And he took what was left, which much be very, very small,
smaller than the size of a cell because the cells were removed by
filtration, and he injected it into another bird. Now, a healthy bird.
And strikingly that bird developed a tumor. So he was able to transfer
whatever agent it was that was associated with tumor development
from this bird to this bird. He concluded that it was something
smaller than a cell, and he concluded that it was a virus.
And indeed he was right. Now, the tumor that these birds
developed is called a sarcoma. It's a tumor of the muscle in this
case. And the virus that was responsible for this tumor was named
after Rous, and it goes by the name of Rous sarcoma virus or RSV.
Now, there was great skepticism about the connections between
viruses and cancer at that time, and actually for many decades
thereafter. And so Rous' work was not fully appreciated
for some time. But it was eventually appreciated.
And Rous actually won the Nobel Prize 50 years after he made this
discovery, when it was confirmed that indeed the stuff that he was
studying was highly relevant to the situation in humans.
Not analogous, necessarily, but highly relevant. Over the years,
since that discovery and discoveries made by others,
demonstrated that the Rous sarcoma virus viral genome carried a set of
genes that were familiar.
These are the same sorts of genes carried by many viruses of this
class. Rous sarcoma virus happens to be a retrovirus.
I'll teach you more about those in a future lecture.
It's the same general class that includes HIV. And there were some
familiar genes that were responsible for the replication functions
of the virus. But then there was a new gene
present in this strain of the virus, specific to Rous sarcoma virus,
which was named Src and was later considered to be the relevant
oncogene, the gene that was responsible for the tumor forming
capabilities of this Rous sarcoma virus. So there was then great
interest in what was this Src gene? What was this special gene that
could cause normal cells to become cancer cells?
And research then went on for, again, a number of years studying
Src, studying its biochemical functions, but the real question
that was of great interest over the years was where did Src come from?
Most viruses in this class don't
carry Src, and it wasn't at all clear what the origins of this
cancer-causing gene were. Was it a rare viral gene or perhaps
did it come from the host cells themselves?
And in experiments that were done, actually by my PhD advisor, Harold
Varmus and his colleague Mike Bishop, it was shown that the Src gene
actually does indeed reside in the cells of the host and gets picked up
by a recombination process by the virus. And I'll just illustrate
that for you. It turns out that there's a related virus fairly
common in chickens called avian leukosis virus.
And this is the predecessor of Rous sarcoma virus.
It's a virus that has in its genome just the replication genes.
And I'll just draw a viral capsid around this virus.
So this is the avian leukosis virus with its genome and replication
genes. Now, what they showed was that the avian leukosis virus,
when it infects cells, here's a normal chicken cell.
Most of the time it just makes more
copies of itself like viruses will do. But occasionally,
through a recombination mechanism, the avian leukosis virus will
actually pick up a gene which is present in the normal genome of the
chicken, a gene which looks very much like the Src gene present in
Rous sarcoma virus. And through this recombination
mechanism, the details of which I won't go through,
very rarely you'll get a recombinant virus produced which has in its
genome the replication genes, but now also Src gene. And this
virus, which modifies the Src gene slightly in the course of the
development of the virus, now, when introduced into birds,
will very efficiently cause tumors. And what this showed,
what this actually Nobel Prize winning experiment showed was that
the oncogene, powerful cancer-causing gene resides in the
DNA of the chicken. And actually there were similar
genes, very analogous genes residing in the DNA of all vertebrate species,
including humans. You are sitting there with two
copies of the Src gene inside of you. So a question is,
if that's true, if we all have these oncogenes in our cells why don't we
get cancer? Why aren't we all sitting there as one large sarcoma?
Anybody? Well, there are two answers. Yeah? Somebody.
Claudette is pointing to somebody. Am I blind? Oh, hi. Excellent.
Yes. So what's different about the situation in the chicken cell and
the situation in the viral genome is how the gene is expressed.
