Haploinsufficiency of Sox2 causes fewer GnRH neurons and delayed puberty in mice

Mutations in the SOX2 gene have been previously linked to a syndromic form of isolated hypogonadotropic hypogonadism, with additional ocular and neurodevelopmental phenotypes. Recently, we reported a functional role for SOX2 in hypothalamic kisspeptin-expressing neurons and established a mechanistic relationship between SOX2 heterozygous variants and isolated hypogonadotropic hypogonadism. To further test the role of Sox2 in the hypothalamic-pituitary-gonadal axis, we generated mice with a whole-body heterozygous knockout of Sox2 (Sox2WT/KO). We found that heterozygous loss of Sox2 significantly delayed pubertal onset in both male and female Sox2WT/KO mice compared to wild-ype (WT) controls. In females, fertility was also compromised, with fewer estrous cycles and a significant delay in time to first litter of Sox2WT/KO dams compared to WT controls. Circulating levels of gonadotropins were normal in both male and female Sox2WT/KO mice, suggesting a functional pituitary. Finally, we assessed the number of kisspeptin and GnRH neurons and found that Sox2WT/KO mice do not differ from controls in the number of kisspeptin neurons but have significantly fewer GnRH neurons. This deficit occurs before birth, as by embryonic day 15.5, there are already fewer GnRH neurons in the Sox2WT/KO mice. Using luciferase assays, we determined that Sox2 increases expression of GnRH in vitro; thus, the decrease in GnRH-expressing neurons in vivo is likely the result of Sox2 haploinsufficiency. Together, these data further substantiate a critical role for SOX2 in the hypothalamic-pituitary-gonadal axis via effects on GnRH neuron development and, therefore, pubertal timing and reproductive function.