RFLPs for the human apolipoprotein B gene: HincII and PvuII

Differential effects of oxidized LDL on apolipoprotein AI and B synthesis in HepG2 cells

Oxidized low-density lipoproteins (Ox-LDL) are key elements in atherogenesis. Apolipoprotein AI (apoAI) is an active component of the antiatherogenic high-density lipoproteins (HDL). In contrast, plasma apolipoprotein B (apoB), the main component of LDL, is highly correlated with coronary risk. Our results, obtained in HepG2 cells, show that Ox-LDL, unlike native LDL, leads to opposite effects on apoB and apoAI, namely a decrease in apoAI and an increase in apoB secretion as evaluated by [(3)H]leucine incorporation and specific immunoprecipitation. Parallel pulse-chase studies show that Ox-LDL impaired apoB degradation, whereas apoAI degradation was increased and mRNA levels were decreased. We also found that enhanced lipid biosynthesis of both triglycerides and cholesterol esters was involved in the Ox-LDL-induced increase in apoB secretion. Our data suggest that the increase in apoB and decrease in apoAI secretion may in part contribute to the known atherogenicity of Ox-LDL through an elevated LDL/HDL ratio, a strong predictor of coronary risk in patients.

APO B 3' HVR polymorphism in healthy population: relationships to serum lipid levels

We have analyzed allele frequency distribution at the hypervariable locus 3' to the apolipoprotein B gene in a healthy population sample (241 women and 246 men) from the Belgrade area. The bimodal distribution of sixteen different hypervariable region (HVR) alleles and the heterozygosity index (average 0.76) in both samples are similar to ones reported for other Caucasian populations. However, we found the hypervariable element containing 34 repeats-HVE34 allele to be the most common one in both female and male samples, and that there was a lower frequency of the HVE>36 alleles. For further analysis of 3'HVR allele frequency and its possible association with serum lipid levels, all HVR alleles were grouped into five main types (HVE<32, HVE32, HVE34, HVE36, and HVE>36), and the samples were divided into lipid quartiles. We found that (1) apo B gene 3'HVR polymorphism is associated with variation of normal lipid levels in males, (2) HVE32 alleles are associated with decreased serum levels of total cholesterol, LDL cholesterol, and triglycerides, and (3) HVR genotypes containing HVE34 and HVE36 alleles are associated with elevated serum levels of total cholesterol, low density lipoprotein (LDL) cholesterol, and triglycerides.

Apolipoprotein B gene DNA polymorphisms (EcoRI and MspI) and serum lipid levels in the Serbian healthy population: interaction of rare alleles and smoking and cholesterol levels

The frequency of restriction fragment length polymorphisms (RFLPs) of the apolipoprotein B (apo B) gene, detected by EcoRI and MspI, and their influence on serum lipids were studied in a total of 239 healthy subjects from the Belgrade area. The influence of interaction between different genotypes and smoking was also studied. The relative frequency of both rare R2 and M2 alleles (lacking the cutting site) was similar to that reported in other groups of Caucasians (0.16 and 0.11, respectively). No association was observed between the apo B genotypes and serum lipid levels adjusted for age, body mass index, and blood pressure either in the whole sample or in either women or men. When smokers and non-smokers were considered separately, smoking had a significant impact on total cholesterol variability in all individuals with genotype M1M2 and high density lipoprotein (HDL) cholesterol variability in women with genotype R1R2. The presence of the rare alleles of these two polymorphisms in smokers was associated with lower lipid levels in the whole sample and in both women and men analyzed separately, except for an increase of HDL cholesterol level in male smokers, heterozygous for EcoRI polymorphism (R1R2).

Frequency of the XbaI, EcoRI, PvuII and MspI polymorphisms of the apolipoprotein B gene in relation to hypercholesterolaemia in the general population

