Most reported cases of BHD have been from Western countries, and reports of cases from Asia are rare, likely due to the lack of awareness and atypical manifestations of this disease in Asian patients. Identification of BHD patients is usually based on dermatologic signs. However, Kunogi et al.  found that among 30 Japanese BHD patients, only 6 (20.0%) had cutaneous lesions, 1 (3.3%) was histologically diagnosed with FFs, and 29 (96.7%) had pneumothorax. Further, Furuya and Nakatani  reported that 13 (28.8%) patients had FFs, 40 (88.9%) had pulmonary cysts, and 9 (20.0%) had RCC among 45 BHD patients from 19 Japanese families. In addition, Murakami et al.  compiled 62 BHD cases from Asia and found that FFs were detected in 17 (27.4%), pulmonary cysts in 49 (79.0%), and RCC in 11 (17.7%). In comparison, Toro et al.  found that among 51 BHD families in the United States, 46 (90.2%) had FFs, 45 (88.2%) had pulmonary cysts, and 25 (49.0%) had renal tumors. Kluger et al.  reported 22 patients from ten unrelated families with BHD in France; 18 (81.8%) patients had FFs, 16 (72.7%) had pulmonary cysts or a history of pneumothorax, and 10 (45.5%) had renal tumors. Therefore, the incidence of FFs may be lower among Asian BHD patients compared with the higher incidence of 80%–90% reported among patients from the United States and Europe, whereas the pulmonary cyst incidences are similar between patients from Asian and Western countries. FLCN mutation screening is recommended for patients with RCC and pulmonary cysts without cutaneous lesions, especially for those with a clear family history of RCC or pulmonary cysts/pneumothorax.
In this report, case 1 had two pathologic types (chromophobe and clear-cell) of RCC. BHD patients are each likely to have various types of RCC , especially those with tumors with composite oncocytoma or chromophobe RCC histology ; these pathologies include the known familial hereditary RCCs, such as von Hippel-Lindau (VHL) syndrome, BHD syndrome, hereditary papillary renal cell carcinoma (HPRCC), hereditary leiomyomatosis renal cell carcinoma (HLRCC), and tuberous sclerosis complex (TSC). Pavlovich et al.  summarized clinical data of 124 BHD patients with 84 resected renal tumors, of which 56 (67%) were hybrid oncocytic tumors, 19 (23%) were chromophobe RCC, 6 (7%) were clear-cell RCC, and 3 (4%) were renal oncocytoma. Urologists should thus be aware that the presence of chromophobe/oncocytic renal neoplasms, especially those that are bilateral or multifocal, may indicate a diagnosis of BHD syndrome, even if dermatologic lesions are absent .
Over 100 unique mutations in the coding region of FLCN have been identified. Most of these mutations are frameshift, nonsense, insertion/deletion, or splice site mutations, resulting in truncation and inactivation of the encoded protein folliculin . However, no clear genotype–phenotype associations between FLCN mutation type or location and skin, lung, or renal manifestations have been identified to date; only a significantly lower frequency of renal neoplasia has been reported in patients with a deleted cytosine in the exon 11 mutational “hot spot” compared with the patients with an inserted cytosine at this location [15, 16]. A definite diagnosis of BHD can only be made when a pathogenic germline FLCN mutation is detected. Detection of an FLCN gene mutation not only confirms the diagnosis in suspected patients but also enables the diagnosis of at-risk family members with this disease. Once RCC is detected in BHD patients, it can be closely monitored as long as the dominant lesion is less than 3 cm in diameter. Surgical resection is recommended when the solid lesion or solid portion of a mixed lesion exceeds the threshold diameter of 3 cm . For patients with bilateral renal tumors measuring greater than 3 cm, staged surgeries in two separate settings are recommended . The tumor growth rate and location are additional factors that should be considered [2, 11]. Recently, a kidney-specific knockout model was developed by disrupting mouse FLCN in proximal tubules . This model provides a valuable tool for further study of FLCN-deficient renal tumorigenesis and for tests of new drugs (such as mTOR inhibitors) for the treatment of BHD-associated renal tumors.
Here, we report two Chinese patients with RCC and spontaneous pneumothorax who were diagnosed with BHD syndrome. Both patients underwent FLCN gene mutation screening and were found to have two novel mutations (c.946-947delAG and c.770-772delCCT) that have not been reported previously. Importantly, skin FFs were absent in both patients. These findings demonstrate, particularly to urologists, the importance of performing FLCN gene mutation screening for patients with RCC and pulmonary cysts.