Circular RNA: A new star of noncoding RNAs.

Circular RNAs (circRNAs) are a novel type of RNA that, unlike linear RNAs, form a covalently closed continuous loop and are highly represented in the eukaryotic transcriptome. Recent studies have discovered thousands of endogenous circRNAs in mammalian cells. CircRNAs are largely generated from exonic or intronic sequences, and reverse complementary sequences or RNA-binding proteins (RBPs) are necessary for circRNA biogenesis. The majority of circRNAs are conserved across species, are stable and resistant to RNase R, and often exhibit tissue/developmental-stage-specific expression. Recent research has revealed that circRNAs can function as microRNA (miRNA) sponges, regulators of splicing and transcription, and modifiers of parental gene expression. Emerging evidence indicates that circRNAs might play important roles in atherosclerotic vascular disease risk, neurological disorders, prion diseases and cancer; exhibit aberrant expression in colorectal cancer (CRC) and pancreatic ductal adenocarcinoma (PDAC); and serve as diagnostic or predictive biomarkers of some diseases. Similar to miRNAs and long noncoding RNAs (lncRNAs), circRNAs are becoming a new research hotspot in the field of RNA and could be widely involved in the processes of life. Herein, we review the formation and properties of circRNAs, their functions, and their potential significance in disease.