oncogenes & tumour suppressors bart vanhaesebroeck cell signalling group

Oncogenes &

tumour suppressors

Bart VanhaesebroeckCell 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) intracellular transducers

create 2nd messengers

transcription factors

DNA

mRNA

proteins

examples:cell cycle controlDNA repairanti-apoptosis

NUCLEUS

transcription

death (apoptosis)

proliferation (cell cycle progression)

growthmetabolism

migrationdifferentiationsurvival

CYTOPLASM

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 messengerseg. - Ras - protein kinases (Tyr, Ser, Thr)

transcription factors eg. Myc, p53

DNA

mRNA

proteins

examples:cell cycle control : Rb, p16, CDKsDNA repair : ATManti-apoptosis : Bcl2, Bad

NUCLEUS

transcription

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

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).

normal cell signalling is de-regulated in cancer

growth factor eg. vascular endothelial growth factor (VEGF)


effector region(often tyrosine kinase) intracellular transducers



DNA

mRNA

proteins


NUCLEUS

transcription

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






DNA

mRNA

proteins


NUCLEUS

transcription

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

= hormone orgrowth factor

receptor

cell membrane

nucleus

(courtesy of Dr. Rob Stein)

gene activation

stimulated normal cell

cellsurvival& division


cancer cell

cell survival& division

gene activation

spontaneous receptordimerisation & activation


effect of = inhibitor of receptor kinase activity


growthinhibition& cell death

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).






DNA

mRNA

proteins


NUCLEUS

transcription


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)


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)






DNA

mRNA

proteins


NUCLEUS

transcription

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)

Rb = retinoblastoma

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

http://akimichi.homeunix.net/~emile/aki/medical/Image/retinoblastoma-1.jpeg

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

RBE2F

G1

P

RB

E2F

S

P PP

cyclin Ec-Mycother

phosphatases

RBE2F

G1

PcyclinD/CDK4

in normal cell:

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

cell cycle arrest

STRESS(irradiation, hypoxia, anoxia, …)

p53

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

protein kinases

PDK1, Akt/PKB, Btk, Itk, …

adaptor proteins

Gab1, Bam32, DAPP1, …

GEFs / GAPs for small GTPases

of Rac, Ras, Arf families

cytosol

PIP2

PI3K PIP3

+

signalling by PI 3-kinases

cancer

inflammation

diabetes

DISEASE:

proliferation

survival

growth

differentiation

migration

CELLS:

ras

PI3K

receptor Akt

deregulation of PI3K signalling in cancer

PI3K PIP3

PIP2

PTEN = lipid phosphatase

deregulation of PI3K signalling in cancer (cont’d)

by loss of function of the PTEN tumour suppressor gene

PTEN +/- mice:

develop cancerwith 100% penetrance

in many

sporadic cancers

e.g. glioblastoma,

endometrium, …

germline PTEN mutations

in some hamartoma

syndromes

e.g. Cowden syndrome

: when inactivated PI3K pathways ‘on’

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)






DNA

mRNA

proteins


NUCLEUS

transcription

summary: oncogenes and tumour suppressor genes can alter every step of cellular signalling

THE END

(thank you for your attention)

oncogenes & tumour suppressors bart vanhaesebroeck cell signalling group

Documents

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cancergrowth factor

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ras protein kinases

cdksdna repair

thrtranscription factors