The MET gene (MNNG-HOS transforming gene; Cooper et al. 1984) located on chromosome 7, encodes a receptor tyrosine kinase (RTK) belonging to the MET/RON family of RTKs. Binding of its ligand, hepatocyte growth factor (HGF; also called scatter factor (SF)), induces a conformational change in the MET receptor that facilitates receptor phosphorylation and activation. Activated MET then phosphorylates its substrates, resulting in activation of multiple downstream pathways within the cell, including the PI3K-AKT-mTOR pathway, which is involved in cell survival, and the RAS-RAF-MEK-ERK pathway, which is involved in cell proliferation (Figure 1). In the context of malignancy, aberrant signaling through the MET receptor promotes pleiotrophic effects including growth, survival, invasion, migration, angiogenesis and metastasis (Birchmeier et al. 2003; Peruzzi and Bottaro 2006).
The MET receptor and/or its ligand HGF have been reported to be aberrantly activated in many human cancers (see http://www.vai.org/met/). Germline mutations in the tyrosine kinase domain of MET occur in 100% of hereditary papillary renal cell carcinoma, and somatic mutations in MET are found in 10–15% of sporadic papillary renal cell carcinoma (Schmidt et al. 1997). Mutations in MET have been reported at low frequencies in head and neck squamous cell carcinoma (Di Renzo et al. 2000), childhood hepatocellular carcinoma (Park et al. 1999), NSCLC (Kong-Beltran et al. 2006; Ma et al. 2003) and small cell lung cancer (Ma et al. 2003). Amplification of MET has been reported in gastric cancer (Nakajima et al. 1999), esophageal cancer (Miller et al. 2006), colorectal cancer (Umeki, Shiota, and Kawasaki 1999), gliomas (Beroukhim et al. 2007), clear cell ovarian cancer (Yamamoto et al. 2011) and NSCLC (Bean et al. 2007; Cappuzzo et al. 2009; Chen et al. 2009; Engelman et al. 2007; Kubo et al. 2009; Okuda et al. 2008; Onozato et al. 2009).
Figure 1. Schematic of the MET signaling pathway. Growth factor binding to MET results in activation of the MAPK signaling pathway (RAS-RAF-MEK-ERK) and the PI3K pathway (PI3K-AKT-mTOR). The letter "K" within the schema denotes the tyrosine kinase domain.