Maintenance DNA methylation in pre-meiotic germ cells regulates meiotic prophase by facilitating homologous chromosome pairing

During spermatogenesis, mammalian spermatogonia undergo mitotic division, to maintain stem cell pool via self-renewal and generate differentiating progenitor cells for entry into meiotic prophase. During the perinatal stage, de novo DNA methylation occurring in pro-spermatogonia plays a key role to complete meiotic prophase and initiate meiotic division. In contrast, the role of the maintenance DNA methylation pathway for regulation of meiotic prophase, or meiotic division, in the adult is not well understood. Here, by using conditional mutants for Np95 (nuclear protein 95 kDa, also known as Uhrf1) or Dnmt1 [DNA (cytosine-5)-methyltransferase 1], two proteins that are essential for maintenance DNA methylation, we reveal that both NP95 and DNMT1 are co-expressed in spermatogonia and that these factors are necessary for meiosis in male germ cells. We found that Np95- or Dnmt1-deficient spermatocytes exhibited spermatogenic defects involving synaptic failure during meiotic prophase. In addition, assembly of pericentric heterochromatin clusters in early meiotic prophase, a phenomenon that is required for subsequent pairing of homologous chromosomes, is disrupted in Np95-deficient as well as Dnmt1-deficient spermatocytes. Based on these observations, we propose that DNA methylation established in pre-meiotic spermatogonia regulates synapsis of homologous chromosomes, and in turn quality control of male germ cells. Maintenance DNA methylation, therefore, plays a role to ensure faithful transmission of both genetic and epigenetic information to offspring. Examination of mRNA expression pattern of control or Np95 knocked out spermatocytes

在精子发生过程中，哺乳动物精原细胞会经历有丝分裂，通过自我更新维持干细胞库，并产生分化祖细胞以进入减数分裂前期。围产期阶段，前精原细胞中发生的从头DNA甲基化，对于完成减数分裂前期并启动减数分裂发挥关键作用。与之相对，成年个体中维持性DNA甲基化通路对减数分裂前期或减数分裂的调控机制，目前尚未完全阐明。本研究借助针对维持性DNA甲基化所必需的两种蛋白——核蛋白95kDa（nuclear protein 95 kDa，又名Uhrf1，下称Np95）与DNA（胞嘧啶-5）-甲基转移酶1（DNA (cytosine-5)-methyltransferase 1，下称Dnmt1）——构建的条件性敲除突变体，证实NP95与DNMT1均在精原细胞中共表达，且二者对雄性生殖细胞的减数分裂过程必不可少。研究发现，缺失Np95或Dnmt1的精母细胞会出现减数分裂前期联会失败相关的生精缺陷。此外，减数分裂早期着丝粒异染色质簇的组装——这一过程是后续同源染色体配对的必要前提——在Np95缺失及Dnmt1缺失的精母细胞中均受到破坏。基于上述观察结果，我们提出：在减数分裂前精原细胞中建立的DNA甲基化，可调控同源染色体联会，进而参与雄性生殖细胞的质量管控。因此，维持性DNA甲基化对于将遗传与表观遗传信息精准传递给子代具有重要意义。本研究还对对照组及Np95敲除精母细胞的mRNA表达模式展开了分析。