Novel Biochemical Markers of Cardiovascular Risk:

October 1, 2004

ABSTRACT: High-sensitivity C-reactive protein (hs-CRP), a marker of low-grade vascular inflammation, reflects baseline inflammatory predilection-a key factor in the genesis and rupture of atheromatous plaque. Measurement of hs-CRP is recommended in persons who have an intermediate (10% to 20%) 10-year risk of coronary artery disease; a level above 3 mg/dL indicates higher cardiovascular risk. Although dietary therapy and statins may lower hs-CRP levels, such reductions have not been shown to prevent cardiovascular events or death. Elevated homocysteine levels have been associated with an increased risk of cardiovascular disease. Consider screening in patients with a personal or family history of cardiovascular disease who do not have well- established risk factors. Supplementation with folic acid and vitamin B12 reduces homocysteine levels by about 30%. Elevated fibrinogen levels have been associated with ischemic heart disease and stroke; however, fibrinogen-lowering therapy has not led to better outcomes than standard treatment regimens.

Data continue to emerge concerning novel biochemical markers of cardiovascular risk. These markers have the potential to provide information on risk or prognosis beyond that available from traditional assessment tools. Numerous studies have evaluated markers of inflammation, such as high-sensitivity C-reactive protein (hs-CRP) and interleukin-6 (IL-6); nontraditional lipid markers, such as lipoprotein a (Lp[a]) and apolipoprotein B (apoB); and markers of hemostasis and thrombosis, such as homocysteine and fibrinogen (Table).

The importance of novel markers stems from the fact that cardiovascular disease is the leading cause of death in the United States and that our current lipid screening protocols fail to identify many high-risk patients. A recent prospective study of nearly 28,000 healthy middle-aged women found that 77% of cardiovascular events occurred in those with low-density lipoprotein cholesterol (LDL-C) values below 160 mg/dL, and 46% occurred in those with levels below 130 mg/dL.1 However, the current guidelines issued by the Adult Treatment Panel (ATP) III of the National Cholesterol Education Program do not recommend initiating therapy even in persons at highest cardiovascular risk until LDL-C values are 130 mg/dL or higher (although therapy may be considered if LDL-C values are in the range of 100 to 130 mg/dL).

In this article we review noteworthy data concerning biochemical markers of cardiovascular risk. Although no clear evidence exists to date that lowering plasma levels of any of these markers reduces the risk of future cardiovascular events, many of these markers provide important insights into the pathophysiology of atherothrombosis.2

hs-CRP

This marker of low-grade vascular inflammation has emerged as a potential indicator of cardiovascular risk that is not measured by the assessment of traditional risk factors. The hs-CRP assay differs from the standard CRP assay in that it accurately measures low levels of CRP rather than elevated levels typical of inflammatory states. It reflects a person's baseline inflammatory predilection-a key factor in the genesis and rupture of atheromatous plaque.

To date, at least 19 large-scale prospective studies have shown that hs-CRP may be an independent predictor of future vascular events, even in apparently healthy persons. In the Physicians' Health Study, baseline hs-CRP levels were compared in 22,071 men who had a myocardial infarction (MI), stroke, or venous thrombosis.3 Patients with hs-CRP levels in the top three quartiles had a relative risk of MI or stroke that was 1.7 to 2.6 times higher than that of controls. There was no increased risk of venous thrombosis. A recent study showed that elevated CRP levels were associated with an odds ratio for coronary artery disease (CAD) of 1.45.4

The American Heart Association (AHA), the American College of Cardiology, and the CDC have endorsed hs-CRP measurement as an adjunct to traditional risk factor screening.5 These groups recommend hs-CRP screening in persons who-based on their Framingham risk score or fam- ily history-have an intermediate (10% to 20%) 10-year risk of CAD. Measurement of hs-CRP in persons with high-risk Framingham scores (higher than a 20% 10-year risk) is unnecessary because treatment should be initiated regardless of the hs-CRP level. According to the AHA/CDC statement, lower, moderate, and higher cardiovascular risk are associated with hs-CRP levels of below 1, 1 to 3, and above 3 mg/dL, respectively.

