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So we're going to talk about prions
today and prion diseases which is a

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fascinating subject and one,
again, of potential medical

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significance.  We'll see how it
plays out in time,

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but there's a great deal of concern
out there about prions and the

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diseases that they cause.
Before we do, I mentioned last time

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at the end of the lecture about the
hope that we can develop vaccines

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for HIV.  And there's a great deal
of effort going on in this country

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and around the world to do that.
That would clearly be the best

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thing to do, to prevent HIV rather
than trying to treat it.

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And there are a number of
strategies that are underway.

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Some have been tested in animal
models of the disease.

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So far nothing has worked.
So there are no vaccines or

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effective vaccines for HIV.
And this just gives you a list of

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what's being tried.
And I put this up in part to remind

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you of the traditional vaccine
strategies.  More or less everything

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that can be tried is being tried for
HIV, including the development of

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what we call attenuated strains.
Strains of the virus which, in

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theory at least,
should be able to replicate but

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might not be pathogenic.
We can take the virus,

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cut out some essential genes.
It will still replicate but it

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won't cause disease.
In theory that would be helpful.

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One could also take the inactivated
HIV approach.  Take some live virus,

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kill it either with heat or with,
say, formaldehyde,

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and use that as the immunogen.
Once could take the component

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vaccine strategy,
that is using recombinant DNA

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technology, purify particular bits
of HIV like the envelope

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glycoproteins,
GP120 or GP41, and inject those in

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the hopes that somebody makes an
antibody response against those.

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One could use heterologous vaccines
like the old vaccinia smallpox

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example.  Because,
in fact, there is a related virus to

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HIV called SIV.
It infects monkeys so it's called

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simian immunodeficiency virus.
It's quite similar.  And it's been

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tried.  Again,
not so far successfully,

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but it's been tried as a potential
immunogen for HIV.

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And finally recombinant vaccines.
To put HIV genes into some other

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virus like vaccinia in the hopes
that, again, the virus will

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replicate and make some HIV proteins
to which the body makes an immune

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response which might be protective.
These have not worked.  And there

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are certain problems that have held
things up.  Firstly is safety.

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There's a lot of worry out there
that if any of these strategies

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aren't fool-proof,
you might actually expose somebody

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to a dangerous virus.
Let's say you think you're

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attenuating HIV but you're really
not or you think you're killing HIV

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but you're really not.
That might be very, very dangerous.

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So there is a lot of worry that
goes into these strategies.

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But more importantly is efficacy.
So far nothing has worked.  And the

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reason we think it doesn't work is
the same reason that we've become

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resistant to, rather become
sensitive to flu virus that we

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reviewed last time.
HIV, like flu, changes very rapidly.

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And there are a lot of HIV strains
out in the world.

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And so if you get vaccinated against
one, it might be protective against

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that one, but it's not going to be
protective against all the other

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ones.  So, so far nobody has come up
with something that will lead to

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resistance sort of across the board,
across many, many strains.  OK.  So

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the subject for today is prion
diseases.  This is a photograph of a

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cow being slaughtered because it
carries a disease called bovine

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spongiform encephalopathy
or Mad Cow Disease.

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And it's an example of a class of
diseases which are called Prion

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Disease or transmissible spongiform
encephalopathies,

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clearly the longest and most
difficult to pronounce word we have

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used in this class.

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Transmissible,
it can be infectious,

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or they can be infectious.
Although, as you'll see, not all of

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this class of diseases is infectious.
Spongiform, because it causes

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sponge-like pathology.
And where does it cause the

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sponge-like pathology?
In the brain.  OK?  The amazing

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part of this story is the agent
which causes this disease.

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Everything else we've taught you
about in this course follows the

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standard central dogma
of molecule biology.

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The organisms has a genome which can
be made out of DNA or,

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as you saw in the context of some
viruses, RNA.  The genome then gives

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rise to RNA in the context of
transcription and protein.

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And the protein then assists in the
replication of the genome.  OK?

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Prions, which cause these diseases,
have no genome.  They have no genome.

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No RNA.  No DNA.  No nothing.

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They're protein only.  And
yet they replicate.

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This is an amazing story,
which is really rather new,

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largely ignored and treated with
some derision by the field,

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but now known to be true, at least
in many respects.

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And the diseases are not just of
cows.  The original disease or at

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least the first one that was studies
is a disease of sheep.

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It's called Scrapie.
And I'll tell you a little bit more

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about it.  There's another disease
very similar of cows.

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Again, bovine spongiform
encephalopathy or BSE,

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commonly known as Mad Cow Disease.
The cows go a little crazy and die.

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And then there's another disease,
actually a class of disease of

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humans.  This one is called
Creutzfeldt-Jakob Disease,

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but it's not the only one.  And,
again, they have the same

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characteristics of sponge-like
morphology pathology in the brain.

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So these are important diseases
agriculturally.

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They're important diseases of
humans, although rather rare,

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but the biggest worry is it appears
that you can get Mad Cow Disease

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from an infected sheep and you might
be able to get Creutzfeldt-Jakob

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Disease from an infected cow.
And you might remember a few years

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ago there was a slaughter.
And I just showed you one picture of

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hundreds of thousands of cows in
Great Britain because there was an

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outbreak of Mad Cow Disease,
and there was an emergence of some

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patients who developed
Creutzfeldt-Jakob Disease.

