Download Molecular Medicine - Oncogenes, Tumor Suppressor Genes

Molecular Medicine Oncogenes, Tumor Suppressor Genes,
- Lecture 3
Dr. Maureen J. O’Sullivan
Consultant Pediatric Pathologist
Our Lady’s Children’s Hospital and National
Children’s Hospital
Oncogenes
Proto-oncogenes [wild-type form of gene] may
become oncogenes through mutation
Oncogenes generally cause neoplastic
transformation by interfering with normal cell
growth or differentiation, often disrupting
control of cell cycle.
Either act by up-regulating cell proliferation OR
immortalising cells by rescue from apoptosis
and senescence.
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Oncogene
‘Activated’ –HOW? – mechanisms include:
Amplification –e.g. N-Myc; ErbB2
Chromosomal rearrangement – translocation:
Promoter exchange or Novel Fusion oncogene in
chromosomal translocations mainly in
hematopoietic and mesenchymal malignancy
Activating mutation – e.g. EGFR; KIT
Epigenetically, including miRNAs, chromatin
remodelling
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Fluorescent In Situ Hybridisation N-Myc
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Oncogene Activation
The type of gene involved may encode
Growth factor,
Receptor,
Signal transducer,
Transcription factor,
Novel fusion protein,
Epigenetic modifier
…….miRNA
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Oncogenes
EGFR Epidermal growth factor
receptor – protein kinase family
member avian erythroblastic
leukemia viral oncogene homolog
Receptor tyrosine kinase – ligand
binding induces dimerisation
[homo- or heterodimerisation],
auto- phosphorylation of residues
resulting in downstream signal
transduction
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EGFR
Amplification, over-expression, mutation of EGFR
occur in human cancers
Mutation of EGFR common in never-smokers’ lung
cancer - signalling through AKT and STAT promote
cell survival
TKI - effective in mutant EGFR lung cancers– targeted
therapy with gefitinib/erlotinib [problem of
resistance]
Family member: ErbB2 gene amplified in breast
carcinoma –HER2 protein targeted with Antibody –
HerCeptin [Trastuzumab]
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Activating mutation in Tyrosine kinase KIT
or PDGFRa– Gastrointestinal stromal
tumor [GIST]
One of the first tumors to be targeted with
molecular therapy
Based on a understanding of the structure of
the receptor and mutation site
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WT KIT Receptor Tyrosine Kinase
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Mutant KIT Receptor Tyrosine Kinase
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KIT Mutation and Sensitivity to
Imatinib Mesylate
Exon 11 mutation [71%] = most sensitive to I.M.
Exon 9 much less so, but can get response by
dose escalation
Problem of treatment resistance
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Oncogenes -RAS
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Fusion Oncogene by Translocation
t(11;22)
Produces
Ews-Fli1
transcript
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Fusion Oncogene
EWS-Fli1
EWS
introns 7-10
N
C
N
introns 3-9
N
Trans. Dom.
C
Fli1
C
EWS-Fli1
DBD
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Fusion Gene from Chromosomal
translocation – A novel transcription
factor
Transcription regulatory domain
DNA Binding Domain
RNA
Polymerase
Complex
DNA Binding Site
TATA Box
mRNA
Exon 1
Exon 2
Exon 3
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Promoter Exchange
P1
P2
Gene 2
Gene 1
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Table of Diagnostic Sarcoma Translocations
Tumour
Translocation
Fusion Gene
Alveolar Rhabdomyosarc
t(1;13)(p36;q14)
t(2;13 )(q35;q14)
t(X;17)(p11;q25)
t(12;16)(q13;p11)
t(12;22)(q13;q12)
t(17;22)(q22;q13)
t(11;22)(p13;q12)
t(11;22)(q24;q12)
t(21;22)(q22;q12)
t(12;15)(p13;q25)
t(1;2)(q22;p23)
t(2;19)(p23;p13)
t(7;16)(q33;p11)
t(9;22)(q22;q12)
t(12;16)(q13;p11)
t(X;18)(p11;q11)
PAX3-FKHR
PAX7-FKHR
ASPL-TFE3
TLS-ATF1
EWS-ATF1
COL1a1-PDGFβ
EWS-WT1
EWS-Fli1
EWS-ERG
ETV6-NTRK3
TPM3-ALK
TPM4-ALK
TLS-CREB3L2
EWS-CHN
TLS-CHOP
SYT-SSX1
SYT-SSX2
SYT-SSX4
Alveol. Soft Part Sarc.
Angiom. Fibrous Histiocytoma
Clear Cell Sarcoma
DFSP/GCF
DSRCT
Ewing sarcoma/PNET
Infantile Fibrosarcoma/CCMN
Inflamm. Myofibrobl. Tumour
Low-grade Fibromyxoid Sarc.
