Differential Diagnosis and a Novel Mutation in Birt-Hogg-Dubé Syndrome: Insights from 2 Cases

Introduction

Birt-Hogg-Dubé syndrome (BHD syndrome, also known as Hornstein-Knickenberg syndrome, OMIM# 135150), characterized by skin abnormalities, pulmonary cysts, spontaneous pneumothorax, and kidney neoplasms, is a rare autosomal dominant disorder predominantly induced by germline mutation in the FLCN gene [1,2]. FLCN is mapped to chromosome 17p11.2 [3,4], encoding the tumor-suppressor protein folliculin [5,6]. Folliculin is involved in the dysregulation of mechanistic target of rapamycin (mTOR) and 5′AMP-activated protein kinase (AMPK) signaling pathways [7–9], and the clinical manifestations of BHD show similarity to other mTOR-related diseases, such as tuberous sclerosis complex (TSC) and lymphangioleiomyomatosis (LAM) [10], and the shared manifestations of these diseases include skin abnormalities, spontaneous pneumothorax, and kidney neoplasms [10,11]. To date, FLCN is the only known causative gene of BHD, and FLCN mutations have been accumulated in several databases, such as Leiden Open Variation Database, gnomAD, skin gene database [12], and the European BHD Consortium [13]. Although FLCN has been known for decades, its functional role is largely unknown.

In 1975, Hornstein and Knickenberg first described this syndrome [14], which is currently regarded as a rare disorder. The epidemiology of this rare disease is largely unknown. Muller et al [15] found the prevalence of BHD in the general population is about 2 per million, with no sex predilection. In a nationwide population-based study performed in South Korea, the prevalence of BHD was 0.57 per million, with no peak age [16]. Clinical manifestations of BHD include dermatological lesions (such as fibrofolliculomas, trichodiscomas, and acrochordons), kidney and lung cysts, spontaneous pneumothorax, kidney neoplasms, colon polyps, and colorectal cancers [6,17,18]. Typical clinical presentations and detection of FLCN mutation can lead to a confirmed diagnosis of BHD, and identification of the pathogenic mutation provides the possibility for predictive evaluation in family members. The purpose of this report is to present 2 Chinese cases of BHD syndrome, with a literature review of this rare disease.

Case Reports

CASE 1:

A 57-year-old Chinese woman was admitted to the Department of Respiratory and Critical Care Medicine in the First Affiliated Hospital of Zhejiang University School of Medicine (Hangzhou, China). Her chief concern was chest tightness for 3 days. She had a history of hypertension for 5 years and diabetes mellitus for 20 years, and no history of smoking, alcohol, or drug abuse. Levamlodipine (2.5 mg, once daily) was prescribed to control blood pressure, and metformin (1000 mg, twice daily), glimepiride (4 mg, once daily), and acarbose (50 mg, twice daily) were given to control blood glucose. She reported a prior history of recurrent spontaneous pneumothorax during 10 years ago, and thoracoscopic bullectomy with pleurodesis was performed to reduce recurrent pneumothorax. She also reported a history of familial pneumothorax in her father, younger brother, and son, but none of them had received genetic evaluation. Physical examination in the patient revealed multiple dome-shaped, whitish or skin-colored papules on the nose, cheeks, neck, and auricles (Figure 1A–1C), and several papules on the mucosal surface of the lower lip (Figure 1D), and the breath sound was completely absent in the left lower lobe. Skin abnormalities on the face were diagnosed as fibrofolliculomas by skin biopsy (Figure 2A, 2B) and papules on the neck were diagnosed as skin tags. Chest high-resolution computed tomography (HRCT) was performed and demonstrated sparsely-distributed, thin-walled lung cysts in the lower lobes, with a large pneumothorax in the left hemithorax and mild pneumothorax in the right hemithorax (Figure 3A–3C). After treatment with insertion of an indwelling drainage device, the pneumothorax was obviously reduced (Figure 3D–3F). Renal magnetic resonance imaging (MRI) revealed multiple renal cysts (Figure 4A, 4B). Laboratory tests found that C-reactive protein was slightly high (11.8 mg/L, normal range <8 mg/L). The patient showed a poor glycemic control, as fasting blood glucose level (8.41 mmol/L, normal range 3.9–6.1 mmol/L), HbA1% (9.9%, normal range 6.3–9.0%) and HbA1c% (8.4%, normal range 4.2–6.2%) were obviously high. There were no obvious abnormalities in the complete blood count, arterial blood gas analysis, liver and kidney functions, autoimmune antibodies, or tumor markers. WES analysis identified a novel mutation of the FLCN gene in exon 12 (NM_144997.5: c.1341delC, p.Thr448Profs*20) (Figure 5A), confirming the diagnosis of BHD. After 1-week treatment, her respiratory symptoms were significantly improved and the chest tube was removed. Poor dietary habits and irregular monitoring for blood glucose levels were the main reasons for poor glycemic control. The patient was advised to have regular follow-up visits in the endocrinology outpatient clinic. During follow-up, she had no recurrence of pneumothorax, but her blood glucose levels were uncontrolled due to poor dietary patterns and non-adherence to monitoring.

