Circular RNAs: an emerging landscape in tumor metastasis

Circular RNAs (CircRNAs), the endogenous long noncoding RNAs, unlike linear RNAs, are structurally continuous, covalently closed loops without 5' cap or 3' polyadenylated tail. High-throughput RNA sequencing has enabled the discovery of several endogenous circRNAs in different species and tissues. The circRNAs mainly act as sponges to cytoplasmic microRNA, aid in protein translation, or interact with RNA-binding proteins to generate RNA-protein complexes which control transcription. Recently, circRNAs have been reported to participate in cancer pathogenesis, particularly tumor metastasis in humans, mainly due to their frequent aberrant expression in cancers. However, the detail molecular mechanism of circRNAs activity in tumor metastasis is still elusive. Some specifically expressed circRNAs can potentially be used as biomarkers and therapeutic targets for tumor treatment. Further understanding of the network interactions and regulation of circRNAs is paving the way for the identification of better therapeutic strategies in tumor metastasis. In this mini review, we have summarized the current state of research on functions and mechanisms of novel circRNAs that regulate tumorigenesis and have evaluated the relationship between dysregulation of circRNAs and tumor metastasis.