An ApaLI restriction site polymorphism is associated with the MB19 polymorphism in apolipoprotein B

Polymorphism of the apolipoprotein B gene and association with plasma lipid and lipoprotein levels in the Mongolian Buryat

Allele frequencies at six RFLP sites (Ins/Del, ApaLI, AluI, XbaI, MspI, and EcoRI) of the apolipoprotein B gene (APOB) and the relationship of genotypes with plasma lipid and lipoprotein levels in the Mongolian Buryat were investigated. Common alleles at these sites in 110 Buryat subjects were I, G, A-, X-, M+, and E+; the frequencies of 0.809-0.991 differed strikingly from those of a few Asians and most Europeans. Five unambiguous haplotypes of all sites were revealed at 74%; haplotype IGA-X-M+E+ (000000) was the most frequent (67%), followed by IGA+X-M+E+ (001000) (19%). The frequency constitution differed significantly from the Chinese, Malaysians, and Caucasians but resembled the Indians. No APOB polymorphisms were associated with cholesterol levels (total, HDL and LDL). Significant associations of genotypes were shown with the triglyceride level only at the AluI and XbaI sites. The lipid level of A-A+ females or X-X+ males was higher than that of A-A- females or X-X- males, respectively.

Immuno-electron cryo-microscopy imaging reveals a looped topology of apoB at the surface of human LDL

A single copy of apoB is the sole protein component of human LDL. ApoB is crucial for LDL particle stabilization and is the ligand for LDL receptor, through which cholesterol is delivered to cells. Dysregulation of the pathways of LDL metabolism is well documented in the pathophysiology of atherosclerosis. However, an understanding of the structure of LDL and apoB underlying these biological processes remains limited. In this study, we derived a 22 Å-resolution three-dimensional (3D) density map of LDL using cryo-electron microscopy and image reconstruction, which showed a backbone of high-density regions that encircle the LDL particle. Additional high-density belts complemented this backbone high density to enclose the edge of the LDL particle. Image reconstructions of monoclonal antibody-labeled LDL located six epitopes in five putative domains of apoB in 3D. Epitopes in the LDL receptor binding domain were located on one side of the LDL particle, and epitopes in the N-terminal and C-terminal domains of apoB were in close proximity at the front side of the particle. Such image information revealed a looped topology of apoB on the LDL surface and demonstrated the active role of apoB in maintaining the shape of the LDL particle.

Identification of the proteoglycan binding site in apolipoprotein B48

An initial event in atherosclerosis is the retention of lipoproteins within the intima of the vessel wall. Previously we identified Site B (residues 3359-3369) in apolipoprotein (apo) B100 as the proteoglycan binding sequence in low density lipoproteins (LDLs) and showed that the atherogenicity of apoB-containing lipoproteins is linked to their affinity for artery wall proteoglycans. However, both apoB100- and apoB48-containing lipoproteins are equally atherogenic even though Site B lies in the carboxyl-terminal half of apoB100 and is absent in apoB48. If binding to proteoglycans is a key step in atherogenesis, apoB48-containing lipoproteins must bind to proteoglycans via other proteoglycan binding sites in the amino-terminal 48% of apoB. In vitro studies have identified five clusters of basic amino acids in delipidated apoB48 that bind negatively charged glycosaminoglycans. To determine which of these sites is functional on LDL particles, we analyzed the proteoglycan binding activity of recombinant human LDLs from transgenic mice or rat hepatoma cells. Substitution of neutral amino acids for the basic amino acids in Site B-Ib (residues 84-94) abolished the proteoglycan binding activity of recombinant apoB53. Carboxyl-truncated apoB80 bound biglycan with higher affinity than apoB100 and apoB48. ApoB80 in which Site B was mutated had the same affinity for proteoglycans as apoB48. These data support the hypothesis that the carboxyl terminus of apoB100 "masks" Site B-Ib, the amino-terminal proteoglycan binding site, and that this site is exposed in carboxyl-truncated forms of apoB. The presence of a proteoglycan binding site in the amino-terminal region of apoB may explain why apoB48- and apoB100-containing lipoproteins are equally atherogenic.

