Download S5.Cell Signaling-Noonan Case Study instructor

Cell Signaling Pathways – A Case Study Approach
L. Emtage, L. Bradbury, N. Coleman, D. Davenport, A. Dunning and J. Grew
Signal
Case Study: Tiger Syndrome
RTK
Plasma membrane
Grb2 Sos (Ras GEF)
(adapter)
Ras
GTP
Ras
(G protein)
GDP
Raf
(MAPKKK)
P
MEK
(MAPKK)
P
ERK
(MAPK)
Case Study: Tiger Syndrome
You are a scientist studying two related congenital diseases, Noonan syndrome and
Tiger syndrome. These syndromes are characterized by developmental heart
defects, multiple skeletal abnormalities, short stature, and dysmorphic facial
features. Individuals with either syndrome are also susceptible to certain types of
cancer, such as leukemia. Some cases of Noonan syndrome are caused by dominant
activating mutations in the MAP kinase-kinase-kinase protein Raf. The genetic bases
of Tiger syndrome are unknown.
Your team is involved in a clinical trial, which finds that drugs that inhibit Raf are
associated with improved outcomes in cancer patients with Noonan syndrome. You
find that Tiger syndrome patients also respond to the treatment, so you sequence
the gene for Raf in several patients, but find no mutations.
Based on what you know about the MAP kinase pathway, you sequence the Sos gene
in Tiger syndrome patients. You find that 5 out of 10 patients in your trial have a
mutation that changes the amino acid Thr266 to Lys (T266K). Introducing this
mutant version of Sos into cells in culture leads to increased levels of
phosphorylated ERK compared to controls.
Cell Signaling Pathways – A Case Study Approach
L. Emtage, L. Bradbury, N. Coleman, D. Davenport, A. Dunning and J. Grew
a) The mutations that cause Noonan and Tiger syndrome are dominant. How do
you think the T266K mutation in Sos might alter its activity? The students
are told that the T266K mutant Sos leads to increased levels of phosphoERK
in cultured cells. Phosphorylated ERK is one measure of positive activity in
the MAPK pathway, so the students should understand that the MAPK
pathway has been activated and, therefore, that Sos has been activated by the
mutation.
b) How does this mutation affect the components of the MAP kinase pathway
upstream of Sos? How does it affect the downstream components?
Presumably, upstream components of the pathway are unaffected. The
T266K mutation is in the RhoGEF domain, responsible for GEF activity, and
has been experimentally determined to increase both Ras and ERK activation
(Roberts et al., 2007). We can assume that Raf and MEK are also activated,
and that the pathway’s cytoplasmic and nuclear targets are appropriately
activated or inhibited.
c) The other five individuals with Tiger syndrome in the trial don’t have
mutations in Raf or Sos. Which components of the pathway would you
sequence next, and why? The observation that Raf inhibitors are effective in
blocking pathway activation is the additional piece of information that the
students should bring to bear on this question. That implies that the
activating mutations must act prior to Raf activation. That leaves Ras, Grb2,
or the receptor itself.
Nature Genetics 39, 70 - 74 (2007)
Germline gain-of-function mutations in SOS1 cause Noonan syndrome
Amy E Roberts1,2,5, Toshiyuki Araki3,5, Kenneth D Swanson3,5, Kate T
Montgomery1, Taryn A Schiripo1, Victoria A Joshi1,4, Li Li1, Yosuf Yassin1, Alex M
Tamburino1, Benjamin G Neel3 & Raju S Kucherlapati1
Cell Signaling Pathways – A Case Study Approach
L. Emtage, L. Bradbury, N. Coleman, D. Davenport, A. Dunning and J. Grew