Sib-pair analysis detects elevated Lp(a) levels and large variation of Lp(a) concentration in subjects with familial defective ApoB

Lipoprotein(a) in hereditary hypercholesterolemia: Influence of the genetic cause, defective gene and type of mutation

BACKGROUND AND AIMS

Lipoprotein(a) [Lp(a)] concentration in heterozygous familial hypercholesterolemia (heFH) is not well established. Whether the genetic defect responsible for heFH plays a role in Lp(a) concentration is unknown. We aimed to compare Lp(a) in controls from a healthy population, in genetically diagnosed heFH and mutation-negative hypercholesterolemia subjects, and to assess the influence on Lp(a) of the genetic defect responsible for heFH.

METHODS

We conducted a cross-sectional study, performed in a lipid clinic in Spain. We studied adults with suspected heFH and a genetic study of FH genes (LDLR, APOB, APOE and PCSK9) and controls from de Aragon Workers' Health Study. HeFH patients from the Dyslipidemia Registry of the Spanish Atherosclerosis Society (SEA) were used as validation cohort.

RESULTS

Adjusted geometric means (95% confidence interval) of Lp(a) in controls (n = 1059), heFH (n = 500), and mutation-negative subjects (n = 860) were 14.9 mg/dL (13.6, 16.4), 21.9 mg/dL (18.1, 25.6) and 37.4 mg/dL (33.3, 42.1), p < 0.001 in all comparisons. Among heFH subjects, APOB-dependent FH showed the highest Lp(a), 36.5 mg/dL (22.0, 60.8), followed by LDLR-dependent FH, 21.7 mg/dL (17.9, 26.4). These differences were also observed in heFH from the SEA cohort. The number of plasminogen-like kringle IV type-2 repeats of LPA, the hypercholesterolemia polygenic score or LDLc concentration did not explain these differences. In LDLR-dependent FH, Lp(a) levels were not different depending on the affected protein domain.

CONCLUSIONS

Lp(a) is elevated in mutation-negative subjects and in heFH. The concentration of Lp(a) in heFH varies in relation to the responsible gene. Higher Lp(a) in heFH is not explained by their higher LDLc.

Lipoprotein metabolism in familial hypercholesterolemia

Familial hypercholesterolemia (FH) is one of the most common genetic disorders in humans. It is an extremely atherogenic metabolic disorder characterized by lifelong elevations of circulating LDL-C levels often leading to premature cardiovascular events. In this review, we discuss the clinical phenotypes of heterozygous and homozygous FH, the genetic variants in four genes (LDLR/APOB/PCSK9/LDLRAP1) underpinning the FH phenotype as well as the most recent in vitro experimental approaches used to investigate molecular defects affecting the LDL receptor pathway. In addition, we review perturbations in the metabolism of lipoproteins other than LDL in FH, with a major focus on lipoprotein (a). Finally, we discuss the mode of action and efficacy of many of the currently approved hypocholesterolemic agents used to treat patients with FH, with a special emphasis on the treatment of phenotypically more severe forms of FH.

Lipoprotein(a) concentration, genetic variants, apo(a) isoform size, and cellular cholesterol efflux in patients with elevated Lp(a) and coronary heart disease submitted or not to lipoprotein apheresis: An Italian case-control multicenter study on Lp(a)

BACKGROUND

Coronary artery disease (CAD) risk is greater with higher plasma lipoprotein(a)[Lp(a)] concentrations or smaller apoisoform size and putatively with increased cellular cholesterol loading capacity (CLC). The relationship between Lp(a) and CLC is not known. Information on Lp(a) polymorphisms in Italian patients is lacking.

OBJECTIVE

The objective of this study was to determine relationships between Lp(a) and CLC, the impact of lipoprotein apheresis (LA), and describe the genetic profile of Lp(a).

METHODS

We conducted a multicenter, observational study in Italian patients with hyperLp(a) and premature CAD with (n = 18)/without (n = 16) LA in which blood samples were analyzed for Lp(a) parameter and CLC. Genetic profiling of LPA was conducted in patient receiving LA.

RESULTS

Mean macrophage CLC of the pre-LA serum was significantly higher than that of normolipidemic controls (19.7 ± 0.9 μg/mg vs 16.01 ± 0.98 μg/mg of protein, respectively). After LA, serum macrophage CLC was markedly lower relative to preapheresis (16.1 ± 0.8 μg/mg protein; P = .003) and comparable with CLC of the normolipidemic serum. LA did not significantly affect average apo(a) isoform size distribution. No anthropometric or lipid parameters studied were related to serum CLC, but there was a relationship between CLC and the Lp(a) plasma concentration (P = .035). DNA analysis revealed a range of common genetic variants. Two rare, new variants were identified: LPA exon 21, c.3269C>G, p.Pro1090Arg, and rs41259144 p.Arg990Gln, c.2969G>A CONCLUSIONS: LA reduces serum Lp(a) and also reduces macrophage CLC. Novel genetic variants of the LPA gene were identified, and geographic variations were noted. The complexity of these polymorphisms means that genetic assessment is not a predictor of CAD risk in hyperLp(a).

Plasma lipoprotein(a) levels in patients with homozygous autosomal dominant hypercholesterolemia

BACKGROUND

Patients with autosomal dominant hypercholesterolemia (ADH), caused by mutations in either low-density lipoprotein receptor (LDLR), apolipoprotein B (APOB), or proprotein convertase subtilisin-kexin type 9 (PCSK9) are characterized by high low-density lipoprotein cholesterol levels and in some studies also high lipoprotein(a) (Lp(a)) levels were observed. The question remains whether this effect on Lp(a) levels is gene-dose-dependent in individuals with either 0, 1, or 2 LDLR or APOB mutations.

