Low-density Lipoprotein Subclasses Research Articles

Low-density lipoprotein particle size decreases during normal pregnancy in association with triglyceride increases.

To characterize the changes in low-density lipoprotein (LDL) peak particle diameter (diameter of the predominant LDL subclass) in relation to changes in serum triglyceride concentration during successive stages of normal gestation and postpartum. Nonfasting venous blood was obtained longitudinally during and after uncomplicated primiparous pregnancy from 10 nonsmoking women with no history of metabolic disorders. Plasma LDL diameter was determined by nondenaturing polyacrylamide gel electrophoresis. Serum concentrations of total cholesterol, triglyceride, apolipoprotein B, apolipoprotein A-I, and LDL-cholesterol were measured. Gestational changes were analyzed by one-way repeated-measures analysis of variance and the paired multiple comparison Student-Newman-Keuls test. Pearson coefficients were computed for correlation of serum lipids and LDL diameter. Low-density lipoprotein diameter decreased progressively with advancing gestation, evident by 16-20 weeks relative to 5-12 weeks. Seven of 10 cases were subclass pattern B (diameter less than 255 A) by term, indicating that small, dense particles predominated. The average diameter decrease from early to late gestation was 13 A. All subjects reverted to subclass pattern A (diameter 255 A or more) by 6-12 weeks postpartum, indicating prevalence of large, buoyant LDL. Low-density lipoprotein diameter correlated inversely with concentrations of serum triglyceride (r = -.61, P < .0001), apo B (r = -.66, P < .0001), cholesterol (r = -.53, P < .001), LDL cholesterol (r = -.45, P < .005), and apo A-I (r = -.39, P < .02). Gestational triglyceride increases are accompanied by progressive decreases in LDL diameter in a majority of cases. These changes undergo reversal postpartum and therefore are transient. Small, dense LDL particles have a number of properties capable of altering vascular function. However, the consequences of the gestational LDL size decrease for maternal and fetal metabolism remain unknown.

Beneficial effect of gemfibrozil on the chemical composition and oxidative susceptibility of low density lipoprotein: a randomized, double-blind, placebo-controlled study

Previous reports have shown that administration of fibrates can reduce coronary events and also improve plasma lipid levels. Oxidative modification of low density lipoprotein has been implicated in the pathogenesis of atherosclerosis, and the resistance of low density lipoprotein (LDL) to in vitro oxidation has been found to be correlated with the extent of atherosclerosis. We performed a double-blind, placebo-controlled intervention trial to establish whether gemfibrozil could improve resistance of LDL to oxidation in patients with hyperlipidemia. Patients were randomly assigned to treatment with gemfibrozil (450 mg, twice a day, n=10) or placebo ( n=9) for 8 weeks. Blood samples were obtained after an overnight (12 h) fast. Gemfibrozil administration significantly reduced total plasma cholesterol and triglyceride levels and changed the LDL from small, dense particles (pattern B, ≤25.5 nm) to larger, more buoyant particles (pattern A, >25.5 nm). Gemfibrozil significantly increased the lag time of LDL oxidation in vitro by 18.2% from 45.5±8.0 min at week 0 to 53.4±11.4 min at week 8, but did not change LDL vitamin E and β-carotene concentrations. Surprisingly, gemfibrozil significantly decreased LDL lipid peroxides by −33.1% and increased the LDL vitamin E/lipid peroxide ratio by 67.6% from 1.3±0.5 at week 0 to 2.1±0.9 at week 8. These results demonstrate that gemfibrozil treatment can render LDL less susceptible to oxidative modification while reducing plasma cholesterol and triglyceride and improving LDL subclass pattern. This antioxidative effect of gemfibrozil on LDL may be one of the factors which could delay the progression of atherosclerosis.

