Src homology domain-containing phosphatase 2 suppresses cellular senescence in glioblastoma.

Glioblastoma multiforme (GBM) is the most common type of malignant primary brain tumour in adults. Around 30 000 new cases are diagnosed every year in the United States and Europe (CBTRUS, 2008). The prognosis is dismal, and despite treatment with the standard of care regimen involving surgery, radiation 'and chemotherapy, median survival remains below 15 months (Stupp et al, 2005), and there is a clear need for improved therapeutic approaches. There has, however, been substantial progress in the understanding of molecular cancer subgroups (Verhaak et al, 2010), pathways involved in gliomagenesis and disease progression (Furnari et al, 2007; Network CGAR, 2008; Parsons et al, 2008; Yan et al, 2009; Verhaak et al, 2010) These efforts to understand the underlying molecular biology of the disease is now paving the way for the development of targeted therapeutics. Epidermal growth factor receptor (EGFR) is overexpressed in a variety of human tumours including GBM, where it has been linked to radiation resistance and poor prognosis. A number of researchers, including us, have shown activation of EGFR to result in cytoprotective and proliferative downstream signalling (Schmidt-Ullrich et al, 2003; Sturla et al, 2005). Studies of receptor tyrosine kinases (RTKs), including EGFR, demonstrate that the overall phosphorylation state is a net result of RTK and protein tyrosine phosphatase (PTP) activities (Reynolds et al, 2003). As the catalytic activity of PTP's can be 1000-fold greater than that of kinases (Haque et al, 1995; Wang et al, 2010), perturbation of activity may have a significantly more profound effect on signal propagation than that of kinases. Most targeting strategies for RTKs emphasise the kinase activity (Shimizu et al, 2008). In autocrine-regulated tumour cells, the Tyr kinase activity is always ‘on', and thus net RTK activity will be mostly regulated by PTP activity. This suggests that the greatest therapeutic gain may be achieved by targeting the counteracting PTP. Src homology domain-containing phosphatase 2 (SHP2) (PTPN11) is a non-receptor PTP, which regulates several of the RTK pathways known to be overexpressed in glioblastoma, including EGFR, FGFR and PDGFR (Grossmann et al, 2010). Perhaps, the most well-studied role of SHP2 is that in the modulation of EGFR phosphorylation, the RTK most widely overexpressed in GBM (Bredel et al, 2010). Here SHP2 has been shown to both antagonise and potentiate the action of its target PTK's and was the first phosphatase described as oncogenic (Bentires-Alj et al, 2004). More than 58 different SHP2 mutations have been identified in various tumours and 18 mutations in Noonan and Leopard syndromes, where patients exhibit disruption of normal cell proliferation and migration during development (Bentires-Alj et al, 2004). In its basal state, SHP2 activity is suppressed by intramolecular interactions between residues in the ‘backside loop' of the N-terminal SH2 domain and the catalytic surface of the PTP domain (Hof et al, 1998). The mutations have been found to cluster mostly in the N-SH2 and PTP domain interface of the protein and are therefore predicted to be activating mutations – suggesting a positive role for SHP2 in tumourigenesis As mutations in SHP2 have been identified in a variety of solid tumours and this phosphatase is an important regulator of multiple RTK's involved in the aetiology of GBM, we decided to examine this phosphatase closely in both established GBM cell lines and the The Cancer Genome Atlas (TCGA) human tissue database. We used both TCGA mutation and expression data to confirm the presence of SHP2 mutations in human GBM and establish a potential role for this phosphatase in the classical, RTK-driven subgroup. We also used an siRNA approach to examine the effects of SHP2 knockdown on cell viability in established GBM cell lines in vitro.