Here it might be expressed properly under what we would call
physiological control, express where it's supposed to be
when it's supposed to be. And here it's been removed from
that regulatory network and is expressed perhaps at two high levels
at the wrong time. And under those conditions it can
push cells to inappropriately divide. So the virus has hijacked this gene
and changes its regulation. That's one explanation. There's
potentially another explanation for why we're not all sitting around
with tumors, and I'll come to that in a moment. Now,
as I said, Varmus and Bishop won the Nobel Prize for this work in 1989.
And the day that they won the Nobel Prize Harold Varmus' sister-in-law
was standing in a cafeteria line at Berkeley and she overheard two guys
talking behind her. One guy said to the other,
what are you going to have for lunch? And the other guy said,
I don't know but it ain't going to be the chicken because two guys just
won the Nobel Prize for showing that chicken causes cancer.
[LAUGHTER] Which is not exactly true. But nevertheless.
Now we know since that work that there are a large number of these
viruses which are called collectively acutely transforming
viruses.
There are a large number of acutely transforming viruses that carry in
their genomes and oncogene which they have cooperated from the host
cell. These viruses are not viruses of human beings.
They're viruses of experimental animals and usually generated in an
experimental setting. Mice, rats, chickens,
turkeys, other species have been used to generate such viruses.
And about 50 or so oncogenes have been discovered through this context,
including one that's hopefully familiar to you already and will
come again, members of the Ras gene family which I've told you about as
an important signaling molecule in mitogenic signaling pathways,
and another important oncogene that I'll mention briefly later called
mic, and about 50 more. And it turns out that they were a
very useful source to identify cancer-associated genes in humans.
Many of the genes that we now know are important in human cancer were
initially discovered through that process. Now,
as I said, most of the time human cancer has nothing to do with
viruses so don't be confused. There are a few human cancers that
are virally associated. The major one is cervical cancer.
About 50% of cervical cancers,
particularly in the Developing World, are associated with the virus called
human papillomavirus or HPV, and specifically the high-risk types.
Not all papillomaviruses will cause
cancer. Papillomavirus is the same virus that causes warts,
for example. And there are many papillomaviruses that are not
associated with true malignancies, but human papillomaviruses of the
high-risk type are. And they can give rise to cervical
cancer. Fortunately, companies are now developing
vaccines against HPVs which are greatly affecting the risk of
cervical cancer around the world. So this is a major step forward in
controlling this particular cancer type. So because most human cancers
are not virally associated, there was still some skepticism
about the importance of this discovery for human cancer.
Maybe it was true of the viruses, maybe it was true of these
experimental animals, but is it true of human beings?
Do these oncogenes have anything to do with human cancer?
And this debate went on for a little while longer until Bob
Weinberg here at MIT in about 1980 did the following experiment.
He took DNA not from a bird but from
a human being who carried a tumor in his bladder. So this individual had
bladder cancer. This cancer was put into cell
culture and turned into a cancer cell line, and it was actually this
material that Weinberg's lab worked with. And they asked the question,
are there genes in the cancer cell line that are cancer-causing?
Are there oncogenes that I can discover within those cancer cells?
So the experiment that they did was to take DNA, they isolated DNA from
the cancer cells, and they introduced it into mouse
cells in the laboratory that were "normal".
And what I mean by that is they
looked pretty normal compared to the cancer cells which had a deranged
sort of architecture, as I mentioned last time.
They grew flat and didn't grow on top of each other like cancer cells
will do. And, importantly, if you were to inject
these cells into an immunocompromised mouse they
wouldn't form a tumor. They were nontumorigenic.
Whereas, these cells, if injected into a mouse, would form tumors.
OK? So they isolated DNA from the cancer cells and put it
on the mouse cells. The mouse cells will,
at some frequency, take up the DNA, incorporate the human DNA into their
own genomes and begin to express the genes from the human DNA.
And they found at low frequency that within this population of
otherwise normal-looking mouse cells abnormal colonies of "transformed"
cells could be found. And these transformed cells looked a
lot like the cancer cells. Their shape was different,
they grew on top of one another, and they had other properties that
also made them similar to cancer cells in the sense that if they
injected these cells into an immunocompromised animal,
whereas the normal cells would not form a tumor, these transformed
cells would. So they were able to convert normal cells into cancer
cells through the addition of DNA from a cancer cell line.
