An isoprecipitation reaction distinguishing human serum-protein types

Lp(a) and the Risk for Cardiovascular Disease: Focus on the Lp(a) Paradox in Diabetes Mellitus

Lipoprotein(a) (Lp(a)) is one of the strongest causal risk factors of atherosclerotic disease. It is rich in cholesteryl ester and composed of apolipoprotein B and apo(a). Plasma Lp(a) levels are determined by apo(a) transcriptional activity driven by a direct repeat (DR) response element in the apo(a) promoter under the control of (HNF)4α Farnesoid-X receptor (FXR) ligands play a key role in the downregulation of APOA expression. In vitro studies on the catabolism of Lp(a) have revealed that Lp(a) binds to several specific lipoprotein receptors; however, their in vivo role remains elusive. There are more than 1000 publications on the role of diabetes mellitus (DM) in Lp(a) metabolism; however, the data is often inconsistent and confusing. In patients suffering from Type-I diabetes mellitus (T1DM), provided they are metabolically well-controlled, Lp(a) plasma concentrations are directly comparable to healthy individuals. In contrast, there exists a paradox in T2DM patients, as many of these patients have reduced Lp(a) levels; however, they are still at an increased cardiovascular risk. The Lp(a) lowering mechanism observed in T2DM patients is most probably caused by mutations in the mature-onset diabetes of the young (MODY) gene and possibly other polymorphisms in key transcription factors of the apolipoprotein (a) gene (APOA).

Lp(a) and the Risk for Cardiovascular Disease: Focus on the Lp(a) Paradox in Diabetes Mellitus

Lipoprotein(a) (Lp(a)) is one of the strongest causal risk factors of atherosclerotic disease. It is rich in cholesteryl ester and composed of apolipoprotein B and apo(a). Plasma Lp(a) levels are determined by apo(a) transcriptional activity driven by a direct repeat (DR) response element in the apo(a) promoter under the control of (HNF)4α Farnesoid-X receptor (FXR) ligands play a key role in the downregulation of APOA expression. In vitro studies on the catabolism of Lp(a) have revealed that Lp(a) binds to several specific lipoprotein receptors; however, their in vivo role remains elusive. There are more than 1000 publications on the role of diabetes mellitus (DM) in Lp(a) metabolism; however, the data is often inconsistent and confusing. In patients suffering from Type-I diabetes mellitus (T1DM), provided they are metabolically well-controlled, Lp(a) plasma concentrations are directly comparable to healthy individuals. In contrast, there exists a paradox in T2DM patients, as many of these patients have reduced Lp(a) levels; however, they are still at an increased cardiovascular risk. The Lp(a) lowering mechanism observed in T2DM patients is most probably caused by mutations in the mature-onset diabetes of the young (MODY) gene and possibly other polymorphisms in key transcription factors of the apolipoprotein (a) gene (APOA).

Lp(a) and the Risk for Cardiovascular Disease: Focus on the Lp(a) Paradox in Diabetes Mellitus

Lipoprotein(a) (Lp(a)) is one of the strongest causal risk factors of atherosclerotic disease. It is rich in cholesteryl ester and composed of apolipoprotein B and apo(a). Plasma Lp(a) levels are determined by apo(a) transcriptional activity driven by a direct repeat (DR) response element in the apo(a) promoter under the control of (HNF)4α Farnesoid-X receptor (FXR) ligands play a key role in the downregulation of APOA expression. In vitro studies on the catabolism of Lp(a) have revealed that Lp(a) binds to several specific lipoprotein receptors; however, their in vivo role remains elusive. There are more than 1000 publications on the role of diabetes mellitus (DM) in Lp(a) metabolism; however, the data is often inconsistent and confusing. In patients suffering from Type-I diabetes mellitus (T1DM), provided they are metabolically well-controlled, Lp(a) plasma concentrations are directly comparable to healthy individuals. In contrast, there exists a paradox in T2DM patients, as many of these patients have reduced Lp(a) levels; however, they are still at an increased cardiovascular risk. The Lp(a) lowering mechanism observed in T2DM patients is most probably caused by mutations in the mature-onset diabetes of the young (MODY) gene and possibly other polymorphisms in key transcription factors of the apolipoprotein (a) gene (APOA).