OBJECTIVE

We set out to study whether Lp(a) levels differ among bi-allelic ADH mutation carriers, and their relatives, in the Netherlands.

METHODS

Bi-allelic ADH mutation carriers were identified in the database of the national referral laboratory for DNA diagnostics of inherited dyslipidemias. Family members were invited by the index cases to participate. Clinical parameters and Lp(a) levels were measured in bi-allelic ADH mutation carriers and their heterozygous and unaffected relatives.

RESULTS

We included a total of 119 individuals; 34 bi-allelic ADH mutation carriers (20 homozygous/compound heterozygous LDLR mutation carriers (HoFH), 2 homozygous APOB mutation carriers (HoFDB), and 12 double heterozygotes for an LDLR and APOB mutation), 63 mono-allelic ADH mutation carriers (50 heterozygous LDLR [HeFH], 13 heterozygous APOB [HeFDB] mutation carriers), and 22 unaffected family members. Median Lp(a) levels in unaffected relatives, HeFH, and HoFH patients were 19.9 (11.1-41.5), 24.4 (5.9-70.6), and 47.3 (14.9-111.7) mg/dL, respectively (P = .150 for gene-dose dependency). Median Lp(a) levels in HeFDB and HoFDB patients were 50.3 (18.7-120.9) and 205.5 (no interquartile range calculated), respectively (P = .012 for gene-dose-dependency). Double heterozygous carriers of LDLR and APOB mutations had median Lp(a) levels of 27.0 (23.5-45.0), which did not significantly differ from HoFH and HoFDB patients (P = .730 and .340, respectively).

CONCLUSION

A (trend toward) increased plasma Lp(a) levels in homozygous ADH patients compared with both heterozygous ADH and unaffected relatives was observed. Whether increased Lp(a) levels in homozygous ADH patients add to the increased cardiovascular disease risk and whether this risk can be reduced by therapies that lower both low-density lipoprotein cholesterol and Lp(a) levels remains to be elucidated.

Structure, function, and genetics of lipoprotein (a)

Lipoprotein (a) [Lp(a)] has attracted the interest of researchers and physicians due to its intriguing properties, including an intragenic multiallelic copy number variation in the LPA gene and the strong association with coronary heart disease (CHD). This review summarizes present knowledge of the structure, function, and genetics of Lp(a) with emphasis on the molecular and population genetics of the Lp(a)/LPA trait, as well as aspects of genetic epidemiology. It highlights the role of genetics in establishing Lp(a) as a risk factor for CHD, but also discusses uncertainties, controversies, and lack of knowledge on several aspects of the genetic Lp(a) trait, not least its function.

Lipoprotein(a): resurrected by genetics

Plasma lipoprotein(a) [Lp(a)] is a quantitative genetic trait with a very broad and skewed distribution, which is largely controlled by genetic variants at the LPA locus on chromosome 6q27. Based on genetic evidence provided by studies conducted over the last two decades, Lp(a) is currently considered to be the strongest genetic risk factor for coronary heart disease (CHD). The copy number variation of kringle IV in the LPA gene has been strongly associated with both Lp(a) levels in plasma and risk of CHD, thereby fulfilling the main criterion for causality in a Mendelian randomization approach. Alleles with a low kringle IV copy number that together have a population frequency of 25-35% are associated with a doubling of the relative risk for outcomes, which is exceptional in the field of complex genetic phenotypes. The recently identified binding of oxidized phospholipids to Lp(a) is considered as one of the possible mechanisms that may explain the pathogenicity of Lp(a). Drugs that have been shown to lower Lp(a) have pleiotropic effects on other CHD risk factors, and an improvement of cardiovascular endpoints is up to now lacking. However, it has been established in a proof of principle study that lowering of very high Lp(a) by apheresis in high-risk patients with already maximally reduced low-density lipoprotein cholesterol levels can dramatically reduce major coronary events.

An ELISA procedure for human Lp(a) quantitation using monoclonal antibodies

The present study describes a monoclonal antibody-based enzyme immunoassay (ELISA) for the quantitation of lipoprotein(a), Lp(a), in human plasma. Two antibodies to Lp(a), 2F4E7 and 8G12G7, were produced and characterized as specific and high affinity antibodies against Lp(a). A reference control serum was utilized to prepare the standard curve in a Lp(a) concentration range from 0.015 to 0.5 ug/ml. A biotinylated monoclonal antibody against apoB-LDL was used as the second antibody. The comparison of the standardized ELISA using mAb 2F4E7 with an ELISA using a characterized mAb against Lp(a) (clone KO9) as capture antibody showed that the Lp(a) concentration of two standard sera was similar with both assays. Furthermore, when compared with an electroimmunoassay kit, similar Lp(a) concentrations for the standard were also obtained.

Lipoprotein(a): intrigues and insights

Lipoprotein(a) is an atherogenic, cholesterol ester-rich lipoprotein of unknown physiological function. The unusual species distribution of lipoprotein(a) and the extreme polymorphic nature of its distinguishing apolipoprotein component, apolipoprotein(a), have provided unique challenges for the investigation of its biochemistry, genetics, metabolism and atherogenicity. Some fundamental questions regarding this enigmatic lipoprotein have escaped elucidation, as will be highlighted in this review.

Dietary and genetic interactions in the regulation of plasma lipoprotein(a)

Lipoprotein(a) is a plasma particle which is considered to be a risk factor for the development of coronary heart disease. Plasma levels of lipoprotein(a) are affected by different types of dietary fat and steroid hormones. Two regions upstream of the apolipoprotein(a) promoter have been isolated which could be the site of regulation of apolipoprotein(a) gene transcription.