This study presents the first population of Polish patients with BHDS. All cases had cystic lung lesions with pneumothoraxes, which were recurrent in all patients with symptomatic pneumothorax. Skin lesions were identified in 27%, and renal changes in 17% of the patients. The lung was the only organ involved in 50% of the patients. The most frequent mutations were in exon 6. In two families, a novel heterozygous sequence variant of c.490delA p.(Arg164GlyTer13) in exon 6, and in one family, a novel c.40delC p.(His14ThrsfTer41) mutation in exon 4 were identified.
The prevalence of specific organ lesions in BHDS differs in relation to the cohorts previously reported by a dermatologist, nephrologist and pulmonologist [1,7,16,18,30,31].
Pneumothorax was reported in 42–88% of pulmonary cohorts and in 23–38% of renal/dermatological cohorts. Recurrent pneumothorax was reported in 75–80% of BHDS patients [8,12], with an average of > 3 episodes [8]. Houweling et al. reported on a group of 115 Dutch patients from 35 families and found a history of pneumothorax in 28% of patients, which was recurrent in 80%, with a mean age of first pneumothorax of 36 years, similar to our group [32]. We want to emphasise the value of chest CT examination in the detection of pulmonary lesions, even in patients without symptoms. This allowed us to diagnose small asymptomatic pneumothoraces in 20% of the cases.
Similar to our group, Liu et al. observed pulmonary cystic lesions in all Chinese patients enrolled, with less frequent skin fibrofolliculomas (10%) and renal tumours (22%) [33]. Of 12 Korean patients, 100%, 18% and 9% presented with pulmonary, skin and renal manifestations of the disease, respectively [34].
In our group, renal cysts were diagnosed in three (20%) patients, and it has been suggested that this may be the only manifestation of the disease [7,9,17]. Generally, renal tumours are detected in > 30% of patients by the mean age of 50 years [6,17]. Of note, our patient developed bilateral clear cell renal cancer, at an earlier age, before the age of 50 years. Zabar et al. evaluated the risk of developing renal cancer in patients with BHDS in comparison with their unaffected family members and disclosed a seven-fold greater risk of kidney tumour, and 50-fold higher risk of pneumothorax [6]. In addition, Nahorski et al. found a higher risk of colon neoplasms in BHDS patients but van de Beek et al. analysed 399 Dutch patients with BHDS, and an increased risk of colon carcinoma was not reported [35,36]. However, in this study, patients with BHDS underwent removal of colon polyps more frequently, but the number of patients with polyps was similar to that of unaffected family members. Similar observations were presented by Zabar et al., who also did not find a predisposition to colon polyps in BHDS patients [6]. Thyroid nodules, lipomas, liver cysts and parotid oncocytomas have been reported in BHDS patients [6,35,36]. However, no previous data presented such a high prevalence of gynaecological tumours (ovarian cancer, leiomyoma uteri, endometriosis, ovarian cyst) as we found in our group of patients. This observation warrants future studies in larger patient populations.
Mutational analyses
There is a wide spectrum of FLCN gene mutations. The gene is located on chromosome 17p11.2 and consists of 14 exons, of which 11 are coding exons [9]. In our study, substitution and deletion mutations were identified, resulting in nonsense and frameshift protein changes, leading to loss of folliculin function. Structurally destabilised, truncated variants of the FLCN protein are degraded by a proteasome [37–39]. The FLCN gene has been implicated in the pathogenesis of BHDS and its role as a tumour suppressor gene is well established. Toro et al. revealed FLCN mutations in 81% (154/190) of patients and 85% (68/80) of BHDS families [12]. However, the lack of a detected mutation in the FLCN gene does not exclude the disease.
In another study, Toro et al. reported that in 17 (48%) patients and 47% of families, mutations were identified in exon 11, which suggested this region as a mutation ‘hot spot’ for BHDS [11]. Similar observations were reported by Nickerson et al., Nahorski et al., Furuyama et al., and Liu et al. [1,33,35,40]. Of note, in our group of BHDS patients, a mutation in exon 11 was found in only one family and detected mutations were most frequently located in exon 6. This may suggest the specific regional appearance of the mutational profile, but further study of a larger group is required. In addition, two novel mutations were found in three families. Furuya et al. investigated the mutation spectrum and clinicopathologic findings of 312 patients from 120 different families (119 Japanese and 1 Taiwanese) [19]. They identified 31 different FLCN sequence variants; almost all patients presented with lung lesions, 13% with renal lesions, and 49% with skin lesions.
Recently, a high number of new mutations was identified by Liu et al. in a Chinese population; of 20 mutations, 14 were novel [33].
A clear association between the disease phenotype and the type of mutation has not been established. Toro et al. noticed that mutations in exon 9 were associated with an increased number of cysts, and mutations in exon 9 and 12 with a higher number of pneumothoraces, but in our group, we did not observe such a correlation [12].
Knowledge about this rare disease and awareness of BHDS by pulmonary physicians are increasing. In Poland, the first case of BHDS was diagnosed in 2016; subsequently, 14 other cases have been identified [29,30]. We noticed a rather long mean diagnosis delay of approximately 20 years between first symptom appearance and the diagnosis of BHDS. None of our patients were referred due to suspicion of the disease. Four of these patients had been observed for many years at our institute, with a diagnosis of lymphangioleiomyomatosis, pulmonary Langerhans cell histiocytosis or emphysema. Most lung lesions had been diagnosed as emphysema, supported not only by an incorrect radiological assessment but also by histological assessment of the lung samples.
The size of the group is the most important limitation of this study. BHDS is a very rare disease, and the symptoms are often wrongly associated with other diseases. However, even in such a small group, two novel FLCN gene mutations have been discovered.
Future studies are needed to assess the prevalence of the disease in the Polish population and to clarify possible linkages to gynaecological tumours.