So what's going on here? What did they do in this experiment?
Yeah. They eventually did. To get to that point they were able
to isolate from the full genome of the cancer cell an individual gene
that had been transferred into these mouse cells. They isolated that
human gene away from the mouse genes, and then they preformed
DNA sequencing. And they discovered the first human
oncogene that was isolated from a cancer as opposed to a virus.
And, as I alluded to last time, the gene that they isolated in this
fashion was a familiar one. It was the Ras gene, a gene that
had been discovered in the context of acutely transforming retroviruses.
It was already known to be cancer-associated,
but now it's in the context of real human cancer. And this
was a major advance. Moreover, they sequenced the Ras
gene from the cancer and compared it to the sequence of the Ras gene from
normal cells and they found a difference. They found that it had
been mutated, just as the genetic theory of cancer would predict.
Compared to the normal sequence of the Ras gene, which is shown above,
the sequence of the Ras gene isolated from the human bladder
cancer sample carried a single mutation which converted a glycine
amino acid to a valine amino acid. And the consequence of that mutation
changed the Ras protein from being a signaling molecule that could be
regulated from an on state to an off state to a signaling molecule that
could not be regulated. Once it go turned on it couldn't'
get turned off. So it was a constitutively active
signaling molecule, which makes sense for a cancer cell
that is continually proliferating. Now, there's an issue here that I
would like you to ponder. And that is that this is a single
point mutation, a single base change.
In all of your cells, during cell division, there's a
particular mutation rate. It's not known exactly, at least
for all cells in your body what that rate is. But let's estimate that
it's about ten to the minus ninth per base pair per cell division.
Now, if you calculate how many cell divisions are going on in your body
at any given time, you can estimate that you have about
ten to the third to ten to the fifth Ras mutant cells inside you as you
sit there today. Maybe as many 100,
00. Maybe more than a million cells that carry that very mutation
in your bodies. So I ask you again,
why is it that you don't all have tumors? This time it's not the
explanation that we heard before about regulation because this is the
native gene which is being changed. It's being regulated the same way.
So why is it that we don't all have bladder cancer,
for example, or some other type of tumor? Well, it relates to what I
told you last time, that cancer is rarely a single-step
event. Cancers rarely are associated with a
single mutation. A single mutation may be involved,
may be involved in initiation, but it's not sufficient to give you all
the steps you need for a full cancer. As I mentioned last time,
you might need five, ten or twenty distinct mutations to get a true
cancer. So you might have mutant cells in you, they might be totally
normal, but if layered on top of those mutations are other mutations
they could progress. OK? Now, that's particularly important
because the mutations that we're talking about are,
at least by some assays, dominant. And the old genetics
terminology of dominant and recessive, these mutations are
dominant. Note, in this assay we're transferring a
single gene into an otherwise normal cell and seeing a phenotype.
That's an example of dominance. And these genes can, in the right
context, function in a dominant fashion on top of a normal copy or
normal copies of the gene. And you'll see why a distinction
between dominant and recessive is important in a moment.
Now, as I hope you remember from earlier lectures about signal
transduction, the Ras proteins fall within a signal transduction pathway
that's very well worked out. They link transmembrane growth
factor receptors through intermediary proteins,
adaptors and exchange factors, to downstream components involved in,
for example, phosphorylation kinases and transcription factors that turn
on the expression of target genes. This is the pathway that I've
illustrated to you before. And we now know that many of the
components of this pathway, this so-called mitogenic signaling
pathway that drives cells to divide are mutated in cancers.
Not just Ras, which is actually mutated at pretty high frequency in
cancers, but many of these components are mutated as well.
And they're mutated through different mechanisms.
