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Birt-Hogg-Dubé syndrome (MIM #135150) is characterized by the development of benign skin tumours called fibrofolliculomas, pulmonary cysts that may lead to pneumothorax and a high risk of developing kidney cancer. BHD is caused by mutations affecting the highly conserved protein folliculin (FLCN), which probably has a role in intracellular transport. Most of the research effort directed towards BHD has focused on understanding how loss of FLCN causes kidney cancer. The cutaneous manifestations have received comparatively little attention. Although understandable, it is unfortunate, as the fibrofolliculomas are highly accessible and thus potentially are an excellent system for trying to understand the basic pathobiology of BHD. Also, patients can be very much burdened by the cosmetic consequences of having hundreds of facial skin tumours. Our lack of insight into what drives fibrofolliculoma growth translates into a very limited therapeutic arsenal. Thus, paying attention to fibrofolliculomas has both basic science and practical benefits. In this review, we will discuss the state of the art regarding our understanding of fibrofolliculoma pathogenesis and indicate future directions for research.

Birt-Hogg-Dubé syndrome (BHD) is an autosomal dominant condition due to germline FLCN (folliculin) mutations, characterized by skin fibrofolliculomas, lung cysts, pneumothorax and renal cancer. We identified a de novo FLCN mutation, c.499C>T (p.Gln167X), in a patient who presented with spontaneous pneumothorax. Subsequently, typical skin features and asymptomatic renal cancer were diagnosed. Probably, de novo FLCN mutations are rare. However, they may be under-diagnosed if BHD is not considered in sporadic patients who present with one or more of the syndromic features. Genetic and immunohistochemical analysis of the renal tumour indicated features compatible with a tumour suppressor role of FLCN. The finding that mutant FLCN was expressed in the tumour might indicate residual functionality of mutant FLCN, a notion which will be explored in future studies.