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Objective: Hepatocellular carcinoma (HCC) is one of the most common malignant tumors.Annually,there are over 0.5 million new patients worldwide and more than half of the new cases occur in China.HCC is notorious for its insidious onset,high malignancy as well as poor prognosis,with most patients dying within 6 to 20 months after diagnosis.Similarly to other solid tumors,HCC development is believed to be a multistep process with accumulation of genetic and epigenetic alterations.RNA editing is one of epigenetic regulatory mechanisms.Its a post-transcriptional processing of RNA molecules,which contributes to the transcriptomic and proteomic plasticity.The most frequently observed RNA editing events in humans are A to I conversions,which are catalyzed by the adenosine deaminase acting on RNA (ADAR) family.Because of technical limitations,only a few RNA editing targets have been discovered currently.Even less is known about the causal relationship between RNA editing defect and cancer development.The recent advent of genomic tools,including RNA sequencing (RNA-Seq),has provided powerful methods to identify on a global level changes in transcriptomes and genomes.This study is going to screen cancer-related RNA editing candidates and explores their roles in HCC carcinogenesis.Methods: RNA editing targets was identified by an integrative RNA-Seq analysis on three pairs of HCC tissues and their matched nontumor tissue coun-terparts.Sanger sequencing and pyrosequencing were adopted to further detect the RNA editing level of AZIN1 (encoding antizyme inhibitor 1) in 181 liver cancer cases.Clinico-pathological variables associated with AZIN1 overediting were analyzed and its prognostic value was also evaluated.The mRNA expression levels of ADAR family members were detected and functional studies were carried out to elucidate ADAR1 s regulation of AZIN1 editing.In vitro and in vivo studies were utilized to characterize the functional phenotypes of wide-type AZIN1 as well as edited AZ1N1.Results: Transcriptome sequencing revealed that adenosine-to-inosine (A→I) RNA editing of AZIN1 is increased in HCC specimens.Clinically,overediting of AZIN1 was detected in about 45% of human HCC samples and the editing frequency was gradually increased during HCC pathogenesis from normal to adjacent non-tumor to clinically verified HCC.Also,the editing level of AZIN1 was associated with advanced liver cirrhosis,tumor recurrence and disease-free survival rate of HCC patients.A→I editing of AZIN1 transcripts,specifically regulated by ADAR1 (encoding adenosine deaminase acting on RNA-1),results in a serine-to-glyeine substitution at residue 367 of AZIN1,predicted to cause a conformational change and induced a cytoplasmic-to-nuclear translocation.Functional studies further demonstrated that RNA editing conferred gain-of-function phenotypes to edited AZIN1,which were manifested by augmented tumor-initiating potential and more aggressive behavior.Compared with wild-type AZ1N1 protein,the edited form has a stronger affinity to antizyme,and the resultant higher AZIN1 protein stability promotes cell proliferation through the neutralization of antizyme-mediated degradation of ornithine decarboxylase (ODC) and cyclin D1 (CCND1).Conclusion: A→I RNA editing of AZIN1 may be a potential driver in the pathogenesis of human cancers,particularly HCC,thus providing potential target for HCC diagnosis and therapy.