We observed that the advent of puberty is associated with increased promoter methylation of both Cbx7 and Eed (Fig. 3c), and that this change was prevented by Aza (Fig. 3c). The regions examined for changes in methylation are shown in Supplementary Fig. 4. The increase in DNA methylation did not result from changes in circulating estradiol levels, because it was distinct at LJ, when plasma estradiol levels are still low, and remained unabated on the day of the first preovulatory surge of gonadotropins (Fig. 3c), when plasma estradiol levels are massively increased (Fig. 3b). Altogether, these results suggest that the onset of female puberty is accompanied by active epigenetic repression of the PcG silencing system in the MBH of female rats.
