Recurrent Primary Spontaneous Pneumothorax in a Large Chinese Family: A Clinical and Genetic Investigation
Primary spontaneous pneumothorax (PSP) is a lung condition characterized by the spontaneous occurrence of pneumothorax without any obvious underlying lung disease. It is a common manifestation of Birt-Hogg-Dubé (BHD) syndrome, an autosomal dominant disorder caused by mutations in the folliculin (FLCN) gene. This study investigates the clinical and genetic characteristics of a large Chinese family with recurrent PSP, identifying a novel mutation in the FLCN gene and expanding the mutational spectrum of BHD syndrome.
Introduction
PSP is a significant clinical issue, particularly in individuals without apparent lung disease. Familial cases of PSP have been reported since 1921, with an estimated 11.5% of individuals with spontaneous pneumothorax having a positive family history. Various genetic causes of PSP have been identified, including alpha-1 antitrypsin deficiency, lymphangioleiomyomatosis, Langerhans cell histiocytosis, cystic fibrosis, Marfan syndrome, Ehlers-Danlos syndrome, and BHD syndrome. Among these, BHD syndrome is the most common genetic cause of familial pneumothorax.
BHD syndrome is a rare autosomal dominant disorder characterized by three main symptoms: multiple pulmonary cysts and/or recurrent pneumothorax, skin fibrofolliculoma, and renal cancer. These symptoms may occur separately and often present atypically. Pulmonary symptoms are usually the earliest to appear, making PSP a dominant phenotype in BHD syndrome, sometimes without skin or renal manifestations.
The FLCN gene, responsible for BHD syndrome, consists of 14 exons and is located on chromosome 17p11.2. It encodes a 579-amino-acid protein highly conserved across species. The FLCN gene is expressed in various tissues, including the skin, kidney, stromal cells, and type 1 pneumocytes of the lung. Although the exact molecular functions of the FLCN gene are not fully understood, it is believed to be a tumor suppressor gene involved in several signaling pathways, including the mammalian target of rapamycin (mTOR) and adenosine monophosphate-activated protein kinase pathways.
Different types of mutations in the FLCN gene, such as insertions, deletions, missense, splicing, and nonsense mutations, have been reported. Most FLCN germline mutations are predicted to truncate the protein, leading to its dysfunction. Given the association between FLCN gene mutations and PSP, genetic testing is imperative in patients with PSP, especially those with a positive family history.
Methods
This study was approved by the Ethics Committee of Beijing Chaoyang Hospital, Capital Medical University. Written informed consent was obtained from all participants. The clinical records of affected family members were collected when available.
A large PSP-affected family from Northeastern China was recruited for this study. The proband, a 29-year-old woman with her first episode of left-lung pneumothorax, was treated with video-assisted thoracoscopic surgery (bullectomy). Other diagnoses, such as lymphangioleiomyomatosis, Marfan syndrome, Ehlers-Danlos syndrome, and other known syndromic pulmonary disorders, were carefully ruled out. Detailed clinical information on the affected members, including their medical history, body weight and height, smoking status, chest computed tomography (CT) imaging, and treatment of pneumothorax, was retrospectively collected.
Peripheral blood samples were collected, and genomic DNA was extracted from blood leukocytes using the QIAamp DNA Blood Mini Kit. The coding regions of the FLCN gene, consisting of exons 4 to 14 and flanking sequences, were amplified using the polymerase chain reaction (PCR) method. The PCR products were purified and sequenced bidirectionally using the Sanger method with the ABI 3700 DNA sequencer. Mutations were described according to the nomenclature recommended by the Human Genome Variation Society (HGVS).
Results
The pedigree of this Chinese family with PSP includes four generations and 14 affected family members. Pedigree analysis revealed an autosomal dominant mode of inheritance. Among the ten affected family members, with a total of 18 episodes of spontaneous pneumothorax, the median age for the initial onset of pneumothorax was 42.5 years (interquartile range: 28.8–57.2 years). There were no significant differences in repeated episodes of pneumothorax and the age at the first episode when the patients were grouped by either sex or smoking history.
Chest CT scans of the proband showed bilateral multiple pulmonary cysts and pneumothorax. Patient II-3 and III-6 died of stroke and lung cancer, respectively. No clinical evidence of skin fibrofolliculoma or renal abnormalities was discovered in the family.
