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Circulation on the Run

Jul 27, 2020

Dr Carolyn Lam: Welcome to Circulation on the Run, your weekly podcast summary and backstage pass to the journal and its editors. I'm Dr Carolyn Lam, associate editor from the National Heart Center and Duke National University of Singapore.

Dr Greg Hundley: And I'm Dr Greg Hundley, associate editor, Director of the Pauley Heart Center from VCU Health in Richmond, Virginia.

Dr Carolyn Lam: Our feature paper today discusses trans-ethnic genome-wide association studies and the insights in the genetic architecture and heritability of long QT syndrome, a massive study that we will be digging into, but only after we talk a little bit about the other papers in this week's issue. And I'm going to start, Greg. Are you ready with your coffee?

Dr Greg Hundley: I am.

Dr Carolyn Lam: The first original paper really represents seminal work, showing that the endothelium can directly regulate obesity and insulin resistance. Now, as obesity develops, there is a decline in adipose tissue vascularity, which seems counterintuitive, and an increase in fibrosis.

So authors, led by Dr Chen from the Irell and Manella Graduate School of Biological Sciences in the City of Hope, speculated that the reduction in vascularity in this adipose tissue might have an adverse effect on adipose tissue function. Now, these authors previously identified Argonaute-1, or AG01, a key component of microRNA-induced silencing complex, as a crucial regulator in hypoxia-induced angiogenesis.

So in the current study, they aim to determine the AG01-mediated endothelial cell transcriptome, the functional importance of AG01-regulated endothelial function in vivo, and the relevance to adipose tissue function and obesity.

A new mouse model with genetic deletion of AG01 in the endothelium was useful to investigate the importance of endothelial regulation of adipose tissue function. The findings were that in mice fed high fat, high sucrose diet, the suppression of endothelial AG01 promoted adipose tissue browning, and led to an anti-obesity phenotype. Endothelial cell AG01 thrombospondin-1 pathway was induced in the endothelium from human donors with insulin resistance.

In total, this study suggests a novel mechanism, by which endothelial cells through AG01 thrombospondin-1 pathway controls vascularization and function of adipose tissues, insulin sensitivity, and whole-body metabolic state.

Dr Greg Hundley: Interesting, Carolyn. So tell me about this clinically. Where do we take this from here?

Dr Carolyn Lam: I thought you would ask. So endothelial dysfunction, per se, can cause metabolic dysregulation, rendering targeting dysfunctional endothelium, a potential therapeutic strategy to counteract obesity, and metabolic disorders. So this study really opens a door to that.

Dr Greg Hundley: Very nice. Well, I've got another basic science paper, and it evaluates single-cell RNA sequencing to dissect the immunological network of autoimmune myocarditis. And it comes from Dr Jiangping Song from the State Key Laboratory of Cardiovascular Disease of Fuwai Hospital, and the National Center for Cardiovascular Disease, Chinese Academy of Medical Sciences, and Peking Union Medical College.

So Carolyn, the study aimed to investigate the immunological network during the transition from myocarditis to cardiomyopathy, and to identify the genes contributing to the inflammatory response to myocarditis.

So mice were treated with myosin heavy chain alpha-peptides to generate an experimental autoimmune myocarditis model. The investigators performed single-cell RNA sequencing analysis of CD45 plus cells extracted from mouse hearts during different experimental autoimmune myocarditis phases, including normal control, acute inflammation, subacute inflammation, and then in the myopathy phase. Also, human heart tissues were collected from surgically removed hearts of patients who had undergone heart transplantation.

Dr Carolyn Lam: So what did they find, Greg?

Dr Greg Hundley: Well, Carolyn, a comparison of the single-cell RNA sequencing data from different experimental autoimmune myocarditis phases suggested that some cell clusters, such as macrophage cluster 2 and Th17 cells, were associated with the inflammatory response in the experimental autoimmune myocarditis model.

The HIF1A expression level correlated with the extent of the inflammatory response, and PX-478, a HIF1A inhibitor, alleviated the inflammation during the different experimental autoimmune myocarditis phases.

Immunohistochemical staining revealed that HIF1A expression was upregulated in autoimmune myocarditis from the tissue samples from the explanted hearts. Thus, the HIF1A inhibitor alleviated inflammatory cell infiltration, and that may serve as a potential therapeutic target in clinical practice.

Dr Carolyn Lam: Wow. That is some serious clinical implications. Well, my next paper is really the first systematic echocardiographic evaluation of consecutive patients requiring hospitalization due to COVID-19, and it comes from Dr Topilsky and colleagues from Tel Aviv Medical Center.

Dr Greg Hundley: So Carolyn, what did they find in this series?

Dr Carolyn Lam: So among a hundred consecutive patients diagnosed with COVID-19 infection who underwent complete echocardiographic evaluation, within 24 hours of admission, only 32% had a normal echocardiogram at baseline. The most frequent abnormality was right ventricular dilatation or dysfunction.

Among patients developing clinical deterioration during follow-up, which were 20% of these hospitalized patients, repeated echocardiograms showed further deterioration of the right ventricular parameters, probably related to increased pulmonary resistance. Five of these patients had deep vein thrombosis.

