When cardiac muscle is lost, the heart mostly heals through the formation of scar tissue. Although the heart cannot undergo large-scale tissue regeneration, stem/progenitor cells with the potential to generate cardiomyocytes in vitro remain in the adult human myocardium, and mature cardiomyocytes have been suggested to have the ability to re-enter the cell cycle and duplicate.

To investigate cardiomyocyte turnover in human hearts, Bergmann et al. used a novel system based on radiocarbon dating of DNA. Following nuclear bomb testing during the early period of the cold war (1945-1963), the levels of ¹⁴C (incorporated into ¹⁴CO₂) in the atmosphere rapidly equalized around the world. After being taken up by plants which are subsequently consumed by humans, the ¹⁴C is then incorporated into DNA, and because DNA is stable after a cell has gone through its last cell division, the concentration of ¹⁴C in DNA serves as a date mark for when a cell was born, and can be used to retrospectively date cells in humans. A similar approach has been used in rodent models previously, but never implemented in humans due to safety concerns.

Using mathematical modeling, the authors suggest that cardiomyocyte renewal is an age-dependent process, with a gradual decrease from 1% annual turnover at the age of 25, to 0.45% at the age of 75. By contrast, non-myocytes in the heart turn over at an estimated rate of 18% per year. By the age of 50, 55% of human cardiomyocytes remain from birth, while 45% were generated afterward. However, overall fewer than 50% of cardiomyocytes are rejuvenated during a normal life span.

This remarkable piece of work exploits a dark period of recent human history to make a novel and exciting scientific discovery. The next step lies in determining the origins of new cardiac myocytes, and the pathways that lead to their development, before looking at how they are activated in response to injury. Knowledge of this process may in the long term lead to methods of rejuvenating damaged myocardium, and preventing heart failure.

· Evidence of Cardiomyocyte Renewal in Humans. Bergmann O, Bhardwaj RD, Bernard S et al. Science 2009:324;p98-102

Secretory phospholipase A2 (sPLA2) enzymes are involved in the production of bioactive lipids that contribute to atherosclerosis. They are highly expressed in human and mouse atherosclerosis, and different groups of the enzyme contribute differently to atherosclerosis. A-002 (1-H-indole-3-glyoxamide, veraslpadib methyl, Anthera pharmaceuticals, CA) is a novel inhibitor of sPLA2 in humand with specificity towards sPLA2-I1A, -V and –X.

In this phase II, randomized, double-blind, placebo-controlled trial, 393 patients were randomly assigned to receive either placebo or one of four doses of A-002 for 8 weeks. The primary endpoint was the change in sPLA2-IIA concentration. 348 patients reached this endpoint, and the mean sPLA2-IIA concentration was found to have fallen by 86.7% in the active treatment group, compared to just 4.8% in the placebo group (p<0.0001). Reductions in LDL cholesterol, C-reactive protein, arachidonic acid, and oxidized-LDL were also seen, even in patients already on statin therapy. This reduction was found to be dose-dependent, but one serious adverse event was seen in with the highest dose (500mg) – an exacerbation of chronic obstructive pulmonary disease. Otherwise the main side-effects of the drug included headache (n=20), nausea (n=17), and diarrhoea (n=12).

These promising initial results suggest that, by acting on a number of atherogenic pathways, sPLA2 inhibition may be able to provide substantial protection from cardiovascular disease. Despite multiple innovations in recent years, deaths from cardiovascular disease continue to rise - the development of novel treatments such as this therefore remains an important goal for drug development.

• Rosenson RS, Hislop C, McConnell D, et al. Effects of 1-H-indole-3-glyoxamide (A-002) on concentration of secretory phospholipase A2 (PLASMA study): a phase II double-blind, randomized, placebo-controlled trial. Lancet 2009; 373:649-658.
• Corson MA. Phospholipase A2 inhibitors in atherosclerosis: the race is on. Lancet 2009; 373: 608-610.

Across the whole spectrum of acute coronary syndromes and in patients undergoing PCI and stenting, dual anti-platelet therapy with aspirin and clopidogrel is the current recognised standard of care. However, significant variation in response to clopidogrel is observed between patients, with those with lesser degrees of platelet inhibition being at higher risk of adverse cardiovascular events.  Two studies in the New England Journal of Medicine investigate the possible reasons for this.

