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Biosynthesis of Circular RNA ciRS-7/CDR1as Is Mediated by Mammalian-Wide Interspersed Repeats (MIRs)

By Rei Yoshimoto, Karim Rahimi, Thomas Hansen, Jørgen Kjems, Akila Mayeda

Posted 07 Sep 2018
bioRxiv DOI: 10.1101/411231 (published DOI: 10.1016/j.isci.2020.101345)

Circular RNAs (circRNAs) are stable noncoding RNAs with a closed circular structure. One of the first and best studied circRNAs is ciRS-7 (CDR1as) that acts as a regulator of the microRNA miR-7, however, the biosynthesis pathway has remained an enigma. Here we delineate the biosynthesis pathway of ciRS-7. The back-splicing events that form circRNAs are often facilitated by flanking inverted repeats of the primate-specific Alu elements. ciRS-7 gene lacks these elements but, instead, we identified a set of flanking inverted elements belonging to the mammalian-wide interspersed repeat (MIR) family. Splicing reporter assays in HEK293 cells demonstrated that these inverted MIRs are required to generate ciRS-7 through a back-splicing and CRISPR/Cas9-mediated deletions confirmed the requirement of the endogenous MIR elements in SH-SY5Y cells. Using bioinformatics searches, we identified several other MIR-dependent circRNAs that we confirmed experimentally. We propose that MIR-mediated RNA circularization constitutes a new widespread biosynthesis principle for mammalian circRNAs.

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