Clinicopathologic characterization of renal tumors with Folliculin (FLCN) biallelic inactivation

Birt-Hogg-Dubé (BHD) syndrome is an autosomal dominant disorder caused by germline inactivation of the folliculin gene (FLCN). Approximately 25% of BHD syndrome patients are diagnosed with renal tumors, which can be multifocal and/or bilateral. These tumors have been reported to span a broad histologic spectrum; however, the majority exhibit a distinctive histology characterized by intermingled cytologic features similar to that seen in oncocytoma and chromophobe renal cell carcinoma. Accordingly, such tumors were previously termed 'hybrid oncocytic chromophobe tumor' (HOCT). To date, most studies have characterized tumors arising in BHD syndrome patients without molecular confirmation of FLCN biallelic inactivation; accordingly, the clinicopathologic characterization of molecularly confirmed FLCN biallelic inactivated tumors remains limited. This study evaluates a multi-institutional cohort of 18 renal tumors from different individuals, including integrated histologic, immunohistochemical, and targeted next-generation sequencing analyses. While all patients exhibited clinical characteristics raising suspicion for BHD syndrome, germline status was not available in 5 patients. Nevertheless, molecular analysis demonstrated findings supportive of FLCN biallelic inactivation in all patients, and there were no other candidate driver alterations or recurrent molecular findings. Histologically, most tumors showed 'conventional' histology consisting of solid or nested growth with a characteristic mosaic population of eosinophilic and clear cells and low-grade nuclear features, including frequent perinuclear halos and binucleation. Two cases showed 'non-conventional' histology including tubulocystic and tubulopapillary architecture. Immunohistochemistry revealed consistent GPNMB expression and a distinctive mosaic staining pattern for KRT7, L1CAM, and GATA3, aiding distinction from other eosinophilic renal neoplasms. Follow-up was available for 17 patients (median 37 months, range 1-110 months). Both patients with non-conventional morphology demonstrated advanced stage at presentation, including one with metastatic disease. Collectively, these findings define one of the largest molecularly-confirmed cohorts of FLCN biallelic inactivated tumors to date and support recognition of FLCN-driven tumors as a distinct, molecularly defined entity.