The Identification and Significance of a Thr→Pro Polymorphism in Kringle IV Type 8 of Apolipoprotein(a)

The effect of LPA Thr3888Pro on lipoprotein(a) and coronary artery disease is modified by the LPA KIV-2 variant 4925G>A

BACKGROUND AND AIMS

High lipoprotein(a) [Lp(a)] concentrations are associated with increased coronary artery disease (CAD) risk. Lp(a) is regulated mainly genetically by the LPA gene but involved genetic variants have not been fully elucidated. Improved understanding of the entanglements of genetic Lp(a) regulation may enhance genetic prediction of Lp(a) and CAD risk. We investigated an interaction between the well-known LPA missense SNP rs41272110 (known as Thr3888Pro) and the frequent LPA splicing mutation KIV-2 4925G>A.

METHODS

Effects on Lp(a) concentrations were investigated by multiple quantile regression in the German Chronic Kidney Disease (GCKD) study, KORA-F3 and KORA-F4 (n_total = 10,405) as well as in the UK Biobank (UKB) 200k exome dataset (n = 173,878). The impact of the interaction on CAD risk was assessed by survival analysis in UKB.

RESULTS

We observed a significant SNP-SNP interaction in all studies (p = 1.26e-05 to 3.03e-04). In quantile regression analysis, rs41272110 as a predictor shows no impact on Lp(a) (β = -0.06 [-0.79; 0.68], p = 0.879), but in a joint model including both SNPs as predictors, rs41272110 is associated with markedly higher Lp(a) (β = +9.40 mg/dL [6.45; 12.34], p = 4.07e-10). Similarly, rs41272110 shows no effect on CAD in UKB (HR = 1.01 [0.97; 1.04], p = 0.731), while rs41272110 carriers not carrying 4925G>A show an increased CAD risk (HR = 1.10 [1.04; 1.16], p = 6.9e-04). This group corresponds to 4% of the population. Adjustment for apolipoprotein(a) isoforms further modified the effect estimates markedly.

CONCLUSIONS

This work emphasizes the complexity of the genetic regulation of Lp(a) and the importance to account for genetic subgroups in Lp(a) association studies and when interpreting genetic cardiovascular risk profiles.

Statin treatment increases lipoprotein(a) levels in subjects with low molecular weight apolipoprotein(a) phenotype

BACKGROUND AND AIMS

We aimed to evaluate the effect of statin treatment initiation on lipoprotein(a) [Lp(a)] levels in patients with dyslipidemia, and the interactions with the apolipoprotein(a) [apo(a)] phenotype, LPA single nucleotide polymorphisms (SNPs) and change in LDL cholesterol.

METHODS

The study population consisted of patients with dyslipidemia, predominantly familial hypercholesterolemia, who first initiated statin treatment (initiation group; n = 39) or were already on stable statin treatment for at least 4 months (control group; n = 42). Plasma Lp(a) levels were determined with a particle-enhanced immunoturbidimetric assay before and at least 2 months after start of statin treatment in individuals of the initiation group, and at two time points with an interval of at least 2 months in the control group. High and low molecular weight (HMW and LMW, respectively) apo(a) phenotype was determined by immunoblotting, and the common LPA SNPs rs10455872, rs3798220 and rs41272110 by Taqman assay.

RESULTS

Plasma Lp(a) levels did not increase significantly in the initiation group (median 20.5 (IQR 10.9-80.7) to 23.3 (10.8-71.8) mg/dL; p = 0.09) nor in the control group (30.9 (IQR 9.2-147.0) to 31.7 (IQR 10.9-164.0) mg/dL; p = 0.61). In patients with the LMW apo(a) phenotype, Lp(a) levels increased significantly from 66.4 (IQR 23.5-148.3) to 97.4 (IQR 24.9-160.4) mg/dL (p = 0.026) in the initiation group, but not in the control group and not in patients characterized by the HMW apo(a) phenotype. Interactions with common LPA SNPs and change in LDL cholesterol were not significant.

CONCLUSIONS

Statins affect Lp(a) levels differently in patients with dyslipidemia depending on the apo(a) phenotype. Statins increase Lp(a) levels exclusively in patients with the LMW apo(a) phenotype.