In this study the frequencies of the genotypes of four restriction fragment length polymorphisms in the apolipoprotein B gene (XbaI, EcoRI, PvuII and MspI) are compared between groups of normolipidaemic and diet resistant hypercholesterolaemic individuals as possible markers for the influence of this gene on plasma cholesterol levels. In the first part of the study genotypes of all four markers were determined in 92 normolipidaemic (mean cholesterol 5.6 + 0.8 mmol/l) and 79 diet resistant hypercholesterolaemic (mean cholesterol 7.8 + 0.7 mmol/l) individuals seen in a local health centre screening programme for coronary heart disease risk factors. No significant difference was seen in the frequencies of the EcoRI and PvuII genotypes between the two groups. There was significant enrichment of both the XbaI X2 (presence of cutting site) allelic frequency and of the MspI M1M2 (M2 absence of cutting site, rarer allele) genotype frequency in the hypercholesterolaemic group. In the second part of the study an independent larger group of individuals, seen in a multicentre screening programme across the city of Glasgow, were genotyped for the two potentially significant polymorphic sites (XbaI and MspI). From this second screening programme 188 age matched normolipidaemic males (mean cholesterol 5.0 +/- 0.8 mmol/l) were compared with 186 males who were still hypercholesterolaemic (mean 8.2 +/- 0.6 mmol/l) after three months dietary intervention. The hypercholesterolaemic individuals in this second study did not show a significant enrichment of the XbaI X2 allele but again showed a highly significant enrichment of the MspI M1M2 genotype. This genetic effect may relate directly to the charge change from arginine to glutamine at amino acid 3611 caused by the MspI mutation or to an as yet unknown functionally significant mutation in linkage disequilibrium with this site.

Patterns of association between genetic variability in apolipoprotein (apo) B, apo AI-CIII-AIV, and cholesterol ester transfer protein gene regions and quantitative variation in lipid and lipoprotein traits: influence of gender and exogenous hormones

Patterns of RFLP association were studied, to identify gene regions influencing quantitative variation in lipid and lipoprotein traits (coronary artery disease [CAD] risk factors or metabolically related traits). Subjects (118 female and 229 male; age 20-59 years) were selected for health. Multiple RFLPs were used to sample variability in regions around genes for apolipoprotein (apo) B (restriction enzymes HincII, PvuII, EcoRI, and XbaI), apo AI-CIII-AIV (BamHI, XmnI, TaqI, PstI, SstI, and PvuII) and cholesterol ester transfer protein (TaqI). Separate analyses were done by gender. The sample was truncated at mean +/- 4 SD, to remove extreme outliers. There was no significant gender difference in RFLP genotype frequency distribution. After trait-level adjustment to maximize removal of concomitant variability, analysis of variance was used to estimate the percentage trait phenotypic variance explained by measured variability in the gene regions studied. Fewer gene regions were involved in men, with less influence on quantitative trait variation than in women, in whom hormone use affected association patterns. Gender differences imply that pooling genders or adjusting data for gender effects removes genetic information and should be avoided. The association patterns show that variability around the candidate genes modulates trait levels: the genes are contributors to the genetics of CAD risk variables in a healthy sample.

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)

Variation at the apolipoprotein (apo) AI-CIII-AIV gene cluster and apo B gene loci is associated with lipoprotein and apolipoprotein levels in Italian children

We have used RFLPs of the apolipoprotein (apo) B gene and apo AI-CIII-AIV gene cluster to estimate the genetic contribution of variation at these loci to the variability of plasmid lipid, lipoprotein, and apolipoprotein levels in 209 children from Sezze in central Italy. The sample was randomly divided into group I (107 children) and group II (102 children). Four site polymorphisms (PvuII, XbaI, MspI, and EcoRI) of the apo B gene and five site polymorphisms (XmnI, PstI, SstI, PvuII-CIII, and PvuII-AIV) of the apo AI-CIII-AIV gene cluster were examined in group I children. After adjustment for gender, age, and body-mass index, polymorphisms at both gene loci (PvuII-B, PvuII-CIII, and PvuII-AIV) were associated with significant effects on the levels of plasma apo AI, apo B, or high-density lipoprotein-cholesterol. RFLPs that showed significant effects in group I were genotyped in group II. All three polymorphisms were associated with similar effects on apolipoprotein levels, though for all RFLPs the magnitude of the effects was smaller in the group II children and only statistically significant for the effect of the PvuII-B genotype on apo AI levels. In the total sample of 209 children 7.4% of the sample variance in apo AI levels was explained by variation associated with the apo B PvuII-B RFLP. In addition, the PvuII-B RFLP was associated with significant effects on plasma apo B levels and explained 5.7% of the sample variance. The PvuII-CIII and PvuII-AIV polymorphisms were both associated with differences in apo AI levels, explaining 3.7%-5.7% of the sample variance. Taken together, the three PvuII polymorphisms explained 17.7% of the phenotypic variance in apo AI levels. There was significant evidence for an effect of nonlinearity of the PvuII-CIII genotypes on apo AI levels, with the individuals heterozygous for the polymorphism having the highest apo AI levels. No evidence of interaction between genotype and gender, age, and body-mass index was shown by covariance analysis. The molecular explanation of this effect is unclear. Our data show that variation at both the apo AI-CIII-AIV and apo B loci are associated with lipoprotein and apolipoprotein levels in this sample of Italian children.

