What is lipoprotein(a)?

What is Lipoprotein(a)?

Lipoprotein(a) [Lp(a)] is a distinct lipoprotein particle consisting of an LDL-like core bound to a unique glycoprotein called apolipoprotein(a), genetically determined and recognized as a causal, independent risk factor for cardiovascular disease and aortic valve stenosis. 1

Structure and Composition

• Lp(a) consists of an LDL-like core lipoprotein containing apolipoprotein B-100 (apoB-100) covalently linked by a disulfide bridge to apolipoprotein(a) [apo(a)], a unique glycoprotein. 1

• The lipid core of Lp(a) is virtually indistinguishable from that of LDL, containing cholesterol esters, free cholesterol, and phospholipids. 1

• Apo(a) has a unique structure consisting of repetitive protein segments called "kringles" that are highly homologous to plasminogen's kringle-IV domains. 1

• The disulfide bridge specifically links Cys4326 in apoB-100 with Cys4057 in apo(a), located in kringle-IV type 9. 1

• Lp(a) exhibits substantial size heterogeneity due to genetic variation in the number of kringle-IV type 2 repeats in apo(a), with 30 or more genetically determined isoforms existing in humans. 1

Genetic Determination and Prevalence

• Lp(a) levels are 70-90% genetically determined by the LPA gene encoding apo(a), with minimal influence from lifestyle factors. 2, 3

• Plasma Lp(a) concentrations show several-hundred-fold interindividual variability, ranging from <0.1 mg/dL to >300 mg/dL. 4

• Elevated Lp(a) levels (>30-50 mg/dL) affect approximately 20-30% of the global population, representing more than one billion people worldwide. 3, 5

• Lp(a) levels increase soon after birth to reach a steady concentration within a few months of life and remain stable throughout adulthood. 6

• Elevated Lp(a) is inherited in an autosomal dominant pattern with high penetrance. 2

Cardiovascular Risk Thresholds

• Traditional thresholds for elevated Lp(a) are >30 mg/dL or >75 nmol/L, representing approximately the 75th percentile in white populations where cardiovascular risk demonstrably increases. 2, 3

• The European guidelines define significant risk at Lp(a) >50 mg/dL (approximately 100-125 nmol/L), representing the 80th percentile. 2, 3

• Risk increases progressively with higher Lp(a) levels, with particularly high risk at >100 mg/dL. 2

Mechanisms of Cardiovascular Pathogenicity

• Lp(a) promotes atherosclerosis through multiple mechanisms: it acts like LDL to promote plaque formation, causes inflammation through oxidized phospholipids, and has anti-fibrinolytic/pro-thrombotic effects. 2

• Lp(a) particles are approximately 7-fold more atherogenic than LDL particles on a per-particle basis. 2

• Mounting evidence from pathophysiological, epidemiologic, and genetic (Mendelian randomization) studies has established a causal relationship between elevated Lp(a) and coronary artery disease, myocardial infarction, stroke, peripheral arterial disease, heart failure, and calcific aortic valve stenosis. 1, 3, 5

• The risk of developing coronary artery disease in Caucasians is more than two times higher in individuals with increased Lp(a). 1

Clinical Significance and Residual Risk

• Evidence from randomized statin and PCSK9 inhibitor trials demonstrates that when Lp(a) is elevated, cardiovascular event rates remain higher at any achieved LDL-C level, confirming unaddressed Lp(a)-mediated residual risk. 2, 3

• Individuals with both elevated Lp(a) (>30 mg/dL) and elevated LDL cholesterol face a 10-fold or higher risk of myocardial infarction compared to those with normal levels of both. 2

• Children with elevated Lp(a) have a fourfold increased risk of acute ischemic stroke, and the risk of recurrent strokes increases more than 10-fold when Lp(a) exceeds the 90th percentile. 2

Laboratory Measurement Considerations

• Standard "LDL-C" laboratory assays cannot separate Lp(a)-cholesterol from true LDL-cholesterol, as Lp(a)-C contributes approximately 30-45% of Lp(a) mass, potentially overestimating true LDL-C in patients with elevated Lp(a). 2, 7

• There is large between-method variation in Lp(a) assays due to lack of a common reference material, making comparisons between laboratories difficult. 2

• Different assays express Lp(a) levels either in mg/dL (mass) or nmol/L (molar concentration), with a conversion factor of approximately 3.17 (1 mg/dL = 3.17 nmol/L). 2

• Serial monitoring of Lp(a) is generally not necessary as levels are genetically determined and remain stable throughout life, except during drug treatment trials or in patients with chronic kidney disease where levels may fluctuate. 2

Physiological Function

• Despite intensive research since its discovery by Berg in 1963, the physiological function of Lp(a) remains elusive. 1

• Lp(a) was initially considered a genetic variant of β-lipoproteins (LDL) but was later recognized as a distinct lipoprotein class. 1

Special Populations

• Lp(a) levels are substantially increased in persons with end-stage renal disease and chronic kidney disease, increasing progressively with worsening renal function. 2

• Lp(a) is an independent predictor of incident coronary heart disease events and mortality specifically in CKD patients. 2

• Patients with familial hypercholesterolemia and elevated Lp(a) have increased cardiovascular risk and may be predisposed to aortic valve calcification. 2

Current Therapeutic Challenges

• Lifestyle modifications (diet, exercise, weight loss) do not significantly lower Lp(a) levels, as 70-90% of variation is genetically determined. 2

• Statins and ezetimibe may paradoxically increase Lp(a) mass levels by approximately 5-10%, despite their overall cardiovascular benefits. 2, 7

• No ICD-10 code exists for the diagnosis of elevated Lp(a) levels in the United States, limiting documentation and research. 3