激活诱导脱氧核苷酸转移酶（AID）是通过类别转换重组（CSR）和体细胞高度变异（SHM）的免疫球蛋白（Ig）基因的印迹免疫记忆的必需酶。AID依赖性的拓扑异构酶1（Top1）减少促进了发生于Ig基因多样化的DNA断裂，而在AID诱导的Top1减少背后的机制仍不清楚。在这里，我们阐明了microRNA-Ago2复合物在AID依赖的Top1降低中的贡献。Ago2与Top1 3'UTR结合，具有两个AID依赖性Ago2结合位点区域（5'-和3'dABs）。B淋巴瘤细胞中的Top1 3'UTR敲除（3'UTRKO）导致IgH基因中的DNA断裂效率降低，伴随CSR和SHM频率降低。此外，3'UTRKO细胞中AID依赖性的Top1蛋白质减少和Ago2结合到Top1 mRNA均被下调。在蔗糖梯度的高度翻译分数中的Top1 mRNA以一种AID依赖性和Top1 3'UTR介导的方式减少，导致Top1蛋白合成减少。AID和Ago2均定位于mRNA结合蛋白的分数，并相互作用。此外，我们发现一些可能与Top1 mRNA中的5'-和3'dAB结合的候选miRNA。其中，miR-92a-3p敲除导致3'UTRKO细胞表现为野生型细胞，而不影响3'UTRKO细胞。综上所述，Ago2-miR-92a-3p复合物将以AID依赖的方式招募到Top1 3'UTR中，并在转录后降低Top1蛋白质合成。这些后果导致非B-DNA结构的增加，在Ig基因上增强Top1的DNA断裂并促进免疫记忆形成。

Activation-induced cytidine deaminase (AID) is the essential enzyme for imprinting immunological memory through class switch recombination (CSR) and somatic hypermutation (SHM) of the immunoglobulin (Ig) gene. AID-dependent reduction of Topoisomerase 1 (Top1) promotes DNA cleavage that occurs upon Ig gene diversification, whereas the mechanism behind AID-induced Top1 reduction remains unclear. Here, we clarified the contribution of the microRNA-Ago2 complex in AID-dependent Top1 decrease. Ago2 binds to Top1 3'UTR with two regions of AID-dependent Ago2-binding sites (5'- and 3'dABs). Top1 3'UTR knockout (3'UTRKO) in B lymphoma cells leads to decreases in DNA break efficiency in the IgH gene accompanied by a reduction in CSR and SHM frequencies. Furthermore, AID-dependent Top1 protein reduction and Ago2-binding to Top1 mRNA are down-regulated in 3'UTRKO cells. Top1 mRNA in the highly translated fractions of the sucrose gradient is decreased in an AID-dependent and Top1 3'UTR-mediated manner, resulting in a decrease in Top1 protein synthesis. Both AID and Ago2 localize in the mRNA-binding protein fractions and they interact with each other. Furthermore, we found some candidate miRNAs which possibly bind to 5'- and 3'dAB in Top1 mRNA. Among them, miR-92a-3p knockdown induces the phenotypes of 3'UTRKO cells to wild-type cells whereas it does not impact on 3'UTRKO cells. Taken together, the Ago2-miR-92a-3p complex will be recruited to Top1 3'UTR in an AID-dependent manner and posttranscriptionally reduces Top1 protein synthesis. These consequences cause the increase in a non-B-DNA structure, enhance DNA cleavage by Top1 in the Ig gene and contribute to immunological memory formation.