Myxoid Chondrosarcoma
Myxoid/round cell Liposarc
Synovial Sarcoma
black=tyrosine kinase activation
pink = promoter exchange, others=fusion TFs
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Tumor Suppressor Genes – Insights
from Familial Cancer
1971 Knudson’s two-hit theory
based on familial versus sporadic
cancer
Retinoblastoma*
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Retinoblastoma
Tumor Suppressor Genes
Knudson’s Conclusion:
Cancer = a recessive effect and need to lose
the function of both alleles of tumor
suppressing genes to get this effect.
First hit deletion leads to hemizygosity,
subsequent mutation produces loss of
heterozygosity = LOH
But note: concept of haploinsufficiency
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Retinoblastoma gene (Rb)
The first TSG to be isolated (1986)
Germline mutation of Rb gene in hereditary
predisposition to retinoblastoma, an eye cancer
occurring in children also at high risk of
osteosarcoma. Sporadically mutated in a variety
of cancers
Re-introduction of WT Rb gene into tumor cells
suppresses their neoplastic properties [– this is not
a requirement for the definition of a TSG]
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Retinoblastoma gene (Rb)
Nuclear phosphoprotein
Rb regulated by phosphorylation by cdc2
kinase family members
Hyper-Phosphorylated form inactive;
Hypo-/Unphosphorylated form blocks cell cycle
progression from G1
S
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Rb role in cell cycle progression
Retinoblastoma gene (Rb)
Therefore, Rb apparently plays a role in gating entry
into the S phase of the cell cycle (gatekeeper gene) –
Not only mutated in retinoblastoma
Inactivation of Rb by mutation e.g in inherited
predisposition (familial Retinoblastoma) or somatically;
abnormal phosphorylation; or binding to viral oncoproteins
(e.g. HPV E7 protein) all can disrupt its role in gated control
of the cell cycle.
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p53
53 kD protein precipitated in 1979
High expression in cells post γ-irradiation and
in chemically induced tumor
Induces cell cycle arrest (G1 arrest to allow
repair pre-replication; or G2 arrest pre-mitosis)
Mediator of apoptosis
Mutated in a large number of tumors, notably with
tumor progression
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p53
Cell cycle arrest mediated through p21WAF1 (CDKI); Gadd45;
Rb
vs. Apoptosis (mediated through FAS/APO1, death
receptor family; Bax = antagonist of bcl-2; IGF-BP3)
Cell type, extent of damage, survival factors, level of
p53…..determine route taken
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p53
Guardian of the genome
– a role in preventing the replication of cells
containing DNA damage –
EITHER permits time for repair by cell cycle
arrest or induces apoptosis if damage
irreparable
Caretaker function
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Viral oncogenesis and p53
HPV
EBNA
HBV
CMV
Adenovirus
E6 protein^
p53 degradation
X protein
IE84 protein
E1B
All form complex with p53
^E7 forms complex with Rb protein
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p53
Roles in:
Cell cycle control
DNA repair
Genome stability
Programmed cell death
[Differentiation, senescence and angiogenesis]
Caretaker and Gatekeeper functions
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Germline p53 Mutation = Li-Fraumeni
Syndrome
Familial cancer predisposition syndrome with
increased risk especially of sarcomas including
osteosarcoma, also breast cancer, choroid plexus
carcinoma, leukemia, adrenocortical carcinoma.
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Gatekeepers vs. Caretakers
Gatekeepers generally have more tissue specificity for tumor
development – RB, APC , VHL, WT1.
Mutated in germline predisposition syndromes and also may be
mutated as somatic events.
Usually need additional mutations to = cancer.
Caretakers germline mutation = cancer predisposition; may occur as
later somatic mutation also.
Classic example = DNA repair genes
NOT quite as tissue-type specific as gatekeepers in terms of tumor
types
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DNA Integrity
Constantly under assault but we have repair
mechanisms to deal with this damage
Flaws in the genetic mechanisms involved in preservation
of the integrity of the genome can provide a
background which facilitates tumorigenesis
mutator phenotype (Secondary to mutation of
Caretaker genes)
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Summary of Lecture
Functions of Oncogenes – the hierarchy of the
signalling pathway and possibility of mutation at various
levels in cascade.
How activating mutation of oncogene can occur –
amplification, chromosomal rearrangement, point mutation
–importance of understanding mutation for targeted
therapy – TKI; Antibody – EGFR, KIT, RAS
Protein structure dictates function – importance of mutation
of key functional domains in genes
Tumor Suppressor Genes – gatekeepers and caretakers
Rb and insights into TSG function from familial cancer
P53 Guardian of the Genome
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