Dizygotic twin sisters with normosmic idiopathic hypogonadotropic hypogonadism caused by an FGFR1 gene variant.

Isolated hypogonadotropic hypogonadism (IHH) is a rare genetic disorder that is clinically and genetically heterogeneous. It is characterized by absent or incomplete pubertal development owing to an isolated defect in the production, secretion, or action of gonadotropin-releasing hormone. The incidence of IHH is estimated at 1:30,000 in males and 1:125,000 in females. Although the vast majority of IHH cases are sporadic, some X-linked recessive, autosomal dominant, and autosomal recessive modes of inheritance have been described. IHH can be classified into Kallmann syndrome with anosmia and normosmic IHH. Here, we report dizygotic twin sisters with normosmic IHH who showed short stature and absence of puberty as a result of a variant of the FGFR1 gene. They had a normal sense of smell, and brain magnetic resonance imaging (MRI) showed well-defined olfactory bulbs. The older sister and the twins' mother had cleft palate, while the younger sister did not. The mother had menarche at the age of 16 years after hormonal replacement owing to delayed puberty. Molecular analysis of the FGFR1 gene identified a missense variant c.874C>G (p.His292Asp) in the twins and their mother. Herein, we described the clinical heterogeneity observed in the 2 affected twins who carry an identical variant in the FGFR1 gene. Further studies of the effects of modifier genes and epigenetic factors on the expression of FGFR1, as well as the various clinical manifestations of its mutations, are warranted.