Circular RNAs in cancer: old tree with new flowers
Circular RNAs (circRNA) is a member of the non-coding RNA kingdom. Unlike linear RNA, circRNA forms a covalently closed continuous loop. It is largely generated from exonic or intronic sequences via multiple mechanisms. Recent studies have discovered thousands of endogenous circRNAs in eukaryotic cells, where circRNAs perform numerous potential functions, such as microRNA (miRNA) sponges, regulators of splicing and transcription, and modifiers of parental gene expression. Specifically, the expression of circRNAs is associated with several pathological states of tumors by participating in the regulation of biological behavior of cancers. Most circRNAs are stable and resistant to RNase R, conserved across species, and often exhibit tissue/developmental-stage-specific expression. Emerging evidence indicates that circRNAs may serve as novel diagnostic or predictive biomarkers of cancer. Similar to miRNAs and long noncoding RNAs (lncRNAs), circRNAs are becoming a new research hotspot in the field of cancer with both agonist and antagonist effects on carcinogenesis. Herein, we illustrate the current understanding about the biogenesis and properties of circRNAs, their functions, and their potential significance in cancer.