非吸烟肺癌是一个全球性的公共健康问题。小鼠自发性肺肿瘤的发生可作为一个人类非吸烟肺癌的动物模型。最近，我们首次发现了基因Xrn2的顺式调控变异与近交小鼠品系的自发性肺肿瘤发生有关，肺组织中的Xrn2高表达增加肺癌发生风险。Xrn2编码5'-3'外切核糖核酸酶2，其表达在多种类型的肿瘤中均是上调的。但是，Xrn2在自发性肺肿瘤的致癌分子机制尚不清楚。一个发表在Nature上的研究特别具有启发意义，他们首次观察到Xrn2在线虫中调控包括Let-7在内的成熟miRNA的降解，敲低Xrn2表达会增加线虫中Let-7水平。本项目研究假设是，Xrn2介导的miRNA-mRNA调控网络的失调是导致自发性肺癌发生的根源之一。主要内容是探索Xrn2促进自发性肺肿瘤发生的分子机制。完成改项目后，我们预期将鉴定多个由Xrn2介导的miRNA及其靶目标，为非吸烟肺癌诊断提供新的生物标记和抗癌治疗的新靶点。

Lung cancer in never smokers (LCINS) is a worldwide public health concern. In comparison to tobacco-driven carcinogenesis, lung carcinogenesis in never smokers may follow very different cellular and molecular pathways towards malignant transformation. However, research on lung cancer in never smokers has long been hindered by the difficulty in assembling large cohorts. We recently identified Xrn2 as a novel candidate gene affecting spontaneous lung cancer susceptibility in inbred mice. Xrn2 encodes 5'-3' exoribonuclease 2, part of a large family of evolutionarily conserved enzymes in all eukaryote systems with crucial functions in RNA metabolism and RNA interference. XRN2 mediates degradation of mature microRNAs (miRNAs) in the nematode Caenorhabditis elegans, affecting functional miRNA homeostasis in vivo. In particular, depletion of Xrn2 increases mature Let-7 level which regulates multiple cancer-associated pathways. The major objective of this project is to characterize the molecular mechanisms by which Xrn2 contributes to spontaneous lung tumorigenesis. Our central hypothesis is that dysregulation of Xrn2-mediated miRNA-mRNA regulatory networks lead to lung tumorigenesis. To test this hypothesis, we propose to accomplish the following aims. Aim 1 will determine if over-expression and/or knockdown of Xrn2 has a global effect on mature miRNA activity and levels in vitro. Aim 2 will identify and validate miRNA targets that are regulated by Xrn2. Aim 3 will determine if over-expression of Xrn2 affects tumor formation in conditional and inducible transgenic mice. Finally, we will integrate multiple levels of genomic and experimental data to be generated in this application to define functional modules associated with lung carcinogenesis in Aim 4. At the completion of this project, we will identify Xrn2-mediated miRNAs and their mRNA targets, and determine the molecular mechanisms of Xrn2 underlying spontaneous lung cancer. Such results are highly likely to provide new biomarkers for diagnosis and targets for anti-cancer therapy in addition to fundamentally advancing the knowledge of lung carcinogenesis in never smokers.

XRN2编码5’-3’外切核糖核酸酶2，与参与同源重组、RNA代谢和干扰的酵母dhp1基因同源。我们前期发现在很多类型肿瘤中XRN2的表达上调。本项目研究发现XRN2表达与肺癌预后相关，XRN2表达越高，肺癌病人的生存期越短。通过大量细胞学体外试验和小鼠体内试验，表明XRN2促进肺癌细胞的上皮-间质转化（EMT）和肿瘤细胞转移。通过小RNA测序、RIP和Northern blot等手段，发现XRN2通过不依赖于DICER加工途径，直接参与处理miRNA前体，促进包括miR-10a等成熟miRNA的生成。miR-10a通过调控其下游的与EMT相关的多个靶基因如HOXA1、MAP3K7、CTNNBIP1和TMPRSS4等，进而影响肺癌细胞转移。本项目揭示了XRN2介导的miRNA-mRNA共表达网络调控肺癌细胞转移的新分子机制，调控网络上主要节点有望成为的新的肺癌生物学标记和抗肿瘤治疗的新靶点。