Involvement of a Novel Variant of FGFR1 Detected in an Adult Patient with Kallmann Syndrome in Regulation of Gonadal Steroidogenesis

Fibroblast growth factor receptor 1 (FGFR1), also known as KAL2, is a tyrosine kinase receptor, and variants of FGFR1 have been detected in patients with Kallmann syndrome (KS), which is a congenital developmental disorder characterized by central hypogonadism and anosmia. Herein, we report an adult case of KS with a novel variant of FGFR1. A middle-aged male was referred for a compression fracture of a lumbar vertebra. It was shown that he had severe osteoporosis, anosmia, gynecomastia, and a past history of operations for cryptorchidism. Endocrine workup using pituitary and gonadal stimulation tests revealed the presence of both primary and central hypogonadism. Genetic testing revealed a novel variant of FGFR1 (c.2197_2199dup, p.Met733dup). To identify the pathogenicity of the novel variant and the clinical significance for the gonads, we investigated the effects of the FGFR1 variant on the downstream signaling of FGFR1 and gonadal steroidogenesis by using human steroidogenic granulosa cells. It was revealed that the transfection of the variant gene significantly impaired FGFR1 signaling, detected through the downregulation of SPRY2, compared with that of the case of the forced expression of wild-type FGFR1, and that the existence of the variant gene apparently altered the expression of key steroidogenic factors, including StAR and aromatase, in the gonad. The results suggested that the novel variant of FGFR1 detected in the patient with KS was linked to the impairment of FGFR1 signaling, as well as the alteration of gonadal steroidogenesis, leading to the pathogenesis of latent primary hypogonadism.