Oncogenes

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Oncogenes tumour suppressorsBart
VanhaesebroeckCell Signalling Group
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cell signalling regulates every aspect of a
cells life death
cancer is a consequence of deregulated cell
signalling
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growth factor
growth factor receptor
effector region (often a tyrosine kinase)
CYTOPLASM
intracellular transducers create 2nd messengers
NUCLEUS
transcription factors
DNA
transcription
mRNA
examples cell cycle control DNA
repair anti-apoptosis
proteins
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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
examples cell cycle control Rb, p16, CDKs DNA
repair ATM anti-apoptosis Bcl2, Bad
proteins
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• normal cell signalling is deregulated in cancer
• this deregulation can occur by
• mutation
• gene amplification
• gene translocation
• gene conversion

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cancer is a disease of DNA (1)
chromosomes of a normal cell
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cancer is a disease of DNA (2)
chromosomes of a cancer cell
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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).

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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
examples cell cycle control Rb, p16, CDKs DNA
repair ATM anti-apoptosis Bcl2, Bad
proteins
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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

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• examples of oncogenes
• Tyrosine kinases EGF-Receptor family members,
  BcrAbl
• Intracellular signalling protein Ras
• transcription factor Myc
• anti-apoptotic protein Bcl2

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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
examples cell cycle control Rb, p16, CDKs DNA
repair ATM anti-apoptosis Bcl2, Bad
proteins
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• 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

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resting normal cell
receptor
nucleus
cell membrane
hormone or growth factor
(courtesy of Dr. Rob Stein)
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stimulated normal cell
(courtesy of Dr. Rob Stein)
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cancer cell
(courtesy of Dr. Rob Stein)
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effect of inhibitor of receptor kinase
activity
(courtesy of Dr. Rob Stein)
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deregulated signalling proteins are increasingly
used for targeted therapies tumours seem to
critically dependon some of these pathways
Achilles heels
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examples of oncogenes (contd) Tyrosine kinases
(contd)

• BcrAbl
• Philadelphia chromosome translocation t(922)
  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).

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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
examples cell cycle control Rb, p16, CDKs DNA
repair ATM anti-apoptosis Bcl2, Bad
proteins
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• examples of oncogenes (contd)
• 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 gt90 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 Burkitts
  B-cell lymphoma in man
• (see lecture D. Linch A. Khwaja)

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examples of oncogenes (contd)

• 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

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• examples of tumour suppressor genes
• gene regulator Rb
• transcription factor p53
• lipid phosphatase PTEN

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• 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)

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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
examples cell cycle control Rb, p16, CDKs DNA
repair ATM anti-apoptosis Bcl2, Bad
proteins
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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) gt 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 gt 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
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• 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
phosphatases
cyclinD/CDK4
P
P
E2F
cyclin E c-Myc other
S
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• examples of tumour suppressor genes (contd)
• 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

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examples of tumour suppressor genes
(contd) p53 (contd) 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
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example of a dose-dependent tumour suppressor
gene PTEN
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signalling by PI 3-kinases
cytosol

PI3K
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deregulation of PI3K signalling in cancer
(contd) by loss of function of the PTEN tumour
suppressor gene
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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
examples cell cycle control Rb, p16, CDKs DNA
repair ATM anti-apoptosis Bcl2, Bad
proteins
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THE END (thank you for your attention)