Congenital hypogonadotropic hypogonadism (CHH) and its anosmia-associated form (Kallmann syndrome [KS]) are genetically heterogeneous. Among the >15 genes implicated in these conditions, mutations in FGF8 and FGFR1 account for ~12% of cases; notably, KAL1 and HS6ST1 are also involved in FGFR1 signaling and can be mutated in CHH. We therefore hypothesized that mutations in genes encoding a broader range of modulators of the FGFR1 pathway might contribute to the genetics of CHH as causal or modifier mutations. Thus, we aimed to (1) investigate whether CHH individuals harbor mutations in members of the so-called "FGF8 synexpression" group and (2) validate the ability of a bioinformatics algorithm on the basis of protein-protein interactome data (interactome-based affiliation scoring [IBAS]) to identify high-quality candidate genes. On the basis of sequence homology, expression, and structural and functional data, seven genes were selected and sequenced in 386 unrelated CHH individuals and 155 controls. Except for FGF18 and SPRY2, all other genes were found to be mutated in CHH individuals: FGF17 (n = 3 individuals), IL17RD (n = 8), DUSP6 (n = 5), SPRY4 (n = 14), and FLRT3 (n = 3). Independently, IBAS predicted FGF17 and IL17RD as the two top candidates in the entire proteome on the basis of a statistical test of their protein-protein interaction patterns to proteins known to be altered in CHH. Most of the FGF17 and IL17RD mutations altered protein function in vitro. IL17RD mutations were found only in KS individuals and were strongly linked to hearing loss (6/8 individuals). Mutations in genes encoding components of the FGF pathway are associated with complex modes of CHH inheritance and act primarily as contributors to an oligogenic genetic architecture underlying CHH.

Mesenchymal stem cells (MSCs)3 have the potential to differentiate into different lineages, including osteoblasts, chondroblasts, and adipocytes (1-7). The osteoblast differentiation program of MSCs is characterized by cell recruitment, which is followed by timely expressed genes including Runx2, alkaline phosphatase (ALP), type I collagen (ColA1), and osteocalcin (OC), which is associated with extracellular matrix mineralization (8 -10). The program of MSC osteogenic differentiation can be induced by soluble molecules such as bone morphogenetic proteins (BMPs) or Wnt proteins that activate several signaling pathways to trigger osteoblast differentiation (11-15). Although various downstream signals are known to promote osteogenic differentiation (16 -20), the molecular mechanisms that control the early stages of MSC osteoblast differentiation are not fully elucidated.Fibroblast growth factors (FGFs) play an important major role in the control of cell proliferation, differentiation, and survival in several tissues including bone (21-24). Notably, FGF2 was found to promote cell growth and osteoblast differentiation in bone marrow-derived mesenchymal cells (25,26). Consistent with an important role of FGF signaling in the control of osteoprogenitor cells, deletion of FGF2 in mice results in decreased bone marrow stromal cell osteogenic differentiation and altered bone formation (27). The actions of FGFs are highly dependent on high affinity FGF receptors (FGFRs) (28). FGF binding to FGFRs leads to receptor dimerization and phosphorylation of intrinsic tyrosine residues, which leads to activation of several signal transduction pathways including phospholipase C␥ (PLC␥), mitogen-activated protein kinases (MAPK), and phosphatidylinositol 3-kinase (PI3K) (29,30). In bone, activation of extracellular-related kinase (ERK1/2) MAPK and protein kinase C (PKC) was found to enhance osteoblast gene expression (31, 32). The important role of FGFR signaling in bone formation is highlighted by the finding that gain-of-function mutations in FGFRs results in premature cranial osteogenesis (33, 34). FGFR1 was recently shown to be an important transducer of FGF signals in proliferating osteoblasts (35). In contrast, activated FGFR2 was shown to enhance osteoblast differentiation in Apert syndromic craniosynostosis (36 -41). However, the role of FGFR2 signaling in osteogenic differentiation of mesenchymal stem cells is yet to be elucidated.In the present study, we investigated the specific role of FGFR2 signaling on osteoblast commitment and differentiation