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The worry being that you could
transmit from the infected cow the

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development of this disease.
OK.  This just shows you some

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pathology.
Again, one similarity between the

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diseases of sheep,
cows, humans, and here's another

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human disease,
is this sort of sponge-like change

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in the context of the brain.
These vacuoles which are caused by

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the death of cells.
And, obviously, if you have dying

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cells in your brain that going to
lead to motor defects,

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as well as cognitive defects,
both of which occur in this disease.

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You might also notice,
especially here and here,

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there are buildups of plaques.

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These are protein aggregates.

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And we actually think it's the
buildup of those protein aggregates

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that kills the cells.
And they turn out to be important

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in the story.  OK.
So let me tell you a little bit

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about the history of this,
which is itself fascinating.

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So, as I said,

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the first disease that was studied
was Scrapie.  And it was observed in

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around 1900.  The animals had this
phenotype of scraping their skin and

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in scraping off their fur.
That's why it's called Scrapie.

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But in general they also had
reduced coordination.

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And it was eventually fatal.
And importantly it seemed to occur

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in herds.

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So something in the environment of
the herd, perhaps the agent that was

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responsible was transmitting the
disease from one effected

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sheep to another.
And it had, as I showed you up there,

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characteristic morphology
in the brain.

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So this was studied.

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And it was considered to be an
important disease in the farming

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community, especially in Europe,
most particularly in the United

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Kingdom.  But it wasn't
largely known.

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More or less confined to that
community.  Until some investigators

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began studying a disease of humans
called Kuru.  This was a disease of

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a particular tribe of peoples called
the Four Highlanders in

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Popua, New Guinea.

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These individuals developed a
disease which was rather similar to

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Scrapie.  Reduced coordination.

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They also developed dementia,
which may well have happened in the

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sheep but you wouldn't have known it.
It was also fatal.

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And it had the very same brain
pathology.  And it also seemed to be

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transmissible because it occurred
within tribes.

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So investigators now started to
study this disease.

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And they discovered,
in fact, that the individuals who

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developed this disease
had engaged in --

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-- ritualistic cannibalism.

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Individuals who died of this
disease were prepared for burial.

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And, in the process of preparing
the body for burial,

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parts of the body was eaten.
That was the way they celebrated

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the death of people in this tribe.
And they were able to link the

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ingestion, specifically of brains of
infected or affected individuals,

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with the subsequent development of
this disease.  And that suggested

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that there was an agent that was
being passed from the affected

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individual to the individual who
became affected,

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suggesting again that there was some
sort of agent responsible.

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When the etiology of this disease
was figured out,

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the tribe was informed and the
process was stopped.

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So in the late 1950s there was no
more ritualistic cannibalism and

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indeed the disease went away,
but it continued to be studied by

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investigators.
And in 1966 they succeeded in

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transmitting it to an animal species,
to monkeys.  It took a long time,

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several years for the monkeys to
exhibit the symptoms

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of the disease.
And it was then attributed to what

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was called a slow virus.
It was known to be very small

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because you could filter the
material before injecting it into

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the animal.  So it was smaller than
a cell and it was assumed to be a

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virus.  And, actually,
the investigators who made that

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discovery went on to win the Nobel
Prize for their “discovery of the

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slow virus” responsible for this and
possibly related diseases.

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Well, that was all well and good,

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but it was very difficult to study
this disease using this experimental

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system.  It took a long time.
It's not easy to do experiments in

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monkeys and so on.
And so it was necessary to find new

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experimental systems.
And this was pioneered by an

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investigator at the University of
California, San Francisco by the

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name of Stan Prusiner.
And his goal was to develop an assay

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for the agent that caused Scrapie.
He called it the Scrapie agent.

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And he probably started by assuming
that this was some sort of virus,

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one of these slow viruses, which has
not been seen by anybody before but

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just been inferred to exist.
And so what he did was to see

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whether he could transfer the agent
from a sheep by taking an extract of

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the brain of an infected sheep and
injecting it into the

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brain of a hamster.
And sure enough that led,

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in time, to disease.  And the
disease manifestations looked very

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much like Scrapie itself.
So now he had an experimental

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system.
And the time it took to get the

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disease in the hamsters was
relatively short,

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just a couple of years as opposed to
maybe ten years in the case of the

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monkeys.  He went further and he
took a brain extract from the

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infected hamsters and he injected it
into mice.  And they,

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too, eventually developed disease.
And importantly,

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over the course of this protocol,
the time to disease development --

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-- reduced each time.

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It took less and less time for the
disease to be seen.

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And so by this point you could do
assays in a couple of months.

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And that's illustrated here.

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So, again, Prusiner took the brains
of infected sheep,

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transferred them to the hamsters
initially.  It took him one to two

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years to develop the brain pathology.
And then he took extracts from them,

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transferred them to mice,
and now it was only six months to

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develop the same sort of lesions.
In doing this he was also able to

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demonstrate that whatever the agent
was, whatever it was,

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in this assay system it could
replicate.

