Genetics of cancer

1. Oncogenes
Dr Sitaram Swain

2. A large body of evidence points to the central role
played by DNA mutations in cancer development.
Some cancer-causing mutations are triggered by
chemicals, radiation, or infectious agents. Others are
spontaneous mutations, DNA replication errors, or in
certain cases, inherited mutations.
But despite these differences in origin, the result is
always the mutation of genes involved in controlling
cell proliferation and survival. The two main classes
of affected genes: oncogenes and tumor suppressor
genes.

3. Oncogenes Are Genes Whose Products Can
Trigger the Development of Cancer
An oncogene is a gene whose presence can trigger the
development of cancer. Some oncogenes are introduced into cells
by cancer-causing viruses, while others arise from the mutation of
normal cellular genes. In either case, oncogenes code for proteins
that stimulate excessive cell proliferation and/or promote cell
survival by inhibiting apoptosis.
The first oncogene to be discovered was in the Rous sarcoma
virus, which causes cancer in chickens and has only four genes.
Mutational studies revealed that mutant viruses with defects in one
of these genes, called src, are still able to infect cells and reproduce
normally but can no longer cause cancer.
In other words, a functional copy of the src gene must be present
for cancer to arise. Similar approaches subsequently led to the
identification of oncogenes in dozens of other viruses.

4. In 1911, Peyton Rous performed experiments on sick
chickens brought to him by local farmers that showed for
the first time that cancer can be caused by a virus. These
chickens had cancers of connective tissue origin, or
sarcomas.
To investigate the origin of the tumors, Rous ground up the
tumor tissue and passed it through a filter with pores so
small that not even bacterial cells could pass through.
When he injected the cell-free extract into healthy
chickens, they developed sarcomas. Since no cancer cells
had been injected into the healthy chickens, Rous
concluded that sarcomas can be transmitted by an agent
that is smaller than a bacterial cell.
This was the first time anyone had detected an oncogenic
virus—that is, a virus that causes cancer. Although Rous’s
findings were initially greeted with skepticism, an 87-year-
old Rous finally received the Nobel Prize in 1966—more
than 50 years after his discovery of the first cancer virus!

5. Evidence for the existence of oncogenes in cancers not
caused by viruses first came from studies in which DNA
isolated from human bladder cancer cells was introduced
into a strain of cultured mouse cells called 3T3 cells.
The DNA was administered under conditions that stimulate
transfection—that is, uptake of the foreign DNA into the
cells and its incorporation into their chromosomes.
After being transfected with the cancer cell DNA, some of
the mouse 3T3 cells proliferated excessively. When these
cells were injected back into mice, the animals developed
cancer. Scientists therefore suspected that a human gene
taken up by the mouse cells had caused the cancer.
To confirm the suspicion, gene cloning techniques were
applied to DNA isolated from the mouse cancer cells. This
resulted in identification of the first human oncogene: a
mutant RAS gene coding for an abnormal form of Ras.

6. RAS was just the first of more than 200 human oncogenes to
be discovered. While these oncogenes are defined as genes
that can cause cancer, a single oncogene is usually not
sufficient.
Introducing the RAS oncogene caused cancer only because
the mouse 3T3 cells used in these studies already possess a
mutation in another cell cycle control gene. If freshly
isolated normal mouse cells are used instead of 3T3 cells,
introducing the RAS oncogene by itself will not cause
cancer.
However, RAS together with other oncogenes that target the
p53 pathway will cause cancer. This observation illustrates
an important principle: Multiple mutations are usually
required to convert a normal cell into a cancer cell.

7. Proto-oncogenes Are Converted into
Oncogenes by Several Distinct Mechanisms
How do human cancers, most of which are not caused by
viruses, come to acquire oncogenes? The answer is that
oncogenes arise by mutation from normal cellular genes
called proto-oncogenes.
Despite their harmful-sounding name, proto-oncogenes are
not bad genes that are simply waiting for an opportunity to
foster the development of cancer.
They are normal cellular genes that make essential
contributions to regulation of cell growth and survival.
If and when the structure or activity of a proto-oncogene is
disrupted by certain kinds of mutations, the mutant form of
the gene can cause cancer.