Influence of an asparagine to lysine mutation at amino acid 3516 of apolipoprotein B on low-density lipoprotein receptor binding

BACKGROUND

Three mutations in the apolipoprotein B (apoB) gene have previously been established as important causes of impaired receptor binding of LDL and, hence, Familial Defective Apolipoprotein B 100 (FDB). Previously, undescribed mutations were sought.

METHODS

Using denaturing gradient gel electrophoresis for mutation detection, DNA from 1852 new patients was examined.

RESULTS

A previously undiscovered mutation was found in codon 3516, located between known FDB mutations at codons 3500 and 3531. The new mutation introduces a positively charged amino acid-lysine-while other FDB mutations remove a positively charged residue, arginine. The phenotype was intriguing, LDL derived from N3516K heterozygotes allowed only poor growth of an LDL cholesterol-dependent cell line. ApoB-100-specific antibody MB47 bound to LDL from N3516K heterozygotes with increased affinity indicating a probable conformational change caused by the substitution. In contrast to these results, a competitive displacement assay in fibroblasts showed normal (or better) binding affinity to LDL receptors and using dynamic laser scattering no preferential accumulation of 3516K LDL particles in plasma was found.

CONCLUSION

Discovery of the mutation and characterisation of N3516K LDL reveals another naturally occurring apoB mutation that influences conformation of LDL apoB and its interaction with the LDL receptor.

A multilocus genotyping assay for candidate markers of cardiovascular disease risk

A number of chronic diseases, including cardiovascular disease, appear to have a multifactorial genetic risk component. Consequently, techniques are needed to facilitate evaluation of complex genetic risk factors in large cohorts. We have designed a prototype assay for genotyping a panel of 35 biallelic sites that represent variation within 15 genes from biochemical pathways implicated in the development and progression of cardiovascular disease. Each DNA sample is amplified using two multiplex polymerase chain reactions, and the alleles are genotyped simultaneously using an array of immobilized, sequence-specific oligonucleotide probes. This multilocus assay was applied to two types of cohorts. Population frequencies for the markers were estimated using 496 unrelated individuals from a family-based cohort, and the observed values were consistent with previous reports. Linkage disequilibrium between consecutive pairs of markers within the apoCIII, LPL, and ELAM genes was also estimated. A preliminary analysis of single and pairwise locus associations with severity of atherosclerosis was performed using a composite cohort of 142 individuals for whom quantitative angiography data were available; evaluation of the potentially interesting associations observed will require analysis of an independent and larger cohort. This assay format provides a research tool for studies of multilocus genetic risk factors in large cardiovascular disease cohorts, and for the subsequent development of diagnostic tests.

A multilocus genotyping assay for cardiovascular disease

In our efforts to develop diagnostic tests for complex multifactorial disorders, and to assist the research community in evaluating genetic markers for predisposition to cardiovascular disease, we have developed a prototype assay to genotype up to 35 variable sites among 15 genes. The candidate markers in this panel were selected from biological pathways likely to contribute to the development and progression of cardiovascular disease. Each sample is amplified in two multiplex polymerase chain reactions that are then hybridized to an array of immobilized oligonucleotide probes. The assay has been applied to a population-based cohort representing 238 families; allele frequencies observed among 455 unrelated parents from this cohort agree with available literature values. Data from a cohort of 142 lipid-clinic patients were used to explore locus associations with arterial occlusion, as measured by quantitative angiography. This prototype assay provides a research tool for studies to assess the association of multiple markers with disease, and for clinical studies to evaluate marker association with patient responsiveness to experimental therapies.

