Hi.
I'm Judy Cole, the Executive
Vice President and CEO
of the MIT Alumni Association.
And I'm delighted to welcome
you to this web production
of the MIT Alumni Association.
Please welcome to
the stage Feng Zhang.
Good morning.
It's really a great
pleasure to be here
and to be able to share with
you some of the work we've
been working on
to be able to edit
DNA sequences in the genome.
So you might be sitting
there and wondering
why do you want to edit DNA
sequences in the genome?
We all have different
DNA and that's
what makes us individuals.
Why do we want to change it?
To get us started, maybe
we'll step back a little bit.
So one of the really
exciting advances
in biology over the
past couple of decades
is the completion
of the human genome.
So more than 10 years
ago the human genome
was completely sequenced,
and from that we're
able to now identify
many of the underlying
genetic bases for disease
and also for our health.
Just to give you some
examples of that.
Based on human genome sequences
or looking at human genetics,
we're able to identify genetic
differences between individuals
that cause disease.
For example, some mutations
cause sickle cell disease,
others cause cystic fibrosis, or
diseases that affect eye health
and cause degeneration.
But not all genetic
differences are harmful.
Some of them are
more beneficial.
Some make us taller,
some make us more agile.
And so there are also
genetic differences
that have been identified to be
protective for certain disease.
For example, a small
percentage of individuals
carry a mutation in
a gene called CCR5.
And those individuals
naturally are very healthy.
They don't have the
CCR5 gene, and also they
don't contract HIV infection.
There are also mutations that
reduce the risk for Alzheimer's
disease.
And there are also
mutations that
reduce the risk for diabetes.
So we know what
mutations are harmful
and we also know what mutations
are potentially beneficial.
So why don't we just go
into the DNA of individuals,
especially the DNA
of patients who
are suffering from some of
these really grievous diseases,
and just get rid of
the genetic mutations?
Well it turns out,
to be able to change
DNA letters in the genome is
actually pretty difficult.
So let me just tell you
a little bit about what
DNA is like inside of a cell.
Each one of our cells
has the complete genome,
and the genome is divided up
into different fragments called
chromosomes.
So this is an artistic
rendering of what
a chromosome looks like.
This is at a microscopic level.
If you zoom in and really look
at what the chromosome is made
of, you can see that it's really
just a long string of four
different letters.
It's a very simple
alphabet, it's just A,
G, T, and C. If you just look at
this long string of DNA letter,
it doesn't really
make any sense.
But based on the work of
many, many laboratories,
many scientists, we're now
able to start to annotate
what the genome looks like.
And so if we abstract
a little bit,
we can think of the genome as
a long string of sentences.
And each one of these sentences
provides an instruction
to the cell for how the
cell should function.
And so you can think
of the long string
as now a paragraph
or a whole book.
And so the genome
is really the book
that provides the
instruction for life.
Now let's look at each one
of these genomic sentences.
Very much like our spoken
language or our written
language, each sentence has
different components to it.
We have scientifically what
we call promoters, enhancers,
genes, and terminators, and
I just put in parentheses
what the analogy might be in
one of our written sentences.
So a promoter would
be like a verb.
It tells the subject what to do.
The adjective is the enhancer.
It amplifies the
activity of the gene,
tells the gene should
it be very active
or should it be not so
active in that cell.
And the subject is the noun,
which is the gene itself.
And so all these things, the
promoters, the enhancers,
and the terminator, which
is a punctuation mark,
sort of revolve around
the function of the gene
and tells the cell what
to do with that gene.
So a mutation in one of
these gene instructions
is equivalent to a typo
one of these sentences
in the book of the genome.
And so typos can happen really
anywhere inside of a sentence.
And so let me give you a very
simple example of this nursery
rhyme sentence, twinkle
twinkle little star.
So the typo in this
sentence is the word little.
So we have twinkle
twinkle big star.
It's not what it's intended
to mean in the nursery rhyme.
And so many ways
have been developed
to be able to rectify this typo.
Older generations of
technology include things
like virus-based gene
therapy, and also include
things like RNAi-mediated
gene knockdown.
So what do those
things really mean?
Gene therapy will perform
changes to the sentence,
but it does something
a little different.
It just puts in
the correct word.
So you end up with something
like twinkle twinkle
little big star.
So we put in the correct
word, but the typo
has not been removed.
And that's what
gene therapy does.
It puts in the correct
gene but it doesn't get rid
of the mutated gene.
Now RNAi knockdown
does the opposite.
It gets rid of the
typo but it doesn't
put in the correct gene.