8. 1.Point Mutation.
The simplest mechanism for converting a proto-oncogene into an
oncogene is a point mutation— that is, a single nucleotide
substitution in DNA that causes a single amino acid substitution in
the protein encoded by the proto-oncogene. The most frequently
encountered oncogenes of this type are the RAS oncogenes that code
for abnormal forms of the Ras protein.
Point mutations create abnormal, hyperactive forms of the Ras
protein that cause the Ras pathway to be continually activated,
thereby leading to excessive cell proliferation. RAS oncogenes have
been detected in several human cancers, including those of the
bladder, lung, colon, pancreas, and thyroid.
A point mutation can be present at any of several different sites
within a RAS oncogene, and the particular site involved appears to
be influenced by the carcinogen that caused it.

9. 2. Gene Amplification.
The second mechanism for creating oncogenes utilizes
gene amplification to increase the number of copies of a
proto-oncogene.
When the number of gene copies is increased, it causes
the protein encoded by the proto-oncogene to be
produced in excessive amounts, although the protein
itself is normal.
For example, about 25% of human breast and ovarian
cancers have amplified copies of the ERBB2 gene,
which codes for a growth factor receptor.
The existence of multiple copies of the gene leads to the
production of too much receptor protein, which in turn
causes excessive cell proliferation.

10. 3. Chromosomal Translocation.
During chromosomal translocation, a portion of one
chromosome is physically removed and joined to another
chromosome.
A classic example occurs in Burkitt lymphoma, a type of
cancer associated with the Epstein–Barr virus (EBV).
Infection with EBV stimulates cell proliferation, but this is
not sufficient to cause cancer by itself.
The disease arises only when a translocation involving
chromosome 8 happens to occur in one of these
proliferating cells.

11. In the most frequent translocation, a proto-oncogene called
MYC is moved from chromosome 8 to 14, where it becomes
situated next to an intensely active region of chromosome 14
containing genes coding for antibody molecules.
Moving the MYC gene so close to the highly active
antibody genes causes the MYC gene to likewise become
activated, thereby leading to overproduction of the Myc
protein—a transcription factor that stimulates cell
proliferation.

12. Although the translocated MYC gene normal Myc protein, it
is still an oncogene because its new location on
chromosome 14 causes gene to be overexpressed.
Translocations can also disrupt gene structure and cause
abnormal proteins to be produced.
One example involves the Philadelphia chromosome, an
abnormal version of chromosome 22 commonly associated
with CML. The Philadelphia chromosome is created by
DNA breakage near the ends of chromosomes 9 and 22,
followed by reciprocal exchange of DNA between the two
chromosomes.
This translocation creates an oncogene called BCR-ABL,
which contains DNA sequences derived from two different
genes (BCR and ABL). As a result, the oncogene produces a
fusion protein that functions abnormally because it contains
amino acid sequences derived from two different proteins.

13. 4. Local DNA Rearrangements.
Another mechanism for creating oncogenes involves
local rearrangements in which the base sequences of
proto-oncogenes are altered by deletions, insertions,
inversions (removal of a sequence followed by
reinsertion in the opposite direction), or transpositions
(movement of a sequence from one location to another).
An example encountered in thyroid and colon cancers
illustrates how a simple rearrangement can create an
oncogene from two normal genes. This example
involves two genes, named NTRK1 and TPM3, that
reside on the same chromosome. NTRK1 codes for a
receptor tyrosine kinase, and TPM3 codes for a
completely unrelated protein, nonmuscle tropomyosin.

14. In some cancers, a DNA inversion occurs that
causes one end of the TPM3 gene to fuse to the
opposite end of the NTRK1 gene.
The resulting gene, called the TRK oncogene,
produces a fusion protein containing the tyrosine
kinase site of the receptor joined to a region of the
tropomyosin molecule that forms a coiled coil
structure that causes two polypeptide chains to join
together as a dimer.
As a result, the fusion protein forms a permanent
dimer and its tyrosine kinase is permanently
activated.

16. 5. Insertional Mutagenesis
Retrovirus can sometimes cause cancer even if
they have no oncogenes of their own.
Retroviruses accomplish this task by integrating
their genes into a host chromosome in a region
where a proto-oncogene is located.
Integration of the viral DNA then converts the host
cell proto-oncogene into an oncogene by causing
the gene to be overexpressed.
This phenomenon, called insertional mutagenesis,
is frequently encountered in animal cancers but is
rare in humans.