Genetic variation in the amino-terminal part of apolipoprotein B: studies in hyperlipidemic patients

Hypertriglyceridemia is a heterogeneous lipid disorder often running in families. Variation in the apolipoprotein B (apo B) gene has been associated with serum triglyceride levels. Recently, a role of the amino-terminal end of apo B in binding with lipoprotein lipase (LPL) has been suggested. We screened the 5' end of the apo B gene in 76 Finnish severely hypertriglyceridemic (> 6 mmol/l) patients, using a single-strand conformation polymorphism (SSCP) screening method. We detected a previously unreported polymorphic C2316-->A change, causing a Val703-->Ile substitution. The minor 703 Ile allele frequency was 0.04 in hypercholesterolemic and normolipidemic population samples. This allele was associated with lower serum triglyceride levels in a normolipidemic population sample. Analysis of two previously reported polymorphisms also located in the amino-terminal domain of apo B (Thr71-->Ile and Val591-->Ala) revealed elevating effects on serum apo B concentrations in hypertriglyceridemic individuals. The 591 Ala allele was associated with elevated apo B (P=0.011), and individuals with both minor alleles (apo B 591 Ala + and apo B 71 Ile +) had higher apo B levels compared to subjects homozygous for both common alleles (P=0.004). Although no DNA sequence change seemed to be the cause of hypertriglyceridemia in our patients, genetic variation in the 5' end of the apo B gene may contribute to changes in serum apo B levels in hypertriglyceridemic patients.

Functional analysis of disulfide linkages clustered within the amino terminus of human apolipoprotein B

We tested the involvement of N-terminal six disulfide bonds (Cys-1 through Cys-12) of human apolipoprotein (apo) B in the assembly and secretion of lipoproteins using two C-terminal-truncated apoB variants, namely B50 and B18. In transfected rat hepatoma McA-RH7777 cells, B50 could assemble very low density lipoproteins (VLDL), and B18 was secreted as high density lipoproteins. When all 12 cysteine residues were substituted with alanines in B50, the mutant protein (B50C1-12) lost its ability to assemble lipid and was degraded intracellularly. However, mutation had no effect on B50C1-12 translation or translocation across the microsomal membrane. Post-translational degradation of B50C1-12 was partially inhibited by the proteasome inhibitor MG132. To determine which cysteines were critical in VLDL assembly and secretion, we prepared three additional mutant B50s, each containing four selected Cys-to-Ala substitutions in tandem (i.e. Cys-1 to Cys-4, Cys-5 to Cys-8, and Cys-9 to Cys-12). Expression of these mutants showed that disruption of disulfide bond formation within Cys-5 to Cys-8 diminished apoB secretion, whereas within Cys-1 to Cys-4 or Cys-9 to Cys-12 had lesser or no effect. In another two mutants in which only one disulfide bond (i.e. between Cys-5 and Cys-6 or between Cys-7 and Cys-8) was eliminated, only secretion of B50 with mutations at Cys-7 and Cys-8 was decreased. Thus, the disulfide bond involving Cys-7 and Cys-8 is most important for VLDL assembly and secretion. In addition, assembly and secretion of VLDL containing endogenous B100 or B48 were impaired in cells transfected with B50s containing Cys-7 and Cys-8 mutation. The Cys-to-Ala substitution abolished recognition of B50 by MB19, a conformational antibody with an epitope at the N terminus of human apoB. The Cys-to-Ala substitution also attenuated secretion of B18, but the effect of the mutation on B18 secretion was less evident than on B50.

Two new immunogenetic polymorphisms of the apoB gene and their effect on serum lipid levels and responses to changes in dietary fat intake

In previous studies, apoB polymorphisms have been shown to modify serum lipid responses to changes in dietary fat intake. The functionally important apoB DNA change or changes underlying these effects have, however, remained unknown. Using a single-strand conformation polymorphism analysis-based screening method, we identified two previously unreported apoB polymorphisms located close to each other in the 5' region of apoB gene exon 26. This DNA segment corresponds to the binding site of monoclonal anti-apoB antibody D7.2. The two A-->G changes at apoB cDNA nucleotides 5869 and 5896 produced an Asn-->Ser change at amino acid 1887 and a His-->Arg change at amino acid 1896. In the Finnish population, allele frequencies of the rare alleles of the apoB 1887 (Asn-->Ser) and apoB 1896 (His-->Arg) polymorphisms were .02 and .11, respectively. Both polymorphisms were shown to have an independent effect on the binding affinity of LDL with monoclonal antibody D7.2. The effect of these polymorphisms on serum lipid levels and responses to changes in dietary fat intake in 102 healthy free-living subjects was assessed. The apoB 1896 Arg allele was associated with a higher serum LDL cholesterol level during a low-fat, low-cholesterol diet in men.

