Plasma lipids in twins. Environmental and genetic influences

Heritability and Genome-Wide Association Study of Plasma Cholesterol in Chinese Adult Twins

Dyslipidemia represents a strong and independent risk factor for cardiovascular disease. Plasma cholesterol, such as total cholesterol (TC), low density lipoprotein cholesterol (LDL-C), and high density lipoprotein cholesterol (HDL-C), is the common indicator of diagnosing dyslipidemia. Here based on 382 Chinese twin pairs, we explored the magnitude of genetic impact on TC, HDL-C, LDL-C variation and further searched for genetic susceptibility loci for them using genome-wide association study (GWAS). The ACE model was the best fit model with additive genetic parameter (A) accounting for 26.6%, common or shared environmental parameter (C) accounting for 47.8%, unique/non-shared environmental parameter (E) accounting for 25.6% for the variance in HDL-C. The AE model was the best fit model for TC (A: 61.4%; E: 38.6%) and LDL-C (A: 65.5%; E: 34.5%). While no SNPs reached the genome-wide significance level (P < 5 × 10^-8), 8, 14, 9 SNPs exceeded the suggestive significance level (P < 1 × 10^-5) for TC, HDL-C, LDL-C, respectively. The promising genetic regions for TC, HDL-C, LDL-C were on chromosome 11 around rs7107698, chromosome 5 around rs12518218, chromosome 2 around rs10490120, respectively. Gene-based analysis found 1038, 1033 and 1090 genes nominally associated with TC, HDL-C, LDL-C (P < 0.05), especially FAF1, KLKB1 for TC, KLKB1 for HDL-C, and NTRK1, FAF1, SNTB2 for LDL-C, respectively. The number of common related genes among TC, HDL-C and LDL-C was 71, including FAF1, KLKB1, etc. Pathway enrichment analysis discovered known related pathways-zinc transporters, metal ion SLC transporters for TC, cell adhesion molecules CAMs, IL-6 signaling for HDL, FC epsilon RI signaling pathway, NFAT pathway for LDL, respectively. In conclusion, the TC and LDL-C level is moderately heritable and the HDL-C level is lowly heritable in Chinese population. The genomic loci, functional genes and pathways are identified to account for the heritability of plasma cholesterol level. Our findings provide important insights into plasma cholesterol molecular physiology and expect future research to replicate and validate our results.

Evidence of differential allelic effects between adolescents and adults for plasma high-density lipoprotein

A recent meta-analysis of genome-wide association (GWA) studies identified 95 loci that influence lipid traits in the adult population and found that collectively these explained about 25–30% of heritability for each trait. Little is known about how these loci affect lipid levels in early life, but there is evidence that genetic effects on HDL- and LDL-cholesterol (HDL-C, LDL-C) and triglycerides vary with age. We studied Australian adults (N = 10,151) and adolescents (N = 2,363) who participated in twin and family studies and for whom we have lipid phenotypes and genotype information for 91 of the 95 genetic variants. Heterogeneity tests between effect sizes in adult and adolescent cohorts showed an excess of heterogeneity for HDL-C (p_Het<0.05 at 5 out of 37 loci), but no more than expected by chance for LDL-C (1 out of 14 loci), or trigycerides (0 out 24). There were 2 (out of 5) with opposite direction of effect in adolescents compared to adults for HDL-C, but none for LDL-C. The biggest difference in effect size was for LDL-C at rs6511720 near LDLR, adolescents (0.021±0.033 mmol/L) and adults (0.157±0.023 mmol/L), p_Het = 0.013; followed by ZNF664 (p_Het = 0.018) and PABPC4 (p_Het = 0.034) for HDL-C. Our findings suggest that some of the previously identified variants associate differently with lipid traits in adolescents compared to adults, either because of developmental changes or because of greater interactions with environmental differences in adults.

Longitudinal Genetic Analysis of Plasma Lipids

The consensus from published studies is that plasma lipids are each influenced by genetic factors, and that this contributes to genetic variation in risk of cardiovascular disease. Heritability estimates for lipids and lipoproteins are in the range .48 to .87, when measured once per study participant. However, this ignores the confounding effects of biological variation measurement error and ageing, and a truer assessment of genetic effects on cardiovascular risk may be obtained from analysis of longitudinal twin or family data. We have analyzed information on plasma high-density lipoprotein (HDL) and low-density lipoprotein (LDL) cholesterol, and triglycerides, from 415 adult twins who provided blood on two to five occasions over 10 to 17 years. Multivariate modeling of genetic and environmental contributions to variation within and across occasions was used to assess the extent to which genetic and environmental factors have long-term effects on plasma lipids. Results indicated that more than one genetic factor influenced HDL and LDL components of cholesterol, and triglycerides over time in all studies. Nonshared environmental factors did not have significant long-term effects except for HDL. We conclude that when heritability of lipid risk factors is estimated on only one occasion, the existence of biological variation and measurement errors leads to underestimation of the importance of genetic factors as a cause of variation in long-term risk within the population. In addition our data suggest that different genes may affect the risk profile at different ages.

Genetic variants in LPL, OASL and TOMM40/APOE-C1-C2-C4 genes are associated with multiple cardiovascular-related traits

BACKGROUND

Genome-wide association studies (GWAS) have become a major strategy for genetic dissection of human complex diseases. Analysing multiple phenotypes jointly may improve both our ability to detect genetic variants with multiple effects and our understanding of their common features. Allelic associations for multiple biochemical traits (serum alanine aminotransferase, aspartate aminotransferase, butrylycholinesterase (BCHE), C-reactive protein (CRP), ferritin, gamma glutamyltransferase (GGT), glucose, high-density lipoprotein cholesterol (HDL), insulin, low-density lipoprotein cholesterol (LDL), triglycerides and uric acid), and body-mass index, were examined.

METHODS

We aimed to identify common genetic variants affecting more than one of these traits using genome-wide association analysis in 2548 adolescents and 9145 adults from 4986 Australian twin families. Multivariate and univariate associations were performed.

