Cell Signaling Pathways – A Case Study Approach

L. Emtage, L. Bradbury, N. Coleman, D. Davenport, A. Dunning and J. Grew

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 dysmorphicfacial 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 proteinRaf. The genetic bases of Tiger syndrome are unknown.

Your team is involved in a clinical trial, which finds that drugs that inhibit Rafare 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.

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