Nonsense mutations in FGF8 gene causing different degrees of human gonadotropin-releasing deficiency

Abstract

Context: FGFR1 mutations cause isolated hypogonadotropic hypogonadism (IHH) with or without olfactory abnormalities, Kallmann syndrome, and normosmic IHH respectively. Recently, missense mutations in FGF8, a key ligand for fibroblast growth factor receptor (FGFR) 1 in the ontogenesis of GnRH, were identified in IHH patients, thus establishing FGF8 as a novel locus for human GnRH deficiency.

Objective: Our objective was to analyze the clinical, hormonal, and molecular findings of two familial IHH patients due to FGF8 gene mutations.

Methods and patients: The entire coding region of the FGF8 gene was amplified and sequenced in two well-phenotyped IHH probands and their relatives.

Results: Two unique heterozygous nonsense mutations in FGF8 (p.R127X and p.R129X) were identified in two unrelated IHH probands, which were absent in 150 control individuals. These two mutations, mapped to the core domain of FGF8, impact all four human FGF8 isoforms, and lead to the deletion of a large portion of the protein, generating nonfunctional FGF8 ligands. The p.R127X mutation was identified in an 18-yr-old Kallmann syndrome female. Her four affected siblings with normosmic IHH or delayed puberty also carried the p.R127X mutation. Additional developmental anomalies, including cleft lip and palate and neurosensorial deafness, were also present in this family. The p.R129X mutation was identified in a 30-yr-old man with familial normosmic IHH and severe GnRH deficiency.

Conclusions: We identified the first nonsense mutations in the FGF8 gene in familial IHH with variable degrees of GnRH deficiency and olfactory phenotypes, confirming that loss-of-function mutations in FGF8 cause human GnRH deficiency.

Figure 1

The p.R127X and p.R129X mutations in the FGF8 gene. Panel A, Pedigree of family 1, a female proband with Kallmann syndrome and her four siblings carrying the p.R127X FGF8 mutation. Two of them reported delayed puberty, whereas the other two had normosmic IHH. They also had other additional abnormalities such as bilateral neurosensorial deafness and cleft lip and palate. Panel B, Pedigree of family 2, a male proband with normosmic IHH who harbors the p.R129X mutation of the FGF8. Arrows indicate proband; squares are males, and circles are females. Panel C, FGF8 genomic and protein structures showing the localization of the novel (top) and the previously described (bottom) FGF8 mutations. The hatched areas indicate the 5′ and 3′ untranslated region. SP, Signal peptide. *, These mutations affected only the isoforms e and f of FG8 protein. Panel D, Analysis of the impact of the p.R127Xand p.R129X mutations on the function of FGF8 ligand based on the FGF8b:FGFR2c (2FDB Protein Data Bank) (16). D2 and D3 of FGFR2c are green and cyan, respectively. The mutations map to the start of β5 strand and are predicted to truncated FGF8 ligands lacking a large portion of the protein that is essential for receptor and HS binding. The expressed regions of the mutant proteins are shown in orange ribbons. The amino acid residues predicted to be absent in both FGF8 mutants are shown in gray. The N terminus and C terminus of FGFR and FGF8b are denoted by the letters N and C, respectively.