Construction of circular miRNA sponges targeting miR-21 or miR-221 and demonstration of their excellent anticancer effects on malignant melanoma cells.

microRNA sponges antagonizing the oncogenic microRNAs are potential candidates for RNA-based cancer therapies. Although the constructed sponges so far are to some extent suitable for biological experiments, they can only express at relative low levels in cells, because they are sensitive to microRNA-mediated activation of deadenylation and subsequent exonucleolytic degradation. Since circular RNA molecules are resistant to exonuclease degradation, we report the production of circular microRNA sponges against miR-21 or miR-221 in cell lines using the self-splicing permuted intron-exon sequences derived from T4 bacteriophage gene td. The circularized microRNA sponges withstand enzymatic degradation and are completely resistant to microRNA-mediated degradation. They are more effective than typical linear microRNA sponges and microRNA inhibitors in derepressing microRNA targets. They also display superior anti-cancer activities compared to the linear sponges in malignant melanoma cell lines. We have provided an alternative method for circular microRNA sponge production and malignant melanoma treatment.