And so that's what we get,
twinkle twinkly empty star.
So both of these are not ideal.
To really fully restore the
meaning of that sentence,
we really want to have
twinkle twinkle little star,
and that's where
editing comes in.
Editing has the ability
to remove the typo
but also put in
the correct word.
And so that's how
we're able to get
twinkle twinkle little star.
So how do you
actually do editing?
Well if the genome
was really a book,
we can load it up with Microsoft
Word and then use the search
and replace function.
We type in the
string, we type in big
and it will take
the cursor exactly
to where that typo
word is, and then
we hit backspace
delete it, and then
we can use the keyboard to
type in the correct word.
What is the equivalent of the
search and replace function?
And what is the
equivalent of a cursor
in the actual genome of a cell?
Well it turns out the
answer to this question
lies in somewhere that's
completely unexpected,
and it's probably
what some of you
have had for breakfast
this morning: yogurt.
Turns out that bacteria that
people use to produce yogurt
carry an immune
system that allows
them to defend themselves
against virus infections.
This is a movie that shows
a bacterial cell getting
infected by a virus.
And when the virus
latches onto the cell,
it injects its own genetic
information into that cell.
Now there's a
system called CRISPR
that has small
fragments of RNA that
are derived from the
previous virus infection.
And so then this RNA
serves as a guide,
and it works with
something called
Cas9, a protein in
the bacterial cell,
to then be able to search
along the invading virus DNA.
And if that RNA
guides the enzyme
to recognize the DNA
by forming this duplex,
then the enzyme will make
a double-stranded cut
in the virus DNA.
That double-stranded
cut in the virus DNA
inactivates the virus.
But that double-stranded
break, if we introduce it
into the genome of
a human cell, it
acts as a cursor for doing
editing on the human genome.
Wherever you place this
double-stranded cut is where
you are placing the
cursor, and then the gene
in that location that gets cut
you can now go and edit it.
So let me show you a different
video of how the system might
work in human cell.
So we can reprogram this
enzyme with a new sequence that
targets the human gene,
and then by introducing it
through the nucleus
of a human cell
we can get this thing to
recognize the human genome.
And so this is the
human DNA here.
And this thing,
because it's programmed
to recognize the human gene, it
will base pair and form a helix
with the human DNA.
When that recognition
happens, the enzyme
then will make a double-stranded
cut in the human genome,
and place in a cursor into
where we want to make the fix.
And so now this double-stranded
broken ends of the DNA
can get repaired.
The first method is called
non-homologous end joining,
which simply rejoins the
two ends together but forms
a small mutation.
And that mutation is
useful for inactivating
a deleterious gene.
Alternatively, we can do
something more precise.
So now you have these
DNA double-stranded cuts.
This is where the cursor is.
You can put in a
new piece of DNA
that carries the
sequence that you
want to switch into the genome.
So this is the correction
that you want to put in.
And now this will match
with the two broken ends
and then allow you to
incorporate a new piece of DNA
directly into where you
want to fix in the genome.
So that's the basic idea.
By taking advantage of a protein
from a bacterial immune system,
we can reprogram it
so that it recognizes
a new sequence in the
human genome, make a cut,
and then stimulate processes
in the cell that would allow us
to then edit the human genome.
So there are many different
applications of this.
This is applicable
in research, it's
already accelerating
research around the world.
You can also use
it in agriculture
to be able to make
new plant species that
are more sustainable, use
less water, more drought
resistant, and so forth.
But one of the most
exciting prospects of this
is if we use it to
treat human disease.
Now there are two
major ways that we
can try to develop
the CRISPR/Cas9 system
as a human therapeutic.
One of the ways is called
ex vivo gene therapy.
The way this works is we
can take some cells out
of the body, especially
those cells that
are involved in the blood
system or in the immune system.
Scientists and doctors
over the several decades
have refined techniques to take
these cells out of the body,
modify them or manipulate
them, and then put them back
into the patient to
do a transplantation.
And so we can do that.
We can take advantage
of that method
to extract cells from the body.
We can put them
into a Petri dish,
introduce the CRISPR/Cas9
reagent into those cells,
fix the mutation, and
then transplant them back
into the patient.
And so prospects of
this include treatment
for sickle cell disease,
for different forms
of immune deficiency, and also
even for cancer treatment.
But not all cells can be
removed from the body.
We can't take the brain out
and fix it and put it back
into the patient.
And so for many of those other
diseases, what we can do is
try to develop ways to do
in vivo genome editing.
