CD&D Science Writer

The most common way to investigate the functions of miRNAs is to inhibit them one at a time.

But in a paper on the online edition of the Proceedings of the National Academy of Sciences, researchers from the University of North Carolina (Chapel Hill), Cold Spring Harbor Laboratory, Baylor College of Medicine (Houston), Chongquing University of Medical Sciences (Chongquing, China), use a more radical approach: knocking out the miRNA processing enzyme Dicer.

"If you take out dicer, you remove most or all of the miRNA," senior author Da-Zhi Wang, assistant professor of cell and developmental biology at the University of North Carolina in Chapel Hill, said.

The rationale for the wholesale of axing miRNAs was twofold. For one thing, when Wang’s team began with their study, the importance of miRNAs was more controversial than it is now, and getting rid of all of them allowed the researchers to show unequivocally that miRNAs are important in the development and functioning of the heart.

Additionally, Wang said that "many miRNAs are redundant." In practical terms, this means that knocking one miRNA at a time, besides being a much greater effort, might not reveal any effects as its relatives took over.

The researchers engineered mice which lack Dicer specifically in the tissues of the heart, and found that while such animals do not die during embryonic development, they do die shortly after birth: none of them survived for longer than four days.

The researchers then studied the hearts of the Dicer knockouts and found that they were larger than those of their control littermates, though the knockouts did not have larger bodies. The hearts had structural and functional defects in their left ventricles, which pump blood into the body. The left ventricles in Dicer knockouts were dilated and showed decreased fractional shortening, which means that the heart is not pumping as efficiently as a normal heart.

The Dicer knockout mice also had a heart rate that was only about half of normal, suggesting that they also had deficits in electrical conduction. Wang and his team checked the expression levels of several connexin proteins, which are important for forming electrical junctions between heart cells; they found that the expression of one connexin was significantly decreased, while another one was significantly increased and a third showed no change.

Wang and his team also studied the tissues of patients with end-stage dilated cardiomyopathy, heart failure, or both, and found that they had lower levels of Dicer than normal heart tissue. When the patients were implanted with a ventricular assist device, levels of Dicer increased. The authors conclude that "these data suggest that Dicer and therefore miRNAs are likely involved the progression and/or regression of DCM and heart failure in human patients."

Wang declined to speculate on which miRNAs are responsible for the effects his team observed, but that "hopefully, our future work will give us some hint" about which miRNAs are the specific culprits.

Wang was optimistic about the potential for applications arising for the work, pointing to a host of efforts at clinical targeting and, on the basic science side, recent reports that miRNAs may be able to target different steps within the same pathway. "MiRNA in general has a very high potential to be targeted," he said.