H255Y and K508R missense mutations in tumour suppressor folliculin (FLCN) promote kidney cell proliferation

Germline H255Y and K508R missense mutationsin the folliculin(FLCN) gene have been identified in patients with bilateral multifocal (BMF) kidney tumoursand clinical manifestations of Birt-Hogg-Dube (BHD) syndrome, or with BMF kidney tumoursas the only manifestation; however, their impact on FLCN function remains to be determined. In order to determine if FLCN H255Y and K508R missense mutationspromote aberrant kidneycell proliferation leading to pathogenicity, we generated mouse models expressing these mutants using BAC recombineering technology and investigated their ability to rescue the multi-cystic phenotype of Flcn-deficient mouse kidneys. Flcn H255Y mutant transgene expression in kidney-targeted Flcn knockout mice did not rescue the multi-cystic kidneyphenotype. However, expression of the Flcn K508R mutant transgene partially, but not completely, abrogated the phenotype. Notably, expression of the Flcn K508R mutant transgene in heterozygous Flcn knockout mice resulted in development of multi-cystic kidneysand cardiac hypertrophy in some mice. These results demonstrate that both FLCN H255Y and K508R missense mutationspromote aberrant kidneycell proliferation, but to different degrees. Based on the phenotypes of our preclinical models, the FLCN H255Y mutant proteinhas lost it tumour suppressive function leading to the clinical manifestations of BHD, whereas the FLCN K508R mutant proteinmay have a dominant negative effect on the function of wild-type FLCN in regulating kidneycell proliferation and, therefore, act as an oncoprotein. These findings may provide mechanistic insight into the role of FLCN in regulating kidneycell proliferation and facilitate the development of novel therapeutics for FLCN-deficient kidney cancer.