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Oncogenes
&
tumour suppressors
Bart Vanhaesebroeck
Cell Signalling Group
cell signalling
regulates
every aspect of a cell’s life & death
cancer is a consequence
of deregulated cell signalling
growth factor
growth factor receptor
effector region
(often a tyrosine kinase)
CYTOPLASM
intracellular transducers
create 2nd messengers
NUCLEUS
transcription factors
DNA
transcription
mRNA
proteins
proliferation (cell
cycle progression)
growth
survival
metabolism
examples:
cell cycle control
DNA repair
anti-apoptosis
differentiation
migration
death
(apoptosis)
growth factor eg. epidermal growth factor (EGF)
growth factor receptor eg. EGF-receptor (EGF-R)
effector region
(often a tyrosine kinase)
intracellular transducers
create 2nd messengers
eg. - Ras
- protein kinases (Tyr, Ser, Thr)
NUCLEUS
transcription factors eg. Myc, p53
DNA
transcription
mRNA
proteins
examples:
cell cycle control : Rb, p16, CDKs
DNA repair : ATM
anti-apoptosis : Bcl2, Bad
normal cell signalling is deregulated in cancer
this deregulation can occur by
• mutation
• gene amplification
• gene translocation
• gene conversion
•…
cancer is a disease of DNA (1)
chromosomes of a normal cell
cancer is a disease of DNA (2)
chromosomes of a cancer cell
normal cell signalling is de-regulated in cancer
this deregulation can occur in
oncogenes
- genes capable of inducing one or more characteristics of cancer cells
- dominant gain-of-function: dominant in genetic terms: have an effect
even if only one of the 2 cellular copies of the gene is altered
- the normal versions of the genes are called ‘proto-oncogenes’
tumour suppressor genes
- genes that inhibit tumour development = ‘brakes’
- recessive loss-of-function: recessive in genetic terms: both copies of the
gene need to be inactivated (this is the ‘classical’ theory – emerging evidence
suggests that this may not be true for all tumour suppressor genes, some (like PTEN;
see later) are ‘haplo-insufficient’, and already ‘cause trouble’ if one copy is lost).
growth factor eg. vascular endothelial growth factor (VEGF)
growth factor receptor eg. EGF-receptor (EGF-R)
effector region
(often tyrosine kinase)
intracellular transducers
create 2nd messengers
eg. - Ras
- protein kinases (Tyr, Ser, Thr)
NUCLEUS
transcription factors eg. Myc, p53
DNA
transcription
mRNA
proteins
examples:
cell cycle control : Rb, p16, CDKs
DNA repair : ATM
anti-apoptosis : Bcl2, Bad
growth factor eg. vascular endothelial growth factor (VEGF)
Avastin
TM
(Genentech)
- blocks action of VEGF, key molecule in angiogenesis
- approved by the FDA in combination with chemotherapy (intravenous 5-fluorouracil [5-FU]based chemotherapy) for treatment of people diagnosed with metastatic colorectal cancer
for the first time
examples of oncogenes
 Tyrosine kinases: EGF-Receptor family members, BcrAbl
 Intracellular signalling protein: Ras
 transcription factor: Myc
 anti-apoptotic protein: Bcl2
growth factor eg. vascular endothelial growth factor (VEGF)
growth factor receptor eg. EGF-receptor (EGF-R)
effector region
(often tyrosine kinase)
intracellular transducers
create 2nd messengers
eg. - Ras
- protein kinases (Tyr, Ser, Thr)
NUCLEUS
transcription factors eg. Myc, p53
DNA
transcription
mRNA
proteins
examples:
cell cycle control : Rb, p16, CDKs
DNA repair : ATM
anti-apoptosis : Bcl2, Bad
oncogenes
EGF-Receptor family members
 overexpressed & constitutively active in breast cancer
 target for (1) antibody therapy:
eg. Herceptin (Genentech) = monoclonal antibody that binds the
extracellular domain of the EGF-R family member HER2
 inhibits the growth of cells that overexpress this EGF-R
(2) tyrosine kinase inhibitor therapy:
eg. IRESSA (Astra Zeneca) = small molecule that inhibits the activity
of the intracellular kinase domain of the EGF-R
resting normal cell
receptor
nucleus
cell membrane
= hormone or
growth factor
(courtesy of Dr. Rob Stein)
stimulated normal cell
gene activation
cell
survival
& division
(courtesy of Dr. Rob Stein)
cancer cell
spontaneous receptor
dimerisation & activation
gene activation
cell
survival
& division
(courtesy of Dr. Rob Stein)
effect of
= inhibitor of receptor kinase activity
growth
inhibition
& cell death
(courtesy of Dr. Rob Stein)
deregulated signalling proteins
are increasingly used for ’targeted therapies’
tumours seem to critically depend
on some of these pathways : ‘Achilles heels’
examples of oncogenes (cont’d)
Tyrosine kinases (cont’d)
BcrAbl
Philadelphia chromosome translocation = t(9;22) : fuses
* part of the bcr gene from chromosome 22
with
* part of the abl tyrosine kinase gene on chromosome 9
 creates the BcrAbl fusion protein in which the Abl tyrosine kinase
(1) has  kinase activity
(2) localised throughout the cells (not only in the nucleus as in normal cells)
 phosphorylation of substrates that  proliferation & protect from apoptosis
 in chronic myelocytic leukemia (CML)
 target for Gleevec (Novartis) = tyrosine kinase inhibitor  almost 100% remission in
chronic phase of disease (but resistance appears to develop).