Triglycerides and atherogenic lipoproteins: rationale for lipid management

Epidemiologic and clinical studies have demonstrated a relation between plasma triglyceride levels and risk of coronary artery disease and an amplification of risk with combined elevations of triglyceride and low-density lipoprotein (LDL) cholesterol. In patients with coronary disease, angiographic progression and clinical events have been correlated with concentrations of smaller very-low-density lipoproteins (VLDL) and intermediate-density lipoproteins (IDL), consistent with evidence for enhanced atherogenicity of lipolytic products of triglyceride-rich lipoprotein metabolism, including postprandial lipoproteins. IDL levels also have been shown to be strongly and independently predictive of progression of carotid artery intimal–medial thickness, a measure of early atherogenesis that is related to coronary disease risk. Although there is evidence that these triglyceride-rich lipoprotein species may have direct atherogenic effects, other lipoprotein changes associated with altered triglyceride metabolism may be of particular importance in the development of coronary artery disease. These include reductions in high-density lipoprotein (HDL) and increases in small, dense LDL particles (LDL subclass pattern B). Because of the strong interrelations among elevated triglyceride, reduced HDL, and small dense LDL, it is difficult to use statistical techniques to determine the independent contributions of these traits to coronary disease risk. Based on their biologic properties, it is likely that each are involved in multiple steps of the disease process. Moreover, this cluster of lipoprotein changes is associated with other conditions that can promote vascular disease, including increases in coagulation factors and reduced insulin sensitivity. Analyses from intervention trials in patients with coronary disease have indicated that measurement of plasma triglyceride and LDL particle distributions can be of value in predicting the benefits of specific lipid-altering therapies on disease progression.

Atherogenicity of Triglyceride-Rich Lipoproteins

There is increasing evidence that alterations in metabolism of triglyceride-rich lipoproteins are of importance in the pathogenesis of atherosclerosis and its clinical consequences. Particles with the characteristics of triglyceride-rich lipoprotein remnants have been related to the extent and severity of atherosclerosis in humans and in animal models. These particles can be identified using ultracentrifugal procedures as small, very low-density lipoprotein (VLDL) and intermediate-density lipoprotein (IDL) with Svedberg flotation rates (S f) of 12–60. Postprandial triglyceride levels also have been related to risk of coronary artery disease, consistent with a pathologic role for remnant lipoproteins. In studies in which measurements of lipoprotein subfractions have been carried out, levels of IDL have been more predictive than low-density lipoprotein (LDL) of atherosclerosis progression as assessed by coronary artery angiography or carotid artery ultrasonography. These findings suggest that a considerable portion of the coronary disease risk attributed to LDL may be accounted for by the IDL particles included in standard LDL measurements. Other metabolic changes associated with increased levels of plasma triglyceride may also adversely affect cardiovascular disease risk. These include reductions in HDL-cholesterol and apoprotein A1, increased levels of small dense LDL particles, redistribution of apoC-III from HDL to apoB-containing lipoproteins, diminished insulin sensitivity, and procoagulant changes, including increased levels of the fibrinolysis inhibitor, plasminogen-activator inhibitor-1 (PAI-1). A predominance of small dense LDL (subclass pattern B) is a discrete marker for this cluster of interrelated abnormalities and is found in 40–50% of patients with coronary artery disease. Therapeutic interventions with favorable effects on components of this dysmetabolic profile appear to be of value in decreasing atherosclerosis risk in a substantial proportion of the population.

Electrophoretic separation of LDL and HDL subclasses.

An extensive literature supports the hypothesis that measurements of lipoprotein subclasses provide a more detailed reflection of lipoprotein metabolism and a more accurate prediction for risk of cardiovascular disease. Lipoprotein subclasses have been resolved on the basis of density fractionation, gel filtration, and immunological and electrophoretic procedures. Perhaps one of the most widely used methods has been the resolution of lipoproteins on the basis of particle size using nondenaturing pore gradient gel electrophoresis (GGE). GGE procedures have been based on highly reliable gradient gels supplied in two formats: PAA2/16 for resolving low-density lipoproteins (LDLs) and PAA4/30 for resolving high-density lipoprotems (HDLs) (Pharmacia, Piscataway, NJ). These gel formats produce repeatable and detailed separations of the two major classes of lipoprotein particles, appropriate for quantitative analyses of lipoprotein phenotypes.

