Tyrosine kinase receptor expression in thymomas

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Abstract
Promising new therapies for neoplasia include tyrosine kinase receptor antagonists. Tyrosine kinase oncogenes present an appealing anti-tumor drug target since they play an integral role in a variety of cellular responses including cell proliferation and differentiation. We previously demonstrated a high rate of epidermal growth factor receptor (EGFR) expression in advanced-stage thymic epithelial tumors. More recently, we have examined c-KIT (CD117) expression in a similar series of tumors. Tumor from 35 patients seen at our institution for treatment of advanced-stage thymoma was available. Twenty thymomas and 15 thymic carcinomas were assessed for c-KIT expression. Tissue sections of tumor were stained immunohistochemically with anti-c-KIT (Oncogene). Either cytoplasmic or membrane staining was considered positive. Appropriate controls were performed. Positive staining for c-KIT was present in 12 tumors (11 thymic carcinomas and 1 thymoma). In distinction to EGFR, c-KIT is expressed more commonly in thymic carcinomas (73% of carcinomas) than in thymoma (5% of thymomas). An EGFR negative/c-KIT positive staining pattern is typical of thymic carcinoma, whereas thymomas are generally EGFR positive/c-KIT negative. Possible therapeutic implications of these observations remain to be determined.