Major locus inheritance of apolipoprotein B in Utah pedigrees

Linkage and association analyses identify a candidate region for apoB level on chromosome 4q32.3 in FCHL families

Familial combined hyperlipidemia (FCHL) is a complex trait leading to cardiovascular disease (CVD) risk. Elevated levels and size of apolipoprotein B (apoB) and low-density lipoprotein (LDL) are associated with FCHL, which is genetically heterogeneous and is likely caused by rare variants. We carried out a linkage-based genome scan of four large FCHL pedigrees for apoB level that is independent of LDL: apoB level that is adjusted for LDL level and size. Follow-up included SNP genotyping in the region with the strongest evidence of linkage. Several regions with the evidence of linkage in individual pedigrees support the rare variant model. Evidence of linkage was strongest on chromosome 4q, with multipoint analysis in one pedigree giving LOD = 3.1 with a parametric model, and a log Bayes Factor = 1.5 from a Bayesian oligogenic approach. Of the 293 SNPs spanning the implicated region on 4q, rs6829588 completely explained the evidence of linkage. This SNP accounted for 39% of the apoB phenotypic variance, with heterozygotes for this SNP having a trait value that was approximately 30% higher than that of the high-frequency homozygote, thus identifying and considerably refining a strong candidate region. These results illustrate the advantage of using large pedigrees in the search for rare variants: reduced genetic heterogeneity within single pedigrees coupled with the large number of individuals segregating otherwise-rare single variants leads to high power to implicate such variants.

Variance decomposition of apolipoproteins and lipids in Danish twins

OBJECTIVE

Twin studies are used extensively to decompose the variance of a trait, mainly to estimate the heritability of the trait. A second purpose of such studies is to estimate to what extent the non-genetic variance is shared or specific to individuals. To a lesser extent the twin studies have been used in bivariate or multivariate analysis to elucidate common genetic factors to two or more traits.

METHODS AND RESULTS

In the present study the variances of traits related to lipid metabolism is decomposed in a relatively large Danish twin population, including bivariate analysis to detect possible common genetic factors of the traits.

CONCLUSIONS

The heritabilities of apolipoprotein B and E, cholesterol, LDL, and high density lipoprotein (HDL) were significant in the general population, although gender-specific levels and significance were detected. Heritabilities of apolipoprotein A1, triglycerides, and very low density protein (VLDL) were only significant when the population was stratified according to gender.

Major gene effects on apolipoprotein B concentrations in families of adolescents—Results from a community-based study in Taiwan

BACKGROUND

Apolipoprotein (Apo) B is considered as a risk factor for atherosclerosis. Previous reports of segregation analyses on the mode of inheritance of Apo B were inconsistent because of heterogeneity in study population or elderly adult diseased probands. We performed complex segregation analysis of Apo B levels in the families of adolescents, systematically ascertained from junior high school students in a rural community in Taiwan.

RESULTS

There is a sex-specific influence in the variation of apo B levels. The mother-daughter (0.216), sister-sister (0.181), sister-brother (0.179) correlations were higher than father-son (0.206), brother-brother (0.002) or cross-sex correlations for the variation in Apo B levels. By the variance component model, the heritability estimate was 26.3+/-6.7% (P<.0001) in Apo B levels. Commingling analysis indicated that a 2-component distribution was needed to account for Apo B variation. Segregation analysis using regressive models revealed that the best-fit model of Apo B was the model of major gene effect plus familial correlation. The gene frequency controlling high Apo B was 0.17, and 3 means of genotypes were 56.3, 54,2, and 117.2 mg/dl.

CONCLUSION

Variations of Apo B levels in the normal range among adolescent families are controlled by major gene, and further identification of this gene locus will be mandatory.

Compendium of genome-wide scans of lipid-related phenotypes: adding a new genome-wide search of apolipoprotein levels

The genetic dissection of complex inherited diseases is a major challenge. Despite limited success in finding genes, substantial data based on genome-wide scan strategies is now available for a variety of diseases and related phenotypes. This can perhaps best be appreciated in the field of lipid and lipoprotein levels, where the amount of information generated is becoming overwhelming. We have created a database containing the results from whole-genome scans of lipid-related phenotypes undertaken to date. The usefulness of this database is demonstrated by performing a new autosomal genomic scan on apolipoprotein B (apoB), LDL-apoB, and apoA-I levels, measured in 679 subjects of 243 nuclear families. Linkage was tested using both allele-sharing and variance-component methods. Only two loci provided support for linkage with both methods: a LDL-apoB locus on 18q21.32 and an apoA-I locus on 3p25.2. Adding those findings to the database highlighted the fact that the former is reported as a lipid-related locus for the first time, whereas the latter has been observed before. However, concerns arise when displaying all data on the same map, because a large portion of the genome is now covered with loci supported by at least suggestive evidence of linkage.

