Xiao‐Ou Zhang

Circular RNAs (circRNAs) derived from back-spliced exons have been widely identified as being co-expressed with their linear counterparts. A single gene locus can produce multiple circRNAs through alternative back-splice site selection and/or alternative splice site selection; however, a detailed map of alternative back-splicing/splicing in circRNAs is lacking. Here, with the upgraded CIRCexplorer2 pipeline, we systematically annotated different types of alternative back-splicing and alternative splicing events in circRNAs from various cell lines. Compared with their linear cognate RNAs, circRNAs exhibited distinct patterns of alternative back-splicing and alternative splicing. Alternative back-splice site selection was correlated with the competition of putative RNA pairs across introns that bracket alternative back-splice sites. In addition, all four basic types of alternative splicing that have been identified in the (linear) mRNA process were found within circRNAs, and many exons were predominantly spliced in circRNAs. Unexpectedly, thousands of previously unannotated exons were detected in circRNAs from the examined cell lines. Although these novel exons had similar splice site strength, they were much less conserved than known exons in sequences. Finally, both alternative back-splicing and circRNA-predominant alternative splicing were highly diverse among the examined cell lines. All of the identified alternative back-splicing and alternative splicing in circRNAs are available in the CIRCpedia database (http://www.picb.ac.cn/rnomics/circpedia). Collectively, the annotation of alternative back-splicing and alternative splicing in circRNAs provides a valuable resource for depicting the complexity of circRNA biogenesis and for studying the potential functions of circRNAs in different cells.

The human 8q24 gene desert contains multiple enhancers that form tissue-specific long-range chromatin loops with the MYC oncogene, but how chromatin looping at the MYC locus is regulated remains poorly understood. Here we demonstrate that a long noncoding RNA (lncRNA), CCAT1-L, is transcribed specifically in human colorectal cancers from a locus 515 kb upstream of MYC. This lncRNA plays a role in MYC transcriptional regulation and promotes long-range chromatin looping. Importantly, the CCAT1-L locus is located within a strong super-enhancer and is spatially close to MYC. Knockdown of CCAT1-L reduced long-range interactions between the MYC promoter and its enhancers. In addition, CCAT1-L interacts with CTCF and modulates chromatin conformation at these loop regions. These results reveal an important role of a previously unannotated lncRNA in gene regulation at the MYC locus.