Impact of apo(a) length polymorphism and the control of plasma Lp(a) concentrations: evidence for a threshold effect

Sequence variation within the KIV-2 copy number polymorphism of the human LPA gene in African, Asian, and European populations

Amazingly little sequence variation is reported for the kringle IV 2 copy number variation (KIV 2 CNV) in the human LPA gene. Apart from whole genome sequencing projects, this region has only been analyzed in some detail in samples of European populations. We have performed a systematic resequencing study of the exonic and flanking intron regions within the KIV 2 CNV in 90 alleles from Asian, European, and four different African populations. Alleles have been separated according to their CNV length by pulsed field gel electrophoresis prior to unbiased specific PCR amplification of the target regions. These amplicons covered all KIV 2 copies of an individual allele simultaneously. In addition, cloned amplicons from genomic DNA of an African individual were sequenced. Our data suggest that sequence variation in this genomic region may be higher than previously appreciated. Detection probability of variants appeared to depend on the KIV 2 copy number of the analyzed DNA and on the proportion of copies carrying the variant. Asians had a high frequency of so-called KIV 2 type B and type C (together 70% of alleles), which differ by three or two synonymous substitutions respectively from the reference type A. This is most likely explained by the strong bottleneck suggested to have occurred when modern humans migrated to East Asia. A higher frequency of variable sites was detected in the Africans. In particular, two previously unreported splice site variants were found. One was associated with non-detectable Lp(a). The other was observed at high population frequencies (10% to 40%). Like the KIV 2 type B and C variants, this latter variant was also found in a high proportion of KIV 2 repeats in the affected alleles and in alleles differing in copy numbers. Our findings may have implications for the interpretation of SNP analyses in other repetitive loci of the human genome.

Combined effects of small apolipoprotein (a) isoforms and small, dense LDL on coronary artery disease risk

BACKGROUND AND AIMS

Lipoprotein (a) [Lp(a)] consists of low-density lipoprotein (LDL) and apolipoprotein (a) [apo(a)]. Both Lp(a) constituents are well-recognized risk factors for coronary artery disease (CAD). This study investigates the interrelationship of apo(a) and LDL size, as well as their possible synergistic effect on the increase of CAD risk.

METHODS

One hundred nine CAD patients and 102 apparently healthy subjects were included in the study. Lp(a) concentration was measured using immunoturbidimetry. The sizes of apo(a) isoforms were determined by SDS-agarose gel electrophoresis followed by immunoblotting. LDL particle size was determined by gradient gel electrophoresis.

RESULTS

We found an inverse correlation between apo(a) size and Lp(a) concentration (r(2) = 31%, p <0.001 in the control group and r(2) = 35%, p <0.001 in the CAD group). Individuals with smaller apo(a) isoforms and small, dense LDL (sdLDL) >50% had the highest risk of CAD development (OR = 4.23, p = 0.017). The synergy index (SIM) for the combination of smaller apo(a) isoforms and sdLDL >50% was 1.2. Adjustment for Lp(a) and triacylglycerol concentrations eliminated smaller apo(a)/sdLDL >50% related risk (p = 0.233 and p = 0.09, respectively).

CONCLUSIONS

Smaller apo(a) isoforms appear to be superior to sdLDL for the assessment of CAD risk. Their combined effect is synergistic.

Elevated Lp(a) with a small apo(a) isoform in children: risk factor for the development of premature coronary artery disease

BACKGROUND

levels of Lp(a) and low-molecular-weight apolipoprotein(a) isoform are strongly associated with the development of early cardiovascular disease. Certain types of apo(a) isoforms in combination with elevated levels of Lp(a) may be important in the determining of premature coronary artery disease. Therefore, we investigated the association of familial history of premature coronary artery disease and apo(a) size and Lp(a) levels in children and adolescents with hypercholesterolemia using a novel method determining apo(a) isoforms.

METHODS AND RESULTS

Isoforms were classified in six phenotype patterns: S1-S4, B, F and according to their K-IV repeats. Apo(a) isoforms were divided into two groups: low-molecular- and high-molecular apo(a) isoforms. In subjects with double-banded apo(a) isoforms containing a small- and a large-isoform Lp(a) each contribution was based on the intensity of staining of the two bands. The percentage of patients with elevated levels of Lp(a) and a small apo(a) isoform (i.e. elevated small-isoform Lp(a)) was 46% in the risk group and 20% in the control group, p < 0.05. The percentage number of children and adolescents with elevated Lp(a) levels was higher in the risk group, reaching statistical significance (p < 0.05).

