ONCOGENESIS REVISITED

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ONCOGENESIS REVISITED Aim Review the molecular
and cellular mechanisms by which cells undergo
neoplastic transformation.
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Neoplastic transformation of a cell results from
cumulative genetic damage, a process that
typically involves at least half a dozen
mutations. The genes that mutate during
neoplastic transformation may be categorised as
follows o Mutation of proto-oncogenes, resulting
in a proliferative stimulus to the cell (e.g.
c-erbB). o Inactivation of tumour suppressor
genes (e.g. p53). o Mutation of genes that
regulate apoptosis (programmed cell death, e.g.
bcl-2). o Mutation of genes encoding enzymes of
DNA repair. Proto-oncogenes may be converted to
oncogenes as a result of o Inherited
mutations. o Environmental factors such as
chemicals, radiation and viruses.
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1. Mutation of proto-oncogenes resulting in a
proliferative stimulus to the cell Five
categories can be identified 1.1. Growth
Factors 1.2. Growth Factor Receptors 1.3.
Non-receptor Signal-Transducing Proteins with
Kinase Activity 1.4. Signal Transducing
G-proteins 1.5. Nuclear Regulatory Factors
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PROLIFERATIVE ONCOGENES
ENDOPLASMIC
RETICULUM
Phosphidyl
inositol
ras
biphosphate
G-proteins
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Ca
IGF
Phospho
Receptor
Lipase C
Calmodulin
Dependent
Diacyl
Kinase
Glycerol
Growth factors
Protein
Kinase C
Tyrosine
PDGF
Kinase
Receptor
NUCLEUS
Tyrosine
GF receptors
EGF
Kinase
Receptor
kinases
Activate transcription
MEMBRANE
Tyrosine Kinase
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1.1. Growth Factors 2 examples have been
implicated in neoplasia PDGF, FGF. Example
Over expression of PDGF, hence excess
secretion by the cell, which feeds back onto
itself, resulting in growth by an autocrine
mechanism. Associated with human astrocytomas
and osteosarcomas. Similarly, hst-1 and int-2
overexpress FGF. Associated with human stomach,
bladder and breast cancers and with
melanoma. 1.2. Growth Factor Receptors Recepto
rs for EGF and CSF-1 have been implicated in
neoplasia. These receptors are normally
trans-membrane receptors and possess a kinase on
their cytoplasmic surface.
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(No Transcript)
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1.3. Non-receptor Signal-Transducing Proteins
with Kinase Activity These kinases are
usually located on the cytoplasmic surface of the
cell membrane, with no cell surface receptor
element. Example Human c-abl possesses
tyrosine kinase activity. In chronic myeloid
leukaemia (CML) and some acute lymphoblastic
leukaemias (ALL), c-abl is translocated (Chr. 9
to Chr. 22) to form the Philadelphia
chromosome forms a chimeric protein with potent
tyrosine kinase activity.
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1.4. Signal Transducing G-proteins (1) see
diagram External ligands bind to cell surface
receptors which activates G-proteins
(intermediary family of proteins on cell membrane
eg ras) G-protein binds GTP which activates
specific effectors which generate second
messengers (e.g. PLC or adenylate cyclase) 2nd
messengers eg cAMP, cGMP, Ca2, IP3
DG Activation of kinases G-protein hydrolyses
GTP to GDP which deactivates G-protein G-protein
cycles again if a ligand-receptor complex is
still present on the cell surface. GTPase-activa
ting proteins (GAPs) accelerate GTPase rate 1,000
x, acting as "brakes" that prevent uncontrolled
ras activity. Therefore normal GAPs are Cancer
Suppressor Genes.
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IGF
IGF Binding
IGF Receptor
Cell Wall
GTP
Ras off
Ras on
Cytopl. kinase
GDP
Bind GTP
Hydrolyse GTP
NF1
ACTIVATION
PO4
GTPase activating proteins (GAPs) eg NF1
increase GTPase rate
Mutant ras slow to hydrolyse GTP
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1.5. Nuclear Regulatory Factors Nuclear
oncoproteins are factors that bind to
DNA-regulatory elements and stimulate or repress
transcription within the nucleus. Malfunction
in transcription factor regulation, expression or
binding leads to deregulation of gene expression
and to neoplastic growth. Regulators of nuclear
transcription myc, myb, jun and fos e.g. myc
normally switches on growth promoting
genes. Increased expression in Burkitt's
lymphoma (via a translocation), small cell lung
cancer and neuroblastoma (via amplification).
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ACTIVATION OF ONCOGENES three mechanisms 1.
Point Mutations Examples ras, erb-B, fms.
In the G-protein ras, a single amino acid
mutation inhibits GTP hydrolysis, prolonging the
activated state, probably by inhibiting the ras
interaction with GAPs (see earlier). Point
mutations also occur in Cancer Suppressor Genes
(e.g. pRb - see later).
