Transcript APC powerpoint

Colon cancer is the second
leading cause of cancer deaths
in the U.S.
Polyps, the first stage
In tumor development
http://www.clevelandclinic.org/registries/inherited/fap.htm
Colorectal cancer
150,000 cases diagnosed per year
(i.e., your chances are 1/18)
> 50,000 deaths (2nd to lung cancer)
Early detection key
>90% 5 year survival with early detection
60% if it has spread locally
<10% if it has metastasized
Data from ACS
Colorectal cancer treatment
1. Surgical removal of tumor
2. This can be followed by either
Radiation or chemotherapy (fluorouracil)
3. Chemotherapy also used to
slow the progress of metastatic disease
Data from ACS
Colon cancer genetics
95% of cases sporadic (no genetic history)
HNPCC 3% of all cases.
Defects in mismatch repair.
Familial adenomatous polyposis (FAP)
1% of all colon cancers
Caused by mutation in a single gene = APC
Familial
Adenomatous
Polyposis (FAP)
Inheriting FAP-related mutations dramatically increases
your risk of developing colorectal cancer.
93% colorectal cancers by age 50
100% colorectal cancers lifetime risk
0.2% colorectal cancers by age 50
6% colorectal cancers lifetime risk
Until recently the only treatment
Was to surgically remove the colon
Treatment with an aspirin-like drug
called celecoxib (a COX2 inhibitor)
led to a 25% reduction in polyp number ,
and 40% reduction in advanced adenomas.
Problem--increased cardiovascular risk.
The hope is this will allow doctors to
Delay surgery till later in life
New England Journal of Medicine 342, 1946 (2000)
And NCI Cancer Bulletin, April 16, 2008.
So what causes
Familial
Adenomatous
Polyposis (FAP)
Figure 7.23 The Biology of Cancer (© Garland Science 2007)
Adenomatous Polyposis Coli (APC)
tumor suppressor
mutated in FAP
and in 70% of
sporadic
cases of colon cancer
colon polyps
APC mutations are
the earliest event
in the development
of the tumor
To find clues to APC’s function
they went fishing for partners
APC
Cell extract
Fishing with APC
Colon cancer
WT
cell line
?
first,
immunoprecipitation
APC
Fishing with APC
Colon cancer
WT cell line
co-immunoprecipitation
p95= -catenin
When they went fishing with APC,
they pulled out Armadillo
Armadillo
(ß-catenin)
APC
Eric Wieschaus and Christiane Nüsslein-Volhard
used genetics to identify proteins that
set up the embryonic body plan
The mighty
fruit fly
How is the body plan specified?
Egg ---> animal
in 24 hours!
Wieschaus and
Nüsslein-Volhard
removed single genes
and looked for effects
on the body plan
2 young scientists
1 microscope
1 year
26,978 flies
139 new genes
Five Signal Transduction Pathways
Shape virtually all cell fates choices
RTK
Wnt
TGF-β
Hedgehog
Notch
Five Signal Transduction Pathways
Shape virtually all cell fates choices
All also play key roles in many cancers
RTK
Wnt
TGF-β
Hedgehog
Notch
Wingless signaling specifies cell fates in
the ventral epidermis
wild type
arm mutant
Wingless signal
influences the fates
of neighboring cells
Cells instruct one another
via cell-cell signaling pathways
Be
Posterior!
Cell-cell signaling can be influenced
by distance between cells
Pardon
me?
Yes
Ma'am!
Be
Posterior!
Turn on
new genes!
Cell surface
Signal transduction
moves information
from the cell surface
to the nucleus and
other cellular targets
Yes
Ma'am!
Nucleus
Turn on
new genes;
pass it on
Turn on
new genes;
pass it on Turn on
new genes;
pass it on
Cell surface
Signal transduction
occurs through
a series of steps
Turn on
new genes;
pass it on
Yes
Ma'am!
Nucleus
Artist’s conception of an Armadillo
Epithelial cells at work
The mammalian core
adherens junction complex
E- cadher in
P la sm a m emb ran e
-catenin
ß-c at enin
Armadillo and dTCF form a
bipartite transcription factor
Armadillo
dTCF
CCTTTGATCTT
Activation of
Wingless-responsive genes
Armadillo and TCF activate Wingless target genes
Without Wingless signal
Groucho
Repression of
Wingless-responsive genes
TCF
CCTTTGATCTT
With Wingless signal
Armadillo
TCF
CCTTTGATCTT
Activation of
Wingless-responsive genes
Elaine Fuch’s Hair Club for Men
Rub ß-catenin here!
Before
After
Wingless signaling modulates
the stability of Armadillo protein
Level of
Wg signal
Armadillo
Proliferate!
Proliferate!
Cell surface
Proliferate!
Proliferate!
Signal transduction
can regulate
cell proliferation
Yes
Ma'am!
Nucleus
In the absence of APC,
levels of Armadillo/ß-catenin
rise dramatically
Wild-type
APC mutant
Our current model for Wnt signaling
Phosphorylation of
Armadillo/ß-catenin
By GSK3
creates a binding site
for an E3 ubiquitin ligase
APC is a complex protein
Arm Repeats
15 AA
rpts
20 AA
rpts
SAMP rpts
Arm repeats protein-protein
interaction motif also found in Arm
!5 and 20 amino acids repeats
bind to Arm
SAMP repeats bind Axin
APC is a complex protein
Arm Repeats
15 AA
rpts
20 AA
rpts
SAMP rpts
Arm Repeats
MCR
All tumors carry one allele that makes
A truncated APC protein
And truncations almost always occur
in the “mutation cluster region” (MCR)
Activated Wnt signaling triggers colon cancer
A quick look at colon architecture
Differentiated
cells
Proliferating
cells
Villus
Crypt
Begin with colon cancer cell line
in which the Wnt pathway is ON
Add an inducible dominant negative form
of TCF4 that turns the Wnt pathway OFF
Look for genes whose transcription
is regulated by Wnt signaling
Hans Clevers
The genes turned ON by Wnt signaling
are expressed in crypts
and those that are turned OFF are
expressed in villi
Gene turned ON by Wnt
Gene turned OFF by Wnt signal
The genes turned on by Wnt signaling
are expressed in crypts
and those that are turned off are
expressed in villi
Gene turned ON by Wnt
Gene turned OFF by Wnt signal
Wnt signaling turns OFF the transcription
of an key cell cycle regulator:
the CDK inhibitor p21
mRNA
The effect on p21 expression is indirectly mediated by
the transcription factor c-myc.
Transcription of the myc gene is directly regulated
By TCF/beta-catenin
The data
The model
Let’s put this together
to form a picture of normal colon biology
and how it is perturbed by APC mutations
As cells migrate
Away from
the crypt, they
differentiate
Cells that receive them
Become stem cells
Stromal cells send Wnt signals