Lipoprotein (a): a historical appraisal

Initially, lipoprotein (a) [Lp(a)] was believed to be a genetic variant of lipoprotein (Lp)-B. Because its lipid moiety is almost identical to LDL, Lp(a) has been deliberately considered to be highly atherogenic. Lp(a) was detected in 1963 by Kare Berg, and individuals who were positive for this factor were called Lpa⁺ Lpa⁺ individuals were found more frequently in patients with coronary heart disease than in controls. After the introduction of quantitative methods for monitoring of Lp(a), it became apparent that Lp(a), in fact, is present in all individuals, yet to a greatly variable extent. The genetics of Lp(a) had been a mystery for a long time until Gerd Utermann discovered that apo(a) is expressed by a variety of alleles, giving rise to a unique size heterogeneity. This size heterogeneity, as well as countless mutations, is responsible for the great variability in plasma Lp(a) concentrations. Initially, we proposed to evaluate the risk of myocardial infarction at a cut-off for Lp(a) of 30-50 mg/dl, a value that still is adopted in numerous epidemiological studies. Due to new therapies that lower Lp(a) levels, there is renewed interest and still rising research activity in Lp(a). Despite all these activities, numerous gaps exist in our knowledge, especially as far as the function and metabolism of this fascinating Lp are concerned.

A historical perspective on the discovery and elucidation of the hepatitis B virus

The discovery in 1965 of the "Australia antigen," subsequently identified as the hepatitis B virus surface antigen (HBsAg), was such a watershed event in virology that it is often thought to mark the beginning of hepatitis research, but it is more accurately seen as a critical breakthrough in a long effort to understand the pathogenesis of infectious hepatitis. A century earlier, Virchow provided an authoritative explanation of "catarrhal jaundice," which did not consider an infectious etiology, but the transmission of jaundice by human serum was clearly identified in two outbreaks in 1885, and the distinction between "infectious" and "serum" hepatitis was recognized by the early 1920s. The inability to culture a virus or reproduce either syndrome in laboratory animals led to numerous studies in human volunteers; by the end of World War II, it was known that the diseases were caused by different filterable agents, and the terms "hepatitis A" and "B" were introduced in 1947 (though some long-incubation cases then designated B must in retrospect have been hepatitis C). The development of a number of liver function tests during the 1950s led to the recognition of anicteric infections and the existence of chronic carriers, but little more could be done until an infectious agent had been identified. Once Blumberg and colleagues had found a specific viral marker, the vast amount of accumulated epidemiologic and clinical data, together with huge numbers of stored serum samples, enabled rapid progress in understanding hepatitis B, and revealed the existence of a vast population of chronically infected people in Asia, Oceania and Africa. In this article, we place the identification of the Australia antigen within the historical context of research on viral hepatitis. Following a chronological review from 1865 to 1965, we summarize how the discovery led to improved safety of blood transfusion, the development of a highly effective vaccine and the eventual identification of the hepatitis C, D and E viruses. This article forms part of a symposium in Antiviral Research on "An unfinished story: from the discovery of the Australia antigen to the development of new curative therapies for chronic hepatitis B."

Immunogenetic polymorphism of lipoproteins in swine. 1. Four additional serum beta-lipoprotein allotypes (Lpp2, Lpp4, Lpp5 and Lpp15) in the Lpp system

Four additional swine serum lipoprotein allotypes are described. Specific anti-allotype reagents were obtained from alloimmune precipitating sera produced in lipoprotein-defined-type recipients immunized with normal sera and subsequently with lipoprotein fractions. Identification studies indicate that the four serologically defined low-density lipoprotein (LDL) variants, designated Lpp2, Lpp4, Lpp5 and Lpp15, are members of a previously described Lpp system. The individual specificities, Lpp2, Lpp4 and Lpp5, are determined by three co-dominant autosomal genes, Lpp2, Lpp4 and Lpp5, respectively, whereas the common specificity, Lpp15, is controlled by a complex of genetic information of the Lpp2 and Lpp4 genes, and by the two previously described alleles, Lpp1 and Lpp3; Lpp15 occurs on the same molecule with respective individual specificity. The Lpp5 and Lpp15 antigens behave as a pair of alternative allotypic specificities. The double immunodiffusion test in agar was employed to demonstrate independent phenotypic expression of each allelic gene in the Lpp heterozygous animals, for the analysis of the immune sera, and for lipoprotein testing of 3305 sera. Marked differences in gene frequencies were found between the swine breeds tested. As a result of characteristic frequencies, only nine of 15 possible Lpp genotypes were found in the breeding herds tested; the remaining six genotypes were obtained from testcross matings.

