Overexpression of KIT (CD117) in chromophobe renal cell carcinoma and renal oncocytoma.

KIT expression has not been studied substantially in renal tumors. We analyzed the immunohistochemical expression for KIT in 256 conventional renal cell carcinomas (RCCs), 29 chromophobe RCCs, 25 papillary RCCs, 6 collecting duct RCCs, 6 unclassified RCCs, 7 renal oncocytomas, 20 urothelial carcinomas, 7 nephroblastomas, and 23 angiomyolipomas. We found that 24 chromophobe RCCs (83%) and 5 renal oncocytomas (71%) revealed membranous immunoreactivity for KIT while none of the RCCs of other types expressed KIT immunohistochemically. Sporadic cases of urothelial carcinoma and nephroblastoma were focally positive for KIT. All angiomyolipomas were negative. Genomic DNA extracted from the chromophobe RCCs and renal oncocytomas was submitted for polymerase chain reaction and direct sequencing of the juxtamembrane (exons 9 and 11) and tyrosine kinase (exons 13 and 17) domains. No mutation was found. Our results demonstrate that KIT could be a useful immunophenotypic marker for chromophobe RCC and renal oncocytoma; therefore, it has value for the precise classification of renal cortical epithelial tumors. However, the therapeutic relevance of KIT overexpression in these tumors is uncertain owing to the lack of mutations that would lead to constitutive activation of the protein.