Comprehensive analysis of circRNA expression profiles in humans by RAISE

Circular RNAs (circRNAs) are pervasively expressed circles of non-coding RNAs. Even though many circRNAs have been reported in humans, their expression patterns and functions remain poorly understood. In this study, we employed a pipeline named RAISE to detect circRNAs in RNA-seq data. RAISE can fully characterize circRNA structure and abundance. We evaluated inter-individual variations in circRNA expression in humans by applying this pipeline to numerous non-poly(A)-selected RNA-seq data. We identified 59,128 circRNA candidates in 61 human liver samples, with almost no overlap in the circRNA of the recruited samples. Approximately 89% of the circRNAs were detected in one or two samples. In comparison, 10% of the linear mRNAs and non-coding RNAs were detected in each sample. We estimated the variation in other tissues, especially the circRNA high-abundance tissues, in advance. Only 0.5% of the 50,631 brain circRNA candidates were shared among the 30 recruited brain samples, which is similar to the proportion in liver. Moreover, we found inter- and intra-individual diversity in circRNAs expression in the granulocyte RNA-seq data from seven individuals sampled 3 times at one-month intervals. Our findings suggest that careful consideration of inter-individual diversity is required when extensively identifying human circRNAs or proposing their use as potential biomarkers and therapeutic targets in disease.