Patent Publication Number: US-11035860-B2

Title: Auto-active and intracellular mutant of MET

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a national stage application under 35 U.S.C. § 371 of International Application No. PCT/EP2015/076989, filed on Nov. 18, 2015. This invention relates to and claims the benefit of priority to European Patent Application No. 14194066.8, filed on Nov. 20, 2014, the disclosures of each of which are explicitly herein incorporated by reference in their entirety. 
     FIELD OF THE INVENTION 
     The present invention relates to the fields of medicine and molecular diagnostics. In particular, it relates to a novel method for the treatment, prevention and/or delay of cancer and to a diagnostic method involving detection of a novel auto-active and intracellular mutant of MET. 
     BACKGROUND OF THE INVENTION 
     The MET proto-oncogene (chromosome 7q31.2) encodes the tyrosine kinase membrane receptor MET (also called Scatter Factor Receptor or Hepatocyte Growth Factor Receptor), which is essential during development. Signaling from the receptor controls epithelial-to-mesenchymal transition (EMT) of myogenic precursor cells during differentiation into skeletal muscle cells (Bladt et al., 1995), a process that involves migration over long distances in the embryo. In adults, MET is involved in tissue regeneration upon injury (Borowiak et al., 2004; Chmielowiec et al., 2007; Huh et al., 2004). 
     MET is produced as a glycosylated single chain precursor protein of ˜190 kDa which, during transport to the membrane, undergoes furin-mediated cleavage in the trans-Golgi network (Komada et al., 1993). The resulting mature receptor consists of an extracellular 50 kDa α-chain, covalently attached via a disulfide bond to a membrane-spanning 140 kDa β-chain (Chan et al., 1988; Liu et al., 1996; Park et al., 1987). The extracellular segment consists of an N-terminal Sema-domain that is involved in ligand binding, a small cysteine-rich domain, and four IPT (Immunoglobulin-like fold shared by Plexins and Transcription factors) domains, which connect the Sema and cysteine-rich domains with the C-terminal β-subunit (Gherardi et al., 2003). 
     Upon binding of the ligand Hepatocyte Growth Factor (HGF, Scatter Factor), receptor dimerization occurs followed by trans-phosphorylation in the intracellular tyrosine kinase (TK) domain at tyrosine (Y) residues 1230, 1234 and 1235 (Ferracini et al., 1991; Longati et al., 1994). The TK domain subsequently induces autophosphorylation of Y1349 and Y1356 which act as docking sites for signal transduction molecules including GAB1, GRB2, phospholipase-C and SRC (Ponzetto et al., 1994). Phosphorylated GAB1 interacts with molecules like PI3-K and SHP2 which together induce several downstream signaling pathways. MET signaling is mediated by, among others, the PI3-K/AKT and RAS/MAPK pathways, which induce cell cycle progression, survival, cytoskeletal changes and invasion (Gherardi et al., 2012). In addition to its role downstream of HGF, MET can also be involved in signaling of other transmembrane receptors, including VEGFR2, CD44, and Plexin B1 (Lai et al., 2009; Lu et al., 2012). Upon ligand-induced receptor dimerization. MET is internalized via endocytosis and may be recycled (Joffre et al., 2011). Phosphorylation of Y1003 in the juxtamembrane (JM) domain of the receptor leads to ubiquitination and subsequent proteasomal degradation (Hammond et al., 2001; Jeffers et al., 1997; Peschard et al., 2001). Thus, levels of MET in the cell are tightly regulated. 
     Aberrant activation of MET signaling is a tumor promoting event in a variety of malignancies and can be induced by several mechanisms including overexpression of its ligand and/or the receptor itself, alternative mRNA splicing and crosstalk with other receptors (Corso and Giordano, 2013). MET amplifications have been found in a number of tumor types including glioblastoma (GBM) (Cancer Genome Atlas Research, 2008; Sottoriva et al., 2013) whereas missense mutations in the Sema, the TK and the JM domain have been reported to affect HGF binding, kinase activation and receptor degradation, respectively (Asaoka et al., 2010; Kong-Beltran et al., 2006; Lee and Yamada, 1994; Ma et al., 2005; Ma et al., 2008; Onozato et al., 2009; Peschard et al., 2001; Petrelli et al., 2002). 
     The significant role that MET plays in tumor progression and metastasis has made it a prime therapeutic target in oncology. There are currently two classes of MET activation inhibitors available which are in different stages of clinical trial: (1) small molecule MET tyrosine kinase inhibitors and (2) therapeutic antibodies against the extracellular domain of MET and against its ligand HGF. The antibodies have been developed to block the interaction between HGF and MET, thereby preventing MET activation. The tyrosine kinase inhibitors inhibit the activity of the intracellular tyrosine kinase domain of the activated MET, thus blocking MET signaling. 
     There is however a group of patients who do not respond to anti-MET-antibodies or anti-HGF-antibodies. In the current era of personalized treatment it is of high importance to select on forehand which patients will respond to what therapies. 
     Accordingly, there is an urgent need for a diagnostic test to assess which subjects are likely not to respond to available antibody therapies but will respond to available tyrosine kinase inhibitors and which subjects may develop resistance against the presently available antibody therapies. 
     DESCRIPTION OF THE INVENTION 
     The present inventors have surprisingly found a novel auto-active and intracellular MET mutant. The present inventors have found the mutant MET to be present in 6% of high-grade gliomas and 14% of non small cell lung carcinomas. Characterization of the MET mutant in a glioma cell line revealed that it comprises a deletion of exons 7 and 8. The encoded protein lacks parts of the extracellular IPT-domains 1 and 2, encoded by these exons 7 and 8. As a result of the mutation, the MET mutant is located predominantly in the cytosol where it is constitutively active, independent of the presence of HGF. The auto-active nature of this protein, in combination with a lack of transmembrane localization, renders the MET mutant non-targetable for antibodies. Testing for the presence of the MET mutant is therefore warranted for defining treatment strategy. Accordingly, in a first aspect, the present invention provides for a method for determining the presence of a MET mutant in a sample, wherein the MET mutant is an auto-active and intracellular mutant MET. Such method is herein referred to as a method according to the present invention; such mutant MET is herein referred to as a MET mutant or mutant MET according to the present invention. A mutant MET according to the present invention is defined as a MET protein that is auto-active and is located intracellularly; included in the term mutant MET are an RNA, DNA or cDNA encoding such mutant MET protein. Auto-active is defined as that the mutant MET does not need its ligand HGF to activate its tyrosine kinase domain; said tyrosine kinase domain is constitutively active. Intracellularly is herein to be construed that the mutant MET lacks transmembrane localization and is predominantly located in the cytosol; preferably at least 50%, 60%, 70%, 80%, 90%, 95%, 96%, 97%, 98%, 99% or most preferably 100% of the MET protein is located in the cytosol. Within the context of the present invention, the term MET mutant includes all genetic, epigenetic, post-transcriptional and post-translational forms, including variants such as, but not limited to, methylated and phosphorylated variants. Within the context of the present invention, the term “presence of a mutant MET” means a mutant MET is detected, preferably at least 0.1%, 0.2%. 0.3%. 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99%, or 100% of the MET detected is mutant MET. 
     A method according to the present invention may be any molecular diagnostic method known to the person skilled in the art that is able to detect whether a MET mutant according to the present invention is present in a sample. Such methods are known to a person skilled in the art. Non-limiting examples of such assays are in situ hybridization, immunofluorescence microscopy, and confocal microscopy to detect whether a mutant MET protein is located intracellularly. An antibody or an antibody conjugate, may be used to determine whether a mutant MET protein is located intracellularly. A nucleic acid detection technique such as, FISH or FuseFISH (Semrau et al 2014) or an amplification technique such as PCR, rolling circle amplification or NASBA or a nucleic acid based signal amplification technique or a sequencing technique may be used to determine whether a mutant MET is present in a sample. The person skilled in art the is aware of these techniques (see e.g. Green and Sambrook (2012)  Molecular Cloning: A Laboratory Manual , Fourth Edition, Cold Spring Harbor Laboratory Press, NY). Altogether, a method according to the present invention may comprise analysis of genomic DNA, analysis of mRNA or cDNA, and/or analysis of MET protein in a sample. A mutant MET protein according to the present invention may be the result of a genomic mutation, such as a deletion, and/or may be the result of exon skipping or alternative splicing. 
     A preferred mutant MET according to the present invention is the result of a chromosomal deletion in chromosome 7, resulting in loss of part of intron 6, exon 7, intron 7 and part of exon 8, thus resulting in a matured mRNA lacking exon 7 and 8 sequences and wherein the mutated MET protein preferably is an uncleaved phosphorylated truncated preform of about 180 kDa. Such mutant MET is herein described in example 1. Preferably, the deletion is about 2114 bp and corresponds substantially to genome positions g. 116395653 and g. 116397766. In the context of these genome positions, the term substantially means that the genome positions may be upstream or downstream of the indicated positions as long as it results in an intact splice donor site of exon 6 and a deletion of splice acceptor sites of exons 7 and 8, making the splice acceptor site of exon 9 the first to accept exon 6. More preferably, the deletion is 2114 bp and corresponds to genome positions g. 116395653 and g. 116397766. 
     A preferred method according to the present invention involves analysis of genomic DNA to detect a deletion which results in a mutated MET protein according to the present invention. Preferably such deletion is about 2114 bp and corresponds substantially to genome positions g. 116395653 and g. 116397766. In the context of these genome positions, the term substantially means that the genome positions may be upstream or downstream of the indicated positions, as long as it results in an intact splice donor site of exon 6 and a deletion of splice acceptor sites of exons 7 and 8, making the splice acceptor site of exon 9 the first to accept exon 6. More preferably, the deletion is 2114 bp and corresponds to genome positions g. 116395653 and g. 116397766. A preferred version of such method is described in example 1. 
     A further preferred method according to the present invention involves analysis of mRNA or cDNA to detect a deletion which results in a mutated MET protein according to the present invention. Preferably such deletion is the substantial absence of exons 7 and 8. The substantial absence of exons 7 and 8 is to be construed herein that exons 7 and 8 are absent partly or completely with the result that the encoded protein is a MET mutant protein according to the present invention, A preferred version of such method are described in examples 1 and 2, preferably example 2. Another preferred method is a FuseFISH assay (Semrau et al, 2014). In such FuseFISH assay, two differently colored sets of fluorescent probes are used to discriminate between wild-type MET and a mutant MET according to the present invention. E.g. a FuseFISH assay can be envisaged where a mixture of green fluorescent oligonucleotides is directed to exon 5/6 and a mixture of red oligonucleotides is directed to exons 7/8; the wild type will be detected as both red and green fluorescence (effectively yellow) since both exon 5/6 and 7/8 are present; the mutant MET is lacking exon 7/8 and will be detected as green fluorescent. Such assay can be performed in situ in cells; alternatively an array or chip of colored oligonucleotides can be used and purified RNA be applied. Mutant MET can alternatively be detected on the transcript level in tissue sections by utilizing PADLOCK probes that allow rolling circle amplification of the exons 6-9 junction (Weibrecht et al, 2013). 
     A further preferred method according to the present invention involves analysis of MET protein to determine whether the MET protein is substantially or exclusively located intracellularly or whether at least part is on the cell surface. Such method may be but is not limited to fluorescence microscopy, confocal microscopy and protein analysis of tissue biopsies, e.g. via SDS-PAGE and western blotting. 
     A preferred method according to the present invention entails analysis of genomic DNA, mRNA and/or cDNA which involves a nucleic acid detection assay comprising an oligonucleotide, wherein a complex of the oligonucleotide with a template polynucleotide is formed, or involves analysis of MET protein involving application of an anti-MET antibody, wherein a complex of the anti-MET antibody and the MET protein is formed. Preferably, these methods are those described here above. A preferred method according to the invention is an amplification assay, preferably PCR that is able to discriminate between a wild-type MET and a MET that lacks exon 7 and 8 (MET delta 7/8). Such assay is presented in example 2 and involves the amplification of cDNA that is generated from RNA that is isolated from tumor tissue, tumors exosomes, or from blood, preferably from blood platelets from a subject. The RNA can be converted into cDNA directly and subsequently amplified by an enzyme having reverse transcriptase and DNA polymerase activity, such as rTth polymerase. The RNA can first be converted into cDNA by a reverse transcriptase using an oligo-dT primer, or random (hexanucleotide) primers, or a METrev2561 (SEQ ID NO: 22) primer or a METrev2393 (SEQ ID NO: 23) primer. Amplification can be performed using a primer that is able to discriminate between a wild-type MET and a MET lacking exon 7 and 8. A preferred primer is forward primer METFW2033junct6_9 (SEQ ID NO: 21) which was designed such that the 3′six nucleotide motif is specific for exon 9 and the remaining nucleotides are specific for exon 6; the primer thus only anneals to a MET mutant cDNA lacking exon 7 and 8, i.e. anneals on the junction of exon 6 and 9. The persons skilled in the art knows that some variation is possible as long as the primer is specific for the junction of exon 6 and 9. Of course, the same strategy can be used using a reverse primer that is specific for the junction between exon 6 and 9. The forward primer can be used in combination with any suitable reverse primer such as METrev2561 (SEQ ID NO: 22) and METrev2393 (SEQ ID NO: 23). The combination of METFW2033junct6_9 and METrev2393 results in a product of 120 bp when the mutation is present that results in a MET lacking exon 7 and 8; the combination of METFW2033junct6_9 and METrev2561 results in a product of 268 bp when the mutation is present that results in a MET lacking exon 7 and 8. Wild-type MET results in no PCR product. A preferred assay using the primers depicted here is the assay set forward in example 2. Detection of a PCR product may be performed using any means known to the person skilled in the art, such as gel electrophoresis and the use of a detectable probe that is specific for the PCR product, such as a fluorescent capture probe. 
     A method according to this aspect of the present invention may be performed in vivo, ex vivo, in situ or in vitro. In all aspects and embodiment of the present invention, a sample may be an in vivo, ex vivo, in situ or in vitro sample. The sample may be any sample from a subject that is suitable for a method according to the present invention. Preferred samples comprises a tissue, a tumor tissue, urine, sperm, saliva, blood, blood plasma, cerebrospinal fluid, blood platelets, and/or exosomes. Such sample may already be available; it may be necessary to acquire such sample. Nucleic acid such as DNA, RNA and, cDNA can be isolated and purified from the sample if necessary before a method according to the invention is performed on such nucleic acid. 
     A method according to the present invention can conveniently be used to determine whether a condition of a subject is susceptible to treatment with a MET activation inhibitor. 
     Accordingly, in a second aspect, the present invention provides for a method to determine whether a MET associated condition, preferably a cancer, of a subject is susceptible to treatment with a MET activation inhibitor comprising:
         a. providing a sample from the subject, and   b. performing on the sample according to (a), a method according to the first aspect of the present invention,
 
