Absence of gene mutations in KIT-positive carcinoma showing thymus-like elements of the thyroid.

KIT, a tyrosine kinase receptor protein encoded by the proto-oncogene c-kit, is overexpressed in human neoplasms such as gastrointestinal stromal tumors and thymic squamous cell carcinoma. However, the role of KIT expression and mutations in carcinoma showing thymus-like elements is not fully understood. In the current study, 8 cases of carcinoma showing thymus-like elements were reported, and immunohistochemical staining with KIT and CD5 was performed. Mutation analyses in the juxtamembrane domains (exons 9 and 11) and in the tyrosine kinase domains (exons 13 and 17) were performed on KIT-positive samples using polymerase chain reaction and direct DNA sequencing. KIT- and CD5-positive stainings were observed in all 8 cases. However, none of the tested samples showed mutations in any of the 4 exons. The primary carcinoma showing thymus-like elements of the thyroid frequently expresses KIT and CD5 proteins; similar to thymic squamous cell carcinoma, the overexpression of KIT does not necessarily indicate a gene mutation in this tumor. KIT, along with CD5, could be a useful marker for the difficult diagnosis of carcinoma showing thymus-like elements, which should be added to the list of KIT-positive neoplasms.