A Case of Birt-Hogg-Dubé (BHD) Syndrome Harboring a Novel Folliculin (FLCN) Gene Mutation

Background

Birt-Hogg-Dubé (BHD) syndrome is a rare autosomal dominant disease originally reported in 1977 [1]. The syndrome is characterized by lung cysts, spontaneous pneumothorax, renal cell cancer, and skin fibrofolliculomas [2,3]. Some patients also may suffer from colorectal polyps and other cancers [4,5]. The gene responsible for BHD syndrome has been mapped on chromosome 17p11.2 and encodes FLCN, which is currently regarded as a tumor suppressor gene [6]. Since the first discovery of a mutation in the FLCN gene, genetic research has further advanced and various different FLCN germline mutations have been identified in Caucasian and Asian families [3,7–9]. Recently, we encountered a Japanese family with a lineage of BHD syndrome and documented histories of pneumothorax in family members. Genetic analysis of one of the family members and a computed tomographic (CT) scan of the chest were performed. Here we report the finding of a patient with BHD syndrome in which mutation analysis subsequently revealed a novel mutation in exon 4 of the FLCN gene.

Case Report

A 56-year-old female non-smoker presented with a sudden onset of dyspnea. Clinical examination and chest X-ray confirmed bilateral pneumothorax (Figure 1A). An intercostal drain was inserted in the left side with complete resolution. Right pneumothorax was reversed without thoracic cavity drainage. She had a past history of right pneumothorax occurring at the ages of 37 years and 45 years. A computed tomography (CT) scan revealed bilateral multiple bullae predominantly located in the subpleural areas in the bilateral lower lobes (Figure 1B), however, no tomographic finding was observed in the abdomen. Physical examination revealed multiple smooth dome shaped skin-colored papules ranging from 0.5 to several mm in diameter spanning the nose and cheek; however, the skin manifestation was inconspicuous and she had not consulted a dermatologist. A detailed history suggested that there was no known inherited or connective tissue disease among the extended family; however, other members of her family, including her mother, younger brother, and her elder, second, and third sons had also suffered spontaneous pneumothorax and undergone surgery (Figure 2). The patient underwent bilateral sequential bulletectomy by video-assisted thoracoscopic surgery (Figure 3). Unfortunately, left spontaneous pneumothorax recurred 5 weeks and 8 weeks after the surgery; for which she was treated with thoracic cavity drainage with pleurodesis and cured. The resected lung specimen showed multiple cysts distributed predominantly in subpleural and mediastinal space, and the air leakage site was not clear. The specimen did not show characteristic findings of lymphangioleiomyomatosis (LAM) and Sjogren’s syndrome, which are pre-disposing factors of pneumothorax in women with multiple lung cysts. Molecular analysis of the FLCN gene was performed after informed consent from the patient and the family. Genomic DNA was extracted from peripheral blood leukocytes and subjected to mutation analysis. The coding region of the FLCN gene consisting of exon 4 to 14 was amplified with polymerase chain reaction (PCR) with oligonucleotide primers and was sequenced by the Sanger method. The sequence analysis revealed a novel deletion mutation (c.57_58delCT) in exon 4 of the FLCN gene of this patient (Figure 4) [10].

Discussion

Nickerson et al. determined in 2002 that the gene responsible for BHD syndrome lies within chromosomal band 17p11.2 and termed this gene BHD [6]. At present, the official symbol for this gene is FLCN. The gene encodes a deduced 579-amino acid protein, which is involved in cell growth [11] and is considered to be a tumor suppressor in the kidney [12]. It has been reported that there are various kinds of germline FLCN mutations in Caucasian and Asian patients with BHD syndrome. The FLCN gene is composed of 14 exons and mutations have been identified in all translated exons. Notably, C insertion or deletion in exon 11 is frequently detected in affected patients [12,13]

In this report, we described a novel mutation found in exon 4. Until now at least nine FLCN germline mutations have been identified in exon 4. Painter et al. reported a 4-bp deletion in the first coding exon (c.235_238delTCGG) predicted to result in a TGA termination codon 50 missense amino acids downstream. This mutation was found in a large Finnish family with a dominantly inherited tendency for primary pneumothorax [13]. Bessis et al. reported a case with sporadic BHD syndrome harboring a novel germline mutation in the initiation codon of the FLCN gene (c.3delG) [14]. The patient had no lung cysts, no renal tumors, and no colon polyps but developed fibrofolliculomas. Although the proband’s son suffered a spontaneous pneumothorax at the age of 4, the patient’s family history was inconspicuous. The other exon 4 mutation (c.59delT) reported by Schmidt et al. was predicted to truncate the protein 34 missense amino acids downstream. Families that had this type of mutation developed fibrofolliculomas without other types of features [2]. In addition to these mutations, Toro et al. have also reported other types of mutations (c.147delA) [3]. In our case study, genetic analysis revealed a unique deletion mutation (c.57_58delCT) in exon 4 that has not been previously described.

It has been reported that more than 80% of patients with BHD syndrome have lung cysts as determined by CT scans of the chest [3]. Pulmonary cysts (of primary spontaneous pneumothorax) are typically located in the apex of the lungs, however, the lung cysts in BHD syndrome are mostly found in the basilar region of the lung [15,16]. Moreover, the characteristic pathological findings of the lung cysts in BHD syndrome are radiologically round and contain blood vessels [17–19]. We confirmed these findings in the case of our patient. In addition to the fact that she was a non-smoker, clinical episodes and radiographic and histopathological findings eliminated the possibility of our patient having pulmonary Langerhans’ cell histiocytosis (PLCH), lymphangioleiomyomatosis (LAM), tuberous sclerosis complex (TSC), lymphoid interstitial pneumonia (LIP), and Sjogren’s syndrome, which can also be causes of multiple lung cysts [20].

Although genetic analysis was performed only on this patient, we are planning to perform genetic analysis of family members of the patient. We consider annual screening for renal tumor to also be important for the patient and her family, so abdominal ultrasonography and magnetic resonance imaging (MRI) of the abdomen should be performed. We have explained to the patient and family that air travel carries a risk of pneumothorax in patients with BHD syndrome [21].

Currently, there are few effective therapies for BHD syndrome of the lung; however, pleural covering combined with resection of cysts is expected to be a safe and effective therapy for BHD patients with intractable pneumothorax [22].

Conclusions

Based on our observations, one should consider the possibility of BHD syndrome in patients with multiple lung cysts that present with discriminative findings on a CT scan of the lungs, as well as patients with familial pneumothorax. Patients suspected of BHD syndrome should be offered molecular analysis, which will not only verify the diagnosis but also inform recommendations for presymptomatic testing of at-risk family members [23].