The role of NSUN2 in tRNA derived small RNA biogenesis and nascent RNA silencing

所有生物体都是由通过细胞分裂繁殖的细胞组成的。定义每种细胞类型的遗传密码储存在DNA分子中。DNA包含称为基因的编码区。基因被转录成信使RNA（mRNA）分子，后者又被翻译成蛋白质，这一过程称为基因表达。重要的是，每个细胞都能产生正确水平的蛋白质，这是通过控制基因表达来实现的。不受控制的基因表达会导致癌症或细胞死亡。基因沉默是基因表达调控的重要机制之一。人类细胞中的基因沉默需要小RNA分子（siRNA）的存在，其导致细胞质中靶mRNA的识别和破坏。另一种类型的基因沉默是在DNA上建立特异性修饰，从而导致基因转录的抑制，也称为转录基因沉默（transformational gene silencing，TGS），导致基因沉默的途径称为RNA干扰（RNA interference，RNAi），这一重大发现于2006年获得诺贝尔奖。RNAi不仅作为一种重要的科学工具，而且作为一种新的治疗剂被广泛探索。随着第一个FDA批准的药物Patisiran，基于sRNA的治疗方法受到越来越多的关注，我们发现了与已知的PTGS或TGS非常不同的新的基因沉默途径。我们发现，对于蛋白质生产至关重要的tRNA分子可以折叠成各种不同的形状。这种非典型结构被Dicer识别，Dicer酶可以将它们切割成tRNA衍生的小RNA分子，称为tsRNA。tsRNA不仅是错误折叠的tRNA的降解产物，而且它们具有功能并抑制许多疾病驱动基因的表达。了解正确的tRNA结构（三叶草叶）和替代结构之间的平衡是如何调节的是非常重要的。我们认为修饰酶NSUN2可以决定tRNA的折叠能力，从而决定Dicer对tRNA的识别和切割。此外，我们假设对tsRNA的修饰可以直接影响新生RNA沉默的效率。这一提议在鉴定新型基于tsRNA的疗法的关键特征方面具有直接翻译潜力。

All organisms consist of cells that multiply through cell division. The genetic code, which defines each cell type, is storedwithin DNA molecules. DNA contains coding regions called genes. Genes are transcribed into messenger RNA (mRNA)molecules, which are in turn translated into proteins, a process called gene expression. It is important that each cell makesthe right levels of proteins, which is achieved by controlled gene expression. Uncontrolled gene expression can lead tocancer or cell death. One of the most important mechanisms for the regulation of gene expression is gene silencing. Genesilencing in human cells requires the presence of small RNA molecules (siRNA) that lead to recognition and destruction oftarget mRNA in the cytoplasm. Another type of gene silencing is the establishment of specific modifications on DNA, whichleads to inhibition of gene transcription, also called transcriptional gene silencing (TGS).Pathway that leads to gene silencing is called RNA interference (RNAi) and this great discovery was recognised by Nobelprize in 2006. RNAi was widely explored not only as an important scientific tool, but also as novel therapeutic agent. Withvery first FDA approved drug, Patisiran, sRNA-based therapeutics are now receiving more and more attention.We have discovered novel gene silencing pathway that is very distinct from well-known PTGS or TGS. We found that tRNAmolecules that are essential for protein production, can fold into various alternative shapes. Such non-canonical structuresare recognised by Dicer, enzyme that can cleave them into tRNA-derived small RNA molecules, called tsRNAs. tsRNAsare not just degradation products of misfolded tRNAs, but they are functional and suppress expression of many diseasedriving genes. It is very important to understand how is the balance between proper tRNA structures (clover leaf) andalternative structures regulated. We propose that modifying enzyme, NSUN2, can determine the foldingabilities of tRNAs and consequently their recognition and cleavage by Dicer. Furthermore, we hypothesise thatmodifications on tsRNAs can have direct impact on efficiency of nascent RNA silencing. This proposal has directtranslational potential in identifying key features of novel tsRNA-based therapeutics.