Circular RNA: an important player with multiple facets to regulate its parental gene expression.

Circular RNAs (circRNAs), a novel type of endogenous RNAs with covalently closed-loop structures, have become a new research hotspot in the RNA world. Their diversity, stability, evolutionary conservation, and cell type- or tissue-specific expression patterns endow circRNAs with various important biological functions. As a consequence, circRNAs are emerging as important regulators of physiological development and disease pathogenesis. Growing evidence has shown that circRNAs can regulate parental gene expression through diverse mechanisms, such as transcription and splicing regulation, microRNA (miRNA) sponges, mRNA traps, translational modulation, and post-translational modification. The study of circRNAs and how circRNAs regulate the expression of parental genes will facilitate a deeper understanding of their biological functions and provide new perspectives on their clinical application. Herein, we review the biogenesis of circRNAs, with a particular focus on the molecular mechanisms of circRNAs regulating their parental gene expression and the biological significance of such regulation.