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So how did he know that?
Well, he took --

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-- a certain amount of diseased

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brain extract,
certain number of milligrams of

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diseased brain extract,
and he extracted from it a certain

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infectious dose.
And that amount was enough to cause

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disease in the injected animal.
If you used less than that it

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wouldn't cause disease.
If he used more than that,

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that was more than enough.  So he
injected it into the brain of a

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mouse, he waited six months.
He then extracted the diseased

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brain, and asked how much
material was there?

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And the answer was at least a
thousand-fold the initial infectious

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dose.  So the only way that could
happen is if “the stuff' that was

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responsible for this,
in this time period, was able to

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replicate.  You had more at the end
than you had at the beginning,

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so it was able to produce more of
itself.

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Again, this thinking was that it was
probably some sort of virus,

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the virus was replicating inside the
brains of these animals,

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and at the end of the disease
process there was a heck of a lot

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more virus than when you started.
Not a big surprise in that respect.

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Well, he then used this to try to
purify further the agent that was

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responsible to get at this virus,
if that's what it was.  So he used

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this assay.  He took fractions of
diseased brain and split them up in

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different ways using different
protocols for purification in order

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to find the thing that
was responsible.

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So he took, for example,
an un-pure diseased brain.

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And he ran that out on a protein
gel.  Now, of course,

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in a brain you're going to have lots
and lots of proteins.

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Some will be more abundant than
others, but there will be lots.

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In fact, there will be a big smear
of proteins here.

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You really won't be able to see a
pattern.  If you took that stuff and

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purified it somewhat,
and he knew that this fraction,

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this partially purified fraction
carried this Scrapie agent because

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if he transferred into a mouse,
that mouse developed disease.  And

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then he looked at the protein
composition of that.

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Now some of the bands went away and
some of them stayed.

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And then he took that stuff and he
purified it still further.

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And now he could see only one band,
one protein band.  He took that out

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and it was found to be infectious.
It would appear that the protein

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was enough to cause infection.
Now, the skeptics said that's

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ridiculous.  Maybe the protein
co-purifies with the thing that

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causes infection,
but there has got to be a virus in

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there or a bacterium.
There has got to be something in

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there, not just a protein.
Proteins cannot replicate

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themselves.  But Prusiner persevered.
He purified this to the point where

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he could sequence it.

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And, much to his surprise,
when he purified that protein --

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-- he found that it was derived from

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the mouse genome.
The protein, which he now believed

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was responsible for the development
of the disease,

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was encoded by the host organism,
not by some as yet uncharacterized

251
00:22:01 --> 00:22:05
infectious agent.
So that was an important

252
00:22:05 --> 00:22:08
breakthrough.  There were still lots
of skeptics who said again maybe so,

253
00:22:08 --> 00:22:12
maybe it's there, maybe it's coming
from the mouse,

254
00:22:12 --> 00:22:15
but there is something else going on
here.  And I have to tell you that

255
00:22:15 --> 00:22:18
right around this time I took a
class from Stan Prusiner.

256
00:22:18 --> 00:22:22
He was at UCSF.  I was a graduate
student at UCSF.

257
00:22:22 --> 00:22:25
And he presented us with his
findings right around this same time.

258
00:22:25 --> 00:22:29
And we basically laughed at him.
We literally did.  We mocked him.

259
00:22:29 --> 00:22:34
We thought it was a joke that you
could suggest that some protein was

260
00:22:34 --> 00:22:40
causing the disease.
And moreover it was replicating.

261
00:22:40 --> 00:22:45
It seemed ridiculous to us.  Again,
he ignored us. A good idea.  And he

262
00:22:45 --> 00:22:51
asked all right,
is this agent which I can purify so

263
00:22:51 --> 00:22:57
well, does it carry a nucleic acid?
One way to do that is to expose the

264
00:22:57 --> 00:23:02
agent to radiation.
Radiation damages DNA and RNA.

265
00:23:02 --> 00:23:07
If this organisms, or whatever the
hell it is, has a DNA or RNA genome

266
00:23:07 --> 00:23:13
it should be affected.
Bacteria are affected when you

267
00:23:13 --> 00:23:18
expose them to UV.
Bacterial viruses are affected.

268
00:23:18 --> 00:23:23
Genes in your cells get mutated.
The Scrapie agents laughs, doesn't

269
00:23:23 --> 00:23:29
care.  You can radiate it till the
cows come home.  It doesn't matter.

270
00:23:29 --> 00:23:33
Again, suggestive that it
didn't have a genome.

271
00:23:33 --> 00:23:40
So he's got a protein that he thinks

272
00:23:40 --> 00:23:44
is causing disease but he has no
idea how.  He then did another

273
00:23:44 --> 00:23:49
important experiment.
He found that the

274
00:23:49 --> 00:23:53
Scrapie-associated form of this
protein was actually different

275
00:23:53 --> 00:23:57
somehow from the normal form of the
protein made in the cells

276
00:23:57 --> 00:24:02
of the organism.
And he found that by looking at the

277
00:24:02 --> 00:24:06
resistance of the protein to
protease.  So he purified the

278
00:24:06 --> 00:24:10
protein now not just from
Scrapie-infected brains but also

279
00:24:10 --> 00:24:15
from normal brains.
So we have the normal version of

280
00:24:15 --> 00:24:19
the protein and the Scrapie version
of the protein.

