The Heart of the Matter
Are miRNAs useful for tracking and treating cardiovascular disease?
Rapid and accurate diagnosis of heart attacks—and the assessment of damage—is critical for improving coronary care. Mature microRNAs (miRNAs) are abundant, easily measured, and relatively stable in blood plasma. If they prove indicative of disease states, miRNAs measured from peripheral blood may be a particularly attractive source for routine clinical assessments.1
Naoharu Iwai’s lab in Osaka, Japan, and Stephane Heymans and Blanche Schroen in Maastricht, Netherlands, found greatly elevated levels of cardiac myocyte–associated miRNAs in the plasma of patients with acute myocardial infarction (AMI).2,3 Increased plasma levels of the mature miRNAs miR-208b and -499 were consistently seen in AMI patients regardless of age, sex, body mass index, systolic blood pressure, and white blood cell count. Importantly, levels of these miRNAs correlated with concentrations of cardiac troponin, currently the best clinical marker for AMI. Unlike troponin, which can also indicate renal dysfunction, these miRNAs appeared to be specific for AMI. Detection of miRNAs, using PCR, is also more sensitive than assays for troponin, although troponin assays have been improved in recent years. These studies suggest that miR-208b and -499 may be useful in tracking both the extent of coronary damage and efficacy of treatment.
The idea of using miRNA as markers of disease is bolstered by the intriguing possibility that miRNAs may participate in cell-cell communication. David Galas and colleagues at the Institute for Systems Biology in Seattle, Washington, found evidence in cell culture that miRNAs were packaged by RNA-binding proteins that protect the nucleic acids from degradation in the bloodstream.4 It is not clear, however, whether these secreted mature miRNAs can be incorporated into silencing complexes (RISCs) in target cells, suggesting that they may not be biologically active. It will be extremely interesting to monitor how this controversy is resolved, and what the implications will be for miRNAs in this setting.
Three manifestations of heart disease—cardiac hypertrophy, heart failure, and myocardial infarction—are each associated with distinct miRNA expression patterns. In instances where attenuation of a particular miRNA appears pathogenic, therapeutically increasing miRNA levels could be beneficial. Regeneration of damaged heart muscle is one of the most appealing therapeutic options to restore myocardial function, and there is some evidence that miRNAs could facilitate the process.
In a fascinating study, Sai Kiang Lim at the Institute of Medical Biology in Singapore, and colleagues, demonstrated that phospholipid vesicles secreted by mesenchymal stem cells, which reduced tissue damage in a mouse model of myocardial ischemia/reperfusion injury,5 contained RNA. Specifically, these vesicles were enriched with pre-miRNAs (~70-nucleotide hairpin structures that are incorporated into RISCs and give rise to mature functional miRNAs), which, unlike single-stranded, mature miRNA, can be readily taken up by other cell types.6 In contrast to secreted mature miRNAs, pre-miRNAs can become biologically active miRNAs once inside the cell, and therefore might be expected to provide some benefit against ischemic injury.
Another example comes from Rakesh Kukreja’s lab at Virginia Commonwealth University Medical Center. Brief episodes of ischemia can protect against future myocardial infarction, in a process known as ischemic preconditioning. Kukreja’s lab isolated and purified miRNAs from preconditioned mice, and injected them into mice with no experience of heart disease. As a result, the levels of endothelial nitric oxide synthase and heat shock protein (HSP)70 increased, which protected the mice against subsequently induced ischemic injury.6
However, overexpression of certain miRNAs may also be pathogenic, and in that case, effective treatments might be found in methods to bind and inactivate miRNAs. Such methods have so far been tested only on a protective miRNA, but could in principle be targeted to any miRNA. In an experiment that demonstrated miRNA reduction, Cesare Peschle and Gianluigi Condorelli, used heart cells that respond to stress by increasing in size and volume. However, experimental overexpression of a protective miR-133 reduced the size of engorged, or hypertrophic, cells. The researchers wanted to test whether this overexpression of miRs could be reversed using antisense miRNA inhibitors (antagomirs), or longer RNA molecules containing multiple binding sites for the miRNA of interest (miRNA sponges). Antagomirs and sponges soak up miRNAs by binding to them, thus preventing them from interacting with their targets. The authors used the sponge and antagomir to successfully inhibit the miRNA-mediated effects in mice expressing the miR-133. They induced sustained cell engorgement by treating mice with an miR-133 antagomir or a sponge delivered inside the cell by adenovirus infection, suggesting a potential therapeutic approach against miRNAs that are pathogenic rather than protective.7
Circulating miRNAs may prove to be diagnostic and prognostic markers superior to those currently in use. However, miRNA-based therapeutics pose challenges. As with most small molecule drugs, miRNAs may be taken up by tissues that do not normally express the miRNA of interest. This raises the possibility that this approach could perturb multiple cellular functions, and some of those perturbations could be pathological. However, miRNA research is a relatively young and growing field, and the recent explosion in interest gives hope for a new wave of miRNA-based medical treatments.
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