Transcript Document
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)