Monogenic Hypocholesterolaemic Lipid Disorders and Apolipoprotein B Metabolism

The study of apolipoprotein (apo) B metabolism is central to our understanding of human lipoprotein metabolism. Moreover, the assembly and secretion of apoB-containing lipoproteins is a complex process. Increased plasma concentrations of apoB-containing lipoproteins are an important risk factor for the development of atherosclerotic coronary heart disease. In contrast, decreased levels of, but not the absence of, these apoB-containing lipoproteins is associated with resistance to atherosclerosis and potential long life. The study of inherited monogenic dyslipidaemias has been an effective means to elucidate key metabolic steps and biologically relevant mechanisms. Naturally occurring gene mutations in affected families have been useful in identifying important domains of apoB and microsomal triglyceride transfer protein (MTP) governing the metabolism of apoB-containing lipoproteins. Truncation-causing mutations in the APOB gene cause familial hypobetalipoproteinaemia, whereas mutations in MTP result in abetalipoproteinaemia; both rare conditions are characterised by marked hypocholesterolaemia. The purpose of this review is to examine the role of apoB in lipoprotein metabolism and to explore the key biochemical, clinical, metabolic and genetic features of the monogenic hypocholesterolaemic lipid disorders affecting apoB metabolism.

Screening of the 3' two-thirds of the coding area of the apo B gene in Finnish hypercholesterolemic patients report of six new genetic variants

Hypercholesterolemia clustering in families not explained by either low density lipoprotein (LDL)-receptor mutations producing familial hypercholesterolemia (FH), or the apolipoprotein B (apo B) Arg3500-->Gln mutation with familial defective apo B (FDB), is common in the Finnish population. In search of previously unknown apo B mutations, we screened exons 26 to 29 of the apo B gene in 68 Finnish severely hypercholesterolemic (> or = 8 mmol/l) non-FH, non-FDB patients, using a single-strand conformation polymorphism analysis based screening method. Four rare and two polymorphic previously unreported DNA variations were detected. The rare variants were a three-nucleotide deletion, with the deletion of Asp2186, an A11961-->G change leading to a Thr3918-->Ala change, a T12922-->G change causing a Val4238-->Ala substitution, and a neutral T12935-->C change leading to a new RsaI cutting site. The polymorphic G12937-->C and G13569-->A changes leading to Arg4243-->Thr and Ala4454-->Thr substitutions, respectively, had minor allele frequencies of 0.03 and 0.02. None of these variants seemed to explain the hyperlipidemia in these patients. A major Finnish mutation causing severe hypercholesterolemia is unlikely to exist in the 3' two-thirds of the coding area of the apo B gene.

Lp(a) lipoprotein: an overview

The Lp(a) lipoprotein, a distinct class of serum lipoproteins, was detected in 1962. It consists of an LDL particle to which a long polypeptide chain is attached by a disulfide bridge. The level of Lp(a) lipoprotein is genetically determined. Single locus control was suggested already in the very first report, and this has been conclusively confirmed by the demonstration of absolute genetic linkage to the plasminogen gene, from which the LPA gene is likely to have evolved. The detection in 1974 of an association between Lp(a) lipoprotein and coronary heart disease has been confirmed in numerous studies. The Lp(a) lipoprotein may have atherogenic as well as thrombogenic properties and thus form the bridge between atherogenesis and thrombogenesis. Genes determining a moderate level of Lp(a) lipoprotein may be longevity genes, and it seems possible that Lp(a) lipoprotein, because of its affinity to vessel walls, may also influence placental function. Lp(a) lipoprotein measurements should be included in the diagnostic work-up of people with premature coronary heart disease or with such disease in close relatives.

