Oncogenic Circular RNAs Arise from Chromosomal Translocations

Recurrent chromosomal translocations have been implicated in multiple tumor types and create fusion genes that can promote oncogenesis. However, the effect of chromosomal translocations on noncoding RNA is not well understood. Guarnerio and colleagues hypothesized that cancer-associated chromosomal translocations could, in addition to creating coding fusion mRNAs, potentially juxtapose distant complementary repetitive intronic sequences that result in aberrant formation of circular RNAs (circRNA) that are generated by back-splice events that favor repetitive sequences. PCR assays and RNA sequencing confirmed in cell lines and patient samples that multiple tumor-associated translocations, such as PML–RARA in promyelocytic leukemia, MLL–AF9 in acute myeloid leukemia, EWSR1–FLI1 in Ewing sarcoma, and EML4–ALK1 in lung cancer, were associated with one or more fusion-circRNAs (f-circRNA). To distinguish the effects of f-circRNAs from other effects of chromosomal translocations, f-circRNAs were expressed in immortalized mouse embryonic fibroblasts (MEF) not harboring chromosomal translocations or expressing fusion proteins. MEFs expressing an f-circRNA associated with PML–RARA (f-circPR) or MLL–AF9 (f-circM9) exhibited an increased proliferation rate, loss of contact inhibition, and increased clonogenicity, suggesting that f-circRNAs may be oncogenic. Although transplantation of f-circM9–expressing hematopoietic stem cells into irradiated mice was not sufficient to induce leukemogenesis in vivo, transplantation of cells expressing f-circM9 together with MLL–AF9 resulted in a higher number of leukemic cells in the bone marrow and spleen compared with MLL–AF9 alone, suggesting that f-circRNAs can contribute to tumor progression. Moreover, f-circM9 protected MLL–AF9-expressing leukemia cells from the cytotoxic effects of arsenic trioxide and cytarabine in vitro and in vivo, and knockdown of endogenous f-circM9 induced apoptosis in human leukemia cell lines, indicating that f-circRNAs can promote cell viability and confer therapeutic resistance. Together, these findings indicate that f-circRNAs can be generated via oncogenic chromosomal translocations and are an added mechanism by which chromosomal rearrangements can drive tumorigenesis.

Guarnerio J, Bezzi M, Jeong JC, Paffenholz SV, Berry K, Naldini MM, et al. Oncogenic role of fusion-circRNAs derived from cancer-associated chromosomal translocations. Cell 2016;165:289–302.