Use of genetic models of idiopathic hypogonadotrophic hypogonadism in mice and men to understand the mechanisms of disease

Abstract

Mutations in the genes encoding the neuropeptides kisspeptin and neurokinin B, as well as their receptors, are associated with gonadotrophin-releasing hormone (GnRH) deficiency and a failure to initiate and/or progress through puberty. Although the total number of patients studied to date is small, mutations in the kisspeptin pathway appear to result in lifelong GnRH deficiency. Mice with mutations in kisspeptin and the kisspeptin receptor, Kiss1(-/-) and Kiss1r(-/-), respectively, appear to be phenocopies of the human with abnormal sexual maturation and infertility. In contrast, mutations in the neurokinin B pathway lead to a more variable adult reproductive phenotype, with a subset of hypogonadotrophic individuals demonstrating paradoxical recovery of reproductive function later in life. While 'reversal' remains poorly understood, the ability to recover reproductive function indicates that neurokinin B may play different roles in the initiation of sexual maturation compared with the maintenance of adult reproductive function. Mice with mutations in the gene encoding the neurokinin B receptor, Tacr3, have abnormal oestrous cycles and subfertility but, similar to their human counterparts, appear less severely affected than mice with kisspeptin deficiency. Further investigations into the interaction between the kisspeptin and neurokinin B pathways will reveal key insights into how GnRH neuronal modulation occurs at puberty and throughout reproductive life.

Figure 1

Kisspeptin and Neurokinin B Pathway Deficiencies and Resulting Phenotypes in Mice and Men. Neurokinin B (NKB) is hypothesized to regulate kisspeptin (Kiss1) release which in turn stimulates gonadotropin-releasing hormone (GnRH) release and normal reproductive function (left panel). It remains unclear at this time whether NKB might also directly regulate GnRH release (dotted line). NKB pathway deficiency (middle panel) is hypothesized to cause a decrease in Kiss1 and subsequently GnRH release resulting in idiopathic hypogonadotropic hypogonadism (IHH) with a subset of patients “reversing” later in life; NKB pathway deficient mice are subfertile. In contrast Kiss1 pathway deficiencies appear to result in a more dramatic decrease in GnRH release causing life-long IHH in humans and complete infertility in mice.