The helicase domain of Dicer ensures strict processing of evolutionarily optimized microRNAs

Ribonuclease Dicer generates small RNAs for RNA interference (RNAi) and microRNA (miRNA) pathways. Mammalian Dicer produces miRNAs but its tripartite N-terminal helicase domain inhibits processing of double-stranded RNA for RNAi. Mouse oocytes express Dicer, which enhances RNAi because it lacks the helicase's proximal subdomain HEL1. Here we show that genetic removal of HEL1 in mice causes embryonic growth retardation, defects in the cardiopulmonary system, and perinatal lethality. HEL1 suppresses biogenesis of mirtrons, a non-canonical low-abundant class of miRNAs, and is required for high-fidelity cleavage and strand selection during biogenesis of canonical miRNAs. Essential is the HEL1 structure, not its helicase activity, because mutations of critical aminoacids do not affect viability or fertility and have minimal impact on miRNA biogenesis. Altogether, HEL1 is a critical structural component of Dicer in its role of a highly conserved structural "mold" that ensures high-fidelity processing and adaptive evolution of mammalian miRNA precursors. Bulk small RNA-seq of 11 mouse E15.5 embryo samples: - 3x WT - 5x DicerX homozygotes - 3x DicerX heterozygotes核糖核酸酶Dicer（Ribonuclease Dicer）可生成用于RNA干扰（RNA interference，RNAi）与微小RNA（microRNA，miRNA）通路的小RNA。哺乳动物来源的Dicer能够产生miRNA，但其三组分N端解旋酶结构域会抑制用于RNAi的双链RNA（double-stranded RNA）的加工过程。小鼠卵母细胞表达的Dicer由于缺失解旋酶的近端亚结构域HEL1，可增强RNAi效应。本研究证实，在小鼠中敲除HEL1会引发胚胎发育迟缓、心肺系统缺陷以及围产期致死。HEL1会抑制内含子源微小RNA（mirtrons）——一类非经典低丰度miRNA——的生物发生，同时在经典miRNA的生物发生过程中，对高保真切割与链选择是必需的。关键在于HEL1的结构而非其解旋酶活性：关键氨基酸突变不会影响小鼠的生存能力与生育能力，且对miRNA生物发生的影响极小。综上，HEL1作为Dicer的关键结构组分，扮演了高度保守的结构“模具”角色，可确保哺乳动物miRNA前体的高保真加工与适应性进化。本研究包含11份小鼠胚胎发育第15.5天（E15.5）胚胎的批量小RNA测序（Bulk small RNA-seq）样本： - 3份野生型（Wild Type，WT）样本 - 5份DicerX纯合子样本 - 3份DicerX杂合子样本