281
00:24:19 --> 00:24:24
He called these proteins PrP for
prion precursor.

282
00:24:24 --> 00:24:28
We'll come to the definition of
prion in a moment.

283
00:24:28 --> 00:24:33
And they could either be normal or
Scrapie associated.

284
00:24:33 --> 00:24:38
And the Scrapie associated he gave
the abbreviation PrPSc.

285
00:24:38 --> 00:24:43
And, as I said, he exposed these to
different conditions looking for a

286
00:24:43 --> 00:24:49
difference, and particularly
exposing to proteases.

287
00:24:49 --> 00:24:54
If he takes the purified protein
and runs it out on a protein gel,

288
00:24:54 --> 00:25:00
in the absence of protease he sees
the band.

289
00:25:00 --> 00:25:04
OK?  Like we saw over there or up
there.  In the presence of protease,

290
00:25:04 --> 00:25:09
the cellular version of the protein
is sensitive so there is no band.

291
00:25:09 --> 00:25:14
It gets digested by the protease.
The Scrapie associated version of

292
00:25:14 --> 00:25:19
the protein in the absence of
protease is there,

293
00:25:19 --> 00:25:24
but in the presence of the protease
is still there.

294
00:25:24 --> 00:25:29
So the Scrapie associated form is
protease resistant.

295
00:25:29 --> 00:25:33
It is somehow different from the
normal.  And maybe it was that

296
00:25:33 --> 00:25:38
difference, whatever it was,
that caused it to be disease-causing.

297
00:25:38 --> 00:25:43
So based on this collection of
evidence and a little bit more,

298
00:25:43 --> 00:25:48
Prusiner set out the Prion
Hypothesis that Scrapie,

299
00:25:48 --> 00:25:52
as well as other TSE, transmissible
spongiform encephalopathies,

300
00:25:52 --> 00:25:57
are caused by a protein-only
infectious agent, which

301
00:25:57 --> 00:26:02
he renamed prion.
The disease causing protein is an

302
00:26:02 --> 00:26:07
altered form a cellular protein
which can cause the cellular protein

303
00:26:07 --> 00:26:12
to adopt the altered conformation,
and in this way the prion can

304
00:26:12 --> 00:26:17
replicate.  He could explain the
production of more of this stuff by

305
00:26:17 --> 00:26:22
suggesting that the interaction of
the altered form with the normal

306
00:26:22 --> 00:26:27
form could turn the normal form in
the altered form.

307
00:26:27 --> 00:26:32
And, as I've told you,
he called the normal protein PrP and

308
00:26:32 --> 00:26:37
the Scrapie associated PrPSc.
So his hypothesis was that there is

309
00:26:37 --> 00:26:43
a normal cellular protein which has
a conformation.

310
00:26:43 --> 00:26:48
And this is PrP.
And there was some evidence,

311
00:26:48 --> 00:26:54
based on analysis of the structure,
although not 3-dimensional structure

312
00:26:54 --> 00:27:00
of this protein,
that it was largely alpha-helical.

313
00:27:00 --> 00:27:07
And it was protease sensitive,
as I said.  And if you ever need to

314
00:27:07 --> 00:27:15
remember which form is which,
another mnemonic, it's alpha-helical,

315
00:27:15 --> 00:27:22
remember helix=happy.
He suggested that this form could

316
00:27:22 --> 00:27:30
convert, it could change it
conformation to a form that was

317
00:27:30 --> 00:27:38
associated with Scrapie.
It had a different structure.

318
00:27:38 --> 00:27:46
It was more of a beta pleated sheet.
It was protease resistant.

319
00:27:46 --> 00:27:54
And, again, if you want to remember
the beta pleated sheet is bad.

320
00:27:54 --> 00:28:01
OK?  So there were two forms.
But more importantly than that he

321
00:28:01 --> 00:28:06
suggested that if you have this form
it can convert this form

322
00:28:06 --> 00:28:23
into this form.

323
00:28:23 --> 00:28:30
So the model was that if you have in
the same cell,

324
00:28:30 --> 00:28:37
PrP in the Scrapie form,
plus normal PrP, the normal gets

325
00:28:37 --> 00:28:44
converted to Scrapie.  And
diagrammatically --

326
00:28:44 --> 00:28:54
-- perhaps through some sort of

327
00:28:54 --> 00:29:00
intermediate in which the two
proteins interact,

328
00:29:00 --> 00:29:06
the abnormal form can change the
conformation of the normal form

329
00:29:06 --> 00:29:12
giving rise to two abnormal forms.
And if this continues and continues

330
00:29:12 --> 00:29:16
and continues in an infected brain,
you're going to have a buildup of

331
00:29:16 --> 00:29:20
this abnormal form which maybe
aggregates and maybe causes the

332
00:29:20 --> 00:29:25
brain's cells to die.
This is a little more detail.

333
00:29:25 --> 00:29:29
Again, this is a theoretical
3-dimensional structure of the

334
00:29:29 --> 00:29:34
cellular form, the normal
form of PrP.

