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December 2006
Introduction to oncogenesand to tumor supressor
genesSusana Balcells
Dpt. Genètica, UB
December 2006
Outline
• 1 The cancer cell
• 2 Oncogenes and tumor supressor genes
• 3 Functions
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1/3 The cancer cell
• Uncontrolled growth/no death• Many types: carcinomas, sarcomas,
lymphomas and leukemias• Clonal origin• Models: cell culture / animal• A multi-step process
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in vitro cell culture
Normal cells
Tumor cells
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=> Some heritable alteration has occured in the genome ofthe progenitor cell
Cancers have a monoclonal (not polyclonal) origin
Progenitor cell
Cell line (clone)
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Thus,
cancer may be viewed as a genetic disease,
even though generally it is not inherited
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The transformed cell is a cultured cell which:
1. is immortal2. can regenerate tumors in nude mice
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Oncogene
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Age
500
400
300
200
100
100806040200
Ann
ual m
orta
lity
(per
mill
ion)
The incidence of human cancers depends on age;It needs 4 to 7 independent events
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Tumor progression occurs in several distinct stages ….
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The karyotypes of tumor cells contain multiple abnormalities
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Cancer is clearly a multi-stage process
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2/3 Oncogenes
and Tumor supressor genes
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We now know…Many different genes- when mutated - do
participate in the genesis of a cancer
Some of them mutateto alleles with gain
of function and dominant -> ONCOGENES
Others do so towards alleles with loss
of function and recessive --> TUMOR SUPRESSORS
December 2006
Oncogenes
• are “activated” alleles of genes that code forfunctions that promote growth or inhibit cell death(apoptosis)
• their presence is easily detected through celltransformation assays
• most of them belong to the host genome
• a few are viral oncogenes
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The cell transformation assay demonstrates thatoncogenes are dominant
oncogene
Normal cell Transformed cell
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The mutations of oncogenes are gain of function mutations
Gene amplification of c-myc in leukemias
Loss of a negative regulatory domain
Overexpression of c-myc in Burkitt’s lymmphomathrough translocation next to enhancers ofimmunoglobulin genes
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• are non functional alleles of genes that encodefunctions that promote apoptosis or inhibit cellgrowth
• may be detected through tumor supressionassays
• some are involved in heritable syndromes withenhanced cancer predisposition
Tumor suppressors
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The tumor suppression assay demonstrates thatthe mutations of tumor supressor genes are
recessive
Normalcell
Transformedcell
Hybrid cell ofnormal aspect
PEG Cell fusion
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The mutations in tumor supressor genesare loss of function mutations
Often, we observe a complete TS gene deletion
When the deletion spans a largechromosomal region, we observe a loss ofheterozygosity (LOH) of markers in thatregion
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Beware that both alleles of a TS need to be lostfor a tumor to progress
This is to say that an individual who is heterozygotefor a non functional allele of a TS is phenotypicallynormal and viable …
…but his/her susceptibility to develop a tumor is higher:
A single mutation in the other allele of the STocurring in a relevant tissue will be enough
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+/+ +/Rb*
*(the tumor is Rb/Rb)
+/+ +/+ +/Rb*+/Rb*+/Rb*
Familial retinoblastoma is an autosomal dominant condition
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3/3 Functions altered in cancer
According to Hanahan & Weinberg, there wouldbe 6 main functions altered in ALL cancers
Ref.: Cell 100:57-70 (2000)
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1. Autosuficiència en elssenyals de creixement
2. Insensibilitatals senyalsd’inhibició delcreixement
3. Evasió del’apoptosi
4. Potencial replicatiuil.limitat
5. Angiogènesisostinguda
6. Invasió imetàstasi
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In the last 25 years: A new view:
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Mitogenic signalsSoluble growth factorsExtracellular matrix componentsCell-cell adhesion molecules
Proliferation:Growth,Replication,Mitosis,Cell division
1. Self-sufficiency in growth signals
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Tumor cells generate many of their own
growth signals
In this way they reduce their dependency on
stimuli from the normal environment
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• Synthesis of GFs to which it can respond
• Overexpression of receptors for the GFs
• Modification of receptors so that they are ligand-
independent
• Changes in integrin expression to favor those
which transmit growth signals
• Alteration of the SOS-Ras-Raf- MAP kinase
pathway
The self-sufficiency of the tumor cell may include:
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≈ 25% of human tumors present with an altered versionof Ras
This allows for a permanent stimulation of growth, independent of upstream signals
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1. Self-sufficiency in growth signals
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Anti-growth signals Soluble inhibitory factorsECM componentsCell-cell adhesion molecules
2 types of inhibition:Arrest at Go (reversible)
Advance to a permanentpostmitotic state (anddifferentiation)