wherein the presence of an auto-active and intracellular mutant MET indicates non-susceptibility to a MET activation inhibitor.
       

     A MET activation inhibitor is herein defined as an agent that is capable of inhibiting the process that brings MET into its activated form (see Background section) wherein typically tyrosine (Y) residues 1230, 1234 and 1235 are phosphorylated. Such agent may interfere with or may block binding of the ligand Hepatocyte Growth Factor (HGF) to the extracellular domain of MET. Preferably, a MET activation inhibitor is an antibody against HGF or against the HGF binding site on MET, thus inhibiting or blocking interaction between HGF and MET, thereby preventing MET activation. A preferred MET activation inhibitor is not a tyrosine kinase inhibitor. Preferred MET activation inhibitors are those described in Surati et al, 2011 and Godzik-Spychalski et al, 2014, such as MetMAb and AMG102 (Rilotumumab). 
     It is evident that the presence of an auto-active and intracellular mutant MET indicates non-susceptibility to a MET activation inhibitor since the mutant MET does not require activation by e.g. binding of HGF. 
     A method according to this aspect of the present invention may be performed in vivo, ex vivo, in situ or in vitro. In all aspects and embodiment of the present invention, a sample may be an in vivo, ex vivo, in situ or in vitro sample. The sample may be any sample from a subject that is suitable for a method according to the present invention. Preferred samples comprises a tissue, a tumor tissue, urine, sperm, saliva, blood, blood plasma, cerebrospinal fluid, blood platelets, and/or exosomes. Such sample may already be available; or it may be necessary to acquire such sample. 
     A method according to the first and second aspect of the present invention can conveniently be implemented in a method of treatment of a subject that has a MET associated condition or is at risk of a MET associated condition. 
     Accordingly, in a third aspect the present invention provides for a method for the treatment of a MET associated condition, preferably a cancer, of a subject comprising: requesting performance or performing a method according to the first or second aspect of the invention on a sample of the subject, thus determining susceptibility of the condition of the subject to a MET activation inhibitor, and treating a subject with a MET associated condition that is susceptible to a MET activation inhibitor by administration of a therapeutically effective amount of at least one MET activation inhibitor and/or treating a subject with MET associated condition that is non-susceptible to a MET activation inhibitor by administration of a therapeutically effective amount of at least one alternative medicament, preferably a tyrosine kinase inhibitor. 
     In the context of all embodiments of the present invention, an alternative medicament is any medicament that is not a MET activation inhibitor as defined herein that interferes with the activity of MET by either acting directly on MET or acting in a MET signaling pathway such as AKT phosphorylation, RAS and pI3K activation, MAPK activation. Such alternative medicament according to the present invention is preferably a tyrosine kinase inhibitor. 
     The subject may be healthy subject, a subject at risk of a MET associated condition, or may suffer from a MET associated condition. 
     In addition, in a third aspect the present invention provides for a medicament for use in the treatment of a MET associated condition, preferably a cancer, of a subject in need thereof, wherein:
         a. a method according to the first or second aspect of the present invention on a sample of the subject is performed or requested to be performed, thus determining susceptibility of the condition of the subject to a MET activation inhibitor, and   b. administration to a subject with a MET associated condition that is susceptible to MET activation inhibitor of a therapeutically effective amount of at least one medicament which is a MET activation inhibitor and/or administration to a subject with MET associated condition that is non-susceptible to a MET activation inhibitor of a therapeutically effective amount of at least one alternative medicament which is not a MET activation inhibitor, preferably a tyrosine kinase inhibitor.       

