Increased phosphorylation and dimethylation of XY body histones in theHr6b-knockout mouse is associated with derepression of the X chromosome

Abstract

Mono-ubiquitylated H2A marks the transcriptionally silenced XY body during male meiotic prophase. Concomitant with H2AK119ub1, the ubiquitin-conjugating enzyme HR6B is also enriched on the XY body. We analyzed H2A and H2B ubiquitylation in Hr6b-knockout mouse spermatocytes, but no global changes were detected. Next, we analyzed phosphorylation of the threonine residues T120 and T119 that are adjacent to the K119 and K120 target sites for ubiquitylation in H2A and H2B, respectively. In wild-type cells, H2AT120ph and H2BT119ph mark meiotically unpaired and silenced chromatin, including the XY body. In Hr6b-knockout spermatocytes, the H2BT119ph signal was unchanged, but H2AT120ph was enhanced from late pachytene until metaphase I. Furthermore, we found increased H3K4 dimethylation on the X and Y chromosomes of diplotene Hr6b-knockout spermatocytes, persisting into postmeiotic round spermatids. In these cells, the X and Y chromosomes maintained an unchanged H3K9m2 level, even when this modification was lost from centromeric heterochromatin. Analysis of gene expression showed derepression of X chromosome genes in postmeiotic Hr6b-knockout spermatids. We conclude that HR6B exerts control over different histone modifications in spermatocytes and spermatids, and that this function contributes to the postmeiotic maintenance of X chromosome silencing.

DNA double strand break repair, chromosome synapsis and transcriptional silencing in meiosis

“…Hr6b knockout male mice are infertile, associated with dysregulation of chromatin structure in meiotic and post-meiotic cells. 63,66,67 In late pachytene nuclei of these cells, the SCs are longer, and the number of MLH1 foci is 20% increased in comparison to controls, 63 but crossover interference is not affected. 68 It might be suggested that Hr6b knockout spermatocytes contain more spontaneous DSB sites that contribute to the increased crossover rates.…”

“…21 In Hr6b knockout spermatocytes, the overall levels of H2B ubiquitylation and H3K4 trimethylation are not changed. 67 However, the regulation of several other histone modifications is disturbed, 67 indicating that changes in chromosome structure may be involved in mediating the enhanced recombination frequency in Hr6b knockout spermatocytes. Recent data have shown that in, C. elegans, a species in which pairing precedes recombination, chromatin condensing complexes regulate meiotic DSB distribution, directly implicating higher order chromosome structure in the regulation of meiotic DSB formation and repair.…”

“…This transcriptionally silenced XY body chromatin is marked by several histone modifications and associated proteins. 67,[83][84][85] The most well known histone modification that marks the XY body from late zygotene onwards is γH2AX, the general marker of DSBs. 86 In mice that lack H2AX, the XY body is not formed.…”

Increased sex chromosome expression and epigenetic abnormalities in spermatids from male mice with Y chromosome deletions

“…polymerase II, and are enriched for several repressive chromatin marks present on the sex body, including histone H3 dimethylated at lysine 9 (H3K9me2) and CBX1 (chromobox protein homolog 1, previously known as HP1) (Baarends et al, 2007;Greaves et al, 2006;Khalil et al, 2004;Namekawa et al, 2006;Turner et al, 2006). However, the X and Y chromosomes are continually remodelled during the transition between meiosis and spermiogenesis, and histone modifications associated with transcriptionally active chromatin [e.g.…”

Flickin’ the ubiquitin switch

“…59 It remains unclear as to whether there is a requirement for HR6B-mediated ubiquitylation of H2B in this process, as Baarends et al detected H2Aub1, but not H2Bub1 during meiotic pachytene and in elongating spermatids. 60,61 An earlier study was able to detect H2Bub1 in elongating rat spermatids however, through the use of less stringent precipitation. 62 Downregulation of several autosomal genes was observed in the HR6B knockout, including many genes that regulate the cell cycle and embryonic development.…”