Cardiac troponin T degradation in end-stage renal disease patients appears to occur in vivo

Background

Cardiac troponin T (cTnT) is a cornerstone biomarker for diagnosing myocardial infarction (MI). However, since the introduction of the high-sensitivity assays, cardiac troponins are also measured in pathologies with cardiac risk factors, such as end-stage renal disease (ESRD), making it more difficult for clinicians to diagnose MI. Previous studies of our group identified specific larger cTnT proteoforms for the acute phase of MI, while a study in serum of ESRD patients showed solely small cTnT fragments. However, others allocated the small cTnT fragments in serum of ESRD patients to a pre-analytic effect due to abundant thrombin generation in serum, as thrombin is known to cause fragmentation of cTnT.

Purpose

This study investigated the effect of multiple anticoagulation methods on cTnT proteoforms in vitro and in the blood of ESRD patients.

Methods

First, cTnT negative serum and lithium-heparinized (LH), ethylenediaminetetraacetic-acid (EDTA), and hirudin plasma tubes were spiked with human cTnT standard, incubated at 37°C till 48 hours, and analysed with Western Blotting using anti-cTnT Roche antibodies. Second, similar blood tubes were collected from ESRD patients (n=10) conform the Declaration of Helsinki, patient samples and spiked cTnT standards were separated using gel filtration chromatography (GFC) and cTnT was subsequently analysed using the Cobas 6000 analyser.

Results

After 48 hours of in vitro incubation of cTnT in hirudin plasma, no cTnT degradation was observed, while LH only showed minor degradation of 10% from intact 40 kDa cTnT to 29 kDa fragments (figure 1). Also, for both EDTA and serum, a time-dependent degradation from 40 kDa cTnT to 29 kDa fragments to 15–18 kDa fragments was observed. Moreover, in ESRD patients, GFC elution profiles of all blood tubes revealed that 85–98% of cTnT corresponded to small 15–18 kDa fragments (41–42 mL), while 2–15% were 29/40 kDa cTnT forms (27–28 mL). For comparison, standards of ternary T-I-C complex and intact cTnT eluted at 21 mL and 28 mL, respectively.

Conclusions

The extent of cTnT degradation in vitro is dependent on the anticoagulation method of the blood tubes, with absence of degradation exclusively in hirudin plasma. Since in ESRD patients for all blood tubes mainly small 15–18 kDa cTnT fragments were found, including hirudin plasma, cTnT degradation appears to occur in vivo. This observation supports the hypothesis that larger cTnT proteoforms might indeed be specific for the acute phase of MI, thereby supporting the opportunity for developing a more specific cTnT method.

Funding Acknowledgement

Type of funding sources: Public grant(s) – National budget only. Main funding source(s): ZonMw Veni grant

High-sensitivity cardiac troponin I and T kinetics after coronary artery bypass grafting in relation to current definitions of myocardial infarction: a systematic review and meta-anal

Background

Cardiac troponins – either cardiac troponin T (cTnT) or cardiac troponin I (cTnI) – are the preferred biomarkers for the diagnosis of myocardial infarction (MI), also after coronary artery bypass grafting (CABG), also known as type 5 MI. Two distinct arbitrarily chosen definitions of MI have been formulated, proposing different diagnostic cTn cut-off concentrations for both cTnT and cTnI (defined by multiples of the 99th percentile upper reference limit of 10x, 35x or 70x URL, with or without need for additional electrocardiographical or imaging findings). Remarkably, these cut-off values are not (yet) validated for the current high-sensitivity (hs-cTn) assays.

Purpose

Investigating the kinetics of hs-cTnT and hs-cTnI in relation to the current definitions of myocardial infarction type 5 in patients undergoing isolated CABG.

Methods

A systematic search was applied to MEDLINE and EMBASE. The search terms were “coronary artery bypass” AND “high-sensitivity cardiac troponin”, including alternative names and abbreviations. All studies reporting hs-cTnT or hs-cTnI on at least two different time points were included. Hs-cTn concentrations were extracted, stratified and normalized to the URL of the assay. The search was performed by 2 independent reviewers, in agreement with the PRISMA guidelines.