     As used herein, and as well understood in the art, “treatment” is an approach for obtaining beneficial or desired results, including clinical results. Beneficial or desired clinical results can include, but are not limited to, alleviation or amelioration of one or more symptoms or conditions, diminishment of extent of disease, stabilized (i.e. not worsening) state of disease, preventing spread of disease, delay or slowing of disease progression, amelioration or palliation of the disease state, and remission. “Treatment” can also mean prolonging survival as compared to expected survival if not receiving treatment. The term “prevention” or “prophylaxis”, or synonym thereto, as used herein refers to a reduction in the risk or probability of a patient develop a disease, preferably a MET associated condition according to the present invention. 
     The term a “therapeutically effective amount”, “effective amount” or a “sufficient amount” of a compound of the present invention is a quantity sufficient to, when administered to the subject, including a mammal, for example a human, obtain beneficial or desired results, including clinical results, and, as such, an “effective amount” or synonym thereto depends upon the context in which it is being applied. In the context of disease, therapeutically effective amounts of the compounds of the present invention are used to treat, modulate, attenuate, reverse, or effect a MET associated condition in a mammal. An “effective amount” is intended to mean that amount of a compound that is sufficient to treat, modulate, attenuate, reverse, or effect a MET associated condition in a mammal. 
     In the context of all embodiments of the present invention, a MET associated condition is any condition that is associated with an aberrant MET wherein an aberrant MET is any MET that is aberrant as compared to normal conditions and preferably involves a mutation of MET, a polymorphism of MET, loss of heterozygosity, duplication or deletion of MET or of its locus or a part thereof and overexpression of MET. A MET associated condition may be any such condition known to the person skilled in the art. A MET associated condition that is non-susceptible to a MET activation inhibitor is preferably a condition associated with an auto-active and intracellular MET mutant according to the present invention. A MET associated condition according to the present invention may be a cancer, but may also be another condition. Preferably, the MET associated condition is amyotrophic lateral sclerosis, systemic sclerosis, ulcerative colitis, autism and/or a cancer selected form the group consisting of lymphoma, leukemia, mycosis fungoide, carcinoma, adenocarcinoma, sarcoma, rhabdomyosarcoma, Ewing sarcoma, castration resistant prostate carcinoma, glioma, astrocytoma, blastoma, neuroblastoma, plasmacytoma, histiocytoma, melanoma, adenoma, hypoxic tumour, myeloma, metastatic cancer, bladder cancer, brain cancer, nervous system cancer, squamous cell carcinoma of the head and neck, neuroblastoma, glioblastoma, ovarian cancer, skin cancer, liver cancer, squamous cell carcinomas of the mouth, throat, larynx, and lung, colon cancer, cervical cancer, breast cancer, cervical carcinoma, epithelial cancer, renal cancer, genitourinary cancer, pulmonary cancer, esophageal carcinoma, lung cancer, head and neck carcinoma, hematopoietic cancer, testicular cancer, colorectal cancer, prostatic cancer, and pancreatic cancer. 
     In a method according to the third aspect of the present invention, the sample may be an in vivo, ex vivo, in situ or in vitro sample. The sample may be any sample from a subject that is suitable for a method according to the present invention. Preferred samples comprises a tissue, a tumor tissue, urine, sperm, saliva, blood, blood plasma, cerebrospinal fluid, blood platelets, and/or exosomes. Such sample may already be available; it may be necessary to acquire such sample. 
     In a fourth aspect, the present invention provides for a biomarker for determining the presence of a MET mutant in a sample, wherein the MET mutant is an auto-active and intracellular mutant MET and wherein the biomarker preferably is an oligonucleotide or a set of oligonucleotides, an antibody or conjugates thereof or an array or chip comprising a multiplicity of such biomarker. 
     The biomarker may be any biomarker capable of detecting an auto-active and intracellular mutant MET. The person skilled in the art knows how to provide such biomarker and how to use it. 
     A preferred biomarker according to the present invention is an oligonucleotide that can hybridize to SEQ ID NO: 15 or its reverse complement. Such oligonucleotide can conveniently be used in a nucleic acid detection technique such as, FISH or FuseFISH (Semrau et al 2014) or an amplification technique such as PCR or NASBA or a nucleic acid based signal amplification technique or a sequencing technique. A further preferred oligonucleotide according to the present invention for the detection of an auto-active and intracellular mutant MET is an oligonucleotide that can hybridize to exon 6 (SEQ ID NO: 16) and/or exon 9 (SEQ ID NO: 17) or its reverse complement; a set of such oligonucleotides can e.g. conveniently be used for PCR or RT-PCR. A further preferred oligonucleotide according to the invention is an oligonucleotide that hybridizes to the boundery between intron 6-7 (SEQ ID NO: 17) or its reverse complement and exon 8 (SEQ ID NO: 18) or its reverse complement, of the aberrant MET delta 7/8. Such oligonucleotide according to the present invention can conveniently be used for e.g. FISH. A further preferred oligonucleotide according to the present invention is capable hybridize to a sequence within exon 6 (SEQ ID NO: 16), intron 6-7 (SEQ ID NO: 17), exon 8 (SEQ ID NO: 18), intron 8-9 (SEQ ID NO: 19) and/or exon 9 (SEQ ID NO: 20), or a reverse complement thereof. A further preferred oligonucleotide according to the present invention is an oligonucleotide as defined in the examples, such as MET1997Fw (SEQ ID NO: 5), MET2342Rv (SEQ ID NO: 6) and MET2414Rev (SEQ ID NO: 8). A preferred set of oligonucleotides is MET1997Fw (SEQ ID NO: 5) and MET2342Rv (SEQ ID NO: 6); a further preferred set is MET1997Fw (SEQ ID NO: 5) and MET2414Rev (SEQ ID NO: 8). An oligonucleotide according to the present invention may comprise or may not comprise a recognition site for a restriction enzyme. An oligonucleotide according to the present invention preferably is an oligonucleotide comprising or consisting of from 15 to 100 nucleotides, more preferred from 15 to 30 nucleotides or preferably comprising or consisting of 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50 nucleotides. In an oligonucleotide according to the present invention, the complementarity to hybridization counterpart preferably is from 90% to 100%. In general this allows for 1 or 2 mismatch(es) in an oligonucleotide of 20 nucleotides or 1, 2, 3 or 4 mismatches in an oligonucleotide of 40 nucleotides, or 1, 2, 3, 4, 5 or 6 mismatches in an oligonucleotide of 60 nucleotides, etc. The complementary part of an oligonucleotide according to the present invention may be flanked by an oligonucleotide part or parts comprising of e.g. 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10 nucleotides that is or are not complementary and may be unrelated to MET. 
     A preferred biomarker is used in a kit, comprising multiple oligonucleotides, preferably e.g. 20, which are designed to specifically hybridize in array to MET sequences, except for exon?/8-encoded sequences, and labeled with a fluorophore which emits green fluorescent light, and multiple oligonucleotides, preferably 20, specifically hybridizing in array to exon 7/8 sequences of MET mRNA and labeled with a fluorophore which emits red fluorescent light. Contacting this panel with RNA in tissue sections in an in situ hybridization experiment, will reveal presence of MET as yellow spots and MET mutant as green spots in individual cells. 
     In a fifth aspect, the present invention provides for a kit for determining the presence of a MET mutant in a sample, wherein the MET mutant is an auto-active and intracellular mutant MET, comprising at least a biomarker as defined in the fourth aspect of the present invention and instructions for use. 
     Such kit according to the present invention can conveniently be used for determining the presence of a MET mutant in a sample, wherein the MET mutant is an auto-active and intracellular mutant MET. Such use preferably involves a method as defined previously herein. 
     In a sixth aspect, the present invention provides for a polynucleotide encoding a MET mutant as defined in the first aspect of the present invention or a part thereof, or a MET mutant protein as defined in the first aspect of the present invention, or a part thereof. A preferred MET mutant protein is a protein or a part thereof with a sequence identity of at least 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, 100% with the polypeptide sequence as set forward in SEQ ID NO: 13. A more preferred MET mutant protein has the polypeptide sequence as set forward in SEQ ID NO: 13. 
     Herein, a polynucleotide is represented by a nucleotide sequence. A polypeptide is represented by an amino acid sequence. The term“polypeptide” or “protein” as used herein refers to any peptide, oligopeptide, polypeptide, or protein. A polypeptide is comprised of consecutive amino acids. The term “polypeptide” encompasses naturally occurring or synthetic molecules. The term “polynucleotide” as herein refers to RNA, DNA, and cDNA molecules. The term is used interchangeably with polynucleotide. An oligonucleotide is a short chain nucleic acid molecule, preferably consisting of at most 100 nucleotides. A primer is an oligonucleotide which is capable of acting as a point of initiation of synthesis when placed under conditions in which synthesis of a primer extension product which is complementary to a nucleic acid strand is induced, (i. e. in the presence of nucleotides and an inducing agent such as DNA polymerase and at a suitable temperature and pH). The primer is preferably single stranded for maximum efficiency in amplification. 
     In this document and in its claims, the verb “to comprise” and its conjugations is used in its non-limiting sense to mean that items following the word are included, but items not specifically mentioned are not excluded. In addition, reference to an element by the indefinite article “a” or “an” does not exclude the possibility that more than one of the element is present, unless the context clearly requires that there be one and only one of the elements. The indefinite article “a” or “an” thus usually means “at least one”. The word “about” or “approximately” when used in association with a numerical value (e.g. about 10) preferably means that the value may be the given value (of 10) more or less 1% of the value. 
     The sequence information as provided herein should not be so narrowly construed as to require inclusion of erroneously identified bases. The skilled person is capable of identifying such erroneously identified bases and knows how to correct for such errors. 
     All patent and literature references cited in the present specification are hereby incorporated by reference in their entirety. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Sequences as set forth in the Sequence Listing 
               
            
           
           
               
               
               
            
               
                 SEQ  
                   
                   
               
               
                 ID NO: 
                 SEQ type 
                 Description 
               
               
                   
               
            
           
           
               
               
               
            
               
                 1 
                 PCR primer 
                 HGF-Fw204 
               
               
                 2 
                 PCR primer 
                 HGF-Rv504 
               
               
                 3 
                 PCR primer 
                 p361-Fw 
               
               
                 4 
                 PCR primer 
                 P425-Rv 
               
               
                 5 
                 PCR primer 
                 MET1997Fw 
               
               
                 6 
                 PCR primer 
                 MET2342Rv 
               
               
                 7 
                 PCR primer 
                 MET1997Fw 
               
               
                 8 
                 PCR primer 
                 MET2414Rev 
               
               
                 9 
                 PCR primer 
                 MET173EcoR1-Fw 
               
               
                 10 
                 PCR primer 
                 MET4421NotI-Rv 
               
               
                 11 
                 PCR primer 
                 MET173Fw 
               
               
                 12 
                 PCR primer 
                 MET3028NheI-Rv 
               
               
                 13 
                 PRT 
                 MET wild-type 
               
               
                 14 
                 PRT 
                 MET delta 7/8 
               
               
                 15 
                 DNA 
                 MET delta 7/8 exon 6 &lt;-&gt; exon 9 
               
               
                 16 
                 DNA 
                 MET delta 7/8 exon 6 
               
               
                 17 
                 DNA 
                 MET delta 7/8 intron 6-7 
               
               
                 18 
                 DNA 
                 MET delta 7/8 exon 8 
               
               
                 19 
                 DNA 
                 MET delta 7/8 intron 8-9 
               
               
                 20 
                 DNA 
                 MET delta 7/8 exon 9 
               
               
                 21 
                 DNA 
                 METFW2033junct6_9 
               
               
                 22 
                 DNA 
                 MET2561R 
               
               
                 23 
                 DNA 
                 MET2393R 
               
               
                   
               
            
           
         
       
     
    
    
     
       FIGURE LEGENDS 
         FIG. 1 . MET is constitutively activated in the E98 cell line and xenograft model 
       A) Sections of an intracranial E98 xenograft were subjected to immunohistochemistry for P-MET (brown staining). Note that tumor shows diffuse infiltration in the brain parenchyma and that the tumor cells are highly positive for this activated MET. 
       B) Western blot analysis of MET expression in serum-starved E98 and A549 cells in absence of presence of HGF. Note that the processed form of MET (arrowhead) in E98 is somewhat smaller than that of A549 while the preform is predominantly present in E98 (arrow). α-tubulin was used as a loading control.
 