Effect of Estrogen on Very Low Density Lipoprotein and Low Density Lipoprotein Subclass Metabolism in Postmenopausal Women1

Estrogen decreases low density lipoprotein (LDL) particle size, and smaller LDL particles are associated with coronary atherosclerosis. To understand the metabolic basis for this change, we studied the effect of oral 17 beta-estradiol (2 mg/day) on apolipoprotein B-100 (apoB) metabolism, in eight healthy postmenopausal women. The study was a randomized, double blinded, placebo-controlled, cross-over trial with intervention sequences of 6 weeks each. ApoB in very low density lipoprotein, intermediate density lipoprotein, and LDL subclasses was endogenously labeled with [D3]L-leucine, and metabolic rates were calculated by computer modeling. The overall effect of oral estrogen therapy on apoB metabolism was to accelerate the fractional catabolic rates of all particles studied and production rates of all except IDL. For light LDL (density = 1.019-1.036 g/mL), estrogen increased the mean fractional catabolic rate by 63% from 0.59 to 0.96 pools/day (P = 0.02), whereas the production rate increased by a lesser amount (42%) from 575 to 817 mg/day (P = 0.10). These metabolic changes reduced light LDL cholesterol and apoB concentrations by 26% (P = 0.005) and 19% (P = 0.03), respectively. In contrast, dense LDL (density = 1.036-1.063 g/mL) cholesterol and apoB concentrations were unchanged by the intervention, as both the apoB fractional catabolic rate and production rate were significantly increased by similar amounts, 39% (from 0.41 to 0.57 pools/day, P = 0.01) and 38% (from 434 to 601 mg/day; P = 0.003), respectively. Estrogen decreased the predominant LDL peak particle size from 273 to 268 A (P = 0.04). Thus, estrogen therapy increases the clearance of both light and dense LDL, counteracting increases in production rates. The reduced plasma residence times of light and dense LDL both may be antiatherogenic, even though, for dense LDL, the concentration did not change.

Effect of Estrogen on Very Low Density Lipoprotein and Low Density Lipoprotein Subclass Metabolism in Postmenopausal Women

Effect of Estrogen on Very Low Density Lipoprotein and Low Density Lipoprotein Subclass Metabolism in Postmenopausal Women

Diet-gene interactions in human lipoprotein metabolism.

Genetic variation can influence the effects of hypocholesterolemic dietary interventions on lipoproteins involved in coronary artery disease (CAD). Individuals with the E4 allelic variant of the apo E gene exhibit greater low-density lipoprotein (LDL) cholesterol reductions on low-fat, low-cholesterol diets than subjects with other alleles. Another apolipoprotein structural variant, apo A-IV-2, attenuates the response of LDL cholesterol to dietary cholesterol. Other studies have associated lipoprotein response to dietary modifications with DNA polymorphisms in the genes for apo B and the LDL receptor, and in the promoter region of the apo A-I gene. Studies in our laboratory involving variation in dietary fat and carbohydrate intake have demonstrated that alleles at the apo E gene locus are associated with changes in large, buoyant but not smaller, denser LDL subclasses. On the other hand, a low-fat diet induces a reduction in small, dense LDL in individuals with a genetically influenced metabolic profile characterized by a predominance of these particles. Moreover, reductions in LDL cholesterol and apo B in these subjects are greater than in the majority of subjects with larger LDL. Since in humans a predominance of small LDL signifies a higher risk for coronary artery disease than that associated with larger LDL, metabolic and genetic factors contributing to the small LDL trait may account for a substantial portion of the coronary risk benefit attributed to reduced-fat diets. Studies in large population groups or in suitable animal models will be necessary to determine the impact of genetic influences on dietary response affecting lipoprotein metabolism and their interactions with other environmental and hormonal factors.

Low-density lipoprotein subclasses: mechanisms of formation and modulation.

A predominance of small, dense LDL has been consistently associated with an increased relative risk of CHD and may be directly involved in the atherogenic process via a number of credible mechanisms (Austin, 1991 ; Griffin, 1995). Whilst plasma triacylglycerols (TAG) measured in the post-absorptive state have emerged in all studies as the major determinant of increased small, dense LDL, postprandial events may be paramount in influencing not only fasting TAG levels, but also the heterogeneity of TAG-rich lipoproteins (TGRL), and the lipolytic and lipid transfer reactions which modulate LDL sub

Associations of age, adiposity, menopause, and alcohol intake with low-density lipoprotein subclasses.