Results

For hs-cTnT, and hs-cTnI, 15 (2646 patients) and 18 studies (1760 patients) were included for meta-analysis, respectively. The mean age of the total hs-cTnT and hs-cTnI patient population was 65 (11) and 63 (11) years, 22% and 21% was female, respectively. Preoperative hs-cTnT and hs-cTnI was 1.2xURL (mean, 95% CI: 0.9–1.4) and 6.1xURL (4.9–7.2), respectively. Peak hs-cTnT and hs-cTnI was reached 6–8h postoperative up to 44.9xURL (28.6–61.1) and 135xURL (109–161), respectively (figure 1). For patients where MI-5 was ruled out, peak hs-cTnT and hs-cTnI level was 43.9xURL (20.1–67.6) and 97.1xURL (20.6–214) respectively. Peak hs-cTnT and hs-cTnI concentrations were dependent on surgical strategy. Patients undergoing off-pump CABG, reached a peak 16–24h post-operatively (hs-cTnT: 13.4xURL, 8.1–18.8) and (hs-cTnI: 60.6xURL, 47.0–74.2). This was notably different from on-pump CABG, where peak hs-cTnT and hs-cTnI was reached 6–8h postoperative (44.8xURL, 37.8–51.9) and (144xURL, 114–174) respectively. MI incidence was 2.7% (range 0–7.0%) and 6.9% (range 0–32%) and the 30-day mortality was ranging from 0–4% and 0–2.5% for hs-cTnT and hs-cTnI respectively.

Conclusion

Postoperative hs-cTn surpasses most of the proposed cut-off concentrations by current definitions of type 5 MI, for hs-cTnT but especially for hs-cTnI. Differences in hs-cTn release following on-pump versus off-pump CABG were also observed, implying the need for different cut-off values for different surgical strategies. Furthermore, future clinical studies should focus on the diagnostic accuracy of cTn in type 5 MI and the re-evaluation of hs-cTn cut-off values in particular.

Funding Acknowledgement

Type of funding sources: Public grant(s) – National budget only. Main funding source(s): ZonMw Veni grant

Cardiac Troponin T: The Impact of Posttranslational Modifications on Analytical Immunoreactivity in Blood up to the Excretion in Urine

Cardiac troponin T (cTnT) is a sensitive and specific biomarker for detecting cardiac muscle injury. Its concentration in blood can be significantly elevated outside the normal reference range under several pathophysiological conditions. The classical analytical method in routine clinical analysis to detect cTnT in serum or plasma is a single commercial immunoassay, which is designed to quantify the intact cTnT molecule. The targeted epitopes are located in the central region of the cTnT molecule. However, in blood cTnT exists in different biomolecular complexes and proteoforms: bound (to cardiac troponin subunits or to immunoglobulins) or unbound (as intact protein or as proteolytic proteoforms). While proteolysis is a principal posttranslational modification (PTM), other confirmed PTMs of the proteoforms include N-terminal initiator methionine removal, N-acetylation, O-phosphorylation, O-(N-acetyl)-glucosaminylation, N(ɛ)-(carboxymethyl)lysine modification and citrullination. The immunoassay probably detects several of those cTnT biomolecular complexes and proteoforms, as long as they have the centrally targeted epitopes in common. While analytical cTnT immunoreactivity has been studied predominantly in blood, it can also be detected in urine, although it is unclear in which proteoform cTnT immunoreactivity is present in urine. This review presents an overview of the current knowledge on the pathophysiological lifecycle of cTnT. It provides insight into the impact of PTMs, not only on the analytical immunoreactivity, but also on the excretion of cTnT in urine as one of the waste routes in that lifecycle. Accordingly, and after isolating the proteoforms from urine of patients suffering from proteinuria and acute myocardial infarction, the structures of some possible cTnT proteoforms are reconstructed using mass spectrometry and presented.

P4503A multi-site coronary vein sampling study on cardiac troponin T degradation in non-ST-elevation myocardial infarction

Background

High-sensitivity cardiac troponin (hs-cTn) assays have reduced specificity for myocardial infarction (MI). In conditions other than MI, cardiac troponin T (cTnT) elevations have been attributed to small cTnT fragments. In search of improved cTnT assay specificity for MI, determination of the in-vivo molecular appearance of cTnT during MI is pivotal.

Purpose

To determine cTnT composition close to its site of release and during the course of MI by multi-site coronary venous system (CVS) and peripheral sampling.

Methods

Lithium-heparinized (LH) plasma and serum samples were obtained from multiple CVS locations in NSTEMI patients. Additionally, samples were drawn from a peripheral artery and vein followed by collections at 6- and 12 hours post-catheterization. cTnT concentrations were measured using the hs-cTnT immunoassay (Roche). The molecular cTnT composition was determined by gel filtration chromatography and Western blotting in an early and late presenting patient.

Results

CVS hs-cTnT concentrations were 28% higher than in the peripheral artery sample (n=71, p<0.001). In contrast to LH plasma, serum caused pre-analytical cTn T-I-C complex disintegration and cTnT degradation. CVS samples demonstrated presence of cTn T-I-C complex, free intact cTnT, and 29 kDa and 15–18 kDa cTnT fragments in higher absolute concentrations than measured peripherally. While the proportion of cTn T-I-C complex decreased and disappeared over time, 15–18 kDa cTnT fragments increased. Moreover, cTn T-I-C complex was more prominent in the early versus the late presenting patient.

