Dr. McCondo: “So, how are you feeling today? You told the nurse you were feeling a little sick?”
Mr. Heart: “Not well, and I think I might be coming down with something serious…”
Dr. McCondo: “Alright, let’s take a look.”
Mr. Heart: “Okay…hey do you think it’s going to be something serious? Do you think it might be because of the new neighbourhood I moved into? Ever since my buddies Turk and Burke helped me move in I started feeling a little weird. I mean, it was nice to relocate to a new place and all because my old one….well let’s just say my old one is a goner. But I feel like I’m not fitting into the new environment too well. I feel…rejected, almost.”
Dr. McCondo: “I see. Well, nothing to worry about right now. I’ll prescribe some medicine right now which should make you feel a bit better. We’ll also run some tests too just to be sure. The result will be available in the next hour or two.”
Mr. Heart: “Okay, thanks doc.”
Dr. McCondo: “No problem, see you in a bit.”
*cue corny fadeout music*
Sounds ludicrous? (That was a rhetorical question.)
But would it be useful if doctors could actually “communicate” with the heart and provide the necessary management at the time of, or better yet, BEFORE the injury (major problem) occurs? Absolutely.
Although the conversation above may never take place in the future despite our current technological advancements, scientists are still exploring, discovering, and utilizing various biological molecules and traits, known as “biomarkers”, as a means to gain insight into the status and wellbeing of the organ and, consequently, the patient.
A biomarker, according to the Merriam-Webster Medical Dictionary Online, is defined as “a distinctive biological or biologically derived indicator of a process, event, or condition.”
More specifically, these biomarkers or “indicators” can also be described as “cellular, biochemical or molecular alterations that are measurable in biological media such as human tissues, cells, or fluids.” [1, 2] This also means specific proteins, metabolites (substances produced from your metabolism), or even unique changes in gene expressions in your body can all be potentially used as biomarkers for diagnosis or even prognosis of injury or diseases.
Whether it’s for the diagnosis of cancer or assessment of organ-specific damage and/or diseases, a myriad of biomarkers have been identified and are currently used clinically everyday. However, as heart / cardiovascular disease is the leading cause of death in North America and a major contributor to the global disease burden, this paper will focus primarily on the (protein) biomarkers of the heart – from the classic markers of heart injury to the ones being used clinically today, and from the ones used today to those scientists are looking into for the use tomorrow. [3, 4]
Past, present, future…and beyond.
First stop, 1954!!
Classic Cardiac Marker(s) – Oldies but Goodies?
One of the most well known cardiovascular diseases is probably myocardial infarction (MI; also known as a “heart attack”). This is not surprising, considering it is thought to be the most common cause of morbidity and mortality in developed countries such as the United States and Canada. 
Myocardial infarction is a condition which occurs when an area of the heart is damaged or dies due to insufficient or complete lack of oxygen / blood supply. [6, 7] Traditionally, myocardial infarctions have often been diagnosed based on clinical presentations (i.e. “Doctor, I feel chest pain, shortness of breath, and squeezing pain which radiates to my jaw and left arm) and electrocardiogram (ECG, which measures electrical activity in the heart over time). [8, 9] However, since these clinical and ECG findings may often be atypical or absent, the use of serum markers / cardiac markers are becoming more and more essential for the diagnosis of myocardial infarction. [8, 9]
I. In the beginning, there was AST
In 1954, Dr. Arthur Karmen and his colleagues reported what is probably now considered the first known marker of AMI, called aspartate transaminase (AST), in the Journal of Clinical Investigation. [5, 10] More specifically, AST, which is a type of enzyme predominantly present in heart tissue, liver and skeletal muscle, was found to be at an elevated level in patients with heart attacks. 
At this point, you might say, “Hey! You said these markers are abundant in liver and skeletal muscle as well! Why would you use it as a cardiac marker!?”
