Epigenetic reprogramming enables the primordial germ cell-to-gonocyte transition (AbaSeq)

Gametes are highly specialised cells that can give rise to the next generation through their ability to generate a totipotent zygote. In mouse, germ cells are first specified in the developing embryo as primordial germ cells (PGCs) starting around embryonic day (E) 6.25. Following subsequent migration into the developing gonad, PGCs undergo a wave of extensive epigenetic reprogramming at E10.5/E11.5, including genome-wide loss of 5-methylcytosine (5mC). The underlying molecular mechanisms of this process have remained enigmatic leading to our inability to recapitulate this step of germline development in vitro. Using an integrative approach, we show that this complex reprogramming process involves the coordinated interplay between promoter sequence characteristics, DNA (de)methylation, Polycomb (PRC1) complex and both DNA demethylation-dependent and -independent functions of Tet1 to enable the activation of a critical set of germline reprogramming responsive (GRR) genes involved in gamete generation and meiosis. Our results also unexpectedly reveal a role for Tet1 in safeguarding but not driving DNA demethylation in gonadal PGCs. Collectively, our work uncovers a fundamental biological role for gonadal germline reprogramming and identifies the epigenetic principles of the PGC-to-gonocyte transition that will be instructive towards recapitulating complete gametogenesis in vitro. AbaSeq on PGCs between E10.5 and E12.5 to detect 5hmC; male and female samples were treated separately following the initiation of sex specification (i.e. from E12.5)

配子（gametes）是一类高度特化的细胞，可通过形成全能合子（totipotent zygote）的能力产生下一代。在小鼠中，生殖细胞最早于胚胎日（embryonic day, E）6.25左右在发育中的胚胎内被特化为原始生殖细胞（primordial germ cells, PGCs）。原始生殖细胞迁移至发育中的性腺后，于E10.5/E11.5发生一轮大规模的表观遗传重编程，包括全基因组范围内的5-甲基胞嘧啶（5-methylcytosine, 5mC）丢失。该过程的潜在分子机制长期不明，导致我们无法在体外重现生殖系发育的这一关键步骤。
本研究采用整合分析策略，证实这一复杂的重编程过程涉及启动子序列特征、DNA（去）甲基化、多梳蛋白（Polycomb, PRC1）复合物，以及Tet1的DNA去甲基化依赖与非依赖性功能之间的协同互作，从而激活一系列参与配子生成与减数分裂的生殖系重编程响应基因（germline reprogramming responsive, GRR）。本研究结果还意外揭示了Tet1在性腺原始生殖细胞中发挥守护而非驱动DNA去甲基化的功能。
综上，本研究阐明了性腺生殖系重编程的核心生物学作用，并明确了原始生殖细胞向性原细胞的表观遗传调控原则，这将为体外重现完整配子发生过程提供重要指导。本研究针对E10.5至E12.5的原始生殖细胞开展AbaSeq测序以检测5-羟甲基胞嘧啶（5hmC）；在性别特化启动后（即自E12.5起），分别对雄性与雌性样本进行独立处理。