Sep 4, 2018
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.
Current guidelines recommend measurement of one of the cardiac specific isoforms of cardiac troponin complex. However, what's the utility of combining measurements of troponins I and T in the early diagnosis of acute myocardial infarction? Well, you have to wait for our upcoming feature discussion, but it's coming right up after these summaries.
The first original paper this week sheds light on the genetic basis and mechanisms of bicuspid aortic valve, the most common congenital heart defect in the population. We know that bicuspid aortic valve is an autosomal dominant trait with variable expression and incomplete penetrants suggestive of genetic and environmental modifiers. In the current study, first author Dr Gharibeh, corresponding author Dr Nemer from University of Ottawa, and authors of the Bicuspid Aortic Valve Consortium assessed cardiac structure and function in mice, lacking a GATA6 allele. They found that GATA6 heterozygous mice had a highly penetrant type of bicuspid aortic valve with right and left leaflet fusion, which is the most frequent type found in humans. GATA6 transcript levels were lower in human bicuspid aortic valve as compared to normal tricuspid valves. Mechanistically, GATA6 haploinsufficiency disrupted valve remodeling and extracellular matrix composition through dysregulation of the importance in the molecules including matrix metalloproteinase nine. Cell-specific inactivation of GATA6 reveal that an essential rule for GATA6 in secondary heart field myocytes. Thus, the study identifies a new cellular and molecular mechanism underlying bicuspid aortic valve.
In the field of cardiac regeneration, c-Kit positive adult progenitor cells were initially reported to produce new cardiomyocytes in the heart. However, more recent genetic evidence suggests that such events are exceedingly rare. Today's paper provides insights into this discrepancy and it is from first author Dr Maliken, corresponding author, Dr Molkentin from Howard Hughes Medical Institute Cincinnati Children's Hospital Medical Center. The authors took a novel approach of deleting the necessary cardiogenic transcription factors, GATA4 and GATA6, from c-Kit expressing cardiac progenitor cells to determine whether true de novo cardiomyocyte formation would occur. They found that deletion of the necessary cardiogenic transcription factors, GATA4 and GATA6, from these c-Kit+ cardiac progenitor cells remarkably resulted in greater apparent cardiomyocyte derivation from the c-Kit+ cells. Deletion of GATA4 from c-Kit–derived endothelial progenitors altered the integrity of the endothelial cell network in the heart, resulting in greater c-Kit+–derived leukocytes entering the heart and fusing with cardiomyocytes.
Thus, they demonstrated a new role for GATA4 in endothelial differentiation, specifically showing for the first time that GATA4 is essential for vascular development by the c-Kit lineage. The study shows that leukocyte to cardiomyocyte fusion is the primary basis for path lineage tracing results, incorrectly suggesting that c-Kit+ cardiac progenitor cells generated de novo cardiomyocytes in the heart.
Lecithin–cholesterol acyltransferase, or LCAT, is the sole enzyme that esterifies cholesterol in the plasma. Its role in the supposed protection from atherogenesis remains unclear, because mutations in LCAT can cause more or less carotid atherosclerosis. Addressing this conundrum, co-first authors Drs. Oldoni and Baldassarre, co-corresponding authors Dr Kuivenhoven from University Medical Center Groningen, Dr Holleboom from Academic Medical Center Amsterdam, and Dr Calabresi from University of Milano in Italy hypothesized that genetic mutations causing complete LCAT deficiency versus partial LCAT deficiency would be differentially associated with carotid atherosclerosis in carriers of LCAT mutations. To study this, they looked at 74 heterozygotes for LCAT mutations who are recruited from Italy and the Netherlands and who were assigned to complete versus partial LCAT deficiency. These were also compared to 280 controls. Using carotid intima-media thickness as a measure of atherosclerosis, the authors demonstrated that carriers of LCAT mutations leading to complete LCAT deficiency exhibited less carotid atherosclerosis, indicating a reduced risk of cardiovascular disease.
By contrast, however, carriers of LCAT mutations leading to partial LCAT deficiency showed marginally more atherosclerosis. The association of mutations in LCAT with subclinical atherosclerosis appeared to be related to the capacity of LCAT to esterify cholesterol on apoB-containing lipoproteins since the abnormal LCAT present in the partial deficiency was only active on this class of lipoproteins. These important findings bear relevance for pharmaceutical strategists that target LCAT.
After a bioprosthesis aortic valve replacement, what is the incidence, correlates, and outcomes of hemodynamic valve deterioration? First author Dr Salaun, corresponding author Dr Pibarot from Quebec Heart and Lung Institute and their colleagues studied 1,387 patients who underwent bioprosthetic aortic valve replacement and found that hemodynamic valve deterioration identified by Doppler echocardiography occurred in one-third of patients and was associated with a 2.2-fold higher adjusted mortality. Diabetes and renal insufficiency were associated with early hemodynamic valve deterioration whereas female sex warfarin use and stented bioprosthetic valve versus the stentless ones were associated with late hemodynamic valve deterioration. These findings suggest that following bioprosthetic valve replacement, a systematic echocardiographic follow-up may be considered to ensure adequate detection and quantitation of hemodynamic valve deterioration.
