A case of Birt-Hogg-Dubé syndrome implying reduced or no wild-type folliculin without mutated protein is pathogenic.

Birt-Hogg-Dubé syndrome (BHDS) is an autosomal dominant cancer syndrome caused by a germline mutation of the folliculin (FLCN) gene. Previous studies have suggested that truncated mutant folliculin proteins generated by disease causing FLCN mutations may retain partial functionality and contribute to disease phenotype. A 38-year-old Russian man presented with a left renal tumor. He underwent a left radical nephrectomy and histological examination confirmed the diagnosis of chromophobe renal cell carcinoma. He had papulae on his face suggestive of fibrofolliculomas, and pulmonary cysts on his computed tomography of the chest. He had a family history of skin manifestations. Genetic analysis identified a genomic deletion including the putative promoter region of FLCN exon 1 in the germline, and the second hit on the remaining wild-type FLCN in the renal carcinoma cells, which is expected to cause the complete lack of folliculin protein. Immunohistochemistry with the use of anti-folliculin antibody showed no antibody-binding on chromophobe renal carcinoma cells. These findings suggest that the decreased FLCN expression itself without producing mutated folliculin proteins can be at risk for developing clinical manifestations of BHDS: fibrofolliculomas, lung cysts, and tumorigenesis in the kidneys. This sheds light on the pathogenesis of BHDS and the role of FLCN as a tumor suppressor gene.