You are right, and that is one of the main reasons AST has not endured the test of time as a cardiac marker – because of its abundance in liver and skeletal muscles.  In fact, the clinical use and significance of AST nowadays is limited mainly to the assessment of hepatocellular (liver) and skeletal muscle diseases. [6, 9] This makes sense as the basic idea behind most of the biomarkers (of organ damage) is that, when cells die, they are no longer biochemically compartmentalized properly. As a result, contents inside the cells (i.e. proteins) can spill out into the blood, which can then be measured and analyzed. 
II. The perfect test
Once released into the blood, the cardiac biomarkers can then be, in an ideal setting, measured by an assay or a biochemical test which is both sensitive and specific (the ability to detect the marker and the ability to distinguish a cardiac and non-cardiac problem, respectively). [8, 9, 11] As well, the test should be widely available and, most importantly, inexpensive! 
As Rolling Stones’ Mick Jagger once sang, “you can’t always get what you want.” Assuming that he was talking about the cardiac marker test (which he wasn’t), he’d be correct; no perfect test exists.
Likewise, the perfect cardiac injury marker has yet to be found (if there is one). Nonetheless, many of markers used today encompass various attributes of “the perfect biomarker.”
III. “The important thing is that it’s from the heart…” – No, I’m not talking about gifts.
Some ideal features of marker of heart injury
As one might imagine based on the example mentioned previously, one of the ideal characteristics of cardiac biomarker / marker of myocardial injury is for it to be found at high concentration in the myocardium (heart muscle tissue), while absent – or present at very low level – in other organs/tissues.  Also, the marker should appear very rapidly in plasma of patients experiencing myocardial infarction or ischemia, but should “not” be present in plasma at any other time.  The reason for this is pretty straightforward. If the marker is quickly elevated in the plasma of MI patients, then there may be less delay in diagnosing the cardiac problem. [8, 9] In other words, physicians can potentially have more time to “save” the myocardium from dying. Lastly, the ideal cardiac marker should remain in plasma / blood long enough to be detected and measured for diagnostic purposes, but it should not remain for too long.
”Why don’t you want it in the blood for too long?” you might ask.
In essence, the reason is to avoid confusion and allow diagnosis of recurrent injury.  Imagine a cardiac marker which was found to be significantly elevated in a patient. The patient was diagnosed with MI and was treated by the doctor. However, this marker is one which continues to be elevated because it doesn’t get cleared or degraded. Then, two days later, the patient comes in, complaining about chest pains again. Now the scenario gets confusing – is this person getting a heart attack or is the marker still elevated from the first heart attack?
Luckily, there are markers, some of which are very organ-specific, for dealing with scenarios such as this.
Modern Marker of Choice? CK vs. Tn – The Showdown
Creatine Kinase (CK), an enzyme generally involved in ATP (a type of energy currency in cells) regeneration in contractile systems and found in muscle of all types (skeletal, cardiac and smooth), is routinely measured as a marker of cardiac injury. [8, 9] In particular, CK-MB, an isoenzyme of CK (imagine the same CK enzyme but with small modification / difference), is found predominantly in cardiac tissues. [8, 9] Although CK-MB can be found in small amounts in other tissue (i.e. skeletal muscle), it’s still relatively specific to myocardium, which is basically the only tissue from which CK-MB “leaks” into the blood in significant amounts (and there’s typically more CK-MB released into blood when cardiac muscle dies than when skeletal muscle dies). [8, 9] More importantly, CK-MB level in blood generally elevates almost as quickly as Troponin (Tn) after cardiac injury, but drops back down (to the reference interval) a lot faster, making it a potentially better marker for assessing reinfarction. [7-9]
Troponins (Tn), on the other hand, are non-enzyme contractile proteins associated with myofibrils (filaments found in muscle cells); each troponin helps regulate muscle contractions and is composed of 3 subunits: TnC, TnI and TnT. [8, 9]
Luckily for the physicians, cardiac-specific TnT and TnI (i.e. cTnT and cTnI) exist! Even better, relative to other cardiac markers, cTnT and cTnI are nearly absent from normal serum!  Although these two markers aren’t exactly specific to the “cause” of the injury (i.e. myocarditis – inflammation of the myocardium, can also cause Tn elevation), they are still the preferred markers and probably the most clinical significant cardiac injury markers in the diagnosis of MI (a.k.a. heart attack). [7, 9] This is because of their superior sensitivity and tissue-specificity compared to the others known at this time. [6, 9, 11]
“But what about the prognosis? What about cardiac injury due to rejection after transplantation?” you ask.