A postulated phylogenetic tree for the human apolipoprotein B gene: unpredicted haplotypes are associated with elevated apo B levels

Using published data on seven polymorphic sites in the human apolipoprotein B (apo B) gene, it is possible to postulate a model phylogenetic tree for this gene, covering the time since the divergence of human beings from other primates. This simple model assumes no obligatory recombination events or multiple occurrences of the same mutation. This model was tested in two samples of Swedish individuals consisting of 143 young, myocardial infarction patients and 90 healthy, age-matched, control individuals. All the haplotypes postulated in the simple model were observed unequivocally. However, in addition, three unpredicted haplotypes were unambiguously observed and a further nine, much rarer haplotypes were deduced to occur in these samples. The frequencies of the haplotypes postulated in the model do not differ between the patient and control samples, however most of the unpredicted haplotypes occur more frequently in the patient group than in the controls. Two of these unpredicted haplotypes, defined by the combination of the Antigen group (a) epitope and the presence of the XbaI cutting site, were associated with raised serum apo B levels in the control group and significantly elevated levels in the patient group. We propose that these observations explain in part the consistent association reported between the XbaI polymorphic site in the apo B gene and levels of plasma lipids.

DNA polymorphism studies. Approaches to elucidating multifactorial ischaemic heart disease: the apo B gene as an example

It is well established that elevated plasma levels of low-density lipoprotein (LDL) particles are a risk factor for ischaemic heart disease with the distribution in LDL levels seen in the general population being the result of interaction between environmental factors, such as dietary fat intake, and genetic variation that is present in different individuals. One of the candidate genes where such variation is likely to occur, is the gene coding for apolipoprotein B (apo B). Many studies have reported an association between a common polymorphism of the apo B gene, detected using the restriction enzyme XbaI, and differences in plasma lipid levels, explaining 3-5% of the variance in LDL-cholesterol levels in samples representative of the healthy population. It has been proposed that the mechanism of this association is due to functional amino acid changes within the apo B protein, that affect LDL catabolism by altering binding affinity to the LDL-receptor. Several amino acid substitutions in the apo B gene have now been characterized, and these form the basis of the different epitopes that create the Ag marker system. Previous studies have reported that the Ag(x) epitope is associated with lower plasma lipid levels, and until recently the molecular basis for this association has been unclear. We have determined that the Ag(x) epitope is associated with both a Pro-Leu2712, and Asn-Ser4311 substitution, with the Leu-Ser allele being associated with significantly lower levels of plasma lipids in a sample of healthy individuals from Sweden.(ABSTRACT TRUNCATED AT 250 WORDS)

Apolipoprotein B polymorphism and altered apolipoprotein B concentrations in Congolese blacks

The immunoreactivity of apolipoprotein B (apo B) in plasma obtained from 238 unrelated black African male subjects from the People's Republic of Congo was analysed by non-competitive Enzyme Linked-Immunosorbent Assay (ELISA) with monoclonal BIP 45 anti-LDL antibody. The polymorphism detected by BIP 45 monoclonal antibody is identical to the Ag(c,g) polymorphism. Antibody BIP 45 distinguishes three apo B allotypes (immunophenotypes) encoded by the two allelic genes apo B Ag(c) and apo B Ag(g). Because of co-dominant transmission, genotypes may be inferred from allotypes, and it has been shown that BIP 45 binds strongly to the Ag(c) factor and only weakly to the allelic Ag(g) factor. Analysis of the Congolese plasma samples indicated that 67.65% of them bound BIP 45 with low affinity (Ag(c-,g+) genotype), 28.15% with intermediate affinity (Ag(c+,g+) genotype) and 4.20% with high affinity (Ag(c+,g-) genotype). According to the Hardy-Weinberg equilibrium, this corresponds to gene frequencies of 0.817 and 0.183 for the type Ag(g)/Ag(c) alleles, respectively. After adjustment for age and body-mass index, it was found that the Ag(c) allele decreases the apo B level by 9.62 mg/dl and that the Ag(g) allele increases apo B by 0.43 mg/dl. Therefore, as much as 4.30% of the genetic variance for apo B level could be accounted for by the Ag(c,g) gene locus.

