Renal Neoplasia in Birt-Hogg-Dubé Syndrome: Integrated Histopathologic, Bulk, and Single-cell Transcriptomic Analysis

It is unclear whether historically diagnosed "hybrid tumors" in patients with Birt-Hogg-Dubé syndrome (BHD) represent unique tumors or a hybrid between oncocytoma and chromophobe renal cell carcinoma (Ch-RCC), and existing diagnostic criteria are ambiguous. We aimed to understand the spectrum of folliculin gene (FLCN) alterations, outcomes for BHD patients with kidney tumors, and the biology of FLCN-mutated tumors (FMTs) to refine diagnostic algorithms.

Germline testing for FLCN alterations and outcomes for 20 BHD patients with 84 kidney tumors were evaluated. Renal tumors were profiled for histopathology and analyzed using a combination of next-generation sequencing, bulk/single-cell transcriptomic analysis, and immunohistochemistry (IHC).

Ninety unique germline FLCN variants in 234 unrelated families included rare deletion events (14/234, 6%), including those of the promoter region. Most patients (17/19, 90%) met the National Comprehensive Cancer Network criteria for germline testing. Almost all cases represented indolent FMTs (n = 81), with metastases seen in two (of three) nonconventional renal cell carcinoma patients. FMTs showed a gene expression profile distinct from both oncocytoma and Ch-RCC characterized by four distinct L1CAM^-/FOXI1⁺ and two L1CAM⁺/FOXI1^- cell populations that showed GPNMB overexpression. IHC panels that include L1CAM, SOX9, and GPNMB can be a reliable screen for conventional FMTs. Limitations include the absence of external transcriptomic datasets to avoid batch effects.

Our results highlight the gaps in current clinical germline testing strategies for BHD, which should include promoter deletion events. Multimodal molecular profiling results can be translated into routine clinical practice using IHC biomarkers to improve the diagnosis of BHD and to separate indolent "conventional" FMTs from "nonconventional tumors," which may be clinically aggressive.