Subtle mutations, like I've just told you about for Ras,
where single nucleotides can be changed and a single amino acid can
be changed to convert a normally regulated protein into an abnormally
regulated protein like Ras. Some of the growth factor receptors
are likewise activated by that mechanism. But there are other
mechanisms, other activation mechanisms for cancer-associated
genes, including gene amplification and translocation which I'll
illustrate on the board. I hope you noticed on the right here,
Review Sessions for Monday night for the exam on Wednesday,
as well as Tutoring Sessions and indications of where you're supposed
to go. So gene amplification is important in cancer.
There are quite a few genes that are amplified in tumors compared to
normal cells. And I'll give you one example which
is relevant to breast cancer and ovarian cancer.
In normal cells, in the DNA of normal cells there's a
gene called HER2 which is one of these growth factor receptors like
number two up there. And of course the normal cells are
diploid so they have two copies of the HER2 gene.
And through normal transcription and translation that's going to give
rise to a particular concentration of the growth factor receptor on the
surface of the cells. And that's going to give rise to a
certain signal emanating from that growth factor receptor which will
cause the cells to divide at the appropriate time and place.
In about 30% of breast cancers and a similar percentage of ovarian
cancers, the number of copies of the HER2 gene has been increased
dramatically, sometimes as much as a hundred fold. And these are due to
errors in DNA replication. Instead of having DNA copied once
in this region, it gets copied again and again and
again and again. And now, in these cancer cells,
you don't have that same concentration of the growth factor
receptor. You have a much higher concentration of the receptor.
And because there's a much higher concentration of the receptor,
you might get as much as ten or a hundred times the level of signaling.
And the consequences of this are that a cell that shouldn't divide
doesn't have actually enough concentration of growth factors
where it should normally divide will now divide anyway.
Inappropriate proliferation, the hallmark of cancer. Now,
the good news here is that there are antibodies against this HER2 protein.
And they are actually effective in the treatment of breast cancer.
They can prolong the life of women with breast cancer,
specifically those who have amplifications of this gene.
This drug actually does nothing for women who don't have amplifications,
but it prolongs the life of women who do.
So that's an important link between basic biology and treatment.
We'll talk more about those next time. Another mechanism of
activation of oncogenes is translocation.
I showed you last time karyotypes of
cancer cells in which one piece of one chromosome gets linked to a
piece of a different chromosome. The reason that happens and is
selected for is that new regulatory networks or new genes can be
produced by those kinds of break and joining reactions.
And one famous one involves this gene called myc which is a
transcription factor like the bottom of this pathway,
like number eight there, transcription factor which is
important in driving cells into the cell cycle. myc is normally
expressed from a "weak promoter".
Which means that not a whole lot of myc mRNA is produced,
which means that not that much myc protein is produced at any given
time in a cell. And this gives rise to regulated
cell division.
However, occasionally in cancers, particularly in certain types of
leukemia and lymphoma, translocation events take place
where a new segment of DNA is produced which joins the normal myc
gene to a very strong promoter.
And now, just like this gentlemen mentioned before about the RSV,
now a gene is not regulated properly. It's now regulated,
in this case, too strongly. So a lot of myc mRNA is produced
which gives rise to a lot of myc protein which gives rise to
unregulated cell growth.
Again, the hallmark of cancer. So the DNA of tumor cells changes
by point mutations, by gene amplifications,
by translocations, as well as by gene deletions,
which I haven't shown you but also occur.
And together these mutations are found in virtually all human cancers.
Sometimes RAS, sometimes in growth factor receptor,
sometimes in transcription factor, but one place or another you would
typically find a mutation in this pathway. So signaling proliferation
is key to tumor development. And that's not too surprising given
how we think about how cancers arise. But these oncogenes,
these positive regulators of tumorogenesis are not the only genes
that are important. Normal cells do,
in fact, use these signals. And I've equated them with like the
gas peddle on your car, go signals, which cause normal cells
to divide when it's appropriate to divide during development or injury
repair or cell replenishment. But there's another class of genes
that turns on when it's appropriate for cells to stop dividing.
So these genes are the equivalent of the stop signals which would
impose themselves when cell division should cease.
