Clear-cell papillary renal cell carcinoma: molecular and immunohistochemical analysis with emphasis on the von Hippel-Lindau gene and hypoxia-inducible factor pathway-related proteins.

Over the past few years several investigators have independently described unique low-grade renal epithelial neoplasms with clear cytoplasm, focal to diffuse papillary architecture, and occasional leiomyomatous stromal metaplasia that are not currently recognized in the World Health Organization classification of renal tumors. These tumors have been referred to by a variety of names including clear-cell papillary renal cell carcinoma and recently "clear-cell tubulopapillary renal cell carcinoma". On the basis of the available data, such tumors are positive for cytokeratin 7 (CK7) and carbonic anhydrase IX (CA9), while being negative for CD10, α-methylacyl-CoA racemase (AMACR), and TFE3. These tumors reportedly lack trisomies of chromosomes 7 and 17, deletions of 3p25, von Hippel-Lindau (VHL) gene mutations, and VHL promoter hypermethylation. Herein, we report on nine cases of this tumor emphasizing detailed studies of the VHL gene and hypoxia-inducible factor (HIF) pathway. Molecular studies performed included VHL mutational analysis, copy number changes assessed using single-nucleotide polymorphism arrays, and qRT-PCR for VHL mRNA expression. Immunohistochemical stains for markers of HIF pathway activation (HIF-1α, CA9, and glucose transporter-1 (GLUT-1)) as well as other relevant markers (CK7, CD10, AMACR, and TFE3) were performed. None of our tumors harbored VHL gene mutations, losses of chromosomal region 3p25, or trisomies of chromosomes 7 or 17. VHL mRNA was overexpressed in our tumors relative to normal renal tissue and clear-cell renal cell carcinoma. All cases showed strong co-expression of CK7, HIF-1α, GLUT-1, and CA9. No expression of TFE3, CD10, or AMACR was seen. The morphological, immunophenotypic, and molecular features of these unique low-grade tumors are sufficiently distinct to allow separation from other renal cell carcinoma subtypes. The co-expression of CA9, HIF-1α, and GLUT-1 in the absence of VHL gene alterations in clear-cell papillary renal cell carcinoma suggests activation of the HIF pathway by non-VHL-dependent mechanisms.