Internal Introns Promote Backsplicing to Generate Circular RNAs from Spinal Muscular Atrophy Gene

Human survival motor neuron 1 (SMN1) codes for SMN, an essential housekeeping protein involved in most aspects of RNA metabolism. Deletions or mutations of SMN1 lead to spinal muscular atrophy (SMA), a devastating neurodegenerative disease linked to a high rate of infant mortality. SMN2, a near identical copy of SMN1 present in humans, cannot compensate for the loss of SMN1 due to predominant skipping of SMN2 exon 7. Restoration of SMN by splicing modulation of SMN2 exon 7 or gene replacement are currently approved therapies of SMA. Human SMN genes produce a vast repertoire of circular RNAs (circRNAs). However, the mechanism of SMN circRNA generation has not yet been examined in detail. For example, it remains unknown if forward splicing impacts backsplicing that generates circRNAs containing multiple exons. Here, we employed SMN as a model system to examine the impact of intronic sequences on the generation of circRNAs. We performed our experiments in HeLa cells transiently transfected with minigenes expressing three abundantly represented circRNAs containing two or more SMN exons. We observed an enhanced rate of circRNA generation when introns joining exons to be incorporated into circRNAs were present as compared to the intronless context. These results underscore the stimulatory effect of forward splicing in the generation of circRNAs containing multiple exons. These findings are consistent with the reported low abundance of SMN circRNAs comprised of single exons. We confirmed our findings using inducible HEK 293 cells stably expressing the SMN circRNAs. Our results support the role of the exon junction complex in the generation of the exon-only-containing circRNAs. We showed that SMN circRNAs were preferentially localized in the cytoplasm. These findings provide new insights regarding our understanding of circRNA generation and open avenues to uncover novel functions of the SMN genes.