Dec 13, 2021
Please join Guest Host Mercedes Carnethon, author Jason Roberts, and Associate Editor Vlad Zaha as they discuss the article "Epigenetic Age and the Risk of Incident Atrial Fibrillation."
Dr. Carolyn Lam:
Welcome to Circulation on the Run, your weekly podcast summary and backstage pass to the journal and its editors. We're your co-host, I'm Dr. Carolyn Lam, Associate Editor from the National Heart Center in Duke National University of Singapore.
Dr. Greg Hundley:
And I'm Dr. Greg Hundley Associate Editor, Director of the Pauley Heart Center at VCU Health in Richmond, Virginia. Well, Carolyn, this week's feature we're going to learn more about the risk of incident atrial fibrillation, but as that pertains to epigenetics. But before we get to that feature, how about we grab a cup of coffee and get started on some of the other articles in the issue. Would you like to go first?
Dr. Carolyn Lam:
I would love to. And the first paper I want to highlight asks the question, are social economic variables associated with 30 day survival after out of hospital cardiac arrest. And this comes from Dr. Jonsson from Karolinska Institute in Stockholm, Sweden, and colleagues. They linked data from the Swedish Registry of Cardiopulmonary Resuscitation with individual level data on social economic factors. In other words, educational level and disposable income, all from statistics, Sweden. And what they found was that both higher disposable income and higher educational level independently associated with better 30 day survival following out of hospital cardiac arrest. The relationship between disposable income and 30 day survival was more robust for mediating factors compared to educational level.
Dr. Greg Hundley:
Oh, wow Carolyn. Really interesting in a very, what I would call hot topic these days. So what are the clinical implications of this particular study?
Dr. Carolyn Lam:
Well, the results really highlight the importance of preventive efforts aimed at patients with lower social economic status. And these preventive actions could include both early recognition and warning signs and for example, CPR and AED training. So very lovely paper there.
Dr. Greg Hundley:
Absolutely. Very nice Carolyn. Well, my first paper comes to us from Dr. Nan Wang from Columbia University Medical Center. And Carolyn this paper focuses on a common genetic variant called link RS 3184504, and it is associated with increased platelet and neutrophil counts, coronary artery disease, thrombotic stroke, and autoimmune diseases. And so this research group previously has shown that hematopoietic link deficiency synergizes with hyperlipidemia to promote platelet production and activation, neutrophilia, platelet neutrophil aggregates, atherosclerosis and arterial thrombosis, all of those things. So platelet activation and platelet neutrophil interactions have been shown to promote neutrophil extracellular traps or net formations. So nets are formed when neutrophils release their contents leading to the formation web-like structures made of DNA, myeloperoxidase, citrullinated histone and proteases that entrap and kill bacteria. Now, while nets may help to suppress infection, the formation of nets called NETosis in blood vessels can promote atherosclerosis and thrombosis. And so this study was undertaken to investigate the hypothesis that linked deficiency might promote NETosis leading to formation of unstable atherosclerotic plaques, and arterial thrombosis.
Dr. Carolyn Lam:
Wow. What a really neat hypothesis and NETosis. I learn new things all the time. So what do they find?
Dr. Greg Hundley:
Right Carolyn. First of all, hypercholesterolemic mice with hematopoietic link deficiency displayed accelerated arterial thrombosis with nets in thrombi and these changes were reversed by PAD4 deficiency or OxPL antibodies. Second, linked deficient platelet from hyperlipidemic mice expose and release increased OxPL when activated promoting NETosis, when incubated with link deficient neutrophils. Third, an AntiOxPL antibody reduced OxPL levels, NETosis and arterial thrombosis specifically in link deficient mice, and finally Carolyn targeting atherothrombotic risk using OxPL antibodies might be particularly effective in genetically defined populations with reduced link function or increased JAK-STAT signaling.
Dr. Carolyn Lam:
Wow. Okay. So they proved their hypothesis. Could you sum it up for us, Greg?
