Detection of Folliculin Gene Mutations in Two Chinese Families with Birt-Hogg-Dube Syndrome

Abstract

Birt-Hogg-Dube syndrome (BHD, OMIM#135150) is a rare disease in clinic; it is characterized by skin fibrofolliculomas, pulmonary cysts with an increased risk of recurrent pneumothorax, renal cysts, and renal neoplasms. Previous studies have demonstrated that variants in folliculin (FLCN, NM_144997) are mainly responsible for this disease. In this research, we enrolled two BHD families and applied direct sequencing of FLCN to explore the genetic lesions in them. Two FLCN mutations were identified: one is a novel deletion variant (c.668delA/p.N223TfsX19), while the other is a previously reported insertion mutation (c.1579_1580insA/p.R527QfsX75). And the pathogenicity of both variants was confirmed by cosegregation assay. Bioinformatics analysis showed that c.668delA may lead to functional haploinsufficiency of FLCN because mRNA carrying this mutation exhibits a faster degradation rate comparing to the wild type. Real-time qPCR also confirmed that the mRNA level of FLCN expression in the proband was decreased significantly compared with the controls, which may disrupt the mTOR pathway and lead to BHD. The insertion mutation (c.1579_1580insA) was predicted to cause a prolonged amino acid sequence of FLCN. The present identification of two mutations not only further supports the important role of tumor suppressor FLCN in BHD and primary spontaneous pneumothorax, but also expands the spectrum of FLCN mutations and will provide insight into genetic diagnosis and counseling of families with BHD.

Figure 1

The clinic and genetic data of Family 1. (a) Pedigree of Family 1 affected with BHD. Squares indicate male family members; circles, female members; N/N normal type; N/M mutation type; arrow, proband. (b) Lung CT testing result of proband in Family 1. Red arrows indicate multiple pulmonary cysts. (c) Kidney B ultrasonic testing results of proband in Family 1. The low echoic area indicates multiple renal cysts. (d) Skin eruptions in the posterior neck of proband in Family 1. (e) Sequencing results of the FLCN mutation in Family 1. Sequence chromatogram indicates an A insertion in nucleotide 1579.

Figure 2

The clinic and genetic data of Family 2. (a) Pedigree of Family 2 affected with BHD. Squares indicate male family members; circles, female members; N/N normal type; N/M mutation type; arrow, proband. Pathological image of the renal mass. (b) Lung CT testing result of proband in Family 2. Red arrows indicate multiple pulmonary cysts. (c) Pathological image of the lung tissue. (d) Kidney contrast-enhanced CT testing result of proband in Family 2. Green arrows indicate renal tumor (right) or renal cyst (left). (e) Pathological image of the renal tumor tissue. (f) Sequencing results of the FLCN mutation in Family 2. Sequence chromatogram indicates an A deletion in nucleotide 668.

Figure 3

RNA expression of FLCN in affected individual and controls. Mean expression (±SEM) of FLCN in affected individual and control measured by real-time qPCR.