Dr Greg Hundley: Carolyn, my next study comes from Dr Stephen Fremes, and it's a modeling study out of the University of Toronto. It modeled TAVR versus SAVR valve durability to determine the effects on life expectancy across a broad range of age.

Dr Carolyn Lam: Interesting. And what were the results?

Dr Greg Hundley: Well, based on their simulation models, the durability of TAVR valves must be 70% shorter than that of surgically replaced valves to result in reduced life expectancy in patients with similar demographics to recent trials.

However, in younger patients, the threshold for TAVR valve durability was substantially higher. In younger patients, life expectancy was reduced when TAVR durability was 30%, 40% and 50% shorter than surgical valves in 40, 50 or 60-year-old patients, respectively.

So Carolyn, these findings suggest that durability concerns should not influence the initial treatment decision regarding TAVR versus SAVR in older low-risk patients, based on current evidence supporting TAVR valve durability. However, in younger low-risk patients, valve durability must be weighed against other patient factors, such as life expectancy.

Dr Carolyn Lam: Thanks Greg, for that summary. Well, let me tell you about other papers in this issue. There are a pair of letters to the editor by Dr Opotowsky, and a response by Dr Goldberg regarding the paper results of the Fontan Udenafil Exercise Longitudinal, or FUEL trial.

There's a research letter by Dr Strik, Validating QT-Interval Measurement Using the Apple Watch ECG to Enable Remote Monitoring During the COVID-19 Pandemic. There are two On My Mind papers, the first, Telemedicine and Forgotten America by Dr Julien, and the second, The COVID-19 Pandemic: Ethical and Scientific Imperatives for "Natural" Experiments by Dr Lewis.

Dr Greg Hundley: Very nice. Well, Carolyn, I've got a research letter evaluating the effect of evolocumab on atherogenic lipoproteins during the peri and early post-infarction period. It's a placebo-controlled randomized trial from Dr Gary Gerstenblith.

Sarah Cuddy also worked through a tough case of cardiac amyloid when a fat biopsy was negative, but imaging studies of the heart suggested cardiac amyloid.

Carolyn, I've also got an On My Mind piece, and it's entitled, Can Old Ally Defeat a New Enemy? And it's by Dr Paul Gurbel, and he discusses the use of inhaled aspirin to treat patients with COVID-19.

And then finally, Carolyn, I have a prospective piece from Dr Robert Lefkowitz who discusses β-arrestin-biased angiotensin II receptor agonists for treatment of COVID-19. Well, Carolyn, what a great issue, and let's get onto that feature discussion.

Dr Carolyn Lam: Yay. Let's go, Greg.

Dr Greg Hundley: Well, listeners. Now we're turning to our feature discussion, and we are very fortunate to have Professor Connie Bezzina from Amsterdam University Medical Center to talk to us about her paper related to long QT syndrome.

Welcome, Connie. And I was wondering, before we get started in discussing your paper, could you tell us a little bit about the background in this area? And then, what was the hypothesis that you wanted to address?

Prof Connie Bezzina: So over the last 20 to 30 years, we've learned a lot about the genetic underpinnings of inherited cardiac disorders associated with sudden cardiac arrest. And basically, we've learned a lot about mutations in specific genes that co-segregate with these disorders within families.

However, two outstanding features have remained unresolved. Essentially, the first unresolved issue is the fact that we observe, oftentimes, a low disease penetrance and variable disease expression within families, which means that not everybody within a family that carries a familial mutation is affected by the disorder.

But two, so among those that are affected, some are affected more severely than others. So some people would have only the ECG abnormality, whereas other people, for instance, would have the ECG abnormality and arrhythmic events. And you could also have individuals, indeed, who don't even manifest any disease manifestations. This is one of the outstanding challenges.

The other outstanding challenge is the fact that, despite extensive genetic testing of the known genes in some probands and some families, they remain genetically lucid, in that we don't find a likely genetic defect in a minority of families. And of course, that hinders genetic testing and implementation of genetic testing in such families.

Dr Greg Hundley: What was the question you were going to answer with your study? And tell us a little bit about your study design and your study population.

Prof Connie Bezzina: Yeah, so essentially, we figured that assigning these disorders to one large genetic defect might be an oversimplification of biological phenomenon. So we hypothesized that even in these Mendelian disorders, the inheritance of additional genetic factors alongside the familial mutation could contribute to risk. Of course, there will be other factors such as environmental factors, which we did not tackle in the study.

The central hypothesis of the study was that common genetic variation, which is present in the germ population, could modulate the effect of the familial genetic defect of the Mendelian mutation.

So in order to do this, we assembled a large consortium of investigators from multiple centers in Europe, in North America and Japan, worldwide, to bring together about 1700 probands with the long QT syndrome. So we tested this hypothesis in the long QT syndrome because we figured, among the rare inherited rhythm disorders, it's one of the more common disorders. Also, because each individual center has too few patients. To do this locally, we put this group of investigators together to come up with 1700 probands.

The study design was a genome-wide association study with a case-control design, where we tested the association of millions of SNPs littered across the genome with susceptibility for the disorder. So this led us to identify three single-nucleotide polymorphisms that are associated with susceptibility for the long QT syndrome.

