The KMT2 complex protein ASH2L controls meiosis through silencing transposon

Meiosis is precisely controlled by complex gene expression programs and involves epigenetic reprogramming and silencing of transposable elements. ASH2L (Absent, small or homeotic 2-like) is a core component of KMT2 complexes conferring H3K4 trimethylation, and plays key roles in mouse embryonic development, neurogenesis, and liver function. However, its role in meiosis remains elusive. Here, we demonstrate an essential role of Ash2l for meiosis. Using a germ cell-specific Ash2l knockout mouse model, we found that Ash2l deficiency leads to meiotic arrest and sterility in both sexes. Ash2l-deficient spermatocytes exhibit failures in chromosomal synapsis, blockades in meiotic recombination, and increased apoptosis. Mechanistically, we found that the Ash2l deficiency results in a global loss of H3K4me3 in promoter regions and significantly decreases expression of thousands of genes, including those for H3K9 di-methylation, DNA methylation and transposon repression, crucial for meiotic prophase I progression. Furthermore, we observed that Ash2l mutant spermatocytes display hypomethylation at transposon sites and ectopic expression of LINE1-ORF1P. Our findings thus reveal a previously unappreciated role of the ASH2L-dependent H3K4me3 modification and provide clues to the molecular mechanisms in epigenetic disorders underlying male infertility. We isolated A4 spermatogonia and leptotene/zygotene spermatocytes from wild-type and Ash2l-gKO mice, and conducted RNA-seq and H3K4me3 ChIP-seq analyses. Additionally, NOMe-seq was performed on zygotene spermatocytes.减数分裂受复杂基因表达程序精准调控，且涉及表观遗传重编程与转座元件沉默。ASH2L（Absent, small or homeotic 2-like）是介导H3K4三甲基化的KMT2复合物（组蛋白赖氨酸甲基转移酶2复合物）的核心组分，在小鼠胚胎发育、神经发生与肝功能调控中发挥关键作用。然而其在减数分裂中的功能仍未被阐明。本研究证实了Ash2l在减数分裂中的核心必需功能。借助生殖细胞特异性Ash2l敲除小鼠模型，本研究发现Ash2l缺失会导致雌雄个体均出现减数分裂阻滞与不育表型。Ash2l缺失的精母细胞表现出染色体联会缺陷、减数分裂重组受阻以及细胞凋亡水平升高的特征。机制层面，本研究发现Ash2l缺失会导致启动子区域全域丧失H3K4三甲基化（H3K4me3），并显著下调数千个基因的表达，其中包括参与H3K9二甲基化、DNA甲基化与转座子抑制的关键基因，而这些过程对减数分裂I期前期的进程至关重要。此外，本研究观察到Ash2l突变的精母细胞在转座子位点出现低甲基化，且出现长散在核元件1开放阅读框1蛋白（LINE1-ORF1P）的异位表达。综上，本研究揭示了此前未被认知的、依赖ASH2L的H3K4me3修饰的生物学功能，并为男性不育相关表观遗传紊乱的分子机制研究提供了全新线索。本研究从野生型与Ash2l生殖细胞特异性敲除（Ash2l-gKO）小鼠中分离得到A4型精原细胞以及细线期/偶线期精母细胞，并开展了RNA测序（RNA-seq）与H3K4me3染色质免疫共沉淀测序（ChIP-seq）分析。此外，本研究还对偶线期精母细胞开展了核小体定位与甲基化测序（NOMe-seq）。