在真核生物中，重复的 DNA 序列通过组蛋白 H3 赖氨酸 9 三甲基化 (H3K9me3) 进行转录沉默。重复元件沉默的丧失会导致基因组不稳定和人类疾病，包括癌症和衰老1-3。尽管 H3K9me3 在异染色质沉默的建立和维持中的作用已被广泛研究4-6，但亲本 H3K9me3 在复制 DNA 链上的分配模式和机制尚不清楚。在这里，我们报道 H3K9me3 优先转移到复制叉的前导链上，这主要发生在长散布核元件 (LINE) 反转录转座子（也称为 LINE-1s 或 L1s）上，理论上，这些复制叉在复制时沿着正面方向转录叉运动。从机制上讲，人类沉默中枢 (HUSH) 复合物与前导链 DNA 聚合酶 Pol ε 相互作用，并有助于 H3K9me3 的不对称分离。缺乏 Pol ε 亚基（POLE3 和 POLE4）或 HUSH 复合物（MPP8 和 TASOR）的细胞表现出 H3K9me3 不对称性受损和 LINE 表达增加。在表达 H3K9me3 结合缺陷的 MPP8 突变体的细胞以及与 Pol ε 相互作用减少的 TASOR 突变体中也获得了类似的结果。这些结果揭示了一种意想不到的机制，即 HUSH 复合物与 Pol ε 一起发挥作用，促进正面 LINE 上的不对称 H3K9me3 分布，从而抑制其在 S 相中的表达。© 2023。作者，获得 Springer Nature Limited 的独家许可。

In eukaryotes, repetitive DNA sequences are transcriptionally silenced through histone H3 lysine 9 trimethylation (H3K9me3). Loss of silencing of the repeat elements leads to genome instability and human diseases, including cancer and ageing1-3. Although the role of H3K9me3 in the establishment and maintenance of heterochromatin silencing has been extensively studied4-6, the pattern and mechanism that underlie the partitioning of parental H3K9me3 at replicating DNA strands are unknown. Here we report that H3K9me3 is preferentially transferred onto the leading strands of replication forks, which occurs predominantly at long interspersed nuclear element (LINE) retrotransposons (also known as LINE-1s or L1s) that are theoretically transcribed in the head-on direction with replication fork movement. Mechanistically, the human silencing hub (HUSH) complex interacts with the leading-strand DNA polymerase Pol ε and contributes to the asymmetric segregation of H3K9me3. Cells deficient in Pol ε subunits (POLE3 and POLE4) or the HUSH complex (MPP8 and TASOR) show compromised H3K9me3 asymmetry and increased LINE expression. Similar results were obtained in cells expressing a MPP8 mutant defective in H3K9me3 binding and in TASOR mutants with reduced interactions with Pol ε. These results reveal an unexpected mechanism whereby the HUSH complex functions with Pol ε to promote asymmetric H3K9me3 distribution at head-on LINEs to suppress their expression in S phase.© 2023. The Author(s), under exclusive licence to Springer Nature Limited.