RESULTS

Multivariate analyses identified eight loci, and univariate association analyses confirmed two loci influencing more than one trait at p < 5 × 10-8. These are located on chromosome 8 (LPL gene affecting HDL and triglycerides) and chromosome 19 (TOMM40/APOE-C1-C2-C4 gene cluster affecting LDL and CRP). A locus on chromosome 12 (OASL gene) showed effects on GGT, LDL and CRP. The loci on chromosomes 12 and 19 unexpectedly affected LDL cholesterol and CRP in opposite directions.

CONCLUSIONS

We identified three possible loci that may affect multiple traits and validated 17 previously-reported loci. Our study demonstrated the usefulness of examining multiple phenotypes jointly and highlights an anomalous effect on CRP, which is increasingly recognised as a marker of cardiovascular risk as well as of inflammation.

Anthropometry, carbohydrate and lipid metabolism in the East Flanders Prospective Twin Survey: heritabilities

AIMS/HYPOTHESIS

We determined the genetic contribution of 18 anthropometric and metabolic risk factors of type 2 diabetes using a young healthy twin population.

METHODS

Traits were measured in 240 monozygotic (MZ) and 138 dizygotic (DZ) twin pairs aged 18 to 34 years. Twins were recruited from the Belgian population-based East Flanders Prospective Twin Survey, which is characterised by its accurate zygosity determination and extensive collection of perinatal and placental data, including information on chorionicity. Heritability was estimated using structural equation modelling implemented in the Mx software package.

RESULTS

Intra-pair correlations of the anthropometric and metabolic characteristics did not differ between MZ monochorionic and MZ dichorionic pairs; consequently heritabilities were estimated using the classical twin approach. For body mass, BMI and fat mass, quantitative sex differences were observed; genetic variance explained 84, 85 and 81% of the total variation in men and 74, 75 and 70% in women, respectively. Heritability estimates of the waist-to-hip ratio, sum of four skinfold thicknesses and lean body mass were 70, 74 and 81%, respectively. The heritability estimates of fasting glucose, fasting insulin, homeostasis model assessment of insulin resistance and beta cell function, as well as insulin-like growth factor binding protein-1 levels were 67, 49, 48, 62 and 47%, in that order. Finally, for total cholesterol, LDL-cholesterol, HDL-cholesterol, total cholesterol:HDL-cholesterol ratio, triacylglycerol, NEFA and leptin levels, genetic factors explained 75, 78, 76, 79, 58, 37 and 53% of the total variation, respectively.

CONCLUSIONS/INTERPRETATION

Genetic factors explain the greater part of the variation in traits related to obesity, glucose intolerance/insulin resistance and dyslipidaemia.

Evidence of age-dependent genetic influences on plasma total cholesterol

BACKGROUND

Causes of variation in cardiovascular risk factors include biological variation within individuals, and more permanent differences between individuals, which are at least partly genetic in origin. We have compared the magnitude of genetic and non-genetic factors within and across occasions through repeated measures of plasma cholesterol in twin subjects, and have also determined how far the same genes affect cholesterol levels at different ages.

METHODS

Data on plasma total cholesterol were extracted for 208 twin pairs who had provided blood on up to six occasions across a period of 17 years. They were aged 18-30 years at the time of first study and 30-47 at the time of the last. Multivariate models of variation due to genetic, shared environmental and unique environmental factors were fitted to the multi-occasion data and the proportions of variation due to these factors were estimated.

RESULTS

One genetic factor influenced plasma cholesterol on all occasions and a second genetic factor only influenced cholesterol results on the fifth and sixth occasions 10-17 years after the first. Environmental factors did not have significant long-term effects.

CONCLUSIONS

We conclude that individuals' long-term mean plasma cholesterol values are strongly genetically determined, but that some of these genes are age-specific in their effects.

Genetics of high-density lipoproteins

PURPOSE OF REVIEW

High-density lipoproteins have multi-factorial anti-atherosclerosis properties: they have potent anti-oxidant effects and prevent the oxidation of low-density lipoproteins; they have anti-inflammatory effects; they modulate vascular endothelial cell function and transport cholesterol back to the liver for excretion into the bile - a process called reverse cholesterol transport. The present review focuses on genetic aspects of high-density lipoprotein metabolism, with genomic approaches used to identify genes that regulate high-density lipoproteins in humans.

RECENT FINDINGS

Disorders of the many genes that code for proteins, including transporters, enzymes, receptors, transfer proteins and lipases, involved in high-density lipoprotein metabolism have been identified in humans as causing extremes of high-density lipoprotein cholesterol, and provide potential novel therapeutic avenues. These, however, explain fewer than 5% of the causes of low high-density lipoprotein cholesterol in the general population.

SUMMARY

Genome-wide linkage studies of large cohorts, with discrete as well as quantitative trait loci analyses, followed by association studies have enabled the identification of large chromosomal regions that may harbor genes that modulate high-density lipoprotein cholesterol levels in humans. Using mouse genetics, the results of the HapMap project and novel genetic approaches will allow the discovery of novel genes in high-density lipoprotein metabolism.

Gene by sex interaction in the etiology of coronary heart disease and the preceding metabolic syndrome

BACKGROUND

Despite decades of research, the genetic basis of coronary heart disease and its metabolic risk factors is poorly understood. Few studies consider that sex may modify the effect of gene variants on disease. Investigation of gene by sex interaction may help to elucidate underlying genetic susceptibilities and explain the sexual dimorphism of these complex traits.

AIMS

The aim of this review is to summarize evidence for gene by sex interaction in the etiology of coronary heart disease and the metabolic syndrome.

DATA SYNTHESIS

Published literature was examined in the areas of familial aggregation of coronary heart disease; heritability of body mass, insulin resistance, hypertension and dyslipidemia; genome-wide linkage analysis in humans and rodents; and large-scale genetic association studies. Possible mechanisms of gene by sex interaction are discussed including X-linked inheritance, confounding by risk factors and the effect of sex hormones.