CONCLUSION

Elevated levels of small-isoform Lp(a) might be a strong and independent risk factor for the development of premature coronary artery disease in children and adolescents with hypercholesterolemia.

Causality of pediatric brainstem infarction and basilar artery fenestration?

Pediatric arterial thromboembolic stroke is an uncommon condition and rarely is reported to be associated with a cerebral artery fenestration. This clinical report discusses the case of a child with brainstem infarction and basilar artery fenestration. A cardiac source of thromboembolic events could be excluded; however, detailed coagulation analysis revealed in addition an apoliopoprotein(a) size polymorphism. Because we assume that the two concurrent pathologies in combination caused the arterial thromboembolic stroke, the evaluation of all potential triggers including vascular anomalies and coagulation disorders should be considered in unexplained pediatric infarction.

Serum lipoprotein(a) levels and apolipoprotein(a) isoform size and risk for first-ever acute ischaemic nonembolic stroke in elderly individuals

In a population-based case-control study, we investigated the association of acute ischaemic stroke with lipoprotein(a) (Lp(a)) levels and apolipoprotein (Apo) (a) isoform size in subjects aged older than 70 years. A total of 163 patients with a first-ever-in-a-lifetime acute ischaemic/nonembolic stroke and 166 controls were included. Compared to controls, stroke patients exhibited higher Lp(a) concentrations (median value, 12.2 mg/dl versus 6.4 mg/dl, p < 0.001) and a higher frequency of small Apo(a) isoforms (44.2% versus 29.5%, p < 0.01). Multivariate logistic regression analysis showed a significant association of acute ischaemic stroke with Lp(a) levels [adjusted odds ratio (OR), 1.37, 95% CI (1.12-1.67); p = 0.002], and small Apo(a) isoform size [OR, 1.74 (1.10-3.03); p = 0.04]. Compared to subjects with Lp(a) levels in the lowest quintile, those within the highest quintile had a 3.2-times adjusted risk to suffer an acute ischaemic/nonembolic stroke (1.60-6.62, 95% CI; p < 0.001). Furthermore, analysis of interaction between lipid variables revealed that in the presence of elevated Lp(a) levels the inverse relationship between HDL-cholesterol levels and ischaemic stroke was negated [OR, 1.01 (1.00-1.03); p = 0.015]. Our study suggests that determination of Lp(a) levels and Apo(a) isoform size may be important in identifying elderly individuals at risk of ischaemic stroke independently of other risk factors and concurrent metabolic derangements.

Lipoprotein(a) as a cardiovascular risk factor

Evidence for the role of lipoprotein(a) (Lp[a]) in atherosclerosis and thrombosis has considerably increased over the past few years. Therefore, Lp(a) is currently classified as an emerging lipid risk factor for cardiovascular disease. High Lp(a) plasma levels carried in particles with small-sized apolipoprotein(a) isoforms are associated with preclinical vascular changes, cardiovascular disease and the mode of presentation of coronary artery disease (acute coronary syndromes). However, randomized clinical trials with an emphasis on agents that specifically lower plasma Lp(a) do not exist. At present, screening for increases in Lp(a) in the general population is not recommended. The measurement of Lp(a) may be of value in individuals with an increased risk of cardiovascular disease, particularly in patients with high low-density lipoprotein cholesterol plasma levels, since a high Lp(a) concentration in such subjects further increases the risk of coronary heart disease.

Relationship between apolipoprotein(a) size polymorphism and coronary heart disease in overweight subjects

BACKGROUND

Overweight is associated with an increased cardiovascular risk which is only partially explained by conventional risk factors. The objective of this study was to evaluate lipoprotein(a) [Lp(a)] plasma levels and apolipoprotein(a) [apo(a)] phenotypes in relation to coronary heart disease (CHD) in overweight subjects.