Identification of apolipoprotein B100 polymorphisms that affect low-density lipoprotein metabolism: description of a new approach involving monoclonal antibodies and dynamic light scattering

Rare mutations in apolipoprotein B (apoB) can cause defective binding of low-density lipoproteins (LDLs) to the LDL receptor, leading to elevated plasma cholesterol levels and premature atherosclerosis. This communication describes a novel approach to study the effects of apoB mutations on LDL metabolism. Monoclonal antibody MB19 identifies a common polymorphism in apoB, an Ile/Thr substitution at residue 71, by binding with a 60-fold higher affinity to apoB(Ile71)-containing LDL. Because each LDL contains a single apoB, a maximum of two LDLs may be bound by the bivalent monoclonal antibody. Thus, at the appropriate concentration, an equivalent amount of MB19 will promote substantial dimer formation of LDL containing the strongly binding apoB(Ile71), but little dimer formation of LDL containing the weakly binding apoB(Thr71). For LDL isolated from heterozygous individuals, the amount of dimer formed, determined by dynamic light scattering, yields an estimate of the allelic ratio of the two forms of LDL. For such individuals, not only the effect of the polymorphism recognized by MB19 but also the effects of other polymorphisms on the LDL allelic ratio can be determined. Examination of six normolipemic MB19 heterozygotes gave percent allelic ratios between 48:52 and 51:49 tight:weak-binding LDL, not significantly different from a 50:50 ratio. These individuals were also heterozygous for six common apoB polymorphisms, allowing calculation of the odds that each of these polymorphisms caused significant alterations in lipid levels. In contrast, the rare mutation at residue 3500 causing defective binding to the LDL receptor and familial defective apoB100 (FDB) resulted in substantial changes (26:74 and 13:87) in LDL allelic ratio in both of two FDB individuals examined.

Apolipoprotein B and low-density lipoprotein structure: implications for biosynthesis of triglyceride-rich lipoproteins

ApoB100 is a very large glycoprotein essential for triglyceride transport in vertebrates. It plays functional roles in lipoprotein biosynthesis in liver and intestine, and is the ligand recognized by the LDL receptor during receptor-mediated endocytosis. ApoB100 is encoded by a single gene on chromosome 2, and the message undergoes a unique processing event to form apoB48 message in the human intestine, and, in some species, in liver as well. The primary sequence is relatively unique and appears unrelated to the sequences of other serum apolipoproteins, except for some possible homology with the receptor recognition sequence of apolipoprotein E. From its sequence, structure prediction shows the presence of both sheet and helix scattered along its length, but no transmembrane domains apart from the signal sequence. The multiple carbohydrate attachment sites have been identified, as well as the locations of most of its disulfides. ApoB is the single protein found on LDL. These lipoproteins are emulsion particles, containing a core of nonpolar cholesteryl ester and triglyceride oil, surrounded by an emulsifying agent, a monolayer of phospholipid, cholesterol, and a single molecule of apoB100. An emulsion particle model is developed to predict accurately the physical and compositional properties of an LDL of any given size. A variety of techniques have been employed to map apoB100 on the surface of the LDL, and all yield a model in which apoB surrounds the LDL like a belt. Moreover, it is concluded that apoB100 folds into a long, flexible structure with a cross-section of about 20 x 54 A2 and a length of about 585 A. This structure is embedded in the surface coat of the LDL and makes contact with the core. During lipoprotein biosynthesis in tissue culture, truncated fragments of apoB100 are secreted on lipoproteins. Here, it was found that the lipoprotein core circumference was directly proportional to the apoB fragment size. A cotranslational model has been porposed for the lipoprotein assembly, which includes these structural features, and it is concluded that in permanent hepatocyte cell lines, apoB size determines lipoprotein core circumference.