CONCLUSIONS

The strongest evidence for gene by sex interaction in relation to coronary heart disease and the metabolic syndrome is in the etiology of body mass, insulin resistance and possibly dyslipidemia. Genetic studies of these traits would benefit from taking sex differences into account. Alternative mechanisms underlying gene by sex interaction, besides obvious sex hormone differences, should be considered.

Less than 50% of variation in HDL cholesterol between and within individuals, is explained by established predictors

Variation in serum high density lipoprotein cholesterol (HDL-C) concentrations is believed to be largely explained by triglycerides, but this has been mainly explored in cross-sectional analyses. Eight hundred and eighty one white male participants in a health screening program attended on a total of 2158 occasions for measurements that included fasting HDL-C, low density lipoprotein cholesterol (LDL-C) and homeostasis model assessment insulin resistance (HOMA-R). Baseline, change between-visit and repeated-measures regression models were used to analyse predictors of between- and within-individual variation in HDL-C. Independent predictors of between-individual variation included serum triglycerides (20.3 or 19.6% of the variance explained, depending on the model used), body mass index (BMI: 4.7 and 4.3%), cigarette smoking (3.3 and 1.5%) and alcohol consumption (0.4 and 1.1%). Within-individual variation in HDL-C was explained by changes in serum triglycerides (4.7 and 7.5%) and BMI (5.3 and 2.9%). In multivariate models, 24.3 and 24.9% of between-individual variation in HDL-C, and 7.9 and 8.8% of within-individual variation could be explained, depending on the model used. Sixty percent of the variation in HDL-C was due to unobserved factors. The majority of variation in HDL-C remains to be explained by influences other than the conventional variables: triglyceride and LDL cholesterol concentrations, insulin resistance, smoking and alcohol.

Genome scan for quantitative trait loci influencing HDL levels: evidence for multilocus inheritance in familial combined hyperlipidemia

Several genome scans in search of high-density lipoprotein (HDL) quantitative trait loci (QTLs) have been performed. However, to date the actual identification of genes implicated in the regulation of common forms of HDL abnormalities remains unsuccessful. This may be due, in part, to the oligogenic and multivariate nature of HDL regulation, and potentially, pleiotropy affecting HDL and other lipid-related traits. Using a Bayesian Markov Chain Monte Carlo (MCMC) approach, we recently provided evidence of linkage of HDL level variation to the APOA1-C3-A4-A5 gene complex, in familial combined hyperlipidemia pedigrees, with an estimated number of two to three large QTLs remaining to be identified. We also presented results consistent with pleiotropy affecting HDL and triglycerides at the APOA1-C3-A4-A5 gene complex. Here we use the same MCMC analytic strategy, which allows for oligogenic trait models, as well as simultaneous incorporation of covariates, in the context of multipoint analysis. We now present results from a genome scan in search for the additional HDL QTLs in these pedigrees. We provide evidence of linkage for additional HDL QTLs on chromosomes 3p14 and 13q32, with results on chromosome 3 further supported by maximum parametric and variance component LOD scores of 3.0 and 2.6, respectively. Weaker evidence of linkage was also obtained for 7q32, 12q12, 14q31-32 and 16q23-24.

Family history of stroke in stroke types and subtypes

Many studies have provided data showing that family history of stroke (FHS) is associated with an increased risk of stroke. The association of the FHS with the various stroke subtypes has not been adequately studied. The purpose of this study was to assess the association of the FHS with the two major stroke types (cerebral haematomas and ischaemic strokes) and the four stroke subtypes (cardioembolic, large artery disease, small artery disease, and undetermined) in a Greek population. The FHS was obtained from 421 consecutive acute stroke patients and from 239 matched control subjects. Positive FHS was observed in 49% of all stroke patients compared with 28% of the control subjects [adjusted OR=2.06 (95% confidence intervals (CI) 1.42-3.00)]. Haematomas, ischaemic strokes, and from the ischaemic strokes, both large and small artery disease strokes were strongly associated with positive FHS compared with the control subjects [adjusted OR=2.06 (95% CI 9-3.04), 2.07 (95% CI 1.09-3.91), 2.05 (95% CI 1.24-3.38), and 2.76 (95% CI 1.55-4.91), respectively]. There was no difference between maternal and paternal heritable contribution.In conclusion, FHS was found in this study to be an independent risk factor for all strokes combined, for each stroke type, and for the large and small-artery disease stroke subtypes, but not for the cardioembolic and undetermined stroke subtypes.

Genome scan for quantitative trait loci linked to high-density lipoprotein cholesterol: The NHLBI Family Heart Study

We conducted a genome-wide linkage scan for quantitative trait loci influencing total HDL-cholesterol (HDL-C) concentration in a sample of 1027 whites from 101 families participating in the NHLBI Family Heart Study. To maximize the relative contribution of genetic components of variance to the total variance of HDL-C, the HDL-C phenotype was adjusted for age, age(2), body mass index, and Family Heart Study field center, and standardized HDL-C residuals were created separately for men and women. All analyses were completed by the variance components method, as implemented in the program GENEHUNTER using 383 anonymous markers typed at the NHLBI Mammalian Genotyping Service in Marshfield, Wis. Evidence for linkage of residual HDL-C was detected near marker D5S1470 at location 39.9 cM from the p-terminal of chromosome 5 (LOD=3.64). Suggestive linkage was detected near marker D13S1493 at location 27.5 cM on chromosome 13 (LOD=2.36). We conclude that at least 1 genomic region is likely to harbor a gene that influences interindividual variation in HDL cholesterol.

The nature of the association between diet and serum lipids in the community: a twin study

Diet is commonly thought to be an environmental influence on serum lipid concentrations. This study evaluated whether total caloric and fat intake predict total cholesterol (TC), low-density lipoprotein cholesterol (LDL), high-density lipoprotein cholesterol (HDL), and triglyceride (TRIG) concentrations for environmental, as compared with genetic, reasons among 137 monozygotic and 67 dizygotic young adult twins. When genetic influences were controlled by correlating differences between monozygotic co-twins, a significant association remained between diet and TC, LDL, and HDL, suggesting that these dietary and serum lipid measures correlate for environmental reasons. Twin structural equation modeling confirmed these results. Overall, these results provide additional support for the hypothesis that diet is an environmental influence on TC, LDL, and HDL.