Genetic variation at the apolipoprotein gene loci contribute to response of plasma lipids to dietary change

Dietary intervention studies (from a low polyunsaturated/saturated fatty acid ratio P/S diet to a high P/S diet), carried out on a group of healthy individuals from North Karelia, Eastern Finland between 1981-1984, provided evidence that there may be a genetic component contributing to variation in response to dietary change. We have resampled blood from 107 individuals involved in the original studies and used Restriction Fragment Length Polymorphisms (RFLPs) to study the genetic contribution of variation at a number of candidate gene loci to the response to dietary change. The genes investigated in this study were the apolipoprotein (apo) genes: apo B, apo AII, apo E (protein polymorphism), apo AI-CIII-AIV gene cluster, and the LDL-receptor gene. On the basal diet the major effect of genotype on lipid traits was due to variation at the apo E gene locus; this protein polymorphism explained 14.6% of the phenotypic variance in LDL cholesterol levels and 12.7% of the phenotypic variance in total cholesterol levels. When switched to low fat high P/S diet, these effects of variation at the apo E gene locus on the phenotypic variation of LDL and total cholesterol levels disappeared. The major effect on the response to dietary change, delta, was seen on the difference in apo AI levels mediated by variation at the apo B gene locus (MspI RFLP) explaining 6.3% of the phenotypic variance in apo AI change. For the RFLPs of the apo AI-CII-AIV gene cluster, small but not significant differences on delta were found. Our results indicate that within the limits of the candidate genes studied, the major effects in response to dietary change was on apo AI levels mediated through variation at the apo B gene locus.

Two distinct truncated apolipoprotein B species in a kindred with hypobetalipoproteinemia

Two novel, distinct truncated forms of apolipoprotein B (apo B) designated as apo B-90 and apo B-40 were found in five members of a kindred with hypobetalipoproteinemia. Sodium dodecyl sulfate gels and immunoblots of plasma or low density lipoprotein (LDL) (d = 1.019 to 1.063 g/ml) of the affected members demonstrated the presence of one or both of the truncated apo B bands. Employing four monoclonal anti-LDL antibodies with defined regional specificities, we demonstrated that amino terminal epitopes of the truncated apo Bs were intact, but that 10% and 60%, respectively, of the carboxyl terminal regions were absent. Thrombin digestion of apo B-90 generated an abnormally small T2 fragment, confirming that approximately 550 amino acids had been deleted from the carboxyl terminus of apo B-100. Restriction fragment length polymorphism analysis and variable number of tandem repeat typing of the 3' flanking hypervariable region of the apo B gene made it possible to distinguish all four parental alleles and therefore to follow the inheritance of the apo B variants through the family. This pedigree analysis confirmed the inheritance of the apo B-90 and apo B-40 identified by monoclonal antibody binding studies. Siblings heterozygous for apo B-90 or apo B-40 exhibited greater than 65% lower concentrations of apo B-90 or apo B-40 relative to apo B-100 and had 5th percentile LDL cholesterol concentrations. Compound heterozygotes (apo B-90/apo B-40) had the lowest LDL levels, and their LDL particles were small in size.(ABSTRACT TRUNCATED AT 250 WORDS)

Homozygous hypobetalipoproteinemia: transcriptional regulation and 5'-flanking sequence analysis in an apolipoprotein B deficiency state