METHODS

A total of 275 overweight (BMI > or = 27 kg/m2) subjects, of which 155 had experienced a CHD event, 337 normal weight subjects with prior CHD and 103 CHD-free normal weight subjects were enrolled in the study. Lp(a) levels were determined by an ELISA technique and apo(a) isoforms were detected by a high-resolution immunoblotting method.

RESULTS

Lp(a) levels were similar in the three study groups. Overweight subjects with CHD had Lp(a) concentrations significantly higher than those without [median (interquartile range): 20 (5-50.3) versus 12.6 (2.6-38.6) mg/dl, P < 0.05]. Furthermore, overweight subjects with CHD showed a higher prevalence of low molecular weight apo(a) isoforms than those without (55.5% versus 40.8%, P < 0.05) and with respect to the control group (55.5% versus 39.8%, P < 0.05). Stepwise regression analysis showed that apo(a) phenotypes, but not Lp(a) levels, entered the model as significant independent predictors of CHD in overweight subjects.

CONCLUSIONS

Our data indicate that small-sized apo(a) isoforms are associated with CHD in overweight subjects. The characterization of apo(a) phenotypes might serve as a reliable biomarker to better assess the overall CHD risk of each subject with elevated BMI, leading to more intensive treatment of modifiable cardiovascular risk factors.

Study of apo(a) length polymorphism and lipoprotein(a) concentrations in subjects with single or double apo(a) isoforms

Cardiovascular risk is associated with high lipoprotein(a) (Lp(a)) concentrations and low molecular weight apolipoprotein(a) (apo(a)) isoforms. We studied the relationship between these two biological parameters, particularly in subjects expressing two apo(a) isoforms. Plasma Lp(a) was measured by immunonephelometry in 530 unrelated Caucasian patients at high cardiovascular risk, and apo(a) size determined by immunoblotting using a recombinant standard. Two, one, or no apo(a) isoforms were detected in 258, 270, and 2 subjects, respectively. Lp(a) concentrations showed a non-Gaussian distribution, being higher in the 'double band' than in the 'single band' group (median 0.42 vs. 0.11 g/l, p < 0.0005). Apo(a) size distribution was bimodal, with two frequency peaks at 18 kringles (K) and 27 K. Small size apo(a) isoforms were more frequently found in the 'double band' group, where major isoforms were of lower size than minor isoforms (median 20 vs. 27 K). Regression analysis showed that apo(a) gene length accounted for 33% of Lp(a) variation, with a threshold effect at 20 K, no correlation being found over this value. The minor apo(a) isoform did not significantly influence Lp(a) concentration. These data confirm the relationship between apo(a) size and Lp(a) concentration and suggest that the assessment of cardiovascular risk should take into account the threshold effect at 20 K and the absence of influence of the minor apo(a) isoform.

Lipoprotein (a) levels and apolipoprotein (a) isoform size in patients with subclinical hypothyroidism: effect of treatment with levothyroxine

The increased risk for ischemic heart disease (IHD) associated with subclinical hypothyroidism (SH) has been partly attributed to dyslipidemia. There is limited information on the effect of SH on lipoprotein (a) [Lp(a)], which is considered a significant predictor of IHD. Serum Lp(a) levels are predominantly regulated by apolipoprotein [apo(a)] gene polymorphisms. The aim of our study was to evaluate the Lp(a) levels and apo(a) phenotypes in patients with SH compared to healthy controls as well as the influence of levothyroxine substitution therapy on Lp(a) values in relation to the apo(a) isoform size. Lp(a) levels were measured in 69 patients with SH before and after restoration of a euthyroid state and in 83 age- and gender-matched healthy controls. Apo(a) isoform size was determined by sodium dodecyl sulfate (SDS) agarose gel electrophoresis followed by immunoblotting and development via chemiluminescence. Patients with SH exhibited increased Lp(a) levels compared to controls (median value 10.6 mg/dL vs. 6.0 mg/dL, p = 0.003]), but this was not because of differences in the frequencies of apo(a) phenotypes. There was no association between thyrotropin (TSH) and Lp(a) levels in patients with SH. In subjects with either low (LMW; 25 patients and 28 controls) or high (HMW; 44 patients and 55 controls) molecular weight apo(a) isoforms, Lp(a) concentrations were higher in patients than in the control group (median values 26.9 mg/dL vs. 21.8 mg/dL, p = 0.02 for LMW, and 6.0 mg/dL versus 3.3 mg/dL, p < 0.001 for HMW). Levothyroxine treatment resulted in an overall reduction of Lp(a) levels (10.6 mg/dL baseline vs. 8.9 mg/dL posttreatment, p = 0.008]). This effect was mainly evident in patients with LMW apo(a) isoforms associated with high baseline Lp(a) concentrations (median values 26.9 mg/dL vs. 23.2 mg/dL pretreatment and posttreatment, respectively; p = 0.03). In conclusion, even though a causal effect of thyroid dysfunction on Lp(a) was not clearly demonstrated in patients with SH, levothyroxine treatment is beneficial, especially in patients with increased baseline Lp(a) levels and LMW apo(a) isoforms.

