FLCN: The causative gene for Birt-Hogg-Dubé syndrome

Abstract

Germline mutations in the novel tumor suppressor gene FLCN are responsible for the autosomal dominant inherited disorder Birt-Hogg-Dubé (BHD) syndrome that predisposes to fibrofolliculomas, lung cysts and spontaneous pneumothorax, and an increased risk for developing kidney tumors. Although the encoded protein, folliculin (FLCN), has no sequence homology to known functional domains, x-ray crystallographic studies have shown that the C-terminus of FLCN has structural similarity to DENN (differentially expressed in normal cells and neoplasia) domain proteins that act as guanine nucleotide exchange factors (GEFs) for small Rab GTPases. FLCN forms a complex with folliculin interacting proteins 1 and 2 (FNIP1, FNIP2) and with 5' AMP-activated protein kinase (AMPK). This review summarizes FLCN functional studies which support a role for FLCN in diverse metabolic pathways and cellular processes that include modulation of the mTOR pathway, regulation of PGC1α and mitochondrial biogenesis, cell-cell adhesion and RhoA signaling, control of TFE3/TFEB transcriptional activity, amino acid-dependent activation of mTORC1 on lysosomes through Rag GTPases, and regulation of autophagy. Ongoing research efforts are focused on clarifying the primary FLCN-associated pathway(s) that drives the development of fibrofolliculomas, lung cysts and kidney tumors in BHD patients carrying germline FLCN mutations.

Keywords: AMPK; Birt-Hogg-Dubé syndrome; DENN domain; FLCN; FNIP1; FNIP2; Folliculin; Kidney tumor; PGC1α; mTOR.

Figure 1

CT-FLCN has structural similarity to DENN domain proteins. A) Crystal structure of carboxy-terminal (CT) domain of FLCN at 2 Å resolution. Two FLCN molecules are in the asymmetric unit. The N-terminus is blue and the C-terminus is red. B) CT-FLCN is structurally similar to the DENN domain of DENN1B protein. CT-FLCN is in blue and DENN domain of DENN1B is in magenta. Adapted from (Nookala et al., 2012).

Figure 2

Clinical manifestations of Birt-Hogg-Dubé syndrome. A) Fibrofolliculomas on the face of a BHD patient. B) Bilateral pulmonary cysts in a BHD patient with a small pneumothorax on the right. C) CT scan showing bilateral multifocal kidney tumors in a BHD patient. Adapted from (Schmidt and Linehan, 2015).

Figure 3

Different somatic second hit FLCN mutations in multiple tumors that developed in the kidneys of a BHD patient with a germline FLCN mutation demonstrating that FLCN is a tumor suppressor gene. Histologic subtypes are color coded. Chromophobe renal carcinoma, yellow; oncocytic hybrid tumor, green; renal oncocytoma, red. LOH, loss of heterozygosity. From (Vocke et al., 2005).

Figure 4

FLCN acts as a GAP for RagC/D to activate mTOR on the lysosome surface in response to amino acids. mTORC1 activation by amino acids requires a lysosome-associated complex of proteins that include the vacuolar (v)-ATPase, the Ragulator complex, and Rag GTPases, which function as obligate heterodimers RagA or B and RagC or D. Amino acids signal from within the lysosomal lumen to Ragulator, which exerts its guanine nucleotide exchange factor (GEF) activity toward RagA/B. GTP-loaded RagA/B then recruits mTORC1 to the lysosomal surface for activation. Additionally, when amino acids are low, FLCN-FNIP associates with the lysosomal surface. When amino acid levels are sufficient, FLCN-FNIP acts as a GTPase activating protein (GAP) towards RagC/D. The GDP-loaded RagC/D can then facilitate mTORC1 recruitment to the lysosome for activation (Petit et al., 2013; Tsun et al., 2013).

Figure 5

Molecular pathways and cellular processes in which FLCN may have a functional role. Research supporting potential functions of FLCN in these pathways and processes is summarized in section 5.