Localization of genes that control LDL size fractions in baboons

LDL phenotypes are strongly associated with risk of cardiovascular disease and are heritable, although little is known about individual genes that influence them. We investigated genetic control of LDL size-related phenotypes in 634 pedigreed baboons fed three diets contrasting in levels of fat and cholesterol. On a high-cholesterol high-fat diet, we obtained significant evidence for a quantitative trait locus (QTL) for cholesterol concentrations of lipoproteins between 27 and 28 nm (LOD=4.22, genomic P=0.0047) on the baboon homologue of human chromosome 22. For baboons fed a low-cholesterol high-fat diet, we obtained suggestive evidence for a QTL for cholesterol concentrations between 26 and 27 nm (LOD=2.67) on the baboon homologue of human chromosome 5. We speculate that this QTL influences LDL size distributions because LDL median diameters and other LDL fractions also showed peak LOD scores in this same chromosomal region. On a low-cholesterol low-fat basal diet we obtained suggestive evidence for a QTL for cholesterol concentrations of lipoproteins between 26 and 27 nm in diameter (LOD=2.15) on the baboon homologue of human chromosome 16. Thus, we have evidence for three putative QTLs that influence variation in baboon LDL size phenotypes on different diets.

Locus controlling LDL cholesterol response to dietary cholesterol is on baboon homologue of human chromosome 6

OBJECTIVE

Cholesterolemic responses to dietary lipids are known to be heritable, but the genes that may affect this response have yet to be identified. Using segregation analysis, we previously detected a potential quantitative trait locus (QTL) in baboons that influenced low density lipoprotein cholesterol response to dietary cholesterol. We performed linkage analyses to locate this QTL by using data on the baboon genetic linkage map.

METHODS AND RESULTS

We obtained evidence for linkage of this potential QTL to the same locus (D6S311) on the baboon homologue of human chromosome 6 by using variance components and parametric linkage analysis methods (2-point lod scores 4.17 [genomic probability value 0.008] and 2.81 [genomic P=0.10], respectively). Linkage analyses of serum levels of apolipoprotein B dietary response, a correlated trait, also gave weak suggestive evidence of linkage to this chromosomal region (maximum 2-point lod score 1.91). Although the LPA locus is nearby, we found no evidence of linkage with LPA.

CONCLUSIONS

This report is the first to localize, in any primate species, a potential QTL that influences low density lipoprotein cholesterol response to dietary cholesterol.

Segregation analysis of apolipoproteins A-1 and B-100 measured before and after an exercise training program: the HERITAGE Family Study

Complex segregation analyses of apolipoproteins (apo) A-1 and B-100 were performed in a sample of 520 individuals from 99 white families who participated in the HERITAGE Family Study. In these sedentary families, plasma apo A-1 and B-100 concentrations were measured before and after a 20-week endurance exercise training program. Baseline apo A-1 and B-100 were adjusted for the effects of age (age-adjusted baseline apo A-1 and B-100) and for the effects of age and BMI (age-BMI-adjusted baseline apo A-1 and B-100). The change in response to training was computed as a simple Delta (posttraining minus baseline) and was adjusted for age and the baseline (age-baseline-adjusted apo A-1 and B-100 responses to training). In the present study, a major gene could not be inferred for baseline apo A-1. Rather, we found a major effect along with a multifactorial effect accounting for 8% to 9% and 51% to 56% of the variance, respectively. In addition, no clear evidence supported a major-gene effect for its response to training, whereas the transmission of a major effect from parents to offspring was ambiguous, ie, genetic in nature or familial environmental in origin. The major effect accounted for 15% of the variance, with an additional 21% and 58% of the variance being accounted for by a multifactorial effect in parents and offspring, respectively. It is interesting to have obtained evidence of a putative recessive major locus for baseline apo B-100, which accounted for 50% to 56% of the variance, with an additional 25% to 29% of the variance due to a multifactorial effect. In contrast, no major effect for its response to training was identified, although a multifactorial effect was found that accounted for 27% of the variance. The novel findings arising from the present study are summarized as follows. Baseline apo A-1 and its response to training were influenced by a major effect and a multifactorial effect. Baseline apo B-100 was influenced by a putative major recessive gene with a multifactorial component, but its response to training was influenced solely by a multifactorial component in these sedentary families.

Etiologic heterogeneity of hyperapobetalipoproteinemia (hyperapoB). Results from segregation analysis in families with premature coronary artery disease

Hyperapobetalipoproteinemia (hyperapoB) is a common familial lipoprotein disorder associated with premature coronary artery disease (CAD). HyperapoB is characterized by an increased number of small, dense LDL particles. Patients with hyperapoB may be normotriglyceridemic (normoTG) or hypertriglyceridemic (hyperTG). We tested the hypothesis that a major locus controls the hyperapoB phenotype by using data from 1035 participants in 145 families enriched for premature CAD. Segregation analysis was conducted, and results suggest etiologic heterogeneity in these families. Families (n = 55) with one or more hyperTG hyperapoB individuals strongly supported mendelian recessive inheritance of hyperapoB. Under this mendelian model, individuals with the high-risk genotype had a baseline risk of 0.78, but parental and spouse's hyperapoB phenotypes did influence the probability of displaying hyperapoB. Low-risk genotypes had virtually no risk of displaying hyperapoB. The other subgroup of families (n = 72), in which all hyperapoB individuals were normoTG, did not show any clear pattern of inheritance. Eighteen families did not have any hyperapoB individual. In the 55 families with hyperTG hyperapoB, diabetes was more prevalent in hyperapoB individuals (18.3% of hyperTG hyperapoB individuals, 9.6% of normoTG hyperapoB individuals) than in normal individuals (4.9%). Both hyperTG hyperapoB and normoTG hyperapoB phenotypes were significant predictors for blood pressure in the 55 families, but not in the total population. These associations further suggest a link between hyperapoB and the small, dense LDL syndromes. This study successfully demonstrated mendelian inheritance of the hyperapoB phenotype and also suggested etiologic heterogeneity of hyperapoB.

