Overactive Reproductive Axis Due to Fragile X Gene Mutation

Abstract

Women carrying a pre-mutation or mutation of the Fragile X mental retardation gene (FMR1) comprise the largest portion of premature ovarian failure (POF) cases due to known genetic factors. FMR1 mutation causes Fragile X syndrome, the most common cause of inherited mental impairment. The mutation inhibits the expression of the fragile X mental retardation protein (FMRP), a ubiquitously expressed mRNA binding protein. The specific molecular mechanism(s) leading to premature ovarian failure in Fragile X carriers are not known. Here, we utilize the complete KO mouse model, to mimic the lack of FMRP in Fragile X mutations and analyze the hypothalamic-pituitary-gonadal axis to uncover causes of POF due to FMR1 mutation. Consistent with mutations in human population, KO females experience early cessation of reproductive function and stop having litters at 150 days of age, compared to controls that stop reproducing at 250 days of age. Since POF can be caused by either insufficient pool of primordial follicles or by increased recruitment in each cycle and early depletion, we analyzed ovaries at 3 weeks of age and determined that the FMR1 KO mice had the same number of primordial follicles when compared to the controls, suggesting that POF is not due to a deficit in primordial follicles. However, at 8 weeks of age, FMR1 KO ovaries had higher number of corpora lutea, and KO females had larger litters, indicating that FMR1 KO mice have more follicles recruited in each estrous cycle. FMR1 KO mice have higher FSH, which corresponds to the high FSH in women. Serum estradiol levels and inhibin b expression levels were unaffected by FMR1 mutation suggesting normally functioning negative feedback signals from the ovaries. Analyses of hypothalamic gene expression demonstrated elevated GnRH mRNA in KO mice. To further investigate alterations is hypothalamic protein levels, western blot analyses determined that FMR1 KO mice have higher levels of NMDAR1 and higher levels of GABAA receptor G2 subunit. Dual label immunofluorescence analyses revealed higher number of NMDAR1 and GABAA receptors specifically in GnRH neurons of FMR1 KO mice when compared to control, suggesting that GnRH neurons themselves are affected by FMR1 mutation. Given that both glutamate and GABA can activate GnRH neurons, alterations in the number of these receptors can potentially cause hyperactivity in the HPG axis at the hypothalamic level leading to elevated FSH and the subsequent POF. In summary, our results reveal a potential mechanism of premature ovarian failure in Fragile X mutation carriers.