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The following content is provided
by MIT OpenCourseWare under a Creative

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Commons license. Additional
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We're going to continue our
discussion about cancer and cancer

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genetics today. One small point
of clarification:

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apparently some people were
confused about my out of balance scales here

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related to proliferation and cell
death, pointing out that if there

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were more proliferation and less
cell death, then the scale should

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shift this way instead of this way.
That's an astute observation.

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It doesn't really matter which way
the scales point really.

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The point was that they are out of
balance, which is critical for

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tumorigenesis. But anyway,
in review,

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last time I told you that cancer
cells arise from normal cells over a

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series of events that relate to
mutations in cellular genes.

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And I gave you lots of evidence
that things that cause cancer affect our

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genes, and that our genes are
disrupted in cancer cells.

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And I briefly mentioned that we
have evidence for mutations in

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cellular genes in cancer cells.
And I'll show you that again today.

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But all of that brings us to the
point of needing to know,

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what are the genes? What are the
genes that are affected in cancer?

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And this is important to help us
understand how cancers arise,

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so from a basic biomedical research
point of view,

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but also as we'll see next time,
this information is actually very

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useful in teaching us how to diagnose
cancer more effectively or

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treat it more effectively. And I'll
just show this slide,

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which I showed at the end again as
a summary. The cancer cells arise

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from normal cells through the
acquisition of

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mutations in cellular genes through
this process which we call clonal

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evolution, more and more abnormal
cells growing out from this

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accumulating mass of cells. And as
we'll discuss today,

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these genes affect proliferation and
cell death, this out of balance

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scheme that I alluded to before, but
not only these processes.

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And I emphasize this because these
other important processes get a

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little bit of short shrift in our
discussions. But angiogenesis,

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this process of recruiting a new
blood supply in cancer is very

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important, and also an important
therapeutic target.

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Cell motility, movement of cells,
as I mentioned, it's important to

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get cells away from the initial
mass in the process of metastasis,

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and invasion likewise. And several
other processes are also affected in

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the course of tumorigenesis.
So what are the genes?

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How do we know which genes are
affected in cancer?

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Well, this set of experiments,
really, has occurred over the course

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of arguably the last century.
I'm trying to find the genes that

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are mutated in cancer cells.
And many people point to landmark

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experiments done in 1910 by an
investigator at the Rockefeller

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University by the name of
Payton Rauss.

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Payton Rauss was a virologist,
studied viruses at Rockefeller

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University. And one day, a farmer
from Long Island brought a

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prize-winning hen to this famous
doctor at Rockefeller University in

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New York because the hen was sick.
Specifically, the hen had a big

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tumor in its breast muscle. And
if you remember, that would

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mean the tumor was a sarcoma.
And he wanted the doctor to cure his

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prize-winning hen.
Payton Rauss said,

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thank you, I'll do what I can,
and then prominently killed the bird

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in order to try to study what was
giving rise to this tumor.

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And he did a famous experiment.

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He took the tumor, ground it up,
filtered it so that

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there were no cells present in the
filtrate, and also used a small

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enough pour filter so that bacteria,
which were also known at that time,

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were removed. So, he had a
filtrate that contained no

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cells, no cancer cells,
and no bacteria.

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And he took this and injected it
into a non-tumor bearing bird.

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And he observed over time that
this bird developed a tumor.

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So he was able to demonstrate the
transmissibility of something which

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could cause cancer in an
otherwise unaffected animal.

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Because it wasn't cells and
it wasn't bacteria,

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he suggested that
it was a virus.

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And it turned out to be, over the
course of the next 50 years

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or so, the virus which was
responsible for this disease was

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identified and given the name
Rauss sarcoma virus,

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named after Payton Rauss. And the
structure of this virus was

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also determined. It was found
to be a retrovirus.

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Retroviruses are in the
class of HIV.

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Their genomes are made of RNA,
and they convert it to a DNA form.

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We'll learn a little bit more
about retroviruses in a few lectures.

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It was a retrovirus, and the
structure of its genome was

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determined eventually. And it
was found to contain in its

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genome the genes that the
virus required for replication.

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And all viruses of this class
has a very similar set of replication

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genes. They need to produce more
virus. But at the end of the viral

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genome, at the extreme right side
of the viral genome,

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there was another gene which was
given the name sarc for sarcoma.

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And it was termed an oncogene,
onco for mass,

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oncology, cancer associated gene,
sarc-oncogene.

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So, the virus it was found can
cause cancer in birds by virtue of

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transferring into those cells a
potent cancer associated gene called

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sarc. And Payton Rauss went onto
win the Nobel Prize,

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actually about 50 years later in
the early 1960s for this work on the

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discovery of Rauss sarcoma virus.
And it was extremely important.

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But it was still a mystery as to
where this cancer associated gene

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came from. Where did the sarc
gene originate? Was it a viral gene

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which the virus used in some
fashion to cause cells to proliferate,

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perhaps for its own replication?
Or did it come from some other

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source, some other origin?
And here in the story entered two

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researchers, Mike Bishop and
Harold Varmus. Mike Bishop is now

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chancellor at UCSF. Harold
Varmus is president at

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Memorial Sloan-Kettering.
They did this work at UCSF,

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and I got quite familiar with it
because I ended up working for

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Harold Varmus as a Ph.D student.
This work was done

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before I got there in about 1975.
And they set out to ask this

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question, what is the origin of
this sarc-oncogene?

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Where did it come from? And what
they were able to show was

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that this Rauss sarcoma virus
derived from a virus that was

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related to it called avian
leucosis virus.