Patients who are identified as at high risk based on their hs-CRP levels and traditional risk factors may benefit from therapy designed to lower these levels. In one study, hs-CRP levels were reduced 10% by a diet low in saturated fat and 28% by a diet high in plant sterols.6 Stat- in therapy lowers hs-CRP levels by 13% to 50% in patients with a history of MI or hyperlipidemia, or as primary prevention.7,8 In patients with CAD, β-blocker therapy decreased hs-CRP by 31%.9 Patients with type 2 diabetes mellitus had significant reductions in hs-CRP after therapy with rosiglitazone.10 Although aspirin does not appear to lower hs-CRP, it reduced the risk of events in men in the highest quartile of hs-CRP levels by 56%. This suggests that the beneficial effects of aspirin may be mediated not only by its antiplatelet activity but also by its anti-inflammatory effects in persons with high hs-CRP levels.3,11

Although levels of hs-CRP can be lowered with dietary measures, β-blockers, and rosiglitazone, current efforts are best directed at modifying traditional risk factors-such as obesity, smoking, and lack of physical activity-that have been definitively shown to lower the risk of CAD.

No randomized controlled trials (RCTs) thus far have shown that lowering hs-CRP levels prevents car- diovascular events or deaths. The JUPITER (Justification for the Use of Statins in Primary Prevention: an Intervention Trial Evaluating Rosuvastatin) trial, a nationwide 15,000- patient RCT, has been designed to study the effects of statin therapy as primary prevention in persons with low LDL-C levels who are at increased cardiovascular risk because of elevated hs-CRP. Results of this trial may provide an evidence base for the use of hs-CRP testing as an adjuvant guide to therapy to complement established risk factors such as lipid levels.

HOMOCYSTEINE

Elevated levels of homocysteine, an amino acid whose metabolism is affected by folate, vitamin B6, and vitamin B12, have been associated with increased risk of ischemic heart disease, stroke, and peripheral arterial disease. Homocysteine levels are higher in smokers and in those with high total cholesterol levels, and high systolic blood pressure.Elevated homocysteine levels may have a genetic component, which may be a factor in some families with a strong history of MI at an early age despite the absence of other risk factors.

A meta-analysis of 30 prospective and retrospective studies showed that patients whose usual homocysteine level (about 0.41 mg/L) was 25% lower than the meanhad an 11% lower risk of ischemic heart disease and a 19% lower risk of stroke.12

Another meta-analysis of 15 studies showed that the odds of CAD in patients with elevated homocysteine were 70% higher than in those with normal levels.13 The odds of stroke were 2.5 times higher in 9 studies and the odds of peripheral vascular disease were 6.8 times higher in 5 studies. The difference in risk of CAD mortality between a patient with a homocysteine level lower than 10 mmol/L and one with a level higher than 15 mmol/L is similar to the risk in a patient whose total cholesterol level is 189 mg/dL compared with one whose level is 275 mg/dL.13

There is strong evidence that folate and vitamin B12 lower homocysteine levels.14 The Homocysteine Lowering Trialists' Collaboration performed a meta-analysis of 12 randomized trials to assess how effectively folic-acid–based supplements lower homocysteine and at which doses.15 Although the results depended on the severity of pretreatment folate deficiency and homocysteine elevation, folic acid supplementation led to a 25% decrease in typical homocysteine levels in Western populations. The addition of vitamin B12 decreased homocysteine levels by another 7%. Adding vitamin B6 did not affect homocysteine levels but did decrease the rise in homocysteine following a methionine load, which would suggest lower postprandial levels of homocysteine in patients treated with vitamin B6.

Lowering homocysteine levels improves endothelial function.16 Nevertheless, there are no large RCTs that demonstrate that lowered homocysteine levels improve end points such as MI, stroke, and mortality. A limited study of patients treated with triple vitamin therapy (folic acid, vitamin B12, and vitamin B6) following percutaneous coronary intervention showed a 38% reduction in need for target lesion revascularization during an average of 11 months of follow-up.17

Several large RCTs in Norway, Australia, the United Kingdom, and the United States are under way; definitive recommendations await these results. Consequently, the AHA currently does not recommend routine screening for homocysteine or routine supplementation with B vitamins. Because most Americans do not meet the recommended daily allowance for folic acid (400 mg), vitamin B12 (2.4 mg) and vitamin B6 (1.7 mg), the AHA continues to emphasize a diet high in fortified cereals, fruits, leafy green vegetables, legumes, fish, poultry, and beef. The AHA does, however, suggest that screening for homocysteine may be useful in patients with a personal or family history of cardiovascular disease who do not have well-established risk factors (smoking, high cholesterol levels, high blood pressure, physical inactivity, obesity, or diabetes).18,19

If screening is warranted, patients should be fasting when samples used to measure homocysteine levels are drawn. Although the risk associated with elevated homocysteine values is continuous, the AHA Science Advisory suggests that levels above 15 mmol/L may suggest an elevated risk of cardiovascular diseaseand that levels below 10 mmol/L may be a reasonable therapeutic target in persons who have established risk factors for cardiovascular disease.18,19

The Homocysteine Lowering Trialists' Collaboration showed that effective doses of folic acid ranged from 0.5 to 5 mg/d.15 The Food and Nutrition Board of the Institute of Medicine recommends using no more than 1 mg/d of folate because of the risk of masking signs of vitamin B12 deficiency.20 The average vitamin B12 dose in the meta-analysis was 0.5 mg/d and the vitamin B6 doses ranged from 50 to 250 mg/d.15 Recheck homocysteine levels after 4 to 7 weeks to confirm a response to therapy.10 If the response is inadequate, some practitioners recommend increasing the folate dose.