What that means is, rather than
taking the organ or the cells
out, we'll directly put
the CRISPR/Cas9 reagent
into the affected organ.
So we can put it into the
liver, or we can inject it
into the bloodstream
so that it can
circulate and target into where
exactly the affected cells are.
And so those are two of the
different approaches that we
can take and we're developing,
to be able to develop CRISPR
into a human therapeutic.
So we thought to
test whether or not
this approach, this
concept, even works.
So what we did is with
we chose a mouse model
to see can we try to treat
the disease in a mouse.
And the disease we chose is
cardiovascular disease, so
elevated levels of cholesterol.
And we're trying to see
can we reduce cholesterol
in these mice.
What we did is we looked at
one of these mutations that
naturally exists in
the human population.
It's a mutation in
a gene called PCSK9.
There are a small number of
individuals who naturally
carry a mutation
that inactivates
the PCSK9 gene in their genome.
And these individuals
happen to have
very low levels of
cholesterol, and then also
very low levels of
cardiovascular disease.
And so drug companies
have already
developed protein-based
therapeutics
like these that target
the PCSK9 protein
and then get rid of
that in the patient.
These are very
expensive therapies,
they cost thousands if not
tens of thousands of dollars,
and you have to periodically
get an injection of this protein
drug.
So it's also not
very convenient.
So we thought maybe we
can use gene editing
to put into patients this
natural mutation that's
healthy, that doesn't affect
the health of human individuals.
Maybe we can do this in
mice and see whether or not
that would work.
And so what we did is
we made a virus that's
called adeno-associated virus.
This is one of the most advanced
vectors for human gene therapy.
It's been shown to
be safe in humans.
And we engineered the
genome of this virus.
This is a linear schematic of
what the genome looks like.
We basically introduced the gene
for Cas9 and also the RNA guide
that directs Cas9 to be able
to target the PCSK9 gene.
And so we made up this
new synthetic genome,
and we made viruses,
and then we injected it
through the tail
vein of the mouse
so that it would target into
hepatocyte cells in the liver.
And if the liver cells
successfully received this,
and it made Cas9
and the RNA guide,
then it should go in and be
able to cut the PCSK9 gene
and then introduce the mutation
that would reduce cholesterol
level in these mice.
So we tested it.
And so this is what we saw.
So we measured the PCSK9 level.
If editing is
successful we should see
a reduction in the PCSK9 level.
So this is a control mouse, you
can see PCSK9 level is sort of,
it fluctuates a little
bit but you can detect it.
It's pretty significant.
Now, what happens if we
inject the virus that
gets rid of PCSK9?
So injection happens
at day 0, and then
you can see that you
can detect PCSK9 level
before the injection,
but then afterwards we
completely deplete PCSK9
level from these mice.
So this shows that
editing is efficacious.
We can achieve
efficient enough editing
to be able to get
rid of the PCSK9
from the blood of these mice.
So if we're reducing
PCSK9, that should also
reduce cholesterol
level in these mice.
And so this is what the
cholesterol level looks like.
This is the control mouse.
You can see it
fluctuates little bit
but it still is
highly detectable.
We inject in the
virus, we're now
able to reduce
cholesterol level by 50%.
And so this shows that we're
not only able to get rid
of the gene, but we can
also get a beneficial effect
when that gene is gone.
Now what's the difference
between CRISPR/Cas9
based editing and also
the previous protein-based
therapeutic?
The difference is that
the gene editing approach
is a single treatment,
it's a one shot treatment,
and then it is a permanent
change, a permanent effect,
within these cells.
And so the nice
thing about this is
that the patient
gets one injection
and then he doesn't have
to worry about it anymore.
The potential is that it will
have a very, very long-lasting
effect.
So this is just the beginning.
There's still a lot
of work to be done.
We and also many other
researchers around the world
are working on further advancing
the technology so that we
can treat many of the over
6,000 genetic diseases that
are identified today.
And this is just a short
list of some of them.
None of them have
treatments, and we're
hoping that gene editing
will be able to usher
in a new class of treatment
for these untreatable diseases.
So finally, this is the
most important slide.
I just want to acknowledge
my team here at MIT,
and also the Brody Institute,
and then our collaborators that
worked with us to develop them,
and also our various funding
agencies.
And last but not
least, it's really
the fantastic
environment here at MIT,
with both the engineering, the
practicality mind, and also
the creative
scientific thinking,
and the spirit to try something
risky with the potential
to make a big impact that
allowed us to really do
all of this interesting work.
Thank you very much.
Thanks again for joining us.
For more information on
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productions, please
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