NRF1-EZH2-DNMT1信号轴的发现及其在肿瘤表观遗传变异形成中的作用

NRF1、EZH2、DNMT1基因在人肿瘤组织中高表达，原因不明，相互关系不清。我们在前期研究中意外发现：在人类肿瘤细胞系等数据库中，NRF1基因与EZH2基因呈肿瘤特异性共转录，相关系数居转录因子之首；NRF1蛋白不仅可直接结合到EZH2启动子DNA上，而且改变NRF1基因表达能够导致EZH2基因转录水平和启动子活性同向变化；发现下调EZH2基因表达会导致细胞DNMT1蛋白含量明显下降和全基因组DNA低甲基化。据此我们提出NRF1是EZH2基因转录因子，NRF1-EZH2-DNMT1轴影响肿瘤细胞表观遗传特征形成的假说。拟对EZH2启动子DNA中NRF1结合序列进行鉴定，明确NRF1的辅助因子，确定NRF1基因敲除小鼠表型及其对肿瘤发生的影响。并且在转录和转录后修饰层面研究EZH2上调DNMT1基因表达的机制，验证NRF1表达上调是否能够触发肿瘤基因组表观遗传改变。

NRF1, EZH2, DNMT1 genes are overexpressed in human cancers. However, the exact reasons for their overexpression are far from clear. In our pilot study, we surprisingly found that the mRNA level of EZH2 gene is cancer-specifically and most significantly correlated with that of NRF1 gene among all the transcription factors in the human cancer cell line encyclopedia (CCLE), the cancer genome atlas (TCGA), and genotype-tissue expression (GTEx) databases. We also observed that NRF1 protein could directly bind to the EZH2 promoter DNA in ChIP-PCR analyses and that upregulation or downregulation of NRF1 expression could promote or repress the transcription and promoter activity of EZH2 gene. In addition, we discovered that downregulation of EZH2 expression resulted in significant decrease of DNMT1 expression and DNA hypomethylation of cancer genome. Based on these findings, we hypothesize that NRF1 is a transcription factor for EZH2 gene and the NRF1-EZH2-DNMT1 axis may play a crucial role in shaping the cancer epigenomes. Here, we plan to further characterize the NRF1 binding sequence within the EZH2 promoter and co-factors. The phenotype and exact effects of NRF1 gene on cancer development will be validated using gene knockout mice. Mechanisms for EZH2 to upregulate DNMT1 expression will also be systemically investigated in both transcriptional and posttranscriptional levels. Whether NRF1 overexpression triggers epigenetic changes in cancer genome will be experimentally confirmed.