Yangyang Song1, Omer An1, Xi Ren2, Tim Hon Man Chan3, Daryl Jin Tai Tay1, Sze Jing Tang1, Jian Han1, HuiQi Hong4, Vanessa Hui En Ng1, Xinyu Ke5, Haoqing Shen1, Priyankaa Pitcheshwar1, Jaymie Siqi Lin1, Ka Wai Leong1, Fernando Bellido Molias1, Henry Yang1, Dennis Kappei2, Leilei Chen6. 1. Cancer Science Institute of Singapore, National University of Singapore, Singapore 117599, Singapore. 2. Cancer Science Institute of Singapore, National University of Singapore, Singapore 117599, Singapore; Department of Biochemistry, Yong Loo Lin School of Medicine, National University of Singapore, Singapore 117596, Singapore. 3. Cancer Science Institute of Singapore, National University of Singapore, Singapore 117599, Singapore; Department of Laboratory Medicine, Molecular Diagnosis Centre, National University Health System, Singapore 119074, Singapore. 4. Cancer Science Institute of Singapore, National University of Singapore, Singapore 117599, Singapore; Department of Physiology, Yong Loo Lin School of Medicine, National University of Singapore, Singapore 117549, Singapore. 5. Cancer Science Institute of Singapore, National University of Singapore, Singapore 117599, Singapore; Department of Anatomy, Yong Loo Lin School of Medicine, National University of Singapore, Singapore 117594, Singapore. 6. Cancer Science Institute of Singapore, National University of Singapore, Singapore 117599, Singapore; Department of Anatomy, Yong Loo Lin School of Medicine, National University of Singapore, Singapore 117594, Singapore. Electronic address: polly_chen@nus.edu.sg.
Abstract
BACKGROUND & AIMS: RNA editing introduces nucleotide changes in RNA sequences. Recent studies have reported that aberrant adenosine-to-inosine RNA editing is implicated in cancers. Until now, very few functionally important protein-recoding editing targets have been discovered. Here, we investigated the role of a recently discovered protein-recoding editing target COPA (coatomer subunit α) in hepatocellular carcinoma (HCC). METHODS: Clinical implication of COPA editing was studied in a cohort of 125 HCC patients. CRISPR/Cas9-mediated knockout of the editing site complementary sequence (ECS) was used to delete edited COPA transcripts endogenously. COPA editing-mediated change in its transcript or protein stability was investigated upon actinomycin D or cycloheximide treatment, respectively. Functional difference in tumourigenesis between wild-type and edited COPA (COPAWTvs. COPAI164V) and the exact mechanisms were also studied in cell models and mice. RESULTS: ADAR2 binds to double-stranded RNA formed between edited exon 6 and the ECS at intron 6 of COPA pre-mRNA, causing an isoleucine-to-valine substitution at residue 164. Reduced editing of COPA is implicated in the pathogenesis of HCC, and more importantly, it may be involved in many cancer types. Upon editing, COPAWT switches from a tumour-promoting gene to a tumour suppressor that has a dominant-negative effect. Moreover, COPAI164V may undergo protein conformational change and therefore become less stable than COPAWT. Mechanistically, COPAI164V may deactivate the PI3K/AKT/mTOR pathway through downregulation of caveolin-1 (CAV1). CONCLUSIONS: We uncover an RNA editing-associated mechanism of hepatocarcinogenesis by which downregulation of ADAR2 caused the loss of tumour suppressive COPAI164V and concurrent accumulation of tumour-promoting COPAWT in tumours; a rapid degradation of COPAI164V protein and hyper-activation of the PI3K/AKT/mTOR pathway further promote tumourigenesis. LAY SUMMARY: RNA editing is a process in which RNA is changed after it is made from DNA, resulting in an altered gene product. In this study, we found that RNA editing of a gene known as coatomer subunit α (COPA) is lower in tumour samples and discovered that this editing process changes COPA protein from a tumour-promoting form to a tumour-suppressive form. Loss of the edited COPA promotes the development of liver cancer.
BACKGROUND & AIMS: RNA editing introduces nucleotide changes in RNA sequences. Recent studies have reported that aberrant adenosine-to-inosine RNA editing is implicated in cancers. Until now, very few functionally important protein-recoding editing targets have been discovered. Here, we investigated the role of a recently discovered protein-recoding editing target COPA (coatomer subunit α) in hepatocellular carcinoma (HCC). METHODS: Clinical implication of COPA editing was studied in a cohort of 125 HCC patients. CRISPR/Cas9-mediated knockout of the editing site complementary sequence (ECS) was used to delete edited COPA transcripts endogenously. COPA editing-mediated change in its transcript or protein stability was investigated upon actinomycin D or cycloheximide treatment, respectively. Functional difference in tumourigenesis between wild-type and edited COPA (COPAWTvs. COPAI164V) and the exact mechanisms were also studied in cell models and mice. RESULTS: ADAR2 binds to double-stranded RNA formed between edited exon 6 and the ECS at intron 6 of COPA pre-mRNA, causing an isoleucine-to-valine substitution at residue 164. Reduced editing of COPA is implicated in the pathogenesis of HCC, and more importantly, it may be involved in many cancer types. Upon editing, COPAWT switches from a tumour-promoting gene to a tumour suppressor that has a dominant-negative effect. Moreover, COPAI164V may undergo protein conformational change and therefore become less stable than COPAWT. Mechanistically, COPAI164V may deactivate the PI3K/AKT/mTOR pathway through downregulation of caveolin-1 (CAV1). CONCLUSIONS: We uncover an RNA editing-associated mechanism of hepatocarcinogenesis by which downregulation of ADAR2 caused the loss of tumour suppressive COPAI164V and concurrent accumulation of tumour-promoting COPAWT in tumours; a rapid degradation of COPAI164V protein and hyper-activation of the PI3K/AKT/mTOR pathway further promote tumourigenesis. LAY SUMMARY: RNA editing is a process in which RNA is changed after it is made from DNA, resulting in an altered gene product. In this study, we found that RNA editing of a gene known as coatomer subunit α (COPA) is lower in tumour samples and discovered that this editing process changes COPA protein from a tumour-promoting form to a tumour-suppressive form. Loss of the edited COPA promotes the development of liver cancer.