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And avian leucosis virus had
only those replication genes,

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the genes that the virus uses to
produce more virus what they were

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able to show was that this
avian leucosis virus,

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when it infects cells, and
here's a chicken cell,

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and here's the nucleus of that cell,
inside that cell are normal copies

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of the sarc gene. They were
able to show that the

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virus actually acquires this
sarc-oncogene from the cells that it

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infects. Now, it doesn't
do that every time.

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In fact, most of the time,
the virus just goes in and replicates

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and produces more virus.

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But rarely, and really it's very
rarely, the virus actually acquires

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a piece of the sarc gene and
sticks it into its own genome,

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thereby creating --

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-- Rauss sarcoma virus,
which has the structure that I

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showed you above with the
replication genes and

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the sarc oncogene. And this
is done through a process

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called transduction, which is
a form of recombination.

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The main point of
this observation,

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and the really important point
of this observation was this potent

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oncogene is present
in chicken cells.

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The sarc is a cellular gene.

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It's not just a viral gene.
It's derived from host cells of chickens,

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and it's not only in chickens.
It's in all vertebrates including

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humans. So, you all have a sarc
gene in your

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cells, too. And this was a
really momentous observation.

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And it led Bishop and Varmus to
win the Nobel Prize back in 1989.

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So, what I've just told you is
that your cells carry a potent oncogene,

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sarc. So, why don't you
all have sarcomas?

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Anybody? Remember,

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we are still being filmed.
Anybody? Yes, somebody's got their

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hand up, but I don't see them.
Yes? Almost right. The gene is

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not expressed properly, OK?
The virus has co-opted this

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gene and placed it into its own
genome. And now this gene is being

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regulated improperly. Maybe it's
expressed too well or at

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the wrong time,
or in the wrong cells.

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And some combination of those
things allows the gene to cause

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cancer in this setting, whereas
when the gene is properly

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regulated, and the cells of
chickens or in you, it's not cancer

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associated. It doesn't cause
cancer in its state in the cellular genome.

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So, this was extremely
important, and as I said,

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led to the Nobel Prize for
these guys. RSV --

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-- is what's called an acutely

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transforming retrovirus,
acutely transforming meaning that it

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can transform cells, turn
them from normal to cancerous

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quickly. And as I said,
it's a retrovirus. We now know of

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many acutely transforming
viruses of animals: mice,

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rats, chickens, turkeys, other things.

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And from them, we have learned
the identity of

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about 50 or so oncogenes, all of
which were derived from the

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host cell. So, we know from this
type of experiment

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of many potential cancer
associated genes. Importantly,

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there are no known acutely
transforming viruses of humans.

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Importantly, the vast majority
of human cancers are not associated

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with viral agents. Occasionally,
there are such cases,

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but they are quite rare. Most
major tumor types are not caused by viral

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infection. But there is one
big exception to this --

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-- human papilloma virus.

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Human papilloma virus, or HPV,
and specifically high-risk types of

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HPV called type 16 and type 18
are associated with cervical cancer in

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the US, but mostly in other parts
of the world. And that's important,

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because if one can control exposure
to HPV, or eradication of HPV,

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one could do something about
cervical cancer,

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and that's actually happening
now. And I'll mention that again next

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lecture. But I want you to
remember that most cancers,

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human cancers, are not virus
associated. But the viruses have

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taught us a great deal about
cancer genetics. OK,

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so if most human cancers don't
arise from a viral infection,

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the infection with an oncogene
carrying virus,

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then where do these mutations
come from?

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What are the mutations in human
cancer? And here,

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just after these experiments
were done, around 1980,

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comes another one of our local
heroes, Bob Weinberg,

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who has a laboratory at the
Whitehead Institute here at MIT.

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He hasn't yet won the Nobel Prize,
but I think he's going to, based on

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this work and a lot of other work
probably in the next several years.

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And what Weinberg's lab did
back in this early 1980s.

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Was to look in human cancers,
so not cancers of animals, but in

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human cancers.
And specifically,

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they took bladder cancers
and isolated the cells,

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and grew them
in the laboratory.

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They assumed that inside of
the cells were altered genes,

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and to become altered through
the life of this individual,

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giving rise to this cancer. And
they wanted to know what those

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genes were. And so, they
isolated the DNA from these

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bladder cancer cells, and they
transfected it into cells

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from mice. They sheared it up,
and they introduced it. That's what

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transfected means, into
the cells of mice.

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And these cells that they put
them into were fairly normal --

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-- mouse cells,

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by which I mean they grew a single
layer on the bottom of the dish.

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There are structures look like
normal cells as opposed to cancer

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cells. They were relatively normal
cells. And what they observed was

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that a small percentage of these
cells that had picked up some bits

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and pieces of the DNA derived from
this human cancer began to

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behave abnormally --

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-- and produced what were
referred to as transformed cells,

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cells that looked more like
cancer cells. So they had converted

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otherwise normal mouse cells to
what looked like cancer cells through the

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introduction of one or
more human genes.

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And they further demonstrated that
these were cancer associated cells,

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cancer-causing cells, by introducing
them into immunocompromised mice,

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and observing that over time these
cells could produce tumors.

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So these were cancer cells indeed.
So that's very interesting because

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it says that there are altered genes
in these cancer cells that can

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convert normal mouse cells into
cancer cells. And then to obvious

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question was, what are those genes?
What is the gene that's responsible

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for this process? So, they --

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-- isolated the human DNA
from the transformed mouse cells.

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And I won't tell you how they
did that, but suffice it to say,

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it's possible to isolate the human
DNA away from the mouse DNA in these

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transformed mouse cells.
They cloned the human DNA into

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bacteria using recombinant DNA
methods that we've talked to you

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about previously.

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And they sequenced the
cancer-causing gene.

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It turns out it was just a
single gene that was providing

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this property.