Segregation analysis of plasma apolipoprotein B levels in familial combined hyperlipidemia

Familial combined hyperlipidemia (FCH) is a heritable lipid disorder that is associated with an increased risk of premature cardiovascular disease. An elevated plasma apolipoprotein (apo) B concentration is reported to be a diagnostic feature of the disorder. Recently we demonstrated a strong relation between plasma apoB concentrations and the cholesterol concentration in VLDL plus LDL, both elevated in FCH families. Therefore, examination of the inheritance of elevated plasma apoB levels in FCH families may reveal important information about the mechanism responsible for the aggregation of elevated plasma lipids in FCH. This study included 663 Dutch family members in 40 families ascertained through FCH probands. Plasma apoB concentration correlated significantly with apoB-related cholesterol both in the probands and the relatives (r=.83 and r=.90, respectively). Adjustment for age, sex, body mass index, and smoking habits accounted for 35.7% of the variation in apoB levels, and there was strong familial aggregation in adjusted apoB levels in these families. Complex segregation analysis was performed to determine the mechanism of inheritance behind this familial aggregation. The aggregation of elevated apoB levels was best explained by a major gene effect inherited by a codominant mechanism. Estimated mean apoB levels for the three supposed genotypes AA, AB, and BB were 111.5, 126.7, and 165.7 mg/dL, respectively, with relative frequencies of 43.5%, 44.9%, and 11.6%, respectively. In conclusion, despite assumed metabolic and genetic heterogeneity of FCH, there is clear evidence for a single gene effect on apoB concentrations in families ascertained through FCH. Linkage studies based on this analysis may further clarify the molecular basis of the apoB regulation in these families.

Genetic contributions to LDL-C, Apo-B and LDL-C/Apo-B ratio in a sample of Israeli offspring with a parental history of myocardial infarction

One hundred and forty sibships consisting of 280 brothers and 256 sisters with a family history of myocardial infarction were investigated for the possible involvement of a major gene in the determination of LDL-C, Apo-B and LDL-C/ Apo-B ratio (as a surrogate for LDL subclasses). The mean ages were 29.5 years (range 15-48) and 29.2 years (range 15-47), for brothers and sisters, respectively, and values were initially adjusted for age effects through multiple regression analysis. Results from commingling analysis indicated that for LDL-C a single normal distribution fitted the data as well as a mixture of two distributions. For Apo-B, a mixture of two normal distributions fitted the data significantly better than a single distribution (chi 2 = 7.8, df = 2, p = 0.02). For LDL-C/ Apo-B ratio a mixture of three normal distributions fitted the data significantly better than two distributions (chi 2 = 9.2, df = 2, p = 0.01). A regression analysis applied to the logarithm of the sex- and age-adjusted mean and variance within sibship, showed no indication of a major gene involvement for LDL-C. For Apo-B and LDL-C/Apo-B ratio, there existed, however, significant linear relationships between the logarithmically transformed means and within sibship variances which support the involvement of major genes. In addition, the Bartlett test applied to the data of within-sibship variances also rejected the null hypothesis of multifactorial transmission for Apo-B and LDL-C/Apo-B ratio (p < 0.0001). Lastly, the results from sib-pair linkage analyses provided significantly positive evidence for linkage between ApoB levels and the Apo-B XbaI restriction site.

The genetic determinants of plasma cholesterol and response to diet

In general, risk factors for multifactorial disorders such as atherosclerosis and hyperlipidaemia show a continuous distribution in the population, and this is the result of both interaction between genetic variation at genetic loci, and genetic and environmental interaction. Therefore, the investigation of the genetics of intermediate phenotypes such as levels of plasma lipid traits is likely to be particularly informative. Once the genes involved in determining the levels of these phenotypes have been identified, it should be possible to use the information to obtain a better understanding of the way these genetic variations determine the clinical end points. In the population it will be possible to identify a number of polygenes that are having a small effect on determining the trait, but for a particular individual, or the relatives of that individual, only a subset of all these polygenes will determine the level of the trait and therefore the risk of developing the disorder. In general, mutations with a large effect on the trait are rare in the population, By contrast, polymorphisms with a small effect on the trait may be common, such as is found with the effect of the apoE alleles and variation at the apoB gene locus on lipid levels. In the field of hyperlipidaemia and atherosclerosis research, molecular techniques have already given a great deal of information on how specific sequence variations in some of the candidate genes are involved in determining levels of plasma apoproteins, lipoproteins and lipids. As more mutations and sequence variations are identified, this will not only aid our understanding of the underlying pathology, but should be useful for identifying individuals who are at risk of developing atherosclerosis because of their particular genotype or combination of genotypes.