A common nonsense mutation in the repetitive Kringle IV-2 domain of human apolipoprotein(a) results in a truncated protein and low plasma Lp(a)

LPA, the gene coding for apolipoprotein(a) [apo(a)], is the major determinant of lipoprotein(a) [Lp(a)] plasma levels, which are associated with risk for coronary heart disease (CHD) and stroke. It is not completely understood how variation in LPA relates to Lp(a) concentrations. One type of variation related to Lp(a) levels is the number of Kringle (K) IV-2 (g.61C>T; GenBank L14005.1) repeats in LPA, but sequence variation may also contribute. Human apo(a) contains from two to >40 nearly identical K IV-2 repeats of genomic size 5.5 kb, which makes it difficult to detect mutations. To elucidate the genetic variation of the apo(a) K IV-2 domain, we isolated a single "nonexpressing" apo(a) allele with 26 K IV-2 repeats, followed by PCR, cloning and sequencing of 96 clones, resulting in an average coverage of each K IV-2 repeat of approximately four-fold. The previously described K IV types 2A and 2B (K IV-2A and K IV-2B) were detected in 74% of the clones. In addition, a new type designated 2C (K IV-2C) was present. A nonsense mutation in the first exon of K IV-2 (g.61C>T) predicted to result in a truncated protein (p.R21X) was found in nine clones on a K IV-2A background. The presence of this mutation was confirmed by analysis of genomic DNA and was shown to represent the rare allele (frequency 0.02) of a SNP. Immunoblot analysis of apo(a) from plasma confirmed the presence of a truncated apo(a) isoform in the index individual and family members. Our data show that SNPs affecting Lp(a) plasma concentrations also exist in the apo(a) K IV-2 domain.

Race-specific association of lipoprotein(a) with vascular access interventions in hemodialysis patients: the CHOICE Study

BACKGROUND

Elevated serum levels of lipoprotein(a) [Lp(a)] and low molecular weight apolipoprotein(a) [apo(a)] isoforms are associated with atherothrombotic disease in the general population and in patients with kidney failure. Lp(a) may be more atherothrombotic in whites than in blacks. Data on the relation of Lp(a) and apo(a) isoform size to hemodialysis vascular access complications are limited.

METHODS

We analyzed the intervention-free survival of the first arteriovenous (AV) access among 215 white and 112 black incident hemodialysis patients participating in the CHOICE Study, a national multicenter prospective cohort study.

RESULTS

Median levels of Lp(a) protein were higher among blacks than whites (81.0 versus 37.5 nmol/L; P < 0.001) and inversely correlated with apo(a) isoform size (r = -0.57; P < 0.001). The incidence rate of access interventions was much higher in synthetic grafts (N = 193) than native fistulae (N = 134; 1.0 vs. 0.5 interventions per access-year; P < 0.001) and in patients with kidney failure primarily due to diabetes mellitus (N = 161) than others (N = 166; 0.9 vs. 0.6; P < 0.01), but did not differ by race. Blacks in the highest race-specific Lp(a) quartile (>145 nmol/L) had a significantly higher incidence rate than other blacks (1.4 vs. 0.7; P = 0.04), whereas no association was found in whites. The association in blacks remained after adjustment for access type and other characteristics (relative hazard = 1.68; 95% confidence interval: 0.98 to 2.86). No association was found with apo(a) isoform size in either race.

CONCLUSIONS

Elevated Lp(a) may be a risk factor for arteriovenous access complications among black hemodialysis patients. Future studies should explore this possibility and be adequately powered to allow race-specific analyses.

The association of serum lipoprotein(a) levels, apolipoprotein(a) size and (TTTTA)(n) polymorphism with coronary heart disease

BACKGROUND

The association between lipoprotein(a) levels, apolipoprotein(a) size and the (TTTTA)(n) polymorphism which is located in the 5' non-coding region of the apo(a) gene was studied in 263 patients with severe coronary heart disease and 97 healthy subjects.

METHODS

Lp(a) levels were measured by ELISA, apo(a) isoform size was determined by SDS-agarose gel electrophoresis, and analysis of the (TTTTA)(n) was carried out by PCR. For statistical calculation, both groups were divided into low (at least one apo(a) isoform with < or = 22 Kringle IV) and high (both isoforms with >22 KIV) apo(a) isoform sizes, and into low number (<10 in both alleles) and high number of (> or =10 at least one allele) TTTTA repeats.

RESULTS

Lp(a) levels were higher (P=0.007), apo(a) isoforms size < or =22 KIV and TTTTA repeats > or = 10 were more frequent (P=0.007 and 0.01) in cases than in controls. Lp(a) levels were found to be increased with low apo(a) weight in both groups (both P<0.0001). In multivariate logistic regression analysis, only the Lp(a) levels (P=0.005) and (TTTTA)(n) polymorphism (P=0.002) were found to be significantly associated with CHD.

