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The Function of NELF and ESR1 in Puberty Development and FertilityReproductive disorders affect 10-15% of the population in the United States and result in debilitating social and economic issues. Reproductive dysfunction may be displayed as delayed puberty or infertility due to defects in the hypothalamic-pituitary gonadal (HPG) axis. This axis is controlled by gonadotropin releasing hormone (GnRH) through the release of GnRH peptide. An important critical barrier to understanding normal puberty is the lack of known specific genes that regulate the development, migration and function of hypothalamic GnRH neurons, which synthesize and secrete GnRH neuropeptide in a pulsatile pattern into the portal blood. Several human diseases occur as the result of impaired GnRH function. Idiopathic hypogonadotropic hypogonadism (IHH) patients present with decreased gonadotropins and sex steroids leading to permanent delayed of puberty and infertility. Kallmann syndrome (KS) combines IHH phenotypes in addition to anosmia/hyposmia as there is impairment in migration o f GnRH neurons and olfactory neurons/axons. To date, the molecular basis has only been identified for approximately 40% o f all IHH/KS patients. The purpose of this study was to understand the role and function o f genes, such as NELF (nasal embryonic LHRH factor), in pubertal development and fertility. NELF, a nuclear protein isolated from migratory GnRH neurons, has been shown to be involved in the pathogenesis of IHH/KS, and knockdown impairs GnRH neuron migration in vitro. The mechanism o f NELF’s role relevant to GnRH migration/signaling and its effects on pubertal development and fertility are unclear. NELF function has been studied in GnRH cell lines, but not in the whole organism. The overall goals of this study are to characterize the phenotypic features of /Ve/^knockout (KO) mice, classify the prevalence of the different variants of NELF reported, and the inheritance patterns of IHH/KS genes including NELF, and finally examine the phenotype and hormonal profile o f an estrogen receptor-a (ESR1) mutation in a woman. Methods: Following breeding of heterozygous mice to generate homozygous N elf knockout mice, pubertal onset was assessed by daily monitoring of weight, vaginal opening (females), and anogenital distance (males). In addition we determined fertility of the mice by doing a continuous 90 day breeding study. Immunocytochemical labeling o f GnRH neurons was performed and analyzed for migration and count; hematoxylin and eosin stains were performed to visualize gonadal morphology. Three additional studies were also performed: 1) NELF splice variants were studied in immortalized human and mouse GnRH neurons; 2) the prevalence of digenic disease in IHH/KS was determined when the 13 most common genes were sequenced in 48 patients; and 3) the first woman with an estrogen receptor-a (ESR1) mutation was characterized clinically and by in vitro analysis. Results: Female NelfK O mice had delayed puberty, small uteri, and a decrease in GnRH neuron distribution distance, but male KO mice had normal puberty and GnRH neuron distribution distance. Both male and female KO mice had decreased litter size. Furthermore, NELF variant 2 was the dominant variant in both mouse and human GnRH neurons. Both nuclear and non-nuclear splice variants were expressed at the RNA and protein levels. DNA sequencing of 13 genes in 48 IHH/KS patients demonstrated that -90% of the cases are monogenic. Finally, the ESR1 mutation in the female patient led to absent pubertal development as ESR1 signaling is profoundly defective.