growth factor eg. epidermal growth factor (EGF)
growth factor receptor eg. EGF-receptor (EGF-R)
effector region
(often tyrosine kinase)
intracellular transducers
create 2nd messengers
eg. - Ras
- protein kinases (Tyr, Ser, Thr)
NUCLEUS
transcription factors eg. Myc, p53
DNA
transcription
mRNA
proteins
examples:
cell cycle control : Rb, p16, CDKs
DNA repair : ATM
anti-apoptosis : Bcl2, Bad
examples of oncogenes (cont’d)
Ras = intracellular signalling protein
 small GTPase
 controls MAP kinase protein cascade  important for proliferation & gene induction
 mutated & constitutively active in many cancers
Myc = transcription factor - in Burkitt lymphoma
 due to Epstein-Barr Virus (EBV): virus carried by >90% of the world's population – in
severely immune-suppressed patients   EBV immune surveillance  B-cell lymphomas
 How does Myc become activated?
 translocation of c-myc proto-oncogene into or near one of the immunoglobulin loci
 found in almost every case of Burkitt’s B-cell lymphoma in man
(see lecture D. Linch & A. Khwaja)
examples of oncogenes (cont’d)
Bcl2 = anti-apoptotic protein = B-cell leukemia-2 (see lecture notes D. Linch & A. Khwaja)
 protects against cell death
 was the first ‘oncogene’ discovered which does not regulate proliferation
 initially identified as a translocation breakpoint common in many B-cell lymphomas
 as a result of this translocation, the bcl-2 gene comes under the control of the
immunoglobulin heavy chain enhancer & is constitutively expressed in B-cells
 the resulting protection from apoptosis apparently permits the survival &
accumulation of aberrant B-cells that ultimately give rise to lymphoid malignancies
examples of tumour suppressor genes
 gene regulator: Rb
 transcription factor: p53
 lipid phosphatase: PTEN
tumour suppressor genes
- genes that inhibit tumour development
- classical theory: recessive (in genetic terms): both gene copies in the cell need to be
inactivated before cancer can arise
 almost all genes in our cells are present in 2 redundant copies (one from mother &
one from father): if one copy is lost, the other copy serves as a backup. In the case of
tumour suppressor genes, this offers a measure of protection.
 loss-of-heterozygosity = LOH = loss of the 2nd allele of a tumour suppressor
(by gene conversion, mutation, gene deletion etc)
 some people carry an inactivating mutation in a tumour suppressor gene in their
sperm or eggs
 offspring is more prone to lose the 2nd allele (eg. by a so-called ‘sporadic’ mutation)
 predisposition to cancer. eg. familial retinoblastoma : carry mutations in Rb gene
(see also lecture notes Dr. Daniel Hochhauser)
growth factor eg. epidermal growth factor (EGF)
growth factor receptor eg. EGF-receptor (EGF-R)
effector region
(often tyrosine kinase)
intracellular transducers
create 2nd messengers
eg. - Ras
- protein kinases (Tyr, Ser, Thr)
NUCLEUS
transcription factors eg. Myc, p53
DNA
transcription
mRNA
proteins
examples:
cell cycle control : Rb, p16, CDKs
DNA repair : ATM
anti-apoptosis : Bcl2, Bad
Rb = retinoblastoma
first identified in the rare eye tumour retinoblastoma (occurs only up to the age of 6-7)
- arises from retinoblasts: cells in the embryonic retina
that will become photoreceptors
- ‘sporadic’ form: afflicted children have no close relatives who
have previously contracted this cancer ( familial form)
Alfred Knutson theory (based on epidemiological studies):
> sporadic form: the 2 mutations occur one after another (either during
embryonic development of shortly after birth), in one of the cells of the retina
 extremely rare & occurs slighly later in life (mean age: 30 months)
 children mostly carry a single retinal tumour in one eye
> familial form: all cells of the embryo carry 1 mutated allele of the Rb gene
(including all cells of the retina).
