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 (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.

ADARs act as potent regulators of circular transcriptome in cancer.

Circular RNAs (circRNAs) are produced by head-to-tail back-splicing which is mainly facilitated by base-pairing of reverse complementary matches (RCMs) in circRNA flanking introns. Adenosine deaminases acting on RNA (ADARs) are known to bind double-stranded RNAs for adenosine to inosine (A-to-I) RNA editing. Here we characterize ADARs as potent regulators of circular transcriptome by identifying over a thousand of circRNAs regulated by ADARs in a bidirectional manner through and beyond their editing function. We find that editing can stabilize or destabilize secondary structures formed between RCMs via correcting A:C mismatches to I(G)-C pairs or creating I(G).U wobble pairs, respectively. We provide experimental evidence that editing also favors the binding of RNA-binding proteins such as PTBP1 to regulate back-splicing. These ADARs-regulated circRNAs which are ubiquitously expressed in multiple types of cancers, demonstrate high functional relevance to cancer. Our findings support a hitherto unappreciated bidirectional regulation of circular transcriptome by ADARs and highlight the complexity of cross-talk in RNA processing and its contributions to tumorigenesis.

MiR-138 is a potent regulator of the heterogenous MYC transcript population in cancers.

3'UTR shortening in cancer has been shown to activate oncogenes, partly through the loss of microRNA-mediated repression. This suggests that many reported microRNA-oncogene target interactions may not be present in cancer cells. One of the most well-studied oncogenes is the transcription factor MYC, which is overexpressed in more than half of all cancers. MYC overexpression is not always accompanied by underlying genetic aberrations. In this study, we demonstrate that the MYC 3'UTR is shortened in colorectal cancer (CRC). Using unbiased computational and experimental approaches, we identify and validate microRNAs that target the MYC coding region. In particular, we show that miR-138 inhibits MYC expression and suppresses tumor growth of CRC and hepatocellular carcinoma (HCC) cell lines. Critically, the intravenous administration of miR-138 significantly impedes MYC-driven tumor growth in vivo. Taken together, our results highlight the previously uncharacterized shortening of the MYC 3'UTR in cancer, and identify miR-138 as a potent regulator of the heterogenous MYC transcript population.

Suppression of adenosine-to-inosine (A-to-I) RNA editome by death associated protein 3 (DAP3) promotes cancer progression.

RNA editing introduces nucleotide changes in RNA sequences. Recent studies have reported that aberrant A-to-I RNA editing profiles are implicated in cancers. Albeit changes in expression and activity of ADAR genes are thought to have been responsible for the dysregulated RNA editome in diseases, they are not always correlated, indicating the involvement of secondary regulators. Here, we uncover DAP3 as a potent repressor of editing and a strong oncogene in cancer. DAP3 mainly interacts with the deaminase domain of ADAR2 and represses editing via disrupting association of ADAR2 with its target transcripts. PDZD7, an exemplary DAP3-repressed editing target, undergoes a protein recoding editing at stop codon [Stop →Trp (W)]. Because of editing suppression by DAP3, the unedited PDZD7WT, which is more tumorigenic than edited PDZD7Stop518W, is accumulated in tumors. In sum, cancer cells may acquire malignant properties for their survival advantage through suppressing RNA editome by DAP3.

Cis- and trans-regulations of pre-mRNA splicing by RNA editing enzymes influence cancer development.

RNA editing and splicing are the two major processes that dynamically regulate human transcriptome diversity. Despite growing evidence of crosstalk between RNA editing enzymes (mainly ADAR1) and splicing machineries, detailed mechanistic explanations and their biological importance in diseases, such as cancer are still lacking. Herein, we identify approximately a hundred high-confidence splicing events altered by ADAR1 and/or ADAR2, and ADAR1 or ADAR2 protein can regulate cassette exons in both directions. We unravel a binding tendency of ADARs to dsRNAs that involves GA-rich sequences for editing and splicing regulation. ADAR1 edits an intronic splicing silencer, leading to recruitment of SRSF7 and repression of exon inclusion. We also present a mechanism through which ADAR2 binds to dsRNA formed between GA-rich sequences and polypyrimidine (Py)-tract and precludes access of U2AF65 to 3' splice site. Furthermore, we find these ADARs-regulated splicing changes per se influence tumorigenesis, not merely byproducts of ADARs editing and binding.

Hepatic Resection Following Selective Internal Radiation Therapy for Colorectal Cancer Metastases in the FOXFIRE Clinical Trial: Clinical Outcomes and Distribution of Microspheres.

The FOXFIRE (5-Fluorouracil, OXaliplatin and Folinic acid ± Interventional Radio-Embolisation) clinical trial combined systemic chemotherapy (OxMdG: Oxaliplatin, 5-fluorouracil and folic acid) with Selective Internal Radiation Therapy (SIRT or radio-embolisation) using yttrium-90 resin microspheres in the first-line management for liver-dominant metastatic colorectal cancer (CRC). We report clinical outcomes for patients having hepatic resection after this novel combination therapy and an exploratory analysis of histopathology. Multi-Disciplinary Teams deemed all patients inoperable before trial registration and reassessed them during protocol therapy. Proportions were compared using Chi-squared tests and survival using Cox models. FOXFIRE randomised 182 participants to chemotherapy alone and 182 to chemotherapy with SIRT. There was no statistically significant difference in the resection rate between groups: Chemotherapy alone was 18%, (n = 33); SIRT combination was 21% (n = 38) (p = 0.508). There was no statistically significant difference between groups in the rate of liver surgery, nor in survival from time of resection (hazard ratio (HR) = 1.55; 95% confidence interval (CI) = 0.83-2.89). In the subgroup studied for histopathology, microsphere density was highest at the tumour periphery. Patients treated with SIRT plus chemotherapy displayed lower values of viable tumour in comparison to those treated with chemotherapy alone (p < 0.05). This study promotes the feasibility of hepatic resection following SIRT. Resin microspheres appear to preferentially distribute at the tumour periphery and may enhance tumour regression.