Direct sequencing of the coding exons of the FLCN gene from the proband’s genomic DNA revealed a novel nonsense mutation in exon 11 (c.1273C>T), which changes a glutamine at codon 425 to a nonsense codon (p.Gln425*). This mutation was identified in the proband and five other affected family members who consented to genetic testing. The mutation is predicted to cause premature termination of the translated FLCN protein and/or trigger nonsense-mediated mRNA decay, leading to a loss-of-function effect. No other sequence variants were detected in the coding regions of this gene.
Discussion
This study presents the clinical and genetic characteristics of a large Chinese family with spontaneous pneumothorax caused by a novel nonsense mutation in exon 11 of the FLCN gene. The genetic analysis confirmed the diagnosis of BHD syndrome in this family, even in the absence of skin lesions or renal tumors.
BHD syndrome typically exhibits clinical heterogeneity, with patients not always presenting all three characteristic manifestations (skin, kidney, and lung involvement). Pulmonary symptoms are often the earliest and most common manifestations. In this study, PSP was the early onset symptom in patients with BHD syndrome, with no clinical evidence of skin fibrofolliculoma or renal abnormalities. This incomplete penetrance of the disease, especially in Asian populations, has been noted in previous studies.
The median age of onset of pneumothorax in this family was 42.5 years, consistent with findings from other large families with BHD syndrome. Smoking history was not a risk factor for the disease, and the recurrent pneumothorax rate in this family was 40%, similar to other reported cases.
Over 100 patterns of mutation in the FLCN gene have been reported, with geographic variation noted. In Caucasian populations, a cytosine deletion or duplication within a poly-C tract (c.1285dupC or delC) in exon 11 of the FLCN gene is common. In Japanese populations, three FLCN mutational hotspots have been identified, including the C8 tract in exon 11, c.1533_1536delGATG in exon 13, and c.1347_1353dupCCACCCT in exon 12. In Chinese populations, the C8 tract in exon 11 is also a mutation hotspot, with the mutation spectrum more extensively distributed over the entire FLCN gene.
The majority of FLCN mutations are predicted to truncate the protein, indicating that BHD syndrome arises through a haploinsufficiency mechanism. Emerging evidence has linked the FLCN gene with several molecular pathways and cellular processes. Studies suggest that the FLCN gene may modulate AKT-mTOR signaling in a context-dependent manner. Down-regulation of FLCN leads to increased cell-cell adhesion and loss of cell polarity, which may result in increased vulnerability to physical forces induced by respiration. Loss of function of FLCN may lead to epithelial apoptosis, alveolar enlargement, and impaired pulmonary function through E-cadherin, liver kinase B1, and the AMP-activated protein kinase signaling pathway, consequently leading to pneumothorax.
This study highlights the importance of considering BHD syndrome in patients with recurrent pneumothorax and/or a positive family history. Genetic testing of the FLCN gene is the most reliable method for the clinical molecular diagnosis of BHD syndrome, especially in Asian patients who do not have skin and renal manifestations. Early detection of renal neoplasm in such patients is recommended, as FLCN mutation carriers have an increased risk of developing renal cell carcinoma.
Limitations
This study has several limitations. First, affected family members were mostly identified from consultations at the Department of Thoracic Surgery and respiratory clinic, so few underwent skin examinations or renal screening. As such, it is possible that diagnoses of skin and renal lesions were missed. Second, the genetic results from this single family and the function of the novel mutation were only predicted here, although this co-segregating nonsense mutation might be strongly pathogenic. Therefore, more studies with larger populations of Chinese patients with BHD syndrome, accompanied by detailed clinical information and further functional analysis of the FLCN gene, are warranted.
Conclusion
This study reports a large Chinese family with spontaneous pneumothorax caused by a novel nonsense mutation in exon 11 of the FLCN gene, confirming the diagnosis of BHD syndrome. The finding of this novel mutant locus expands the mutation spectrum of BHD syndrome in the Chinese population. Genetic testing of the FLCN gene should be conducted in patients with spontaneous pneumothorax, particularly those with a positive family history, to enable early diagnosis and management of BHD syndrome.
doi.org/10.1097/CM9.0000000000000442
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