It gets more complicated.
New playing field, new challenges – Cardiac Biomarkers in Transplantation
For patients with life-threatening, end-stage heart failure, cardiac transplantation, the surgical procedure of putting a donor heart into the “foreign” environment of the recipient body, is currently considered as the primary therapy of choice.  Unfortunately, after transplantation, the donor’s heart, known as a cardiac allograft, may be attacked by both immune and non-immune elements in the recipient’s body, leading to dysfunction or rejection of the organ.  Heart rejection may occur in 20-50% of recipients at least once within the first year after transplant, while10-year cardiac allograft survival is only about 50%. [12, 13]
Normally, endomyocardial biopsies (essentially cutting a small piece of the heart muscle for examination!) are carried out post-transplant to monitor potential rejection episodes.  Not only is this a very invasive procedure, but the results often also suffer from reproducibility and interpretation issues. [13, 14]
This is where biomarkers come into play.
Researchers are attempting to use them to develop non-invasive, inexpensive and reliable tests to contribute to the diagnosis (is there rejection?), as well as prognosis (is there “going” to be a rejection?) of the patient and the transplanted heart. [13, 14]
”Why not just use Tn or CK?” you might ask.
One reason is because some of the aforementioned cardiac markers, such as cardiac troponin, are elevated post-heart transplant but are “not” specific to rejection. Also, it has been suggested that troponins may lack diagnostic sensitivity during the early post-operative period. [13, 15] This is also why researchers are looking into other areas for markers – such as markers of inflammation / immune process. But even those markers have their own potential advantages (i.e. prognosis of rejection before sufficient immune process occurs to damage the heart) and disadvantages (i.e. the immune marker is elevated, but is it actually a rejection or is it just an infection which caused the immune system to become activated). 
From the present….to the future….and beyond!
The general consensus among the research community is that, instead of finding that single “miracle” marker of rejection, multiple biomarkers (i.e. proteins, genes, metabolites) are going to be necessary to achieve the desired prognostic and diagnostic values. [11, 14] But just exactly which “ones” are clinically most useful and relevant? That’s the million dollar question researchers are trying to answer.
Large-scale “biomarker-searching” projects, such as the one funded by Genome Canada (called “the Better Biomarkers of Acute and Chronic Allograft Rejection” project), are currently underway and are using cutting-edge technologies to discover biomarkers (i.e. genes, proteins, metabolites) which might help pave the way for future diagnosis and prognosis of organ rejection. Furthermore, in addition to finding better biomarkers, the path to finding them has also helped scientists gain insight into the mechanisms involved in transplant rejection.
Conclusion – They are just cardiac markers, not doctors
After this brief overview of some of the classic markers of cardiac injury and their advantages and disadvantages, as well as a quick look at the current investigation for markers of cardiac allograft rejection and the rationale behind it, it’s not difficult to see how cardiac biomarkers can have such an important impact, both clinically and in regards to research.
It’s important to remember, however, that even with the availability of these newer, better biomarkers in the future, diagnosis for cardiac injuries will still require astute physicians’ clinical observations and the correct interpretation of the serum marker test results.
Nonetheless, the search continues.
Who knows, maybe that perfect cardiac marker is out there after all.
Mr. Heart: “Hey Doc”
Dr. McCondo: “Hello. So, how are you feeling now?”
Mr. Heart: “A little better I think…Did the test result come back?”
Dr. McCondo: “Good….Yes, the biomarkers test came back. Everything seems normal”
Mr. Heart: “Phew, that’s good news”
Dr. McCondo: “Yup. That is. Mr. Heart, don’t worry about feeling rejected. I think you are going to fit into the new environment just fine. Enjoy your new life there.”
*cue corny fade out music, part 2*
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