C) RT-PCR for HGF on E98, TOV-112D and HEK-293T cell line cDNA. HMBS was used as a control housekeeping gene.
 
         FIG. 2 . E98 MET contains an intronic deletion, resulting in a truncated transcript 
       A) RT-PCR on cDNA from different cell lines using full length MET primers, using HMBS as a housekeeping gene for reference. 
       B) Ion-torrent sequencing analysis exposes a high-copy amplification of MET in E98 DNA. Plotted on the x-axis are all 409 genes sequenced, in chromosomal order. The number of reads per gene was compared to blood-obtained DNA from a glioma patient as a reference. The dotted line represents the level of heterozygous losses, as can be seen for the X-chromosome (the reference blood sample was female, while the tumor DNA was male). The inset shows the relative number of reads per MET exon. Note the loss of exon 7 and part of exon 8.
 
C) FISH analysis of the number of MET (red) and chr. 7 centromere (green) copies in the genome of an E98 xenograft. D) Amplification of MET using exon 6 and 9-specific primers on genomic DNA of E98 and U87 cells. METAΔ7-8 and wtMET amplification leads to 910 and 3041 bp products in E98 and U87 respectively.
 
D) Schematic overview of the METΔ7-8 deletion found in E98 cDNA. Splicing of exon 6 to exon 9 is indicated with the dotted orange lines. The orange solid line represents the deletion.
 
         FIG. 3 . Predicted protein structure of METΔ7-8 
       A) Schematic overview of MET and METΔ7-8. The red lined box represents the deletion, comprising parts of the IPT1 and 2 domains (represented by the red and yellow boxes respectively in METΔ7-8). 
       B) Yasara modeling of the remaining IPT1 and 2 domains in METΔ7-8 was performed based on wtMET IPT1. The wtMET SEMA domain and IPTs are shown in blue, with the novel putative IPT domain consisting of IPT1 (red) and 2 (yellow) of METΔ7-8, superimposed on wtMET. An enlargement of the fusion part in the overview figure (arrow) is shown in the right panel. The arrow indicates the additional loop in red that is formed by the novel IPT domain. 
         FIG. 4 . Aberrant processing and localization of METΔ7-8 
       A) Western blot analysis of MET expression in HEK-293T and TOV-112D cells, transfected with pIRESneo-MET or pIRESneo-METΔ7-8. Note that HEK-293T cells have endogenous MET expression. As a loading control, α-tubulin was used. 
       B) Analysis of MET localization in E98 by confocal microscopy, using CD44 as a membrane marker (green) in combination with C-terminal MET antibodies (red), or phosphorylated MET (1234/1235, red) as indicated. U87 cells were also analyzed for MET/CD44 colocalization for comparison. Q-nuclear deep red stain was used to stain all nuclei.
 
C) Subcellular localization of MET (green) with early endosome and RER markers EEA-1 and CLIMP-63 (both in red).
 
D) Western blots of biotinylated E98 and A549 cells or cell lysates. MET was immunoprecipitated and analyzed on western blot for biotinylation. The α-chain of MET is shown.
 
E) His-tagged ectodomains of both wildtype MET and METΔ7-8, biotinylated via a biotin-acceptor peptide at the carboxyterminus, were expressed in HEK-293T cells. Ni-purified culture media and extracts of the transfected cells were used to analyze MET localization. Both fractions were subjected to western blotting using N-terminal MET antibodies, recognizing both the preform and the cleaved α-portion of the ectodomain, and directly labeled streptavidin to visualize the preform and the n-chain. GAPDH was used as a loading control for the RIPA extracts (red signal at 36 KDa).
 
         FIG. 5 . METΔ7-8 is blocked by TKIs but not by inhibiting antibodies E98 and A549 cells were left untreated or treated with the MET VHH G2, cabozantinib (XL; MET/VEGFR2 inhibitor), or HGF. Protein lysates were analyzed for MET and AKT phosphorylation. GAPDH was used as a loading control. Note the prominent uncleaved MET protein in E98, which is absent in A549 cells. 
         FIG. 6 . METΔ7-8 is expressed in glial tumors 
       A) PCR analysis of cell line cDNAs using primers located in exon 6 and exon 9. This PCR results in a product of 105 bp for the METΔ7-8, while wtMET gives a 345 bp product. 
       B) Immunohistochemical staining for MET of the patients&#39; tumor from which the E98 cell line has been generated. 
       C) FISH analysis of the number of MET (red) and chromosome 7 (green) copies in the original E98 patient tumor. D) PCR analysis for. METΔ7-8 on a random set of glioma and NSCLC biopsies. Note that in tumors with METΔ7-8 expression, also wtMET is observed. 
         FIG. 7 . Discriminative PCR for the detection of MET RNA lacking exon 7 and 8 (METΔ7-8). 
       PCR analysis of wild-type MET cDNA and MET A7-8 cDNA. When using primer pair METFW2033junct6_9 and MET2393R, a PCR product of 120 bp is generated from MET. Δ7-8 cDNA, no PCR product is generated from wild-type MET. Likewise, when using primer pair METFW2033junct6_9 and MET2561R, a PCR product of 268 bp is generated from MET A7-8 cDNA, no PCR product is generated from wild-type MET. 
     
    
    
