Published in the March 31 advance online issue of Cell, a team led by investigators at Beth Israel Deaconess Medical Center (BIDMC) reveal that circular RNAs are also affected by genomic rearrangements in cancer, like their protein counterparts, resulting in abnormal fusions.

Moreover, they appear to promote tumour growth and progression, underscoring their role in the disease.
"Cancer is essentially a disease of mutated or broken genes, so that motivated us to examine whether circular RNAs, like proteins, can be affected by these chromosomal breaks," said senior author Pier Paolo Pandolfi, MD, PhD, Director of the Cancer Center at BIDMC and George C. Reisman Professor of Medicine at Harvard Medical School.

"Our work paves the way to discovering many more of these unusual RNAs and how they contribute to cancer, which could reveal new mechanisms and druggable pathways involved in tumour progression."

RNA is perhaps best known for its role ferrying instructions out of the nucleus,  yet only 2 percent of the genome is transcribed from DNA into RNA and then translated into protein.

Scientists now recognize that much, if not all, of the remaining 98 percent -- which had previously been deemed non-functioning-- is in fact transcribed into RNA.

The roles this vast swath of so-called "non-coding RNA" might play in human biology and disease now signify an area of intense research, including the work of Dr Pandolfi’s team to detect relevant changes in tumours known to harbor distinct fusion proteins.

The researchers examined acute promyelocytic leukemia, which often carries a translocation between the PML and RARα genes; and acute myeloid leukemia, which can harbor a translocation between the MLL and AF9 genes, finding abnormal fusion-circular RNAs (f-circRNAs) corresponding to different exons associated with the PML-RARα gene fusion as well as the MLL-AF9 gene fusion.

f-circRNAs we also found in solid tumour samples from Ewing sarcoma, a form of soft tissue cancer, and lung cancer.

"Our ability to readily detect these fusion-circular RNAs -- and their normal, non-fused counterparts -- will be enhanced by advances in sequencing technology and analytic methods," said first author Jlenia Guarnerio, PhD, also of BIDMC.

"Indeed, as we look ahead to cataloguing them comprehensively across all cancers and to deeply understanding their mechanisms of action, we will need to propel these new methodologies even further."

To determine whether f-circRNAs play a functional role in cancer, the researchers introduced them experimentally into cells, causing the cells to increase their proliferation and tendency to overgrow. When the researchers blocked f-circRNA activity, the cells' normal behaviors were restored.

The researchers also conducted experiments using a mouse model of leukemia, focusing on a specific f-circRNA associated with the MLL-AF9 fusion gene, called f-circM9.

Although insufficient on its own to trigger leukemia, f-circM9 appears to work together with other cancer-promoting signals (such as the MLL-AF9 fusion protein) to cause disease.

Additional studies suggest that f-circM9 may also help tumour cells persist in the face of anti-cancer drugs.
"These results are particularly exciting because they suggest that drugs directed at fusion-circular RNAs could be a powerful strategy to pursue for future therapeutic development in cancer," said Pandolfi.

Circular RNAs were first identified more than three decades ago and largely dismissed as a rare cellular oddity, until a study published in 2012 by Patrick Brown's group at Stanford University showed that they are present at high levels in diverse cell types, igniting scientists' efforts to study and understand them.

Surprisingly, circular RNAs -- are among the most abundant non-coding RNAs in cells, driven in part by the molecules' unusual chemical stability.

Unlike linear RNAs, circular RNAs are not susceptible to RNA-degrading enzymes.

This ability to persist makes them not only an interesting therapeutic target, but also a potential molecular beacon or biomarker that can facilitate the diagnosis of disease.

"Our knowledge of circular RNAs is really in its infancy," explained Pandolfi. "We know that normally, they can bind proteins as well as DNA and microRNAs, but much more needs to be done to understand how fusion-circular RNAs work. We have only scratched the surface of these RNAs and their roles in cancer and other diseases."

Article: Beth Israel Deaconess Medical Centre

Source: Cell Advance Online