Heterogeneous lipoprotein (a) size isoforms differ by their interaction with the low density lipoprotein receptor and the low density lipoprotein receptor-related protein/alpha 2-macroglobulin receptor

Lipoprotein (a) (Lp(a)) is a complex of low density lipoprotein (LDL) with apolipoprotein (apo) (a). To examine the size distribution of Lp(a), plasma was separated by fast flow gel filtration and Lp(a):B complexes were determined in the eluate by enzyme immunoassays, in which detection was performed with monoclonal antibodies specific for apoB. Lp(a):B particles displayed apparent molecular masses (M(r)) of 2 x 10(6) to at least 10 x 10(6). Lp(a) size isoforms differed by the expression of apoB epitopes and their interaction with cultured human skin fibroblasts. LDL was more effective in inhibiting binding, uptake, and degradation of low M(r) Lp(a) than of high M(r) Lp(a). In contrast, Glu-plasminogen, alpha 2-macroglobulin and tissue-type plasminogen activator were more effective in competing for the cellular degradation of high M(r) Lp(a) than of low M(r) Lp(a). Ligand blotting revealed that Lp(a) bound to the low density lipoprotein receptor, the low density lipoprotein receptor-related protein/alpha 2-macroglobulin receptor (LRP) and to two other endosomal membrane proteins. We propose that the LDL receptor preferentially internalizes low M(r) Lp(a), whereas LRP may have a role in the clearance of high M(r) Lp(a).

Variable expression of the mutation in familial defective apolipoprotein B-100

Although most subjects with familial defective apolipoprotein B-100 (FDB) have raised plasma low-density lipoprotein (LDL) levels, a few have LDL levels within the normal range. We have previously identified two normocholesterolemic FDB heterozygotes in an affected family. Results obtained from a study of this family are compatible with a major genetic contribution to the normocholesterolemia in the two heterozygotes. However, our findings are not compatible with inheritance of a variant normal allele at the apolipoprotein B locus in this family that neutralizes the effect of an FDB allele on the plasma LDL level. Polymorphic variations at the apolipoprotein E and LDL receptor loci did not explain the presence of normal LDL levels in the two heterozygous FDB subjects.

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.

StyI polymorphism in an enhancer region of the second intron of the apolipoprotein B gene in hyper- and hypocholesterolemic subjects

The regulation of the human apolipoprotein (apo) B gene that plays a crucial role in lipid metabolism is apparently very complex, with multiple cis- and trans-acting regulatory factors. One of these factors is an enhancer region in the second intron. In this region a point mutation at position + 722 has been found that is detectable by the restriction enzyme StyI. The report of Levy-Wilson et al. (1991) could suggest that the mutant allele (abolished StyI site) is associated with hypocholesterolemia. To investigate further the possible effect of this mutation on plasma cholesterol levels, we have compared the frequency of the mutant allele between 206 hypercholesterolemic Norwegian or Czech subjects on one hand, and 165 hypocholesterolemic Norwegian or Czech subjects on the other hand. No significant difference in frequency was found between the hypercholesterolemic and the hypocholesterolemic groups. This finding indicates either that the mutation at position + 722 does not affect the enhancer activity or that this in vitro enhancer activity is of little or no clinical significance. One of the Norwegian hypercholesterolemic subjects who was of Czech descent possessed the apoB 3500 mutation that leads to defective binding of low density lipoprotein (LDL) to the LDL receptors. Haplotype analysis of the apoB gene in her family showed that the mutation-bearing allele was identical to that reported in other countries, indicating a common gene source.

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.

Apolipoprotein B gene mutations in Austrian subjects with heart disease and their kindred

In a group of 110 subjects with severe coronary artery disease, two were heterozygous for the apolipoprotein (apo) B arginine3,500----glutamine mutation that characterizes familial defective apo B-100. Both affected subjects were moderately hypercholesterolemic, and their low density lipoproteins (LDLs) were deficient in binding to the LDL receptor. Pedigree analysis of the two probands' families established a correlation between the apo B mutation, defective LDL, and a particular apo B haplotype that was characterized by 10 apo B gene markers. In addition to having one allele carrying the arginine3,500----glutamine mutation, one family member may harbor a second mutant apo B allele that causes its gene product to be present in plasma at a lower than normal level, despite the fact that the affinity of the protein for the LDL receptor appears to be normal. The metabolic basis for the underrepresentation of this second allotype remains to be elucidated.

