Circular RNAs: a new class of cancer drivers Circular RNAs (circRNAs) are a recently discovered type of non-coding RNA molecule. Unlike linear messenger RNAs, they form a closed loop structure that makes them highly stable and resistant to the cell's normal RNA degradation machinery.
circRHOT1 identified in HCC Using computational analysis of two circRNA expression datasets from hepatocellular carcinoma (HCC) patients, researchers identified circRHOT1 (also called hsa_circRNA_102034) as among the most dramatically upregulated circRNAs in HCC tumors compared to normal liver tissue.
Study objectives The study investigated whether circRHOT1 drives HCC tumor growth and spread, how it does so mechanistically, and whether it could serve as a prognostic biomarker or therapeutic target in HCC patients.
Core mechanism discovered circRHOT1 works as a molecular scaffold, recruiting the histone acetyltransferase enzyme TIP60 to the promoter region of the transcription factor NR2F6, switching on NR2F6 gene expression and promoting tumor proliferation, migration, and invasion.
Bioinformatics screen Differentially expressed circRNAs were identified by analyzing two publicly available HCC circRNA expression datasets (GSE94508 and GSE97332). Conserved circRNAs upregulated in both datasets were prioritized, with circRHOT1 emerging as the most consistently elevated candidate.
Expression validation Northern blotting confirmed circRHOT1's circular structure. qRT-PCR, in situ hybridization (ISH), and RNA-FISH quantified circRHOT1 expression in 100 paired HCC and adjacent normal tissue samples.
Functional assays CCK-8 cell viability assays, colony formation, EdU proliferation labeling, and Transwell migration/invasion assays were used to measure how circRHOT1 affects HCC cell behavior. CRISPR-Cas9 knockout of circRHOT1, TIP60, and NR2F6 was used to verify causal roles in vitro and in vivo xenograft models.
Mechanism studies RNA pulldown with biotin-labeled circRHOT1 probes followed by mass spectrometry identified TIP60 as a physical binding partner. ChIP (chromatin immunoprecipitation) assays confirmed TIP60 enrichment at the NR2F6 promoter in a circRHOT1-dependent manner. EMSA assays confirmed direct RNA-protein binding.
Significant upregulation confirmed circRHOT1 was significantly overexpressed in HCC tumor tissues compared to adjacent normal liver across multiple independent datasets and 100 patient samples analyzed by the authors. Expression was conserved between human and mouse HCC, supporting its functional importance.
Prognostic significance HCC patients with high circRHOT1 expression had significantly shorter overall survival and disease-free survival compared to patients with lower circRHOT1 levels. This association held even after accounting for tumor stage and other clinical variables.
Predominantly nuclear localization RNA-FISH and ISH showed that circRHOT1 localizes preferentially in the nucleus of HCC cells, in contrast to cytoplasmic circRNAs. This nuclear localization was an important clue to its mechanism of action - it functions in gene regulation rather than as a cytoplasmic microRNA sponge.
Clinical parameter correlations High circRHOT1 expression correlated with larger tumor size, advanced tumor stage, presence of vascular invasion, and higher AFP levels - all established markers of aggressive HCC biology.
TIP60 as the binding partner RNA pulldown using a biotin-labeled circRHOT1 probe, followed by mass spectrometry of captured proteins, identified TIP60 (Tat-interacting protein 60 kDa) as a direct physical binding partner. TIP60 is a histone acetyltransferase enzyme that activates gene transcription by adding acetyl groups to histone proteins.
NR2F6 as the downstream target RNA sequencing of circRHOT1-knockout cells identified NR2F6 as the primary downstream gene whose expression depended on circRHOT1. NR2F6 is a transcription factor previously identified as a suppressor of anti-tumor immune responses in other contexts, but here acts as an oncogene.
The promoter recruitment mechanism ChIP assays demonstrated that TIP60 was specifically enriched at the NR2F6 gene promoter in a circRHOT1-dependent manner. When circRHOT1 was knocked out, TIP60 failed to accumulate at the NR2F6 promoter and NR2F6 transcription was suppressed.
Functional necessity of NR2F6 Knockout of NR2F6 phenocopied circRHOT1 knockout - suppressing HCC cell proliferation, migration, and invasion. Critically, restoring NR2F6 expression in circRHOT1-knockout cells fully rescued the proliferative and metastatic defects, proving NR2F6 is the essential downstream effector.
In vitro effects on HCC cells CRISPR-Cas9 knockout of circRHOT1 significantly reduced HCC cell proliferation, colony formation, EdU incorporation (a measure of DNA synthesis), migration, and invasion across multiple HCC cell lines tested.
Apoptosis induction circRHOT1 knockout increased apoptosis (programmed cell death) rates in HCC cells, as measured by flow cytometry with annexin V staining. This anti-apoptotic function of circRHOT1 may contribute to HCC therapy resistance.
In vivo xenograft validation Subcutaneous injection of circRHOT1-knockout HCC cells into nude mice produced significantly smaller tumors than control cells. This in vivo confirmation that circRHOT1 is required for tumor growth in a living system greatly strengthens the therapeutic rationale.
Rescue experiments confirm specificity Restoring NR2F6 expression in circRHOT1-knockout cells rescued the proliferation and metastasis defects observed in vitro, confirming that the entire oncogenic activity of circRHOT1 in HCC is channeled through the TIP60-NR2F6 axis.
Mechanism of NR2F6 as an oncogene This study establishes that NR2F6 is a critical effector of circRHOT1 in HCC, but the downstream pathways through which NR2F6 drives proliferation and invasion in this context need further investigation, as NR2F6 has also been reported to suppress T cell activation.
Delivery of circRNA-targeting therapeutics Translating circRHOT1 as a therapeutic target requires developing methods to silence circular RNAs in vivo. Standard antisense oligonucleotides work differently against circular vs. linear RNAs, and specialized approaches for in vivo circRNA knockdown are still maturing.
Biomarker development Because circRNAs are highly stable and detectable in blood, circRHOT1 has potential as a liquid biopsy biomarker. Prospective studies measuring circRHOT1 in HCC patient serum or plasma and correlating with clinical outcomes are needed.
Broader circRNA landscape circRHOT1 is one of thousands of circRNAs expressed in HCC. A systematic survey of which circRNAs have functional roles in HCC through similar nuclear chromatin-regulatory mechanisms could reveal additional therapeutic targets.