circFL-seq reveals full-length circular RNAs with rolling circular reverse transcription and nanopore sequencing

Circular RNAs (circRNAs) act through multiple mechanisms with their sequence features to fine-tune gene expression networks. Due to overlapping sequences with linear cognates, identifying internal sequences of circRNAs remains a great challenge, which hinders comprehensive understanding of circRNA functions and mechanisms. Here, based on rolling circular reverse transcription (RCRT) and nanopore sequencing, we developed circFL-seq, a full-length circRNA sequencing method, to profile circRNA at the isoform level. With a customized computational pipeline circfull to directly identify full-length sequences from rolling circular reads, we reconstructed 77,606 high-quality circRNAs from seven human cell lines and two human tissues. Benefiting from rolling circles and long-read sequencing, circFL-seq showed more than tenfold enrichment of circRNA reads and advantages for both detection and quantification at the isoform level compared to short-read RNA sequencing. The concordance of RT-qPCR and circFL-seq results for the identification of differential alternative splicing suggested wide application prospects for functional studies of internal variants in circRNAs. Moreover, the detection of cancer-related fusion circRNAs at the omics scale may further expand the application of circFL-seq. Together, the accurate identification and quantification of full-length circRNAs make circFL-seq a potential tool for large-scale screening of functional circRNAs.