Elizabeth Murchison: Fighting a contagious cancer

48,645 views ・ 2011-09-22

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Everyone's familiar with cancer,
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but we don't normally think of cancer
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as being a contagious disease.
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The Tasmanian devil has shown us
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that, not only can cancer be a contagious disease,
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but it can also threaten
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an entire species with extinction.
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So first of all, what is a Tasmanian devil?
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Many of you might be familiar with Taz, the cartoon character,
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the one that spins around and around and around.
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But not many people know
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that there actually is a real animal called the Tasmanian devil,
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and it's the world's largest carnivorous marsupial.
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A marsupial is a mammal with a pouch
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like a kangaroo.
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The Tasmanian devil got its name
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from the terrifying nocturnal scream that it makes.
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(Screaming)
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(Laughter)
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The Tasmanian devil is predominantly a scavenger,
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and it uses its powerful jaws
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and its sharp teeth
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to chomp on the bones of rotting dead animals.
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[The] Tasmanian devil is found only on the island of Tasmania,
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which is that small island
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just to the south of the mainland of Australia.
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And despite their ferocious appearance,
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Tasmanian devils are actually
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quite adorable little animals.
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In fact, growing up in Tasmania,
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it always was incredibly exciting
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when we got a chance to see
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a Tasmanian devil in the wild.
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But the Tasmanian devil population
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has been undergoing a really extremely fast decline.
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And in fact, there's concern
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that the species could go extinct in the wild
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within 20 to 30 years.
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And the reason for that
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is the emergence of a new disease,
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a contagious cancer.
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The story begins in 1996
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when a wildlife photographer took this photograph here
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of a Tasmanian devil
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with a large tumor on its face.
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At the time, this was thought to be a one-off.
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Animals, just like humans,
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sometimes get strange tumors.
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However, we now believe
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that this is the first sighting of a new disease,
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which is now an epidemic spreading through Tasmania.
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The disease was first sighted
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in the northeast of Tasmania in 1996
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and has spread across Tasmania like a huge wave.
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Now there's only a small part of the population,
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which remains unaffected.
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This disease appears first as tumors,
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usually on the face or inside the mouth
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of affected Tasmanian devils.
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These tumors inevitably grow into larger tumors,
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such as these ones here.
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And the next image I'm going to show
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is quite gruesome.
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But inevitably, these tumors progress
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towards being enormous, ulcerating tumors like this one here.
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This one in particular sticks in my mind,
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because this is the first case of this disease
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that I saw myself.
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And I remember the horror of seeing this little female devil
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with this huge ulcerating, foul-smelling tumor
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inside her mouth
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that had actually cracked off her entire lower jaw.
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She hadn't eaten for days.
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Her guts were swimming with parasitic worms.
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Her body was riddled with secondary tumors.
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And yet, she was feeding three little baby Tasmanian devils
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in her pouch.
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Of course, they died along with the mother.
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They were too young to survive without their mother.
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In fact, in the area where she comes from,
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more than 90 percent of the Tasmanian devil population
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has already died of this disease.
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Scientists around the world
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were intrigued by this cancer,
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this infectious cancer,
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that was spreading through the Tasmanian devil population.
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And our minds immediately turned to cervical cancer in women,
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which is spread by a virus,
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and to the AIDS epidemic,
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which is associated with a number of different types of cancer.
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All the evidence suggested that this devil cancer
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was spread by a virus.
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However, we now know -- and I'll tell you right now --
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that we know that this cancer is not spread by a virus.
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In fact, the infectious agent of disease in this cancer
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is something altogether more sinister,
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and something that we hadn't really thought of before.
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But in order for me to explain what that is,
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I need to spend just a couple of minutes
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talking more about cancer itself.
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Cancer is a disease
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that affects millions of people around the world every year.
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One in three people in this room
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will develop cancer at some stage in their lives.
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I myself had a tumor removed from my large intestine
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when I was only 14.
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Cancer occurs
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when a single cell in your body
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acquires a set of random mutations in important genes
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that cause that cell to start to produce
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more and more and more copies of itself.
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Paradoxically, once established,
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natural selection actually favors
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the continued growth of cancer.
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Natural selection is survival of the fittest.
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And when you have a population of fast-dividing cancer cells,
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if one of them acquires new mutations,
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which allow them to grow more quickly,
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acquire nutrients more successfully,
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invade the body,
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they'll be selected for by evolution.
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That's why cancer is such a difficult disease to treat.
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It evolves.
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Throw a drug at it,
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and resistant cells will grow back.
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An amazing fact is that,
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given the right environment and the right nutrients,
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a cancer cell has the potential
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to go on growing forever.
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However cancer is constrained
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by living inside our bodies,
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and its continued growth,
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its spreading through our bodies
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and eating away at our tissues,
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leads to the death of the cancer patient
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and also to the death of the cancer itself.
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So cancer could be thought of
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as a strange, short-lived, self-destructive life form --
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an evolutionary dead end.
