Expression and mutational status of c-kit in thymic epithelial tumors

Abstract

Background: Overexpression of c-kit, a tyrosine kinase receptor protein encoded by the protooncogene kit, has been previously reported in thymic epithelial tumors and in other neoplasms such as gastrointestinal stromal tumors, myeloproliferative disorders, melanoma, and seminoma. Mutations in the kit gene have been related to response to imatinib in gastrointestinal stromal tumor and one case report of thymic carcinoma. We studied expression of c-kit in a large retrospective series of thymic epithelial malignancies and sequenced the whole gene in a subset of patients.

Methods: Thymic epithelial tumors from 120 patients (13 thymic carcinomas and 107 thymomas) were examined. Immunohistochemical staining with an antic-kit polyclonal antibody was performed on a tissue microarray. Mutation analyses of exons 1 to 20 were conducted by direct DNA sequencing of polymerase chain reaction products in eight thymic carcinomas, five thymomas, and one thymic carcinoma cell line.

Results: The percentage of c-kit positive cells was significantly higher in thymic carcinoma (46%) than in thymoma (4%). Decreased disease-related survival and progression-free survival were observed in c-kit positive tumors. No mutations were detected.

Conclusion: c-kit expression is strongly but not exclusively related to thymic carcinoma histotype, and it is of prognostic value. Mutations are very rare.

FIGURE 1.

A, Membrane c-kit positive staining, ×40 original magnification. B, c-kit negative staining, ×40 original magnification. C, Kaplan-Meier plot for disease-related survival of patients positive or negative for c-kit expression. D, Progression-free survival related to c-kit expression. E, Disease-related survival of thymoma and thymic carcinoma patients. F, Progression-free survival of thymoma and thymic carcinoma patients.