CASE 2:

A 58-year-old Chinese woman visited our outpatient clinic for respiratory disease in the First Affiliated Hospital of Zhejiang University School of Medicine (Hangzhou, China), and she had recurrent episodes of pneumothorax. Thoracoscopic bullectomy with pleurodesis was performed and there was no recurrence of spontaneous pneumothorax after this treatment. She reported no chronic diseases, and her family history of kidney neoplasms or genetic disorders was negative. Physical examination revealed dome-shaped skin papules on her face and neck (Figure 1E, 1F). Complete blood count, liver and kidney functions were normal. Chest HRCT was demonstrated multiple thin-walled lung cysts, mainly distributed in the subpleural and basilar medial regions of the lung (Figure 3G–3I). Abdominal MRI or CT was not performed in this patient. WES analysis identified a mutation of FLCN gene in exon 14 (NM_144997.5: c.1579_1580insA, p.Arg527Glnfs*75) (Figure 5B), leading to the diagnosis of BHD. During follow-up, she had no obvious respiratory symptoms and her pulmonary function was normal.

Discussion

In 1975, Hornstein and Knickenberg first described perifollicular fibromatosis with polyps of the colon in 2 sibs[14]. In 1977, Birt, Hogg, and Dubé reported a family in which several members had skin abnormalities, consisting of fibrofolliculomas, trichodiscomas, and acrochordons (skin tags) [19]. This disorder was subsequently known as BHD syndrome, and it is currently considered an autosomal dominant hereditary disease characterized by dermatologic lesions, lung cysts, spontaneous pneumothorax, and kidney neoplasms [20]. The prevalence of BHD is 1.86 per million in men and 1.88 per million in women [15]. Here, we reported 2 Chinese cases of BHD syndrome with typical dermatological lesions, spontaneous pneumothorax, pulmonary cysts, and mutation of the FLCN gene. Most notably, a novel pathogenic variant of FLCN mutation in exon 12 (NM_144997.5: c.1341delC, p.Thr448Profs*20) was first identified.

The tumor-suppressor gene FLCN, encoding for the tumor-suppressor protein folliculin, is the only causative gene of BHD [21]. Thus far, no previous studies have demonstrated any definitive genotype-phenotype associations between mutation type or location within FLCN. However, several interesting findings of FLCN mutations showed that mutation of exon 9 was associated with more lung cysts, and more episodes of spontaneous pneumothorax occurred in people with mutations of exons 9 and 12 [22]. A previous study demonstrated a greater risk of colorectal cancer in people with the c.1285dupC mutation than in those with the c.610delGCinsTA mutation [23], and, compared with those carrying the FLCN hotspot mutation c.1285dupC, an increased incidence of pneumothorax was observed in patients with mutations c.1300G>C or c.250-2A>G [24].

Folliculin-interacting protein 1 (FNIP1) and FNIP2 form a complex with the C-terminus of FLCN, and the complex interacts with AMPK [8,25], which can act as a critical sensor of cellular metabolism and a negative regulator of mTOR [8,26,27]. Previous studies demonstrated a conflicting role for FLCN-deficiency in mTOR signaling pathway, suggesting that functional loss of FLCN can result in either inhibition or activation of mTOR [8,27–31], and the role of FLCN in mTOR seems to depend upon cell types and nutritional/energy status [31]. Studies with FLCN-deficient cell and animal models support a potential role of FLCN in regulating the AKT-mTOR signaling pathway [28–33]. Emerging evidence demonstrates that FLCN acts in association with a series of molecular pathways and cellular processes, including regulating pluripotency of embryonic stem cell by mediating TFE3/TFEB transcriptional activity [34], modulating PGC1α45;-driven mitochondrial biogenesis [35], exerting a tumor-suppressive effect via TGF-β-mediated apoptosis [36,37], regulating Rag-mediated amino acid sensing [38,39], autophagy [40,41], cell polarity, and cell–cell adhesion [42,43]. Although the functional role of FLCN has been widely explored, the details of the molecular network and mechanism of BHD syndrome remain largely unknown.

