• pleural disease
• rare lung diseases

Individuals with pneumothoraces are frequently divided into young patients with the so-called primary spontaneous pneumothorax and older patients with lung disease causing secondary pneumothoraces. In truth, the younger patients may also have underlying lung disease either acquired through smoking, particularly cannabis, or inherited. Ten per cent of individuals presenting with apparently primary pneumothoraces will have a family history of pneumothorax, indicating a large heritable component.1 The most common genetic diagnosis in patients with familial pneumothoraces is Birt-Hogg-Dubé syndrome (BHD), an autosomal dominant condition caused by mutations of the FLCN gene encoding the protein folliculin. Two articles in this edition of Thorax address different aspect of BHD biology with potential implications for the diagnosis and management of these patients.2 3

Folliculin is so named because its deficiency can cause fibrofolliculomas to form. These hamartomas of the skin can aid diagnosis of BHD. Mutation of FLCN can also cause renal cell carcinomas to develop in up to a third of patients which if not treated early can be lethal. The diagnosis of BHD can often be made clinically based on a family history of pneumothoraces and/or renal cancer, dermatological examination revealing fibrofolliculomas and other skin lesions, and characteristic cross-sectional imaging of the thorax with predominantly basal cysts. Confirmation of the diagnosis involves analysis of the FLCN gene, which can be disrupted by point mutations or deletions.1

Folliculin is a widely expressed protein that controls cell growth and metabolism. It regulates the growth-promoting kinase mTORC1 and the nutrient-sensitive kinase AMPK.4 Deregulation of mTORC1 is important for the tumour suppressor function of folliculin, and renal tumours develop owing to somatic loss of the remaining wild-type allele of FLCN, accounting for the incomplete penetrance and later onset of renal malignancy in BHD, typically occurring from the fifth decade onwards. Pneumothorax tends to occur earlier offering an opportunity to diagnose BHD prior to the onset of cancer and enables life-saving renal surveillance.

Pulmonary cysts are a typical feature and account for the pneumothoraces that occur in BHD. Unlike the apical blebs and bullae observed in most patients with primary spontaneous pneumothoraces, individuals with BHD characteristically have irregular, thin-walled cysts found inferiorly, predominantly below the level of the carina. While the role of folliculin as a classical tumour suppressor is reasonably well understood, in renal tumourigenesis, the mechanism linking FLCN mutation to cyst formation is not entirely unclear. Homozygous Flcn^−/− mice are embryonic lethal, while heterozygous Flcn^+/− mice spontaneously develop renal cancers without pulmonary pathology.5 Although expressed widely, in the lung folliculin protein is detected most easily in type II alveolar epithelial cells (AECII) and in the underlying mesenchymal tissue. Previous work showed that deletion of Flcn from AECIIs, but not from bronchial airway epithelium, caused enlargement of alveolar airspaces throughout the lungs.6 This was attributed to abnormal epithelial cell–cell contacts and increased cell death, suggesting these processes might be involved in cyst formation in patients with BHD. The study of Min et al reveals that loss of FLCN disrupts the expression of several mircoRNAs.2 The expression of two, miR-424–5 p and let-7d-5p, is suppressed in cells expressing folliculin, while their levels are elevated in the cells surrounding pulmonary cysts in BHD. Since expression of these microRNAs appears to induce cell death and reduce epithelial wound repair in vitro, the authors speculate that cyst formation might be driven by apoptosis in cells deficient in folliculin. Remarkably, levels of miR-424–5 p and let-7d-5p were also detectable in the plasma of individuals with known BHD raising the possibility of a novel screening test for this disease.

The strikingly basal distribution of cysts in BHD led to the suggestion that dynamic stretch might play a role. Owing to gravity, tension is unevenly distributed throughout the lung with more superior parts being pulled down more than basal portions. Consequently, alveoli closer to the apex are prestretched and less able to inflate further during inspiration. By contrast, basal alveoli, being unstretched, are more compliant so account for the majority of ventilation during tidal breathing. The ‘stretch hypothesis’ was proposed to link this unequal ventilation with the formation of basal cysts.7 If deficiency of folliculin were to alter cell–cell contacts, then cysts might form in these regions of more variable distention. Indeed, it has been shown that cells lacking folliculin have abnormally strong cell–cell contacts, either by dysregulation of the cadherin-binding protein PKP4 and/or through altered AMPK function.7 8 Interestingly, in addition to its role in monitoring nutrient availability, AMPK senses the strength of cell–cell stretch and regulates cell stiffness accordingly. Increased cell–cell adhesion or altered cell stiffness in FLCN deficient cells might therefore lead to abnormal stretch during respiration and thus promote cyst formation.

But loss of Flcn in epithelia impairs barrier function and AECII-specific deletion causes enlarged airspaces without cyst formation.6 The new study of Chu et al now examines the consequence of folliculin loss in non-epithelial cells.3 Mice generated to lack Flcn in mesenchymal cells were smaller than wild-type littermates and survived only 1 month. Nevertheless, their lungs showed abnormal alveolar development with cyst formation. Alveolarisation requires myofibroblast differentiation and proliferation to provide the extracellular matrix needed for septi to form. Myofibroblast formation was defective in the animals lacking Flcn in mesenchymal cells resulting in less elastin and type III collagen to be deposited in alveolar septi. The precise mechanisms accounting for these effects are not fully worked out, although both mTOCR1 and AMPK activation were abnormal in the lungs of the mutant mice. Since folliculin has recently been shown to regulate the exit from pluripotency, perhaps some of the pathogenesis of BHD involves the failure of mesenchymal differentiation and a subsequent failure to form alveolar septi.9

Loss of lung parenchyma in disease generally occurs when cellular loss or injury exceeds repair capacity. Taken together, the findings point towards a failure of lung development and reduced epithelial viability in the areas of the lung undergoing high levels of mechanical strain, resulting in cyst formation and a tendency to pneumothorax.