Novel biochemical risk factors
The classic risk factors and the derived algorithms and scores for global risk estimation described in the previous section are valid for risk estimation in presymptomatic patients.
Since only one third of MIs occur in individuals with an estimated risk above 20%, there is a considerable need for additional biomarkers that will help to identify patients at increased risk, for example, to further stratify risk among persons in the category of intermediate risk. In addition, the classic risk factors and the algorithms are not suitable for assessing the prognosis of patients with manifest acute or chronic heart disease. Among the many novel risk factors that are currently being evaluated, we will focus on those examples that have been best investigated so far.
Lipoprotein (a)
Lipoprotein (a) (Lp(a), pronounced “lipoprotein small A” or “L-P small A”), consists of an LDL particle, the apolipoprotein B moiety of which is covalently attached to the glycoprotein apo A. This lipoprotein was discovered by the Norwegian Kare Berg in 1963. The level of Lp(a) is genetically determined and can vary widely. In most people, about 6% of circulating cholesterol is carried by Lp(a).
High serum levels of Lp(a) are a risk factor for CHD. A meta-analysis of data on more than 4,000 cases showed that patients within the upper third of Lp(a) levels had a risk of coronary events 70% greater than that of patients with levels in the lower third of the Lp(a) distribution. By convention, most laboratories agree on a cut-off of 30 mg/dL, above which cardiovascular risk is considered to be increased. The importance of a high level of Lp(a) as a coronary risk factor increases in the presence of other risk factors. In PROCAM, for example, elevated Lp(a) increased risk in men with elevated LDL cholesterol, low HDL cholesterol, arterial hypertension and high overall cardiovascular risk to a greater extent than in men with low or normal LDL cholesterol, high HDL cholesterol, normal blood pressure or low estimated global risk.
Elevated Lp(a) has been identified as a risk factor for stroke in the elderly and in the young. In the latter, raised Lp(a) seems to be of special relevance if it coincides with thrombophilic risk factors such as the presence of factor V Leiden, a mutation in a clotting factor that is associated with an increased risk of deep vein thrombosis and pulmonary embolism.
The Lp(a) level in a person is genetically determined and therefore usually invariable. However, renal insufficiency and proteinuria may increase Lp(a) levels, reflecting the role of the kidney in the elimination of apo A. Lp(a) levels are little influenced by currently available drugs, with the exception of nicotinic acid and sex steroids.
An international Lp(a) standard became available only recently. However, different tests have yielded varying results using even this standard, and thus the Lp(a) level measured in the same sample may vary between different laboratories. This lack of standardization has also hindered the incorporation of Lp(a) into standard risk algorithms.
Homocysteine
Homocysteine is an independent cardiovascular risk factor, but does not show the strength of association to CHD of the classic risk factors or of the novel risk factors CRP, Lp(a) or fibrinogen. Homocysteine further increases the risk of high-risk individuals, such as those with pre-existing CHD or at high global CHD risk. Recommended cut-offs for homocysteine vary from 10 to 16 μmol/L. Folate, either alone or in combination with vitamins B6 and B12, lowers homocysteine levels, but this has not been shown in studies to reduce CHD risk. It is important to note, however, that a normal homocysteine level rules out deficiencies of folate and vitamins B6 and B12 and that these studies did not include patients on the basis of their homocysteine levels. As a consequence, many patients received treatment without having a vitamin deficiency.
C-reactive protein
Mildly elevated serum levels of C-reactive protein (CRP) are a significant and independent cardiovascular risk factor. A CRP level above 1 mg/L indicates a moderate increase in risk, and a level above 3 mg/L is associated with high risk. Since CRP is strongly influenced by inflammation, levels above 10 mg/L may not be used for cardiovascular risk assessment. Several studies indicate that CRP may improve global risk estimation using conventional risk factors. Post hoc analyses of intervention trials indicate that men with elevated CRP benefit to a disproportionate degree from aspirin and statin therapy. Measurement of CRP is recommended in individuals at intermediate CHD risk, but not for general screening or for individuals at low (low case finding) and high risk (intervention required regardless of CRP).
It is important to note that CRP has a better prognostic efficacy in patients with manifest CHD than in asymptomatic patients. In patients with either acute coronary syndromes or stable angina pectoris, increases in CRP are associated with poorer prognosis in both the short (<30 days) and long terms. Some authorities recommend measurement of sensitive CRP, together with troponins and B-type natriuretic peptides, for risk stratification in patients with acute coronary events. However, consensus on this approach does not exist at the present time.
In November 2008, the results of the Justification for the Use of statins in Primary prevention: an Intervention Trial Evaluating Rosuvastatin (JUPITER) were represented1. In this trial, 17,802 patients in late middle age (men older than 50 years, women older than 60 years) in 1,315 centers in 26 countries received either 20 mg rosuvastatin daily or placebo. Inclusion criteria were an LDL cholesterol below 130 mg/dL and a CRP above 2 mg/dL. Patients on statin showed a reduction in LDL cholesterol of 55 mg/dL and a 37% drop in CRP compared to controls. This was associated with a 44% drop in the combined endpoint of myocardial infarction, stroke, revascularization, hospitalization for unstable angina or death, as a result of which the study was terminated prematurely after 1.9 years instead of 4 years as planned. There have been claims that this benefit was in part causally related to the statin-induced drop in CRP, but there is at present little evidence to support this view, as pointed out by a report in Science published in late November 2008.2
Other novel risk factors
Fibrinogen and D-dimer
In several prospective studies, fibrinogen has been identified as an independent cardiovascular risk factor. Like CRP, fibrinogen is an acute-phase reactant and is not useful for assessing risk in patients with acute inflammatory disease. There is no international consensus on a diagnostic cutoff for fibrinogen, although most studies used 3.5 g/L. Fibrinogen was found to further increase the risk of men at high global risk in the Framingham Heart Study and the PROCAM study. Information on therapeutic interventions based on elevated fibrinogen is not available.