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And it turned out that this gene
was H-RAS, RAS being a gene that

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we've talked to you about before,
and important signaling protein in

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cells. Moreover, they
sequenced the copy of RAS that

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was present in the cancer cells,
and compared it to the sequence in

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normal cells in the other
cells of this individual.

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00:20:15,000 --> 00:20:20,000
And they found a single alteration.
OK, so what you are looking at here

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00:20:20,000 --> 00:20:24,000
now is the sequence of the H-RAS
gene in normal people or in the

229
00:20:24,000 --> 00:20:28,000
normal cells of this cancer patient
has a particular sequence.

230
00:20:28,000 --> 00:20:32,000
It encodes a protein, H-RAS,
which if you recall is an important

231
00:20:32,000 --> 00:20:36,000
signaling protein that shuttles
between a GTP-bound form

232
00:20:36,000 --> 00:20:41,000
and a GDP bound form.
In its GTP bound form,

233
00:20:41,000 --> 00:20:45,000
it's active and signals, and
then it undergoes hydrolysis

234
00:20:45,000 --> 00:20:49,000
reaction. So, the GTP is
converted to GDP,

235
00:20:49,000 --> 00:20:53,000
and becomes inactive. The mutant
copy found in the cancer cells and

236
00:20:53,000 --> 00:20:57,000
in these transformed mouse cells
has been alteration which blocks the

237
00:20:57,000 --> 00:21:01,000
ability of the proteins undergo
GTP hydrolysis. Therefore,

238
00:21:01,000 --> 00:21:05,000
the protein gets locked into
this active signaling state,

239
00:21:05,000 --> 00:21:09,000
and stimulates
proliferation continuously.

240
00:21:09,000 --> 00:21:14,000
And that's why it's a
cancer associated gene.

241
00:21:14,000 --> 00:21:19,000
OK, so here we have for the
first time evidence that a normal,

242
00:21:19,000 --> 00:21:24,000
cellular gene gets mutated,
presumably by an error or by

243
00:21:24,000 --> 00:21:29,000
exposure to some mutagen that
is responsible at least far an aspect

244
00:21:29,000 --> 00:21:34,000
of the transformed state.
OK, so that's very,

245
00:21:34,000 --> 00:21:40,000
very interesting. It also
raises an important question.

246
00:21:40,000 --> 00:21:45,000
If you note that the change
between the normal gene and the abnormal

247
00:21:45,000 --> 00:21:51,000
gene is signal-base change that
converts this alanine residue,

248
00:21:51,000 --> 00:21:56,000
sorry, this glycemic residue to
a valine residue: single base change.

249
00:21:56,000 --> 00:22:02,000
Such single base changes occur
in your cells all the time.

250
00:22:02,000 --> 00:22:07,000
It's estimated that the mutation
rate inside your cells is about ten

251
00:22:07,000 --> 00:22:12,000
to the minus ninth per cell division.
And if you calculate how many cells

252
00:22:12,000 --> 00:22:17,000
you have in your body at any
given time based on how many cell

253
00:22:17,000 --> 00:22:22,000
divisions have arisen, we can
estimate that there are about

254
00:22:22,000 --> 00:22:28,000
ten to the third to
ten to the fifth --

255
00:22:28,000 --> 00:22:35,000
-- rasmutin

256
00:22:35,000 --> 00:22:41,000
cells in all of U as you sit
there now. You are all carrying 1,

257
00:22:41,000 --> 00:22:46,000
00-100,000 cells that have
this very mutation. So,

258
00:22:46,000 --> 00:22:52,000
why don't all of you have cancer?
Why don't all of you have bladder

259
00:22:52,000 --> 00:22:58,000
cancer or some other form of
cancer? Anybody?

260
00:22:58,000 --> 00:23:05,000
The reason is that cancer
is not a

261
00:23:05,000 --> 00:23:09,000
single step process.
Cancer cells arise from normal

262
00:23:09,000 --> 00:23:13,000
cells to the acquisition of
many mutations that occur over the

263
00:23:13,000 --> 00:23:17,000
lifetime of the individual.
So a single mutation in RAS is not

264
00:23:17,000 --> 00:23:21,000
enough to give you a cancer.
It might be enough to cause the

265
00:23:21,000 --> 00:23:25,000
cells to behave somewhat abnormally.
But that in and of itself does not

266
00:23:25,000 --> 00:23:29,000
produce even hyperplasia
necessarily, let alone an adenoma,

267
00:23:29,000 --> 00:23:34,000
an adenocarcinoma and
invasive cancer.

268
00:23:34,000 --> 00:23:37,000
It's necessary to acquire multiple
mutations. So you may have cells

269
00:23:37,000 --> 00:23:41,000
that are on their way.
But they are not fully there.

270
00:23:41,000 --> 00:23:45,000
And hopefully they'll never
get there. Now, I want to remind you

271
00:23:45,000 --> 00:23:49,000
that this pathway, this
signaling pathway that involves

272
00:23:49,000 --> 00:23:53,000
RAS is something we taught you
about previously. This should look

273
00:23:53,000 --> 00:23:57,000
familiar based on the cell
signaling lectures that we had before.

274
00:23:57,000 --> 00:24:01,000
RAS sits right here, this important
signaling molecule that links events

275
00:24:01,000 --> 00:24:05,000
that take place at
the cell membrane.

276
00:24:05,000 --> 00:24:09,000
Sound at all familiar? Yes?
Transmembrane receptors?

277
00:24:09,000 --> 00:24:13,000
Growth factor receptors that
bind to ligands and cause a series of

278
00:24:13,000 --> 00:24:17,000
events that for example convert
RAS to its GTP bound form,

279
00:24:17,000 --> 00:24:22,000
and then initiate a kinase
cascade of protein kinases,

280
00:24:22,000 --> 00:24:26,000
map kinase, kinase, kinase,
kinase, kinase, kinase, remember the

281
00:24:26,000 --> 00:24:30,000
little movie? Moves into the
nucleus, phosphorylates

282
00:24:30,000 --> 00:24:34,000
transcription factors that initiate
the expression of target genes that,

283
00:24:34,000 --> 00:24:39,000
for example cause cells
to proliferate.