Laboratory issues: use of nutritional biomarkers

Biomarkers of nutritional status provide alternative measures of dietary intake. Like the error and variation associated with dietary intake measures, the magnitude and impact of both biological (preanalytical) and laboratory (analytical) variability need to be considered when one is using biomarkers. When choosing a biomarker, it is important to understand how it relates to nutritional intake and the specific time frame of exposure it reflects as well as how it is affected by sampling and laboratory procedures. Biological sources of variation that arise from genetic and disease states of an individual affect biomarkers, but they are also affected by nonbiological sources of variation arising from specimen collection and storage, seasonality, time of day, contamination, stability and laboratory quality assurance. When choosing a laboratory for biomarker assessment, researchers should try to make sure random and systematic error is minimized by inclusion of certain techniques such as blinding of laboratory staff to disease status and including external pooled standards to which laboratory staff are blinded. In addition analytic quality control should be ensured by use of internal standards or certified materials over the entire range of possible values to control method accuracy. One must consider the effect of random laboratory error on measurement precision and also understand the method's limit of detection and the laboratory cutpoints. Choosing appropriate cutpoints and reducing error is extremely important in nutritional epidemiology where weak associations are frequent. As part of this review, serum lipids are included as an example of a biomarker whereby collaborative efforts have been put forth to both understand biological sources of variation and standardize laboratory results.

Sequence and functional changes in a putative enhancer region upstream of the apolipoprotein(a) gene

Two enhancer regions upstream of the apolipoprotein(a) (apo(a)) gene were amplified and sequenced from subjects who were known to express abnormally high or low amounts of lipoprotein(a) (Lp(a)). No base changes were found in the DHII region situated 28 kb from the apo(a) gene. Three common base substitutions were found in the DHIII region about 20 kb from the gene. One, an A to G change at position -1230, increased the activity of reporter-gene constructs approximately 2.5-fold. The other two, a C to A change at -1617 and a G to T change at -1712, decreased reporter activity by 30 and 40%, respectively. The sites at -1230 and -1617 were in linkage disequilibrium with each other and also with a polymorphic site near the DHII enhancer and sites in the apo(a) promoter and gene. The rarer G variant at -1230 was associated with smaller alleles. After correcting for the effect of allele size, values of Lp(a) concentration for alleles associated with the G variant at -1230 were 70% higher than those associated with the more common A variant. In contrast, the corrected values for alleles associated with the rare T variant at -1712 were 40% lower than those associated with the common G variant. Thus, overall the changes observed in this enhancer could influence apo(a) gene transcription up to 4-fold and could provide a significant contribution to the variation in Lp(a) concentrations in plasma.

Specificity determinants in the interaction of apolipoprotein(a) kringles with tetranectin and LDL

Lipoprotein(a) is composed of low density lipoprotein and apolipoprotein(a). Apolipoprotein(a) has evolved from plasminogen and contains 10 different plasminogen kringle 4 homologous domains [KIV(1-110)]. Previous studies indicated that lipoprotein(a) non-covalently binds the N-terminal region of lipoprotein B100 and the plasminogen kringle 4 binding plasma protein tetranectin. In this study recombinant KIV(2), KIV(7) and KIV(10) derived from apolipoprotein(a) were produced in E. coli and the binding to tetranectin and low density lipoprotein was examined. Only KIV(10) bound to tetranectin and binding was similar to that of plasminogen kringle 4 to tetranectin. Only KIV(7) bound to LDL. In order to identify the residues responsible for the difference in specificity between KIV(7) and KIV(10), a number of surface-exposed residues located around the lysine binding clefts were exchanged. Ligand binding analysis of these derivatives showed that Y62, and to a minor extent W32 and E56, of KIV(7) are important for LDL binding to KIV(7), whereas R32 and D56 of KIV(10) are required for tetranectin binding of KIV(10).