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.

Recent progress in understanding apolipoprotein B

For the past 5 years, investigators from many different laboratories have contributed to a greatly increased understanding of two very important lipid-carrying proteins in plasma--apo B-100 and apo B-48. Apo B-100, an extremely large protein composed of 4,536 amino acids, is synthesized by the liver and is crucial for the assembly of triglyceride-rich VLDL particles. Apo B-100 is virtually the only protein of LDL, a cholesteryl ester-enriched class of lipoproteins that are metabolic products of VLDL. The apo B-100 of LDL serves as a ligand for the LDL receptor-mediated uptake of LDL particles by the liver and extrahepatic tissues. The LDL receptor-binding region of apo B-100 is located in the carboxyterminal portion of the molecule, whereas its lipid-binding regions appear to be broadly dispersed throughout its length. Apo B-48 contains the amino-terminal 2,152 amino acids of apo B-100 and is produced by the intestine as a result of editing of a single nucleotide of the apo B mRNA, which changes the codon specifying apo B-100 amino acid 2,153 to a premature stop codon. Apo B-48 has an obligatory structural role in the formation of chylomicrons; therefore, its synthesis is essential for absorption of dietary fats and fat-soluble vitamins. Both apo B-48 and apo B-100 are encoded on chromosome 2 by a single gene that contains 29 exons and 28 introns. An elevated level of apo B-100 in the plasma is a potent risk factor for developing premature atherosclerotic disease. In the past 3 years, many different apo B gene mutations that affect the concentrations of both apo B and cholesterol in the plasma have been characterized. A missense mutation in the codon for apo B-100 amino aid 3,500 is associated with hypercholesterolemia. This mutation results in poor binding of apo B-100 to the LDL receptor, thereby causing the cholesteryl ester-enriched LDL particles to accumulate in the plasma. This disorder is called familial defective apo B-100, and it is probably a cause of premature atherosclerotic disease. Familial hypobetalipoproteinemia is a condition associated with abnormally low levels of apo B and cholesterol; affected individuals may actually have a reduced risk of atherosclerotic disease.(ABSTRACT TRUNCATED AT 400 WORDS)

Rapid typing of apolipoprotein B DNA polymorphisms by DNA amplification. Association between Ag epitopes of human apolipoprotein B-100, a signal peptide insertion/deletion polymorphism, and a 3'flanking DNA variable number of tandem repeats polymorphism of the apolipoprotein B gene

We present here rapid and efficient methods for the analysis of multiple variable apolipoprotein (apo) B loci using polymerase chain reaction based techniques. For illustrative purposes, we have applied these methods to establish an association between these polymorphisms and the apo B Ag immunological epitopes. The 5 DNA polymorphisms include 3 restriction endonuclease sites (for XbaI, EcoRI and MspI), a variable number of tandem repeat (VNTR) locus at the 3' end of the apo B gene, and an insertion/deletion polymorphism involving the signal peptide region of apo B. The latter two newly described polymorphisms are directly detectable following amplification and may have physiological effects on apo B expression because of their critical locations. All of these sites were typed using flanking oligonucleotides and the newly developed polymerase chain reaction. Amplification products were typed either directly (3' VNTR and signal peptide insertion/deletion alleles), or following specific enzyme digestion (for the restriction sites), or by allele specific oligonucleotides. The detailed methods presented will prove generally useful for rapidly typing DNA variation in the apo B gene. Using these techniques, we found a significant linkage disequilibrium between the Ag(t/z) locus and the 3' VNTR, and the Ag(c/g) locus and the signal peptide length polymorphism. Future association studies using these DNA polymorphisms should take into consideration that observed effects may be related to its linkage disequilibrium with the Ag loci and vice versa.