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

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Abstract

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.

Introduction

Birt-Hogg-Dubé syndrome (BHDS) is an autosomal dominantly-inherited syndrome caused by a germline mutation of the folliculin (FLCN) gene and is characterized by cutaneous fibrofolliculomas, renal tumors, and multiple lung cysts with or without spontaneous pneumothorax. The FLCN gene is believed to function as a tumor suppressor gene. Therefore, patients with BHDS are predisposed to renal neoplasms, which are caused by the “second-hit” mutation on the wild-type FLCN allele. Earlier studies suggested that the mutated folliculin protein may retain partial functionality or show dominant negative effects and therefore contribute to clinical manifestations of BHDS (Hasumi et al., 2017; Luijten et al., 2013). Here we report a case with BHDS whose germline FLCN mutation was predicted to result in no protein synthesis and yet who developed all three manifestations of BHDS, facial papules, lung cysts, and chromophobe renal cell carcinoma (RCC). Additionally, the analysis of “second hit” in RCC suggested a complete lack of folliculin with no mutant protein.

Section snippets

Clinical report

A 38-year-old man of Russian descent was referred to our hospital for a left renal tumor. He had flesh-colored papules on his face (Fig. 1A). Laboratory tests revealed mild anemia (hemoglobin 12.5 g/dL), elevated lactase dehydrogenase (440 IU/L), and elevated C-reactive protein (2.74 mg/dL). Urinary cytology showed no malignant cells. Enhanced computed tomography (CT) revealed a heterogeneously-enhancing solid mass in the upper pole of his left kidney which was 70 mm in size (Fig. 1B). No lymph

Histopathologic and immunohistochemical examinations

The patient's resected renal tissues were fixed in 10% buffered formalin, embedded in paraffin after routine processing, and stained with hematoxylin and eosin (H&E). Immunostaining was performed with antibodies directed to folliculin (dilution 1:150, Santa Cruz, Paso Robles, CA, USA). 3, 3′-diaminobenzidine tetrahydrochloride (dilution 1:200, Abcam, Cambridge, UK). Fast Red (Nichirei Bioscience Inc., Tokyo, Japan) was used as the chromogen.

Mutation analysis of the FLCN gene

Genomic DNA isolated from peripheral blood leukocytes

Results

A FLCN genetic test was performed for the patient and his family members, which confirmed the diagnosis of BHDS (Fig. 2, Fig. 3A). The mutation of the FLCN gene we identified in the proband (II-3 in Fig. 2) was a large genomic deletion including FCLN exon 1, which segregated in the affected sister and her asymptomatic children (II-2, III-1, and III-2 in Fig. 2). The proband's father did not undergo genetic testing (I-1 in Fig. 2). This genomic deletion has already been submitted to ClinVar by

Discussion

The FLCN gene, encoding folliculin protein, consists of 14 exons; the coding sequence is included in exons 4 to 14 (Nickerson et al., 2002). Earlier studies demonstrated that patients with BHDS had various germline FLCN mutations, most of which contain frameshift, nonsense or splice site mutations and are expected to generate truncated and presumably inactivated form of folliculin (Lim et al., 2010; Vocke et al., 2005). Less frequently, large intragenic deletions or duplications and FLCN

Consent

Written informed consent was obtained from the patient to publish this case report, including any accompanying images. A copy of the written consent is available upon request.

Authors' contributions

YE drafted the manuscript and provided the medical care. YN and KS helped draft the manuscript. YH, KM, EK, and KS performed the molecular genetic studies and immunohistochemical analyses. YK provided medical care and helped creat some figures. HK provided the medical care and participated in the study design and coordination. YM and TU provided histopathological diagnosis and prepared the microphotographs of renal tumor. YH conceived of the study and helped draft the manuscript. All authors

Declaration of competing interest

The authors declare that there is no conflict of interests regarding the publication of this study.

Acknowledgements

We thank Ms. Elaine Blumberg for her excellent proofreading and editing of English.

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