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2. Chromosomal Translocations two mechanisms
(i) Translocation leading to overexpression of a
proto-oncogene e.g. in Burkitt's lymphoma
c-myc from Chr. 8 is translocated to Chr. 14
close to the immunoglobulin heavy chain (IgHC)
gene, a region where there is hectic
transcriptional activity, hence overexpression of
normal myc protein. (ii) Translocation and
genetic alteration of a proto-oncogene e.g.
Philadelphia chromosome in chronic myeloid
leukaemia (CML), is a translocation of part of
the abl gene (a tyrosine kinase) on Chr. 9 to
Chr. 22 to form a hybrid (chimeric) protein with
the bcr (breakpoint cluster region) gene on Chr.
22. The abl-bcr 210 kDa chimera has potent
tyrosine kinase activity.
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3. Activation by Gene Amplification Reduplicati
on of proto-oncogenes up to several hundred times
over on their chromosome Results in the
appearance of "homogeneous staining regions"
(HSR's) on the chromosomes, and/or the presence
of small lumps of DNA called "double minutes"
(dms). Amplification of a proto-oncogene will
result in increased expression of its protein
product, predisposing to neoplastic
transformation. Examples include myc
(neuroblastoma and small cell lung cancer) and
neu (also called c-erb-B2) (breast carcinoma).
The extent of amplification may correlate with
survival.
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CANCER SUPPRESSOR GENES (ANTI-ONCOGENES) Cancer
suppressor genes are a more recent addition to
our understanding of oncogenesis. One of the
stars of this category, p53, was voted 1993
Molecule of the Year by Science magazine!
Essentially, tumour suppressor genes apply
brakes to cell proliferation. Thus, if these
genes or their products are inactivated, cell
proliferation may increase, predisposing to
oncogenesis. Usually two hits that inactivate
both alleles are required. Often, the first is
inherited and the second is a somatic mutation in
susceptible tissue eg epithelium.
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Ca SUPPRESSOR GENES The retinoblastoma (Rb)
gene Abnormalities in the Rb gene are
essential for retinoblastoma, but also seen in
cancers of the lung, breast and bladder. pRb,
the protein product of Rb, is a nuclear
phosphoprotein that regulates the G1 to S
transition in the cell cycle. The
underphosphorylated form of pRb prevents cell
replication by binding nuclear transcription
factors such as c-myc. Neoplastic mutations of
the Rb gene have been shown to involve the
transcription factor binding domain (e.g. for
c-myc).
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CANCER SUPPRESSOR GENES p53 Is claimed
to be affected in up to 50 of all human cancers.
p53 is a nuclear phosphoprotein that arrests
cells in G1 if a mutagenic agent has caused DNA
damage. If cellular repair does not occur, the
cell goes on to apoptosis (programmed cell
death). Missense mutations usually cause the
inactivation of p53. Thus, inactivation of p53
will result in proliferation and a predisposition
to neoplasia. Mutant inactivated p53 builds up
in affected cells this may be detected in tumour
cells by immunocytochemical staining. NOTE
certain oncogenic DNA viruses encode proteins
that bind to and neutralise Rb and/or p53. The
list includes oncogenic strains of human
papilloma virus, adenoviruses, SV40 and related
polyomaviruses.
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CANCER SUPPRESSOR GENES NF-1 A GAP protein
(GTPase-activating protein) that increases the
GTPase rate of G-proteins (eg ras). Increasing
the GTPase rate of ras inactivates ras faster,
thus NF-1 is a Cancer Suppressor Gene. Other
examples of tumour suppressor genes
adenomatous polyposis coli (APC) and deleted
in colon carcinoma (DCC) are involved in cell
adhesion. Loss interferes with cell-cell
communication and may alter intercellular
adhesiveness, facilitating invasion and
metastasis. WT-1 in Wilm's tumour of the kidney
in children is involved in nuclear transcription.
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GENES THAT REGULATE APOPTOSIS The bcl-2 protein
product prevents apoptosis or programmed cell
death, probably by acting as a free radical
scavenger. If excessive quantities of bcl-2 are
expressed, then an affected cell will survive
longer than it should, and the probability that
other potentially transforming events may occur
increases. Example Follicular B cell lymphoma
-gt chromosomal translocation -gt bcl-2 (Ch. 18)
onto the Ig region (Ch. 14), causing
overexpression of bcl-2. bax accelerates cell
death and is induced by p53
Cell survival
Apoptosis
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DNA REPAIR GENES These genes normally exercise
surveillance over the integrity of genetic
information by participating in the cellular
response to DNA damage. Thus, the loss of these
gene functions renders the DNA susceptible to the
progressive accumulation of mutations. Hereditar
y Nonpolyposis Colon Carcinoma (HNPCC) syndrome
defect in genes involved in DNA mismatch repair
(excision and replacement of mismatched
nucleotides after DNA replication). e.g. Msh-2
on Chr. 2 one defective allele is inherited,
damage to the other allele occurs in colonic
epithelial cells Associated with colonic cancer
in caecum and proximal colon and also stomach,
endometrial and ovarian cancers.