Identification of the base substitution responsible for the Ag(x/y) polymorphism of apolipoprotein B-100

The identification of the base substitution responsible for Ag(x/y) completes the description of the antigen group polymorphisms associated with the apolipoprotein B polypeptide. Surprisingly, all five antigen group polymorphisms alter restriction endonuclease cleavage sites and have associated restriction fragment length polymorphisms, thereby providing a convenient alternative for antigen group phenotyping.

Relationships between DNA and protein polymorphisms of apolipoprotein B

The associations between four restriction fragment length polymorphisms (RFLPs) of the gene for human apolipoprotein B (apo B) and five antigen group (Ag) protein-polymorphisms of apo B have been investigated in 24 unrelated Finnish individuals. In this sample a complete correlation exists between the EcoRI RFLP and the Ag(t/z) polymorphism. There is strong association between the alleles of the XbaI RFLP and Ag(c/g) and a weaker one of the same XbaI site with Ag(x/y). Linkage disequilibrium is observed between the PvuII RFLP and the Ag(a1/d) polymorphism. These associations confirm that the Ag variants are true protein sequence polymorphisms of apo B.

Association between epitopes detected by monoclonal antibody BIP-45 and the XbaI polymorphism of apolipoprotein B

An epitope of Apolipoprotein B (ApoB), recognised by a monoclonal antibody BIP-45, is associated with the development of ischaemic heart disease (Duriez et al. 1988). We have examined the genetic relationships between this epitope and three Restriction Fragment Length Polymorphisms (RFLPs) of the gene for ApoB detected with the enzymes EcoRI, PvuII and XbaI in a sample of 53 unrelated individuals from France. There is an association between binding affinity to BIP-45 and the XbaI RFLP; the 8.6 kb XbaI allele (absence of cutting site) being associated with low-affinity binding to BIP-45. In this sample of individuals there is no significant association between serum cholesterol levels and BIP-45 binding affinity, but there is a significant correlation between serum cholesterol levels and XbaI genotype, with individuals of the genotype X1X1 having the highest and those with the genotype X2X2 having the lowest levels of serum cholesterol. This suggests that variation at the ApoB locus may be involved independently in the determination of serum lipid levels and in the development of ischaemic heart disease.

Two monoclonal antibodies that discriminate between allelic variants of human low density lipoprotein

Human low density lipoprotein shows a genetic polymorphism, the so-called Ag-system. it consists of 5 pairs of allelic epitopes, x/y, al/d, c/g, t/z, and h/i, which are localized on apolipoprotein B. We have generated a large number of monoclonal antibodies against low density lipoprotein. Two of them, D2E1 and H11G3, recognize epitopes related to this genetic polymorphism. Direct ELISA and ELISA inhibition experiments with different low density lipoproteins of known phenotype showed that D2E1 is directed against the allelic epitope c and H11G3 against d. The two antibodies were used for the characterization of low density lipoprotein in sera from different blood donors and the results compared to those obtained by passive hemagglutination using human allotypic anti-sera. Sera from homo- or heterozygous donors (which display the relevant epitope) could be distinguished from the sera of homozygous donors (which lack the epitope) with the monoclonal antibodies described.

A monoclonal antibody (BIP 45) detects Ag(c,g) polymorphism of human apolipoprotein B