LDL physical and chemical properties in familial combined hyperlipidemia

Familial combined hyperlipidemia (FCHL) is characterized by elevations of triglyceride and/or cholesterol within families and an elevation in apoB. Although small dense LDL has been consistently associated with hypertriglyceridemia, small dense LDL persists despite reductions in triglyceride after treatment with gemfibrozil in FCHL. The current study evaluated potential differences in the distribution and chemical composition of LDL species in patients with FCHL and normolipidemic control subjects. LDL from FCHL patients was characterized by a relative abundance of a discrete LDL species with a mean peak analytic ultracentrifuge flotation rate (S0f) of 4.7 +/- 0.5 (SEM), a density of 1.041 +/- 0.001 g/mL, and a particle diameter of 250 +/- 1 A as assessed by gradient gel electrophoresis. The major LDL species in the control subjects had a higher mean S0f rate (6.3 +/- 0.4), was more buoyant (density, 1.037 +/- 0.001 g/mL), and was larger (diameter, 262 +/- 2 A). In addition, in a series of six LDL fractions separated by equilibrium density gradient ultracentrifugation, particle diameters were significantly smaller in all fractions from FCHL patients compared with those from control subjects. LDL particles from patients contained less free cholesterol, cholesteryl ester, and phospholipid than LDL from control subjects. The amount of triglyceride per LDL particle, however, did not differ between FCHL patients and control subjects. Differences in flotation rate and mass of the major LDL species between patients and control subjects could not be fully accounted for by differences in plasma triglyceride levels. Thus, LDL particles from FCHL patients are smaller and more dense with less cholesterol and phospholipid. Many of these differences appear to be independent of plasma triglyceride.(ABSTRACT TRUNCATED AT 250 WORDS)

Genetic predictors of FCHL in four large pedigrees. Influence of ApoB level major locus predicted genotype and LDL subclass phenotype

The genetic basis of familial combined hyperlipidemia (FCHL) has eluded investigators for 20 years, despite the apparent segregation of FCHL as an autosomal dominant disorder affecting 1% to 2% of individuals. Etiologic heterogeneity and additive effects of traits controlled by other genetic loci have been suggested. Two traits have been implicated in FCHL. The first is the predominance of a small, dense low-density lipoprotein (LDL), LDL subclass phenotype B, which segregates as a mendelian trait. The second is a mendelian locus with large effects on apolipoprotein (apo) B levels that is defined by complex segregation analysis (predicted apoB level genotype). This study shows that these factors appear to be separate genetic effects, both of which aid in the prediction of FCHL in four large pedigrees. The results suggest that FCHL may be best predicted by a threshold model in which apoB level genotype and LDL subclass phenotype each act to increase the risk of FCHL. Heterogeneity in the transmission of apoB levels among families is suggested, supporting the etiologic heterogeneity of FCHL. These results emphasize the advantages inherent in the study of large pedigrees when disease heterogeneity is suspected.

Familial combined hyperlipidemia in children: clinical expression, metabolic defects, and management

The first evidence that elevation of plasma levels of cholesterol is a risk factor for the development of atherosclerosis in children came from the Bogalusa Heart Study in 1986, which reported an association between aortic fatty streaks in 3- to 26-year-old subjects and increased plasma levels of low-density lipoprotein cholesterol (LDL-C). The most compelling evidence of a cause-and-effect relationship has come from the multicenter cooperative study called the Pathobiological Determinants of Atherosclerosis in Youth. When the investigators examined the abdominal aorta and the right coronary artery of adolescents and young adults who had died of trauma, they found a significant relationship between the sum of the very low density lipoprotein (VLDL) plus LDL-C level and both fatty streaks and raised atherosclerotic lesions. They also found an inverse relationship between those lesions and increased high-density lipoprotein cholesterol (HDL-C) levels. In addition, their studies showed that smoking (as assessed by the serum thiocyanate level) promotes atherogenesis in children as young as age 15 years. Thus many pediatricians have now accepted the importance of identifying children with significant hypercholesterolemia so that appropriate dietary and life-style modifications can be recommended. This is especially important because there is often a major genetic component to the hyperlipidemia seen in children.

Plasma apolipoprotein concentrations in young adults with a parental history of premature coronary heart disease and in control subjects. The EARS Study. European Atherosclerosis Research Study

The European Atherosclerosis Research Study (EARS) is a multicenter collaborative project within the European community. Its main objective is to study in young individuals the biological expression of a paternal history of premature coronary heart disease and to analyze the relative contribution of genetic and environmental factors to this expression. This study was carried out in 14 centers in 11 European countries, where the offspring of fathers who suffered a documented myocardial infarction before the age of 55 years (cases) were compared with age- and sex-matched control subjects. Plasma apolipoproteins A-I, B, A-II, A-IV, and E and lipoprotein (Lp) A-I lipoparticles were measured in this student population. Comparison of the values between cases and control subjects showed significantly higher apo B levels in cases compared with control subjects, and these differences were homogeneous throughout Europe. Regional differences were observed for apo E levels with an increasing north-south gradient, which was inversely related to that observed for triglycerides. A stepwise regression analysis including the lipid and apolipoprotein variables showed that apo B and triglycerides were the strongest discriminators between offspring of fathers with premature coronary heart disease and control subjects.

