Matrix metalloproteinases (MMP)
MMP are endogenous zinc dependent endopeptidases required for structural integrity of extracellular matrix of myocardium. TIMP (Tissue Inhibitors of Metalloproteinases) regulates MMP. MMPs may degrade myocardial ECM leading to the development of LV dilatation and heart failure and their inhibition in experimental models of AMI has been associated with reduced LV dilatation and wall stress. In a study of patients with acute myocardial infarction, TIMP-1 and MMP-9 correlated with echocardiographic parameters of LV dysfunction and remodelling after AMI and identified patients at risk of subsequent LV remodeling and associated with severe extensive CAD.
Placental Growth Factors (PGF)
Placental Growth Factor is a member of VEGF (vascular endothelial growth factor) subfamily – a key molecule in angiogenesis and vasculogenesis, in particular during the embryogenesis. Placental growth factor expression within human atherosclerotic lesions is associated with plaque inflammation and neovascular growth. Recent studies established the role of different inflammatory markers such as hsCRP, sr amyloid A, IL-6 not only gets elevated during acute coronary syndrome (ACS) but predicts its adverse outcomes. PGF was recently shown that it is upregulated in all forms of atherosclerotic lesions. PGIF induces the following
• Vascular smooth muscle cell growth.
• Recruits macrophages into atherosclerotic lesions.
• Upregulates production of TNF alpha.
• Monocyte chemotactic protein 1 by macrophages.
• Pathological angiogenesis.
Plasma PIGF levels may be an independent inflammatory biomarker of poor outcome in patients with suspected ACS. A single initial measurement of plasma PIGF appears to extend the predictive and prognostic information gained from traditional inflammatory markers.
Pregnancy associated plasma protein alpha (PAPP-A)
PAPP-A was originally identified in the serum of pregnant women. PAPP-A is produced by placental tissue. Circulating PAPP-A levels increase during pregnancy and they are used in the fetal diagnosis of Down syndrome. Only recently has PAPP-A been identified in nonplacental tissues. The concentrations in the sera of nonpregnant human beings being several orders of magnitude lower than during pregnancy. The physiological role of PAPP-A is only beginning to be unraveled. PAPP-A is a high-molecular-weight, zinc-binding metalloproteinase, which acts as a specific protease of IGF binding protein-4 (IGFBP-4). There is histological evidence, using specific monoclonal antibodies, that PAPP-A is abundantly expressed in both eroded and ruptured coronary plaques, but not in stable plaques, in patients who have died suddenly of cardiac causes. Furthermore, accumulating evidence suggests that PAPP-A may play a pivotal role in the development of atherosclerosis and subsequent plaque instability in ACS patients. PAPP-A is markedly elevated in the earliest hours after the onset of symptoms in patients with STEMIs treated with heparin and primary percutaneous coronary intervention, and in animal studies, heparin administration is associated with a significant increase in PAPP-A levels, presumably because of the detachment of PAPP-A from the vessel wall. If future studies confirm that concomitant heparin administration also increases PAPP-A levels in humans, the prognostic role of PAPP-A in patients with ST elevation myocardial infarction needs to be reevaluated.
Markers of ischemia
An ideal marker is one in which there is a specific easily measurable increase that clearly aligns with a predictable outcome be it evidence of ischemia, inflammation, myocardial necrosis, plaque rupture, plaque destabilization, or heart failure. Because of the underlying shared etiologies related to the process of arteriosclerosis and the complexity of the pathological processes giving rise to adverse thrombotic outcomes, a single marker that relates to each stage is unlikely. Rather more probable would be use of multiple markers with varying decision levels to either rule-in or rule-out a clinical decision. As the understanding of the niceties between ACS, inflammation, and coronary artery process develops, the ongoing search for better cardiac markers will continue.
Ischemia modified albumin (IMA)
The only ischemia marker that has been approved by the FDA is the modified albumin (IMA) using the albumin cobalt binding test (ACB) for assessment of myocardial ischemia. IMA occurs in 2 forms: 1.in which human serum albumin (HAS) binds mostly copper and 2.in which the damage to the N-terminus prevents metal binding. Patients without ischemia have more available metal binding sites on their HSA, than those from ischemic patients. This alteration is most likely due to damage caused by oxidative free radicals prevalent during ischemic events, and resulting in altered binding of trace metals resulting in IMA. As mentioned earlier, the FDA approved method for IMA (ACB Test by Ischemia Technologies) uses cobalt in its assay. Normal HSA will bind cobalt when it is added to a sample, leaving little residual cobalt. However, IMA cannot do the same due to its altered binding site. Patients having transient ischemic episodes without parallel myocyte death increases IMA which causes less cobalt binding and more residual unbound cobalt available. This can complex with chromogen (dithiothreitol) which can be measured photometrically. It is estimated that approximately 1% to 2% of the total albumin concentration in the normal population is IMA compared to 6% to 8% in patients experiencing ischemia. The clinical utility of IMA appears to be in its negative predictive value for ischemia and ACS, particularly when used in conjunction with other tests. While the optimum cutoff for IMA for ruling out ACS is 85kU/L, the manufacturer has suggested a higher value of 100 kU/L for risk stratification. Some of the limitations to be taken into consideration includes: a. there is an overlap between the normal population and that of individuals with cardiac ischemia; b. IMA is not specific for cardiac ischemia; c. false positive can occur in patients with cirrhosis, bacterial and viral infections, advanced cancers, stroke and end-stage renal disease and d. interpretation of IMA in certain populations, including those with peripheral vascular disease and in marathon runners is not yet clear. Hence IMA appears at this time as a potential marker in certain clinical situations but a number of possible interferences may limit its utility in patients with suspected ACS.