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But that is where the Tasmanian devil cancer
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has acquired an absolutely amazing
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evolutionary adaptation.
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And the answer came from studying
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the Tasmanian devil cancer's DNA.
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This was work from many people,
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but I'm going to explain it through a confirmatory experiment
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that I did a few years ago.
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The next slide is going to be gruesome.
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This is Jonas.
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He's a Tasmanian devil that we found
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with a large tumor on his face.
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And being a geneticist,
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I'm always interested to look at DNA and mutations.
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So I took this opportunity
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to collect some samples from Jonas' tumor
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and also some samples from other parts of his body.
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I took these back to the lab.
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I extracted DNA from them.
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And when I looked at the sequence of the DNA,
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and compared the sequence of Jonas' tumor
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to that of the rest of his body,
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I discovered that they had a completely different genetic profile.
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In fact, Jonas and his tumor
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were as different from each other
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as you and the person sitting next to you.
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What this told us was that Jonas' tumor
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did not arise from cells of his own body.
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In fact, more genetic profiling
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told us that this tumor in Jonas
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actually probably first arose
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from the cells of a female Tasmanian devil --
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and Jonas was clearly a male.
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So how come
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a tumor that arose from the cells of another individual
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is growing on Jonas' face?
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Well the next breakthrough
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came from studying hundreds of Tasmanian devil cancers
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from all around Tasmania.
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We found that all of these cancers
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shared the same DNA.
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Think about that for a minute.
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That means that all of these cancers
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actually are the same cancer
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that arose once from one individual devil,
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that have broken free of that first devil's body
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and spread through the entire Tasmanian devil population.
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But how can a cancer spread in a population?
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Well the final piece of the puzzle came
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when we remember how devils behave when they meet each other in the wild.
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They tend to bite each other,
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often quite ferociously and usually on the face.
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We think that cancer cells
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actually come off the tumor, get into the saliva.
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When the devil bites another devil,
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it actually physically implants living cancer cells into the next devil,
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so the tumor continues to grow.
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So this Tasmanian devil cancer
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is perhaps the ultimate cancer.
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It's not constrained by living within the body that gave rise to it.
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It spreads through the population,
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has mutations that allow it to evade the immune system,
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and it's the only cancer that we know of
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that's threatening an entire species with extinction.
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But if this can happen in Tasmanian devils,
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why hasn't it happened in other animals
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or even humans?
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Well the answer is, it has.
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This is Kimbo.
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He's a dog that belongs to a family in Mombasa in Kenya.
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Last year, his owner noticed some blood
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trickling from his genital region.
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She took him to the vet
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and the vet discovered something quite disgusting.
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And if you're squeamish, please look away now.
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He discovered this,
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a huge bleeding tumor
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at the base of Kimbo's penis.
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The vet diagnosed this as transmissible venereal tumor,
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a sexually transmitted cancer
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that affects dogs.
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And just as the Tasmanian devil cancer is contagious
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through the spread of living cancer cells,
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so is this dog cancer.
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But this dog cancer is quite remarkable,
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because it spread all around the world.
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And in fact, these same cells
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that are affecting Kimbo here
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are also found affecting dogs
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in New York City,
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in mountain villages in the Himalayas
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and in Outback Australia.
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We also believe this cancer might be very old.
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In fact, genetic profiling tells
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that it may be tens of thousands of years old,
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which means that this cancer
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may have first arisen
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from the cells of a wolf
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that lived alongside the Neanderthals.
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This cancer is remarkable.
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It's the oldest mammalian-derived life form that we know of.
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It's a living relic
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of the distant past.
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So we've seen that this can happen in animals.
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Could cancers be contagious between people?
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Well this is a question
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which fascinated Chester Southam,
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a cancer doctor in the 1950s.
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Ad he decided to put this to the test
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by actually deliberately inoculating people
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with cancer from somebody else.
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And this is a photograph of Dr. Southam in 1957
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injecting cancer into a volunteer,
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who in this case was an inmate
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in Ohio State Penitentiary.
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Most of the people that Dr. Southam injected
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did not go on to develop cancer
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from the injected cells.
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But a small number of them did,
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and they were mostly people who were otherwise ill --
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whose immune systems were probably compromised.
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What this tells us,
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ethical issues aside,
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is that ...
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(Laughter)
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it's probably extremely rare
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for cancers to be transferred between people.
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However, under some circumstances,
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it can happen.
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And I think that this is something
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that oncologists and epidemiologists
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should be aware of in the future.
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So just finally,
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cancer is an inevitable outcome
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of the ability of our cells
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to divide
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and to adapt to their environments.
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But that does not mean that we should give up hope
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in the fight against cancer.
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In fact, I believe, given more knowledge
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of the complex evolutionary processes that drive cancer's growth,
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we can defeat cancer.
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My personal aim
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is to defeat the Tasmanian devil cancer.
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Let's prevent the Tasmanian devil
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from being the first animal
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to go extinct from cancer.
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Thank you.
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(Applause)
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