Central illustration

Conclusions

In NSTEMI, cTnT is released as a combination of cTn T-I-C complex, free intact cTnT and multiple cTnT fragments. Over time, the composition of observed cTnT forms changes due to in-vivo degradation. These findings serve as a stepping stone to improve diagnostic accuracy of the hs-cTnT assay for MI.

Multi-Site Coronary Vein Sampling Study on Cardiac Troponin T Degradation in Non-ST-Segment-Elevation Myocardial Infarction: Toward a More Specific Cardiac Troponin T Assay

Background Cardiac troponin T ( cTnT ) is seen in many other conditions besides myocardial infarction, and recent studies demonstrated distinct forms of cTnT . At present, the in vivo formation of these different cTnT forms is incompletely understood. We therefore performed a study on the composition of cTnT during the course of myocardial infarction, including coronary venous system sampling, close to its site of release. Methods and Results Baseline samples were obtained from multiple coronary venous system locations, and a peripheral artery and vein in 71 non- ST -segment-elevation myocardial infarction patients. Additionally, peripheral blood was drawn at 6- and 12-hours postcatheterization. cTnT concentrations were measured using the high-sensitivity- cTnT immunoassay. The cTnT composition was determined via gel filtration chromatography and Western blotting in an early and late presenting patient. High-sensitivity - cTnT concentrations were 28% higher in the coronary venous system than peripherally (n=71, P<0.001). Coronary venous system samples demonstrated cT n T-I-C complex, free intact cTnT , and 29 kD a and 15 to 18 kD a cTnT fragments, all in higher concentrations than in simultaneously obtained peripheral samples. While cT n T-I-C complex proportionally decreased, and disappeared over time, 15 to 18 kD a cTnT fragments increased. Moreover, cT n T-I-C complex was more prominent in the early than in the late presenting patient. Conclusions This explorative study in non- ST -segment-elevation myocardial infarction shows that cTnT is released from cardiomyocytes as a combination of cT n T-I-C complex, free intact cTnT , and multiple cTnT fragments indicating intracellular cTnT degradation. Over time, the cT n T-I-C complex disappeared because of in vivo degradation. These insights might serve as a stepping stone toward a high-sensitivity- cTnT immunoassay more specific for myocardial infarction.

A multi-site coronary sampling study on CRP in non-STEMI: Novel insights into the inflammatory process in acute coronary syndromes

BACKGROUND AND AIMS

Inflammation has become a key element in cardiovascular disease, and recently, new anti-inflammatory interventions have shown promising results. In this context, CRP levels have been thoroughly studied in vitro and in animals, but studies in humans are scarce and insights into its release, site(s) of production and uptake are not uniform.

METHODS

We performed a biomarker study with multi-site sampling in the coronary circulation, in non-ST elevation MI (NSTEMI) patients with coronary angiography and right-sided catheterisation. Trans-lesional gradients were obtained by sampling distal to the culprit lesion, in patients with a suitable anatomy. To asses trans-cardiac gradients, blood was sampled from the systemic circulation, coronary sinus (CS) and great cardiac vein. Concentrations of CRP were measured with a high-sensitivity assay.

RESULTS

In 42 patients, a median systemic venous CRP concentration of 4.97 mg/L was observed. There was no evidence of a trans-lesional gradient (4.59 mg/L versus 4.56 mg/L, p = 0.278; n = 14). A significant decrease in CRP concentration was observed between systemic arterial and CS samples (4.88 mg/L versus 4.44 mg/L; p < 0.001; n = 42). This trans-cardiac gradient was irrespective of time of presentation, infarct size and culprit lesion location. The gradient was not only driven by blood that ran through the injured myocardium, but also by lower CRP concentrations in the coronary veins that drain non-infarcted myocardium.

CONCLUSIONS

In the context of NSTEMI, we observed a trans-cardiac decrease in CRP, which may indicate the first human in vivo proof of a net CRP uptake by the myocardium, with a role for CRP both in the injured and adjacent myocardium.

Inosine Triphosphate Pyrophosphohydrolase Expression: Decreased in Leukocytes of HIV-Infected Patients Using Combination Antiretroviral Therapy

OBJECTIVE

In HIV-infected patients, the enzyme Inosine triphosphate pyrophosphohydrolase (ITPase), involved in purine nucleotide homeostasis, was found to be decreased in erythrocytes. Since purine analogues are pivotal in the HIV treatment, a better understanding of ITPase expression in CD4 lymphocytes may lead to better understanding of nucleotide metabolism and (adverse) effects.

DESIGN

Cross-sectional, cohort, observational study.