BHD has great clinical heterogeneity, ranging from asymptomatic to variable phenotypic features of the skin, lungs, and kidneys. Our 2 patients presented with typical clinical features of BHD syndrome, including lung cysts, recurrent pneumothorax, cutaneous abnormalities, and renal cysts. Irregular and variable pulmonary cysts is a characteristic feature of BHD, affecting 80% or more of BHD patients [6,22,44]. Compared with sporadic primary pneumothorax, where pulmonary cysts are mainly distributed in the apical predominance, lung cysts of BHD tend to be located in the basal lung regions [2]. BHD-affected patients have an increased risk of recurrent spontaneous pneumothorax, which may be associated with the rupture propensity of lung cysts [45]. Familial pneumothorax is found in about 35% of patients with BHD, and approximately 24% of BHD patients have a history of spontaneous pneumothorax [46], and the average age at the first episode of pneumothorax is 36–38 years [22,46,47]. Studies suggested that Asian patients with BHD had a higher incidence of pneumothorax but a lower incidence of renal or cutaneous manifestations [48,49], but further assessment is still needed to demonstrate genetic, epidemiological, and clinical differences between Asian and White populations. Skin abnormalities of BHD are mainly manifested as fibrofolliculomas, trichodiscomas, acrochordons, angiofibromas, and intraoral lesions [45]. Fibrofolliculomas and trichodiscomas are benign tumors of perifollicular mesenchyme, which can appear as multiple small, dome-shaped, yellowish or skin-colored papules on the face, neck, scalp, and upper trunk [3]. Fibrofolliculomas occur in 82–92% of BHD-affected patients [50], and skin lesions were histologically confirmed as fibrofolliculomas in 1 of our cases. Multiple cutaneous fibrofolliculomas, at least 1 histologically confirmed, are considered as a major diagnostic criterion for the syndrome [2]. Different types of kidney neoplasms were found in BHD, including chromophobe/oncocytic hybrid, chromophobe, clear cell, oncocytoma, and papillary, and the most common form is a chromophobe–oncocytoma hybrid [51]. Kidney tumors are observed in about 30% of BHD-affected individuals, and occur at an average age of 50 years [6,20,44]. Older age and male sex are risk factors for renal tumor development in BHD [52]. Additionally, renal cysts also occur in BHD patients [18], and occasionally are the only renal manifestation [53]. About 45% of people with BHD have asymptomatic renal cystic changes without any clear symptoms [53]. Other findings, such as colon polyps or cancer, parotid oncocytomas, lipomas, thyroid nodules/cancer, and liver cysts, have been infrequently reported, but none of those are considered to be a clinical phenotype of BHD [21,45].

In clinical practice, BHD is easily underdiagnosed, or mistakenly diagnosed as pulmonary emphysema, LAM, pulmonary Langerhans cell histiocytosis, and other cystic lung diseases. The identification of skin lesions, especially fibrofolliculomas, should raise the suspicion of BHD and prompt further investigations to narrow the differential diagnosis [45]. For definitive diagnosis, biopsy of dermatological lesions, investigation of personal and family history of cutaneous lesions, spontaneous pneumothorax, lung cysts, renal neoplasia, and evaluation of genetic testing are important [45]. Confirmative diagnosis of BHD needs evidence of clinical manifestations and identification of FLCN germline mutations. To date, more than 300 pathogenic variants have been identified in FLCN and documented in the Leiden Open Variation Database, and the most common pathogenic variants are deletion (c.1285del) or duplication (c.1285dup) in exon 11. Here, we identified a FLCN gene in exon 14 (NM_144997.5: c.1579_1580insA), which has been previously reported [48]. In addition, we identified a novel pathogenic mutation of the FLCN gene in exon 12 (NM_144997.5: c.1341delC), which expands the known mutational spectrum of FLCN in people with BHD.

Clinical management of BHD aims at early recognition and the diagnosis and treatment of pneumothorax and kidney neoplasms. Currently, there is no specific therapy for BHD-related lung cysts. Treatment of pulmonary cysts is limited to prevention and management of pneumothorax, and management of BHD-related pneumothorax is similar to that of sporadic primary pneumothorax [2]. Pleurodesis is recommended after the first episode of spontaneous pneumothorax in BHD [46]. However, pleurodesis only reduced the rate of recurrent pneumothorax by half [54]. Both of our patients underwent pleurodesis, but it was only effective in preventing recurrent pneumothorax in 1 patient. BHD-affected patients should be discouraged from piloting aircraft and deep-sea diving, as exposure to large ambient pressure changes can induce pneumothorax [2]. There is no evidence that BHD-affected individuals should be advised against air travel unless they have an ongoing or recently treated pneumothorax [54,55]. Most BHD-associated renal tumors have a low malignant potential and a favorable prognosis. A delayed nephron-sparing strategy, which can preserve kidney function over time, is recommended in treatment of BHD-related renal cancers [50,56]. When surgery is not applicable, other techniques such as radiofrequency ablation and cryotherapy are alternative methods. It has been suggested that BHD-affected individuals are more vulnerable to cigarette smoke-induced pulmonary function impairment [57], and some evidence shows that smoking can increase the risk of pneumothorax and renal tumors in BHD; therefore, smoking cessation is encouraged in people with BHD [2]. BHD skin abnormalities do not need specific treatment, as they are basically benign. However, excision, debulking, laser ablation using fractional CO2 or erbium-YAG laser, or electrodessication can be applied for aesthetic reasons [2].

Conclusions

We report 2 Chinese cases of BHD syndrome and a previously unknown causative mutation of the FLCN gene that contributes to BHD, which can facilitate further understanding of this rare disorder. As an autosomal dominant hereditary disease, BHD should be distinguished from emphysema, TSC, LAM, and other diffuse cystic lung disease for differential diagnosis. When BHD is suspected, identification of pathogenic FLCN germline mutations is essential for a definitive diagnosis. Due to its rarity, a multidisciplinary team (MDT) approach involving radiologists, respiratory physicians, dermatologists, nephrologists, thoracic surgeons, pathologists, and geneticists is needed to make the diagnosis and to implement optimal follow-up.