Pulmonary cysts and pneumothorax are key indicators of BHD syndrome. However, very little is known about the pathophysiology of these lung cysts. A recent Japanese study of 9 families has now shown that not only are BHD cysts different from other blebs and bullae in the lung, they are also associated with dysfunctional mTOR signalling (Furuya et al., 2012).
The authors noted that pulmonary cysts and pneumothoraces were the most frequent presentation of BHD syndrome in these families, followed by fibrofolliculomas and then kidney cancer. However, it is important to note that this finding may reflect an ascertainment bias (as 7 families were diagnosed due to pneumothoraces and only 2 from kidney cancer). Genetic testing was then conducted to determine the FLCN mutations present in this cohort, and it was noted that there were 5 heterozygous mutation patterns (of which 3 have been previously described only in Japanese patients so far).
Histopathological analysis showed that the BHD-associated lung cysts were lined with differentiated pneumocytes and had alveolus-like structures within them, both of which could help differentiate BHD syndrome from other cystic lung conditions. Immunohistochemical analysis using a rabbit polyclonal antibody against full-length FLCN showed that the epithelial cells lining the cysts expressed FLCN, leading the authors to suggest that it could be haploinsufficient in these tissues. However, the possibility of a dominant negative mutation has not been discussed. When compared to controls, these cells also weakly expressed phosphorylated mTOR (p-mTOR, which is an active form of the protein) and strongly expressed phosphorylated ribosomal protein S6 (p-S6, which is an indicator of mTOR activation). Consequently, could deranged mTOR signalling play a role in the development of BHD lung cysts?
Furuya et al. also examined kidney tissues from these patients and saw that the tumour-free regions had cysts and oncocytic nodules. Additionally, immunostaining (using the same rabbit polyclonal FLCN antibody described above) showed that FLCN was expressed in the normal-looking tubules, as well as the cyst epithelial cells. In contrast, there were numerous FLCN-negative cells in the tumour tissue. Similarly, full-length FLCN was also visible in the normal-looking kidney tissue by western blot using the rabbit polyclonal FLCN antibody, but was almost undetectable in the BHD tumour sample. However, the presence of truncated forms of FLCN cannot be ruled out.
Furthermore, a large increase in p-S6 was observed by western blot in the BHD kidney tumour when compared to controls. This result suggests that mTOR is activated in BHD kidney tumours, which adds to the discussion regarding the functional role of FLCN in mTOR signalling (see here for more information). Moreover, FLCN haploinsufficiency may also be associated with this signalling pathway in both lung and kidney cysts, which could play a role in the initial stages of tumourigenesis in BHD syndrome.
- Furuya M, Tanaka R, Koga S, Yatabe Y, Gotoda H, Takagi S, Hsu YH, Fujii T, Okada A, Kuroda N, Moritani S, Mizuno H, Nagashima Y, Nagahama K, Hiroshima K, Yoshino I, Nomura F, Aoki I, & Nakatani Y (2012). Pulmonary cysts of Birt-Hogg-Dubé syndrome: a clinicopathologic and immunohistochemical study of 9 families. The American journal of surgical pathology, 36 (4), 589-600 PMID: 22441547