00:01
What do we mean by that?
Okay.
00:03
In the lower left hand corner
is the normal wheel of fortune
or the wheel of the cell cycle.
00:09
And at each stage along the way
where the arrows stop,
and there's a gap,
there's a pause that is involved
in the cell deciding
whether
to go forward or to stop.
00:21
And one of the
first pauses that occur
in the normal cell cycles
between the growth phase, G1,
and the synthesis phase, S.
00:29
That G1, S transition
is actually regulated by
retinoblastoma gene products
or the retinoblastoma protein.
00:42
Till a little bit more involved,
very briefly,
we need to cause
the breakdown,
retinoblastoma is kind of a break
on the entire system.
00:52
It's saying, "Nope, we're not
going to go anywhere,
we're not going to go
beyond G1 into S, until I say so."
So we need to make retinoblastoma
go away, or the Rb protein go away.
01:02
To do that, we have to have
upregulated activity
of Cyclin-dependent kinase 2, Cdk2.
01:09
And that happens because we
increase the activity of cyclin E.
01:13
When we talk about pathology
and other settings,
we'll get into more detail here.
01:17
But basically,
it's going to be the upregulation
and increased activity
of these proteins
that will allow us to interact
and make Rb go away.
01:29
Rb normally sits,
so the RB gene
that kind of read lips
normally sits
on top of elongation factor
and like a
big sumo wrestler says
"Nope, you're not
going to do your job.
01:40
I'm going to keep you
in an inactive form."
Well, if we want the cell
to go through cell cycle,
we have to convince
the sumo wrestler
to step away from
elongation factor 2.
01:51
So in order to do that,
what we do is cyclin E and cdk2,
they work together to phosphorylate
the retinoblastoma gene,
and when it does that,
that hyperphosphorylation means
it goes away.
02:02
It is released from
elongation factor 2.
02:05
And now, E2F, elongation factor 2
can allow the synthesis of the DNA.
02:13
We will get past that block
the G1 to S block.
02:16
Okay, that's
what normally Rb does.
02:19
It sits like a break
on the system and says,
"Nope, you're not going anywhere."
And it's not until
we've specifically activated
by phosphorylation,
that it releases E2F to do its job.
02:31
So what goes on in retinoblastoma
when there's mutations?
Well, in that setting,
the sumo wrestler has been convinced
to never bind
the elongation factor 2.
02:42
It's just going to go, you know,
I don't care anymore,
and it's an abnormal form
of protein.
02:47
So E2F is free to do
its thing all the time.
02:50
We've taken away
the brake on the cell cycle.
02:54
And so, by having that mutant form,
we constantly drive cells
through the cell cycle.
03:01
The familial form of retinoblastoma
actually makes a lot of sense
when we think about
genetic mutations
and how things
are passed down.
03:12
The familial form
means
that either mom or dad
has one bad copy of the gene.
03:16
That's indicated here in blue.
03:18
Normal,
is indicated as red.
03:21
So the normal retinoblastoma gene
on chromosome 13
is going to be red.
03:26
Dad, in this case,
it could be mom,
but dad, in this case
is the carrier.
03:30
He doesn't have
any disease necessarily.
03:34
But half of his sperm will carry
that mutant form of retinoblastoma.
03:40
Now, when he and mom get together
and make a zygote,
every gene or every cell
in this new zygote
will have one bad copy.
03:51
So all the somatic cells
of the child will have this.
03:56
Now, all it takes is a second hit.
03:58
So, with just one bad
when there is just one bad copy,
that means there's one good copy
of the sumo wrestler
that can sit on E2F
and keep it from doing its job.
04:11
So with one bad copy
were good,
but if every cell in the body
has one bad copy,
all it takes is an additional hit
of the normal chromosome,
a mutation of the Rb
of the normal one,
and now we have no sumo wrestler.
04:29
We have no ability
to turn off E2F,
no ability to regulate
cellular proliferation.
04:35
So, this frequently
the second mutation
of a somatic cell
will occur in retinal cells.
04:41
The exact reason for this
is not exactly no
because every cell in the body
has got one bad gene.
04:51
Why is the retina so prone?
Well, do you remember
that I told you that
you are kids with
the art the familial form,
the hereditary form of Rb,
a retinoblastoma
are also more prone to glioblastoma,
to other tumors such as sarcomas.
05:09
So, okay,
that kind of makes sense.
05:11
But why is this hidden
in the retina?
And part of the reason is
that we think that the retina,
because of light
going in through the pupil
is more susceptible
to secondary UV damage.
05:26
This is a little hand wavy, okay.
05:28
And when I waved my hands furiously,
that means we're not exactly sure.
05:32
This is an example where
we're not exactly sure.
05:35
But nevertheless,
these kids will get a second hit
at a certain frequency,
and now developed
loss of the regulation
of the cell cycle.
05:47
And then the consequence is
we get retinoblastoma.
05:51
So the familial form
of retinoblastoma.
05:54
This hereditary form constitutes
about 40% of the cases
of retinoblastoma.
05:59
So one in 15,000 kids,
40% of those will be due to this.
06:04
And it tends to
as you would imagine,
because we carry that mutation
in every somatic cell
in the kid's body,
it will tend to be bilateral
or multi-focal, okay.
06:15
There are sporadic forms.
06:16
The sporadic forms
are a little bit harder to achieve,
because it requires mutations,
multiple mutations in the same cell.
06:26
So, both parents are all red
in their chromosome 13 RB gene.
06:33
And so all of the germ cells
are going to be normal.
06:35
They're all going to be red,
all the way through.
06:38
The zygote is going to be red-red.
06:40
All the somatic cells of
the child completely normal.
06:43
But somewhere along the line,
one of the cells in the retina
gets a mutation in Rb.
06:50
So one cell out of that
entire population
gets that mutation.
06:56
And then we have to get
a second hit.
06:58
In the sec, in that same cell
that has already lost
the original wild type Rb,
in order to get retinoblastoma.
07:07
In yet it happens.
07:08
In about 60% of the cases
of retinoblastoma, overall,
are due to this sporadic double
mutation in a single cell.
07:17
Tends to be unilateral,
and, you know,
it's just incredibly bad luck.
07:21
And so it tends to be
unifocal and unilateral
because only
very few cells ever get
the double mutation
in their RB gene.