ADAR2-Mediated Editing of miR-214 and miR-122 Precursor and Antisense RNA Transcripts in Liver Cancers

A growing list of microRNAs (miRNAs) show aberrant expression patterns in hepatocellular carcinoma (HCC), but the regulatory mechanisms largely remain unclear. RNA editing catalyzed by members of the adenosine deaminase acting on the RNA (ADAR) family could target the miRNA precursors and affect the biogenesis process. Therefore, we investigate whether RNA editing could be one mechanism contributing to the deregulation of specific miRNAs in HCC. By overexpression of individual ADARs in hepatoma cells, RNA editing on the precursors of 16 miRNAs frequently deregulated in HCC was screened by a sensitive high-resolution melting platform. The results identified RNA precursors of miR-214 and miR-122 as potential targets edited by ADAR2. A subset of HCC showing elevated ADAR2 verified the major editings identified in ARAR2 overexpressed hepatoma cells, either with A-to-I or U-to-C changes. The unusual U-to-C editing at specific residues was demonstrated as being attributed to the A-to-I editing on the RNA transcripts complementary to the pri-miRNAs. The editing event caused a decrease of the RNA transcript complementary to pri-miR-214, which led to the decrease of pri-miR-214 and miR-214 and resulted in the increased protein level of its novel target gene Rab15. In conclusion, the current study discovered ADAR2-mediated editing of the complementary antisense transcripts as a novel mechanism for regulating the biogenesis of specific miRNAs during hepatocarcinogenesis.越来越多的微小RNA（microRNAs, miRNAs）被发现在肝细胞癌（hepatocellular carcinoma, HCC）中存在异常表达模式，但其调控机制在很大程度上仍未阐明。由RNA腺苷脱氨酶（adenosine deaminase acting on RNA, ADAR）家族成员催化的RNA编辑，可靶向miRNA前体并影响其生物发生过程。为此，本研究探讨RNA编辑是否为导致HCC中特定miRNA失调的潜在机制之一。研究人员通过在肝癌细胞中过表达单个ADAR家族成员，利用高灵敏度的高分辨率熔解检测平台，对16种在HCC中常发生失调的miRNA前体的RNA编辑情况进行了筛选。结果鉴定出miR-214与miR-122的RNA前体为ADAR2编辑的潜在靶点。在ADAR2表达升高的部分HCC样本中，研究人员验证了ADAR2过表达肝癌细胞中所发现的主要编辑事件，包括A-to-I与U-to-C两类编辑类型。特定位点上罕见的U-to-C编辑，被证实源于与初级miRNA（pri-miRNAs）互补的RNA转录本所发生的A-to-I编辑。该编辑事件可降低与pri-miR-214互补的RNA转录本水平，进而导致pri-miR-214及成熟miR-214的表达量下调，并最终使其新型靶基因Rab15的蛋白水平升高。综上，本研究发现ADAR2介导的反义转录本编辑是肝癌发生过程中调控特定miRNA生物发生的全新机制。