Indeed, FGF8 and FGFR1 deficiency severely compromises vertebrate reproduction in mice and humans and is associated with Kallmann Syndrome (KS), a congenital disease characterized by hypogonadotropic hypogonadism associated with anosmia.
In Kallmann syndrome (KS), according to the presence of certain accompanying clinical features, genetic screening for particular gene(s) may be prioritized: synkinesia (KAL1), dental agenesis (FGF8/FGFR1), bony anomalies (FGF8/FGFR1), and hearing loss (CHD7, SOX10).
Both our patients had additional bilateral cryptorchidism, a key phenotypic feature in males with FGF8 associated Kallmann syndrome.Each mutation was paternally inherited.
To date, five genes for KS have been identified: KAL1, located on the X chromosome, and FGFR1, PROKR2, PROK2 and FGF8, which are involved in autosomally transmitted forms of KS.
Finally, recent additional observations, notably using animal models, but also the description of human mutations in a specific ligand of FGFR1, FGF8, are beginning to shed light on the pathogenesis of GnRH deficiency in general, not just KS.
Substantial variation in clinical expression, from complete anosmia and hypogonadotropic hypogonadism to delayed puberty and normosmia, of the same Kallmann syndrome gene defects including in newer ones (FGF8 and CHD7) continues to be repeatedly observed.