Now, oncogenes promote cell division. Too much oncogene product,
a mutant oncogene product will cause normal cells to divide too often.
But in addition to those we find mutations in the stop signals,
the signals that would normally halt proliferation.
And this allows an even greater accumulation of cells.
The go signals are the equivalent of oncogene mutations and the stop
mutations are the equivalent of another class of genes known as
tumor suppressor genes.
Tumor suppressor genes have been known for about 20 years now.
And the first one comes from this tumor here. This is a child with a
tumor called retinoblastoma. It's a tumor of the retina. It's
actually really rare.
It occurs in about one in 40, 00 births in the general population,
but there are individuals who are familialy predisposed to
retinoblastoma. And in these kids 90% of the time
they will develop retinoblastoma actually affecting both eyes.
So can be actually a fairly common and severe disease in that sense.
This gene, the gene responsible for retinoblastoma was cloned,
again in Bob Weinberg's lab several years ago. And we now know that
it's an important cell cycle regulator, as is indicated here.
So the retinoblastoma gene encodes a retinoblastoma protein.
The retinoblastoma protein is called pRB. And,
as is illustrated on this slide, the RB protein is a negative
regulator of the cell cycle. As you recall,
cells normally cycle from M phase to G1/S phase through again.
And the RB protein acts normally to restrain cell division.
It blocks cells in the G1 phase of the cell cycle when it's active.
When cells receive signals from growth factor pathways they
inactivate the RB gene through phosphorylation,
the RB protein through phosphorylation.
And this occurs by cyclin-CDK complexes, which I told you about in
the cell cycle lectures. There are actually two different
cyclin-CDK proteins which phosphorylate RB and inactivate it.
So when you need to divide you inactive this brake on the cell
cycle and the cells can divide. When you need to stop dividing you
activate certain growth inhibitory pathways that inhibit the kinases
that block the phosphorylation of RB so RB stays in its active state.
And now the cells cannot divide anymore. So RB is a molecular brake
on the cell cycle. RB mutations, and mutations in
other components of this pathway occur in about 90% of human tumors.
It's a very, very commonly affected pathway in tumor development.
What kind of mutations would you expect to see in the RB gene in
human cancer? Would you expect to see gene amplifications,
translocations that cause too much of the RB protein to be produced?
Does that sound right? Remember, it's a brake.
So what kinds of mutations do you expect to find?
Nonsense mutations. Or more generally inactivating
mutations.
Nonsense mutations. Maybe deletions which would remove
the gene all together. Now, are these mutations dominant
or recessive? Inactivating mutations are recessive.
So what's going on here?
We all have two copies of the RB gene, and yet we're saying that the
mutations that take place in this gene are loss of function mutations,
inactivating mutations. So how is it that you ever find RB
mutations in cancer?
Well, the answer is that one mutation is not enough.
One mutation of one copy of the RB gene is not enough.
For tumor suppressor genes were the mutations are recessive,
you need two hits. Two mutational events are required
to now fully deprive the cell of the brake. One mutation is not enough.
And this is now known in the field as the two-hit model of tumor
development, at least for tumor suppressor genes. And it's
illustrated here. Because we carry two copies of all
of our genes, two mutational events are required to eliminate the
function of a tumor suppressor gene. And this is what it looks like. If
this is a normal cell which has two normal copies of the RB gene,
the first thing to happen is that one copy of the RB gene incurs a
mutation. It might be a nonsense mutation or a deletion,
but whatever it is it eliminates the function of the RB gene.
But this cell itself is normal because it still has one normal copy
of the RB gene and this is a recessive mutation.
So this cell here is actually normal, but it's one step away from lacking
the RB gene all together. And there are a number of ways that
that second copy of the gene can be lost. And they're illustrated by
these different arrows. The simplest thing to think about
is that that second copy of the RB gene picks up its own mutation.
