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Q&A: Stanford Philip Tsao on miR-21 and Abdominal Aortic Aneurysms


Philip Tsao

Professor, cardiovascular medicine, Stanford University School of Medicine

• Associate professor, medicine, Stanford University School of Medicine — 2006-2011
• Assistant professor, medicine, Stanford University School of Medicine — 2002-2005
• Postdoc, Stanford University School of Medicine — 1991-1995
• PhD, cardiovascular physiology, Thomas Jefferson University — 1991
• BS, exercise physiology, Pennsylvania State University — 1987

Researchers from Stanford University School of Medicine this week published a report linking a ubiquitous microRNA, miR-21, with the progression of abdominal aortic aneurysms.

According to the paper, which appeared in Science Translational Medicine, the miRNA proved to be a “key modulator of proliferation and apoptosis of vascular wall smooth muscle cells during development of [the disease] in two established murine models.”

The findings also suggest that modulation of miR-21 could prove to be a “new therapeutic option to limit AAA expansion and vascular disease progression.”

This week, Gene Silencing News spoke with Philip Tsao, the senior author of the paper, about the results.

Let's start with some background on your research and on abdominal aortic aneurysm.

I've always been a vascular biologist, and most of the work up until now has concentrated on the molecular mechanisms of atherosclerosis — clogging and hardening of the arteries. Abdominal aortic aneurysms, or AAAs, are believed to be related to atherosclerosis as they are often associated with the same type of plaques on the blood vessel wall, but instead of it being an occlusive disease … the wall begins to weaken and bulge from the pressure from the blood inside.

Over time … as the structural integrity of the blood vessel decreases further, that ballooning will continue until … rupture of the blood vessel. Since it's the major highway from your heart to your legs, it carries quite a lot of blood and [rupture] can be quite catastrophic.

What is the general treatment process?

It's a little bit bleak. First of all, it is a fairly common disease … [primarily among] individuals over 55. The prevalence rates, depending on which study you read, can be from 3 up to 16 percent for men. In women, it's about half that. That's a lot of older adults.

The disease is usually asymptomatic, and you don't know you have this until it gets to such a large scale that it may cause a patient to see a doctor for significant back pain or some type of abdominal pain. [Alternatively], you could feel a throbbing mass in your belly. Those are cardinal signs that you have a large AAA. … [For the most part], they go undiagnosed … until they get to be a reasonable size.

[Currently], there are no specific medical therapies directed for AAAs. If you are diagnosed, the first step is to mitigate risk factors such as smoking cessation and blood pressure medication. Otherwise, you reach a point of watchful waiting, watching it grow over years until it gets to the point where the risk of rupture outweighs the risk of the major surgery needed to repair the AAA.

Can you talk about the latest work and how miR-21 came onto your radar?

One of the approaches we've used over the years is to look at both in vitro cell-based models, as well as predictive animal models of disease … and take a generally holistic viewpoint. In other words, looking at the whole genome using transcriptional profiling or some other approach that would give you a broad swath of what patterns of genes are going up and down.

Using that technique, it is quite easy to make lists of genes that go up with disease and lists of genes that go down with disease. However, the questions remain, 'Which ones are causative of the disease?' and 'If we believe that these complex diseases are regulated by coordinated pathways of genes, then how are those pathways regulated?'

With the discovery [that] microRNAs ... can block the translation of potentially dozens, if not hundreds, of genes simultaneously … these become attractive regulatory mechanisms to account for patterns of gene expression.

With that as the rationale, we started to profile the different microRNAs that might be altered during the course of this disease and overlay that on top of the gene expression. One of the signals that came to the top was microRNA-21.

It became attractive for two reasons. One, it was one of the most regulated microRNAs over the time course of our disease models. And number two, some published work has shown that it is involved in processes such as cell proliferation and cell survival, potentially matrix production, and even inflammation — three of the main aspects of AAA that everyone agrees is part of the pathophysiology of the disease.

We took a two-pronged approach, looking at cultured cells we think are altered in the course of the disease — smooth muscle cells, fibroblasts, endothelial cells — and taking advantage of our predictive animal models. Using lentiviruses to over-express miR-21, as well as locked nucleic acid-based antagonists to block the expression thereof, we were able to demonstrate that miR-21 was significantly involved with the growth rate of these aneurysms.

Was it down-regulation of miR-21 that had an effect on the aneurysms' growth?

We saw that miR-21 went up with the course of the disease, so your natural inclination would be [to block its expression] to inhibit disease progression. But we found the opposite. When we suppressed miR-21 expression, we got expanded aneurysms. And the converse was true, as well; if we over-expressed miR-21, we actually limited the course of the disease.

We interpret these results to indicate that miR-21 has an endogenous, protective effect. [In AAA], it's turned on, but not turned on sufficiently to mitigate the progression of this disease.

[In terms of translating these findings to humans], I should say that miR-21 is a ubiquitous microRNA that appears to be involved with many different processes, and has been studied in the past in cardiac hypertrophy, as well as in different cancers via pro-fibrotic and pro-proliferative mechanisms.

In our situation, [when miR-21 was over-expressed], we had beneficial effects on AAAs. However, due to the global nature of the therapy (intravenous injections), we also saw effects in other areas where the microRNA accumulated — the liver and the heart. There, we saw what we would predict to be negative consequences: fibrosis in both of those tissues.

Certainly, from a mechanistic insight, miR-21 is important for AAA. Whether we can direct miR-21 therapy specifically to that area would become the next bottleneck in terms of translating this particular microRNA into an AAA therapy.

There are potential platforms that can be used to direct therapy, whether it be catheters that would deliver microRNAs in a particular location or … a [drug-eluting] stent graft.

That is the approach being taken by certain therapeutic companies. Are you working with anyone in industry on this?

Not yet. We'd love to, and we think that AAA is one of those beachfronts that could both test a delivery platform, as well as specific microRNAs to mitigate AAA growth.

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