     The present invention is further described by the following examples which should not be construed as limiting the scope of the invention. 
     Unless stated otherwise, the practice of the invention will employ standard conventional methods of molecular biology, virology, microbiology or biochemistry. Such techniques are described in Sambrook et al. (1989) Molecular Cloning, A Laboratory Manual (2 nd  edition), Cold Spring Harbor Laboratory, Cold Spring Harbor Laboratory Press; in Sambrook and Russell (2001)  Molecular Cloning: A Laboratory Manual , Third Edition, Cold Spring Harbor Laboratory Press, NY; in Volumes 1 and 2 or Ausubel et al. (1994)  Current Protocols in Molecular Biology, Current Protocols , USA; and in Volumes I and II of Brown (1998)  Molecular Biology LabFax , Second Edition, Academic Press (UK);  Oligonucleotide Synthesis  (N. Gait editor);  Nucleic Acid Hybridization  (Hames and Higgins, eds.). 
     EXAMPLES 
     Example 1. A novel MET mutation in high-grade glioma and non-small cell lung carcinoma resulting in an auto-active intracellular protein 
     Abstract 
     MET is a tyrosine kinase membrane receptor that has gained interest as a therapeutic target for a number of malignancies, including glioma, because of its involvement in tumorigenesis, invasion and metastasis. At present, a number of inhibitors, both antibodies against MET or its ligand hepatocyte growth factor (HGF), and small molecule MET tyrosine kinase inhibitors are in clinical trials. 
     We here describe a novel mutation in MET that we found to be present in 6% of high-grade gliomas and 14% of non small cell lung carcinomas. Characterization of this mutation in a glioma cell line revealed that it consists of an intronic deletion, resulting in a splice event connecting an intact splice donor site in exon 6 with the next splice acceptor site being that of exon 9. The encoded protein lacks parts of the extracellular IPT-domains 1 and 2, encoded by exons 7 and 8, and is named METΔ7-8. As a result of the mutation, METΔ7-8 is located predominantly in the cytosol where it is constitutively active, independent of the presence of HGF. The auto-activating nature of this protein, in combination with a lack of transmembrane localization, renders METΔ7-8 not targetable using antibodies. Genetic testing of MET-expressing tumors for the presence of this variant is therefore warranted for treatment decision making. 
     Introduction 
     The MET proto-oncogene (chromosome 7q31.2) encodes the tyrosine kinase membrane receptor MET (also called Scatter Factor Receptor), which is essential during development. Signaling from the receptor controls epithelial-to-mesenchymal transition (EMT) of myogenic precursor cells during differentiation into skeletal muscle cells (Bladt et al., 1995), a process that involves migration over long distances in the embryo. In adults, MET is involved in tissue regeneration upon injury (Borowiak et al., 2004; Chmielowiec et al., 2007; Huh et al., 2004). 
     MET is produced as a glycosylated single chain precursor protein of ˜190 kDa which, during transport to the membrane, undergoes furin-mediated cleavage in the trans-Golgi network (Komada et al., 1993). The resulting mature receptor consists of an extracellular 50 kDa α-chain, covalently attached via a disulfide bond to a membrane-spanning 140 kDa β-chain (Chan et al., 1988; Liu et al., 1996; Park et al., 1987). The extracellular segment consists of an N-terminal Sema-domain that is involved in ligand binding, a small cysteine-rich domain, and four IPT (Immunoglobulin-like fold shared by Plexins and Transcription factors) domains, which connect the Sema and cysteine-rich domains with the C-terminal β-subunit (Gherardi et al., 2003). 
     Upon binding of the ligand Hepatocyte Growth Factor (HGF, Scatter Factor), receptor dimerization occurs followed by trans-phosphorylation in the intracellular tyrosine kinase (TK) domain at tyrosine (Y) residues 1230, 1234 and 1235 (Ferracini et al., 1991; Longati et al., 1994). The TK domain subsequently induces autophosphorylation of Y1349 and Y1356 which act as docking sites for signal transduction molecules including GAB1, GRB2, phospholipase-C and SRC (Ponzetto et al., 1994). Phosphorylated GAB1 interacts with molecules like PI3-K and SHP2 which together induce several downstream signaling pathways. MET signaling is mediated by, among others, the PI3-K/AKT and RAS/MAPK pathways, which induce cell cycle progression, survival, cytoskeletal changes and invasion (Gherardi et al., 2012). In addition to its role downstream of HGF, MET can also be involved in signaling of other transmembrane receptors, including VEGFR2, CD44, and Plexin B1 (Lai et al., 2009; Lu et al., 2012). Upon ligand-induced receptor dimerization, MET is internalized via endocytosis and may be recycled (Joffre et al., 2011). Phosphorylation of Y1003 in the juxtamembrane (JM) domain of the receptor leads to ubiquitination and subsequent proteasomal degradation (Hammond et al., 2001; Jeffers et al., 1997; Peschard et al., 2001). Thus, levels of MET in the cell are tightly regulated. 
     Aberrant activation of MET signaling is a tumor promoting event in a variety of malignancies and can be induced by several mechanisms including overexpression of its ligand and/or the receptor itself, alternative mRNA splicing and crosstalk with other receptors (Corso and Giordano, 2013). MET amplifications have been found in a number of tumor types including glioblastoma (GBM) (Cancer Genome Atlas Research, 2008; Sottoriva et al., 2013) whereas missense mutations in the Sema, the TK and the JM domain have been reported to affect HGF binding, kinase activation and receptor degradation, respectively (Asaoka et al., 2010; Kong-Beltran et al., 2006; Lee and Yamada, 1994; Ma et al., 2005; Ma et al., 2008; Onozato et al., 2009; Peschard et al., 2001; Petrelli et al., 2002). 
     The significant role that MET plays in tumor progression and metastasis has made it a prime therapeutic target in oncology. MET tyrosine kinase inhibitors and therapeutic antibodies against the extracellular domain of MET and against HGF, all preventing HGF-mediated MET activation, are currently in clinical trial (www.clinicaltrials.gov). In a previous study we have shown that the combined VEGFR2/MET tyrosine kinase inhibitor cabozantinib (XL-184, CoMETRIQ) potently inhibits MET phosphorylation, cell proliferation and migration and consequently prolongs survival of mice carrying orthotopic E98 GBM xenografts. Here we identify a novel MET mutation which results in a truncated protein that is constitutively active and lacks membranous expression, thereby having important implications for therapeutic strategies targeting MET. 
     Methods 
     Cell Lines and Xenografts 
     The E98 cell line and xenograft model and genetic analysis thereof have been described before (Caretti et al., 2011; Claes et al., 2008). E98, U87, A549, HEK-293T and TOV-112D cells were cultured in DMEM+4.5 g/L glucose medium (PAA Laboratories, Pasching, Austria) supplemented with 10% fetal calf serum (FCS) (PAA) and gentamycin (40 μg/mL). All cell lines were maintained at 37° C. in the presence of 5% CO2. To examine HGF-induced MET activation, E98 and A549 cells were seeded in 6 wells plates. The next day cells were serum-starved overnight. The following day cells were treated for 10 minutes (min) with HGF (50 ng/mL, Miltenyi Biotec, Bergisch Gladbach, Germany). In some experiments, prior to HGF incubation cells were incubated with the anti-MET llama VHH G2 (Heukers et al., 2014) or cabozantinib (XL-184, Exelixis, San Francisco, Calif.) for 60 min. 
     Protein Expression Analysis 
     Cell lysates were prepared using RIPA buffer containing protease and phosphatase inhibitors (Cell Signaling Technology, CST, Danvers, Mass., USA). RIPA extracts (20-40 μg per lane) and His-tagged proteins from conditioned media (equivalent of 20 μl conditioned medium) were subjected to PAGE on 8-10% polyacrylamide gels and western blotting according to standard procedures. Nitrocellulose blots (Whatman Optitran BA-S85, GE Healthcare, Little Chalfont, UK) were blocked with PBS/Odyssey Blocking Buffer (LI-COR Biosciences, Lincoln, Nebr., USA) (1:1), followed by overnight incubation with primary antibodies at 4° C. Antibodies used were against MET N-terminus (clone EP1454Y, Epitomics, Abcam, Cambridge, UK), MET C-terminus (clone D1C2), phosphorylated (P)-MET (Y1234/1235, clone D26) (CST), GAPDH (clone 6C5, Abcam), and α-tubulin (clone 236-10501, Molecular Probes, Life Technologies). Biotin groups and primary antibodies were visualized using respectively streptavidin-680 (Molecular Probes, Life Technology) and appropriate secondary antibodies [goat-anti-rabbit-IRDye800 (Rockland Immunochemicals, Gilbertsville, Pa., USA) or Alexa Fluor 680 goat-anti-mouse IgG (Molecular Probes, Life Technologies)]. Blots were scanned on the Odyssey imager (LI-COR Biosciences). 
     Immunohistochemistry 
     Immunohistochemistry on formalin fixed paraffin embedded (FFPE) slides was performed as previously described (Navis et al., 2011), using antibodies against MET and P-MET (clone D1C2 and D26 respectively, both CST). Bound antibodies were visualized via sequential incubations with biotinylated secondary antibodies, avidin-biotin complexes (Vector laboratories, Burlingame, Calif., USA) and 3,3′-diaminobenzidine solution (Power-DAB, ImmunoLogic, Duiven, The Netherlands). 
     RNA Analysis 
     RNA was isolated from cell lines using TRIzol Reagent (Life Technologies) according to the manufacturer&#39;s instruction. Glioma tissues were in part obtained from the Radboud UMC and from the University of Cologne (n=80). Sarcoma (n=25) and castration resistant prostate cancer (CRPC) tissues (n=43) were obtained from the Radboud UMC. Non small cell lung cancer (NSCLC) samples were obtained from the VU University Medical Centre Amsterdam. For RNA isolation from tissues, the mirVANA RNA isolation kit was used (Life Technologies), according to standard procedures. For cDNA synthesis, 1 μg RNA was transcribed using oligo-dT primers (Invitrogen, Carlsbad, Calif., USA) or via the Quantitect Reverse Transcription Kit (QIAGEN, Venlo, The Netherlands), containing both oligo-dT and random primers. HGF was PCR-amplified using Phusion High-Fidelity DNA polymerase (Finnzymes, ThermoFisher Scientific, Waltham, USA) according to the manufacturers&#39; instructions. Primers were HGF-Fw204 (5′-CTGCAGCATGTCCTCCTGCA-3′; SEQ ID NO: 1) and HGF-Rv504 (5′-GAGGTCAAATTCATGGCCAA-3′; SEQ ID NO: 2) (30 cycles, annealing at 55° C., 20 sec; extension at 72° C., 20 sec). Control PCR reactions were performed for housekeeping gene HMBS (Hydroxymethylbilane Synthase) (Chretien et al., 1988), using primers p361-Fw (5′-TGCCAGAGAAGAGTGTGGTG-3′; SEQ ID NO: 3) and p425-Rv (5′-GTTAAGCTGCCGTGCAACATC-3′; SEQ ID NO: 4) primers. To distinguish wtMET from METΔ7-8-transcripts, a PCR was performed in AmpliTaq Gold 360 mastermix (Applied Biosystems, Life Technologies) using primer MET1997Fw (5′-CTCCTTGGAAATGAGAGCTG-3′; SEQ ID NO: 5, forward, located in exon 6), and primer MET2342Rv (5′-CAGTTGAAATGGTTTGGGCTG-3′; SEQ ID NO: 6, reverse, located in exon 9). This PCR results in a 105 bp product for METΔ7-8 and a 345 bp product for wtMET. Conditions were: denaturation 95° C., followed by 35 cycli of 95° C. denaturation; annealing at 58° C., 30 sec; elongation at 72° C., 1 min. A final elongation step of 7 min was done at 72° C. 
     Genetic Analysis 
     DNA was isolated from E98 cells using the DNeasy Blood &amp; Tissue Kit (QIAGEN), according to the manufacturer&#39;s protocol. 10 ng was subjected to PCR, using primer MET1997Fw (5′-CTCCTTGGAAATGAGAGCTG-3; SEQ ID NO: 7, forward, exon 6, genome location chr7q; g116395517-36) and a reverse primer MET2414Rev (5′-GGGATCTTCACGGTAACTG-3′; SEQ ID NO: 8, located in exon 9, CHR7q; g. 116398565-45). 
     E98 was genetically analyzed by semi-conductor sequencing (IonPGM, Life Technologies) using the comprehensive cancer panel (Life Technologies) that targets 409 cancer-related genes. The IonPGM E98 library generation was performed according to the manufacturer&#39;s protocol. In short, 10 ng of DNA per pool, was amplified in 21 cycles by PCR using the Ion AmpliSeq™ mastermix, followed by barcode and adapter ligation. Amplified products were purified with Agencourt AMPure XP beads (Beckman Coulter Genomics, High Wycombe, UK). The library was diluted to 20 pM. Emulsion PCR was performed using the Ion OneTouch™ 200 Template kit following the protocol of the Ion OneTouch™ System. Next, Ion Sphere Particles (ISPs) were recovered and enriched for template positive ISPs using Dynabeads MyOne Streptavidin Cl beads (Life Technologies) in the Ion OneTouch™ ES instrument (Life Technologies). ISP enrichment was quantified using the Qubit 2.0 fluorometer (Life Technologies). Sequencing primer and polymerase were added to the final enriched spheres before loading onto an Ion 318 chip according to the Ion PGM™ 200 sequencing kit protocol. The gene copy number analysis was performed as follows. The relative number of sequence reads aligned to a specific gene (of the total number of aligned reads of E98) was determined for each gene and divided over the relative number of sequence reads aligned to a specific gene (of the total number of aligned reads of an unrelated blood sample). The relative ratios are plotted in a graph based on the genomic position of the gene. 
