Circular RNAs as novel biomarkers with regulatory potency in human diseases

Circular RNAs (circRNAs) are a large class of noncoding RNAs characterized with closed loop structures without 3' and 5; polar ends. They can roughly be divided into exonic circRNAs, exon-intron circRNAs and circular intronic RNAs. CircRNAs are characterized with stability, prevalence, specificity and conservation, which arouse great interest in circRNAs as disease biomarkers. Their abilities to sponge to miRNAs, cis-regulate parent genes, bind to proteins and encode proteins endow circRNAs a critical role of regulation in eukaryotic cells. This concise review focuses on circRNAs as functional biomarkers and therapeutic targets in both tumor and nontumorous diseases. Lay abstract: Although they were discovered in 1970s, circular RNAs (circRNAs) have attracted great interest only relatively recently. Instead of genome 'junk matters', circRNAs are now considered as promising biomarkers and treatment targets. CircRNAs are involved in numerous cancer-related and noncancer diseases, such as lung cancer, gastric cancer, cardiovascular diseases, diabetes mellitus and so on. This review outlines the classification, characterization and function of circRNAs, with a specific focus on recent studies concerning the role of circRNAs in human diseases.