The protein encoded by this gene is a member of the fibroblast growth factor (FGF) family. FGF family members possess broad mitogenic and cell survival activities, and are involved in a variety of biological processes, including embryonic development, cell growth, morphogenesis, tissue repair, tumor growth and invasion. This protein was isolated as a secreted factor that exhibits a growth-stimulating effect on cultured glial cells. In nervous system, this protein is produced mainly by neurons and may be important for glial cell development. Expression of the mouse homolog of this gene was found to be dependent on Sonic hedgehog (Shh) signaling. Mice lacking the homolog gene displayed a male-to-female sex reversal phenotype, which suggested a role in testicular embryogenesis. This gene is involved in the patterning of sex determination, lung development, and skeletal development.
FGF9 has also been shown to play a vital role in male sex development. FGF9’s role in sex determination begins with its expression in the bi-potent gonads for both females and males. Once activated by SOX9, it is responsible for forming a feedforward loop with Sox9, increasing the levels of both genes. It forms a positive feedback loop upregulating SOX9, while simultaneously inactivating the female Wnt4 signaling pathway. The absence of Fgf9 causes an individual, even an individual with X and Y chromosomes, to develop into a female, as it is needed to carry out important masculinizing developmental functions such as the multiplication of Sertoli cells and creation of the testis cords.
In lung development, FGF9 is expressed in the mesothelium and pulmonary epithelium, where its purpose is to retain lung mesenchymal proliferation. Inactivation of FGF9 results in diminished epithelial branching. By the end of gestation, the lungs that are developed cannot sustain life and will result in a prenatal death.
Another biological role presented by this gene is its involvement in skeletal development and repair. FGF9 and FGF18 both stimulate chondrocyte proliferation. FGF9 heterozygous mutant mice had a compromised bone repair after an injury with less expression of VEGF and VEGFR2 and lower osteoclast recruitment. One disease associated with this gene is multiple synostoses syndrome (SYNS), a rare bone disease that has to do with the fusion of the fingers and toes. A missense mutation in the second exon of the FGF9 gene, the S99N mutation, seems to be the third cause of SYNS. A mutation in Noggin (NOG) and the Growth Differentiation Factor 5 (GDF5) are the other two causes of SYNS. The S99N mutation results in cell signaling irregularities that interfere with chondrogenesis and osteogenesis causing the fusion of the joints during development.
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