Variation in genetic and environmental influences in serum lipid and apolipoprotein levels across the lifespan in Swedish male and female twins

The contribution of genetic and environmental factors to variation in lipids and apolipoproteins has been estimated in previous twin and family studies. However, it is unclear whether there are sex and/or age differences in parameter estimates. We investigated a sample selected from the population-based Swedish Twin Registry of 725 like- and unlike-sex twin pairs, ages 17-85. Quantitative genetic methods were used to evaluate sex and age differences in genetic and environmental variation in lipid and apolipoprotein levels in three age groups, 17-49, 50-69, and 70-85. Heritabilities for lipids and apolipoproteins ranged from 35%-74%. Consistent sex differences were found in triglycerides. Females had higher heritabilities (56%) than males (35%) across the age groups. Total phenotypic variation increased across the age groups for cholesterol and apolipoprotein B due to an increase in unique environmental variance components. In contrast, in apolipoprotein A1 variance was highest in the middle age group and no differences were found in the phenotypic variance between age groups for triglycerides. We concluded that differences in phenotypic variation for cholesterol and apolipoprotein B were almost entirely due to the accumulation of environmental experiences throughout life, whereas there were no consistent patterns of differences in phenotypic variance for apolipoprotein A1 and triglycerides.

Dissecting the genetic architecture of lipids, lipoproteins, and apolipoproteins: lessons from twin studies

We review the ways in which twin studies have been used to investigate the genetic architecture of lipids, lipoproteins, and apolipoproteins. We focus on the age dependency of genetic effects and the importance of pleiotropy for the lipid system. Finally, consequences are discussed of age dependency and pleiotropy for the design and power of twin studies aimed at detecting the actual quantitative trait loci (QTLs) involved. It is concluded that twin studies have played an important role and will remain highly valuable for the elucidation of the genetic architecture of lipids, lipoproteins, and apolipoproteins. Twins can efficiently be used to identify the location and function of QTLs. Taking account of pleiotropy and age-dependent gene expression in study design and data analysis will improve the power and efficiency to find these QTLs for components of the lipid system.

Genetic and environmental causes of covariation among blood pressure, body mass and serum lipids during young adulthood: a twin study

OBJECTIVE

To determine the extent to which the correlation of systolic blood pressure, diastolic blood pressure, body mass, fasting total cholesterol and fasting triglycerides in young adulthood reflects common genetic or environmental influences.

DESIGN

Cardiovascular risk factors were measured in a community sample of 129 monozygotic and 67 dizygotic twin pairs, ages 18-30 years.

METHODS

Multivariate twin structural equation modelling allows estimation of the extent to which the covariation of two or more variables is attributable to common genetic and environmental factors and was used to analyse the correlation among systolic blood pressure, diastolic blood pressure, body mass index, fasting total cholesterol and triglycerides.

RESULTS

The covariation of risk factors was partially attributable to a single common genetic factor, while the covariation of systolic blood pressure, body mass index and triglycerides was also, in part, attributable to a common non-shared environmental factor.

CONCLUSIONS

Genetic and, to a lesser extent, non-shared environmental factors contribute to the covariation of cardiovascular risk factors in young adult twins. Nonetheless, it should be noted that these common influences account for a relatively small percentage of the variance in each risk factor compared to genetic and environmental factors that are risk factor-specific.

Human pedigree-based quantitative-trait-locus mapping: localization of two genes influencing HDL-cholesterol metabolism

Common disorders with genetic susceptibilities involve the action of multiple genes interacting with each other and with environmental factors, making it difficult to localize the specific genetic loci responsible. An important route to the disentangling of this complex inheritance is through the study of normal physiological variation in quantitative risk factors that may underlie liability to disease. We present an analysis of HDL-cholesterol (HDL-C), which is inversely correlated with risk of heart disease. A variety of HDL subphenotypes were analyzed, including HDL particle-size classes and the concentrations and proportions of esterified and unesterified HDL-C. Results of a complete genomic screen in large, randomly ascertained pedigrees implicated two loci, one on chromosome 8 and the other on chromosome 15, that influence a component of HDL-C-namely, unesterified HDL2a-C. Multivariate analyses of multiple HDL phenotypes and simultaneous multilocus analysis of the quantitative-trait loci identified permit further characterization of the genetic effects on HDL-C. These analyses suggest that the action of the chromosome 8 locus is specific to unesterified cholesterol levels, whereas the chromosome 15 locus appears to influence both HDL-C concentration and distribution of cholesterol among HDL particle sizes.

Familial aggregation of cardiovascular disease risk factors: the Cuenca Study

OBJECTIVE

The familial aggregation of lipid levels, blood pressure, and body mass index (BMI) was studied in schoolchildren in Cuenca, Spain.

METHODS

A cross-sectional observation study was made of 307 schoolchildren of both sexes, age range 9-12 years, from three schools in Cuenca, Spain, and of 346 parents. Social and demographic variables, weight, height, body mass index, systolic blood pressure, diastolic blood pressure, and fasting plasma concentrations of total cholesterol, LDL-cholesterol, HDL-cholesterol, and triglycerides were evaluated.

RESULTS

The Spearman coefficients of correlation for total cholesterol, LDL-cholesterol, and BMI for parents and daughters were 0.34-0.42 (P < 0.01). These coefficients of correlation for parents and sons were lower (P > 0.05). The coefficient of correlation for blood pressure in parents and sons was low (P < 0.05). None of the variables showed any coefficient of correlation between spouses. The sexual differences in the correlations between the levels of the different variables were confirmed by multiple regression analysis. Total cholesterol and LDL-cholesterol levels and BMI accounted for larger percentages of variability in these parameters in daughters than in sons. The paternofilial aggregation of HDL-cholesterol and triglyceride levels was weak. The only variable that accounted for a significant variability in blood pressure (systolic and diastolic) was weight in children of both sexes.

CONCLUSIONS

The familial aggregation of lipid levels and body mass index showed sex differences. The paternofilial aggregation of blood pressure was weak. There was no relation between spouses.

