Summary
Birt-Hogg Dube (BHD) syndrome is characterized by skin lesions, kidney tumors, and pulmonary cysts that may be associated with pneumothorax. It is a rare clinicopathologic condition named after the three Canadian physicians who reported the syndrome in 1977. The prevalence of BHD is estimated at 1/200,000 but the exact incidence is unknown. There have been more than 100 families affected by BHD. The kidney tumors in BDH patients range from benign oncocytomas to malignant renal cell carcinomas including chromophobe, clear cell or papillary subtypes. Hybrid tumors may also sometimes develop in the kidney of a BDH patient. The typical skin lesions are called fibrofolliculomas. Fibrofolliculomas are characterized by a circumscribed proliferation of collagen and fibroblasts surrounding distorted hair follicles from which basaloid cells protrude into the surrounding fibromucinous stroma. Other characteristic skin anomalies are trichodiscomas and acrochordons. Dermatological manifestations of BHD usually develop in the third and fourth decades of life and persist indefinitely. Moreover, cutaneous manifestations usually have an earlier onset than associated kidney tumors. The pulmonary cysts are characterized by cystic dilatation of alveolar spaces, ranging from microscopic foci to a few millimeters in diameter. The thin-wall cysts are lined by cuboidal epithelium. The cysts rupture under pressure of inhalation, which leads to pneumothorax. BHD syndrome is transmitted in an autosomal dominant fashion. A potential causative gene, FLCN, has been located on chromosome 17p11.2. This BHD gene encodes folliculin and although the function of the BHD gene product is still unknown, it is believed to be a component of the mTOR pathway (like other hamartoma syndrome-related proteins) and play a role in the development of renal neoplasms and possibly other associated lesions. Diagnosis relies on recognition of the clinical manifestations, together with histological analysis to confirm the presence of the trichodiscomas, perifollicular fibromas and fibrofolliculomas associated with BHD. The diagnosis can be confirmed by detection of DNA mutations in the BHD (FLCN) gene. The differential diagnosis of the multiple firm papules of BHD is dependent on whether the skin lesions are epithelial, mesodermal or mixed in origin. BHD patients and their relatives should seek genetic counseling and testing if possible. There is no specific medical treatment for the dermatologic manifestations of BHD. The definitive treatment for solitary perifollicular fibromas is surgical removal. In addition, dermabrasion and electro-desiccation have been suggested as treatment options, but lesions may recur. Patients with BHD should be screened and pulmonary, renal, and gastrointestinal findings should be managed. The prognosis for patients with BHD is dependent on the degree of internal involvement and the type of kidney tumor involved. *Authors: Drs X.J. Yang and T. Lertsburapa (November 2008)*.