CONCLUSION

Nevertheless, these results indicate that in CHD patients the (TTTTA)(n) polymorphism has an effect on Lp(a) levels which is independent of the apo(a) size.

Sequence polymorphisms in the apolipoprotein(a) gene and their association with lipoprotein(a) levels and myocardial infarction. The ECTIM Study

Lp(a) concentrations are largely determined by apo(a) isoform size, but several studies have shown that apo(a) isoforms could not entirely explain the increase of Lp(a) levels observed in patients with coronary heart disease (CHD). Since up to 90% of the variance in Lp(a) levels has been suggested to be attributable to the apo(a) locus, the hypothesis that polymorphisms of the apo(a) gene other than size could contribute to the increase of Lp(a) levels in CHD patients must be considered. This hypothesis was tested in the ECTIM Study comparing 594 patients with myocardial infarction and 682 control subjects in Northern Ireland and France. In addition to apo(a) phenotyping, five previously described polymorphisms of the apo(a) gene were genotyped: a (TTTTA)n repeat at position -1400 from the ATG, a G/A at -914, a C/T at -49, a G/A at -21 and a Met/Thr affecting amino acid 4168. As reported earlier [Parra HJ, Evans AE, Cambou JP, Amouyel P, Bingham A, McMaster D, Schaffer P, Douste-Blazy P, Luc G, Richard JL, Ducimetiere P, Fruchart JC, Cambien F. A case-control study of lipoprotein particles in two populations at contrasting risk for coronary heart disease. The ECTIM study. Arterioscler Thromb 1992; 12:701-707], mean Lp(a) levels were higher in cases than in controls (20.7 vs 14.6 mg/dl in Belfast, 17.2 vs 8.9 mg/dl in France, P < 0.001 for case-control and population differences). In the present study, mean apo(a) isoform size differed significantly between cases and controls (25.7 vs 26.6 kr in Belfast, 25.9 vs 27.4 kr in France, P < 0.001 for case-control and P = 0.13 for population difference). After adjustment for apo(a) isoforms, Lp(a) levels remained significantly higher in cases than in controls (difference, 4.6 mg/dl; P < 0.001). Genotype and allele frequencies did not differ significantly between cases and controls for any of the five polymorphisms studied. The five polymorphisms were in strong linkage disequilibrium and had a combined heterozygosity of 0.83. In multivariate regression analysis adjusted for apo(a) isoforms, only the (TTTTA)n polymorphism was significantly associated with Lp(a) levels; it explained 4.5% of Lp(a) variability in cases and 3.1% in controls. The Lp(a) case/control difference was not reduced after taking into account the (TTTTA)n effect. We conclude that the increase of Lp(a) levels observed in MI cases, and which was not directly attributable to apo(a) size variation, was not related to the five polymorphisms of the apo(a) gene considered.

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.

Atherothrombogenicity of lipoprotein(a): the debate

Although lipoprotein(a) (Lp[a]) has been recognized as an atherothrombogenic factor, the underlying mechanisms for this pathogenicity have not been clearly defined. Plasma levels have received most of the attention in this regard; however, discrepancies among population studies have surfaced. Particularly limited is the information on the fate of Lp(a) that enters the arterial wall, in terms of mechanisms of endothelial transport and interactions with cells and macromolecules of the extracellular matrix. A typical Lp(a) represents a low-density lipoprotein (LDL)-like particle having as a protein moiety apo B-100 linked by a single interchain disulfide bond to a unique multikringle glycoprotein, called apolipoprotein(a) (apo[a]). In vitro studies have shown that Lp(a) can be dissected into its constituents, LDL and apo(a). In turn, the latter can be cleaved by enzymes of the elastase and metalloproteinase families into fragments that exhibit a differential behavior in terms of binding to macromolecules of the extracellular matrix: fibrinogen, fibronectin, and proteoglycans. By immunochemical criteria, apo(a) predominantly localizes in areas of human arteries affected by the atherosclerotic process, where elastase and metalloproteinase enzymes operate and where apo(a) fragments are potentially generated. The accumulation of these fragments in the vessel wall is likely to depend on their affinity for the constituents of the extracellular matrix. Thus, factors that modulate inflammation and inflammation-mediated fragmentation of Lp(a)/apo(a) may play an important role in the cardiovascular pathogenicity of Lp(a). This pathogenicity may be attenuated by measures directed at preventing the activation of those vascular cells that secrete enzymes with a proteolytic potential for Lp(a)/apo(a), namely, leukocytes, macrophages, and T cells.