Linkage between the APOB gene and serum ApoB levels in a large pedigree from the Bogalusa Heart Study

Maximum likelihood linkage analyses were performed to test for linkage between serum apoB levels and several candidate gene markers including apolipoprotein B, lipoprotein lipase, hepatic lipase, cholesterol ester transfer protein, and apolipoprotein AI in a large pedigree. Parameters of general Mendelian inheritance derived from maximum likelihood segregation analysis of the serum apoB levels were used in the linkage analysis. The highest two-point lod score between the quantitative trait and a marker defined by a single restriction digest was 1.86 at recombination fraction (theta) = 0. This was observed for linkage between serum apoB levels and the presence or absence of a PvuII digestion site in the apoB gene. Linkage between serum apoB levels and polymorphisms of the apoB gene defined by the two restriction digests EcoR1 and PvuII was supported by a lod score of 3.30, while inclusion of VNTR typings led to a lod score of 2.33. None of the other candidate genes gave positive evidence of linkage.

Host and environmental effects on plasma apolipoprotein B

In this study, levels of apo B in an unselected sample of 487 middle-aged Caucasian spouses of patients and spouses of the patients' relatives are described. In males, apo B levels increased with age until the 7th decade, then declined; apo B levels in females, which were lower than in males, increased linearly with age across the entire life-span. Height and weight, smoking, and presence of noninsulin-dependent diabetes mellitus significantly influenced age- and gender-adjusted apo B levels in this sample, whereas use of alcohol, diuretics, beta-blockers, or insulin did not. Age, gender, height, weight, smoking, and noninsulin-dependent diabetes mellitus account for 21% of the total variation in apo B levels in this sample.

Familial combined hyperlipidemia in children: clinical expression, metabolic defects, and management

Familial combined hyperlipidemia (FCHL) is a dominantly inherited hyperlipidemia that occurs in at least 1% of the adult population and is responsible for 10% of premature coronary artery disease. In families referred for evaluation because of primary hyperlipidemia in a child, FCHL is expressed three times more commonly than familial hypercholesterolemia and half of the siblings are affected. Several metabolic defects apparently are associated with the FCHL phenotype. Most commonly, excess production of very low density lipoprotein apolipoprotein B can be demonstrated. In other families, reduced lipoprotein lipase activity is associated. One allele at a locus influencing apolipoprotein B levels predicts FCHL in a large proportion of families ascertained through affected children. Whether this allele is responsible for the excess of very low density lipoprotein apolipoprotein B detected in metabolic studies has not been elucidated. Management of FCHL in children begins with dietary modification. A bile acid sequestrant may be considered as well if diet cannot reduce the plasma low-density lipoprotein cholesterol level to less than 4.13 mmol/L (160 mg/dl) after the age of 10 years. Although the hydroxymethylglutaryl-coenzyme A reductase inhibitors are not currently recommended for children younger than 19 years of age, we speculate that they will be increasingly utilized for the management of FCHL in teenage boys who continue to have low density lipoprotein cholesterol levels greater than 4.13 mmol/L (160 mg/dl) after dietary modification.

Prevalence of hyperapobetalipoproteinemia and other lipoprotein phenotypes in men (aged < or = 50 years) and women (< or = 60 years) with coronary artery disease

The prevalence and clinical characteristics of hyperapobetalipoproteinemia (hyperapoB) and other phenotypes of dyslipoproteinemia were examined in 99 men (aged < or = 50 years) and 104 women (< or = 60 years) undergoing elective diagnostic coronary arteriography. HyperapoB was the most common phenotype (34%) associated with premature coronary artery disease (CAD). Only 20.2% of patients with CAD had a normal lipoprotein phenotype. The significant odds ratios for CAD were as follows: hypertriglyceridemic hyperapoB 17.45 (p < 0.0001), type IV 6.54 (p = 0.0001), type IIa 4.73 (p = 0.008), normotriglyceridemic hyperapoB 2.54 (p = 0.03) and type IIb 8.73 (p = 0.05). The strong association of hypertriglyceridemic hyperapoB with CAD reflected the multiplicative effect of increased low-density lipoprotein apolipoprotein B and endogenous hypertriglyceridemia, and was independent of the effects of age, sex, diabetes mellitus, systemic hypertension, body mass index and cigarette smoking. The ratio of apolipoprotein B to A-1 was better than those of low-density to high-density lipoprotein cholesterol and total to high-density lipoprotein cholesterol at discriminating dyslipidemic phenotypes from normal. Obesity was increased approximately 1.5 to two-fold in the hypertriglyceridemic phenotypes, diabetes was more prevalent in hypertriglyceridemic hyperapoB (6.8-fold; p < 0.001) and type IV (4.4-fold; p = 0.02), and hypertension was increased 1.5- to twofold in most dyslipidemic groups. The data indicate that hyperapoB and endogenous hypertriglyceridemia both contribute to the risk of premature CAD.

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.

Inheritance of plasma apolipoprotein B levels in families of patients undergoing coronary arteriography at an early age

An elevated plasma level of apolipoprotein B (apoB), the major protein of low density lipoproteins, is a risk factor for coronary artery disease. This study tested the hypothesis, suggested by previous studies, that the apoB level is strongly influenced by a major gene. The study population included 832 family members of 116 subjects who had undergone elective coronary arteriography at an early age. The apoB level was adjusted for age, gender, body mass index, alcohol consumption, and cigarette smoking (R2 = 20%). ApoB levels revealed strong familial aggregation with correlations among spouses of 0.23, parent-offspring of 0.16, and siblings of 0.21. Regressive models were used to examine inter-individual variation in adjusted apoB levels. In the total sample, familial aggregation of the apoB level was consistent with two models: (1) a major gene model and (2) a polygenic model with a mixture of non-transmitted "types". Comparison of these two models in each family showed that 57 families supported the first model over the second. Segregation analysis in these 57 families conclusively favored a major gene model with codominant transmission. Genotypic means were 124, 164, and 208 mg/dl with relative frequencies of 45%, 44%, and 11%. Linkage studies in these families can be used to clarify the molecular basis of apoB regulation. However, in the whole population the genetic control of apoB levels may be quite complex.