2. Insensitvity to anti-growth signals
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Antiproliferative signals act on the cell cycle and inparticular on the G1 transition
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At the molecular level, anti-proliferative signalsactivate a brake of the cell cycle:
The dephosphorylated p105Rb protein
S phase genetranscription
TranscriptionRepression
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TGFβ:
Blocks expressionof c-Myc
And also
stimulates thesynthesis of p15and p21 (blockersof cyclin-CDKcomplexes)
Ref. Cell 103:295-309 (2000)
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• Down-regulation of the receptors for TGFβ
• Loss-of-function mutation of the receptors
• Loss of Smad4 (efector protein of this pathway)
• Deletion of the p15 locus
• Activating mutation of CDK4 to make it
insensible to p15
• Loss of p105Rb (or block by viral proteins)
The insensitivity of a tumor cell to anti-proliferative signalsmay include:
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2. Insensitivity to anti-growth signals
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3. Evading apoptosis
Apoptosis refers tothe process of
physiological death
A highly regulatedprocess
Used to eliminateexcess cells,
dangerous cells ordamaged cells
Ref. Zörnig M et al. BBA 1551:F1-F37 (2001)
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Signals
External signals of survivalExternal signals of deathInternal signals
damaged DNAsignal imbalance
Interactions with ECMCell-cell interactions
Cytochrome C:The catalyst of apoptosis
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FasL: external signal of death
DNA damage:internal signal
Caspases:Apoptosis-specific proteases
Ref. Zörnig M et al. BBA 1551:F1-F37 (2001)
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3. Evading apoptosis
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4. Limitless replicative potential
normal cells in culture
finite replicative potential60-70 divisions
Senescence: growth arrest
Crisis: - massive death- karyotypic disarray with end-to-end fusions ofchromosomes- 1 in 107 cells emerges immortalitzed
Inactivation of p105Rb or p53
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4. Limitless replicativepotential
The problem
The solution
Recombination:a viable
alternative
3’
5’
3’
5’
3’
5’
3’
5’
Telomerase:The preferred pathway
3’
5’
5’
3’
Adapted from: Smith, S. (2000) Nat. genet. 26:388-89.
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Mortal cells• Do not display telomerase activity• telomeres get shorter at each cycle• Once lost, naked chromosomal ends
undergo fusion and crisis happens
Cancer cells
• 85-90% upregulate telomerase• 10-15% activate the ALT pathway• all manitain telomeres • are immortal
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5. Sustained angiogenesis
There are over two dozen angiogenic inducer factorsAnd a similar number of inhibitors
VEGF:
Trombospondin-1:
Integrins: (of endotelial cells)
angiogenesis-initiating signal
prototypical angiogenesis inhibitor
depending on the class, the vessels willremain quiescent or grow
December 2006Ref. Veikkola & Alitalo (1999). Sem.Canc.Biol. 9:211-220
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The importance of sustained angiogenesis in cancer
• Tumor explants require angiogenesis to fully develop
• Anti-VEGF antibodies (or dominant negative mutantreceptors) are able to block neovascularitzation as well asthe growth of subcutaneous tumors in mice
• Several antiangiogenic substances are being tried andthey are able to block tumor growth
• Many tumors display an increased expression of VEGF
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December 2006Veikkola & Alitalo (1999). Sem.Canc.Biol. 9:211-220
Tumor induced angiogenesis
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6. Invasion and metastasis
Avanced stage of tumor development
some cells migrate
and colonize a new tissue: metastasis
Metastases are responsible for 90% of cancer deaths
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6. Invasion and metastasis
Metastases areresponsible for 90%of cancer deaths
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Metastasis involves changes in:
1. physical coupling of cells to their environment
2. Activation of extracellular proteases
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CAMs (cell adhesion molecules) cell-cell interaction
immunoglobulin familycadherins (Calcium depenent)
Interaction of cells with the ECM
Integrins
All these adherent interactions send regulatory signalsto the cells
Cell surface proteins involved in interactions with environment
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E-cadherin
E-cadherin function is lost in a majority ofepithelial cancers (mutation, repression,proteolysis)
Forced expression of E-cadherin in transgenicmodels of carcinogenesis impairs invasive andmetastatic phenotypes
• ubiquitously expressed on epithelial cells
• homotypic cell-cell interactions
• transmits antigrowth signals via β-catenin
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Activation of extracellular proteases
Quiescent cell
Invasive cell
Matrixmetalloproteases
Inhibitors ofMatrixMetalloproteases
Invasive cell
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Dialog between stromal cells and malignant epithelial cells
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6. Invasion and metastasis
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7. Genomic instability
A means to acquire the 6 alterations is through genomic instability
1. Malfunction of DNA repair mechanisms2. Malfunction of mechanisms that maintain karyotypehomeostasis
e.g. The p53 DNA damage signaling pathway is lost inmost cancers
In other cancers other DNA repair genes or genes thatensure the correct segragation of chromosomes at mitosisare lost
These genes belong to the tumor supressor family
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the sixfunctions are
acquired indifferentorders indifferent
tumors
