Circular RNAs are miRNA sponges and can be used as a new class of biomarker.

Circular RNAs (circRNAs) are a class of non-coding RNAs (ncRNAs) that are involved in transcriptional and posttranscriptional gene expression regulation. The development of deep sequencing of ribosomal RNA (rRNA)-depleted RNA libraries, associated with improved computational tools, has provided the identification of several new circRNAs in all sorts of organisms, from protists, plants and fungi to animals. Recently, it was discovered that endogenous circRNAs can work as microRNA (miRNA) sponges. This means that the circRNAs bind to miRNAs and consequently repress their function, providing a new model of action for this class of ncRNA, as well as indicating another mechanism that regulates miRNA activity. As miRNAs control a large set of biological processes, circRNA sponge activity will also affect these pathways. Several studies have associated miRNA sponges with human diseases, including osteoarthritis, diabetes, neurodegenerative pathologies and several types of cancer. Additionally, high stability, abundance and tissue-specific expression patterns make circRNA sponges very attractive for clinical research. Herein, we review the biogenesis, properties and function of endogenous circRNA sponges, with a special focus on those related to human cancer. A list of web tools available for the study of circRNAs is also given. Additionally, we discuss the possibility of using circRNAs as molecular markers for the diagnosis of diseases.