RNA editing mediates the functional switch of COPA in a novel mechanism of hepatocarcinogenesis.

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 (COPAvs. COPA) 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, COPA switches from a tumour-promoting gene to a tumour suppressor that has a dominant-negative effect. Moreover, COPA may undergo protein conformational change and therefore become less stable than COPA. Mechanistically, COPA 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 COPA and concurrent accumulation of tumour-promoting COPA in tumours; a rapid degradation of COPA 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.