Scope and strategies of genetic epidemiology: analysis of articles published in Genetic Epidemiology, 1984-1991

Genetic epidemiology is a relatively new discipline that seeks to unravel the role of genetic factors and their interactions with environmental factors in the etiology of diseases, using population and family study approaches. To characterize the overall direction and emphasis of research strategies used in this field, we reviewed original research articles published in the journal Genetic Epidemiology since its inception in 1984 until the end of 1991. Of 259 published original articles, 92 (35%) focused primarily on methodologic/statistical developments, most commonly in the area of linkage analysis/gene mapping, and 167 (65%) articles were applied or data-derived. Only 42 articles (16%) were population studies, and 217 (84%) were family studies. Most family studies dealt with genetic analysis of pedigree data using segregation and linkage analyses. Of the 137 applied family studies, 73 (53%) were drawn from well-defined populations, and only 40 (29%) considered specific environmental factors in their analyses. These findings clearly indicate a rapid growth in the methodologic and statistical aspects of genetic epidemiology, and in the emphasis on family-based studies and genetic analysis methods. Further developments in genetic epidemiology will require greater integration of epidemiologic approaches of study design and analyses into population and family studies of disease etiology.

Genotype at a major locus with large effects on apolipoprotein B levels predicts familial combined hyperlipidemia

A sample enriched for familial combined hyperlipidemia (FCHL) was examined for evidence of an association between genotype at an apolipoprotein B (apoB) elevating locus defined by complex segregation analysis and FCHL. Complex segregation analysis detected a locus with a large effect on plasma apoB levels and was used to compute the most probable genotype of family members. None of the 35 normolipidemic adults carried a copy of the allele associated with elevated apoB levels, yet 58% of the 109 adults with FCHL carried 1 (29%) or 2 (28%) copies. Two of 28 (7%) normal children had 1 copy of this allele and none had 2 copies, while 88 of 182 (48%) children with FCHL had 1 (26%) or 2 (22%) copies. Further, 41 of 48 (85%) individuals classified as having hyperapobetalipoproteinemia did not carry a copy of this "elevated apoB" allele. Therefore, the presence of the allele associated with elevation of apoB level is highly predictive of FCHL and this association cannot be explained solely by the presence of elevated apoB levels in FCHL, suggesting that the locus controlling apoB levels may play an etiologic role in FCHL.

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)

The polymorphism ApoB/4311 in patients with myocardial infarction and controls: the ECTIM Study

The polymorphism affecting codon 4311 of the apolipoprotein B gene (ApoB/4311) was investigated in a large case-control study in two French and one Northern Irish geographically defined populations. Cases were recruited 3 to 9 months after a myocardial infarction (MI) and controls were randomly selected from the population. The polymorphism was assessed using allele-specific oligonucleotides (ASO). The genotype frequencies of the ApoB/4311 polymorphism did not differ in Northern Ireland and France and were in Hardy-Weinberg equilibrium in all groups; strong associations with three other polymorphisms of the ApoB gene (XbaI, EcoRI, VNTR(34 repeats)) were observed and it was possible to identify highly sensitive and specific markers of the ApoB/4311 rare variant. Homozygotes for the ApoB 4311 rare variant were slightly less frequent in cases than in controls: 22 (4.4%) and 35 (6.7%) respectively (population adjusted chi 2 = 3.3 P less than 0.07), especially in Belfast: 6 (3.1%) and 12 (7.6%), respectively (P less than 0.06). Several lipid and lipoprotein parameters were measured. Consistently among control groups, rare homozygotes had lower mean levels of ApoB (P less than 0.02), triglycerides (P less than 0.02), and lipoprotein particles containing ApoE and ApoB (LpE:B; P less than 0.001) and a higher mean level of lipoprotein particles containing ApoAI and not ApoAII (LpAI; P less than 0.02) than heterozygotes and frequent homozygotes combined. The strong association between the ApoB/4311 polymorphism and LpE:B was also observed in patients with MI. When present in the homozygous form, the ApoB/4311 Asn----Ser variant is associated with a lipoprotein profile that is apparently favourable.

Two-locus mitochondrial and nuclear gene models for mitochondrial disorders

Stimulated by a large pedigree with a cochlear form of deafness, for which we considered a two-locus mitochondrial and nuclear gene model, we have extended the classic methods of segregation analysis to these classes of two-locus disorders. Based on the unique maternal transmission pattern of the mitochondria, we demonstrate that utilization of the maternal line pedigree allows us to simplify the various two-locus mitochondrial models to "one nuclear locus" models. Classifying the nuclear families into different independent groups by the mother's phenotypes allows us to estimate the nuclear gene frequency in one group and to use this estimate as the expected value to test the fitness of the model on the other group. In addition, if we restrict the analysis to specific subsets of the mating type(s), we can also test the model on specific groups of nuclear families without estimating the gene frequency. Goodness-of-fit tests can be performed on pooled sibship data as well as individual sibship data. These methods of analysis should assume increasing importance as more disorders with features of mitochondrial inheritance are identified.

