Circular RNA: features, functions and their correlation with diseases especially cancer

In early 2012, the world of science saw a fascinating discovery called circular RNA as a transcription product of thousands of genes in mice and humans. These circular RNAs have recently been grouped as the encoding RNA in an independent group that their remarkable difference with other RNAs is that these RNAs are not linear, in which two ends connect with a covalent connection creating a loop-shaped structure. These molecules play a role in regulating the expression of genes in mammals, and also, unlike other RNAs, they are very stable. These RNAs are created quite differently by means of a mechanism called back splicing. In this mechanism, in a molecule of RNA, axons or lariat- shaped introns are bonded at two ends of '3 and '5 with a covalent bond, creating circular structures that, unlike other common RNAs in the cytoplasm, are very stable. The circular RNAs act as mRNA sponge and form a set of RNA and protein that engage in transcription regulation by binding to RNA-related proteins. This suggests that these RNAs regulate gene expression at the transcriptional level and then they react by miRNAs. In fact, those circular RNAs that play a role in regulating the function of miRNAs also play a role in the onset and progression of the cancer. In tumor tissues, circular RNAs are reduced in comparison with normal tissues, and this may be for reasons like error occurs in the back splicing mechanism, degradation by unregulated miRNAs or increased cell proliferation. Recently circular RNAs have been identified in exosomic studies and in the movement of chromosomes in cancers, and there is found that the incorrect and abnormal attachment of these types of RNAs is related to drug resistance. Although it is thought that circular RNAs are non-coding, some of them are translated into functional proteins. So far, there are many unknowns about circular RNAs and the mechanism for regulating the expression of genes by them, but there is a lot of evidence to convince us that they will soon be used as biomarkers to diagnose diseases and therapeutic targets for cancers.