18. Most Oncogenes Code for Components
of Growth-Signaling Pathways
We have just seen that alterations in proto-oncogenes can
convert them into oncogenes, which in turn code for
proteins that either are structurally abnormal or are
produced in excessive amounts.
Although more than 200 oncogenes have been identified to
date, many of the proteins they produce fit into one of six
categories: i.growth factors, ii.receptors, iii. plasma
membrane GTP-binding proteins, iv. Non-receptor protein
kinases,v. transcription factors, and vi. cell cycle or
apoptosis regulators.
These six categories are all related to steps in growth-
signaling pathways. The following sections provide
examples of how oncogene-produced proteins in each of the
six groups contribute to the development of cancer.

20. Regulation of the cell cycle by Ras consists of four steps:
1 Binding of a growth factor to its receptor, leading to
activation of Ras protein; 2 activation of a cascade of
cytoplasmic protein kinases (Raf, MEK, and MAPK); 3
activation or production of nuclear transcription factors
(Ets, Jun, Fos, Myc, E2F); and synthesis of cyclin and
Cdk molecules.
The resulting Cdk-cyclin complexes catalyze the
phosphorylation of Rb and hence trigger passage from G1
into S phase (MAPK = Map kinases).

21. 1. Growth Factors. Normally, cells will not divide unless they have
been stimulated by an appropriate growth factor. But if a cell possesses
an oncogene that produces such a growth factor, the cell may stimulate
its own proliferation.
One oncogene that functions in this way is the v-sis gene (―v‖ means
viral) found in the simian sarcoma virus, which causes cancer in
monkeys. The v-sis oncogene codes for a mutant form of platelet-derived
growth factor (PDGF). When the virus infects a monkey cell whose
growth is normally controlled by PDGF, the PDGF produced by the v-sis
oncogene continually stimulates the cell’s own proliferation (in contrast
to the normal situation, in which cells are exposed to PDGF only when it
is released from surrounding blood platelets).
A PDGF-related oncogene has also been detected in some human
sarcomas. These tumors possess a chromosomal translocation that
creates a gene in which part of the PDGF gene is joined to part of an
unrelated gene (the gene coding for collagen). The resulting oncogene
produces PDGF in an uncontrolled way, thereby causing cells containing
the gene to continually stimulate their own Proliferation.

22. 2. Receptors.
Several dozen oncogenes code for receptors involved in growth-
signaling pathways. Many receptors exhibit intrinsic tyrosine
kinase activity that is activated only when a growth factor binds to
the receptor.
Oncogenes sometimes code for mutant versions of such receptors
whose tyrosine kinase activity is permanently activated, regardless
of the presence or absence of a growth factor. Another example is
the v-erb-b oncogene, which is found in a virus that causes a red
blood cell cancer in chickens.
The v-erb-b oncogene produces an altered version of the epidermal
growth factor (EGF) receptor that retains tyrosine kinase activity
but lacks the EGF binding site. Consequently, the receptor is
constitutively active—that is, it stays active as a tyrosine kinase
whether EGF is present or not, whereas the normal form of the
receptor exhibits tyrosine kinase activity only when bound to EGF.

24. Other oncogenes produce normal receptors but in excessive
quantities, which can also lead to hyperactive growth
signaling. An example is provided by the human ERBB2 gene.
Amplification of the ERBB2 gene in certain breast and ovarian
cancers causes it to overproduce a growth factor receptor. The
presence of too many receptor molecules causes a magnified
response to growth factor and hence excessive cell
proliferation.
Some growth-signaling pathways, such as the Jak-STAT
pathway utilize receptors that do not possess protein kinase
activity. With such receptors, binding of growth factor causes
the activated receptor to stimulate the activity of an
independent tyrosine kinase molecule.
An example of an oncogene that codes for such a receptor
occurs in the myeloproliferative leukemia virus, which causes
leukemia in mice. The oncogene, called v-mpl, codes for a
mutant version of the receptor for thrombopoietin, a growth
factor that uses the Jak-STAT pathway to stimulate the
production of blood platelets.