That wraps up on the summaries this week. Now for our feature discussion.
We are recognizing the critical role that cardiac troponins play for the early diagnosis of acute myocardial infarction. We also know that there are different isoforms of cardiac troponins, the cardiac troponins T and I. Now, have you ever considered combining the two? How does that help the early diagnosis of acute myocardial infarction? Well, I am delighted to have with us the corresponding author of our feature paper today, Dr Christian Mueller from University Hospital Basel in Switzerland, a very familiar voice on this podcast. Welcome, Christian, and thank you so much for publishing yet another wonderful paper with us.
Dr Christian Mueller: Thank you very much for highlighting this important work and allowing me to comment on it in the podcast.
Dr Carolyn Lam: Christian, first of all, could you paint the background to help us understand what's the difference between the two isoforms, I mean, in terms of diurnal variation, the way that they may be released earlier or later, the way they may or may not be impacted by comorbidities like renal dysfunction or hemolysis? Could you help us understand why there may be rational to combine the two in looking at their impact on the diagnosis of acute myocardial infarction?
Dr Christian Mueller: The measurement of cardiac troponin as a structural protein unique to the heart clear is a central piece in our early diagnosis of acute myocardial infarction, so both for the early rule out in patients who present with chest pain and are finally found to have more benign disease as well as the early ruling. In general, I think it's important to highlight that there are two isoforms exactly as you have mentioned, so there is cardiac troponin T and cardiac troponin I. So these two proteins are cardiac specific and are used in the diagnosis of acute myocardial infarction. Now with the development of high-sensitivity methods or measurements of both cardiac troponin T and cardiac troponin I concentrations, we have been able to get a little bit of a better understanding of in fact differences in the pathophysiology as well as analytical details between cardiac troponin T and I.
Before I start highlighting the differences, I think it's important, I mean, both signals show a very strong correlation, so still very, very similar to each other. However, the small differences that have begun to emerge kind of allow to suggest that possible we could use them together as two pieces of information in the diagnosis.
So, what are the differences? First, exactly as you have highlighted, that if in fact that diurnal rhythm with cardiac troponin T, which means that cardiac troponin T concentrations are higher in the morning hours as compared to the evening, we still have no clue why that's the case, but it's a relevant difference about 25% and it has been shown in two cohorts and a group from Maastricht who was the first one highlighting this. This rhythm has not been found for cardiac troponin I. The second difference is that, again, probably understood in many, many population studies cardiac troponin T concentrations are even stronger predictors of death as compared to cardiac troponin I concentration. Then the third difference it seems that if we measure it with high-sensitivity assays, for example high sensitivity, it seems to rise or if you released from injured cardiomyocytes even slightly earlier as compared to T and possibly even less injuries necessary to release I as compared to T.
Then you mentioned renal function. Cardiac troponin T concentration shows slightly higher correlation with renal function as compared to I. Also, other pre-analytical issues, hemolysis seems to affect T and I concentration in a different way. So a lot of small tiny differences that have emerged and that underlie the hypothesis that possibly by combining the two signals we could be even more accurate in the diagnosis rather than relying on one on its own.
Dr Carolyn Lam: That's good. That really sets up the rational very well. I think in and of itself is a learning lesson, because I think most clinicians sort of take the two equivalently. So could you tell us what you found?
Dr Christian Mueller: I would like to of course thank the fantastic team that has allowed us to generate this data. It's a collaboration between the APACE investigators, the ADAPT investigators and experts in clinical chemistry from Maastricht University and Noreen Fandalin and Karen Villa of the first office. So we used two large diagnostic studies, APACE and ADAPT. We measured high-sensitivity cardiac troponin T and I and both of them and compared the diagnostic performance as compared to the final adjudicated diagnosis by two independent cardiologists who, of course, had all information, cardiac imaging and whatever you need to adjudicate.
So, what we found is that in general if you look at diagnostic accuracy, overall is quantified by the area under the curve. Combining the two signals did not consistently increase overall diagnostic accuracy as compared to the individual isoforms. However, we were able to document some improvement for the rule out for the very early rule out of acute myocardial infarction. So the concept that is extremely attractive of course from a medical as well as from an economic perspective is to rule out the presence of acute myocardial infarction with a single blood draw. So, we can do this if we assess the ECG. The ECG doesn't show relevant changes. Then if the troponin concentration measured with a high-sensitivity assay is very low, then the likelihood that the patient would have an acute myocardial infarction again is extremely low or in scientific term sort of a negative predictive value approach is 99 to 100%. By combining very low concentration for high-sensitivity T and very low concentration for I, we were able to increase the efficacy of the early rule out and that seemed to be the most likely possible clinical utility of combining the two signals.
