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Elevated plasma lipoprotein(a) [Lp(a)] level has been established as an independent risk factor for atherosclerosis and coronary heart disease. Considerable ethnic group differences in the distribution of plasma Lp(a) levels have raised public health concerns. Recently, we have reported that Samoans have the lowest plasma Lp(a) levels of any population group. In the present investigation, we report the contribution of two apolipoprotein(a) (APOA) polymorphisms, the kringle 4 type 2 (K4) repeat and the pentanucleotide repeat (PNR), in affecting plasma Lp(a) levels in an American Samoan sample (n = 309). The K4 repeats ranged in size from 15 to 40. The common alleles contained repeats ranging from 26 to 36 with allele frequencies between 5.5% to 9.7%, and these accounted for 82% of all alleles. An inverse relationship between K4 repeat number and plasma Lp(a) level was observed for single-banded (r = -0.59, p = 0.0001) and double-banded phenotypes (r = -0.50, p = 0.0001). This polymorphism explained 60% of the variation in plasma Lp(a) level in American Samoans. For the PNR polymorphism, five different repeat alleles and eight different genotypes were identified; the most common allele was eight repeats. The *8 PNR allele was associated with a wide range of K4 repeats, the *9 PNR allele with larger K4 repeats (25-40), and the *10 PNR with smaller K4 repeats (15-24). Analysis of variance (ANOVA) revealed that the PNR polymorphism accounts for 2.1% of the variability in plasma Lp(a) levels in this sample, when the K4 repeat polymorphism was taken into account. Our data show that common polymorphisms in the APOA gene are major determinants of plasma Lp(a) variation in American Samoans.

Apolipoprotein J (apoJ, protein; APOJ, gene) is found in serum associated with high-density lipoprotein (HDL) subfractions, which also contain apolipoprotein A-I (apoA1) and cholesteryl ester transfer protein. ApoJ has been shown to be involved in a variety of physiological functions, including lipid transport. In earlier studies we reported the existence of a common genetic polymorphism (APOJ*1 and APOJ*2 alleles) using isoelectric focusing (IEF) and immunoblotting. In this study we determined the molecular basis of this polymorphism and together with another polymorphism at codon 328 (G--A) evaluated its relationship with serum HDL cholesterol and apoA1 levels in 767 African blacks stratified by staff level: junior (less affluent, n = 450) and senior (more affluent, n = 317). The molecular analysis of the cathodally shifted APOJ*2 allele on IEF gels revealed an amino acid substitution of asparagine by histidine resulting from a missense mutation (A--C) at codon 317 in exon 7. The frequency of the APOJ*2 (C) allele of codon 317 in the total sample was 0.267, whereas that of the less common allele A of codon 328 was 0.04. Despite their close proximity, no linkage disequilibrium was observed between the 2 polymorphisms. The impact of the codon 317 polymorphic variation was significant on serum HDL cholesterol (p = 0.003) and HDL3 cholesterol (p = 0.001) in junior staff. The adjusted mean values of these traits were higher in the codon 317 APOJ*2/*2 genotype than in the *1/*1 and *1/*2 genotypes. Overall, the APOJ codon 317 polymorphism explained 10.2% and 8.3% of the phenotypic variation in HDL cholesterol and HDL3 cholesterol, respectively, in junior staff. The codon 328 polymorphism showed a significant effect on HDL2 cholesterol (p = 0.039) and apoA1 (p = 0.007) only in junior women and accounted for 2.5% and 4.2% of the phenotypic variation in HDL2 cholesterol and apoA1, respectively. We also analyzed the combined effects of these genotypes at the 2 polymorphic sites. Significant effects on HDL cholesterol (p = 0.004) and HDL3 cholesterol (p = 0.008) in junior men and on HDL2 cholesterol (p = 0.003) in junior women were observed in the combined genotype data. The 2-locus genotypes explained 6.0% and 5.3% of the residual phenotypic variation of HDL cholesterol and HDL3 cholesterol in junior men and 10.4% of HDL2 cholesterol in junior women. These data indicate that the effect of the APOJ polymorphism on HDL cholesterol levels is modulated by socioeconomic status, as measured by staff level. Given the association of HDL and its subfractions with cardiovascular disease, these polymorphisms may lead to a better understanding of interracial differences in the risk of cardiovascular disease.