284
00:24:39,000 --> 00:24:43,000
And when RAS gets stuck in its
on state as it does here,

285
00:24:43,000 --> 00:24:47,000
these signals are sent
constitutively so that the cell is

286
00:24:47,000 --> 00:24:51,000
being told to divide all the time
instead of in a regulated fashion.

287
00:24:51,000 --> 00:24:55,000
Now, I point out all the different
components which I've told you about

288
00:24:55,000 --> 00:24:59,000
in previous lectures because,
in fact, it's not just RAS, this one

289
00:24:59,000 --> 00:25:04,000
signaling protein that gets
mutated in cancers.

290
00:25:04,000 --> 00:25:08,000
Many different of these signaling
proteins get mutated in cancers.

291
00:25:08,000 --> 00:25:12,000
It is a very common pathway that's
affected in not necessarily all but

292
00:25:12,000 --> 00:25:16,000
a very high percentage of
cancers of different types.

293
00:25:16,000 --> 00:25:20,000
The RAS gene is mutated at
about 30% of cancers.

294
00:25:20,000 --> 00:25:24,000
30% of all cancers have mutations
in one of the RAS genes including,

295
00:25:24,000 --> 00:25:28,000
for example, 90% of pancreatic
cancer, 50% of colon cancers,

296
00:25:28,000 --> 00:25:33,000
30% of lung cancers carry
mutations in this gene.

297
00:25:33,000 --> 00:25:36,000
But other components get
mutated as well. For example,

298
00:25:36,000 --> 00:25:40,000
the growth factor or growth factor
receptor genes get altered in

299
00:25:40,000 --> 00:25:43,000
cancers. They become amplified.
And I'll mention what I mean by

300
00:25:43,000 --> 00:25:47,000
that in a second period.
We observe translocations,

301
00:25:47,000 --> 00:25:50,000
these chromosomal alterations
in different genes,

302
00:25:50,000 --> 00:25:54,000
including the transcription factor
genes that lie at the bottom of this

303
00:25:54,000 --> 00:25:57,000
pathway. There can be deletions
that affect the function of these

304
00:25:57,000 --> 00:26:01,000
proteins, make the function
abnormally, and subtle mutations

305
00:26:01,000 --> 00:26:04,000
like the one in RAS itself.
And there are other examples,

306
00:26:04,000 --> 00:26:08,000
subtle mutations that
lock the proteins, for example,

307
00:26:08,000 --> 00:26:21,000
in an active
signaling state.

308
00:26:21,000 --> 00:26:27,000
So, DNA amplification is one such
mechanism to alter the function of a

309
00:26:27,000 --> 00:26:33,000
cellular gene in cancer.
And an example of that is in a case

310
00:26:33,000 --> 00:26:45,000
of a gene called HER2.

311
00:26:45,000 --> 00:26:51,000
It encodes a growth factor
receptor like item number two up here.

312
00:26:51,000 --> 00:26:58,000
It encodes a growth factor receptor.
And when it's present in single

313
00:26:58,000 --> 00:27:04,000
copy or two copies per cell,
it produces a certain concentration

314
00:27:04,000 --> 00:27:11,000
of this growth factor
receptor protein. And it signals

315
00:27:11,000 --> 00:27:17,000
at normal levels. And that
gives rise to a normal

316
00:27:17,000 --> 00:27:21,000
amount of proliferation.
And this is particularly important

317
00:27:21,000 --> 00:27:25,000
in cells of the mammary gland.
Mammary epithelial cells depend on

318
00:27:25,000 --> 00:27:30,000
this signaling pathway to be
produced in the right amounts.

319
00:27:30,000 --> 00:27:36,000
Unfortunately at some frequency
in cancer cells, and specifically

320
00:27:36,000 --> 00:27:42,000
cancer cells of the breast, and
sometimes ovary, an

321
00:27:42,000 --> 00:27:48,000
amplification event takes place
so that now instead of having just one

322
00:27:48,000 --> 00:27:54,000
copy of this HER2 gene on the
chromosome, there are many copies.

323
00:27:54,000 --> 00:28:00,000
An error takes place in the
replication process so that instead

324
00:28:00,000 --> 00:28:06,000
of just copying this gene one
time, it gets copied multiple times.

325
00:28:06,000 --> 00:28:12,000
So, now we have too many copies
of this gene, and therefore in these

326
00:28:12,000 --> 00:28:19,000
cells, the concentration of this
receptor is significantly higher

327
00:28:19,000 --> 00:28:26,000
than in normal cells such that
in these cells, there might be ten

328
00:28:26,000 --> 00:28:33,000
times or a hundred times the amount
of signaling, or ten to a hundred

329
00:28:33,000 --> 00:28:38,000
times the amount of proliferation.
So, the dysregulation of this gene's

330
00:28:38,000 --> 00:28:42,000
function by making too much of the
protein, it's an otherwise normal

331
00:28:42,000 --> 00:28:46,000
protein. There's just too much
of it, causes the cells to get

332
00:28:46,000 --> 00:28:50,000
overstimulated and divide too often.
That's bad news for cancer. The

333
00:28:50,000 --> 00:28:54,000
good news is we now have a therapy,
and I'll put you about that next

334
00:28:54,000 --> 00:28:58,000
time, directed
against this protein.

335
00:28:58,000 --> 00:29:07,000
I'll mention another
mechanism of

336
00:29:07,000 --> 00:29:11,000
activation, and that's translocation.
I showed you slides of cancer cells

337
00:29:11,000 --> 00:29:16,000
that have broken chromosomes or
rearranged chromosomes.

