Download APC/β – CATENIN PATHWAY

NEOPLASIA
V
INSENSITIVITY TO GROWTH INHIBITION
AND ESCAPE FROM SENESCENCE :
TUMOR SUPPRESSOR GENES
INTERNAL CONTROLS OF CELL CYCLE - “CHECKPOINTS”
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1. G1/S checkpoint : checks for DNA damage ,prevents replication of
cells that have defects in DNA ; cell cycle arrest mediated through p53
2. G2/M checkpoint : monitors completion of DNA replication ; cell
cycle arrest by p53- dependent & p53-independent mechanisms
Defects in cell cycle checkpoints are major cause of genetic instability
in cancer cells
TUMOR SUPPRESSOR GENES
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Products of tumor suppressor genes
apply “brakes” to cell proliferation –
network of check points
Tumor suppressors like RB & p53 , are
part of regulatory network that recognize
genotoxic stress and respond by shutting
down prolif
Another set of tumor suppressors
involved in cell differentiation causing
cells to enter postmitotic differentiated
pool without replicative potential
SELECTED TUMOR SUPPRESSOR GENES INVOLVED IN HUMAN NEOPLASMS
RB GENE
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RB - The 1st and prototypic tumor
suppressor gene
60% retinoblastomas are sporadic
Familial Retinoblastoma inherited as an
autosomal dominant trait.
Patients with familial retinoblastoma are
also at greatly increased risk of developing
osteosarcoma and other soft-tissue
sarcomas.
“two-hit” hypothesis of oncogenesis
proposed by Knudson
Knudson's hypothesis can be stated as
follows :
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Two mutations (hits) involving both alleles
of RB at chromosome locus 13q14, are
required to produce retinoblastoma
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In familial cases, children inherit one
defective copy of the RB gene in the germ
line (one hit ) ; the other copy is normal.
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Knudson's hypothesis :
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Retinoblastoma develops when the normal
RB allele is mutated in retinoblasts as a
result of spontaneous somatic mutation
(second hit).
In sporadic cases both normal RB alleles
must undergo somatic mutation in the
same retinoblast (two hits). A retinal cell
that has completely lost RB function
becomes cancerous.
PATHOGENESIS OF RETINOBLASTOMA
ROLE OR RB IN REGULATING G1/S TRANSITION
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RB (RB protein) – product of Rb gene is nuclear
phosphoprotein.
Active form – hypophosphorylated in quiescent
cells
Inactive form – hyperphosphrylated in G1/S
transition
Key role in G1/S checkpoint ; if RB is absent as
a result of gene mutation the “molecular brakes”
on cell cycle are released
ROLE OR RB IN REGULATING G1/S TRANSITION
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Initiation of DNA replication requires activity
of cyclinE – CDK2 ; expression of cyclin E is
dependent on E2F family of transcription
factors
In its active form RB binds to E2F & blocks E2F
mediated transcription of cyclin E
RB couples control of cell cycle progression at
G1 with diferentiation ; diff is associated with
exit from cell cycle
Role of RB in checking G 1- S checkpoint of cell cycle
P 53 – GUARDIAN OF THE GENOME
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p53 gene is located on chromosome 17p
Most common target for genetic alteration
over 50% of human tumors contain
mutations in this gene
Homozygous loss of p53 occurs in virtually
every type of cancer, including CA lung,
colon breast—the three leading causes of
CA death
P53 protein functions as a transcription
factor
P 53 – GUARDIAN OF THE GENOME
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In response to DNA damage, p53 is
phosphorylated by genes that sense the
damage & are involved in DNA repair.
p53 links cell damage with DNA repair, cell
cycle arrest, and apoptosis.
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Assists in DNA repair by causing G1 arrest
and inducing DNA-repair genes.
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A cell with DNA damage beyond repair is
directed to undergo apoptosis.
P 53 – GUARDIAN OF THE GENOME
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In view of these activities, p53 has been
rightfully called a “guardian of the genome.”
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With loss of function of p53, DNA damage
goes unrepaired, mutations accumulate in
dividing cells, and the cell marches along a
one-way street leading to malignant
transformation.
The role of p53 in maintaining the integrity of the genome.
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p53 prevents neoplastic transformation
by three interlocking mechanisms:
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activation of temporary cell cycle
arrest (quiescence)
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induction of permanent cell cycle
arrest (senescence) or
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triggering of programmed cell death
(apoptosis)
Li – Fraumeni syndrome :
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individuals who inherit one mutant
allele of p53
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have a 25 fold greater chance of
developing malignant tumor by age
50 than general population
APC/β – CATENIN PATHWAY
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Adenomatous polyposis coli genes (APC)
is a tumor suppressor ; down-regulate
growth-promoting signals.
Germ-line mutations at the APC (5q21)
loci are associated with familial
adenomatous polyposis
Almost invariably, one or more of these
polyps undergoes malignant transformation, giving rise to colon cancer.