335
00:29:34 --> 00:29:38
See the alpha-helical structure?
And this is the proposed beta

336
00:29:38 --> 00:29:43
pleated sheet structure of the PrP
Scrapie form.  Note,

337
00:29:43 --> 00:29:48
the beta pleated sheets.
And the idea was that the abnormal

338
00:29:48 --> 00:29:52
form, shown in blue,
could interact with the normal form,

339
00:29:52 --> 00:29:57
shown in red, and convert the normal
to the abnormal.

340
00:29:57 --> 00:30:02
And in so doing perhaps created
these aggregates which built up in

341
00:30:02 --> 00:30:07
the brain's brain cells and
caused them to die.

342
00:30:07 --> 00:30:11
So that seems reasonable.
And, as I said, a great deal of

343
00:30:11 --> 00:30:16
evidence went on to support it.
There was one additional definitive

344
00:30:16 --> 00:30:21
experiment about the nature of the
agent which comes a little bit later

345
00:30:21 --> 00:30:26
in the story.  And it was really
after that, that Prusiner went on to

346
00:30:26 --> 00:30:37
win the Nobel Prize.

347
00:30:37 --> 00:30:42
So, as I said,
there are several human diseases.

348
00:30:42 --> 00:30:47
This is not just a disease of sheep.
There are several human diseases

349
00:30:47 --> 00:30:52
that look similar,
and we now believe are all caused by

350
00:30:52 --> 00:30:58
this exact same mechanism.
There's Kuru which is infectious.

351
00:30:58 --> 00:31:02
If you eat the brain of somebody
with Kuru you will get Kuru,

352
00:31:02 --> 00:31:07
which is not a good thing to get.
It actually stands for, I believe,

353
00:31:07 --> 00:31:11
this name means “laughing death”.
These individuals become demented

354
00:31:11 --> 00:31:16
and then die.  So Kuru
is infectious.

355
00:31:16 --> 00:31:33
The one that's been in the news more

356
00:31:33 --> 00:31:37
recently is Creutzfeldt-Jakob
Disease.  And this can be caused by

357
00:31:37 --> 00:31:42
different mechanisms.
You can get it through iatrogenic

358
00:31:42 --> 00:31:46
exposure.  Does anybody have any
idea what iatrogenic means?

359
00:31:46 --> 00:31:51
This is one of my favorite words in
medical terminology.

360
00:31:51 --> 00:31:55
It means a disease you get in the
hospital.  OK?

361
00:31:55 --> 00:32:00
You go in with one problem,
you come out with a worse problem.

362
00:32:00 --> 00:32:04
OK?  Iatrogenic.
And specifically the way that some

363
00:32:04 --> 00:32:09
of these patients,
and it's a very, very small number,

364
00:32:09 --> 00:32:14
I assure you, developed
Creutzfeldt-Jakob Disease is they

365
00:32:14 --> 00:32:19
went in for an electroencephalogram
where they put a brain probe into

366
00:32:19 --> 00:32:24
your head.  Unfortunately,
take, can I help you?  Hello?

367
00:32:24 --> 00:32:29
Someone's watching me.  [LAUGHTER]
Not a good topic to have someone

368
00:32:29 --> 00:32:34
watching you teach.
Brain probe in the head.

369
00:32:34 --> 00:32:38
OK?  Several years later the person
develops Creutzfeldt-Jakob Disease.

370
00:32:38 --> 00:32:42
Go back, as who used that brain
probe last?  Turns out it was

371
00:32:42 --> 00:32:47
somebody with Creutzfeldt-Jakob
Disease.  The brain probes are

372
00:32:47 --> 00:32:51
stored in formalin,
a fixative, it doesn't kill the

373
00:32:51 --> 00:32:55
agent.  It's remarkably resistant to
almost anything you could do to it.

374
00:32:55 --> 00:33:00
And so these patients, regrettably,
got exposed to that thing.

375
00:33:00 --> 00:33:04
Another way is through corneal
transplants.  Every once in a while

376
00:33:04 --> 00:33:08
somebody with a corneal transplant
develops Creutzfeldt-Jakob Disease

377
00:33:08 --> 00:33:12
because the cornea came from
somebody who had or went on to

378
00:33:12 --> 00:33:17
develop Creutzfeldt-Jakob Disease.
There are also sporadic forms.  So

379
00:33:17 --> 00:33:21
sometimes people just get
Creutzfeldt-Jakob Disease as though

380
00:33:21 --> 00:33:25
spontaneously.
Their normal PrP protein flips to

381
00:33:25 --> 00:33:30
the Scrapie form.
And once that happens it's catalytic,

382
00:33:30 --> 00:33:35
it keeps happening.
And then, interestingly enough,

383
00:33:35 --> 00:33:40
there are familial forms, familial
forms of CJD.  It runs in some

384
00:33:40 --> 00:33:45
families.  They're not very common
but there are some.

385
00:33:45 --> 00:33:50
And there are two other diseases,
which I'm not going to write out

386
00:33:50 --> 00:33:55
because you don't need to know their
names, but one of them is called GSS

387
00:33:55 --> 00:34:00
and it's also familial.
And another is called FFH.