Dr. Greg Hundley:
You bet Carolyn. So this foundational work suggests that perhaps future studies targeting NETosis and OxPL in patients carrying the common link loss of function variant, could reduce atherothrombotic risk.
Dr. Carolyn Lam:
Wow. Thanks, Greg. My next paper is super interesting in its approach. Listen up. Now the assessment of the relationship between myocardial ATP production and cardiac workload. We know is important for better understand disease development and choice of nutritional or pharmacological treatment strategies. So what Dr. Berndt from Charity University and colleagues did, was they developed a comprehensive physiology based mathematical model of cardiac energy metabolism. And this model is called cardiokine one. And what it does is it recapitulates numerous experimental findings on cardiac metabolism obtained with isolated cardiomyocytes, perfused animal hearts and in vivo studies with humans. The model encompassed all pathways along, which the possible energy delivering substrates like glucose, long chain fatty acids, keto bodies, acetate, branch chain, amino acids are utilized.
Dr. Carolyn Lam:
They use the proteomic space, the abundance of metabolic enzymes and cardiac tissue to generate individualized metabolic models of cardiac energy metabolism. And so to prove their case, they further applied this approach to the left ventricles of controls in patients with mitral insufficiency and aortic stenosis, and showed that despite overall preserved systolic function, the ATP producing capacity of these left ventricles of patients with valvular dysfunction was generally diminished and correlated positively with mechanical energy demand and cardiac output.
Dr. Greg Hundley:
So Carolyn really interesting findings. Sort of linking metabolism them with ventricular dysfunction in those with valvular heart disease. So what were the clinical implications here? What's the take home?
Dr. Carolyn Lam:
Well, this methodology is just awesome, but what they also found I think is a very important physiological principle. And that is, while metabolic capacity have a significant correlation with biomechanical properties like myocardial power and cardiac output, they can also vary considerably between individual patients and therefore help us to understand in future perhaps why some patients develop heart failure over time while others with similar hemodynamic conditions do not. So just interesting. I think it just opens the space to a lot more.
Dr. Greg Hundley:
Absolutely beautiful summary there Carolyn. Well, in the rest of the mailbag for this issue, we have an exchange of letters between Professors Hu and Trifon on the previously published paper, entitled “Short Term Treatment with Aspirin plus Clopidogrel Compared to Monotherapy of Aspirin May Not Significantly Decrease the Risk of Stroke Recurrence.” Also, there's a Research Letter from Professor Catalucci entitled, “Nano miR-133A Replacement Therapy, Blunts Pressure Overloaded Induced Heart Failure.” And then finally Carolyn, there's an In-Depth article from Professor Aengevaeren entitled, “Exercise-Induced Cardiac Troponin Elevations From Underlying Mechanisms to Clinical Science.” Well Carolyn, how about we get onto that feature discussion and learn more about incident atrial fibrillation and the age of epigenetics.
Dr. Carolyn Lam:
Let's go.
Dr. Mercedes Carnethon:
Welcome to this episode of Circulation on the Run, where we're going to have a very exciting discussion about a paper on epigenetic age and the risk of incident atrial fibrillation. We're extremely excited to have the lead author here with us, Dr. Jason Roberts from the Population Health Research Institute, McMaster University and Hamilton Health Sciences in Ontario Canada. And I am really excited to host this episode alongside the handling editor. My name is Mercedes Carnethon and I'm the professor and vice chair of Preventive Medicine at the Northwestern University School of Medicine. And I'm pleased to be hosting this with Dr. Vlad Zaha from UT Southwestern Medical School, who was the associate editor who handled the piece. So I'm really excited to jump right into this because I think there's a lot that we can all learn from this. So welcome Jason, and thank you so much, Vlad.
Dr. Jason Roberts:
Thank you so much for having me, it's a delight to be here.
Dr. Mercedes Carnethon:
So Jason, tell us a little bit about the rationale for this study, what you found and what it means.
