Circular RNAs (circRNAs) are a subclass of noncoding RNAs widely expressed in mammalian cells. We report here the tumorigenic capacity of a circRNA derived from angiomotin-like1 (circ-Amotl1). Circ-Amotl1 is highly expressed in patient tumor samples and cancer cell lines. Single-cell inoculations using circ-Amotl1-transfected tumor cells showed a 30-fold increase in proliferative capacity relative to control. Agarose colony-formation assays similarly revealed a 142-fold increase. Tumor-take rate in nude mouse xenografts using 6-day (219 cells) and 3-day (9 cells) colonies were 100%, suggesting tumor-forming potential of every cell. Subcutaneous single-cell injections led to the formation of palpable tumors in 41% of mice, with tumor sizes >1 cm in 1 month. We further found that this potent tumorigenicity was triggered through interactions between circ-Amotl1 and c-myc. A putative binding site was identified in silico and tested experimentally. Ectopic expression of circ-Amotl1 increased retention of nuclear c-myc, appearing to promote c-myc stability and upregulate c-myc targets. Expression of circ-Amotl1 also increased the affinity of c-myc binding to a number of promoters. Our study therefore reveals a novel function of circRNAs in tumorigenesis, and this subclass of noncoding RNAs may represent a potential target in cancer therapy.

Circular RNAs are a large group of noncoding RNAs that are widely expressed in mammalian cells. Genome-wide analyses have revealed abundant and evolutionarily conserved circular RNAs across species, which suggest specific physiological roles of these species. Using a microarray approach, we detected increased expression of a circular RNA circ-Dnmt1 in eight breast cancer cell lines and in patients with breast carcinoma. Silencing circ-Dnmt1 inhibited cell proliferation and survival. Ectopic circ-Dnmt1 increased the proliferative and survival capacities of breast cancer cells by stimulating cellular autophagy. We found that circ-Dnmt1-mediated autophagy was essential in inhibiting cellular senescence and increasing tumor xenograft growth. We further found that ectopically expressed circ-Dnmt1 could interact with both p53 and AUF1, promoting the nuclear translocation of both proteins. Nuclear translocation of p53 induced cellular autophagy while AUF1 nuclear translocation reduced Dnmt1 mRNA instability, resulting in increased Dnmt1 translation. From here, functional Dnmt1 could then translocate into the nucleus, inhibiting p53 transcription. Computational algorithms revealed that both p53 and AUF1 could bind to different regions of circ-Dnmt1 RNA. Our results showed that the highly expressed circular RNA circ-Dnmt1 could bind to and regulate oncogenic proteins in breast cancer cells. Thus circ-Dnmt1 appears to be an oncogenic circular RNA with potential for further preclinical research.