The first study tested the association between functional genetic variants in CYP genes, plasma concentrations of the active drug metabolite and platelet inhibition in response to clopidogrel in 162 healthy controls. Subsequently, they evaluated the association between these genetic variants and cardiovascular outcomes in 1477 patients with acute coronary syndromes who were treated with clopidogrel in the Trial to Assess Improvement in Therapeutic Outcomes by Optimising Platelet Inhibition with Prasugrel-Thrombolysis in Myocardial Infarction (TRITON-TIMI 38). The primary efficacy outcome was a composite of death from cardiovascular causes, myocardial infarction or stroke. An important pre-specified secondary outcome was definite or probable stent thrombosis as defined by the Academic Research Consortium. In healthy individuals treated with clopidogrel, carriers of at least one reduced function CYP2C19 allele (30% of the study population) had a relative reduction of 32.4% in plasma exposure to the active metabolite of clopidogrel when compared to non-carriers (p < 0.001). Carriers demonstrated an absolute reduction in maximal platelet aggregation in response to clopidogrel that was 9% less than that see with non-carriers (p < 0.001). In the TRITON-TIMI 38 population, in those treated with clopidogrel, carriers had a relative increase of 53% in the composite primary efficacy outcome as compared with non-carriers (12.1% vs 8%, HR for carriers 1.53, 95% CI 1.07-2.19, p = 0.01). An increase by a factor of 3 times was observed for risk of stent thrombosis (2.6% vs 0.8%, HR 3.09, 95%CI 1.19-8.0, p = 0.02).

The results from this study provide strong evidence that CYP genetic variation leads to reduced exposure to the active drug metabolite of clopidogrel, less platelet inhibition and decreased protection from recurrent ischaemic events. Common polymorphisms in the CYP2C19 gene are seen in 30% of whites, 40% of blacks and >55% of East Asians resulting in decreased pharmacokinetic and pharmacodynamic responses to clopidogrel in approximately 1/4 to 1/3 of individuals. In patients with ACS treated with clopidogrel, there was a greater then 50% increase in the risk of the primary outcome and a 3 fold increase in the risk of stent thrombosis.

The second study evaluated the relationship between known polymorphisms of relevant genes and clinical outcome after myocardial infarction in patients receiving clopidogrel. The polymorphisms of genes involved in clopidogrel absorption (ABCB1), metabolic activation (CYP3A5 and CYP2C19), and biological activity (P2RY12 and ITGB3) were assessed against death or ischaemic events during the first year of follow-up of the French Registry of Acute ST elevation and Non-ST elevation Myocardial Infarction (FAST-MI). 2208 patients were enrolled and assessed for the previously mentioned allelic variants to risk of death from any cause, non-fatal stroke, or myocardial infarction during 1 year follow-up. 225patients died and 94 suffered a non-fatal MI or stroke. None of the selected single nucleotide polymorphisms (SNPs) in CYP3A5, P2RY12, or ITGB3 were associated with risk of adverse outcome. Patients homozygous for the variant of ABCB1 were observed to have a higher rate of cardiovascular events at 1 year than their wild-type counterparts (15.5% vs 10.7%, HR 1.72, 95% CI 1.20-2.47). Patients who were homozygous for CYP2C19 loss of function alleles also had a higher event rate compared to wild-type (21.5% vs 13.3%, HR 1.98, 95% CI 1.10-3.58). 1535 patients underwent percutaneous coronary intervention during admission and the rate of cardiovascular events in patients homozygous for CYP2C19 was 3.58 times that seen with wild-type (95% CI 1.71-7.51).

Previously there has been some concern that there may be reduced antiplatelet activity of clopidogrel in patients prescribed omeprazole a CYP2C19 inhibitor. This was not observed in this study and is an important finding as there is a high frequency of co-prescription of anti-platelet agents and proton pump inhibitors. This study was observational in design and therefore does not allow discernment of cause and effect relationships. It is therefore possible that both ABCB1 and CYP2C19 may have an effect on atherothrombosis directly rather than modulating platelet response to clopidogrel.

Both these studies further expand the understanding of the pharmacogenetics of clopidogrel and the advent of newer anti-platelet therapies such as prasugrel will provide possible alternatives for patients not achieving an appropriate anti-platelet response with current therapies.

·      Mega JL, Close SL, Wiviott SD et al. Cytochrome P-450 Polymorphisms and Response to Clopidogrel. N Engl J Med 2009;360 (online-early)

·      Simon T, Verstuyft C, Mary-Krause M et al. Genetic Determinants of Response to Clopidogrel and Cardiovascular Events. N Engl J Med 2009;360 (online-early)

Lipoprotein-associated phospholipase A2 (Lp-PLA2) is an enzyme that is produced by inflammatory cells in atherosclerotic lesions.  It has previously been shown to be present at higher levels in the core of ruptured atherosclerotic plaques, although it was unclear whether this was merely an association or whether the enzyme played a key role in plaque vulnerability.