388
00:34:00 --> 00:34:05
FF, sorry, FFI.
And it's also familial.

389
00:34:05 --> 00:34:10
This is a funny one. Not funny if
you have it, but the FFH stands for

390
00:34:10 --> 00:34:15
fatal familial insomnia.
Now, I've had insomnia sometimes I

391
00:34:15 --> 00:34:21
feel like dying at the end of it,
but these patients actually do.

392
00:34:21 --> 00:34:26
This is a degenerative disease
associated somehow with insomnia,

393
00:34:26 --> 00:34:31
and it's ultimately fatal.
GSS has some similarities with

394
00:34:31 --> 00:34:35
Creutzfeldt-Jakob Disease but it's a
little bit different.

395
00:34:35 --> 00:34:40
And so all three of these diseases
have similarities,

396
00:34:40 --> 00:34:44
but they also have their own unique
features.  And interestingly they

397
00:34:44 --> 00:34:48
all have a common cause.
All of the familial forms of these

398
00:34:48 --> 00:34:53
diseases have a common cause.
Can anybody tell me what it is?

399
00:34:53 --> 00:34:57
Why might you get this disease?
Something in your genome,

400
00:34:57 --> 00:35:14
why might you get it?

401
00:35:14 --> 00:35:18
They carry mutations in the PrP gene.
So if you sequence the genome of

402
00:35:18 --> 00:35:22
these patients,
you find that they carry a PrP gene

403
00:35:22 --> 00:35:26
which is different from the normal
one.  And the different diseases

404
00:35:26 --> 00:35:30
carry different mutations
in the PrP gene.

405
00:35:30 --> 00:35:33
Some of them cause this
manifestation.

406
00:35:33 --> 00:35:37
Other mutations cause this
manifestation.

407
00:35:37 --> 00:35:41
And still other mutations cause
this manifestation.

408
00:35:41 --> 00:35:45
But it's all more or less the same.
PrP protein is flipping and causing

409
00:35:45 --> 00:35:49
other like proteins around it,
other versions, normal versions to

410
00:35:49 --> 00:35:53
flip, too.  The fact that there are
differences is intriguing and a

411
00:35:53 --> 00:35:57
little bit hard to explain.
I don't think there is a

412
00:35:57 --> 00:36:01
satisfactory explanation for how
different mutations in the same gene

413
00:36:01 --> 00:36:05
cause apparently different
conformations.

414
00:36:05 --> 00:36:10
And those different conformations
can be propagated faithfully upon

415
00:36:10 --> 00:36:15
interaction with the normal protein.
So here's a diagram of that.

416
00:36:15 --> 00:36:21
Here's the point mutant,
let's say it's CJD.  And here's a

417
00:36:21 --> 00:36:26
point mutant FFI.
It's not the same point mutation.

418
00:36:26 --> 00:36:32
This one folds in its abnormal
conformation into this structure.

419
00:36:32 --> 00:36:36
This one folds in a slightly
different structure.

420
00:36:36 --> 00:36:40
And maybe this can propagate its
structure, and it can propagate its

421
00:36:40 --> 00:36:44
own structure on interaction with
the normal protein.

422
00:36:44 --> 00:36:48
And those different structures may
be aggregated different subsets of

423
00:36:48 --> 00:36:52
cells or the different cells are
sensitive in some ways to these

424
00:36:52 --> 00:36:56
different structures.
So some die in one disease,

425
00:36:56 --> 00:37:00
others die in a different disease,
and that's what causes the different

426
00:37:00 --> 00:37:04
manifestations.
And so I've given you examples now

427
00:37:04 --> 00:37:09
of these diseases which are caused
by the same general mechanism,

428
00:37:09 --> 00:37:14
although with slightly different
etiologies.  In the case of Kuru,

429
00:37:14 --> 00:37:20
you can ingest an abnormal copy of
the protein.  It makes its way to

430
00:37:20 --> 00:37:25
the brain, interacts with normal
protein and converts

431
00:37:25 --> 00:37:30
it to abnormal.
There are probably examples of

432
00:37:30 --> 00:37:34
spontaneous or sporadic conversion
of normal protein to abnormal

433
00:37:34 --> 00:37:38
protein.  Again,
a process that gets propagated.

434
00:37:38 --> 00:37:42
Or you can carry a mutation which
will increase the likelihood that

435
00:37:42 --> 00:37:46
this conversion takes place.
And when it does it gets propagated

436
00:37:46 --> 00:37:50
inside of you.
And maybe sometimes people pick up

437
00:37:50 --> 00:37:54
sporadically a mutation in their PrP
gene which causes those cells again

438
00:37:54 --> 00:37:59
to flip this conformation
more rapidly.

439
00:37:59 --> 00:38:03
So lots of different mechanisms but
the same basic idea,

440
00:38:03 --> 00:38:07
PrP becoming abnormal and causing
cells around it,

441
00:38:07 --> 00:38:12
or rather proteins around it to
become abnormal,

442
00:38:12 --> 00:38:16
too.  Why has this caused increased
attention recently?