We used nondenaturing polyacrylamide gradient gel electrophoresis to examine the associations of age, adiposity, menopause, and alcohol intake with LDL subclasses in 355 individuals. The absorbency of protein stain was used as an index of mass concentrations at intervals of 0.05 nm within seven LDL subclasses: LDL-IVB (22.0 to 23.2 nm), LDL-IVA (23.3 to 24.1 nm), LDL-IIIB (24.2 to 24.6 nm), LDL-IIIA (24.7 to 25.5 nm), LDL-II (25.5 to 26.4 nm), LDL-I (26.0 to 28.5 nm), and intermediate-size lipoproteins (ISL, 28.0 to 32.0 nm). Age and alcohol intake were obtained from questionnaires, and body mass index was computed from clinic measurements of weight and height. In adult men, body mass index correlated positively with LDL-III, and alcohol intake correlated positively with larger LDL-I. Age was positively correlated with LDL-IIIA and ISL in both men and women and with LDL-IIIB and LDL-II in women. Postmenopausal women had higher LDL-IIIA, LDL-II, and ISL than both premenopausal and premenarchal females. Adult males, > or = 18 years old, had higher levels of LDL-IIIA and LDL-II than younger males. Adjustment for fasting plasma triglyceride levels eliminated the significant associations between age and LDL-IIIA in both men and women and between age and LDL-II in women. Partial correlation analyses showed that reductions in the LDL peak diameter associated with increasing age, male sexual maturation, menopause, and adiposity are attributable to increases in the LDL-IIIA subclass. Thus, densitometric measurements of protein-stained gradient gels reveal specific relationships between LDL subclasses and age, adiposity, and alcohol intake beyond those identified by the LDL peak or average diameter.

Low‐density lipoprotein subfraction profiles in dialysis patients

Summary: Uraemic dyslipidaemia is a major risk factor for cardiovascular disease in end‐stage renal failure patients. In patients without renal failure, high levels and qualitative abnormalities of low‐density lipoprotein (LDL) are known to be atherogenic. Recently, LDL subfraction analysis has associated premature coronary artery disease with a high prevalence of small, dense LDL particles characterizing the LDL subclass phenotype B. We therefore examined the lipid profiles, LDL subfraction distribution and phenotypes in our population of haemodialysis (HD; n= 30) and peritoneal dialysis patients (PD; n= 17), and compared them to 40 asymptomatic, non‐uraemic volunteers. Dialysis patients had significantly higher triglyceride and VLDL cholesterol concentrations and lower HDL cholesterol and smaller LDL peak particle diameters. PD patients had significantly higher total cholesterol, glycated haemoglobin and fasting blood glucose levels with smaller LDL peak particle diameters (24.4 [0.1] vs 24.8 [0.1 nm] than HD. Both groups showed significant negative correlations between plasma triglyceride and LDL peak particle diameter, and positive correlations between HDL cholesterol and LDL peak particle diameter. All the PD patients expressed the B phenotype (LDL peak diameter ± 25.5 nm) compared to 73% of HD patients. This study demonstrates that HD and especially PD patients have atherogenic lipid profiles which are associated with a predominance of small dense LDL particles and the highly atherogenic LDL subclass phenotype B.

Reduced beta-strand content in apoprotein B-100 in smaller and denser low-density lipoprotein subclasses as probed by Fourier-transform infrared spectroscopy.

The secondary structure of apolipoprotein B-100 in low-density lipoprotein (LDL) subfractions was analysed by Fourier-transform IR spectroscopy. LDLs were isolated in three density ranges by gradient centrifugation of human plasma from healthy volunteers. The spectra revealed differences in the lipid content and composition of the three LDL fractions. The secondary structure of apolipoprotein B-100 was the same in the two fractions corresponding to the large less-dense LDL particles, whereas a lower content of beta-strands was found in the third fraction corresponding to the smaller denser LDL particles. Analysis of the spectroscopic data suggests that, in the same set of LDL subfractions, the particle size is probably the cause of the observed differences in apolipoprotein B-100 secondary structure.

Characterization of low-density lipoprotein subclasses in children

Low-density lipoprotein (LDL) particles are heterogeneous in density, size, and chemical composition, and this heterogeneity is thought to be genetically influenced. In the present study, plasma LDL subclasses in 248 children aged 7 to 13 years were analyzed by gradient gel electrophoresis. The prevalence of small dense LDL (SDLDL), a potent atherogenic LDL, was 9.3%, which is lower than that reported in adults. Furthermore, children with this LDL subclass showed increased body fatness and dyslipidemia, including elevated plasma triglyceride and apolipoprotein (apo) B concentrations and decreased plasma high-density lipoprotein (HDL) cholesterol and apo A-I concentrations, compared with children without this phenotype. These findings suggest that in addition to genetic factors, environmental factors that affect these cardiovascular risk factors may also influence expression of the SDLDL subclass.