Apolipoprotein (apo) B is the principal apolipoprotein of chylomicrons, very-low-density lipoproteins (VLDL) and low-density lipoproteins (LDL). Patients with homozygous hypobetalipoproteinemia (HBL), characterized by apoB deficiency, have markedly decreased levels of hepatocyte mRNA as well as intracellular B apolipoprotein, and a virtual absence of plasma apoB. We have cloned, sequenced and analyzed the 5' regulatory region of the human apoB gene from -899 to +121 bp in normal and hypobetalipoproteinemic subjects. TATA and CAAT boxes were located at -30 and -61, respectively, and two GC-like boxes were identified at positions +56 and +108. The analysis of the HBL sequence revealed two substitutions at positions -838 and -517, when compared to the normal sequence. These substitutions were not present in any known apoB regulatory elements. The transcriptional activities of the homozygous hypobetalipoproteinemic and normal regulatory regions were compared by chloramphenicol acetyltransferase (CAT) assays in Hep G2 cells, and were found to be the same. Therefore, we conclude that the 5' regulatory region of the HBL apoB gene in this kindred is normal, and the two base substitutions do not affect promoter activity of the apoB gene. These studies suggest that a coding region abnormality in the apoB gene may lead to HBL.

Apolipoprotein B amino acid 3611 substitution from arginine to glutamine creates the Ag (h/i) epitope: the polymorphism is not associated with differences in serum cholesterol and apolipoprotein B levels

A G- to A-DNA sequence change in exon 26 of the human apolipoprotein B (apo B) gene leads to a glutamine substitution for arginine at codon 3611 of the mature apolipo-protein B100 and causes a loss of an MspI site. In 106 Finnish individuals, a complete correspondence exists between this MspI polymorphic site and the Ag (h/i) immunochemical polymorphism. Linkage disequilibrium was found between this MspI polymorphic site and the apo B XbaI and EcoRI variable sites and the Ag (al/d) and (c/g) epitope pairs; there is apparent linkage equilibrium with the apo B PvuII variable site. Based on three population studies (samples from London. Finland and Italy), no significant association was found between this RFLP and serum cholesterol and apo B levels. These data suggest that the arginine 3611----glutamine 3611 substitution has no significant effect on apo B function.

Restriction fragment length polymorphisms in the apo B gene in relation to coronary artery disease

We have determined the frequencies of the alleles at the EcoRI (E), XbaI (X) and PvuII (P) polymorphic restriction sites in the apo B gene in 124 white men with coronary artery disease (CAD) and in 146 white men free from CAD. The frequencies of the E- (restriction site absent) and X- alleles were both significantly higher in normocholesterolaemic men with CAD than in those without CAD, but the frequency of the P+ allele (restriction site present) was similar in the 2 groups. The frequency of the E- allele was significantly higher in CAD men with hypertriglyceridaemia than in normal men without hypertriglyceridaemia. In the normocholesterolaemic men without CAD, the mean serum cholesterol concentration was higher in those with genotype X++ than in those with genotype X--. Mean serum LDL-apo B and LDL-cholesterol concentrations did not differ significantly between men with different XbaI or EcoRI genotypes. Serum apo A-I levels differed significantly between normal men with different XbaI genotypes. Serum HDL-cholesterol levels differed significantly between CAD men with different XbaI genotypes. These results suggest that in white men the E- and X- alleles are in linkage disequilibrium with a nearby allele that is causally related to CAD. It is also possible that the amino acid substitution at position 4154 in apo B, brought about by the nucleotide change responsible for the EcoRI polymorphism, has a direct effect on the atherogenicity of LDL.

XbaI restriction fragment length polymorphism of apolipoprotein B in Swedish myocardial infarction patients

Apolipoprotein B (apoB) is a major importance to the metabolism of lipoproteins, and there is also evidence which suggests that apoB plays a central role in atherogenesis. In order to study whether there is a link between one of the mutations of the apoB gene and premature coronary heart disease, the frequency of the XbaI RFLP for the apoB gene was analysed in 52 male myocardial infarction patients. These were compared with a control group matched for age and sex (n = 52), and a random population sample of middle-aged men (n = 106). Two alleles were identified by the presence (X2) or the absence (X1) of an XbaI cleavage site. A somewhat higher frequency of the X2 allele was seen among the patients, however there was no significant difference between patients and controls regarding the genotypes or allele frequencies. This observation does not confirm one earlier report where a higher frequency of the X1 allele was seen in myocardial infarction patients. Differences between the studied populations or epidemiological designs of the studies might explain the diverging results. Further studies are evidently needed to fully resolve the relation between the XbaI RFLP and risk of atherosclerotic disease or lipoprotein metabolism.