APC /β –C ATENIN PATHWAY
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Both copies of the APC gene must be lost /
mutated for tumor to arise
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APC (& β-catenin ) are component of the WNT
signaling pathway, which has a major role in
controlling cell fate, adhesion, and cell polarity
during embryonic development
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An important function of the APC protein is to
down-regulate β-catenin
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Cells with loss of APC behave as if they are
under continuous WNT signaling
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A, The role of APC in regulating the stability and function of β-catenin. APC and β-catenin are
components of the WNT signaling pathway. In resting cells (not exposed to WNT), β-catenin
forms a macromolecular complex containing the APC protein. This complex leads to the
destruction of β-catenin, and intracellular levels of β-catenin are low.
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B, When cells are stimulated by WNT molecules, the destruction complex is deactivated, βcatenin degradation does not occur, and cytoplasmic levels increase. β-catenin translocates to
the nucleus, where it binds to TCF, a transcription factor that activates genes involved in cell
cycle progression.
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C, When APC is mutated or absent, the destruction of β-catenin cannot occur. β-catenin
translocates to the nucleus and coactivates genes that promote entry into the cell cycle, and
cells behave as if they are under constant stimulation by the WNT pathway.
EVASION OF APOPTOSIS
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Accumulation of neoplastic cells may also
result from mutations in the genes that
regulate apoptosis.
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Apoptosis represents a barrier that must be
surmounted for cancer to occur.
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In the adult, cell death by apoptosis is a
physiologic response to several pathologic
conditions that might contribute to
malignancy if the cells remained viable.
EVASION OF APOPTOSIS
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CD95 receptor- induced (extrinsic pathway)
and DNA damage–triggered (intrinsic)
pathways of apoptosis & mechanisms used
by tumor cells to evade cell death.
1.
Reduced CD95/Fas (death receptors) level.
2.
Inactivation of death-induced signaling
complex by FLICE protein (caspase 8)
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Reduced egress of cytochrome c from
mitochondrion as a result of up-regulation of
BCL2.
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Reduced levels of pro-apoptotic BAX resulting
from loss of p53.
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Loss of apoptotic peptidase activating factor
1 (APAF1).
6.
Up-regulation of inhibitors of apoptosis (IAP).
LIMITLESS REPLICATIVE POTENTIAL: TELOMERASE
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Most normal human cells have a capacity of 60
to 70 doublings only, then become senescent
due to progressive shortening of telomeres
Short telomeres are recognized by the DNArepair machinery as double - stranded DNA
breaks, and this leads to cell cycle arrest
mediated by p53 and RB.
In cells in which the checkpoints are disabled
by p53 or RB1 mutations, the nonhomologous
end-joining pathway is activated to save the cell,
joining the shortened ends of two chromosomes
LIMITLESS REPLICATIVE POTENTIAL: TELOMERASE
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At mitosis the dicentric chromosomes are pulled
apart, generating random double-stranded breaks,
activating DNA-repair pathways, leading to the random
association of double-stranded ends and the
formation, again, of dicentric chromosomes
Genomic instability from the repeated “bridge-fusionbreakage cycles” eventually produces mitotic
catastrophe, characterized by massive cell death.
For tumors to grow indefinitely tumor cells must
Reactivate / Re-expression of telomerase
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Re-expression of telomerase allows the cells to escape
the bridge-fusion-breakage cycle
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Sequence of events in the development of limitless replicative potential.
• Replication of somatic cells, which do not express telomerase, leads to
shortened telomeres.
• In the presence of competent checkpoints, cells undergo arrest and enter
nonreplicative senescence.
• In the absence of checkpoints, DNA-repair pathways are inappropriately
activated, leading to the formation of dicentric chromosomes.
ANGIOGENESIS
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Solid tumors cannot enlarge beyond 1 to 2
mm in diameter unless they are vascularized
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Cancer cells can stimulate neoangiogenesis
or in some cases vasculogenesis
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Tumor angiogenesis is controlled by the
balance between angiogenesis promoters
and inhibitors (e.g p53)
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Tumor vasculature is abnormal, vessels are
leaky , dilated and have a haphazard pattern
of connection
ANGIOGENESIS
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Most human tumors do not induce angiogenesis early
remain small /in situ, possibly for years, until the
angiogenic switch terminates this stage
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The molecular basis of the angiogenic switch involves
increased production of angiogenic factors and/or
loss of angiogenic inhibitors
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These factors may be produced directly by the tumor
cells ,or by inflammatory cells (e.g., macrophages) or
stromal cells
ANGIOGENESIS
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Neovascularization has a dual effect on tumor
growth:
1. Perfusion - supplies needed nutrients & O2
2. Endothelial cells secrete growth factors (IGFs,
PDGF, GMCSF) promoting growth of adjacent
tumor cells
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Angiogenesis is required also for access to the
vasculature and hence for metastasis.
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Angiogenesis is thus a necessary biologic
correlate of malignancy