Genetic epidemiology of dyslipidaemia and atherosclerosis.

The clinical relevance of the heterogeneity in the size and density of low-density lipoprotein (LDL) particles is now widely recognized. The evidence from epidemiological studies, family studies and twins studies demonstrates that small, dense LDL (LDL subclass phenotype B) is a common, genetically influenced risk factor for coronary heart disease (CHD). Several atherogenic mechanisms have been proposed to explain the association of small, dense LDL with CHD, including evidence that small, dense LDL is an integral feature of the insulin resistance syndrome. Furthermore, a recent study in elderly Finnish men and women has shown that phenotype B prospectively predicts non-insulin-dependent diabetes mellitus (NIDDM). In addition, ongoing studies of large Japanese-American kindreds will provide valuable data for evaluating small, dense LDL as a marker for genetic susceptibility to both CHD and NIDDM in a high-risk ethnic group.

Identification and cholesterol quantification of low density lipoprotein subclasses in young adults by VAP-II methodology

Low density lipoprotein (LDL) particles are heterogeneous in size, density, and chemical composition; small, dense LDL may be more atherogenic than large, buoyant LDL. We have developed a rapid microscale method called LDL VAP-II (Vertical Auto Profile-II) for quantification of cholesterol in LDL subclasses. The method is based upon a short (1 h) single vertical spin density-gradient ultracentrifugation and on-line VAP-II analyzer. LDL VAP-II is rapid and reproducible. Using this method five LDL subclasses, designated as LDL-1 (most buoyant) through LDL-5 (most dense), have been identified in a population consisting of 195 medical students (ages, 22-29 years). The Rf (relative position of the major LDL peak in the density gradient; the higher the Rf value, the lower the peak density) was significantly positively correlated with cholesterol levels of high density lipoprotein (HDL) (r = 0.594), HDL3 (0.350) and HDL2 (0.625), and significantly negatively correlated with triglycerides (TG) (-0.355) and cholesterol levels of very low density lipoprotein (VLDL) (-0.386) and intermediate density lipoprotein (IDL) (-0.432). These results are consistent with those obtained by other investigators. The Rf value was significantly correlated with peak particle diameter as determined by non-denaturing gradient gel electrophoresis (r = 0.859). In a forward stepwise multivariate analysis comparing Rf with sex, VLDL, LDL, Lp[a], IDL, HDL3, HDL2, and triglyceride, only HDL2 remained in the model.

Autoantibodies against malondialdehyde-modified LDL are elevated in subjects with an LDL subclass pattern B

Small low density lipoproteins (LDL) are more susceptible to in vitro oxidation than larger LDL. To study whether this leads to more oxidation of small LDL in vivo, we determined the level of autoantibodies against malondialdehyde-modified LDL (MDA-LDL) in subjects with small or large LDL (LDL subclass pattern B or A) by ELISA. The study group consisted of 92 subjects with coronary heart disease without severe hypercholesterolemia (mean total plasma cholesterol 5.9 ± 0.8 mM), 46 with an LDL subclass pattern A and 46 with an LDL subclass pattern B. In the subjects with LDL subclass pattern B the titre of autoantibodies of the IgM class against MDA-LDL was 29% higher than in the subjects with LDL subclass pattern A ( P < 0.0001). The concentration of the anti-MDA-LDL autoantibodies of the IgM class was 58% higher in the patients with the pattern B than in the patients with the pattern A ( P < 0.0001). There was no statistically significant difference in the titre or concentration of autoantibodies of the IgG class between subjects with LDL subclass patterns A and B. Besides plasma triglyceride and HDL cholesterol, the titre and concentration of the IgM autoantibodies were found to be independent predictors of the LDL subclass pattern. These results show that small LDL are associated with higher autoantibody levels than large LDL. Based on the assumption that the level of autoantibodies against MDA-LDL represents the rate of LDL oxidation in vivo, we conclude that in vivo small LDL is more readily oxidised than larger LDL. These results support the hypothesis that the LDL subclass B represents an atherogenic condition in itself.

Associations of hepatic and lipoprotein lipase activities with changes in dietary composition and low density lipoprotein subclasses.