     Fish 
     FFPE sections (4 μm) on SuperFrost glass. (dried&gt;45 min at 56° C.) were deparaffinized and rehydrated in ddH2O. After boiling in a microwave in sodiurncitrate buffer (pH 6), slides were allowed to cool to RT, washed in ddH2O and incubated for 5 min in 10 mM HCl. Proteins were digested with pepsin (200 U/mL, Sigma) for 15 min at 37° C. Subsequently, slides were rinsed in 10 mM HCl and PBS and postfixed for 5 min in 1% paraformaldehyde (PFA, Merck)/PBS. Sections were washed in ddH20, dried and hybridized with 10 μl probe mix (1 μl cep7 Spectrum Green (06J37007)+1 μl LSI MET Spectrum Red (06N05-001)+7 μl hybridization buffer in MQ, all Vysis) under a cover slip. Sections were denatured at 80° C. for 10 min, followed by hybridization o/n at 37° C. in a Hybridizer (Dako). After removing the coverslip by soaking for 5 min in 2×SSC buffer (Maxim Biotech) at 42° C. slides were washed 3 times in 2×SSC buffer at 73° C., once in 2×SSC (5 min) and once in ddH2O. After dehydration in EtOH, slides were air-dried in the dark and mounted in Vectashield/DAPI (3 parts Vectashield/DAPI+1 part Vectashield, all Vector). Slides were analyzed on a Leica Fluorescence microscope. 
     Protein Domain Modeling 
     A homology model for the new hybrid IPT-domain using the WHAT IF &amp; YASARA Twinset was generated (Krieger et al., 2002; Vriend, 1990). We used the experimentally solved 3D-structure 2uzx, which contains the human tyrosine kinase MET. The first 60 residues of the hybrid domain are identical to this structure, whereas the following 38 residues were modeled based on homology between the 2 domains. 
     Cloning 
     The METΔ7-8 open reading frame was PCR amplified (Phusion DNA polymerase, Finnzymes) from E98 cDNA using primers flanking the start and stop codons (MET173EcoR1-Fw: 5′-CGAATTCGATAAACCTCTCATAATGAAGG-3′; SEQ ID NO: 9 and MET4421NotI-Rv 5′-AGCGGCCGCCTATGATGTCTCCCAGAAGG-3′; SEQ ID NO: 10). The 3,961 bp PCR product was purified, digested with NotI and cloned as blunt-NotI fragment in pIRESneo-EcoRV-NotI (Clontech Laboratories, Inc, CA, USA). wtMET cDNA was PCR-cloned from the MET overexpressing ovary carcinoma cell line TOV-112D-MET using MET173Fw (5′-GATAAACCTCTCATAATGAAGGC-3′; SEQ ID NO: 11) and MET4421NotI-Rv. The extracellular parts of METΔ7-8 and wtMET were PCR-amplified from cDNA using primers MET173EcoR1-Fw and MET3028NheI-Rv (5′-CGCTAGCCTGATCTGGTTGAACTATTAC-3′; SEQ ID NO: 12), and cloned in vector pHLsec-BAPHIS, a derivative from pHLsec-HIS (Addgene, Cambridge, Mass., USA). The resulting vector adds a biotin-acceptor peptide and His-tag to the carboxyterminus of the MET extracellular domains, thus resulting in pHLsec-METΔ7-8ED-BAPHIS and pHLsec-METED-BAPHIS. 
     Transfection 
     pIRESneo-MET or pIRESneo-METΔ7-8 were transfected into HEK-293T or TOV-112D cells in 6-well culture dishes (Greiner Bio-One, Krëmsmunster, Austria) using Fugene HD transfection reagent (Promega, Fitchburg, Wis., USA) according to the manufacturer&#39;s instructions. After 48 hrs, cell monolayers were washed with PBS and cell extracts prepared in RIPA buffer. In separate experiments, pHLsec-METΔ7-8ED-BAPHIS or pHLsec-METED-BAPHIS were cotransfected in a 1:1 ratio with the, biotin-ligase expression construct pDISPLAY-BirA-ER (Addgene) and cells were cultured in the presence of 10 μM biotin (Sigma-Aldrich, St Louis, Mo., USA). After 48 hours cytosolic extracts were made in RIPA buffer, and His-containing proteins were purified from the conditioned media using Nickel beads. 100 ul Ni-NTA sepharose slurry (IBA, Goettingen, Germany) was incubated with 1 ml conditioned medium for 1 hour at 4° C. After 3 washes with buffer (500 mM NaCl, 50 mM phosphate buffer, pH 7.4) beads were loaded onto a Poly-Prep column (Bio-Rad, Hercules, Calif., USA) and aspecifically bound proteins were eluted with 2 ml 10 mM imidazole. Finally, protein constructs were eluted with 0.5 ml 0.5 M imidazole, and dialysed o/n at 4° C. to 50 mM TRIS/150 mM NaCl pH 7.5. 
     Confocal Microscopy 
     E98 and U87 cells, grown on Nunc Lab-Tek chamber slides (Sigma-Aldrich) to 40% confluence, were fixed with 2% PFA in 0.1 M phosphate buffer (pH 7.4) for 15 min at RT, followed by three washes with PBS, 30 min glycine incubation (100 mM in PBS) to quench PFA-induced autofluorescence, three PBS washes and permeabilization with digitonin for 10 min (100 μM in PBS). Aspecific binding was blocked by incubation with 20% normal goat serum in PBS/1% BSA for 20 min, and cells were incubated overnight at 4° C. in a humidified chamber with PBS/1% BSA containing (combinations of): mouse monoclonal anti-CD44 antibody (as a surface marker, clone Hermes-1, ThermoFisher Scientific); rabbit anti-MET (clone D1C2, CST); rabbit anti-P-MET (clone D26, CST); mouse anti-EEA-1 (clone 14/EEA1, BD Biosciences, early endosome marker); anti-CLIMP-63 (clone G1/296, rough ER marker, a kind gift of J. Fransen). Primary antibodies were detected using Alexa Fluor 488- or 594-labeled secondary goat-anti-mouse and Alexa Fluor 488- or 568-labeled goat-anti-rabbit IgGs (all Life Technologies), 1:200 diluted in PBS/1% BSA. Q-nuclear deep red (1:200, Life Technologies) was used to stain nuclei, and cells were mounted in Fluoromount G with DAPI (Southern Biotech, Birmingham, Ala., USA). Cells were analyzed using a confocal laser scanning microscope (Leica SP2 CLSM) and Leica confocal software. 
     Biotinylation Assay 
     E98 and A549 cells were allowed to adhere for 24 hrs until 80% confluence in a 6-wells plate (Cellstar, Greiner Bio-One, Kremsmünster, Austria), washed 3× with ice cold PBS and incubated for 30 min at 4° C. with 0.5 mg/ml EZ-Link Sulfo-NHS-LC-Biotin (ThermoFisher Scientific) to biotinylate exposed membrane proteins. In parallel, RIPA extracts prepared from equivalent numbers of E98 cells and freed of Tris by repeated washing over Amicon Ultra K10 centrifugal filters (Merck Millipore, Billerica, Mass., USA) were treated similarly to biotinylate all cellular proteins. The reaction was quenched by washing cells or cytosolic proteins three times with 100 mM glycine/PBS and two times with PBS (using K10 columns in the case of protein lysates). Treated adherent cells were then subjected to lysis with RIPA buffer. Protein concentrations in all lysates were determined using the BCA protein concentration assay (ThermoFisher Scientific) according to standard procedures. MET was immunoprecipitated from 200 μg total protein in 200 μl using anti-MET (1:50, clone D1C2, CST) for 1 hour at 4° C. Immune complexes were captured by incubation for 30 min at 4° C. with 10 μl prot A agarose slurry (Roche Diagnostics, Basel, Switzerland), followed by centrifugation (14.000 rpm, 4 min) and three PBS washes. Immune complexes were solubilized by heating (5 min 95° C.) in 30 μl 2×SDS-PAGE sample buffer (0.2% SDS, 62.5 mM Tris-HCl pH 6.8, 10% glycerol and 0.2 μM DTT). Samples were subjected to 10% SDS-PAGE and western blotting as described above. 
     Results 
     E98 MET Protein is Auto-Activated in an HGF-Independent Fashion. 
     Our previous experiments have shown that MET in E98 xenografts is phosphorylated in tumor cells in a homogeneous fashion ( FIG. 1A , see also (Navis et al., 2013)). In vitro, E98 cells also show high levels of phosphorylated MET when grown under serum-free conditions ( FIG. 1B ). HGF treatment did not further increase phosphorylation levels of MET, in contrast to A549 control cells in which HGF was required for MET activation. MET phosphorylation in E98 cells was not the result of endogenous HGF expression as revealed by RT-PCR analysis ( FIG. 1C ), while analysis of E98 xenograft RNA revealed the presence of mouse HGF only, as determined by Sanger sequencing (not shown). Since mouse HGF is not an activating ligand for human MET (Ohgaki and Kleihues, 2005; Zhang et al., 2005) we conclude that constitutive activation of MET in E98 cells and xenografts is not the result of an autocrine HGF-activation loop. 
     E98 Cells Express a Truncated Version of an Amplified MET Gene. 
     To examine the underlying mechanism of MET auto-activation in this model, we PCR-cloned MET cDNA from E98 cells, using primers flanking the open reading frame (ORF, NM_000245.2). The MET PCR product from E98 was 240 bp smaller than that from U87 and CaCo2 cDNA ( FIG. 2A ), and Sanger sequencing analysis revealed an in-frame deletion of nt 2050-2289 in the coding sequence, corresponding to exons 7 and 8. The same transcript was also found in E98 xenografts (not shown). Genomic analysis of E98 cells using semi-conductor sequencing revealed high copy MET amplification ( FIG. 2B ). Average amplification of MET was about 14-fold. FISH analysis using probes specific for MET and chromosome 7 centromere confirmed the amplification ( FIG. 2C ). Consistent with Sanger sequencing, amplicons in exon 7 and the first part of exon 8 were absent (see insert in  FIG. 2B ), suggesting that the lack of exons 7 and 8 results from a genomic rearrangement. (similar to the EGFR variant III (Gan et al., 2013)) instead of alternative splicing. PCR on genomic DNA using exon 6 and 9 specific primers resulted in amplification of a 910 bp fragment (predicted size from the wild-type allele, present in U87, is 3,014 bp, see  FIG. 2D ). Sequencing of this product revealed an intronic deletion of 2,114 bp (between position g. 116 395 653 located in intron 6, and 116 397 766, located in exon 8). This deletion results in an intact splice donor site from exon 6, juxtaposed to the exon 9 splice acceptor site, and explains the lack of exons 7 and 8 in the resulting mRNA ( FIG. 2E ). Of note, a wtMET allele could not be detected in E98 cells ( FIG. 2D ). 
     On the protein level, this rearrangement leads to loss of the C-terminus of IPT1 and the N-terminus of IPT2 ( FIG. 3A ). Yasara modeling, using X-ray crystallographic data of the MET ectodomain, predicts the formation of a novel IPT, composed of the remaining parts of IPT1 and 2. Interestingly, in the new IPT1/2 fusion a small stretch of 5 extra amino acids loops out towards the Sema domain ( FIG. 3B ). 
     METΔ7-8 is Aberrantly Processed 
     MET is synthesized as a 190 kDa precursor protein which is proteolytically cleaved by furin between residues 307 and 308, to yield an extracellular α-chain of approximately kDa, covalently linked via a disulfide bridge to the transmembrane β-chain. Reducing SDS-PAGE, followed by western blot analysis of E98 cell extracts showed that the majority of MET protein was in a 180 kDa phosphorylated form, corresponding to the uncleaved truncated preform ( FIG. 1B , arrow). In contrast, in A549 cells the matured, cleaved MET protein was the predominant form (arrowhead in  FIG. 1B ). To investigate whether this was a specific feature of E98 cells, we analyzed the protein structure in cells after transfection with the full-length cDNAs encoding wtMET or METΔ7-8. In both HEK-293T and TOV-112D cells, wtMET was properly processed to an α- and β-chain, indicating that these cells are not defective in furin-mediated processing. In contrast, METΔ7-8 was predominantly present in the uncleaved preform in both cell types ( FIG. 4A , arrow). Thus, improper MET cleavage is an intrinsic property of METΔ7-8. Overexpression studies in HEK-293T and TOV-112D cells resulted in phosphorylated Y1234/1235 residues in both wtMET and METΔ7-8 proteins ( FIG. 4A , P-MET). Because both cell lines produce HGF ( FIG. 1C ) this may be a result of HGF-dependent autocrine activation. 
     MET is cleaved by the endoprotease Furin, which is localized predominantly in the Trans Golgi Network, but also in vesicles and near the plasma membrane (Komada et al., 1993; Molloy et al., 1994; Schafer et al., 1995). To test whether the inefficient cleavage of METΔ7-8 in E98 cells is related to intracellular transport defects, we analyzed the subcellular localization of METΔ7-8 in detail via confocal microscopy. METΔ7-8 did not co-localize with the cell surface marker CD44 and was confined to the cytosol ( FIG. 4B ). In contrast, on U87 cells wtMET did co-localize with CD44. Additional intracellular staining in U87 cells reflects de novo synthesized material that is being processed for constitutive secretion. Immunostainings with the early endosome marker EEA-1 and the rough endoplasmic reticulum (RER) marker CLIMP-63 suggested that METΔ7-8 in E98 cells is predominantly retained in the RER ( FIG. 4C ). 