Caloric restriction lowers plasma lipoprotein (a) in male but not female rhesus monkeys

Many age-associated pathophysiological changes are retarded by caloric restriction (CR). The present study has investigated the effect of CR on plasma lipoprotein (a) [Lp(a)], an independent risk factor for the age-associated process of atherosclerosis. Rhesus monkeys were fed a control diet (n=19 males, 12 females) or subjected to CR (n=20 males, 11 females fed 30% less calories) for >2 years. All female animals were premenopausal. Plasma Lp(a) levels in control animals were almost two fold higher for males than females (47+/-9 vs 25+/-5mg/dl mean+/-SEM, p=0.05). CR resulted in a reduction in circulating Lp(a) in males to levels similar to those measured in calorie-restricted females, (27+/-5 vs 24+/-4 mg/dl mean+/-SEM). For all animals, plasma Lp(a) was correlated with total cholesterol (r=0.27, p=0.03) and LDL cholesterol (r=0.50, p=0.0001) whether unadjusted or after adjustment for treatment, gender or group. These studies introduce a new mechanism whereby CR may have a beneficial effect on risk factors for the development of atherosclerosis in primates.

Impact of apolipoprotein(a) phenotypes on long-term renal transplant survival

The long-term success of renal transplantation is limited because of chronic rejection (CR), which shows histologic parallels to atherosclerosis. Lipoprotein(a) [Lp(a)] is an independent risk factor for atherosclerosis, but its role in CR has not been investigated. Plasma levels of Lp(a) are determined mainly by the inherited isoform (phenotype) of apolipoprotein(a) [apo(a)] and show an inverse correlation with the molecular weight of apo(a). Apo(a) isoforms were identified in frozen sera of 327 patients who received a renal transplant during 1982 to 1992. Long-term graft survival in recipients with high molecular weight (HMW) or low molecular weight (LMW) apo(a) phenotypes were compared retrospectively. Mean (95% confidence interval) transplant survival was 12.8 yr (range, 11.9 to 13.6 yr) in patients with HMW and 11.9 yr (range, 10.8 to 13.1 yr) in patients with LMW apo(a) phenotypes (P = 0.2065). In patients who were 35 yr or younger at the time of transplantation, mean transplant survival was more than 3 yr longer in recipients with HMW apo(a) phenotypes compared with those with LMW apo(a) phenotypes (13.2 yr [range, 12.1 to 14.4 yr] versus 9.9 yr (range, 8.5 to 11.5 yr); P = 0.0156). In a Cox's proportional hazards regression model, the presence of LMW phenotypes-but not gender, immunosuppression, or HLA mismatches-in young patients was associated with a statistically significant risk of CR (P = 0.0434). These retrospective data indicate that young renal transplant recipients with LMW apo(a) phenotypes have a significantly shorter long-term graft survival, regardless of the number of HLA mismatches, gender, or immunosuppressive treatment.

Lipoprotein (a) and stroke

Strokes are one of the most common causes of mortality and long term severe disability. There is evidence that lipoprotein (a) (Lp(a)) is a predictor of many forms of vascular disease, including premature coronary artery disease. Several studies have also evaluated the association between Lp(a) and ischaemic (thrombotic) stroke. Several cross sectional (and a few prospective) studies provide contradictory findings regarding Lp(a) as a predictor of ischaemic stroke. Several factors might contribute to the existing confusion--for example, small sample sizes, different ethnic groups, the influence of oestrogens in women participating in the studies, plasma storage before Lp(a) determination, statistical errors, and selection bias. This review focuses on the Lp(a) related mechanisms that might contribute to the pathogenesis of ischaemic stroke. The association between Lp(a) and other cardiovascular risk factors is discussed. Therapeutic interventions that can lower the circulating concentrations of Lp(a) and thus possibly reduce the risk of stroke are also considered.

Are lipid-dependent indicators of cardiovascular risk affected by renal transplantation?

