The folliculin-FNIP1 pathway deleted in human Birt-Hogg-Dubé syndrome is required for murine B-cell development

Abstract

Birt-Hogg-Dubé (BHD) syndrome is an autosomal dominant disorder characterized by cutaneous fibrofolliculomas, pulmonary cysts, and kidney malignancies. Affected individuals carry germ line mutations in folliculin (FLCN), a tumor suppressor gene that becomes biallelically inactivated in kidney tumors by second-hit mutations. Similar to other factors implicated in kidney cancer, FLCN has been shown to modulate activation of mammalian target of rapamycin (mTOR). However, its precise in vivo function is largely unknown because germ line deletion of Flcn results in early embryonic lethality in animal models. Here, we describe mice deficient in the newly characterized folliculin-interacting protein 1 (Fnip1). In contrast to Flcn, Fnip1 ؊/؊ mice develop normally, are not susceptible to kidney neoplasia, but display a striking pro-B cell block that is entirely independent of mTOR activity. We show that this developmental arrest results from rapid caspaseinduced pre-B cell death, and that a Bcl2 transgene reconstitutes mature B-cell populations, respectively. We also demonstrate that conditional deletion of Flcn recapitulates the pro-B cell arrest of Fnip1 ؊/؊ mice. Our studies thus demonstrate that the FLCN-FNIP complex deregulated in BHD syndrome is absolutely required for B-cell differentiation, and that it functions through both mTORdependent and independent pathways. (Blood. 2012;120(6):1254-1261) IntroductionHuman Birt-Hogg-Dubé (BHD) syndrome is an autosomal dominant hereditary cancer syndrome characterized by the development of cutaneous fibrofolliculoma, multiple pulmonary cysts, spontaneous pneumothorax, and increased risk of renal cell neoplasia. 1,2 Affected individuals carry germ line mutations in FLCN, a recently characterized gene that encodes a 64-kDa protein, folliculin. 2 In kidney tumors, the FLCN wild-type (WT) allele is deleted by somatic mutation or loss of heterozygosity, [3][4][5] supporting the idea that FLCN is a classic tumor suppressor gene that follows the Knudson 2-hit hypothesis. 6 FLCN lacks predicted functional domains or homology to known proteins; however, it is also highly conserved across species indicating that it might play an essential role in development. 2 In agreement with this notion, homozygous loss of Flcn in mice causes visceral endoderm defects and embryonic lethality at E5.5-E6.5. 5 Although its precise in vivo function is unknown, folliculin may regulate cellular energy sensing by controlling mTOR signaling. [7][8][9][10] For instance, loss of Flcn in mouse kidneys leads to cellular hyperproliferation and overactivation of mTORC1, mTORC2, and MAPK pathways, whereas rapamycin treatment partially rescues this phenotype. 5,[11][12][13][14] In this context, FLCN is not unlike the 6 additional genes that have so far been implicated in kidney cancer (VHL, MET, TSC1, TSC2, FH, and SDH), in that they all interact with metabolic pathways that respond to stress or nutrient stimulation (reviewed by Linehan et al 15 ). However, mounting evidence indicates that folliculin may also reg...

Mutation of Fnip1 is associated with B-cell deficiency, cardiomyopathy, and elevated AMPK activity

“…Our investigation of the hamel allele reinforces the nonredundant roles of FNIP1 in B-cell development (7,8), skeletal muscle composition (9), and cardiac function (6). We reveal that BCL2 overexpression can partially, but not entirely, rescue B-cell development in the absence of FNIP1, and we provide evidence that FNIP1 is an endogenous negative regulator of AMPK.…”

“…Fnip1 has been reported to play an essential role in B-cell development (7,8). Mouse FNIP1 consists of 1,165 aa and shares 91% amino acid identity with human FNIP1 and 49% identity with mouse FNIP2 (Fig.…”

“…In one report (9), FNIP1-deficient skeletal muscle exhibited enhanced phosphorylation of the catalytic α subunit of AMPK (at residue Thr172-a requirement for its activation) (9) but reduced phosphorylation in another (6). Similarly, mTOR activity was reported to be increased in B-cell precursors in one Fnip1 mutant (8) but normal in a second model (7). Although phosphorylation of mTOR or the S6 ribosomal protein (a downstream mediator of mTOR signaling) was consistently increased in renal carcinomas of BHD syndrome patients or Flcn knockout mice (15)(16)(17), this observation may reflect a direct effect of transformation rather than the predisposing mutation.…”

Glycogen: A must have storage to survive stressful emergencies

“…In accordance, heightened glycogen levels were also reported in the muscle tissues of muscle-specific Flcn KO mice as compared to the controls. 21,22 A dual role for glycogen…”

Metabolic regulator Fnip1 is crucial for iNKT lymphocyte development

“…Previous studies indicated that Fnip1 deficiency completely blocks B-cell development at the large pre-B-cell stage, whereas conventional αβ T cells develop normally (22,28). In this study, we extended our analyses of Fnip1 −/− mice by examining if Fnip1 is important for the development and function of γδ, NK, and iNKT cells and dendritic cells.…”