Bimodality of plasma apolipoprotein B levels in familial combined hyperlipidemia

To investigate possible genetic influences on plasma apolipoprotein (apo) B levels in familial combined hyperlipidemia (FCHL), commingling analysis was performed on data from seven large kindreds, including 183 individuals. The overall frequency distribution of apo B was skewed and was compatible with the presence of two normally distributed subdistributions (mean values, 117 and 172 mg/dl). The analysis was repeated after stratification of individuals by low density lipoprotein (LDL) subclass phenotype. Among subjects with phenotype A (predominance of large, buoyant LDL), a single apo B distribution was found (mean, 115 mg/dl). Among subjects with phenotype B (predominance of small, dense LDL), the distribution was bimodal, with mean values, 116 and 167 mg/dl, similar to the unstratified data set. Thus the skewing of the overall apo B distribution in FCHL family members may be due to a distinct subset of individuals with phenotype B who are genetically susceptible to even higher elevations of apo B. The higher apo B/phenotype B subjects also showed significantly higher levels of triglyceride and LDL-cholesterol than the lower apo B/phenotype B subjects. The lower apo B/phenotype B subjects had higher triglyceride and lower LDL-cholesterol than the phenotype A subjects. The enhanced information regarding apo B and lipid levels in the three subgroups of individuals identified here may facilitate a better understanding of genetic susceptibility to coronary heart disease.

Atherogenic lipoprotein phenotype. A proposed genetic marker for coronary heart disease risk

In a community-based study of 301 subjects from 61 nuclear families, two distinct phenotypes (denoted A and B) were identified by nondenaturing gradient gel electrophoretic analysis of low density lipoprotein (LDL) subclasses. Phenotype A was characterized by predominance of large, buoyant LDL particles, and phenotype B consisted of a major peak of small, dense LDL particles. Previous analysis of the family data by complex segregation analysis demonstrated that these phenotypes appear to be inherited as a single-gene trait. In the present study, the phenotypes were found to be closely associated with variations in plasma levels of other lipid, lipoprotein, and apolipoprotein measurements. Specifically, phenotype B was associated with increases in plasma levels of triglyceride and apolipoprotein B, with mass of very low and intermediate density lipoproteins, and with decreases in high density lipoprotein (HDL) cholesterol, HDL2 mass, and plasma levels of apolipoprotein A-I. Thus, the proposed genetic locus responsible for LDL subclass phenotypes also results in an atherogenic lipoprotein phenotype.

Inheritance of low density lipoprotein subclass patterns in familial combined hyperlipidemia

The inheritance of low density lipoprotein (LDL) subclass patterns was investigated in 234 members of seven large kindreds with familial combined hyperlipidemia (FCHL), a disorder characterized by elevated LDL cholesterol and/or triglyceride and increased coronary disease risk in families. Analysis of LDL subclasses by nondenaturing gradient gel electrophoresis showed a predominance of large, buoyant LDL particles (pattern A) in 71% of the family members and a predominance of small, dense LDL particles (pattern B) in 29% of family members. Based on complex segregation analysis, pattern B appeared to be inherited as an autosomal trait with either a dominant or an additive mode of inheritance and a small, but significant, multifactorial inheritance component. The proposed allele for pattern B was common (frequency = 0.3), and reduced penetrance was observed among men under age 20 and among women under age 50. These results in these FCHL families are consistent with those from a previously reported population-based sample of families, in which pattern B showed an apparent dominant mode of inheritance. In that study, reduced penetrance was observed for men under age 20 and for premenopausal women, but a somewhat lower allele frequency was found for pattern B (0.25). In the FCHL family members, LDL subclass pattern B was associated with significantly increased plasma levels of apolipoprotein B and triglyceride and decreased high density lipoprotein cholesterol. In comparison with a group of controls, the FCHL family members with pattern A had similar mean triglyceride levels, but higher mean apolipoprotein B. Thus, in families with FCHL, a predominance of small, dense LDL particles appears to be inherited as a common, single-gene trait, which is closely associated with the higher plasma triglyceride levels found in these families. The increased plasma apolipoprotein B levels found in FCHL cannot, however, be accounted for by this proposed locus.

Genetic epidemiology of differences in low-density lipoprotein (LDL) cholesterol concentration: possible involvement of variation at the apolipoprotein B gene locus in LDL kinetics

Circulating levels of low-density lipoprotein (LDL) vary considerably within and between populations, paralleled by differing coronary heart disease (CHD) mortality rates. We have previously shown that variation in the apolipoprotein (apo) B gene as associated with certain restriction fragment length polymorphisms (RFLPs) influences the metabolism of LDL in the U.K. population. To investigate a possible genetic contribution to variation in LDL levels in differing populations we have extended this original study. RFLPs of the apo B gene were determined in samples of individuals from the United Kingdom, Finland, Italy, Spain, and Africa. Significant associations of LDL fractional catabolic rate with the apo B EcoRI and XbaI RFLP genotypes were detected only in the two North European populations. In the African population sample, the XbaI RFLP displayed a significant association with LDL apo B synthesis. The data suggest that variation in the apo B gene influences the metabolism of LDL and that it is different in individuals of different ethnic background.