We call that a de novo mutation. And now you have a cell which has
two independently mutated copies of RB, and that's a cell that now has
lost control of proliferation. But there are other mechanisms,
which are illustrated here, where the second copy of the RB gene is
actually lost all together because the chromosome that carries that
gene is itself lost giving rise to a cell that only has one copy of the
chromosome. And it's the one that has the mutant RB gene.
Or there's a recombination event which replaces the good copy of the
RB gene with the bad copy of the RB gene giving rise to a cell that is
lacking functional RB once again. And this term here is called loss
of heterozygosity or LOH.
And it's a hallmark of tumor suppressor gene mutations.
It's an indication that there's a tumor suppressor gene mutation
in the region. It's usually detected not by a
change in the tumor suppressor gene itself but rather by some linked
polymorphic marker like a SNP which is close by to the RB gene or
whatever tumor suppressor gene it is. This individual has a big A,
little A, and therefore is heterozygous.
He or she is heterozygous, big A, little A. But if you notice
in the tumors, if the tumors arise by chromosome
loss or by mitotic recombination, the individual has lost the little A
allele. Either there's only big A or there are two copies of big A,
in which case we've seen loss of heterozygosity,
LOH. And that's a common feature of tumor suppressor gene mutations.
And I suspect you'll hear about it in problem sets and beyond.
So LOH is an important mechanism which allows us to go from one hit
to two hits. OK. Now, RB is not the only tumor
suppressor gene that's mutated in cancer. And proliferation is not
the only process that affected in tumorigenesis.
As I told you, at the end of the day in cancer we
care about how many cells we have. And you can have too many cells by
increasing proliferation. And the RAS mutations and RB loss
are examples of how you can have too much proliferation.
But you can also have too little cell death. And we actually have
examples of both oncogenes, which are activated to prevent cell
death, and tumor suppressor genes that are lost to prevent cell death.
So let me tell you a little bit about those.
The first gene that was associated
with apoptosis and cancer was this gene called Bcl2.
It's related to one of the first apoptosis genes discovered in the
Horvitz lab here at MIT, and it's involved in regulating
caspases. You've probably forgotten by now but caspases are proteases
that are kept normally inside your cells in an inactive state.
And they get activated by cell death signals.
Signals that would trigger the cells to commit apoptosis which causes the
caspases to become active.
And this then leads to the program of cell death.
The Bcl2 gene functions to block, downstream of the death signals, the
activation of caspases. Is Bcl2 an oncogene or a tumor
suppressor gene? Does it get activated during cancer
or inactivated during cancer? Activated. You want to suppress
cell death in tumorigenesis. And Bcl2 does that, so Bcl2 gets
activated. It's an oncogene. And it's typically activated by
translocation, in the cases where it is involved.
Translocation, like with myc, too much Bcl2,
inappropriate cell survival. Another very important gene in
tumorigenesis that regulates apoptosis is p53.
p53 is actually the most commonly
affected gene in all human cancer. 50% of all tumors --
-- carry p53 mutations.
That's a remarkable number. And p53, we now know, is an
important regulator of apoptosis. It doesn't *only* regulate
apoptosis but it is an important regulator of apoptosis.
p53 is like a molecular policeman inside your cells.
It's present at very low levels normally, and it's responsive to
various stress signals. When cells get stressed,
for example when their DNA gets damaged or they recognize that
they're proliferating abnormally or they're starved of oxygen,
the levels of p53 protein go way up. And when that happens the cells
either arrest to try to repair the damage or they will commit suicide
killing themselves so they cannot go on to become life-threatening
tumors. So is p53 an oncogene or a tumor
suppressor gene? Is it activated in cancer or
inactivated in cancer? Inactivated. It's a tumor
suppressor gene. Cancer cells don't want to die so
they get rid of p53 to prevent the death.
So p53 is a tumor suppressor gene and a very frequently mutated one in
human cancers. You can study p53 in a variety of
ways. My lab has been studying it for a dozen years using mice that
carry mutations in p53 that were generated by gene targeting
technology. You don't need to know the details of this,
but suffice to say it's possible to create mice which are either
heterozygous for a p53 mutation or homozygous for a p53 mutation.