Hyperlipoproteinemia has been reported to frequently occur in kidney transplanted patients, thus possibly explaining, at least in part, the increased incidence of cardiovascular disease in this population. To evaluate the impact of renal transplantation (Tx), and related immunosuppressive therapy, on plasma lipoprotein and Lp(a) profile, we selected a cohort of kidney transplanted patients (36 M/14 F; age 33.8 + 12.0 yr, range 13-62) lacking significant causes of hyperlipidemia. All patients received a triple immunosuppressive regimen and showed a stable renal function after Tx (plasma creatinine: 1.36 +/- 0.35 mg/dL). One year after Tx, we found a significant increase of total cholesterol (TC), LDL, HDL, ApoB and ApoA-I (p < 0.005), while plasma triglyceride levels remained unmodified. Lp(a) plasma levels after Tx were within the normal range and displayed a significant inverse relationship with apo(a) size. Noteworthy, LDL/HDL ratio and ApoB/ ApoA-I ratio in kidney transplanted patients were almost superimposable with those of normal controls. Specifically, LDL/HDL ratio significantly decreased in 64% of patients after Tx, due to a prevalent increase of HDL, and was associated with a moderate amelioration of plasma TG. In a multiple linear regression model, post-Tx HDL level was significantly related to recipient's age, gender, BMI and cyclosporine (CyA) trough levels (Adj-R2 = 0.35, p = 0.0002), with gender and CyA trough levels being the better predictors of HDL. In conclusion, immunosuppressive regimens, in themselves, do not appear to significantly increase the atherogenic risk related to lipoproteins. Rather, other factors can affect the lipoprotein profile and its vascular effects in renal transplant recipients.

Genetic architecture and evolution of the lipoprotein(a) trait

Apolipoprotein(a) is coded by one of the most polymorphic genes known in humans. In white and Asian populations variation in this gene is the major determinant of the plasma concentrations of the atherogenic lipoprotein(a) which varies enormously between individuals and considerably across populations. Recent studies have shown that the genetic architecture of the quantitative Lp(a) trait differs among major human groups. In Africans there is evidence for a transacting factor. Three types of variation have been identified in the apo(a) gene: a size polymorphism in the coding region (K IV type 2 repeats), a pentanucleotide repeat polymorphism in the promoter (5'PNRP) and sequence variation in coding and non-coding regions of the gene including a C/T polymorphism at +93 which creates an additional ATG start codon but also affects transcription. The causal +93 C/T effect is masked by linkage disequilibrium in white populations. Analysis of apo(a) K IV 6-10 exons revealed the existence of population-specific spectra of polymorphism in this domain. However further sequence variation which may provide clues for the understanding of the regulation of apo(a) concentrations still needs to be identified. DNA sequencing and phylogenetic analysis have demonstrated that two types of apo(a) exist, in phylogenetically distant mammalian lineages a K IV derived primate form and a K III-derived hedgehog form which are products of convergent evolution.

Predictors of plasma lipoprotein(a) concentration in the West of Scotland Coronary Prevention Study cohort

An elevated plasma lipoprotein(a) (Lp(a)) concentration is an independent risk factor for coronary heart disease (CHD). Plasma Lp(a) levels are believed to be predominantly controlled by the APO(a) gene, which encodes the apo(a) glycoprotein moiety of the Lp(a) particle. However, other parameters in the lipoprotein profile as well as co-existing disease states or personal traits have been proposed as co-varieties. In order to examine these potential controlling factors in greater detail than previously possible, 1760 unrelated Caucasian subjects were studied, from which were identified 907 with a single expressing APO(a) allele. This strategy was followed to obviate the difficulty in dealing with the co-expression of different apo(a) isoforms and the resulting compound plasma Lp(a) level. After cube-root transformation of the plasma Lp(a) levels to normalise their distribution, a series of correlates were computed. There was no good correlation between Lp(a) concentration and any other measured lipid or lipoprotein in the lipid profile or with any other variable examined, with the important exception of the length of the expressed apo(a) isoform (r = -0.491, P = 0.0001). We conclude that in this population the plasma Lp(a) concentration is not predicted by the plasma lipid profile, alcohol intake, or smoking status but is predicted, albeit incompletely, by the length polymorphism of the APO(a) gene.