The Activated Targets of mTOR Signaling Pathway Are Characteristic for PDGFRA Mutant and Wild-type Rather Than KIT Mutant GISTsSápi, Zoltán MD, PhD; Füle, Tibor PhD; Hajdu, Melinda MD, PhD; Matolcsy, András MD, DSc; Moskovszky, Linda MD; Márk, Ágnes PhD; Sebestyén, Anna PhD; Bodoky, György MD, DScAuthor Information *1st Department of Pathology and Experimental Cancer Research, Semmelweis University †Department of Oncology, Szent László Hospital, Budapest, Hungary A part of this study was financially supported by Novartis Ltd., Hungarian division. Reprints: Zoltán Sápi, MD, PhD, 1st Department of Pathology and Experimental Cancer Research, Semmelweis University, 1085 Budapest, Üllői út 26, Hungary (e-mail: email@example.com). Diagnostic Molecular Pathology: March 2011 - Volume 20 - Issue 1 - p 22–33 doi: 10.1097/PDM.0b013e3181eb931b Buy Metrics Abstract The therapy for gastrointestinal stromal tumors (GISTs) has been revolutionized by tyrosin kinase inhibitors. Clinicopathologic studies have been conducted to assess therapeutical responses in cases with KIT and platelet-derived growth factor receptor α (PDGFRA) gene mutations. Cell culture data suggest that Akt/mammalian target of rapamycin (mTOR) kinase signaling may be important in GIST. The aim of our study was to determine the activity of the mTOR pathway in a larger series of GISTs (108 different cases) with different exon mutation types. The KIT and/or PDGFRA mutation status of 108 GIST patients was analyzed by direct DNA sequencing. Immunohistochemistry was performed on tissue microarrays using antibodies for phospho-p70S6 kinase, phospho-4EBP1, and phospho-S6, which are downstream target proteins of mTOR. DNA sequencing identified 73 cases with mutations of KIT and 12 cases with PDGFRA mutations. Wild-type receptors were present in 23 cases. KIT exon mutations were accompanied by the activation of the mTOR pathway in 28 of 73 (38.4%) cases, whereas PDGFRA mutant GISTs showed activation in 10 of 12 (83.3%) cases. Wild-type cases were accompanied by mTOR activation in 17 of 23 (73.9%) cases. Our results indicate that the activation of the mTOR pathway is not a general hallmark of GIST with KIT mutations. However, mTOR signaling seems to be activated in PDGFRA mutants and in wild-type cases, which suggests that mTOR or upstream mTOR inhibitors may be therapeutically useful in primary resistant GISTs and confirms earlier data that mTOR is a crucial survival pathway in resistant GISTs. © 2011 by Lippincott Williams & Wilkins.