To test whether lipoprotein lipase or hepatic lipase activities are associated with lipoprotein subclasses, and to assess the effects of dietary manipulations on these associations, enzyme activities were measured in postheparin plasma (75 U heparin/kg) from 43 healthy men who were randomly allocated to a low-fat (24% fat, 60% carbohydrate) and a high-fat (46% fat, 38% carbohydrate) diet for 6 weeks each in a cross-over design. The high-fat diet significantly increased both lipoprotein lipase (+20%, P = 0.02) and hepatic lipase (+8%, P = 0.007) activities. On both diets, hepatic lipase activity was significantly positively correlated (P < 0.01) with plasma apolipoprotein (apo)B concentrations, and with levels of small dense low density lipoprotein (LDL) III, measured by analytic ultracentrifugation as mass of lipoproteins of flotation rate (Sof) 3-5, while lipoprotein lipase activity was inversely associated with levels of LDL III (P < 0.05). Despite the cross-sectional correlations, increased hepatic lipase activity was not significantly correlated with the reduction in LDL III mass observed on the high-fat diet. Rather, changes in hepatic lipase were correlated inversely with changes in small very low density lipoproteins (VLDL) of Sof 20-40, and small intermediate density lipoproteins (VLDL) of Sof 10-16. Moreover, changes in lipoprotein lipase activity were not significantly correlated with changes in small LDL, but were positively associated with changes in small IDL of Sof 10-14, and large LDL I of Sof 7-10. Thus, while increased levels of small dense LDL are associated with a metabolic state characterized by relatively increased hepatic lipase and decreased lipoprotein lipase activity, changes in these enzymes do not appear to be primary determinants of diet-induced changes in levels of this LDL subfraction. On the other hand, increased lipoprotein lipase activity induced by high-fat feeding may contribute to the accumulation in plasma of both large LDL I and small IDL, whereas increased hepatic lipase may promote catabolism or clearance of triglyceride-rich lipoprotein remnants.

Modulation of low density lipoprotein subclasses by alimentary lipemia in control and normotriglyceridemic non-insulin-dependent diabetic subjects

Conventional factors do not fully account for the increased cardiovascular risk in NIDDM but, because of the underlying disorders in lipid metabolism, the postprandial state can be expected to induce temporary changes of a potentially atherogenic nature. The response to a 1000-kcal meal (70% lipid; 100 000 IU vitamin A) over 8 h was compared in 10 normoponderal, normotriglyceridemic NIDDM male patients and 12 controls. In patients lipolysis was normal, but remnant clearance was delayed ( P < 0.02) and apo E concentrations were lower. LDL-C decreased postprandially, more in patients ( P < 0.05), while LDL-PL accumulated in controls but not in patients. As a result UC:PL decreased in controls ( P < 0.05) not in patients. The distribution of LDL subclasses shifted towards large particles in controls (LDL-I, 42%; LDL-II, 50%; LDL-III, 7.6% at 6 h) and smaller ones in patients (LDL-I, 29%; LDL-II, 56%; LDL-III, 16% at 6 h). In controls only, the percentage of LDL-III correlated negatively with apo E ( r = − 0.97, P < 0.001) suggesting that apo E promotes removal of light particles before they reach LDL-III and may be a limiting factor in patients. We conclude that the postprandial state is potentially more atherogenic in normoponderal, normotriglyceridemic patients than in controls: remnant clearance is delayed, the UC:PL ratio of LDL fails to decrease postprandially as it does in controls, limiting the acceptor capacity of LDL for UC, and the distribution of LDL subclasses is shifted towards a more atherogenic profile.

Characterization of low-density lipoprotein subclasses: methodologic approaches and clinical relevance.

Emerging evidence suggests that subclasses of LDL, characterized by variations in density, size, and chemical composition of LDL particles, are of important clinical significance. Accumulating case-control studies demonstrate that a predominance of small, dense LDL particles (LDL subclass phenotype B) is associated with an increased risk of coronary heart disease, and several potential atherogenic mechanisms have been proposed. New studies also demonstrate that LDL subclass phenotype B is an integral feature of the insulin resistance syndrome. In addition to the well-documented genetic influences on LDL subclass distributions, lipid-altering drugs, diet, and exercise all appear to affect LDL subclasses. A better understanding of this combination of genetic and environmental influences could lead to the development of effective intervention strategies for the prevention of coronary heart disease.

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.