Genetics defects in GNRH1: a paradigm of hypothalamic congenital gonadotropin deficiency

Gonadotropin-releasing hormone (GnRH) is a crucial neurohormone regulating puberty and reproduction in non primate mammals, monkeys and humans. This neuropeptide is released in synchronized pulses from the nerve endings of several hundred GnRH neurons into the hypophyseal portal system to stimulate the biosynthesis and secretion of LH and FSH from pituitary gonadotrophs. These two dimeric glycoprotein hormones in turn travel through the circulation to the gonads, stimulating the synthesis and secretion of male and female sex steroid hormones and gametogenesis in both genders. Congenital hypogonadotropic hypogonadism (CHH) is a clinical syndrome that results from a decrease in gonadotropin secretion and is characterized by a complete or partial lack of pubertal development. CHH is caused mainly by defective GnRH production or release by the hypothalamus or by defective GnRH-receptor function in the pituitary. The syndromes of GnRH deficiency, either with anosmia or hyposmia, termed Kallmann Syndrome (KS), or with a normal sense of smell, called normosmic non syndromic CHH, (nCHH) are important disease models that have revealed respectively much about the developmental and functional abnormalities that can befall the GnRH neurons. For more than three decades, GNRH1 was an obvious candidate gene for nCHH in human beings. Indeed, in the natural hpg mouse model discovered in 1977, the hypogonadotropic hypogonadal phenotype, secondary to a hypothalamic GnRH deficiency, was caused by a homozygous deletion within the ortholog Gnrh1 gene. In 2009, homozygous inactivating mutations in the GNRH1 gene causing hypothalamic nCHH patients were at last reported in one female and two young male subjects, validating definitively the pivotal role of GnRH in human pubertal development and reproduction.