Dissecting the molecular signatures underlying hybrid oncocytic tumors in Birt-Hogg-Dubé syndrome.

707 Background: Birt-Hogg-Dubé (BHD) syndrome is associated with an increased risk of renal tumors including the hybrid oncocytic tumors (HOT), renal oncocytoma (RO) and chromophobe renal cell carcinoma (CHRCC). HOT are known to exhibit morphological features overlapping between RO and CHRCC. The molecular underpinnings of HOT and the corresponding morphologic correlates remain poorly understood. Herein we aimed to understand the distinct molecular features underlying HOT and how the genomic phenotype differs from CHRCC. Methods: We performed next generation sequencing and single cell sequencing analysis to understand the clinico-pathologic aspects and tumor biology of HOT and CHRCC, including tumor development. This was followed by spatial transcriptome analysis of the distinct HOT tumor compartments, using nanoString GeoMX digital spatial profiler. The candidate biomarker expression was confirmed by immunohistochemistry (IHC) and RNA in situ hybridization (RNA-ISH), a unique gene expression visualization molecular technology. Results: We discovered unique gene expression signatures underlying the two distinct morphological patterns seen in HOT. FOXI1 and L1CAM show a striking mutually exclusive expression pattern in the two cellular populations seen within this tumor. The signature genes of RO like cells seen within HOT resembles the principal cells of the distal nephron seen in normal kidney; and the signature genes of the CHRCC like cells resembles the intercalated cells of the distal nephron. This suggests distal nephron as the cellular site of origin for HOT. We also nominated tumor specific biomarkers for HOT. Conclusions: We identified unique gene expression signatures which are definitional for HOT and nominated candidate diagnostic biomarkers to help distinguish HOT from other renal tumor subtypes with overlapping morphology. This finding would improve the diagnostic accuracy, assist familial genetic monitoring, and facilitate disease management.