Researchers have recently identified a circRNA that increases activity in soft tissue and connective tissue tumors.
DNA manufactures RNA, which unlike the double-helix structure of DNA, is single-stranded. Messenger RNA (mRNA) behaves like a courier, taking the DNA code’s instructions to ribosomes, where the proteins are actually made. However, there are other forms of RNA. They don’t carry DNA code but perform other important functions. They are known as non-coding RNAs.
In the 1970s, a class of non-coding RNA called circular RNA (circRNA) was discovered. At first, researchers thought circRNA was a virus. Researchers have recently identified a circRNA that increases activity in soft tissue and connective tissue tumors.
“Under certain conditions, RNA processing systems can get tricked into thinking they are supposed to join the ends,” said Mona Batish, a molecular biologist at the University of Delaware, who collaborated with scientists at Harvard Medical School and the University of California, Los Angeles (UCLA) on the study.
“When this error occurs,” Batish went on, “it creates a backwards loop in the RNA’s genetic sequence and then keeps on going—kind of like when you get a kink in the middle of a necklace.”
That loops splits off and remains as circular RNA. The process is called back splicing, and for quite some time researchers thought the circRNA had no effect. However, since the 1990s and the development of DNA sequencing, they began to find circRNA in brain tissue and other tissues. Since 2014, researchers found that circRNA was important and there is an entire field focused on identify circRNA as biomarkers for disease.
In the study, which was published in the journal Cell Research, Batish and her team found a new circRNA created by the Zbtb7a gene, which is found in soft tumors, such as mesenchymal tumors. When the RNA is linear, it produces a tumor-suppressing protein. But in its circular form, it acts independently to stimulate tumor growth. As they point out, theoretically, the linear and circular RNA strands should do the same thing because they come from the same genetic material. However, they don’t.
It also caused some technical problems, because the RNA strands have the same code, but different structures.
“You don’t ‘see’ RNA, per se,’ so you have to label it,” Batish said. “But, if you label it with something that is sequence-specific, it’s hard to tell if it’s linear or circular because the genetic code looks the same.”
Batish adapted a fluorescence microscopy technique to distinguish circular RNA from linear RNA that leverages a color combination technique.
“Essentially, it’s like creating a pattern of beads on a necklace,” Batish said. “Say the RNA we are working with contains red and green beads. We know that circular RNA is a closed circle of green beads only, so we add probes for both red and green beads and then image them under a fluorescence microscope. If we see a signal for both red and green at the same spot, which appear as yellow (combination of green and red) in the sample, we know it is linear RNA. If it doesn’t have red, it must be circular RNA.”
Where the ends meet in the circRNA is unique, so Batish thinks it may be possible to develop treatments that uniquely target the circRNA but ignore linear RNA. Since every cancer is known to have circular RNA, it’s possible that the technology could have widespread implications for cancer therapies, biomarkers, as well as for other disease states.