Heritability analysis of lipids and three gene loci in twins link the macrophage scavenger receptor to HDL cholesterol concentrations

We studied 100 healthy monozygotic and 72 dizygotic twin pairs (mean age, 34 +/- 14 years) to test for genetic influences on blood lipids and to examine relevant gene loci. Total cholesterol (TC), LDL cholesterol (LDL-C), HDL cholesterol (HDL-C), and triglyceride (TG) levels were determined after a 12-hour fast. Zygosity was determined with the use of microsatellite markers. Heritability estimates were conducted by using the lisrel 8 program; a sib-pair analysis was conducted by using the sibpal program. Linear regression analyses were carried out between identical-by-descent status and squared within-pair differences of TC, LDL-C, HDL-C, and TG values. Heritability estimates of the lipid serum concentrations ranged from .58 to .66. A significant linkage relationship was found for HDL-C (P = .008) and TGs (P = .05) with D8S261 on chromosome 8p. However, no linkage was found between any of the lipid variables and the lipoprotein lipase gene locus (LPL GZ14/15 and D8S282). Because D8S261 is located approximately halfway between the LPL and macrophage scavenger receptor genes, we examined the nearby markers D8S549 and D8S1731. Linkage was found for HDL-C and D8S549 (P = .001) and for HDL-C and D8S1731 (P = .04). On the other hand, we found no linkage between the LDL receptor gene locus and LDL-C serum concentrations nor between the LPL gene locus and the various other lipid fractions. Our data suggest a significant influence of the macrophage scavenger receptor gene locus on HDL-C and weak influence on TG levels. We suggest that inherited variability in the macrophage scavenger receptor gene has an influence on serum lipid concentrations.

A twin study of polycystic ovary syndrome and lipids

Our objective was to assess the relative contribution of genetic and environmental factors (particularly androgens) on circulating levels of lipid fractions and to determine the effect, if any, of polycystic ovary syndrome (PCOS) on lipid fractions. The study was carried out in the outpatient clinic of the Royal Hospital for Women, Paddington, Sydney, Australia. A group of 19 monozygotic (MZ) and 15 dizygotic (DZ) twin pairs was identified from the National Twin Register. Ultrasound clinical and biochemical parameters were used to define polycystic ovaries. Serum androgen and lipid fractions were also measured. Eleven pairs of twins (five MZ, six DZ) were scan discordant (i.e. one twin had polycystic ovaries and the co-twin did not). Serum levels of the lipoprotein fractions in twins with polycystic ovaries were not significantly different from the levels found for their co-twins with normal ovaries. There were no significant correlations between androgen-related hormones and any of the lipid measurements. Body mass index (BMI) was positively correlated with triglycerides and lipoprotein (a), and negatively correlated with high-density lipoprotein cholesterol (HDL-C). Sex hormone-binding globulin (SHBG) levels were negatively correlated with triglycerides and lipoprotein (a) and positively associated with HDL-C. Fasting insulin levels were significantly correlated with triglycerides and negatively with HDL-C. The MZ intraclass correlation exceeded that of the DZ twin pairs for all the lipid variables measured. The heritability estimates for lipoprotein (a), apolipoprotein B, total cholesterol and HDL-C were 0.95, 0.56, 0.48 and 0.54, respectively. However, the intraclass correlation coefficient for triglycerides was not significantly different between MZ and DZ twins, but maximum likelihood analysis indicated that at least 10% of the variance of the circulating triglyceride concentration is determined by genetic factors. We conclude that twins discordant for the PCOS do not have significantly different lipid fractions.

Is family history an independent risk factor for stroke?

OBJECTIVE

To estimate the influence of family history on the occurrence of stroke.

METHODS

A case-control study was carried out from August 1992 to January 1994. The study population comprised 502 patients with a first stroke, aged between 20 and 70 years, who were treated at 48 affiliated hospitals. The same number of age and sex matched controls were selected from outpatients. Diagnoses were based on CT findings and clinical signs. There were 155 case-control pairs for subarachnoid haemorrhage, 158 for intracerebral haematoma, and 159 for cerebral infarction. Information about the patients and their families was obtained from a questionnaire which included the family histories of each subtype of stroke and other potential risk factors for stroke. The data were analysed focusing on the role of the family histories in the occurrence of stroke.

RESULTS

In univariate analysis, the family histories of subarachnoid haemorrhage and intracerebral haematoma were positively associated with each of the subtypes of stroke (odds ratios 11.24 for subarachnoid haemorrhage, 2.39 for intracerebral haematoma), whereas family history of cerebral infarction was not a significant risk factor for its occurrence (odds ratio 1.41). Family history of intracerebral haematoma was correlated with a personal history of hypertension and habitual alcohol consumption. After adjustment for potential risk factors (hypertension, diabetes, hyperlipidaemia, obesity, alcohol consumption, and regular smoking), family history of subarachnoid haemorrhage still remained the most powerful risk factor for subarachnoid haemorrhage, whereas family history of intracerebral haematoma no longer showed a significant association with haematoma.

CONCLUSION

Genetic factors play a major part in the pathogenesis of subarachnoid haemorrhage, and family history of subarachnoid haemorrhage is the strongest independent risk factor for the disease. On the other hand, family history of intracerebral haematoma was not an independent risk factor for haematoma, but it might be a good predictor, which indirectly influences the pathogenesis of intracerebral haematoma via certain hereditary components such as hypertension, and even lifestyle factors such as alcohol consumption. In cerebral infarction, genetic factors play a minor part in its pathogenesis.

Plasma HDL cholesterol, triglycerides, and adiposity. A quantitative genetic test of the conjoint trait hypothesis in the San Antonio Family Heart Study

BACKGROUND

The conjoint trait hypothesis proposes that combined low HDL cholesterol (HDL-C) and high triglyceride (TG) levels represent a single, inherited phenotype that adiposity may influence in an unspecified manner. We conducted formal statistical genetic tests of the conjoint trait hypothesis and the relation of the conjoint trait to adiposity using data for 569 subjects in 25 pedigrees from the San Antonio Family Heart Study.