     To confirm the absence of cell surface expression of METΔ7-8 on E98 cells, we labeled intact E98 cells or cell lysates with NHS-biotin and immunoprecipitated MET using specific antibodies, followed by SDS-PAGE/western blot and staining for biotin and MET. Whereas METs N-terminal α-chain was readily biotinylated in E98 cell lysates, only very little biotinylated MET was detected when intact cells were labeled ( FIG. 4D , lane 1). In contrast, biotinylated MET was readily detected in A549 cells upon labeling of intact cells, as shown by the biotin-labeled α-chain ( FIG. 4D , lane 3). Thus, these data confirm that METΔ7-8 is predominantly localized intracellularly and is poorly exposed on the cell surface of E98 cells. 
     To further confirm a defect in intracellular trafficking of METΔ7-8 we analyzed secretion patterns of extracellular domains of wtMET or METΔ7-8 (ending with residue D929, numbering according to MET variant 2 (NP_000236.2), containing a C-terminal biotin tag. Whereas the extracellular domain of wtMET was properly processed and secreted into the culture medium (as illustrated by the presence of the 309-929 biotinyated extracellular β-chain and the MET25-308 α-chain,  FIG. 4E , lane 4), no secreted MET products were found in medium of cells, transfected with the METΔ7-8 ectodomain ( FIG. 4E , lane 3). Instead, all biotinylated MET products were located intracellularly ( FIG. 4E , lane 1). 
     185 kDa METΔ7-8 is not Affected by Antagonistic Anti-MET Antibodies but is Inhibited by Cabozantinib 
     It was previously reported that VHH G2, a recombinant single domain llama antibody against MET, results in low MET activation levels, while inhibiting the strong activation which is induced by HGF (Heukers et al., 2014). To test whether and how G2 affects phosphorylation of METΔ7-8, we treated serum-starved E98 cells with G2, either or not followed by HGF, using A549 cells as control. As shown in  FIG. 5 , HGF did not increase overall MET phosphorylation levels, although, interestingly, a slight activation was seen in the minority of processed β-fragment of MET. For G2 a similar effect was observed. Consistent with high overall MET phosphorylation levels in all samples, neither G2 nor HGF treatment resulted in altered levels of pAKT, a direct target of MET. In A549 cells G2 and HGF induced MET phosphorylation, which increased levels of pAKT. Thus, in contrast to A549 cells that express wtMET, E98 cells are not responsive to antibodies against or ligands of MET. However, both E98 and A549 cells responded well to the MET tyrosine kinase inhibitor cabozantinib ( FIG. 5 ). 
     Prevalence of METΔ7-8 
     PCR with deletion-spanning primers revealed the sole presence of the deletion-specific 105 bp fragment in E98 cDNA, while a number of other cell lines presented with the wt 345 bp amplicon only ( FIG. 6A ). Since frozen material from the patient tumor that was used to generate E98 is unavailable, we could unfortunately not obtain genomic DNA and cDNA of sufficient quality to confirm the presence of the mutation in the originating tumor. We did however perform immunohistochemistry on formalin-fixed, paraffin-embedded tumor material from this patient and observed a highly heterogeneous staining for MET, with only a small percentage of strongly positive tumor cells, apparently with intracellular staining ( FIG. 6B ). Of note, such immunostainings cannot discriminate between wtMET and METΔ7-8. FISH analysis confirmed the presence of the MET amplification in the original tumor ( FIG. 6C ). 
     To investigate the prevalence of the METΔ7-8 mutation further, we performed the exon 6-9 PCR on cDNA, generated from a series of gliomas (n=102) and a number of other tumor types in which MET has been suggested to play an important role (castration resistant prostate carcinomas, n=43; Ewing sarcoma, n=21; rhabdomyosarcoma, n=22; NSCLC, n=21) (Humphrey et al., 1995; Knudsen et al., 2002). METΔ7-8 was found in 6 out of 102 gliomas, both grade III and IV (5.8%; 2 out of 5 anaplastic oligodendrogliomas, 2 out of 16 anaplastic astrocytomas and 2 out of 61 glioblastomas), and was not detected in the other tumor types tested, except for NSCLC (example in  FIG. 6D  and Table 2). Most tumors that contained the mutation were heterozygous, containing also the wild-type transcript, whereas in one sample of NSCLC only the mutated MET transcript could be detected ( FIG. 6D ). Sanger sequencing of the smaller PCR products of one patient with glioma and NSCLC confirmed the deletion of exons 7 and 8 (not shown). Of note, due to the lack of high quality genomic DNA from these clinical samples we could not discriminate in these samples whether METΔ7-8 resulted from a mutation or from exon skipping or alternative splicing, such as suggested for MET variants lacking exon 10 or 14 (Ma et al., 2003). 
     Discussion 
     A major problem in glioma treatment is diffuse growth in the neuropil, and it has been suggested that the MET oncogene is causally involved in this phenotype in subsets of tumors. The MET signal transduction pathway is also increasingly recognized as a rescue pathway, generating resistance to EGFR-targeted therapies (Corso and Giordano, 2013). We previously reported that MET is crucially implicated in proliferation, survival and migration of E98 glioma cells in vitro and showed that treatment with the MET tyrosine kinase inhibitor cabozantinib prolongs survival of mice carrying orthotopic E98 xenografts (Navis et al., 2013). Based on the observation that the pattern of MET phosphorylation in E98 xenografts was remarkably homogeneous in the absence of its ligand HGF, we analyzed the MET product in E98 cells in more detail and found that the protein is expressed as a truncated product, which lacks effective furin cleavage and is predominantly retained intracellularly in its active, phosphorylated form. The truncation generates a novel IPT domain consisting of a fusion between the carboxyterminus of IPT1 and the amino terminus of IPT2. Modeling of the novel fusion IPT domain suggests that it adopts a similar structure as the IPT domains in wild type MET, although a small stretch of extra amino acid residues is accommodated in a loop, extending towards the Sema domain. Since this loop approaches the furin-cleavage site in the Sema domain, it may directly impact on furin cleavage, but this requires further investigation. 
     The METΔ7-8 mutant resembles the auto-active EGFRvIII variant. This variant results from a genetic deletion of exons 2-7 and its activation is also independent of the ligands EGF or TNFα. Like the EGFRvIII mutation (Gan et al., 2013), the METΔ7-8 alteration appears to be restricted to only few cancer types, as we detected it in glioma and NSCLC. The mutation was detected in grade III gliomas of both oligodendroglial and astrocytic origin and GBMs. 
     Interestingly, we did not detect a wtMET transcript in E98 cells, in contrast to the clinical gliomas which were analyzed in this study and which all showed abundant wtMET, also in METΔ7-8 tumors. Only one NSCLC sample also showed exclusive METΔ7-8 expression. We formally cannot exclude that the wild type MET amplicons in our PCR derive from ‘contaminating’ non-neoplastic stromal cells in the tumor biopsies. There may however be another explanation for the relatively low levels of METΔ7-8 in clinical tumors: in the patient tumor from which the E98 model was generated, a low percentage of MET-expressing tumor cells was detected, although it is impossible to determine whether these cells carry the METΔ7-8 mutation because the antibodies used do not discriminate between MET and METΔ7-8. It is tempting to speculate that during the generation of the E98 model, a small subset of MET-mutated tumor cells in the primary tumor experienced a growth advantage, ultimately resulting in clonal outgrowth and xenografts. This scenario fits with the notion that clinical tumors not only show inter—but also intratumoral heterogeneity (Sottoriva et al., 2013), and derived preclinical tumor models may only be representative for the most malignant population of tumor cells. Our finding may have important consequences for therapy. MET is increasingly recognized as an important target in multiple tumor types, and therapeutic antibodies against HGF or the HGF binding site on MET have been developed. Since HGF is not involved in METΔ7-8 activation and METΔ7-8 is retained intracellularly, MET-mutated cells will not be responsive to these therapies. Indeed we were able to show that E98 cells do not respond to the anti-MET VHH G2. Selection of MET-mutated cells in tumors that initially respond to antibody-based MET-directed treatment is expected to result in recurrence of treatment-resistant clones. In this respect, it will be important in future studies to assess the occurrence of the A7-8 mutation in paired samples of primary and recurrent tumors after MET-antibody-based therapies. With this in mind, the use of specific tyrosine kinase inhibitors of MET may have preference over antibody-based therapy, since METΔ7-8 is sensitive to these inhibitors (Navis et al., 2013). Such inhibitors have already shown to improve overall survival of patients with non-small cell lung carcinoma with MET amplification and renal papillary carcinoma with MET mutations (Choueiri et al., 2013; Ou et al., 2011). A recent phase II study with the antibody MetMab for lung cancer failed to meet the primary end point of prolonged survival (Spigel et al., 2013). A study of MET mutations and intracellular localization patterns in this patient group may be highly informative for future therapeutic directions (Hirsch et al., 2014). 
     The Δ7-8 alteration leads to a reduction in furin-mediated cleavage of the MET precursor. Interestingly, the colorectal adenocarcinoma cell line Lovo lacks furin protease and consequently cannot properly process MET (Mark et al., 1992). In this cell line, the unprocessed preform is still expressed on the cell surface, binds HGF and is capable of signaling. Thus, defective processing alone does not explain intracellular retention of METΔ7-8 and we therefore hypothesize that the novel IPT1-2 fusion domain in. METΔ7-8 is involved in defective furin cleavage, intracellular retention of the uncleaved perform and auto-activation of the uncleaved preform. 
     Recently, effects of various activating mutations in the MET TK domain have been analyzed in detail. Interestingly, these mutants are expressed on the cell surface but are subject to increased rates of turnover, resulting in accumulation in early endosomes where they can still signal (Joffre et al., 2011). METΔ7-8 did not accumulate in early endosomes, suggesting that the underlying mechanism of intracellular retention is different (Mu et al., 1995). Our confocal microscopy and biotinylation experiments strongly suggest that only the very small fraction of METΔ7-8 that is cleaved reaches the cell membrane, but this fraction is insignificant with respect to the auto-active component in the cytosol. 
     Deletions in the extracellular domains of tyrosine kinase receptors are not uncommon in glioma, a prime example being EGFRvIII. The vIII mutation, concomitant with EGFR amplification, occurs in 25-64% of all GBMs (Aldape et al., 2004; Biemat et al., 2004; Feldkamp et al., 1999; Saikali et al., 2007; Viana-Pereira et al., 2008). Furthermore, 40% of the GBMs with amplified PDGFRA contain an in-frame deletion in the ectodomain, leading to constitutive activation of the TK domain (Kumabe et al., 1992; Ozawa et al., 2010). Recently, also in pediatric high grade gliomas ligand-independent and tumorigenic in-frame PDGFRA deletion variants have been found (Kumabe et al., 1992; Paugh et al., 2013; Rand et al., 2005; Verhaak et al., 2010). Data on the subcellular localization of mutated oncoreceptors is frequently lacking, and our data call for in-depth analysis of the cellular localization of receptors that are considered targetable. Interestingly, activating mutations in the RON tyrosine kinase receptor have been identified which resemble the METΔ7-8 mutation in that it also involves the first IPT domain (Ma et al., 2010). Of note, these alterations do not lead to a loss of expression at the cell surface. 
     In conclusion, we describe a highly active, non-ligand dependent mutant of MET in a subset of patients with glioma and NSCLC, which is not exposed on the cell surface and is predicted to be non-targetable with therapeutic antibodies against MET and/or HGF. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 METΔ7-8 expression in glioma; sarcoma and CRPCs 
               
