Analysis of Ovaries from FMR1 CGG Repeat Mice

Research Summary: National Fragile X Foundation – Translational Research Grant
Renate Hukema PhD – $50,000
Erasmus MC, The Netherlands

SPECIFIC AIMS: The CGG repeat sequence located in the 5’ untranslated region (UTR) of the Fmr1 gene leads to three major clinical phenotypes: 1) Fragile X syndrome (FXS), 2) ovarian insufficiency (FXPOI), and 3) Fragile X-associated tremor/ataxia syndrome (FXTAS). The expression of each phenotype depends on the size of the repeat expansions and the consequent molecular outcome. In this project, we will focus on premutation associated ovarian insufficiency. Using the molecular mechanism established for FXTAS as a precedent, we hypothesize that reduction in the ovarian reserve is due to the accumulation of the toxic effect of the CGGs in the FMR1 transcript (rCGGs), perhaps leading to increased follicular artresia and/or oocyte degeneration, resulting in a faster depletion of the pool. Elevated levels of mRNA are already present during embryonic life and this might influence the pool of primordial follicles as well.

We will use a direct approach to test these hypotheses using an established premutation (PM) mouse model. This specific PM knock-in (KI) mouse recapitulates the mutation in humans: 1) the CGG repeat sequence in unstable during transmission, 2) it leads to increased transcript levels and decreased FMRP levels, and 3) it models the (neuro)pathology and phenotype measures related to FXTAS.

Our key objective is using the well defined PM KI mouse to conduct a study to examine the hypothesized toxic effect of the premutation on ovarian function. We will use state-of-the-art methods to characterize the follicle pool in these mice at different ages from the time of birth throughout the reproductive life. We will test for PM-associated inclusions in ovarian tissue, and, if present, determine if their numbers are associated with CGG repeat size and/or transcript levels.

At least three hypotheses that could explain FXPOI can be recognized:

  1. initial decrease in the primordial follicle pool.
  2. altered initial recruitment (of primordial follicles) or cyclic recruitment (of primary, secondary, and tertiary follicles).
  3. increased oocyte degeneration and follicular artresia.