And the question we asked in those experiments is would these
heterozygous mutant mice or homozygous mutant mice be cancer
prone? Should they be? Would you expect an animal that's
lacking p53, one copy or both, to be cancer prone? It's a tumor
suppressor gene so, yeah, absolutely. And sure enough
it is. If you look at a population of mice
and plot their percent survival as a function of time,
one year, two years, three years, mice live about three
years, normal mice with two copies of p53 will live a normal lifespan.
Mice that have one functional copy of p53 die sooner.
They live about a year and a half. And if you look in the tumors of
these mice, the normal copy of p53 is missing, which is the prediction
of the two-hit model of tumorigenesis.
You need to get rid of both copies. And p53 minus-minus mice only make
it about four to six months and die from cancer. So p53 is an
incredibly important tumor suppressor gene.
If you're mutated in it, your risk of getting cancer goes way,
way up. OK. Now, in the last five minutes I just want
to mention the fact that cancer, while typically a sporadic disease --
-- by which I mean there's
no family history --
-- about 5% of the time there's familial predisposition.
Cancer gets passed through the
family, through the generations. And you can see that in this
pedigree here. This is actually a pedigree of
familial retinoblastoma. The individuals who inherit the
mutant copy of the gene most of the time will develop the tumor,
and they'll pass along that predisposition to their children.
If you were to look at this pedigree for a while, you would see that it
looks like an autosomal dominant pedigree, if you remember back to
that portion of the course. If you inherit the mutation,
you have a high likelihood of developing the tumor.
And it doesn't matter whether the mutation is coming from your mother
or father or whether you're a boy or a girl. What's surprising,
despite that fact, is that most familial cancer syndromes like that
one are caused by tumor suppressor gene mutations.
So what's being passed through the
generations here is a mutant copy of a tumor suppressor gene,
the RB gene, or the breast cancer susceptibility gene,
or colon cancer susceptibility gene. A loss of function copy of that gene
such that these individuals who are developing cancer and passing the
mutation onto their kids are heterozygotes.
And, as you think about it, you have to wonder why are they
cancer prone? Most of their cells should be normal because most of
their cells are heterozygous for the mutation.
But, importantly, those cells are one step away from
mutating the other copy of the gene. In contrast to the general
population, it carries two normal copies of the tumor suppressor gene.
And in whose cells two mutational events have to happen in succession
to mutate the first copy and then the second copy.
In these people all of their cells carry the first mutation.
They're only one mutational event away.
And the likelihood that that will happen in the retinas of their eyes
or in the developing breast or colon is sufficiently high that there is a
near certainty that they will develop cancer.
OK? So what event must happen during tumor development?
A mutation in the second copy of the gene, one of these LOH events or
a second de novo mutation. I also wonder what might explain
the fact that this gentleman here, who must have the mutation because
he passed it onto his sons, did not himself develop the tumor.
Why is that? What's going on in that guy? Excellent.
That's exactly the way I phrase it, he was lucky. In his developing
retina by chance no cell underwent the second hit,
and so his cells were just like normal and he developed normally,
but he still passed the mutation onto his children and they
weren't so lucky. And, finally, what would happen if
two individuals who were heterozygous produced a homozygous
mutant offspring? What do you think would happen in
those individuals? Well, if they survive they should
be mighty cancer prone because no mutational event is required to
eliminate the function in them. They might look like these p53
minus-minus mice. In fact, most tumor suppressor genes
are required for normal development. So you cannot do that experiment.
The embryo would never survive because the gene is important for
proliferation control all over the place. So the embryo doesn't make
it. So most of the time you cannot do that experiment.
And, finally, I'd mentioned last time and I just want to emphasize
that the genes that we've focused on for you in this class are genes that
involve proliferation control and cell death.
And we'd mentioned briefly DNA damage control.
But there are many other processes that are important in tumorigenesis
that are responsive to mutations in other kinds of genes like the
process of invasion and angiogenesis and the all important process of
metastasis. So I'll stop there.