Clinic Examination and Gene Diagnosis for a Birt–Hogg–Dubé Syndrome Family With a Novel flcn Frameshift Mutation Causing Nonsense‐Mediated mRNA Degradation

Background: Birt–Hogg–Dubé syndrome (BHD) was an autosomal dominant disorder caused by a mutation in the folliculin ( FLCN ) gene and characterized by benign cutaneous fibrofolliculomas in the head and neck, pulmonary cysts, spontaneous pneumothorax, and combined renal tumors. Methods: This study reported a familial case presenting multiple pulmonary bullae, recurrent spontaneous pneumothorax, diffuse cystic lesions in both lungs, and renal cysts. To further clarify the diagnosis, next‐generation sequencing (NGS) was performed in conjunction with the clinical diagnostic criteria for Birt–Hogg–Dubé. The eukaryotic recombinant expression vectors of pEGFP‐C1‐ FLCN and knock‐in FLCN mutation by CRISPR/Cas9 were conducted in 293 T and BEAS‐2B cell lines. The mRNA and protein expression of the FLCN mutation were verified by fluorescence quantitative PCR and Western blot assay. Nonsense‐mediated mRNA decay (NMD) assays and immunohistochemical assays were conducted to elucidate the pathogenicity of the mutation and explore potential mechanisms. Results: A unique, novel, unspecified significance FLCN mutation NM_144997.7: c.21_22del (p. Cys8 Profs ∗ 28) in Exon 4 was detected in both patients. The results demonstrated that the newly identified FLCN frameshift mutation significantly decreased FLCN mRNA and protein expression. The NMD complex recognized and degraded mRNAs containing a premature termination codon (PTC) in the open reading frame of the FLCN frameshift mutation, resulting in haploinsufficiency and ultimately contributing to the manifestation of BHD. Protein expression on the AMP‐activated protein kinase (AMPK), Wnt/ β ‐catenin, and mammalian target of rapamycin (mTOR) signaling pathways by immunohistochemistry indicated that FLCN frameshift mutations were responsible for BHD through the activation of AMPK, Wnt/ β ‐catenin, and mTOR signaling pathways. Conclusion: The study demonstrated that a novel FLCN frameshift mutation was responsible for the pathogenesis of BHD and preliminarily demonstrated that FLCN causes BHD through the AMPK, Wnt/ β ‐catenin, and mTOR signaling pathways.