Dr. Jason Roberts:
Absolutely. So as a cardiac arrhythmia specialist, I see a lot of patients with atrial fibrillation. And in 2021, our understanding of its underlying pathophysiology still remains modest. Our treatment strategies for the condition are also somewhat modest, although catheter ablation and antiarrhythmic drugs can potentially be very effective. In the context of these limitations, they're also exacerbated to some extent by the prevalence of atrial fibrillation, increasing dramatically in developed countries. Part of this is related to the obesity epidemic. Things like hypertension increasing becoming more common, but because atrial fibrillation is age dependent and because of our aging populations in developed countries, this is felt to have a major contribution to the growing prevalence of atrial fibrillation. Unlike obesity and hypertension and other risk factors, which are potentially modifiable, chronological aging is viewed as non-modifiable. It's not something that we can tackle. That said, we know within the population and just from personal experience that people age at different rates. There are some people that are 65 who behave more like they're 50, other people that are 50 who behave more like they're 65.
Dr. Jason Roberts:
And in that context, biological aging, we wondered whether or not, does biological aging independent of chronological aging potentially impacts the risk of atrial fibrillation. If that was the case, because there are gradually accumulating to suggest that biological aging is potentially modifiable, that could potentially open up the possibility of tackling aging as a respective for atrial fibrillation. So that drove us to ask this question. In terms of what we found in the approach that we used. So we used our biological marker of aging, was something called an epigenetic clock. So it's been found that modifications to DNA, specifically methylation at CpG at dinucleotides, they correlate with aging. This has been appreciated for a few decades. It was initially felt that with aging, methylation levels gradually reduced over time. But with more careful interrogation, it's shown that there's patterns. Some methylation areas increase, other methylation areas there's decreases.
Dr. Jason Roberts:
And Steve Horvath, who is a scientist at UCLA has found that using mathematical algorithms, you're able to very accurately ascertain chronological age based on the patterns of DNA methylation, he's called these things epigenetic o'clock. That said, even though they very accurately ascertain chronological age, they aren't perfect in each individual in terms of matching up to their chronological age, but that's actually turned out to be a good thing. So when people, their epigenetic age is older than their chronological age, they're said to have positive epigenetic age acceleration. They may be biologically older than their actual chronological age. And then the reverse also holds. So using this concept of epigenetic age acceleration, we ask whether or not do people that are older biologically on the basis of their epigenetic age, do they have an increased risk of atrial fibrillation? And then we tackle that using a few different core works that I'm certainly happy to elaborate on in terms of what we found.
Dr. Jason Roberts:
So we used three population based cohorts from the United States, the well known Framingham Heart Study, the Cardiovascular Health Study and Eric as well. There were approximately just under 6,000 people from those studies that had undergone genome wide methylation analysis that in the enabled us to calculate their epigenetic ages. The follow period for these people was just under 13 years. And then we look to see whether or not these epigenetic clocks associated with instant atrial fibrillation. In these cohorts, we look at five different clocks. So there's the Horvath Clock and the Hannum clock that were designed to predict chronological aging. The more recent clocks, things like DNAm PhenoAge and DNAm GrimAge are more designed to predict aspects of clinical phenotype and also mortality. We found that in unadjusted analyses, all of these clocks were associated with atrial fibrillation. When we then adjusted for multiple different clinical variables, we found that the DNAm PhenoAge clock and the DNAm GrimAge clock continued to exhibit statistically significant associations with atrial fibrillation.
Dr. Jason Roberts:
Interestingly, the multi-variable adjustment, one concern is, do these clinical factors, are they confounders where we should be adjusting, or are they potentially mediators. If we adjust for mediators that potentially masks the effect of the clock. But regardless of how we treat them both DNAm PhenoAge and DNAm GrimAge, we're associated with increased risks of incident atrial fibrillation. Alluding to the possibility that biological aging independent of chronological aging is important in terms of determining risk for atrial fibrillation. And it may be that if we're able to modify biological aging, we could potentially reduce the risk of atrial fibrillation. So that's the study in a nutshell.