338
00:29:16,000 --> 00:29:20,000
Those rearrangements often
produce abnormal genes,

339
00:29:20,000 --> 00:29:25,000
sometimes fusions between one
gene and another. Other times,

340
00:29:25,000 --> 00:29:29,000
rearrangements of the promoter
of the gene such that the normal

341
00:29:29,000 --> 00:29:34,000
promoter of the gene, which is
responsible for its level

342
00:29:34,000 --> 00:29:38,000
of expression is replaced by a
different promoter,

343
00:29:38,000 --> 00:29:43,000
which might give too much
expression. And that's true
for an important

344
00:29:43,000 --> 00:29:48,000
oncogene called MIC.
The MIC oncogene is a

345
00:29:48,000 --> 00:29:53,000
transcription factor. It's like
number eight down there.

346
00:29:53,000 --> 00:29:58,000
It's a transcription factor
that binds to DNA and regulates the

347
00:29:58,000 --> 00:30:03,000
expression of other genes.
And it's normally expressed from a,

348
00:30:03,000 --> 00:30:09,000
let's call it, weak promoter.
You don't want to have too much of this

349
00:30:09,000 --> 00:30:15,000
protein produced. It needs to
be produced at normal

350
00:30:15,000 --> 00:30:21,000
levels. So, we get a certain
concentration of the

351
00:30:21,000 --> 00:30:32,000
protein, which gives --

352
00:30:32,000 --> 00:30:37,000
-- regulated cell division.
You get as much television as you

353
00:30:37,000 --> 00:30:42,000
need based on the appropriate signals
in the environment of those

354
00:30:42,000 --> 00:30:47,000
cells. In certain types of
leukemia, one sees a translocation.

355
00:30:47,000 --> 00:30:52,000
The DNA is broken and rejoined so
that the MIC oncogene doesn't have

356
00:30:52,000 --> 00:30:57,000
its own promoter anymore.
Instead, it's next to a very strong

357
00:30:57,000 --> 00:31:04,000
promoter.

358
00:31:04,000 --> 00:31:09,000
And now, again, the concentration
of this protein is

359
00:31:09,000 --> 00:31:15,000
increased. And this gives rise
to deregulated cell

360
00:31:15,000 --> 00:31:21,000
division, same idea. Change the
proper regulation of

361
00:31:21,000 --> 00:31:27,000
this signaling network, and now
the signaling network is

362
00:31:27,000 --> 00:31:31,000
broken. And as I said,

363
00:31:31,000 --> 00:31:35,000
there are many, many such
genes that are known.

364
00:31:35,000 --> 00:31:39,000
We know about 50 or so oncogenes
from these acutely transforming

365
00:31:39,000 --> 00:31:42,000
viruses. We probably know of 50
more oncogenes because of their

366
00:31:42,000 --> 00:31:46,000
changes in the DNA of cancer cells.
And by the way, there's a lot of

367
00:31:46,000 --> 00:31:50,000
overlap between these two sets.
Many of the genes that were found

368
00:31:50,000 --> 00:31:53,000
in the context of viruses were later
found to be mutant in human cells

369
00:31:53,000 --> 00:31:57,000
having nothing to do with viruses.
And RAS is one such example. MIC,

370
00:31:57,000 --> 00:32:01,000
actually, is another example, and
a relative of this HER2 gene

371
00:32:01,000 --> 00:32:08,000
is another example.

372
00:32:08,000 --> 00:32:16,000
OK.

373
00:32:16,000 --> 00:32:21,000
OK, so the slide is meant to
reinforce this notion that cell

374
00:32:21,000 --> 00:32:27,000
division, normal production of cells,
is tightly regulated through these

375
00:32:27,000 --> 00:32:32,000
signaling networks, and that
oncogenes function normally,

376
00:32:32,000 --> 00:32:38,000
their normal cellular function
is to control this process.

377
00:32:38,000 --> 00:32:42,000
That's what they do in normal
development. That's what they do in

378
00:32:42,000 --> 00:32:47,000
normal individuals. But
they can be subverted in cancer

379
00:32:47,000 --> 00:32:51,000
cells by mutation. It makes
them act in a deregulated

380
00:32:51,000 --> 00:32:56,000
or uncontrolled fashion. There
is another set of genes that

381
00:32:56,000 --> 00:33:01,000
act, in a sense, in opposition
to the oncogenes.

382
00:33:01,000 --> 00:33:05,000
These genes function to inhibit
this process of cell division.

383
00:33:05,000 --> 00:33:09,000
And they go by another name:
tumor suppressor genes.

384
00:33:09,000 --> 00:33:13,000
Tumor suppressor genes function
to block the normal cell division

385
00:33:13,000 --> 00:33:18,000
process or to inhibit the
production of cells. And they,

386
00:33:18,000 --> 00:33:22,000
too, are important in cancer.
You can think of these genes as the

387
00:33:22,000 --> 00:33:26,000
on switch and the off switch
of an electrical circuit.

388
00:33:26,000 --> 00:33:31,000
In cancer cells,
oncogenes become hyperactive.

389
00:33:31,000 --> 00:33:36,000
When the lights switch gets stuck
in the on state. OK,

390
00:33:36,000 --> 00:33:41,000
it gets taped open. So now,
you're getting signaling

391
00:33:41,000 --> 00:33:46,000
through this circuit continuously.
Tumor suppressor genes normally

392
00:33:46,000 --> 00:33:51,000
inhibit cell division. And so,
in cancer, these genes get

393
00:33:51,000 --> 00:33:56,000
inactivated. It's like the light
switch getting turned off.