Long-term randomized, controlled clinical trials of vitamin therapy versus placebo are in order before recommendations concerning screening for and treatment of hyperhomocysteinemia can be offered.

FIBRINOGEN

Fibrinogen plays a well-known role in coagulation and, as an acute phase reactant, is the principal determinant of the erythrocyte sedimentation rate. Elevated fibrinogen levels have been associated with smoking, lack of exercise, hypertriglyceridemia,21 and insulin resistance.22 Fibrinogen is also an independent risk factor for ischemic heart disease and stroke.

A meta-analysis published in 1998 reviewed data from 18 prospective studies that included more than 4000 patients.23 The analysis showed a relative risk of 1.8 for cardiovascular disease in persons with fibrinogen levels in the upper tertile compared with those with levels in the lower tertile. More recent observational studies confirm that elevated fibrinogen levels are associated with cardiovascular disease.24,25

Currently, however, fibrinogen cannot be recommended as a marker in cardiovascular risk stratification. Evidence that targeted fibrinogen-lowering therapy leads to better outcomes than the current ATP goals is lacking. Moreover, there is no widely available, standardized, accurate measurement method. The most frequently used method results in levels that vary as much as 17.8%.26 These variations are unacceptable, because the mean difference in fibrinogen levels between groups with and without cardiovasculardisease is just over 9%.

OTHER MARKERS

ApoB is a component of the atherogenic lipoproteins (LDL, intermediate-density lipoprotein, very-low-density lipoprotein, and Lp[a]). Some experts have suggested that apoB may be a better predictor of cardiovascular risk than LDL-C levels alone. In one trial with more than 175,000 participants, apoB was a better predictor of risk than LDL-C.27 Other studies have shown conflicting results. The AHA does not recommend testing for apoB. The Canadian national guidelines, in contrast, began recommending testing for apoB in 2001. Statins do lower apoB levels, but screening for and treatment of apoB must await consistent study results, standardized assays, and agreed-on thresholds and treatment targets.

Nitric oxide–derived oxidants may be the link between inflammation and the development of atherosclerosis. Nitrotyrosine, a specific marker for protein modification by nitric oxide–derived oxidants, is elevated in human atherosclerotic lesions and LDL recovered from human atheromas. A recent case-control interventional study showed an association between higher levels of nitrotyrosine and CAD that was independent of other established cardiovascular risk factors.28 Statin therapy reduced nitrotyrosine levels by 25% compared with no therapy. Research on the effects of lowered nitrotyrosine levels on cardiovascular morbidity and mortality is needed before this marker can be used for routine screening.

IL-6 may be a marker for increased risk of cardiovascular disease. This molecule is an inflammatory cytokine and, like hs-CRP, a marker of systemic inflammation. Increased IL-6 levels are associated with advanced age, hypertension, smoking, and insulin sensitivity. Plasma levels of IL-6 are elevated in patients with unstable angina compared with stable angina as well as in those with acute MI. A number of observational studies have shown a higher risk of cardiovascular events and death among patients with the highest levels of IL-6 compared with those with the lowest levels, independent of CRP levels or traditional cardiovascular risk factors.29 More research is needed to determine whether IL-6 will help predict cardiovascular disease and which interventions might affect plasma levels.

Adiponectin, a plasma protein derived from adipose tissue, exhibits anti-inflammatory and antiatherosclerotic properties. Adiponectin levels correlate negatively with percentage of body fat, central fat distribution, fasting plasma insulin levels, and oral glucose tolerance. Adiponectin levels are also reduced in patients with CAD, which may indicate a role in vasculopathic states.30 More research is needed to delineate the role of this marker in cardiovascular disease.

Plasminogen activator inhibitor–1 (PAI-1) is the principal physiologic inhibitor of tissue- and urokinase-type plasminogen activators. Some experts believe that increased levels of PAI-1 at sites of atherosclerotic lesion formation might disturb the balance between coagulation, anticoagulation, and fibrinolysis, thus leading to thrombus formation. However, PAI-1 is also thought to protect against atheromatous plaque growth. Because of this multifaceted biochemistry, the role of PAI-1 in atherosclerosis remains to be determined.31 n

References:

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