The FLCN/BHD gene consists of a 3717 nt mRNA (using NM_144997 derived from BQ423946 and AF517523, the coding sequence extends from nt499 to nt2238) and contains 14 coding exons. The initiation codon is located within exon 4.

Northern blot analysis revealed a 3.8 kb FLCN/BHD mRNA transcript expressed in most tissues
Alternate splicing of FLCN/BHD results in two transcript variants encoding two different isoforms. Transcript 1 is the full-length isoform. Transcript 2 has a shorter and distinct C-terminus from Transcript 1.

The BHD protein, folliculin (FLCN), consists of 579 amino acids with a central glutamic acid-rich coiled-coil domain, one N-glycosylation site and three myristoylation sites, and an estimated molecular weight of 64.5 kDa.

expressed in most major adult tissues, including kidney, lung and skin, which are involved in the BHD phenotype.

Epitope-tagged FLCN expressed in HEK293 cells localized in both the nucleus and cytoplasm by fluorescence in situ hybridization..

FLCN is a novel protein, with no characteristic domains to suggest function. Coimmunoprecipitation studies have identified a novel folliculin-binding partner, FNIP1, which also interacts with 5AMP-activated protein kinase (AMPK), a key molecule for energy sensing and a negative regulator of mTOR (mammalian target of rapamycin). FLCN exists in phosphorylated forms, which are enhanced by FNIP1 overexpression, and suppressed by inhibitors of mTOR signaling including rapamycin and amino acid starvation, and by an AMPK inhibitor, Compound C. These data suggest that FLCN and its interacting partner, FNIP1, may be involved in energy and nutrient-sensing through the AMPK and mTOR signaling pathways.
Using a genetic approach in Drosophila, RNA interference studies to decrease expression of the fly BHD homolog, DBHD, have established a requirement for DBHD in male germline stem cell maintenance in the fly testis. Further genetic studies to examine the interaction between DBHD and the JAK/STAT pathway, which is necessary for germline stem cell self-renewal, suggested that DBHD may regulate maintenance of germline stem cells downstream of or in parallel with the JAK/STAT and Dpp(a TGFbeta family member) signaling pathways. Thus the work with the Drosophila homolog of FLCN/BHD supports a potential role for DBHD in stem cell maintenance and raises the possibility that dysregulation of FLCN in human tumors may result from aberrant modulation of stem cells.

Folliculin shows no strong homology to any known proteins but is evolutionarily conserved, and orthologs have been identified in chimpanzee, dog, cow, rat, mouse, red jungle fowl, frog, fly, and worm.

All FLCN/BHD germline mutations identified in Birt-Hogg-Dubé (BHD) patients are predicted to truncate the mutant protein, including frameshift (insertions/deletions), nonsense and splice-site mutations. To date, no missense germline mutations have been identified. The mutation detection rate in BHD families is about 84%. Mutations are located along the entire length of the coding region, with no genotype-phenotype correlations noted between type of mutation, location within the gene and phenotypic disease manifestations (BHD skin lesions, lung cysts/spontaneous pneumothorax and renal tumors). The most frequent mutation found in the germline of BHD patients is the insertion or deletion of a cytosine in a C8 tract located in exon 11, predicted to cause a frameshift and prematurely truncate the mutant protein. This hot spot mutation occurs in about half of all BHD patients. Among BHD patients with the exon 11 mutation, significantly fewer renal tumors developed in patients with the C-deletion than those with the C-insertion mutation.
Germline FLCN/BHD mutations have been reported in primary spontaneous pneumothorax (PSP) families with nearly 100% penetrance in family members in which lung blebs or bullae indicated affected status. The PSP-associated mutations, including 2 nonsense and one 4-bp deletion, are predicted to prematurely truncate the protein and are located in exons 9, 12 and 4, respectively.

FLCN/BHD somatic mutations have been found at only a very low frequency (0-10%) in sporadic renal tumors and therefore, may not represent a major mechanism for the development of sporadic renal carcinoma. Loss of 17p DNA including p53 (36%) or partial methylation (28%) of the FLCN/BHD promoter were reported in sporadic renal carcinomas with various histologies.
Mutations have been identified in the mutational hot spot in exon 11 of the FLCN/BHD gene in other tumor types exhibiting microsatellite instability, including colorectal carcinoma (20%), endometrial carcinoma (12%) and gastric carcinoma (16%).