Inactivation or non-reactivation: what accounts better for the silence of sex chromosomes during mammalian male meiosis?

Abstract

During the first meiotic prophase in male mammals, sex chromosomes undergo a program of transcriptional silencing called meiotic sex chromosome inactivation (MSCI). MSCI is triggered by accumulation of proteins like BRCA1, ATR, and γH2AX on unsynapsed chromosomes, followed by local changes on the sex chromatin, including histone modifications, incorporation of specific histone variants, non-histone proteins, and RNAs. It is generally thought that MSCI represents the transition of unsynapsed chromatin from a transcriptionally active state to a repressed state. However, transcription is generally low in the whole nucleus during the early stages of the first meiotic prophase, when markers of MSCI first appear, and is then reactivated globally during pachytene. Thus, an alternative possibility is that MSCI represents the targeted maintenance and/or reinforcement of a prior repressed state, i.e., a failure to reactivate. Here, we present an analysis of the temporal and spatial appearance of transcriptional and MSCI markers, as well as chromatin modifications related to transcriptional regulation. We show that levels of RNA pol II and histone H3 acetylated at lysine 9 (H3K9ac) are low during leptotene, zygotene, and early pachytene, but increase strongly in mid-pachytene, indicating that reactivation occurs with some delay after synapsis. However, while transcription markers appear abundantly on the autosomes at mid-pachytene, they are not directed to the sex chromosomes. Interestingly, we found that chromatin modifications related to transcriptional silencing and/or MSCI, namely, histone H3 trimethylated at lysine 9 (H3K9me3), histone H3 monomethylated at lysine 4 (H3K4me1), γH2AX, SUMO1, and XMR, appear on the sex chromosomes before autosomes become reactivated. These results suggest that the onset of MSCI during late zygotene and early pachytene may prevent sex chromosome reactivation during mid-pachytene instead of promoting inactivation de novo. Additionally, we found temporal differences between the X and Y chromosomes in the recruitment of DNA repair and MSCI markers, indicating a differential regulation of these processes. We propose that many of the meiotic defects attributed to failure to silence sex chromosomes could be interpreted as a more general process of transcriptional misregulation that occurs under certain pathological circumstances in zygotene and early pachytene.

Dynamic localization of SMC5/6 complex proteins during mammalian meiosis and mitosis implies functions in distinct chromosome processes

“…RNAs to the sex body (Page et al, 2012). However, an alternative possibility recently proposed assumes that MSCI represents the maintenance of a prior repressed state (Page et al, 2012).…”

Chromatin Organization and Remodeling of Interstitial Telomeric Sites During Meiosis in the Mongolian Gerbil (Meriones unguiculatus)

“…In mammals, the phosphorylated form of histone H2AX (gH2AX) is one of the characteristic responses to this damage, and thus it is used as a common marker of DNA damage during meiosis (Mahadevaiah et al 2001). Additionally, gH2AX is accumulated on sex chromosomes from pachytene up to metaphase I, where it functions in the meiotic silencing of sex chromosomes (Mahadevaiah et al 2001;Turner et al 2004;Page et al 2012).We followed the location of gH2AX during pachytene in combination with labeling of the centromeres to ascertain if the centromeres were specifically marked for DNA damage and repair during these stages (Figure 6, A-D). We found that during early pachytene gH2AX was abundantly distributed on the sex chromosomes, which showed a fuzzy appearance ( Figure 6A).…”

“…It is then possible that the particular dynamics that RAP1 and TRF1 exhibit during prophase I could be related to protection and integrity maintenance of the ITSs, especially from pachytene up to diplotene, when chromatin in autosomes becomes decondensed and transcriptionally active (Page et al 2012). This becomes particularly important since meiosis is a critical process for genome stability.…”

Escape of X-linked miRNA genes from meiotic sex chromosome inactivation

“…This post-translational modification of histone H2AX is believed to trigger a positive feedback loop between H2AX and MDC1 to promote the additional recruitment of epigenetic factors responsible for silencing the sex chromosomes during meiosis (FernandezCapetillo et al, 2003;Ichijima et al, 2011). It has been reported that molecular events leading to MSCI are initiated at the zygotene to pachytene transition during prophase I in spermatocytes, and that the actual gene silencing effects are first manifest at the early pachytene stage (Page et al, 2012;Royo et al, 2013).…”