443
00:38:16 --> 00:38:20
Because of the outbreak of this
related disease bovine spongiform

444
00:38:20 --> 00:38:25
encephalopathy or BSE.
This is a story from about five

445
00:38:25 --> 00:38:29
years ago.  It was just after the
height of the fear of the increase

446
00:38:29 --> 00:38:34
in the incidence of this disease in
Great Britain.

447
00:38:34 --> 00:38:37
Some of the cows in Great Britain
had made their way to other

448
00:38:37 --> 00:38:41
countries and,
therefore, were slaughtered.

449
00:38:41 --> 00:38:44
And throughout Great Britain cows
were slaughtered in remarkable

450
00:38:44 --> 00:38:48
numbers to try to limit the scope of
this disease.  Why were people so

451
00:38:48 --> 00:38:51
worried about the outbreak of a
disease in cows?

452
00:38:51 --> 00:38:55
Because it turned out,
and this just shows you the outbreak.

453
00:38:55 --> 00:38:59
This is the number of BSE cases in
this timeframe.

454
00:38:59 --> 00:39:04
And you can see the number of
affected cows dramatically increased.

455
00:39:04 --> 00:39:09
The reason that they dramatically
increased is that the cows were

456
00:39:09 --> 00:39:14
being fed parts of infected sheep.
The sheep had the disease, or some

457
00:39:14 --> 00:39:19
of them, and they passed it onto the
cows.  And then the cows developed

458
00:39:19 --> 00:39:24
the disease.  When they noticed this
practice, they started prohibiting

459
00:39:24 --> 00:39:29
the use of other animal products in
the feeding of these cows.

460
00:39:29 --> 00:39:33
And so eventually the incidence
dropped.  And these are various

461
00:39:33 --> 00:39:37
safeguards that were put into place
at different times to limit what the

462
00:39:37 --> 00:39:41
cows actually got exposed to.
And nowadays there are very strict

463
00:39:41 --> 00:39:46
practices about what you can feed
your cows and your sheep to try

464
00:39:46 --> 00:39:50
prevent the spread of this disease.
And it's not just a spread of the

465
00:39:50 --> 00:39:54
disease in cows,
but here in this pink line tracking

466
00:39:54 --> 00:39:58
with the incidence of BSE that was
going up around this same time

467
00:39:58 --> 00:40:03
period was the incidence
of CJD in the pink.

468
00:40:03 --> 00:40:07
And indeed it was documented that
these individuals developed this

469
00:40:07 --> 00:40:11
disease because of prior exposure to
infected meat.

470
00:40:11 --> 00:40:15
So you can get this disease through
exposure to an animal that carries

471
00:40:15 --> 00:40:20
this disease.  However,
I need to tell you it's still rather

472
00:40:20 --> 00:40:24
rare.  It doesn't happen very often
so you don't need to panic.

473
00:40:24 --> 00:40:28
In fact, there was great fear that
this would skyrocket after 2000

474
00:40:28 --> 00:40:32
because it was feared that there was
some incubation period taking place

475
00:40:32 --> 00:40:37
and maybe lots more people would
develop the disease.

476
00:40:37 --> 00:40:42
That turned out not to be true.
If you look at the peak in 2000,

477
00:40:42 --> 00:40:47
it actually drops back down again.
So the epidemic of CJD that was

478
00:40:47 --> 00:40:52
feared actually didn't materialize.
Still, there are great fears that

479
00:40:52 --> 00:40:58
any exposure could give you some
risk of developing this disease.

480
00:40:58 --> 00:41:02
And you may remember just earlier
this year, a big scar in this

481
00:41:02 --> 00:41:07
country because two cows in Canada
were found to have BSE,

482
00:41:07 --> 00:41:11
and maybe that cow meat had made its
way into our food supply.

483
00:41:11 --> 00:41:16
So, again, there's tremendous worry
out there.  But it's important for

484
00:41:16 --> 00:41:20
you to know that the incidence,
even for people known to be infected,

485
00:41:20 --> 00:41:25
is very, very small.
And because it often comes up,

486
00:41:25 --> 00:41:29
how do you go from ingesting
something to get a disease

487
00:41:29 --> 00:41:34
in the brain?
We now think that the cells in the

488
00:41:34 --> 00:41:38
gut, and probably dendritic cells,
cells of the immune system pick up

489
00:41:38 --> 00:41:43
the Scrapie protein and transfer it
up to the brain.

490
00:41:43 --> 00:41:48
And once there somehow it gets into
the neurons and then spreads from

491
00:41:48 --> 00:41:52
neuron to neuron,
thereby causing the disease in the

492
00:41:52 --> 00:41:57
brain.  OK.
The last thing I want to tell you in

493
00:41:57 --> 00:42:02
the last five minutes is that there
seems to be some form of species

494
00:42:02 --> 00:42:07
barrier.  So if you get some human
Scrapie protein,

495
00:42:07 --> 00:42:12
PrP Scrapie, the likelihood that
you're going to get disease is

496
00:42:12 --> 00:42:17
pretty high because that protein
interacts well with your PrP protein.

497
00:42:17 --> 00:42:22
But if you get a PrP Scrapie
protein from a different species,

498
00:42:22 --> 00:42:27
it doesn't interact so well with
yours.  And this is called

499
00:42:27 --> 00:42:32
a species barrier.
And you can see it's shown here.