  chance of loss of 2nd allele (LOH)
  frequency of retinoblastoma & occurs early (mean age: 14 months)
 often multiple tumours in both eyes
Rb = retinoblastoma protein
 ‘pocket’ protein: binds & inhibits E2F transcription factors
 ‘super’ phosphorylation of Rb (by cyclin-dependent kinases that act in cell cycle)
 release of E2F from the DNA  brake is gone  allows transcription of genes
important for cell cycle progression
in normal cell:
P
P
RB
P
P
P
RB
E2F
G1
RB
E2F
S
E2F
cyclin E
c-Myc
other
G1
in Rb -/- cell: loss-of-expression of Rb  brake is lost  no brakes on cell cycle progression
examples of tumour suppressor genes (cont’d)
p53
 = transcription factor
 in 50% of tumours: lost or (in most cases) mutated such that it can no longer bind DNA
 = ‘GUARDIAN OF GENOME’: ‘senses’ DNA damage, stress
if damage is moderate: stalls cells in cell cycle until DNA is repaired
if damage is severe: induces cell death programme
STRESS
(irradiation, hypoxia, anoxia, …)
p53
cell cycle arrest
cell death
examples of tumour suppressor genes (cont’d)
p53 (cont’d):
Not entirely clear how p53 works, but a very plausible pathway goes as follows:
damage of cellular DNA  activation of ATM / DNA-PK (DNA-dependent protein
kinase)  phosphorylation of p53  increased p53 stability  p53 accumulation &
activation  induction of
* cell cycle inhibitors (such as p21)
* apoptosis-inducing proteins (such as Bax, Fas-receptor, ..)
* IGF-BP3 (a secreted binding protein for the survival factor IGF-1)
EXPRESSION OF THESE NEGATIVE REGULATORS IS LOST UPON LOSS OF p53
example of a dose-dependent tumour suppressor gene: PTEN
signalling by PI 3-kinases
cytosol
PI3K
receptor
PIP2
ras
+
PIP3
CELLS:
protein kinases
PDK1, Akt/PKB,
Btk, Itk, … proliferation
Akt
adaptor proteins
Gab1, Bam32, DAPP1, …
cancer
survival
growth
differentiation
GEFs / GAPs for small GTPases
of Rac, Ras, Arf families
DISEASE:
migration
deregulation of PI3K signalling in cancer
inflammation
diabetes
deregulation of PI3K signalling in cancer (cont’d)
by loss of function of the PTEN tumour suppressor gene
PIP3
= lipid phosphatase : when inactivated  PI3K pathways ‘on’
PIP2
germline PTEN mutations
in many
in some hamartoma
sporadic cancers
syndromes
e.g. glioblastoma,
e.g. Cowden syndrome
endometrium, …
PTEN +/- mice:
develop cancer
with 100% penetrance
under those conditions, the wild-type PTEN allele is retained, and only the dose of
PTEN enzyme is altered
Apparently, lowering the dose of a tumour suppressor gene can already have dire
effects for cancer development, and it is thus not always necessary to lose BOTH
copies of a tumour suppressor gene !!! (( Knutson theory)
summary: oncogenes and tumour suppressor genes can alter
every step of cellular signalling
growth factor eg. epidermal growth factor (EGF)
growth factor receptor eg. EGF-receptor (EGF-R)
effector region
(often tyrosine kinase)
intracellular transducers
create 2nd messengers
eg. - Ras
- protein kinases (Tyr, Ser, Thr)
NUCLEUS
transcription factors eg. Myc, p53
DNA
transcription
mRNA
proteins
examples:
cell cycle control : Rb, p16, CDKs
DNA repair : ATM
anti-apoptosis : Bcl2, Bad
THE END
(thank you for your attention)