Several epidemiological studies have established that elevated serum lipoprotein (a) [Lp(a)] levels are independent risk factors for coronary heart disease, stroke, and restenosis of coronary lesions in white and Asian populations. Serum Lp(a) levels vary over a 1000-fold range among individuals and are under strict genetic control. Serum Lp(a) levels are significantly higher in populations with African ancestry than in populations of European ancestry. The APOA gene exhibits hypervariable length polymorphism resulting from a variable number of expressed kringle 4 repeats. An inverse relationship exists between the size of kringle 4 repeats and serum Lp(a) levels. However, most studies have been conducted in whites, and the data are scanty in African populations. To explore this relationship among Africans, we determined serum Lp(a) levels and APOA Kringle 4 size polymorphisms in 781 unrelated Africans (490 men, 291 women) from Benin City, Nigeria. Mean and median serum Lp(a) values were 25.6 +/- 0.6 mg/dl and 20.9 mg/dl, respectively. Although there was no difference in mean Lp(a) values between men and women, median Lp(a) values were higher in women than in men (p = 0.02). Using SDS-agarose gel electrophoresis, we detected 38 APOA isoforms, the highest number recorded to data. There were 10 consecutive medium-size alleles whose frequencies ranged between 4.2% and 10.9%, and together they accounted for 72.8% of the alleles observed in this population. Spearman's correlation coefficients showed an inverse relationship between the size of the APOA isoform and Lp(a) levels using either single-banded (r = 0.46; p0.0001) or double-banded (r = 0.42; p0.0001) phenotypes. Using random effects analysis of variance on the entire sample, the APOA size polymorphism explained about 15% of the phenotypic variation in Lp(a) levels. These data suggest that despite significant correlation between the APOA kringle 4 size polymorphism and Lp(a) levels, other sequence variations either in the APOA gene or closely linked genes may account for relatively higher Lp(a) levels found in Africans.

Two DNA polymorphisms, insertion/deletion (ins/del) and XbaI, of the apolipoprotein B (APOB) gene and their relationships with serum lipoprotein and lipid levels were examined in two Nigerian populations from Benin (n = 828) and Sokoto (n = 394). Allele frequencies for both polymorphisms were comparable between the two populations, and genotype frequencies conformed to Hardy-Weinberg equilibrium expectations. Because lipid profiles are affected by many known sex differences, males and females were analyzed separately. Analysis of variance was performed to test equality of means of quantitative traits across genotypes; the dependent variables were first adjusted by stepwise linear regression for environmental covariates in both populations. Plasma apoB and HDL2 cholesterol levels varied significantly between ins/del genotypes in Benin males (p = 0.05) and Sokoto males (p = 0.02), respectively. No significant variation between XbaI genotypes was observed for any lipid trait in either population. Combining the ins/del and XbaI polymorphisms to form haplotypes revealed significant linkage disequilibrium in both the Benin (p0.0001) and Sokoto (p0.001) populations. Analysis of the two-site haplotype showed a significant effect on serum lipoprotein (a) levels in Benin females (p0.04) and on HDL2 cholesterol among Sokoto males (p = 0.01). The haplotype explained 4% of the adjusted phenotypic variation in lipoprotein (a) levels in Benin females and 8.5% of the adjusted phenotypic variation in HDL2 cholesterol levels in Sokoto males. These data indicate that the ins/del and XbaI polymorphisms in the APOB gene affect interindividual variation in serum lipoprotein and lipid levels in African populations.