Unbound free fatty acids
Fatty acids are essential building blocks for many lipid molecules and are energy stores that can be utilized during times of fasting or increased metabolic demand. Fatty acids exist in body as esterified form (bound to glycerol or other alcohol), non-esterified form bound to albumin, and to a much smaller extent, as an unbound soluble form. Evidence exists that unbound free fatty acids increase significantly in ischemic-related events. It is not certain as of today what role the unbound free fatty acid plays in cardiac disease. It possibly partakes in the developing necrotic process and is released as a result of cell rupture or other precipitating conditions. Currently, a fluorescent probe assay is available but its clinical utility at this time is not established.
Fatty acid binding proteins (FABPs)
Fatty acid binding proteins (FABPs) are transport proteins that carry fatty acids and other lipophilic molecules like eicosanoids and retinoids across the membranes. They occur in nine different isoforms in a predictable tissue distribution and fairly long half-life of several days. The heart-type FABP (H-FABP) is released following myocardial death within 6 hours and is not specific to the heart, similar to myoglobin. It is released to a smaller extent in skeletal muscle, distal tubular cells of the kidney, specific parts of the brain, lactating mammary glands, and the placenta. It has been found that H-FABP may perform better and reach its upper reference limit sooner than either myoglobin or troponin. A number of enzyme immunoassays are available for H-FABP testing. Its relation to ischemia and prognosis for adverse events is likely to expound in near future.
Phospholipase
Phospholipase are the enzymes that release fatty acids from the second carbon group of glycerol. They are grouped into 4 major categories: A to D. Phospholipase A2 and D have drawn much attention in their role in assessing ischemia associated coronary artery disease. Lipoprotein-associated phospholipase A2 (Lp-PLA2) also known as platelet-activating factor acetylhydrolase (PAF-AH) is a phospholipase A2 enzyme that in humans is encoded by the PLA2G7 gene. In human atherosclerotic lesions, 2 main sources of Lp-PLA2 can be identified, including that which is brought into the intima bound to LDL (from the circulation), and that which is synthesized de novo by plaque inflammatory cells (macrophages, T cells, mast cells). I A meta-analysis involving a total of 79,036 participants in 32 prospective studies found that Lp-PLA2 levels are positively correlated with increased risk of developing coronary heart disease and stroke. Recently, there has been a renewed interest in this molecule, not for use in cardiac assessment as it was originally approved by the FDA, but rather in stroke prediction after it was found that elevated levels of Lp-PLA2 were associated with an almost 2-fold increase in stroke in the selected population coupled with a 6-fold increase in hypertensive individuals.
Phospholipase D (PLD) is an enzyme that catalyzes the hydrolysis of membrane bound phospholipids into phosphatidic acid and choline. In addition, it is also involved in endorsement of fibrinogen binding to platelets. Increased levels of plasma (PLCHO) and whole blood choline (WBCHO) levels have been seen in tissue ischemia in patients with negative troponin values. Choline is not a marker for myocardial necrosis but indicated high-risk unstable angina in patients without acute myocardial infarction (sensitivity 86.4%, specificity 86.2%). Therefore obtaining levels of both plasma PLCHO and WBCHO may prove to be a useful aid in patients suspected of ACS.
Conclusion
Cardiac markers have been implicated in the diagnosis and risk stratification of patients with chest pain and suspected acute coronary syndrome (ACS). Among the markers of cardiac necrosis, troponins have become the cardiac markers of choice for patients with ACS. In fact, cardiac troponin has become central to the definition of acute myocardial infarction (MI) in the consensus guidelines from the American College of Cardiology (ACC) and the European Society of Cardiology (ESC). Current focus is on finding appropriate upstream markers which may aid in detection of myocardial ischemia and a variety of events involved in the process of pathophysiology of acute coronary syndrome especially in relation to plaque destabilization and rupture. The ideal biomarkers that offer early detection, risk stratification, selection of therapy, monitoring disease progression, and treatment efficacy remain to be elucidated.