METHODS AND RESULTS

We conducted multivariate genetic analyses to detect the effects of genes and environmental factors on variation in plasma concentrations of HDL-C and TG, fat mass (as percent body weight [FM%], determined by bioelectric impedance), and body mass index (BMI). We used maximum-likelihood methods to simultaneously estimate the phenotypic means and SDs, heritabilities (h2), effects of sex, age-by-sex, eight dietary and medical covariates, and genetic and environmental correlations. Likelihood ratio tests disclosed significant heritabilities (P < .001) for all traits (h2HDL-C = 0.55, h2TG = 0.53, h2FM% = 0.37, h2BMI = 0.44) but significant genetic correlations (P < .001), indicating pleiotropy, between two trait pairs only: HDL-C and TG (PG = -0.52) and fat mass and BMI (PG = 0.86). We obtained significant environmental correlations between all trait pairs except HDL-C and BMI (P > .05).

CONCLUSIONS

Both shared genes (pleiotropy) and shared environmental factors contribute to the commonly observed inverse phenotypic association between plasma levels of HDL-C and TG. Rather than low HDL-C and high TG being a single, genetically transmissible entity, it is the inverse relation between these two phenotypes throughout their normal ranges of variation as well as at the extremes that is influenced by shared genes and shared environments. However, common environmental factors, not shared genes, account for reported associations of plasma HDL-C and TG levels with measures of adiposity.

A major locus influencing plasma high-density lipoprotein cholesterol levels in the San Antonio Family Heart Study. Segregation and linkage analyses

To detect and measure the effects of a single locus on quantitative variation in plasma concentrations of HDL cholesterol (HDL-C), we conducted statistical genetic analyses on data from 526 Mexican American individuals in 25 randomly ascertained pedigrees. By using maximum-likelihood complex segregation analysis, we found evidence for a major locus with a codominant mixture model that included the phenotypic means, standard deviations, relative frequency of a low HDL-C allele, and heritability for plasma HDL-C levels, plus the effects of sex (genotype specific), age-by-sex, age2-by-sex, plasma concentrations of apolipoprotein (apo)AI and triglycerides (genotype specific), exogenous sex hormone use, and menopausal status under an unrestricted general model. Inclusion of the four covariates (in addition to the sex and age-by-sex effects) accounted for nearly 79% of the variance in total plasma HDL-C levels. Of the remaining 21% of the variance, the detected major locus accounted for approximately 55% in men and 21% in women; the total genetic contributions to the variance by genes were approximately 82% in men and 69% in women. Linkage analyses with penetrance parameter estimates from the segregation analysis excluded tight linkage between the detected major locus and markers for the following candidate loci: the apoAI/apoCIII genomic region (P < .05), apoB (P < .01), hepatic lipase (P < .001), lipoprotein lipase (P < .001), and the LDL receptor (P < .001). While not excluding the apoE locus (LOD = -0.348, P < .21), the analysis provided no support for tight linkage between it and the detected major locus.

The importance of family history in cerebrovascular disease

BACKGROUND AND PURPOSE

The role of genetics in cerebrovascular disease remains controversial. The purpose of this study was to assess the influence of family history on atherothrombotic infarction or transient ischemic attack.

METHODS

Ninety patients with stroke or transient ischemic attack and 90 age- and sex-matched community control subjects were studied prospectively. Medical and family histories were obtained from all subjects, and a complete physical examination was performed.

RESULTS

Eighty-five patients and 86 control subjects knew their family history for ischemic heart disease and stroke. A positive history for ischemic heart disease was present in 62 (73%) of the patients and 46 (53%) of the control subjects (P = .019), and a positive family history for stroke was present in 38 (47%) of the patients and 21 (24%) of the control subjects (P = .014).

CONCLUSIONS

Although a positive vascular family history was not an independent risk factor in a multivariate analysis, it was an excellent marker of the presence of other established vascular risk factors. Personal histories of ischemic heart disease, hypertension, and hyperlipidemia were found to be significant independent risk factors for stroke.

Genetic and environmental influences on serum lipid levels in twins

BACKGROUND

The extent to which serum lipid levels are affected by genetic and environmental factors remains a point of controversy. We examined both genetic and environmental influences on serum lipid levels in twins reared either together or apart who participated in the Swedish Adoption/Twin Study of Aging.

METHODS

We studied 302 pairs of twins (mean age, 65.6 years; range, 52 to 86); 146 pairs had been reared apart. We simultaneously compared the twins on the basis of both zygosity and rearing status, which allowed joint estimation of genetic and environmental influences on serum lipid levels. Genetic influence was expressed in terms of heritability, the proportion of the population variation attributable to genetic variation (a value of 1.0 indicates that all of the population variation is attributable to genetic variation). The serum lipids and apolipoproteins measured included total cholesterol, high-density lipoprotein cholesterol, apolipoproteins A-I and B, and triglycerides.

RESULTS

Structural-equation analyses revealed substantial heritability for the serum levels of each lipid measured, ranging from 0.28 to 0.78. Comparisons of the twins reared together with those reared apart suggested that the environment of rearing had a substantial impact on the level of total cholesterol (accounting for 0.15 to 0.36 of the total variance). Sharing the same environment appeared to affect the other lipid measures much less, however, than did genetic factors and unique environmental factors not shared by twins. Comparisons of younger with older twins suggested that heritability for apolipoprotein B and triglyceride levels decreased with age.

CONCLUSIONS

The effect of genetic factors on the serum levels of some but not all lipids appears to decrease with age. Early rearing environment appears to remain an important factor in relation to levels of total cholesterol later in life, but it has less effect on other serum lipids and apolipoproteins in the elderly.