Familial resemblance of plasma apolipoprotein B: the Nancy study

The familial resemblance of plasma apolipoprotein B (apo B) was investigated in a sample of 102 families including 419 members who volunteered for a free health checkup in the Preventive Center of Vandoeuvre-lès-Nancy, France. The mean levels (+/- SD) of apo B were 141.0 (+/- 32.6), 121.8 (+/- 27.7), and 98.6 (+/- 22.6) mg/dl in fathers, mothers, and offspring, respectively. The familial correlations were 0.04, 0.13, 0.21 (P less than .01), and 0.47 (P less than .001) between spouses, father-offspring, mother-offspring, and siblings, respectively, after adjustment on age, body mass index, and sex. A genetic analysis was performed using the approach proposed by Bonney, which indicated that a recessive and a dominant major-locus model appeared nearly equally supported by the data. Under the recessive model, the frequency q of the most common allele was estimated as 0.825, with a mean difference of 60.4 mg/dl between high and low homozygotes. Under the dominant model, q was estimated as 0.875, with a mean increase of 34.2 mg/dl in heterozygotes and high homozygotes. However, the hypothesis of Mendelian transmission and the environmental hypothesis could not be formally tested because of great numeric difficulties encountered in the estimation of the three transmission probabilities. Given these analytical restrictions, we cannot conclude in favor of a major locus influencing apo B level in our population, even though the evidence is suggestive. The genetic heterogeneity underlying the familial aggregation of apo B level, suggested by several recent publications, might explain the difficulty in discerning a single major locus in a population sample of small nuclear families, not ascertained through patients enriching the sample in high values of apo B. These findings call into question the relevance of the approach through "healthy" populations in the search for major loci influencing biological traits.

Apolipoprotein, low density lipoprotein subfraction, and insulin associations with familial combined hyperlipidemia. Study of Utah patients with familial dyslipidemic hypertension

Familial dyslipidemic hypertension (FDH) is a syndrome recently described from sibships selected for early familial hypertension and found to have one or more of three fasting lipid abnormalities [high triglycerides, low high density lipoprotein (HDL) cholesterol, high low density lipoprotein (LDL) cholesterol]. In further analyses of these same 131 hypertensive subjects, apolipoprotein A-I and B, fasting plasma insulin (adjusted for body mass index), and detailed anthropometrics were different in two subgroups of FDH. Of 63 FDH patients, 19 met the criteria for familial combined hyperlipidemia (FCHL); 44 did not, but still had high triglyceride and/or low HDL cholesterol levels. When compared to 20 normolipidemic hypertensive patients, the 19 hypertensive patients with FCHL had 196% higher very low density lipoprotein cholesterol (p = 0.0001), 33% higher apolipoprotein B (p = 0.0002), smaller LDL particles (p = 0.007), and 73% higher fasting insulin (p = 0.003), but no significant differences in body mass index or skinfold thicknesses. The other 44 FDH patients without FCHL had 33% lower HDL (p = 0.0001), with only 8% lower apolipoprotein A-I levels (p = 0.20); significantly higher subscapular skinfolds (p = 0.02), weights (p = 0.002), body mass index (p = 0.006), knee widths (p = 0.0007), and wrist circumferences (p = 0.0009); smaller, denser LDL subfractions (p = 0.001); and increased apolipoprotein B levels (p = 0.01) compared to the normolipidemic hypertensive group. Increased fasting insulin levels were similar to the normolipidemic group and significantly lower than the FCHL group after adjustment for body mass index, suggesting a relationship between obesity and fasting insulin levels only in the non-FCHL group. We conclude that FDH consists of at least two subgroups: 1) FCHL with high apolipoprotein B, small LDL particles, and increased fasting plasma insulin levels, and 2) a less well-defined residual having upper central obesity with low HDL cholesterol and high triglyceride levels. Elevated insulin levels found in both groups, but possibly originating through different physiological mechanisms, may provide the pathophysiological connections between dyslipidemia, obesity, and hypertension.

Estimation of genetic model parameters: variables correlated with a quantitative phenotype exhibiting major locus inheritance

A major locus that is detected through its effect on one phenotype (a primary trait) may also affect other quantitative phenotypes or qualitative disease endpoints (secondary traits). The pattern of effects of the major locus on a set of primary and secondary traits suggests candidate defects for the mutant allele. The effects are directly estimable when "measured genotypes" or a tightly linked marker allow unambiguous assignment of major locus genotypes. When genotypes assignments are ambiguous for a major locus detected through its effect on a quantitative primary trait, we propose estimators using genotypic probabilities. Making certain reasonable assumptions, we demonstrate asymptotic unbiasedness of these genotypic probability estimators of the genotypic means and variances for either the quantitative primary or secondary traits, of the covariances between quantitative primary and secondary traits, and of prevalences for the secondary qualitative traits. An important application of genotypic probability estimators is to define an effect of a major locus that cannot be detected upon analysis of the variable; for example, major locus effects may be defined for hypertension or blood pressure as secondary traits, but not detected as primary traits.