Targeted therapy of cancer: new roles for pathologists in identifying GISTs and other sarcomas.

Once a poorly understood pathologic entity, gastrointestinal stromal tumor (GIST) has emerged in recent years as a distinct oncologic-molecular paradigm that is now a leading model for kinase-targeted therapies in oncology. Most GISTs are KIT-expressing and KIT signaling-driven mesenchymal tumors, many of which have KIT-activating mutations. A small subset of GIST show activating mutations in PDGFRA, encoding a related member of the type III receptor tyrosine kinase family. The revelation of KIT expression as a diagnostic signature of GIST has not only revolutionized the pathologic criteria in classifying GIST, but also shed light on the histogenesis of these tumors. The similarities in KIT immunoreactivity and ultrastructural appearance between GISTs and the intestinal pacemaker, the interstitial cells of Cajal (ICC), suggested that GISTs derive from or differentiate toward the ICC lineage. KIT plays a significant role in proliferation, survival, and differentiation of hematopoietic stem cells, mast cells, melanocytes, and interstitial cells of Cajal; activating KIT mutations have been identified in tumors affecting most of these cell lineages. This review will include a summary of the biology behind the specific targeted therapies, emphasizing the central role of KIT and PDGFRA oncogenic mutations in GISTs and their clinical and pathologic correlates. The role of KIT immunohistochemistry vs mutation testing will be discussed, with an insight into the indications for KIT/PDGFRA genotyping in GIST. The morphologic and molecular changes that appear with imatinib treatment, such as response and acquired imatinib resistance, are being discussed. The success GIST story based on targeted molecular paradigm may be applied in other imatinib-responsive sarcoma, such as dermatofibrosarcoma protuberans.