394
00:33:56,000 --> 00:34:01,000
And the process is normally
regulated that way, and

395
00:34:01,000 --> 00:34:06,000
these get stuck off. OK, so want
to tell you a little bit

396
00:34:06,000 --> 00:34:10,000
about tumor suppressor genes now
which is the other major category of

397
00:34:10,000 --> 00:34:14,000
cancer associated genes that
we'll cover here. Briefly,

398
00:34:14,000 --> 00:34:18,000
just to reiterate that point,
in addition to the light switch

399
00:34:18,000 --> 00:34:23,000
analogy, people often use the
gas pedal and breaks analogy,

400
00:34:23,000 --> 00:34:27,000
or the go signal and stop signal
analogy. So, I'll use that as well.

401
00:34:27,000 --> 00:34:31,000
In normal cells, when they
receive the signals to divide,

402
00:34:31,000 --> 00:34:35,000
they are stimulated to go,
that is, to divide to make more such

403
00:34:35,000 --> 00:34:40,000
cells. This happens in
development when you

404
00:34:40,000 --> 00:34:44,000
need to make more cells.
It happens in wound healing.

405
00:34:44,000 --> 00:34:48,000
It happens in normal homeostasis.
And then, when the process is

406
00:34:48,000 --> 00:34:53,000
completed, there are stop signals
sent. The go signals represent

407
00:34:53,000 --> 00:34:57,000
oncogenes, normally regulated,
stop signals tumor suppressor genes

408
00:34:57,000 --> 00:35:02,000
normally regulated.
In cancer, oncogenes become

409
00:35:02,000 --> 00:35:06,000
deregulated. And pushed too strong
they send constitutive signals.

410
00:35:06,000 --> 00:35:11,000
And as a consequence,
too many cells are produced.

411
00:35:11,000 --> 00:35:15,000
And likewise, the stop signs, the
stop signals are lost so that

412
00:35:15,000 --> 00:35:20,000
one produces even more cells.
OK, so hopefully these are useful

413
00:35:20,000 --> 00:35:24,000
analogies to think about these two opposing sets of oncogenes and tumor

414
00:35:24,000 --> 00:35:29,000
suppressor genes. OK, so to get into tumor suppressor

415
00:35:29,000 --> 00:35:33,000
genes, sorry, this just makes the point, oncogene mutations,

416
00:35:33,000 --> 00:35:38,000
tumor suppressor gene mutations. To get into tumor suppressor genes,

417
00:35:38,000 --> 00:35:42,000
the second class, an important class of human cancer genes,

418
00:35:42,000 --> 00:35:46,000
let me tell you a little bit about this disease which is called

419
00:35:46,000 --> 00:35:50,000
retinoblastoma. Retinoblastoma is a childhood tumor

420
00:35:50,000 --> 00:35:54,000
of the eye, of the retina. It's not very common. Only about

421
00:35:54,000 --> 00:35:58,000
one in 40,000 children develop this tumor. But it's very important what

422
00:35:58,000 --> 00:36:03,000
it's taught us about cancer genes. And you can see in the normal eye a

423
00:36:03,000 --> 00:36:09,000
normal looking retina, and in the cancer containing eye,

424
00:36:09,000 --> 00:36:15,000
these blobs of tumors. We now know the gene that's responsible for this

425
00:36:15,000 --> 00:36:21,000
disease, the gene that's mutated in the formation of this disease.

426
00:36:21,000 --> 00:36:28,000
And that gene is the gene called RB, named after retinoblastoma.

427
00:36:28,000 --> 00:36:44,000
And this gene functions as an

428
00:36:44,000 --> 00:36:51,000
important regulator of cell cycle progression. If you remember cell

429
00:36:51,000 --> 00:36:58,000
cycle progression, mitosis, S phase mitosis,

430
00:36:58,000 --> 00:37:05,000
the G1 phase, the G2 phase, again, dim memories from earlier in

431
00:37:05,000 --> 00:37:11,000
the class. The RB protein,

432
00:37:11,000 --> 00:37:16,000
which goes by the name of PRB, this is the protein encoded by this

433
00:37:16,000 --> 00:37:22,000
tumor suppressor gene, RB, acts to inhibit cell cycle

434
00:37:22,000 --> 00:37:27,000
progression. That's its function. It blocks cell cycle progression

435
00:37:27,000 --> 00:37:32,000
literally like the brakes on a car. Now, you might ask,

436
00:37:32,000 --> 00:37:37,000
if it's there and functioning, then how do you ever get cell cycle

437
00:37:37,000 --> 00:37:43,000
progression? And the answer is that this active inhibitor can be

438
00:37:43,000 --> 00:37:48,000
inactivated through phosphorylation. When this protein gets

439
00:37:48,000 --> 00:37:54,000
phosphorylated, it becomes inactive.

440
00:37:54,000 --> 00:38:00,000
And now, cell cycle progression

441
00:38:00,000 --> 00:38:14,000
can occur.

442
00:38:14,000 --> 00:38:19,000
It becomes inactive when the cell receives growth stimulatory signals

443
00:38:19,000 --> 00:38:24,000
like through that signaling network that I mentioned previously.

444
00:38:24,000 --> 00:38:29,000
These signals, then, act on cycline CDK complexes,

445
00:38:29,000 --> 00:38:34,000
cell cycle associated kinases that you were taught about in the cell

446
00:38:34,000 --> 00:38:39,000
cycle lecture, and these go about inactivating the

447
00:38:39,000 --> 00:38:44,000
RB protein, allowing the cells to cycle.

