Substrate-selective mTORC1 regulation and gene dosage-dependent tissue divergence in Birt-Hogg-Dubé syndrome.

Birt-Hogg-Dubé (BHD) syndrome, caused by mutations in the tumor suppressor gene FLCN, has traditionally been classified as a classic "mTORopathy" characterized by global mTORC1 activation. Recent structural and multi-omic studies have fundamentally challenged this view, revealing that FLCN functions as a GAP for RagC/D to govern substrate-selective mTORC1 regulation. Synthesizing emerging evidence, we describe an integrated pathogenic framework: biallelic FLCN inactivation drives renal tumorigenesis via constitutive MiT/TFE nuclear accumulation and metabolic reprogramming, whereas haploinsufficiency suffices to disrupt structural integrity in the lung and skin. By reconciling the "mTORC1 paradox" through the lens of gene dosage and temporal signaling dynamics, we highlight novel therapeutic vulnerabilities targeting MiT/TFE factors and kinase rewiring, providing a rationale for organ-specific precision medicine in BHD.