Birt-Hogg-Dubé syndrome: from gene discovery to molecularly targeted therapies.

Since the hallmark dermatologic features of Birt-Hogg-Dubé (BHD) syndrome were first described by three Canadian physicians in 1977, the clinical manifestations of BHD have been expanded to include hamartomas of the hair follicle, lung cysts, increased risk for spontaneous pneumothorax and kidney neoplasia. Twenty-five years later the causative gene FLCN was identified, and the mutation spectrum has now been defined to include mainly protein truncating mutations, but also rare missense mutations and large gene deletions/duplication. Second "hit" FLCN mutations in BHD kidney tumors and loss of tumorigenic potential of the FLCN-null UOK257 tumor cell line when FLCN is re-expressed underscore a tumor suppressor role for FLCN. The identification of novel FLCN interacting proteins FNIP1 and FNIP2/L and their interaction with 5'-AMP activated protein kinase (AMPK) has provided a link between FLCN and the AMPK-mTOR axis and suggested molecular targets for therapeutic intervention to treat BHD kidney cancer and fibrofolliculomas. The generation of FLCN-null cell lines and in vivo animal models in which FLCN (or FNIP1) has been inactivated have provided critical reagents to facilitate mechanistic studies of FLCN function. Research efforts utilizing these critical FLCN-deficient cell lines and mice have begun to uncover important signaling pathways in which FLCN and its protein partners may play a role, including TGF-β signaling, TFE3 transcriptional regulation, PGC1-α driven mitochondrial biogenesis, apoptotic response to cell stress, and vesicular transport. As the mechanisms by which FLCN inactivation leads to BHD manifestations are clarified, we can begin to develop therapeutic agents that target the pathways dysregulated in FLCN-deficient fibrofolliculomas and kidney tumors, providing improved prognosis and quality of life for BHD patients.