Dr Carolyn Lam: Even that so-called neutral findings are very important. It's an important question to ask and important answer to get. Could you give us an idea for the rule-out part? How much do we gain? How much exactly do we gain by using both assays instead of just one?
Dr Christian Mueller: So, the efficacy of the early rule-out depends to some extent on the assay used and the cut off applied. So the current you see algorithm uses cut-off that has been shown to be very safe. However, they are regarding their efficacy not very high. So the current you see recommended cut-offs and approach, allows the rule-out only in about perhaps 10 or 15% of patients. That number can be significantly increased, likely doubled or perhaps even increased threefold by using the combination approach. So this has been consistently showed both in the derivation and the validation cohort.
Dr Carolyn Lam: Yeah. Do you think this is ready for prime time? I noticed a very balanced discussion actually calling for future studies, but perhaps you could state it better now.
Dr Christian Mueller: The main limitation regarding prime time is the fact that currently manufacturers either of a high-sensitivity TSA or of a high-sensitivity high method, which means that the vast majority of hospitals at this point in time do only have one method available. It would require quite substantial investment in both hardware as well as changing of the logistics in the lab to implement measurement of both assays. So I think it's likely feasible, but it would be associated with relevant investment from a hospital perspective. In addition, I mean, also the rule-out approaches that use of only one assay also there are studies ongoing in trying to further increase the efficacy of the single marker approach. So I think it's the best tool marker strategy that we were able to come up with recently, because many of the other biomarkers that we had tested really didn't work out. Still, as you mentioned, I think it's also important to be very, very honest that it will be difficult to implement tomorrow in most institutions.
Dr Carolyn Lam: Yeah, and perhaps a little bit more work needs to be done to sort first identify perhaps special situations where these may be particularly helpful. I supposed like you just said when we're thinking of the ESCs to review one-hour type algorithm, who knows maybe we should be having that extra insurance of the second test in those that test it negative in the first or something like that. Do you plan further work? I always ask you because you're always in the forefront of these things and we just love touching your work.
Dr Christian Mueller: We have several additional analyses ongoing. Again, I think the main part is for just to go ... I go back from a clinical perspective. So I think for many hospitals that are using T at the moment, it's important to have I available for certain situations. So for example if you have a patient in whom you have evidence of chronic skeletal muscle disease, most of these disorders are rare but some of them have been shown to be associated with increasingly highly troponin T that do not seemed to be related for cardiac diseases but from skeletal muscle. This is rare but if you have a patient with that kind of history, then the dual mark measurement is I think mandatory.
The same applies to iso that the other reasons to have false positive results for iso whenever you are ... If your hospital is using I, you should have the T method also available because once in a while you will identify patients in whom you have an I result that doesn't really match the clinical setting, then it's so easy and often so helpful to get the T result to decide on the most appropriate measurement of patient.
For which patients are kind of a standard that measures T and I would be justified, I think that's something to tease out in future study. I think that the rational is there and likely it will depend also on kind of which T or which I method we might use in the future. So at the moment, we have one method for high-sensitivity T, but there are several other methods in development and kind of applying for FDA approval for high-sensitivity I and possibly combination of these might be even more beneficial regarding the single measurements and I think that has to be teased out in future studies.
Dr Carolyn Lam: Exactly, but what great insights for us to consider as clinicians now for specific cases where we may consider find those if we have those in our institutions. At the end of the day, I supposed cost-effectiveness analysis will need to be done. Agree?
Dr Christian Mueller: Absolutely, absolutely. The good thing about troponin, it's extremely inexpensive. So as compared to most of the new fancy biomarkers that are usually, rather prices of troponin is a routine marker. It's inexpensive. It's there for very likely that if we are able to document some clinical value that also the cost-effectiveness study that's definitely unnecessary will show also some economic benefit.
Dr Carolyn Lam: Oh, Christian, thank you for publishing yet another impactful and clinically relevant paper with us here in Circulation. I mean, it's exactly the kinds of papers that we really treasure here, because they directly inform clinicians and open our eyes to actually things that we should be considering in our everyday practice. Clod I ask you maybe cheekily to share about your experience with publishing at Circulation? Someone like you will be the best person to tell the world what it's like.
Dr Christian Mueller: Oh, of course. I mean, for us as a research group and for me as a researcher, it's fantastic. It's perfect to have some of our work published in Circulation that has fantastic impact factor, fantastic readership and ensures that the research catch the attention that's fantastic. Also, I think for us as a research group, the recognition of being able to publish in Circulation is outstanding and it helps us continue in the research group that we do. The comments made to large extent also by the editors. Also, on this manuscript, I think we're incredibly insightful and definitely had a major contribution to the final product to make it as attractive and also as balanced and insightful I think as it is at this point in time.
Dr Carolyn Lam: Thank you so much for providing that feedback, because it is our aim, explicit aim to put a partner authors in getting the best of the manuscript and working really closely with you. So thank you once again, Christian, for your time today. Audience, I know you've heard many times from this favorite person that we have on our podcast.
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