The complex genetics of Kallmann syndrome: KAL1, FGFR1, FGF8, PROKR2, PROK2, et al

Kallmann syndrome (KS) combines hypogonadotropic hypogonadism and anosmia. Anosmia is related to the absence or hypoplasia of the olfactory bulbs and tracts. Hypogonadism is due to gonadotropin-releasing hormone (GnRH) deficiency, which presumably results from a failure of the embryonic migration of neuroendocrine GnRH cells from the olfactory epithelium to the forebrain. This failure could be a consequence of the early degeneration of olfactory nerve and terminal nerve fibres, because the latter normally act as guiding cues for the migration of GnRH cells. Defects in GnRH cell fate specification, differentiation, axon elongation or axon targeting to the hypothalamus median eminence may, however, also contribute to GnRH deficiency, at least in some genetic forms of the disease. To date, five KS genes have been identified, namely, FGFR1, FGF8, PROKR2, PROK2, and KAL1. Mutations in these genes, however, account for barely 30% of all KS cases. Mutations in FGFR1, encoding fibroblast growth factor receptor 1, underlie an autosomal dominant form of the disease. Mutations in PROKR2 and PROK2, encoding prokineticin receptor-2 and prokineticin-2, have been found in heterozygous, homozygous or compound heterozygous states. These two genes are likely to be involved both in monogenic recessive and digenic or oligogenic KS transmission modes. Finally, KAL1, encoding the extracellular glycoprotein anosmin-1, is responsible for the X chromosome-linked form of the disease. It is believed that anosmin-1 acts as an enhancer of FGF signalling and perhaps of prokineticin signalling too.