Trithorax Group (TrxG)-Dependent Gene Transcription Is a Major Stimulatory Epigenetic Force Controlling the Timing of Female Puberty
Presentation Number: OR17-1
Date of Presentation: April 2nd, 2017
Carlos Alejandro Toro*, Hollis Wright, Carlos Francisco Aylwin, Sergio R Ojeda and Alejandro Lomniczi
Oregon Health & Science University/Oregon National Primate Research Center (OHSU/ONPRC), Beaverton, OR
We previously showed that an epigenetic mechanism of transcriptional repression underlies the process by which GnRH release is kept in check before the advent of female puberty. Our results identified the Polycomb group (PcG) of transcriptional silencers as a major contributor to this repressive mechanism and showed that the PcG complex operates in the arcuate nucleus (ARC) of the hypothalamus to modulate the activity of Kiss1, a gene required for puberty to occur. Our results also showed that as PcG proteins are evicted from the Kiss1 promoter at the initiation of puberty, the abundance of histone modifications catalyzed by (and associated with) TrxG proteins increases. These findings suggest that the pubertal activation of key puberty-inducing genes, such as Kiss1, depends on the balance that exists between the PcG and TrxG complexes, two fundamental forces of epigenetic regulation. The present study provides physiological and molecular evidence supporting this view. Double fluorescent in situ hybridization revealed that all key TrxG components, including the two COMPASS genes SET Domain Containing 1, Set1a and Set1b, and the four COMPASS-like Mixed-Lineage Leukemia genes Mll1, Mll2, Mll3 and Mll4 are expressed in ARC kisspeptin neurons. Using massively parallel sequencing and female rats as our animal model we singled out Mll1 among eighteen TrxG members as a major element of the TrxG complex exhibiting increased expression in the ARC before the onset of female puberty. A similar increase was observed in the hypothalamus of female rhesus monkeys entering puberty. Chromatin immunoprecipitation assays demonstrated that as puberty approaches recruitment of MLL1 to the Kiss1 and Tachykinin 3 (Tac3) promoters, and MLL3 to a Kiss1 distal enhancer increases. These changes were accompanied by increased di-and tri-methylation of histone 3 at lysine 4 (H3K4me2 and H3K4me3), two bona fide TrxG-dependent “activating” histone marks, at the Kiss1 and Tac3 promoters. Concomitantly, the functional status of Kiss1 enhancer changed from poised to active as evidenced by increased abundance of histone 3 acetylated at lysine 27 (H3K27ac), a hallmark of active enhancers, greater recruitment of the histone acetyltransferase p300, and eviction of both Embryonic Ectoderm Development (EED, a PcG protein required for PcG-dependent silencing) and the PcG-dependent repressive modification histone 3 trimethylated at lysine 27 (H3K27me3). Reducing Mll1 expression in the ARC of juvenile rats via lentiviral-mediated, targeted delivery of shRNAs, markedly delayed the age at vaginal opening and first ovulation, and disrupted estrous cyclicity. These results suggest that an epigenetic switch from PcG-mediated repression to TrxG-dependent activation is a key regulatory mechanism that, operating within the confines of the medial basal hypothalamus, controls the timing of mammalian puberty.
Nothing to Disclose: CAT, HW, CFA, SRO, AL