Wilensky et al. selectively inhibited Lp-PLA2 in a swine model of atherosclerosis by the administration of darapladib, and found levels of Lp-PLA2 in the both plasma and atherosclerotic plaque were lowered.  Furthermore, a significant decrease in plaque area was noted, and in particular a decrease in necrotic core area and reduced medial destruction.  In addition, coronary gene expression analysis demonstrated that darapladib had a general anti-inflammatory action, reducing the expression of genes associated with macrophage and T-lymphocyte functioning.

This is the first paper to prove the pathological role of Lp-PLA2 in unstable plaque formation.  Studies of darapladib in humans are currently underway; early data has already shown the ability of the drug to reduce inflammatory biomarkers in patients already on statin therapy, the effect this will have on patient outcomes is awaited.

• Wilensky RL, Shi Y, Mohler ER et al.  Inhibition of lipoprotein-associated phospholipase A2 reduces complex coronary atherosclerotic plaque development.  Nat Med 2008;14:1059-1066

Telomeres are specialised DNA-protein structures at the end of chromosomes, and are vital to preserve chromosome integrity and stability.  Telomeres are known to shorten with age, but prematurely shortened telomeres have been found in cells from human atherosclerotic plaques, and a previous relationship between telomere length and coronary artery disease has been described.

Using data from the second European Atherosclerosis Research Study (EARS II), Salpea et al. looked at the association between mean telomere length in various populations across Europe and their association with a parental history of premature myocardia infarction (age <55 years).  The mean leukocyte telomere length was measured in 369 male students aged between 18-28 years, all of whom had a paternal history of premature myocardial infarction, and compared to a control group of 396 with no family history.  After adjustment for age and geographical region, the controls had a shorter telomere length by a mean length of 550 base pairs (p=0.05).

The findings of this study suggest that, in young men at least, a familial risk of myocardial infarction is associated with short telomeres, although whether this is a causative association remains unknown.  Indeed, short telomeres have been found in many major diseases, particularly cancer, and it is currently unclear whether they play any biological role in specific diseases or are merely markers of an increased “genetic age” and hence an increased risk of disease in general.  Prospective studies with follow-up measurements of telomere length will help to determine whether a genetic tendency to short telomeres in leukocytes are at an increase risk for developing ischaemic heart disease, independent of traditional risk factors.

• Salpea KD, Nicaud V, Tiret L et al.  The association of telomere length with paternal history of premature myocardial infarction in the European Atherosclerosis Research Study II. J Mol Med 2008;86:815-824

The discovery of the protective effects of myocardial preconditioning has stimulated much research into the molecular mechanism of this phenomenon.   A number of cardiac proteins have been found whose levels or state of phosphorylation change with cardioprotection, but do any of these play a critical role?

Chen et al. used unbiased proteomic analysis to identify an enzyme that consistently correlated with cardioprotection in  a rat model, mitochondrial aldehyde dehydrogenase 2 (ALDH2).  Further research identified a small molecule, Alda-1, which enhanced levels of ALDH2 and led to a 60% reduction in infarct size when it was injected into rats.

Several animal studies have found important cardioprotective proteins before and it remains to be seen whether ALDH2 and Alda-1 will be of any use in limitng myocardial ischaemia.  On the other hand, the mechanisms described here provide attractive targets for future pharmaceutical agents on several fronts - ALDH2 is also important in the metabolism of alcohol.

• Chen C-H, Budas GR, Churchill EN et al. Activation of aldehyde dehydrogenase-2 reduces ischemic damage to the heart. Science 2008; 321: 1493-1495.

Metabolic technologies allow the profiling of low-molecular weight biochemicals such as amino acids, sugars, and lipids that are substrates and products in important biological processes, such as ischaemia.  To examine whether this techology could be used to allow earlier detection of myocardial ischaemia and infarction, Lewis et al. applied mass spectrometry-based metabolite profiling to 36 patients undergoing alcohol septal ablation for hypertrophic cardiomyopathy.

Blood samples were collected before and at various intervals after the procedure, and coronary sinus sampling was used to distinguish cardiac-derived from peripheral metabolites.  Alterations in the levels of multiple metabolites were detected as early as 10 minutes after infarction, even in patients who had no rise in contemporary bimarkers (e.g. troponin) at 12 hours.  Furthermore, the same pattern of metabolites was not seen in a control group of nine patients undergoing elective cardiac catheterisation.

Multiple biomarker assays such as this are likely to play a major role in the future detection of cardiac damage, providing early warning of impending ischaemic damage and facilitating early intervention.  However, it is too early to consider mainstream use of such assays; the fundamental next step will be examining the metabolic profiles of patients undergoing genuine acute coronary syndromes.

• Lewis GD, Wei R, Liu E, et al. Metabolite profiling of blood from individuals undergoing planned myocardial infarction reveals early markers of myocardial injury. J Clin Invest 2008:DOI: 10.1172/JCI35111.