A group led by researchers from Sanford-Burnham Medical Research Institute published data last week showing that inhibiting a single microRNA, miR-25, could stop the progression of heart failure in a mouse model while boosting the animals' cardiac function and survival.
The findings implicate miR-25 as a key player in the decline of cardiac function during heart failure and demonstrate its promise as a therapeutic target for the condition.
“Currently, heart-failure medications do not effectively address the underlying mechanisms that weaken contractile function and lead to the enlargement of heart-muscle cells,” Roger Hajjar, a researcher at the Icahn School of Medicine at Mount Sinai and co-author of the report, said in a statement.
“Our study provides us with the key evidence we need to begin developing miR-25 as an important new therapeutic target, while adding our successful technique to block this microRNA to our growing arsenal of promising heart failure therapies that we will further develop and test in future clinical trials,” he said.
Heart failure marks the culmination of a range of cardiovascular disorders including hypertension, ischemic disease, and atherosclerosis, and it is characterized by the progressive loss of contractile function and reserve.
A key feature of heart failure is impaired uptake of the calcium ion Ca2+ in cardiomyocytes due to the decreased expression and activity of SERCA2a, a calcium-transporting ATPase, the researchers wrote in a paper appearing in Nature. "Accordingly, restoration of SERCA2a by gene transfer has proven effective in improving key parameters of heart failure in animal models and more recently in clinical trials."
To explore the potential role of miRNAs in this process, the scientists functionally screened a whole-genome collection of miRNAs for the selective downregulation of the Ca2+ pump. In the first pass, they identified 82 of the small, non-coding RNAs, but it was miR-25 that had the most profound effect.
In fact, the miRNA was found to elicit a physiological effect on par with an siRNA specifically designed to silence SERCA2a, they noted in their paper, and its upregulation was confirmed in myocardial samples from patients with severe heart failure.
To establish a link between miR-25 and cardiac function, the group computationally searched for putative targets of the miRNA involved in Ca2+ handling, identifying SERCA2a and the inositol IP3R1, which is believed to mediate Ca2+ release.
The team then designed siRNAs against SERCA2a and IP3R1 to see if RNAi silencing would replicate the effect of miR-25 on cardiac Ca2+ transient kinetics in vitro and found that they did, mimicking the roughly 1.5- to 2-fold decline in the targets in ventricular cardiomyocytes isolated from failing human hearts.
Notably, siRNAs against IP3R1 "nominally" affected Ca2+ transient kinetics, suggesting that the bulk of the effect of miR-25 on this pathway is mediated through SERCA2a downregulation.
To test their findings in vivo, miR-25 was expressed in mice using an adeno-associated viral vector, which boosted levels of the miRNA by approximately 50 percent while correspondingly decreasing SERCA2a in the ventricular myocardium. The animals also displayed a progressive decline in fractional shortening, an index of cardiac function.
Further analysis after six weeks confirmed a decline in the left ventricular functioning in the animals. Meanwhile, mice treated with AAV-delivered miR-92a, which has the same seed sequence as miR-25, displayed no changes to cardiac function.
These results, the investigators wrote, "point to a selective interaction between miR-25 and SERCA2a mRNA that is consistent with the importance of non-seed sequences for target specificity, and also indicate that elevated miR-25 can depress cardiac function."
Further supporting this connection were experiments in SERCA2a-knockout mice, in which anti-miR-25 oligos failed to improve cardiac morphometric or hemodynamic function.
To examine the potential clinical application of their findings, the researchers intravenously dosed mice in which heart failure had been established with either miR-25 antagonists or control oligos.
Echocardiography showed "substantial improvements" in cardiac function in the treated animals 4.5 and 5.5 months following the induction of heart failure, despite constant pressure overload in their hearts. Untreated animals, in contrast, experienced "severe deterioration."
At 5.5 months, anti-mir-25-treated mice also had substantially improved left ventricular functioning and greater survival compared with control animals.
"Given that anti-miR-25 had no salutary effect on cardiac morphometric or hemodynamic para- meters in SERCA2a knockout mice, did not affect expression of other miRNAs with homologous seed sequences, and that AAV9-mediated cardiac transfer of miR-25 selectively affected SERCA2a, we propose that the beneficial effect of anti-miR-25 is due to the inhibition of pathologically upregulated endogenous miR-25 and the subsequent restoration of SERCA2a activity," the scientists wrote in Nature.
As a result, miR-25 inhibition may be a "novel therapeutic strategy for the treatment of heart failure," they concluded.