25. 3. Plasma Membrane GTP-Binding Proteins.
In many growth-signaling pathways, the binding of a
growth factor to its receptor leads to activation of the
plasma membrane, GTP-binding protein called Ras.
Oncogenes coding for mutant Ras proteins are one of
the most common types of genetic abnormality detected
in human cancers.
The point mutations that create RAS oncogenes usually
cause a single incorrect amino acid to be inserted at one
of three possible locations within the Ras protein.

26. The net result is a hyperactive Ras protein that
retains bound GTP instead of hydrolyzing it to
GDP, thereby maintaining the protein in a
permanently activated state.
In this hyperactive state, the Ras protein
continually sends a growth-stimulating signal to
the rest of the Ras pathway, regardless of
whether growth factor is bound to the cell’s
growth factor receptors.

28. 4. Nonreceptor Protein Kinases. A common feature
shared by many growth-signaling pathways is the use of
protein phosphorylation reactions to transmit signals
within the cell.
The enzymes that catalyze these intracellular
phosphorylation reactions are referred to as nonreceptor
protein kinases to distinguish them from the protein
kinases that are intrinsic to cell surface receptors.

29. In Ras pathway, the activated Ras protein triggers a
cascade of intracellular protein phosphorylation reactions,
beginning with phosphorylation of the Raf protein kinase
and eventually leading to the phosphorylation of MAP
kinases.
Several oncogenes code for protein kinases involved in
this cascade. An example is the BRAF oncogene, which
codes for a mutant Raf protein in a variety of human
cancers.
Oncogenes coding for nonreceptor protein kinases
involved in other signaling pathways have been identified
as well. Included in this group are oncogenes that produce
abnormal versions of the Src, Jak, and Abl protein
kinases.

31. 5.Transcription Factors.
Some of the non-receptor protein kinases activated in
growth-signaling pathways subsequently trigger
changes in transcription factors, thereby altering gene
expression.
Oncogenes that produce mutant forms or excessive
quantities of various transcription factors have been
detected in a broad range of cancers.
Among the most common are oncogenes coding for
Myc transcription factors, which control the expression
of numerous genes involved in cell proliferation and
survival.

32. For example, we have already seen how the
chromosomal translocation associated with Burkitt
lymphoma creates a MYC oncogene that produces
excessive amounts of Myc protein.
Burkitt lymphoma is only one of several human cancers
in which the Myc protein is overproduced. In these
other cancers, gene amplification rather than
chromosomal translocation is usually responsible.
For example, MYC gene amplification is frequently
observed in small-cell lung cancers and to a lesser
extent in a wide range of other carcinomas, including
20–30% of breast and ovarian cancers.

34. 6. Cell Cycle and Apoptosis Regulators. In the final step
of growth-signaling pathways, transcription factors activate
genes coding for proteins that control cell proliferation and
survival.
The activated genes include those coding for cyclins and
cyclin-dependent kinases (Cdks). For example, a cyclin-
dependent kinase gene called CDK4 is amplified in some
sarcomas, and the cyclin gene CYCD1 is commonly
amplified in breast cancers and is altered by chromosomal
translocation in some lymphomas.
Such oncogenes produce excessive amounts or hyperactive
versions of Cdk–cyclin complexes, which then stimulate
progression through the cell cycle (even in the absence of
growth factors).

35. Some oncogenes contribute to the accumulation of
proliferating cells by inhibiting apoptosis rather than
stimulating cell division.
One example involves the gene that codes for the
apoptosis-inhibiting protein Bcl-2.
Chromosomal translocations involving this gene are
observed in certain types of lymphomas.
The net effect of these translocations is an excessive
production of Bcl-2, which inhibits apoptosis and
thereby fosters the accumulation of dividing cells.

36. The MDM2 gene, which codes for the Mdm2 protein that
targets p53 for destruction, can also cause a failure of
apoptosis when the gene is amplified or abnormally
expressed.
Excessive production of Mdm2 leads to a destruction of the
p53 protein, thereby inhibiting the p53 pathway that is
normally used to trigger cell death by apoptosis.
Most oncogenes code for a protein that falls into one of the
six preceding categories. Some of these oncogenes produce
abnormal, hyperactive versions of such proteins. Other
oncogenes produce excessive amounts of an otherwise
normal protein. In either case, the net result is a protein that
stimulates the uncontrolled accumulation of dividing cells.