448
00:38:44,000 --> 00:38:50,000
In addition, when the cells shouldn't be cycling in the presence

449
00:38:50,000 --> 00:38:56,000
of growth inhibitory signals, inhibitory signals are sent to the

450
00:38:56,000 --> 00:39:02,000
cycline dependent kinase proteins, and they don't function, thereby

451
00:39:02,000 --> 00:39:08,000
blocking the inactivation of RB. RB stays in its active state and

452
00:39:08,000 --> 00:39:13,000
blocks cell cycle progression. OK, so that's how this really

453
00:39:13,000 --> 00:39:19,000
important cell cycle regulator functions more or less.

454
00:39:19,000 --> 00:39:24,000
And it's a very important human tumor suppressor gene,

455
00:39:24,000 --> 00:39:30,000
which is itself mutated and probably 30 or 40% of human cancers.

456
00:39:30,000 --> 00:39:36,000
And the pathway that regulates this gene, this pathway,

457
00:39:36,000 --> 00:39:43,000
is mutated in probably 95% of human cancers very, very commonly affected.

458
00:39:43,000 --> 00:39:49,000
OK, so given what I've told you so far, would you expect the mutations

459
00:39:49,000 --> 00:39:56,000
that we find in the RB gene in human cancer to be activating mutations or

460
00:39:56,000 --> 00:40:01,000
inactivating mutations? Inactivating. It's the brakes.

461
00:40:01,000 --> 00:40:04,000
Cancer cells want to get rid of the brakes.

462
00:40:04,000 --> 00:40:15,000
And so we find inactivating

463
00:40:15,000 --> 00:40:23,000
mutations in the RB gene in human cancers like deletions or nonsense

464
00:40:23,000 --> 00:40:31,000
mutations, that is, stop codon mutations.

465
00:40:31,000 --> 00:40:35,000
OK, that's the kind of mutations we find in this gene.

466
00:40:35,000 --> 00:40:40,000
For the oncogenes, which we focused on at the beginning,

467
00:40:40,000 --> 00:40:45,000
are the mutations in oncogenes dominant or recessive?

468
00:40:45,000 --> 00:40:49,000
Are the mutations in the oncogenes dominant or recessive?

469
00:40:49,000 --> 00:40:54,000
The oncogene mutations are dominant. When you acquire mutation in an

470
00:40:54,000 --> 00:40:59,000
oncogene, whether it's a subtle mutation or an amplification,

471
00:40:59,000 --> 00:41:04,000
it doesn't matter what's happening to the other allele.

472
00:41:04,000 --> 00:41:08,000
This thing has new function, gain of function. It will transform

473
00:41:08,000 --> 00:41:12,000
or start the process of transformation,

474
00:41:12,000 --> 00:41:16,000
regardless of what is happening to the other allele.

475
00:41:16,000 --> 00:41:21,000
And that's evident, by the way, in this transfection

476
00:41:21,000 --> 00:41:25,000
experiment. This would only work if it were a dominant mutation.

477
00:41:25,000 --> 00:41:29,000
So, oncogene mutations are dominant mutations. Are tumor suppressor

478
00:41:29,000 --> 00:41:34,000
gene mutations dominant or recessive? These are recessive mutations.

479
00:41:34,000 --> 00:41:38,000
Here, it matters, the state of the other allele of the

480
00:41:38,000 --> 00:41:43,000
RB gene. As long as you have one functional copy of the brakes,

481
00:41:43,000 --> 00:41:48,000
you can do this. You can control cell cycle regulation.

482
00:41:48,000 --> 00:41:52,000
In order to make this dysregulated, you need to get rid of both copies.

483
00:41:52,000 --> 00:41:57,000
You need to get rid of the brakes entirely. So, these mutations

484
00:41:57,000 --> 00:42:02,000
are recessive. And this class of mutations are

485
00:42:02,000 --> 00:42:08,000
recessive. Tumor suppressor gene mutations are recessive.

486
00:42:08,000 --> 00:42:14,000
So this raises an interesting question. Tumor suppressor gene

487
00:42:14,000 --> 00:42:19,000
mutations are recessive. I've told you that tumor suppressor

488
00:42:19,000 --> 00:42:25,000
gene mutations occur commonly in human cancer, not just this RB gene

489
00:42:25,000 --> 00:42:31,000
but others too, and yet you are all diploid.

490
00:42:31,000 --> 00:42:36,000
You all carry two copies of the tumor suppressor genes.

491
00:42:36,000 --> 00:42:41,000
So, how do we ever find mutations in these genes in human cancer?

492
00:42:41,000 --> 00:42:46,000
Is it enough to mutate just one copy? No, because they are

493
00:42:46,000 --> 00:42:52,000
recessive mutations. So, to find mutations in tumor

494
00:42:52,000 --> 00:42:57,000
suppressor genes in human cancer, it's not enough to mutate just one

495
00:42:57,000 --> 00:43:03,000
copy. Both copies need to be mutated.

496
00:43:03,000 --> 00:43:13,000
Recessive mutations require there be

497
00:43:13,000 --> 00:43:18,000
two mutations. And this is what typically happens

498
00:43:18,000 --> 00:43:23,000
in the development of a tumor with a mutation in the RB gene,

499
00:43:23,000 --> 00:43:27,000
or any other tumor suppressor gene. Again, there are probably 20 to 50

500
00:43:27,000 --> 00:43:32,000
different tumor suppressor genes. Initially, a cell, which has a

501
00:43:32,000 --> 00:43:37,000
normal copy of RB on both copies of chromosome 13,

502
00:43:37,000 --> 00:43:42,000
that's where this gene sits, and here we are looking at a linked

503
00:43:42,000 --> 00:43:47,000
polymorphic marker, big A little a, if you remember

504
00:43:47,000 --> 00:43:52,000
polymorphic markers, big A little A.

505
00:43:52,000 --> 00:44:00,000
First, one copy of RB gets mutated. In this cell, there is still a

506
00:44:00,000 --> 00:44:06,000
normal copy of RB. And therefore,

507
00:44:06,000 --> 00:44:10,000
this cell that has acquired this mutation is normal.