We examined the distributions of seven polymorphic sites in three apolipoprotein genes (APOE, APOA4, and APOH) and their relationships with quantitative lipid levels (total cholesterol, LDL cholesterol, HDL cholesterol, and triglycerides) among the Evenki reindeer herders of central Siberia. The polymorphism data reveal several distinctive features that differentiate the Evenki from white populations: the near absence of the APOE*2 allele, the highest ever recorded frequency of the APOH*3 allele, the complete absence of the APOA*2 allele at codon 360, and significantly different frequencies at three other APOA4 polymorphic sites. Our analyses of the relationships of common apolipoprotein polymorphism and plasma lipid levels also revealed interesting results. The well-established positive association between the APOE*4 allele and LDL cholesterol level reported in white populations was not seen in the Evenki despite a comparable frequency of the APOE*4 allele. Because the Evenki have significantly lower cholesterol levels than Westernized whites, this difference in allelic effect probably reflects gene-diet interaction, which modulates the effect of APOE polymorphism on LDL cholesterol. At the APOA4 locus the HincII polymorphism at codon 127 shows a significant impact on plasma triglyceride variation in the Evenki sample: The HincII - allele was associated with higher triglyceride levels than the HincII + allele. Our data indicate that both the genetic and the environmental factors conventionally associated with cardiovascular disease risk in Western societies are different in the Evenki.

The Samoan islands were politically separated into American Samoa and Western Samoa in the early 1900s. Economic modernization is far more extensive in American Samoa. However, the Samoan archipelago has maintained a remarkable degree of sociocultural homogeneity, including intermarriage. The sociocultural exchanges presumably led to genetic homogeneity between the two Samoas. Detailed genetic comparisons and characterizations of Samoans are scanty, however. As part of a multidisciplinary study of modernization and cardiovascular risk factors in adults, we analyzed nine hypervariable nuclear DNA (HVR) and four serum protein polymorphisms in the two Samoan groups. The average heterozygosities at both DNA and serum protein loci are comparable in the two groups. As expected, the HVR loci reveal a high degree of variability (heterozygosity 30-87%) compared with the serum protein loci (heterozygosity 1-52%). A large proportion of alleles at the HVR loci, ranging from 50% to 100%, are shared between American and Western Samoa. With the exceptions of the D1S80 locus in American Samoa and the D13S118 locus in Western Samoa, the genotype distributions at all loci conform to their respective Hardy-Weinberg expectations. Sporadic occurrence of the F13B*2 allele at the F13B locus in Samoans indicates a low level of European admixture because this allele is unique to Europeans. The calculated zero values of kinship coefficients and standard genetic distances indicate minimal population differentiation between the two Samoan groups.

Apolipoprotein A-IV exhibits a common two-allele polymorphism in several human populations studied to date. Using isoelectric focusing and immunoblotting, we have analyzed plasmas from 188 non-insulin-dependent diabetic and 238 normoglycemic Hispanic individuals from the San Luis Valley, Colorado, to determine APOA4 genotype frequencies and to estimate the impact of the genotypes on quantitative traits. The frequencies of the two common alleles, APOA4*1 and APOA4*2, were 0.929 and 0.069, respectively, in normal subjects and 0.901 and 0.096, respectively, in diabetics. The third rare allele, APOA4*3, was detected sporadically in both groups. We studied the impact of APOA4 polymorphism on the levels of total plasma cholesterol, HDL cholesterol and its subfractions (HDL3 and HDL2), LDL cholesterol, triglycerides, glucose, and insulin. We observed no significant effect of the APOA4 polymorphism on any trait in diabetics. However, we did note a significant sex-specific effect in normoglycemic females on the level of total HDL cholesterol (p = 0.001) and its subfractions HDL2 (p = 0.043) and HDL3 (p = 0.001). The effect of the APOA4*2 allele in normal Hispanic females was to lower the total HDL, HDL2, and HDL3 cholesterol by 8.75 mg/dl, 2.37 mg/dl, and 5.36 mg/dl, respectively, compared to the common APOA4*1 allele.