Emerging roles of circular RNAs in cancer: a narrative review

Circular RNAs (circRNAs) constitute a novel class of endogenous noncoding RNAs characterized by a covalently closed structure and involved in multiple biological processes. The main biological functions and properties of circRNAs can be defined by five features: a "sponging" effect on other RNA species, post-transcriptional regulation, rolling circle translation, generation of pseudogenes, and splicing interference. Although circRNAs were first detected decades ago, the role of circRNAs and the mechanisms underlying their actions remain incompletely characterized. Recently, circRNAs were reported to play indispensable roles in regulating metabolic and signal transduction events controlling the proliferation, migration, and survival of cells. Importantly, many studies demonstrated that dysregulated circRNA expression is associated with the development of multiple diseases, including cancer. In this review, we summarize current knowledge on the roles and mechanisms of circRNAs in cancer and discuss their functions as oncogenes or tumor suppressors in different tumor types.