508
00:44:10,000 --> 00:44:14,000
It's functionally normal. However, it now has just one copy

509
00:44:14,000 --> 00:44:18,000
of the RB gene. And so, it's predisposed to

510
00:44:18,000 --> 00:44:22,000
becoming a cancer cell if that normal copy gets lost.

511
00:44:22,000 --> 00:44:26,000
And what this shows you are various mechanisms by which the

512
00:44:26,000 --> 00:44:31,000
normal copy of the RB gene can be lost.

513
00:44:31,000 --> 00:44:36,000
The other copy can acquire its own mutation. Or,

514
00:44:36,000 --> 00:44:42,000
the chromosome that carries the normal copy of the RB gene can get

515
00:44:42,000 --> 00:44:47,000
lost in the developing cancer cell. Or, it can be a recombination of

516
00:44:47,000 --> 00:44:53,000
that, that replaces the normal copy of the gene with the abnormal copy

517
00:44:53,000 --> 00:44:59,000
of the gene. And all three of these things happen in the development of

518
00:44:59,000 --> 00:45:05,000
cancer cells. And depending on which mechanism is used --

519
00:45:05,000 --> 00:45:22,000
-- one can observe something

520
00:45:22,000 --> 00:45:30,000
referred to as loss --

521
00:45:30,000 --> 00:45:34,000
-- of heterozygosity, loss of heterozygosity within the

522
00:45:34,000 --> 00:45:39,000
cancer cell. You will notice that this cancer cell,

523
00:45:39,000 --> 00:45:44,000
sorry, this normal cell, is heterozygous for this A allele,

524
00:45:44,000 --> 00:45:48,000
big A little A. And you will notice that here and here,

525
00:45:48,000 --> 00:45:53,000
the cell has become, the cell carries only one version of that A

526
00:45:53,000 --> 00:45:58,000
allele, big A. So, there was heterozygosity.

527
00:45:58,000 --> 00:46:03,000
And in the cancer cell, there's now loss of heterozygosity.

528
00:46:03,000 --> 00:46:09,000
Loss of heterozygosity is a hallmark for tumor suppressor genes,

529
00:46:09,000 --> 00:46:16,000
the involvement of tumor suppressor genes in the development of cancer.

530
00:46:16,000 --> 00:46:22,000
OK, now I told you several times that proliferation is important and

531
00:46:22,000 --> 00:46:33,000
cell death is important, too.

532
00:46:33,000 --> 00:46:38,000
Cell death control in cancer: I don't have a lot of time to tell you

533
00:46:38,000 --> 00:46:43,000
more about this in this class, but I want to just emphasize that

534
00:46:43,000 --> 00:46:48,000
the pathways that lead from cell death signals,

535
00:46:48,000 --> 00:46:53,000
the signals that tell cells to commit suicide to ultimately undergo

536
00:46:53,000 --> 00:46:59,000
this process called apoptosis are also dysregulated in cancer.

537
00:46:59,000 --> 00:47:07,000
For example, there's a gene called

538
00:47:07,000 --> 00:47:13,000
BCL2, which becomes mutated in certain types of cancer.

539
00:47:13,000 --> 00:47:20,000
BCL2 normally blocks this process of cell death.

540
00:47:20,000 --> 00:47:26,000
And in this type of cancer, it becomes hyperactive. So there's

541
00:47:26,000 --> 00:47:33,000
not as much cell death. BCL2 is an oncogene.

542
00:47:33,000 --> 00:47:41,000
Another gene, very important gene in human cancer is P53.

543
00:47:41,000 --> 00:47:49,000
It stimulates cell death. P53 gets lost in a high percentage

544
00:47:49,000 --> 00:47:57,000
of cancers. It's a tumor suppressor gene, and therefore gets

545
00:47:57,000 --> 00:48:03,000
inactivated. BCL2 is an oncogene.

546
00:48:03,000 --> 00:48:08,000
It gets hyperactivated in cancers. I'll talk more about this pathway

547
00:48:08,000 --> 00:48:14,000
next time, but just to put in your minds, P53 is a very important

548
00:48:14,000 --> 00:48:19,000
signaling protein responsive to many signals upstream,

549
00:48:19,000 --> 00:48:24,000
giving rise to signals that lead to death as well as arrest.

550
00:48:24,000 --> 00:48:30,000
Hang on, please. Hang on. Two minutes left.

551
00:48:30,000 --> 00:48:34,000
And this slide is just a summary of what we've been talking about so far.

552
00:48:34,000 --> 00:48:38,000
Control of proliferation, control of cell death, the normal

553
00:48:38,000 --> 00:48:42,000
balance, perturbations in these pathways, oncogene mutations that

554
00:48:42,000 --> 00:48:46,000
stimulate proliferation, tumor suppressor gene mutations that

555
00:48:46,000 --> 00:48:50,000
stimulate proliferation. Alterations in apoptosis,

556
00:48:50,000 --> 00:48:54,000
oncogene mutations that block apoptosis, tumor suppressor gene

557
00:48:54,000 --> 00:48:58,000
mutations that block apoptosis. All of these are important in

558
00:48:58,000 --> 00:49:03,000
cancer. Next time I'm going to tell you that

559
00:49:03,000 --> 00:49:07,000
some cancers are caused by inherited mutations. And this is a pedigree

560
00:49:07,000 --> 00:49:12,000
of such an example, and lastly to remind you,

561
00:49:12,000 --> 00:49:16,000
mutations affect proliferation in cell death. But they also affect

562
00:49:16,000 --> 00:49:21,000
many other things: invasion, angiogenesis, as well as metastasis.

563
00:49:21,000 --> 00:49:24,000
And I'll stop there.