Circulating circular RNAs as biomarkers of cancer

Circular RNA (circRNA) is a kind of novel non-coding RNA, which is widely present in eukaryotes and has a special circular structure formed by 3'- and 5'-ends linking covalently. CircRNA has many features, including high stability, conservation, etc. Since the discovery of circRNA, it has been considered a “by-product” of gene expression. In recent years, many studies have confirmed that circRNAs play an essential role in the pathogenesis of many diseases. CircRNA is involved in various cancer processes including the regulation of cell growth, proliferation, invasion, metastasis, etc., and is differentially expressed in tumor tissues and even in the circulation of patients. Because of these characteristics, circRNA has become a competitive candidate in being a novel biomarker of cancer. This article reviews the role of circulating circRNA in cancer and its diagnostic significance.