Genetic and environmental covariance of serum cholesterol and blood pressure in female twins

Blood pressure elevation is frequently associated with elevated cholesterol, triglyceride or low density lipoprotein (LDL-C) or low high density lipoprotein (HDL-C). The relative importance of genetic and environmental factors in these associations is unclear. We examined the relative contribution of genetic and environmental influences to the association between blood pressure and serum lipids in 75 pairs of female twins using path analysis and maximum-likelihood model fitting. Associations between systolic blood pressure and total cholesterol (r = 0.44, P < 0.001), and LDL-C (r = 0.38, P < 0.001), but not HDL-C (r = 0.05, N.S.), remained significant after age and body mass index adjustment. Univariate models suggested genetic effects contributed 60-70% to the variance of total cholesterol, LDL-C, HDL-C and systolic blood pressure. The remaining variance was explained by age and/or unique environmental influences. Using bivariate models, we demonstrated genetic (P = 0.017) and unique environmental covariance (P = 0.011) of cholesterol and systolic blood pressure. Significant genetic covariance (P = 0.038) was observed between LDL-C and systolic blood pressure. The association between blood pressure and total cholesterol in these twins results from shared genetic and similar unique environmental influences. The association between LDL-C and blood pressure is partly due to shared genetic influences. We conclude that both additive genetic and environmental factors unique to the individual are important determinants of the relationships between serum lipids and blood pressure.

A study of twins and stroke

BACKGROUND AND PURPOSE

Although there are strong genetic contributions to coronary artery disease, only a few studies have considered heritable influences on stroke.

METHODS

We investigated the role of genetic factors in stroke using the Twin Registry maintained by the National Academy of Sciences-National Research Council. The registry includes 15,948 male twin pairs born between 1917 and 1927. In 1985, 9,475 twins responded to a mailed questionnaire, which covered vascular risk factors, cardiac events, and stroke.

RESULTS

Analysis of twin pairs in which both responded to the questionnaire, and a question on stroke, indicated proband concordance rates of 17.7% for monozygotic pairs and 3.6% for dizygotic pairs (relative risk = 4.3; chi 2 = 4.94, df = 1; p less than 0.05).

CONCLUSIONS

This nearly fivefold increase in the prevalence of stroke among the monozygotic compared with the dizygotic twin pairs suggests that genetic factors are involved in the etiology of stroke. The twin study paradigm holds considerable promise for identifying both genetic and environmental influences on stroke.

Factors associated with high density lipoprotein cholesterol in a population with high high density lipoprotein cholesterol levels

A cross-sectional study of a random sample of 976 coloureds (mixed race) of the Cape Peninsula, ages 15 to 64 years old, revealed a population with unexpectedly high levels of high density lipoprotein (HDL) cholesterol. The mean level for men was 55.4 +/- 16.1 mg/dl (SD) and for women, 60.8 +/- 16.0 mg/dl. The ratio of HDL cholesterol to total cholesterol expressed as a percentage was 26.3% +/- 9.5% for men and 28.1% +/- 9.3% for women. The HDL cholesterol levels were apparently lower than those of black and Negro populations, yet higher than those of Caucasian populations. Men with levels of HDL cholesterol above the median reported a personal history and a family history of coronary heart disease less frequently than did men with lower levels, while women with high levels of HDL cholesterol were less likely to have a history of hypertension or diabetes. Stepwise multiple regression analysis of variables significantly associated with HDL cholesterol levels showed that they explained 29.7% and 24.7%, respectively, of the variation in HDL cholesterol in men and women. Those variables independently associated with HDL cholesterol in both men and women were: serum triglyceride (-), cigarette consumption (-), alcohol, body mass index (-), age, and serum low density lipoprotein cholesterol levels (-). The reasons for the relatively high HDL cholesterol levels in this population are unknown. However, it would seem possible that these levels offer some protection against the high risk factors of smoking, hypertension, and hypercholesterolemia.

Twin study of genetic and environmental effects on lipid levels

A study of 106 pairs of monozygotic (MZ) and 94 pairs of dizygotic (DZ) twins tested the hypothesis that part of the previously described genetic influence on blood lipid levels can be ascribed to closer similarities among MZ than among DZ twin pairs in environmental factors that affect lipid levels. Participants were adult twin volunteers (age 17-66; 64 male and 136 female pairs) who were selected from the NH & MRC Twin Registry or were respondents to advertisements. They completed a 4-day weighed food diary from which mean nutrient intake was derived. Information on lifestyle and demographic variables was obtained by questionnaire and a nonfasting blood sample was taken for measures of total, low-density lipoprotein (LDL) cholesterol, and high-density lipoprotein (HDL) cholesterol and the HDL2 and HDL3 subfractions. Height and weight were measured, and body mass index (BMI) was calculated (kg/m2). Estimates of the heritability of sex-adjusted lipid levels were 0.72 for total cholesterol, 0.79 for HDL cholesterol, 0.69 for HDL2, 0.20 for HDL3, 1.06 for LDL cholesterol, and 0.44 for sex-adjusted BMI. In all cases except for HDL3, genetic variance was statistically significant. After adjusting for the effects of environmental variables in three different ways, the estimates of heritability were somewhat lower for total cholesterol, HDL2, and BMI, and those for HDL cholesterol (borderline) and LDL cholesterol (definitely) remained statistically significant but were decreased. A genetic influence on HDL3 was not found. Adjusted heritability estimates obtained from one method of analysis were 0.35 for total cholesterol, 0.49 for HDL, 0.04 for HDL2, -0.34 for HDL3, 0.66 for LDL, and 0.32 for BMI. These results suggest that the assumptions made in the classical twin study approach are not appropriate when examining genetic effects on lipid levels or BMI, or indeed on any biological variable that may be affected by environmental factors that tend to be more similar in MZ twins than in DZ twins. In these circumstances, more complex models may be needed to differentiate between genetic and environmental influences.

Lifestyle factors affecting intrapair differences of serum apoproteins and cholesterol concentrations in adult identical twins

83 pairs of twins, aged 50-74 years, were studied with respect to their serum apoproteins (A-I, A-II, B, C-II, C-III and E) and cholesterol concentrations. Each of the variables showed a higher intraclass correlation in the MZ twins than in the DZ twins. HDL cholesterol showed the highest intraclass correlation among the variables and indicated strong heritability. In the MZ pairs discordant for alcohol consumption, the higher consumers within the pair showed a significantly higher level of apo A-I and HDL cholesterol, and a lower level of apo B concentration. In the MZ pairs discordant for cigarette smoking, the higher consumers within the pair showed a significantly lower level of apo C-III. In the MZ pairs discordant for obesity, the heavier twins within the pair showed higher levels of non-HDL cholesterol, total cholesterol, apo B, apo C-II and apo C-III. In the MZ pairs discordant, for occupation, the twins of 'heavy work' showed significantly higher levels of apo B, total cholesterol and non-HDL cholesterol than the twins of 'light work'.