Dr. Mercedes Carnethon:
No, that is really exciting. You said something early on about chronological age being immutable. And I would have to say, both Vlad and I are not aging. And in fact, we are going in the opposite direction. If only this were not just an audio podcast, you would see that I steadily gotten younger and younger and I'm suddenly about 25 now. But no, these are really important findings. I really like the innovation of using multiple different strategies to characterize epigenetic age and genetic aging. So tell me Vlad, I want to turn to you. When this came across your desk, what excited you about this particular piece and why did you think that it would be of great interest to our readership?
Dr. Vlad Zaha:
Good morning Merci and Jason. This is a great question. And as in associate editor at Circulation for the bridging discipline section, it was fascinating to see this topic coming on my desk, thinking about all the genome wide association studies in nature of fibrillation and predisposition to atrial fibrillation, that in that case would not be changed by interventions because of different loci that would be determined. This was coming as a completely new perspective that was opening some new potentials. And it was very interesting to see some of the findings.
Dr. Mercedes Carnethon:
Certainly. So Jason, I have a question. So what surprised you about the findings of this particular study?
Jason Roberts:
Yeah, that's a great question. So we had hoped that biological aging would be associated with atrial fibrillation. I think the concept of being able to tackle biological aging is exciting. In terms of what surprised us, I guess we were hoping for these results, I guess.
Dr. Mercedes Carnethon:
Yeah.
Dr. Jason Roberts:
But we were…Yeah. So I guess we were pleasantly surprised that our hypothesis was born out. It's important to note that the epigenetic clocks don't tell the full story with chronological aging. So after we insert the clock into the model, chronological age continues to remain associated with instant atrial fibrillation. So this measure biological aging is just part of the story. So I think that's very important. I had wondered whether or not inserting the epigenetic clocks would that potentially eliminate the subsequent association of chronological aging. So that finding suggests it's part of the story.
Dr. Jason Roberts:
I think that in terms of the overall concept, the idea of this being reversible really excites me. In terms of the approach of how to reverse biological aging. Right now healthy lifestyle seems to be very important. I think it provides more evidence to suggest to patients with atrial fibrillation, living healthy from a diet perspective, from exercise, keeping your weight under control, all of these things that seem to impact epigenetic aging and biological aging can be helpful for preventing atrial fibrillation. So I think that can help reinforce this message to our patients.
Dr. Jason Roberts:
I think ultimately in terms of where we'll be at in 15 to 20 years, it's possible that new therapies in the future are developed that are able to more powerfully address biological aging. As you alluded to, will it be possible to reverse biological aging as you and Vlad are experiencing that?
Dr. Mercedes Carnethon:
Most definitely. Yes.
Dr. Jason Roberts:
I think it may be possible. This is an intense area of investigation that's being pursued and it's still in its relative infancy. But I think that could it be small molecules? Could it be potentially gene editing that can help adjust biological aging and not only increase lifespan, but also health span? I think those concepts are really exciting.
Dr. Mercedes Carnethon:
I completely agree. There's a lot of richness in this paper and I think our readership is going to really enjoy digging in. Part of the richness is the use of three different cohorts and the use of multiple measures of epigenetic age. And I think you provided a really nice description of the unique information that each of these markers of epigenetic age provide. One thing I note are differences in the strength of association across the different measures of epigenetic age, which I think makes sense, because you said they characterize different aspects of the phenomenon, but I also see what looks like some variability across the cohorts with Framingham in particular seeming to stand out. And that being the only cohort that is 100% one race. It's white. Versus both the cardiovascular health study and the Eric study, which have more diverse study populations. I'm wondering what your hypothesis is about the differential strength of association that it seems Framingham is demonstrating and what you think is possibly the source of those differences.
Dr. Jason Roberts:
Yeah. I think those are great questions for all of genetics. The question is, does it apply to all races? For example, polygenic risk scores. It seems like when a polygenic risk scores develop for one race, it may not perfectly translate over to other races. So how relevant is that for epigenetic age acceleration. In this study, I think it's difficult to make definitive conclusions about it. We needed the three cohorts to have adequate statistical power in terms of being able to determine a differential effect of race. I think it would really be primarily hypothesis generating. We weren't really powered to look at the different races. So it's difficult for me to comment.