               
                 The incidence of METΔ7-8 in different subtypes and malignancy grades 
               
               
                 of diffuse glioma, sarcomas, and castration resistant prostate carcinomas 
               
               
                 (CRPC). Abbreviations: O-II, oligodendroglioma WHO grade II; O-III, 
               
               
                 oligodendroglioma (WHO grade III); A-II, astrocytoma WHO grade II; 
               
               
                 A-III, anaplastic astrocytoma, WHO grade III; GBM, glioblastoma. 
               
            
           
           
               
               
               
            
               
                   
                 Tumor type 
                 Δ7/8 occurance 
               
               
                   
               
               
                   
                 Glioma 
                 6/102 (5.8%) 
               
               
                   
                 O-II 
                 0/3 
               
               
                   
                 A-II 
                 0/7 
               
               
                   
                 OA-II 
                 0/5 
               
               
                   
                 O-III 
                 2/5 (40%) 
               
               
                   
                 A-III 
                 2/16 (12.5%) 
               
               
                   
                 OA-III 
                 0/5 
               
               
                   
                 GBM 
                 2/61 (1.64%) 
               
               
                   
                 Sarcoma 
                 0/25 
               
               
                   
                 Ewing tumor 
                 0/5 
               
               
                   
                 Cell line 
                 0/3 
               
               
                   
                 Rhabdo tumor 
                 0/15 
               
               
                   
                 Cell line 
                 0/2 
               
               
                   
                 CRPC 
                 0/43 
               
               
                   
                 NSCLC 
                 3/21 
               
               
                   
               
            
           
         
       
     
     Example 2. A Robust Discriminative PCR for the Detection of the Novel MET Mutation (METΔ7-8) Resulting in an Auto-Active Intracellular Protein. 
     To develop a robust, discriminative PCR, primer METFW2033junct6_9 was designed in which the last six nucleotides are specific to exon 9 and consequently do not hybridize to exon 7, whereas the remaining nucleotides hybridize to exon 6. This means that this primer specifically recognizes METdelta7/8 cDNA, but not wild-type MET cDNA. The combination of METFW2033junct6_9 and METrev2393 results in a product of 120 bp when the mutation is present that results in a MET lacking exon 7 and 8; the combination of METFW2033junct6_9 and METrev2561 results in a product of 268 bp when the mutation is present that results in a MET lacking exon 7 and 8. Wild-type MET results in no PCR product. 
       FIG. 7  demonstrates that this primer, in combination with either MET2393rev or MET2561rev (both located in exon 9) indeed recognizes METdelta7/8 and not wild-type MET; this is demonstrated at three different annealing temperatures. Lanes 5 comprises a mixture of templates; METd7/8: wild-type MET cDNA (1:99) and shows that under the conditions used, only the METd7/8 is amplified in absence of wild-type MET. PCR was carried out using standard conditions on 1 ng of template DNA/reaction. The cycle protocol was: an initial denature step of 30″ at 94° C., followed by 25 cycles of 15″ at 94° C.; 30″ at 58° C., 60° C. or 62° C.; 30″ at 72° C., followed by a final elongation step of 2′ at 72° C. 
     The gel lanes in  FIG. 7  correspond to the following PCR reactions at three different annealing temperatures: 
     
       
         
           
               
               
               
               
             
               
                   
               
               
                   
                   
                   
                 reverse  
               
               
                 lane 
                 template 
                 forward primer 
                 primer 
               
               
                   
               
             
            
               
                 1: 
                  1 ng METdelta7/8 
                 x MET2033FWjunct6_9 
                 x MET2393R 
               
               
                 2: 
                  1 ng METwt 
                 x MET2033FWjunct6_9 
                 x MET2393R 
               
               
                 3: 
                  1 ng METdelta7/8 
                 x MET2033FWjunct6_9 
                 x MET2561R 
               
               
                 4: 
                  1 ng METwt 
                 x MET2033FWjunct6_9 
                 x MET2561R 
               
               
                 5: 
                 10 pg METd78 + 
                 x MET2033FWjunct6_9 
                 x MET2393R 
               
               
                   
                  1 ng METwt 
                   
                   
               
               
                 6: 
                 No template 
                 x MET2033FWjunct6_9 
                 x MET2393R 
               
               
                 7: 
                 No template 
                 x MET2033FWjunct6_9 
                 x MET2561R 
               
               
                   
               
            
           
         
       
     
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