500
00:42:32 --> 00:42:36
If you take a hamster with an
infected brain and give it to a

501
00:42:36 --> 00:42:40
mouse, it takes six months for that
mouse to develop the disease.

502
00:42:40 --> 00:42:44
OK?  It takes a while for the
hamster PrP Scrapie to interact

503
00:42:44 --> 00:42:49
sufficiently with the mouse protein,
which is present in the mouse's

504
00:42:49 --> 00:42:53
cells, to give you disease.
But if you now take this

505
00:42:53 --> 00:42:57
preparation which is PrP Scrapie
from mouse, because it's the mouse

506
00:42:57 --> 00:43:01
protein that's been converted here,
and inject it into mouse, now it

507
00:43:01 --> 00:43:06
happens much, much faster.
So there's a species barrier.

508
00:43:06 --> 00:43:10
And once it's been passed through
one species, you overcome that

509
00:43:10 --> 00:43:14
barrier.  Now,
how could you test this notion that

510
00:43:14 --> 00:43:19
there's a species-specific
difference.  Well,

511
00:43:19 --> 00:43:23
the way it was done by Prusiner and
others is to introduce a hamster

512
00:43:23 --> 00:43:27
gene into this mouse here.
So this mouse now makes not just

513
00:43:27 --> 00:43:32
mouse PrP but also hamster PrP.
And what would be the expectation

514
00:43:32 --> 00:43:36
for that experiment?
If you now take this transgenic

515
00:43:36 --> 00:43:40
mouse that carries a hamster PrP
gene and introduce the Scrapie agent

516
00:43:40 --> 00:43:44
from hamster, what would happen?
It would happen faster because now

517
00:43:44 --> 00:43:48
you could template off of the
hamster protein,

518
00:43:48 --> 00:43:52
and sure enough this animal develops
disease faster.

519
00:43:52 --> 00:43:56
And when you purify the PrP Scrapie
from that infected brain,

520
00:43:56 --> 00:44:00
it's composed of both mouse and
hamster.

521
00:44:00 --> 00:44:05
OK?  Now, what would happen if you
made a mouse that didn't have a PrP

522
00:44:05 --> 00:44:10
gene?  You knocked it out.
A, you might be surprised if that

523
00:44:10 --> 00:44:15
mouse lived because you might think
that the PrP gene is important for

524
00:44:15 --> 00:44:20
something.  Well,
it turns out you can make a mouse

525
00:44:20 --> 00:44:25
that lacks PrP all together.
You can delete the PrP gene.

526
00:44:25 --> 00:44:30
So let's say you had a PrP
deficient mouse and you introduced

527
00:44:30 --> 00:44:36
the Scrapie agent into it,
what would happen?

528
00:44:36 --> 00:44:40
Well, you need a PrP protein to be
converted in order to develop the

529
00:44:40 --> 00:44:44
disease.  If you don't have the gene,
you don't have the protein,

530
00:44:44 --> 00:44:49
so it cannot be converted.  And sure
enough that's the case.

531
00:44:49 --> 00:44:53
If you do this experiment,
you propagate some PrP in a mouse

532
00:44:53 --> 00:44:57
and then transfer it into a mouse
that's lacking PrP altogether,

533
00:44:57 --> 00:45:02
the animal is disease-free.
And this was the experiment that

534
00:45:02 --> 00:45:06
really sealed the deal and led to
the Nobel Prize for Prusiner.

535
00:45:06 --> 00:45:11
At this point nobody could really
argue that it was the cellular

536
00:45:11 --> 00:45:15
protein being converted in the
absence of any genome that was

537
00:45:15 --> 00:45:20
responsible.  So the problem doesn't
go away.  Here's another species

538
00:45:20 --> 00:45:25
which has been observed to have
another one of these diseases.

539
00:45:25 --> 00:45:29
It's clearly a Scrapie Disease.
It's called Chronic Wasting Disease

540
00:45:29 --> 00:45:33
or CWD.
It occurs in herds of deer,

541
00:45:33 --> 00:45:37
as well as in elk.  And if you look
at the incidence of animals that are

542
00:45:37 --> 00:45:41
affected, there are several herds in
our country that are affected.

543
00:45:41 --> 00:45:44
And, again, there's a lot of worry
out there, it's a little below the

544
00:45:44 --> 00:45:48
surface now, but there's a lot of
worry out there that if you eat that

545
00:45:48 --> 00:45:52
stuff you might develop CJD.
Importantly, there are no

546
00:45:52 --> 00:45:55
documented cases,
no documented cases so far.

547
00:45:55 --> 00:45:59
There are some reported cases but
they've been investigated and found

548
00:45:59 --> 00:46:03
probably not to be associated with
exposure to this meet.

549
00:46:03 --> 00:46:07
Nevertheless, there's a fear that
the species barrier might be

550
00:46:07 --> 00:46:12
breached in some individuals,
and those individuals would develop

551
00:46:12 --> 00:46:16
this disease.  So with that I will
stop.  Thank you for you attention.

552
00:46:16 --> 00:46:19
Good luck on the final.  And don't
forget office hours.  [APPLAUSE]