Lack of correlation between the apolipoprotein B XbaI polymorphism and blood lipid levels in a Swedish population

The possible connections between the apolipoprotein B (apo B) XbaI polymorphism and the serum levels of total cholesterol, triglycerides, LDL, HDL and apo B have been investigated among 187 randomly selected subjects from Gothenburg, Sweden. The interferences of age and sex on the serum lipoproteins and apo B concentrations were considered. Using multiple regression analysis, we compared the different lipid levels and the levels of apo B with the genotypes X1X1, X1X2 and X2X2 (X1 = without the XbaI restriction site, X2 = with the site), with age and with sex and with those factors combined with each other. A significantly higher concentration of serum cholesterol and LDL among men than among women was found and total serum cholesterol, LDL and apo B were positively correlated with age. The allele frequency of the XbaI polymorphism in the sample was 0.45 for the allele without the XbaI restriction site. No correlation was found between the apo B genotypes and the levels of serum lipoproteins or apo B.

Genetic evidence from two families that the apolipoprotein B gene is not involved in abetalipoproteinemia

Abetalipoproteinemia (ABL) is a recessive disorder in which affected individuals have extremely low or undetectable levels of serum apo B-containing lipoproteins. Using restriction fragment length polymorphisms, we have studied two families, each with two children with classical ABL born of normal parents. In each of these families, the two affected children have inherited different apo B alleles from at least one parent, whereas the siblings would be anticipated to share common alleles if this disorder were due to an apo B gene mutation. This linkage study shows that in these families, the apo B gene is discordant with ABL and therefore the disorder is caused by a defect in another gene, which is important for the normal synthesis or secretion of apo B-containing lipoproteins from both the liver and intestine.

DNA polymorphisms of the apolipoprotein genes--their use in the investigation of the genetic component of hyperlipidaemia and atherosclerosis

DNA probes for all eight of the major apolipoprotein genes are now available. The chromosomal location, the basic structure and in many cases the nucleotide sequences of the normal genes are known. Common DNA polymorphisms of all of the genes have been detected. These have been been used in a number of ways to investigate rare inherited defects of the apolipoprotein genes, to study the potential involvement of different variants of the genes in the development of hyperlipidaemia in patients, and to investigate the contribution of common variation in these genes in the determination of serum lipid levels in the normal population.

A study of DNA polymorphism in the apolipoprotein B gene in a Japanese population

A Japanese group comprising 53 hyperlipidemic and 54 normolipidemic subjects was genotyped for DNA restriction fragment length polymorphisms (RFLPs) at the apo B gene locus. The polymorphisms with XbaI and PvuII were present at allelic frequencies of 0.04 (X1 allele) and 0.96 (X2 allele), 0.94 (P1 allele) and 0.06 (P2 allele), respectively. Unlike the previous reported association of the X1 allele with hypercholesterolemia found in Caucasians there was no difference in the frequency of the X1 allele between normolipidemic and hypercholesterolemic Japanese. Among the Japanese, two RFLPs appear to be in linkage equilibrium and can be used in conjunction as a haplotype. There is no strong population association in our patient group between any allele of the RFLPs studied and hyperlipidemia.

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.

A PvuII polymorphism in the 5' flanking region of the apolipoprotein AIV gene: its use to study genetic variation determining serum lipid and apolipoprotein concentration

We have used a 1.05-kb unique genomic fragment from the 5' end of the apolipoprotein (apo) CIII gene to identify a restriction fragment length polymorphism (RFLP) detected with the restriction enzyme PvuII, in the apoCIII-apoAIV intergenic region. In a sample of 220 normolipidaemic individuals from the UK population, the frequency of the rare allele, VB2 is 0.054. The PvuII polymorphism is in apparent linkage equilibrium with three other RFLPs of this gene cluster, detected with the restriction enzymes XmnI, PstI and SstI, but in linkage disequilibrium with an RFLP in the apo-CIII gene also detected with PvuII. Taken together, these five RFLPs have a PIC (polymorphism information content) value of 0.8, and therefore are informative for genetic studies. Individuals with the genotype VB1VB2 had lower mean concentrations of apoAI, and HDL-cholesterol than individuals with the genotype VB1VB1. However these differences were not statistically significant.

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.