METHODS

HIV-infected and control patients above 18 years were included. All DNA samples were genotyped for the 2 functional ITPA SNPs; c.94C>A (rs1127354) and g.IVS+21A>C (rs7270101). ITPase expression was determined by flow cytometry in all leukocyte subsets.

RESULTS

Fifty-nine HIV-infected patients and 50 controls were included. Leukocyte subtype distribution showed no difference in monocytes and granulocytes, but lymphocytes were higher in HIV-infected patients (P < 0.001). ITPase expression was highest in activated monocytes and lowest in lymphocytes. In HIV-infected patients, the percentage of ITPase positive cells was less in all leukocyte and lymphocyte subsets compared with controls (P < 0.01). In HIV-infected patients, 97.4% of CD4 lymphocytes were ITPase positive versus 99.9% in controls (P = 0.002) and 85.9% versus 99.6% of CD8 lymphocytes (P < 0.0001), respectively. Stratification according to genotype revealed no significant differences in ITPase expression in leukocytes in HIV-infected and control patients.

CONCLUSIONS

HIV-infection seems to be interfering with the nucleotide metabolism in leukocytes, including CD4 lymphocytes, by decreasing ITPase expression, independently of ITPA genotype. Given that active metabolites of purine-analogue reverse transcriptase inhibitors are potential substrates for ITPase, these results warrant further research towards effectiveness and adverse events of purine analogues and ITPase activity.

Inosine triphosphate pyrophosphohydrolase activity: more accurate predictor for ribavirin-induced anemia in hepatitis C infected patients than ITPA genotype

BACKGROUND

ITPA polymorphisms have been associated with protection against ribavirin-induced anemia in chronic hepatitis C (HCV) patients. Here we determined the association of inosine triphosphate pyrophosphohydrolase (inosine triphosphatase or ITPase) enzyme activity with ITPA genotype in predicting ribavirin-induced anemia.

METHODS

In a cohort of 106 HCV patients, hemoglobin (Hb) values were evaluated after 4 weeks (T4) and at the time of lowest Hb value (Tnadir). ITPase activity was measured and ITPA genotype determined. Single-nucleotide polymorphisms (SNPs) tested were c.124+21A>C and c.94C>A. ITPase activity ≥1.11 mU/mol Hb was considered as normal.

RESULTS

After 4 weeks of treatment, 78% of the patients with normal ITPase activity were anemic and 21% of the patients with low ITPase activity (p<0.001). Stratified by genotype, the percentages of anemic patients were: wt/wt 76%, wt/c.124+21A>C 46% (p=0.068), and wt/c.94C>A 29% (p=0.021). At Tnadir, virtually all patients with normal ITPase activity were anemic, compared to only 64% of the patients with low activity (p=0.02). Thirteen patients had wt/c.124+241A>C genotype. Within this group all five patients with normal ITPase activity and only four of eight with decreased activity developed anemia. Presence of HCV RNA did not influence ITPase activity.

CONCLUSIONS

This study is the first to report that ITPase activity predicts the development of anemia during ribavirin treatment. ITPase activity and ITPA genotype have high positive predictive values for development of ribavirin-induced anemia at any time during treatment, but ITPase activity predicts ribavirin-induced anemia more accurately.

A novel multiparameter flow cytometric assay for inosine triphosphatase expression analysis in leukocytes

The aim of this study was to assess inosine triphosphate (ITPase) expression in the different leukocyte populations present in peripheral blood samples of a nonimmune compromised control group. For this purpose, a multiparameter flow cytometric assay was developed and performed to study ITPase expression in peripheral leukocyte subpopulations of healthy volunteers (n = 20). Qualitative ITPase expression was assessed by determining the percentage of ITPase-positive cells. Quantitative data were obtained by measuring the median fluorescent intensity (MFI). Subcellular localization of ITPase was analyzed using immunocytochemistry. Immunocytochemistry showed that ITPase is present in all leukocytes and localized intracellular. Based on this finding, a multiparameter flow cytometric assay was developed using a Fix & Perm strategy. Qualitative and quantitative ITPase expression remained stable (variation, <10%) for at least 48 h after blood sampling. MFI values showed that activated monocytes contained significantly more ITPase when compared to the total monocyte fraction (P < 0.0001), which subsequently had a higher amount of expression than granulocytes (P < 0.0001). In addition, the phagocyte subpopulations ([activated] monocytes and granulocytes) contained significantly higher levels of ITPase when compared to lymphocytes (P < 0.0001). Within the lymphocyte fraction, it appeared that T-helper cells contained significantly higher ITPase levels when compared to cytotoxic T cells, B lymphocytes, and natural killer cells (P < 0.0001). Our study is the first which describes a flow cytometry assay to analyze ITPase expression in leukocytes qualitatively as well as quantitatively and visualizes the intracellular localization of ITPase in leukocytes. © 2012 International Society for Advancement of Cytometry.