Gastrointestinal stromal tumor (GIST)

KIT is a sensitive diagnostic marker for GIST (Sarlomo-Rikala et al. 1998) and in the majority of GISTs activating mutations in KIT can be detected. In various national and international studies KIT mutation rates of 69-89% percent in GIST have been reported (Antonescu et al. 2003; Heinrich et al. 2003a; Wozniak et al. 2012). A subgroup of about 5%-13% GIST cases has mutations in the closely related PDGF-receptor alpha (PDGFRA), mutually exclusive with mutations in KIT (Heinrich et al. 2003b; Corless et al. 2005; Wozniak et al. 2012). The remaining fraction of cases often is classified as

“wild-type GIST” based on the absence of KIT and PDGFRA mutations and containing the majority of pediatric GIST.

Within the WT GIST class, KIT is usually still expressed (Antonescu et al. 2003;

Agaram et al. 2008a) and a few candidate driver mutations have been detected. The au-tosomal dominant disorder neurofibromatosis type 1 (NF-1; von Recklinghausen's dis-ease) had occasionally been associated with large, solid stromal tumors of the gastrointestinal tract (Schaldenbrand and Appelman 1984; Fuller and Williams 1991).

In a subset of cases with germline NF1 mutations, GIST development without KIT and PDGFRA mutations has been reported (Kinoshita et al. 2004; Miettinen et al. 2006). As NF1 is a negative regulator of the RAS/MAPK pathway, mutations in this pathway might contribute to GIST tumorigenesis. Indeed, BRAF^V660E mutations have been identi-fied in some cases of WT GIST (Agaram et al. 2008b; Hostein et al. 2010; Daniels et al.

2011). Also, amplification of insulin-like growth factor 1 receptor (IGF1R) has been implicated in WT/pediatric GIST (Prakash et al. 2005; Agaram et al. 2008a). Recently,

germline and spontaneous inactivating mutations in the succinate dehydrogenase subu-nit genes SDHB and SDHC have been found in patients with WT/KIT-expression posi-tive GIST (Janeway et al. 2011; Miettinen et al. 2011). Notably, this diverse spectrum of mutations apparently gives rise to the same clinical tumor entity, GIST, implying common oncogenic pathways beneath the dominating roles the expression and activat-ing mutations of KIT play in GIST.

GIST are refractory to radiation therapy as well as standard chemotherapy and up to today only surgery can be curative in about half the cases presenting with localized dis-ease (Silberhumer et al. 2009; Joensuu and DeMatteo 2012). Metastatic and/or surgical-ly unresectable GIST could not be treated effectivesurgical-ly until 2000. The first targeted ther-apy using a small molecule kinase inhibitor, imatinib (formerly CGP 57148B and STI571), had produced promising results in chronic myelogenous leukemia (CML).

(Druker et al. 2001). This stimulated the initiation of clinical trials in ad-vanced/metastatic GIST, as imatinib was know to inhibit the KIT and PDGF receptor besides the fusion protein BCR-ABL underlying and defining CML (Carroll et al. 1997;

Tefferi and Vardiman 2008). Imatinib proved to be efficacious also in advanced solid stromal tumors of the gastrointestinal tract and as adjuvant treatment after surgery pro-longing recurrence-free survival (Joensuu et al. 2001; Demetri et al. 2002; Dematteo et al. 2009). As imatinib is not curative, continuous treatment is necessary and patients over time relapse, most often with second-site mutations in KIT (Antonescu et al. 2005).

The second-site mutations in acquired imatinib-resistant GIST tend to be single ami-no acid substitutions in KIT, located on the allele with the primary mutation (Tamborini et al. 2004; Antonescu et al. 2005; Nishida et al. 2008). Though not the most frequent, one specific type of second-site mutation gained fame as the “gatekeeper” mutation, as it confers resistance to most known TKIs and occurs in the same conserved residue of the ATP-binding pocket of different targeted kinases and across various malignancies (Tab. 2). The gatekeeper mutation in KIT is a T670I conversion, caused by a missense mutation of ACA to ATA in the codon for this amino acid residue. As of July 2012, the database Catalogue of somatic mutations in cancer (Cosmic) was listing 21 tumors of 14 individual cases of GIST with mutation of the KIT gatekeeper residue, 13 cases had the T670I mutation and 1 case of the T670E mutation (e.g., the database lists 3 tumors from

Nishida et al. 2008, but all are from the same case; Wardelmann et al. cite the T670E case in two publications, which is listed as 4 tumors in the database)¹.

Tab. 2: Overview of “gatekeeper” mutations conferring resistance to kinase inhibitors, as detected clinically and with special emphasis on GIST.

Cancer/syndrome Kinase inhibitor Gatekeeper mutation Reference CML Imatinib/dasatinib BCR-ABL(T315I) (Gorre et al. 2001)

HES Imatinib FIP1L1-PDGFRA(T674I) (Cools et al. 2003)

(von Bubnoff et al. 2005) NSCLC Gefitinib/erlotinib EGFR(x;T790M) (Kobayashi et al. 2005)

(Pao et al. 2005) EGFR(T790M) (Bell et al. 2005) GIST Imatinib/dasatinib KIT(x;T670I) (Tamborini et al. 2004)

(Debiec-Rychter et al. 2005) (Antonescu et al. 2005) (Wardelmann et al. 2006) (Nishida et al. 2008) (Liegl et al. 2008) (Cameron et al. 2010) (Bauer et al. 2010)

Abbreviations: CML, chronic myelogenous leukemia; HES, hypereosinophilic syndrome; NSCLC, non small-cell lung cancer; x, primary mutation.

The central role of KIT activating mutations in GIST was experimentally proven by the introduction of KIT mutations found in familial cases of GIST into the germline of mice by targeted mutagenesis of the Kit locus. Three such mouse models have been reported, all of which developed ICC hyperplasia and cecal GIST lesions (Sommer et al. 2003;

Rubin et al. 2005; Nakai et al. 2008). Advantages of knock-in models like the Kit^V558Δ/+

mouse in contrast to transgenic mice are that the targeted gene is expressed from its en-dogenous locus (including all possible tissue specific regulators/enhancers); as the tar-geted chromosome is known, mutations on other chromosomes can be combined by in-tercrossing; phenotypes/tumors develop orthotopic in an immunocompetent environ-ment; and unphysiologic alteration/silencing of the mutant allele is not expected in germline transmissions over the generations.

1URL:http://www.sanger.ac.uk/perl/genetics/CGP/cosmic?action=bygene&ln=KIT&start=670&end=670

&coords=AA%3AAA (July 30, 2012).