Dr. Jason Roberts:
I think ultimately and I want to believe anyways, that epigenetic age acceleration is relevant to all races, but in terms of, was it race that drove the differential impacts that we saw to some extent in terms of the magnitude of the hazard ratios, it's difficult to know in terms of tests for interaction and were these actually truly statistically different. We weren't adequately powered to address that hypothesis. So it's difficult for me to comment in a definitive matter I'd say. And sorry to cop out on…
Dr. Mercedes Carnethon:
No, not at all. I mean, I think there are a lot of things where there is no firm answer and that was just one of my hypotheses when I saw what was going on differently across the cohorts. I think that's a perfectly reasonable answer that sets us on a course for thinking about how we set up future studies. So I wanted to turn to you Vlad for the closing frame around this. As the editor, how do you hope that our readership will use these findings?
Dr. Vlad Zaha:
That is an excellent question. I was going to follow on this excellent unpacking of the core messages of the manuscript by Jason here to get his perspective as an electrophysiologist into what these type of work may represent for the everyday life of an electrophysiologist in the connecting with the patients and how would this type of approach influence, and maybe now, maybe later when our treatment for atrial fibrillation.
Dr. Jason Roberts:
Yeah. So that's a great question. I think, as I alluded to some extent before, as far as reinforcing healthy lifestyle, I think this provides more evidence in that respect. So we know that things like excessive alcohol consumption, being excessively obese, poor diets, not engaging in enough exercise, all of those things seem to accelerate your epigenetic age. And those are all things that we think or feel that are important with atrial fibrillation in terms of driving the path of physiology and people progressing. So I think this gives more data to us to reinforce the patients that in addition to the treatments that we're offering in terms of catheter ablation and antiarrhythmic drugs, the concern is that the substrate can continue you to progress. And that's likely driven by to some extent these modifiable risk factors. So keeping all of these under best control as possible, and hence trying to slow your biological aging as much as possible.
Dr. Jason Roberts:
I think that this will provide us more motivation to push these messages to our patients. A lot of patients can sometimes be like, "Let's just get on with a catheter ablation and I want to get on with my life…" but it really I think, provides more data to suggest that modifying these very important risk factors that can lead to accelerated biological agents, is very important. And in terms of the future as mentioned, so chronological aging, as people get older, people view it as, "Well, there's nothing I can do, and I'm just going to get gradually more and more unhealthy." I think, and this is somewhat futuristic, but to what extent can we slow biological aging? Can we potentially reverse it in the future? There's certainly lots of very compelling and interesting animal work and people are starting to delve into this in a big way.
Dr. Jason Roberts:
And not only to increase lifespan, will we some day live until we're 200. Who knows? But the concept of prolonging your health span as well. So the number of healthy years that you have before your body starts to gradually give way, I guess to some extent. Hopefully in the future will have therapies that will help keep us healthy. And if we do that increased health span, I think this data suggests that atrial fibrillation will be one thing that benefits from this. So hopefully in the future, maybe in terms of curbing the AFib pandemic, being able to address biological aging will help push things in the right direction.
Dr. Mercedes Carnethon:
Well, thank you so much Jason. And thank you so much Vlad for your thoughtful questions. I really like that the final bottom line leans towards my area as an epidemiologist, which is maintaining and promoting healthy lifestyles as a way to hopefully help prevent some of the difficulties of atrial fibrillation and its long-term outcomes. Really pleased to have you on this episode of Circulation on the Run, Jason, and thank you again Vlad, and I hope everyone enjoys this episode of the journal and has an opportunity to really dig into this piece. This is Mercedes Carnethon from Northwestern University Feinberg School of Medicine, saying thanks for listening today.
Dr. Greg Hundley:
This program is copyright of the American Heart Association 2021. The opinions expressed by speakers in this podcast are their own and not necessarily those of the editors or of the American heart association for more visit ahajournals.org.