What we immediately saw is that, actually, these three SNPs, perhaps not surprisingly at all, had been previously associated with the extent of the QT interval, with QT interval in the general population. This is not surprising, of course, because repolarization is a central part of physiological mechanism in the long QT syndrome. So this basically indicated overlap between genetic control of the QT interval in the germ population and susceptibility to the long QT syndrome.

So the fact that the three SNPs that we identified as long QT syndrome susceptibility SNPs had been associated with QT interval duration in the germ population, we felt that that was pointing to assure genetic underpinnings between these two phenotypes.

So we went on to investigate that by looking at the correlation between the odds ratio for long QT syndrome susceptibility and the effect that these SNPs have on QT interval in the germ population. And in fact, we found a very high correlation between those. So essentially, this pointed to sure genetic factors between QT interval in the germ population and long QT syndrome susceptibility.

Of course, we wanted to look for disease variability. The next thing we wanted to do was whether these SNPs could actually explain disease variability. Now, this was perhaps the most disappointing part of the study, because when we constructed a polygenic risk score based on SNPs that impact on the QT in the germ population, we found no relation to QT interval among patients, and also no relation to life-threatening arrhythmic events among the patients.

We think that this is because our patients... or probrands. They're primarily probands, so they are all more sick. So we didn't have enough variability in our patient set to identify an association with disease variability. And in fact, this is at variance with previous studies that tested individual SNPs, and even our own studies with smaller polygenic risk scores that did find an association between a polygenic risk score based on QT SNPs and QT prolongation and events among patients.

So we think that this is certainly something to study further in the future, in larger patient sets where we not only have the probands, but also their relatives, their mutation-carrying relatives, which will give us a bigger variability to actually test this hypothesis. So we think that looking at probands actually was a very good design to find susceptibility variance but was not maybe a good design to find SNPs or polygenic risk scores to test their effect on disease variability.

Dr Greg Hundley: It sounds like you've found certain gene low PSI that indicate a predilection for prolongation of the QT interval, but not necessarily are those gene low PSI consistent with who's going to experience an adverse cardiovascular event as a result of their genetic constitution. Is that a fair statement?

Prof Connie Bezzina: Well, I think that the setting, because we had probands, they were the most sick people in their families. I think to have stronger conclusions on that, we need to test the polygenic risk scores in families where there are people who are differentially affected.

Dr Greg Hundley: I see. I-

Prof Connie Bezzina: We had too-narrow of a variability in a probands-only design, as opposed to a study where we would have probands who are severely affected and mutation-carrying relatives who are less severely affected.

Dr Greg Hundley: Very nice. So that puts that clearly into context. This was a massive effort. You have quite a list of investigators, and you mentioned you had to gather so many sites. How would you conduct that next study? Would you need another large collection of individuals and many sites to take that on?

Prof Connie Bezzina: Yes. I'm a geneticist, and geneticists always want larger, larger numbers, and I'm also one of those. So I'm interested in explaining as much as possible into individual variability. And I think to do that properly, I think we should go preferably for a similar design where we will approach the same centers. And hopefully, we can organize the next study, which will have these probands and their relatives.

Dr Greg Hundley: Now, just quickly, for us working in the clinic, how should we approach genetic testing in patients with long QT?

Prof Connie Bezzina: At the moment, I think our findings don't have an immediate impact. I think our findings tell us about the genetic architecture of the disorder. And actually, one thing I haven't gone into yet is the fact that what we also found is that patients who do not have mutations in the no-long QT genes, which were called mutation-negative, which are about 20% of all long QT syndrome probands, actually have a higher burden of these common variants that prolong the QT interval.

So we think, actually, that mutation-negative long QT syndrome probands will not have a Mendelian large effect variant but will have perhaps a higher burden of these QT-prolonging alleles.

Therefore, I think this has direct implications for clinical genetics of these patients, because if you have a proband in whom you don't find a mutation in the known genes, you could think that maybe it is not monogenic, which has implications because you don't have a single genetic defect to test on that family. One would need to keep follow-up of more family members until we understand more about the genetics of those individuals.

Dr Greg Hundley: So Connie, this has been just a wonderful discussion. Any additional studies examining the genetic architecture of individuals that we need to think about for the future?

Prof Connie Bezzina: Sure. So for long QT syndrome in particular, as additional SNPs that modulate the QT interval in the germ population are identified, it will be very important to incorporate these into larger polygenic risk scores, and see whether we could have a better discriminative capacity of such polygenic risk scores in discriminating between severely affected and less severely affected people, or who is more at risk for an arrhythmic event.

Outside of long QT syndrome, I think there's a lot of work to be done with respect to the likely complex inheritance of many of these disorders that we previously considered to be Mendelian. So for instance, ongoing work in our group concerns Brugada syndrome, where we're seeing the same kind of thing, and hypertrophic cardiomyopathy, where we're seeing the same kind of inheritance.

Dr Greg Hundley: Well listeners, on behalf of both Carolyn and myself, we look forward to catching you on the run next week. Take care. This program is copyright the American Heart Association 2020.