Genotype-influenced changes in serum HDL cholesterol after short-term overfeeding in man: association with plasma insulin and triglyceride levels

Six pairs of male monozygotic (MZ) twins were submitted to a 22-day overfeeding period during which they ingested a daily surplus of 1,000 kcal above their individual daily energy needs in the form of a mixed diet. Serum lipids, lipoproteins, and apoprotein A and B concentrations were measured before and after the overfeeding period. Percentage of body fat, fasting plasma glucose, and insulin levels as well as plasma glucose and insulin concentrations after a glucose challenge were also measured before and after overfeeding. Results showed that before overfeeding, MZ twins exhibited a significant intrapair resemblance for total serum cholesterol (CHOL), triglycerides (TG), low density lipoprotein cholesterol (LDL-C), and for the high density lipoprotein-cholesterol/total cholesterol ratio (HDL-C/CHOL) (8.2 less than or equal to F ratios less than or equal to 32.7, P less than .01). The overfeeding experiment induced significant increases only in serum CHOL (P less than .01) and in serum LDL-C (P less than .05). However, although mean group values of serum TG, HDL-C, and HDL-C/CHOL ratio were not significantly modified by overfeeding, there were large interindividual variations in the response of these variables to the experiment. Results suggest that changes in serum TG, HDL-C, and in the HDL-C/CHOL ratio were significantly associated with the genotype of the subjects as a significant intrapair resemblance in the response to overfeeding was observed for these variables (0.69 less than or equal to r less than or equal to .85, P less than .05).(ABSTRACT TRUNCATED AT 250 WORDS)

Plasma cholesterol variation in the National Heart, Lung and Blood Institute Twin Study

Plasma cholesterol was measured in the fifth decade of the life of 249 pairs of monozygotic (MZ) and 262 pairs of dizygotic (DZ) World War II veteran twins and 70% of the same cohort 10 years later. There were no significant differences between the mean cholesterol values for MZ and DZ twins, and the within DZ pair mean squares were significantly larger than the within MZ pair mean squares for all of the cholesterol variables measured. However, the DZ twins were found to have greater total variance, positive skewness, and leptokurtosis than the MZ twins for total and high-density lipoprotein cholesterol, and the total/high-density ratio. Comparisons with published data revealed that the variance of DZ twins was similar to that of singletons while the MZ twins have smaller total variance, perhaps owing to a missing component of variation. Hypotheses for the source of the differences in the zygosity distributions are proposed including environmental influences (pre- or post-natal and within- or among-families), genetic differences, and selection at the time of induction into the armed services. Because of the differences in total variance of the two zygosities it is difficult to know which estimates of genetic variance or heritability have the least bias. However, these data provide clues that may lead to further understanding of sources of plasma cholesterol variation that could be important to the future understanding of risk for coronary heart disease.

Familial associations of lipids and lipoproteins in a highly consanguineous population: the Limone sul Garda study

The impact of genetic factors on the levels of plasma lipids and lipoproteins was evaluated in the total population of a small village. Limone sul Garda, separated up to recent years from neighboring communities, shows a high degree of consanguineity: major blood group phenotypes differ from nearby provinces for at least three gene frequencies. The absence of a difference in plasma lipid correlations between parent-pediatric offspring and parent-adult offspring, is consistent with the uniformity of living habits in the Limone community. Correlation coefficients proved highly significant only for plasma total cholesterol (range of r: 0.223 to 0.359). Differently from other reports, correlations for low density and high density cholesterolemias (LDL and HDL-C) were mostly nonsignificant in the parent-offspring comparisons. Similarly, only intersibling correlations for triglycerides (TG) were statistically significant. As indicated in other genetic studies on plasma lipoprotein levels (also in the Limone sul Garda study), TG and HDL-C were negatively correlated, whereas the correlation was positive between TG and LDL-C levels. The examined population does not, therefore, show a metabolic behavior at variance from other investigated groups. This study, offering a unique opportunity for enucleating genetic from environmental factors, suggests that most familial clusterings of lipoprotein concentrations are environmentally determined, whereas total cholesterolemia is controlled by autosomal mechanisms, without significant variations between sexes.

Genetic and environmental influences on the size and number of cells in the blood

The heritabilities of human blood cell characteristics were estimated in a study of 206 pairs of young adult twins, male and female. White cell numbers, indices related to circulating red cell mass (haemoglobin, red cell count, and haematocrit), and platelet numbers and size all appeared to be accounted for by genetic and nonshared environmental influences only. Mean cell volume (of the erythrocytes) appeared to be influenced by environmental factors shared by siblings as well as the other two sources of variation. Correlation between red cell count and haemoglobin is modulated by both genetic and environmental factors, but the negative correlation between red cell numbers and size is due mainly to genetic factors independent of those influencing haemoglobin. A significant negative correlation also exists between platelet numbers and size. In males, alcohol consumption increased mean cell volume, and genetic factors influencing alcohol consumption are partly responsible for the correlation between them.

Plasma cholesterol response to a change in dietary fat intake: a collaborative twin study

The rise of plasma cholesterol in response to an increase from 8 to 22.3% in saturated fatty acid of total intake was studied by comparing the concordance of change in pairs of young English and Italian monozygotic (33 pairs) and dizygotic (22 pairs) twins. LDL and total cholesterol rose about 0.6 mmol/l. All methods of analyses showed a marked genetic component influencing total and LDL cholesterol level. However, the genetic component of the increase in LDL and total cholesterol levels was small in Italy and absent in England. These results suggest that the response of plasma cholesterol to this dietary change is largely determined by environmental factors rather than inheritance.