18 Chapter 1 The ectodomain consists of a large SEMA domain, required for ligand binding and receptor dimerization (80), which is separated from four immunoglobulin-like domains by a small cysteine-rich domain. Next comes the transmembrane domain, followed by a long juxtamembrane sequence, the kinase domain and the COOH-terminal sequence, of which the latter three hold essential motifs for downstream signaling (72). HGF binding induces MET’s kinase activity by facilitating receptor dimerization and trans-phosphorylation of juxtamembrane catalytic residues Tyr1234 and Tyr1235. Subsequent phosphorylation of the carboxy-terminal ‘docking’ residues Tyr1349 and Tyr1356, allows MET to recruit a variety of downstream signal-relay transducers. On the other hand, phosphatases antagonize MET signaling by dephosphorylating either the catalytic or docking tyrosine residues. The majority of MET-mediated downstream signaling modules is transduced through interaction of the receptor with the multi-adaptor protein GAB1 (GRB2-associated-binding protein 1). Ultimately, downstream signaling pathways, including MAPK, PI3K-Akt, and STAT, are effectively activated by MET and its signal transducers (Figure 3) (71, 72). MET is aberrantly activated by mutations and amplifications in approximately 25% of HNSCC. Approximately 4% of HNSCC carry exon 14 deletion. The resulting exon 14 skipping delays MET ubiquitination and down-regulation, having MET overexpression and kinase activation as a consequence (81). Somatic mutations affecting the kinase domain occur at a rate of around 8%, which include Y1230C, Y1235D, and Y1253D (81). Mutations Y1230C and Y1235D constitutively activate MET and were found in RLNMs of HNSCC (82). Interestingly, while transcripts of the Y1235D mutant alleles are highly represented in RLNMs, they are hardly detectable in the corresponding primary tumors. This suggests clonal expansion of cells carrying mutated MET during HNSCC disease progression (82). MET activating point mutation Y1253D has been described to be significantly associated with decreased metastasis-free survival of HNSCC patients treated with radiotherapy or CRT (83). Progression-free and overall survival of HNSCC patients with exon 14 deletions, mutations of the kinase domain (V1110I, H1112Y, V1333I), or juxtamembrane domain mutations (R988C and T1010I), that were included in a phase III randomized trial investigating the effectivity of the EGFR inhibitor gefitinib, was shorter compared to patients with MET wild-type cancers irrespective of the treatment arm (84). Seen differences in used scoring methods and definitions, it has been reported that MET gene amplifications occur in 1-13% of HNSCC. Although known, the biological and clinical consequences of MET amplification in HNSCC need to be further explored (81). If a molecular subtype of HNSCC, and more specifically OSCC, exists that is driven by MET mutation and/or amplification conferring susceptibility to targeted agents, requires further examined
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