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Saving Failing Hearts

Inhibiting a small regulatory RNA appears to improve cardiac function in mice with surgically induced heart problems.

By | March 12, 2014

WIKIMEDIA, RAMADuring chronic heart failure, the muscles of the vital organ slowly lose their ability to contract. Blocking the action of microRNA-25 (miR-25)—a noncoding RNA molecule that regulates gene expression—helps restore contractility to cardiac muscles in a mouse model of chronic heart failure, according to a paper published today (March 12) in Nature. The researchers hypothesize that miR-25 exacerbates heart failure by binding and preventing translation of the mRNA that encodes the calcium pump SERCA2a, a protein that is key to heart muscle contraction. The flow of calcium in and out of the cytosol of heart muscle cells helps regulate the rhythmic muscle contractions that cause the heart to beat.

Study coauthor Mark Mercola, a cardiology researcher at the Sanford-Burnham Medical Research Institute in La Jolla, California, said there is reason to hope the path to the clinic will be smooth for the RNA-based therapeutic approach his team presents. A Phase 2 clinical trial has already demonstrated that boosting SERCA2a in human hearts via gene therapy can slow progression of chronic heart failure. “From a pharmaceutical standpoint, we have a clinically validated target,” Mercola said.

“These experiments are very interesting and exciting,” Stefanie Dimmeler, director of the Institute of Cardiovascular Regeneration at Goethe University Frankfurt in Germany, wrote in an e-mail to The Scientist. Dimmeler was not involved in the study. “SERCA2a is a very important target for treating heart failure and its augmentation by anti-miRs [miRNA inhibitors] may be much easier than the currently tested gene therapy approaches,” she said.

But others warned that the study’s results seem to contradict those of other studies on miR-25’s role in the failing heart. A paper published last year in Nature Cell Biology, for example, showed that miR-25 was downregulated during heart failure, and that inhibiting miR-25 appeared to provoke heart disease in mice. “Before there can be any serious discussion of therapeutic opportunities for miR-25 inhibition, I would think these seeming disparities need to be reconciled,” Eric Olson, a molecular biologist at the University of Texas Southwestern Medical Center in Dallas, wrote in an e-mail.

Mercola argued that the differences arose because his team studied a mouse model of more chronic heart failure, while the Nature Cell Biology research involved mice with more acute heart problems.

Mercola and his colleagues zeroed in on miR-25 following a screen of 875 miRNAs. The researchers knew that SERCA2a levels drop during human heart failure, causing abnormalities in how calcium travels within heart muscle cells. They suspected that miRNA might play a role in repressing the pump. Throughout the body, miRNAs regulate gene expression by binding to mRNAs and either causing them to be destroyed or preventing their translation into proteins.  

The researchers fused nucleotides encoding SERCA2a to nucleotides encoding green fluorescent protein, transfected the sensor into human embryonic kidney cells, and systematically added miRNAs. They saw a strong decrease in the reporter’s fluorescence in the presence of miR-25, indicating that it was binding SERCA2a mRNA and repressing it.

The researchers then tested miR-25 levels in the cells of humans with severe heart failure, finding that it was upregulated. When the researchers raised the levels of miR-25 in live mice by genetically inserting extra copies, they found that SERCA2a levels decreased in their heart muscles and that heart function declined. The researchers next suppressed miR-25 in mice using an anti-miR, finding that SERCA2a levels rose.

Finally, the researchers experimentally induced heart failure in mice by constricting the aorta. When they injected their anti-miR into the mice with heart failure, the animals showed improved cardiac function compared to mice with heart failure that did not receive the treatment. “It halted progression [of heart failure], and we even saw an improvement,” said Mercola.

Leon de Windt, a molecular biologist at Maastricht University in the Netherlands, questioned the team’s conclusions. De Windt led the group behind last year’s Nature Cell Biology paper that arrived at seemingly opposite results. He pointed to several additional papers, aside from his own, which found that miR-25 is downregulated in heart failure. Further, de Windt said that after Mercola’s team performed the surgery to induce heart failure in the new study, mouse heart function did not decrease enough to be comparable to the functional defects associated with human heart failure.

“Their model is formally not even in heart failure, while the animal models (plural) we used in our study clearly are in heart failure and we saw exactly the opposite,” he wrote in an e-mail.

Dimmeler, de Windt’s coauthor on the Nature Cell Biology paper, suggested another explanation for the discrepancies: it could be that Mercola’s team used a slightly different method to silence miR-25 than was used in the contradictory paper. She noted that her own lab recently found that blocking both miR-92a and miR-25 with a single anti-miR improved heart function in pigs, suggesting that, in this model system, decreased miR-25 did not have a negative effect.

De Windt is now attempting to better understand Mercola’s results by genetically knocking out miR-25 in mice and observing its effects, and by studying the differences between the methods used to suppress miR-25 in the two studies.

Mercola, meanwhile, plans next to repress miR-25 in other animal models of induced heart failure. This will help determine whether the therapy might be viable in humans, he said.

“Our present work shows a potential RNA therapeutic,” Mercola said. “So the logical next step is to address the pharmacological and pharmaceutical issues of how to make a drug out of anti-miR-25.”

C. Wahlquist et al., “Inhibition of miR-25 improves cardiac contractility in the failing heart,” Nature, doi:10.1038/nature13073, 2014.

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