A monoclonal antibody (BIP 45) against human apolipoprotein B (apo B) was used to study the polymorphism of apo B in families and in unrelated subjects. BIP 45 bound to apo B-containing lipoprotein particles in one of three distinct patterns of immunoreactivity (strong, weak and intermediate). Family studies showed that these binding patterns result from co-dominant transmission of apo B allelic pairs which are temporarily designated allele BIP- and allele BIP+; allele BIP+ would code for the apo B BIP 45 epitope. Analysis of plasma samples from 244 unrelated men randomly chosen from the North French population indicated that 46.7% of them bound BIP 45 with low affinity (weak reactors), 44.7% with intermediate affinity (intermediate reactors) and 8.6% with high affinity (strong reactors). According to the Hardy-Weinberg equilibrium, this corresponds to gene frequencies of 0.690/0.310 for the type BIP-/BIP+ alleles. This corresponds to the gene frequencies of 0.695/0.305 at the Ag(g)/Ag(c) locus previously found in a Caucasian population. Furthermore, the investigation of Ag(c,g) and of monoclonal BIP 45 antibody immunoaffinity for 30 individual plasma samples showed that BIP 45 bound strongly to Ag(c) factor, whereas it bound weakly to the allelic Ag(g) factor. This monoclonal antibody will be useful for the detection of the two corresponding apo B species designated apo B (Ag(c) factor, BIP+) and apo B (Ag(g) factor, BIP-).

Detection of two apolipoprotein B species (apoBc and apoBg) by a monoclonal antibody

A monoclonal antibody (MB-19) was used to investigate the polymorphism of apolipoprotein B in a large East Finnish family and in unrelated subjects. Apolipoprotein B was shown to exhibit high, intermediate or low affinity binding to this antibody. Thus, MB-19 bound strongly to the Ag(c) epitope, an Ag antigenic domain previously characterized by human antisera, while it bound only weakly to the allelic epitope Ag(g). It proved useful for the detection of the two corresponding allelic apoB species designated apoBc (= high affinity binding) and apoBg (= low affinity binding), and for confirming their co-dominant transmission. Intermediate binding resulted from the presence of a mixture of both apoB populations in heterozygous subjects.

Immunogenetic polymorphism of apolipoprotein B in humans: studies with a monoclonal anti-Ag(c) antibody

In studies that use a monoclonal antibody (MB-19), apolipoprotein B exhibited one of three immunophenotypes: high, intermediate, or low affinity binding to this antibody. The distribution of these immunophenotypes (allotypes) in families was compatible with a codominant transmission of two alleles, one coding for the high and the other for the low affinity binding allotype. The high affinity binding allotype coincided with antigen Ag(c) and the low affinity binding allotype with Ag(g), two allelic antigenic determinants previously defined by human antisera. Preliminary studies did not reveal differences in plasma lipid levels in association with apolipoprotein B allotypes. Young Finnish men with low affinity binding apolipoprotein B had slightly lower plasma apolipoprotein B levels than those with the intermediate affinity binding phenotype.

Genetic linkage between the antigenic group (Ag) variation and the apolipoprotein B gene: assignment of the Ag locus

The antigenic group (Ag) system of homospecific human serum antigens of low density lipoprotein is detected by antiserum from multiply transfused patients. A complex series of common Ag alleles has been described, but the biochemical nature of this polymorphism is uncertain. Here we report that DNA polymorphisms at the human apolipoprotein B (apoB) locus are very closely linked to alleles of the Ag system. We also show a strong association between Ag(x) and a polymorphism detected with the restriction endonuclease Xba I. We conclude that the immunologically determined Ag system represents protein polymorphism of apoB rather than primary genetic differences in posttranslational processing or lipid binding. These studies therefore demonstrate that the Ag locus is located on the short arm of human chromosome 2 in the region p23-p24 to which the apoB gene has been assigned. Since the Ag(x) antigen is associated with altered plasma lipid levels, this determinant may indicate a functionally important domain of apoB.

Monoclonal antibody detects Ag polymorphism of apolipoprotein B

A monoclonal antibody (MB-19) was used to investigate the polymorphism of apolipoprotein B in a large family and in unrelated subjects. Apolipoprotein B was shown to exhibit high-, intermediate- or low-affinity binding to this antibody. Thus, MB-19 bound strongly to the Ag(c) epitope, An Ag antigenic domain previously characterized by human antisera, while it bound only weakly to the allelic epitope Ag(g). It proved therefore useful for the detection of the two corresponding allelic apoB species designated apoBc (high-affinity binding) and apoBg (low-affinity binding), and for confirming their co-dominant transmission. Intermediate binding resulted from the presence of a mixture of both apoB populations in heterozygous subjects.