elizabeth - It's extraordinary. About 1% of dogs worldwide in places where this is endemic, which is most places, have this disease so it's extraordinarily common. It's quite shocking to think how many dogs have this disease. But it's different to the Tasmania devil cancer in that it is not nearly as deadly. Dogs with this disease, they tend to have localised tumours which remain in the external genitalia. They don't metastasise or spread other parts of the body that frequently. And also the other wonderful thing is that this disease is highly curable with a course of simple chemotherapy. Most of these dogs, almost 100% of them will undergo a complete cure. And we think that that's actually something to do with the immune system rather than to do with the way that chemotherapy drugs act per se.
Chris - Hannah Siddle was telling us about the Tasmanian devils that there's evidence that in each case, because there were two diseases, they've originated from one individual. Is the same true with the dog tumour?
Elizabeth - Exactly. And that's the really fascinating thing about this dog cancer that all the tumors and all these dogs all around the world, they all carry the cells of one individual dog that first gave rise to this cancer. Extraordinarily, we think that that one dog lived about 6,000 years ago. That dog was a dog which got a cancer rather than that cancer dying together with its dog host, as most cancers do, those cancer cells spread to another host and from there that one single cancer lineage has survived for thousands of years and has spread all around the world. It's by far the most prolific and widespread cancer that we know of in nature, it's a really remarkable phenomenon.
Chris - How do you know it's 6,000 years old?
Elizabeth - Well, that's an estimate and it's based on counting mutations in the genome of this cancer. So as cancer cells - or actually indeed any cell - divides, it will accumulate mutations with time. So what we've been able to do is estimate how quickly cells of this dog cancer accumulate mutations. And we've done that by collecting tumours where we know the tumour was spread from one dog to another dog and we know when that happened. So by counting the number of mutations in these short intervals, we've been able to estimate how many mutations occur per unit time, and then by counting the number of mutations back to that original dog and applying this mutation rate, we've been able to come up with this estimate, which does have huge error associated with it. So we think around 6,000 years ago, give or take a few thousand years.
Chris - Why do you think that the Tasmanian devil tumour is a disaster for them, but in dogs as you've pointed out, it's eminently treatable and much more mild as a disease?
Elizabeth - Well, this is an interesting question because it does make you wonder whether thousands of years of evolution have potentially caused this dog cancer to adapt to being a less aggressive cancer. If you think about it, a cancer which comes in and kills its host is going to limit its own possibilities for onward transmission whereas a cancer that comes in and causes a fairly mild disease lives alongside its host for potentially years has many more transmission opportunities. So although we can't time travel back to that original cancer that first arose from that dog several thousand years ago to see whether it was actually more aggressive, it's a really interesting possibility that this cancer may have adapted to being less deadly.