Birt-Hogg-Dubé syndrome with c.1579_1580insA variant in a Chinese family: a case report

Birt-Hogg-Dubé (BHD) syndrome is a rare inherited genodermatosis characterized by hair follicle hamartomas, kidney tumors, and spontaneous pneumothorax. Recombination mapping in BHD families delineated the susceptibility locus to 700 kb on chromosome 17p11.2. Protein-truncating mutations were identified in a novel candidate gene in a panel of BHD families, with a 44% frequency of insertion/deletion mutations within a hypermutable C(8) tract. Tissue expression of the 3.8 kb transcript was widespread, including kidney, lung, and skin. The full-length BHD sequence predicted a novel protein, folliculin, that was highly conserved across species. Discovery of disease-causing mutations in BHD, a novel kidney cancer gene associated with renal oncocytoma or chromophobe renal cancer, will contribute to understanding the role of folliculin in pathways common to skin, lung, and kidney development.

Spontaneous pneumothoraces due to lung cyst rupture afflict patients with the rare disease Birt-Hogg-Dubé (BHD) syndrome, which is caused by mutations of the tumor suppressor gene folliculin (FLCN). The underlying mechanism of the lung manifestations in BHD is unclear. We show that BHD lungs exhibit increased alveolar epithelial cell apoptosis and that Flcn deletion in mouse lung epithelium leads to cell apoptosis, alveolar enlargement, and an impairment of both epithelial barrier and overall lung function. We find that Flcn-null epithelial cell apoptosis is the result of impaired AMPK activation and increased cleaved caspase-3. AMPK activator LKB1 and E-cadherin are downregulated by Flcn loss and restored by its expression. Correspondingly, Flcn-null cell survival is rescued by the AMPK activator AICAR or constitutively active AMPK. AICAR also improves lung condition of Flcn(f/f):SP-C-Cre mice. Our data suggest that lung cysts in BHD may result from an underlying defect in alveolar epithelial cell survival, attributable to FLCN regulation of the E-cadherin-LKB1-AMPK axis. Copyright © 2014 The Authors. Published by Elsevier Inc. All rights reserved.

Birt-Hogg-Dubé (BHD) syndrome is an autosomal dominant disorder characterized by fibrofolliculomas, renal tumors, and pulmonary cysts with recurrent pneumothorax. Multiple pulmonary cysts and pneumothorax are the key signs for diagnosing BHD syndrome. The pathologic features of BHD pulmonary cysts, however, are poorly understood. This disorder is caused by mutations in the gene that encodes folliculin (FLCN). FLCN is regarded as a tumor suppressor; it mediates cellular activities by interacting with the mammalian target of rapamycin (mTOR). In this study, we investigated the lungs of 11 patients from 9 BHD families. The majority of patients consulting doctors were women between 30 and 60 years of age who had pulmonary cysts and repeated pneumothoraces. Genomic DNA testing revealed 5 different mutation patterns. Histopathologic examination found that the inner surface of cysts was lined by epithelial cells, sometimes with a predominance of type II pneumocyte-like cuboidal cells. The cysts occasionally contained internal septa consisting of alveolar walls or showed an "alveoli within an alveolus" pattern. The cells constituting the cysts stained positive for phospho-S6 ribosomal protein expression, suggesting activation of the mTOR pathway. Although BHD pulmonary cysts are frequently misdiagnosed as nonspecific cystic diseases, they are distinctly different in histopathology from other bullous changes. Mechanical stress such as rupture and postrupture remodeling allows mesothelial invagination and fibrosis. Such modified BHD pulmonary cysts are virtually indistinguishable from nonspecific blebs and bullae. We propose a new insight, namely, that the BHD syndrome-associated pulmonary cyst may be considered a hamartoma-like cystic alveolar formation associated with deranged mTOR signaling.