Gastrointestinal stromal tumors (GISTs) represent a distinct and the most important subset of mesenchymal
tumors of the GI tract. These
tumors are both phenotypically and genotypically different from true
leiomyomas and usually express CD34, a hematopoietic progenitor cell
antigen. CD34, however, is also present in a wide variety of fibroblastic and endothelial cell
tumors. In this immunohistochemical study of CD117, we evaluated 85 cases of GIST and more than 150 other mesenchymal
tumors, including
leiomyomas and
schwannomas. CD117, the c-kit proto-oncogene product, is expressed in subsets of hematopoietic stem cells, mast cells, melanocytes, and interstitial cells of Cajal of the GI tract. CD117 was almost always (85%) expressed in both benign and malignant GISTs. CD117 was observed both in the spindle cell and epithelioid subtypes of GISTs in all locations. In addition to reacting with the CD34-positive GISTs, CD117 was positive in some CD34-negative cases. Approximately one-third of GISTs coexpressed CD117 and smooth muscle
actins. In contrast, true
leiomyomas (
desmin and actin-positive) and
schwannomas in both GI and peripheral locations were consistently negative for CD117.
Solitary fibrous tumors and Kaposi's
sarcomas, which are typically CD34 positive, were consistently CD117 negative. Among the CD34-positive
tumors that showed occasional CD117 reactivity were
dermatofibrosarcoma protuberans (1 of 7) and
hemangiopericytoma (2 of 10). Other mesenchymal
tumors that were variably CD 117 positive included
clear cell sarcoma (7 of 15), metastatic
melanoma (9 of 25), and
malignant fibrous histiocytoma (1 of 20). These results indicate that CD117 is a specific marker for GIST among
tumors that occur in the GI tract and adjacent regions. CD117 expression also separates GISTs from true
leiomyomas and gastric
schwannomas.