Histone chaperone CAF-1 is essential for retrotransposon silencing by mediating histone H4K20me3 deposition in mouse preimplantation embryos. Mus musculus

Retrotransposons are widely spread in the mammalian genome and are usually silenced during development to avoid transposition-inducing mutations. But how they are repressed in embryos shortly before implantation remain to be identified, since the genome at this stage is globally hypomethylated. Here we show a histone chaperon, CAF-1, is responsible for retrotransposon silencing at the morula-blastocyst stages by depositing histone H4 lysine 20 trimethylation (H4K20me3). Knockdown of CAF-1 with a specific siRNA resulted in derepression of LINE-1, SINE-B2 and IAP associated with the decreased H4K20me3 level, and arrested embryonic development at the morula stage. The identical results were obtained with siRNAs against Suv420h1/2, H4K20 methyltransferases. Treatment with reverse transcriptase inhibitors rescued at least a part of these embryos. Thus, CAF-1 ensures the genomic integrity of preimplantation embryos by establishing repressive histone marks in the multiple retrotransposon classes. Overall design: Comparative gene expression analyses using P150 knockdown (P150KD) embryos at morula stage were performed by microarray. P150KD embryos were produced with the injection of P150 siRNA into 1-cell embryos. As controls, siControl embryos were produced by the injection of control siRNA. These embryos were subjected to gene expression microarray.

逆转录转座子（Retrotransposons）广泛分布于哺乳动物基因组中，通常在发育过程中被沉默以避免转座诱导突变的产生。但在着床前短时间的胚胎中，这类逆转录转座子的阻遏机制仍有待阐明——因为该阶段的基因组处于全基因组低甲基化状态。本研究证实，组蛋白伴侣（histone chaperon）CAF-1可通过沉积组蛋白H4赖氨酸20三甲基化（H4K20me3），在桑椹胚-囊胚阶段负责逆转录转座子的沉默。采用特异性siRNA敲低CAF-1后，LINE-1、SINE-B2与IAP出现去阻遏现象，该过程伴随H4K20me3水平降低，且使胚胎发育阻滞于桑椹胚阶段。针对H4K20甲基转移酶Suv420h1/2的siRNA亦可得到完全一致的实验结果。经逆转录酶抑制剂处理后，至少可挽救部分受影响的胚胎。综上，CAF-1通过在多类逆转录转座子家族中建立阻遏性组蛋白修饰，保障了着床前胚胎的基因组完整性。实验设计概述：采用基因芯片对桑椹胚阶段的P150敲低（P150KD）胚胎开展比较基因表达分析。P150KD胚胎通过向1细胞期胚胎注射P150 siRNA制备得到；对照组胚胎则通过注射对照siRNA（siControl）制备得到。所有胚胎均接受基因芯片表达检测。