In contrast to fibrinogen, D-dimer is a marker for coagulation activation and fibrin formation. Increased D-dimer levels have been associated with cardiovascular disease (CVD) in men, although data on D-dimer as a risk factor in the general population do not exist. A generally valid cutoff value for all assays and disease situations is not available. No information currently exists on therapeutic interventions based on elevated D-dimer values.
Microalbuminuria
Urinary excretion of between 30 and 300 mg albumin/24 h (microalbuminuria) is a marker of micro- and macrovascular damage in patients with diabetes mellitus or hypertension. For this reason, and because of the proven benefit of treatment with angiotensinconverting enzyme inhibitors or angiotensin II receptor antagonists in patients with microalbuminuria, some guidelines recommend the measurement of albuminuria in hypertensive or diabetic patients. Evidence is accumulating that microalbuminuria also indicates cardiovascular risk in the general population and interacts with classic risk factors. However, lack of agreement on the optimal sample type and large intra-individual variation due to fever, stress and menstrual bleeding have limited the use of microalbuminuria. Twenty-four h urine collections (cut-offs: >30 mg/24 h or >20 μg/min) are neither practical nor well accepted by patients. Albumin concentrations in spot urine show a good correlation with 24-h albumin excretion if taken at a defined time point (second morning urine), but disagreement exists on whether the albumin-to-creatinine ratio (i.e., 2 mg/mmol creatinine) or absolute albumin concentration (i.e., >20 mg/L) should be used. The former depends on muscle mass and requires age- and sex-specific cut-offs; the latter is confounded by variation in urine amount. Despite these drawbacks, microalbuminuria should be assessed annually in all patients with diabetes mellitus and in patients with poorly controlled hypertension.
Natriuretic peptides
B-type natriuretic peptide (BNP) is secreted by the heart muscle cells in response to increased wall tension due to ventricular volume expansion and pressure overload. BNP stimulates natriuresis and diuresis. Biologically active BNP is released into the circulation together with an inactive aminoterminal peptide, N-terminal proBNP (NT-proBNP), which is cleaved from the high molecular weight precursor protein. Both BNP and NT-proBNP are diagnostic and prognostic markers of congestive heart failure and left ventricular dysfunction. In addition, even mildly elevated plasma levels of either BNP or NT-proBNP indicate increased risk of cardiovascular events in patients with previous MI and among the healthy population. In combination with troponin and CRP, BNP predicts recurrent events and death in patients with acute coronary syndrome. BNP is also elevated in patients with pulmonary embolism, chronic kidney disease and sepsis. It is not clear at present whether BNP or NT-proBNP is the superior diagnostic marker. BNP is the biologically active hormone, but has the shorter half-life and is less stable than NT-proBNP. In addition, the lack of standardization leads to discrepant BNP values obtained with tests from different manufacturers. The measurement of NT-proBNP is protected by patent, and thus all tests use identical antibodies, improving assay standardization.
Biomarkers of oxidation
Oxidative damage to lipids and proteins is an important feature of atherosclerosis, and oxidation-related molecules are candidate biomarkers for improving cardiovascular risk assessment. The best-evaluated markers to date are lipoprotein-associated phospholipase A2 (Lp-PLA2) and asymmetric dimethylarginine (ADMA).
Lp-PLA2, also known as platelet-activating factor acetylhydrolase, is secreted by various inflammatory cells, including monocyte-macrophages, T cells and mast cells. Several prospective population studies have shown that elevated plasma levels of Lp-PLA2 are associated with increased risk of coronary events and stroke. The association with CHD is independent of traditional risk factors and other inflammatory markers. Lp-PLA2 is also a risk factor for recurrent events in subjects with coronary artery disease.
ADMA is an endogenous inhibitor of all major isoforms of endothelial nitric oxide synthase and may thereby contribute to endothelial dysfunction. ADMA plasma levels are increased in patients with impaired renal function, hypercholesterolemia, hypertriglyceridemia, insulin resistance, diabetes mellitus, hyperhomocysteinemia and hypertension.ADMA has been identified as an independent risk factor for cardiovascular events in healthy populations and as a marker for atherosclerosis progression, cardiovascular death and total mortality in patients with CHD and renal failure. The value ot Lp-PLA2 and ADMA in overall risk assessment is unclear at the present time.
- Ridker, P.M., Danielson, E., Fonseca, F.A. et al. Rosuvastatin to prevent vascular events
in men and women with elevated C-reactive protein. N Engl J Med 2008; 359: 195-2207. - Couzin, J. Cardiovascular health. Statin therapy reduces disease in healthy volunteers–but how, exactly? Science 2008; 322: 1039.