Novel genes for familial combined hyperlipidemia

Clinical and laboratory assessment of cardiovascular risk in children: Guidelines for screening, evaluation, and treatment

The early lesions of atherosclerosis begin in childhood and are related to antecedent cardiovascular disease (CVD) risk factors. Environmental and genetic factors (eg, diet, obesity, exercise, and certain inherited dyslipidemias) influence progression of such lesions. Identification of youth at risk for atherosclerosis includes an integrated assessment of these predisposing factors. Treatment starts with a diet low in total and saturated fat and cholesterol, use of water-soluble fiber, plant stanols and plant sterols, weight control, and exercise. Drug therapy, for example, with inhibitors of hydroxymethylglutaryl-CoA reductase, bile acid sequestrants, and cholesterol absorption inhibitors, can be considered in those with a positive family history of premature CVD and low-density lipoprotein cholesterol >160 mg/dL after dietary and hygienic measures. Candidates for drug therapy often include those with familial hypercholesterolemia, familial combined hyperlipidemia, the metabolic syndrome, polycystic ovarian syndrome, type 1 diabetes, and the nephrotic syndrome. Such dietary and drug therapy appears safe and efficacious. Early identification and treatment of youth with CVD risk factors and dyslipidemia are likely to retard the atherosclerotic process. Optimal detection and treatment of high-risk children either from the general population or from families with premature CVD will require a comprehensive universal screening and evaluation program.

Association of stearoyl-CoA desaturase 1 activity with familial combined hyperlipidemia

OBJECTIVE

Stearoyl-CoA desaturase 1 (SCD1) is the rate-limiting enzyme involved in the synthesis of monounsaturated fatty acids, and in mice SCD1 activity is associated with plasma triglyceride levels. We used the fatty acid desaturation index (the plasma ratio of 18:1/18:0) as a marker of SCD1 activity to investigate the relationship of SCD1 to familial combined hyperlipidemia (FCHL).

METHODS AND RESULTS

The fatty acid desaturation index was measured in 400 individuals from 18 extended FCHL pedigrees. FCHL-affected individuals exhibited increased SCD1 activity when compared to unrelated controls (P < 0.0001). The fatty acid desaturation index was found to be highly heritable (h(2) = 0.48, P = 2.2 x 10(-11)) in this study sample. QTL analysis in 346 sibling pairs from 18 FCHL families revealed suggestive linkage of the desaturation index to chromosomes 3p26.1 to 3p13 (z = 2.7, P = 0.003), containing the peroxisome proliferator-activated receptor gamma (PPARgamma) gene, and 20p11.21 to 20q13.32 (z = 1.7, P = 0.04), containing the hepatocyte nuclear factor 4, alpha (HNF4alpha) gene. A specific haplotype of HNF4alpha was found to be associated with the desaturation index in these FCHL families (P = 0.002).

CONCLUSIONS

Our results demonstrate that the fatty acid desaturation index is a highly heritable trait that is associated with the dyslipidemia observed in FCHL.

Apolipoprotein E polymorphism influences lipid phenotypes in Chinese families with familial combined hyperlipidemia

BACKGROUND

Apolipoprotein E (apoE) polymorphism is associated with changes in the lipoprotein profile of individuals with familial combined hyperlipidemia (FCHL), but its effects on the lipoprotein profiles of members of Chinese families with FCHL remain uncertain.

METHODS AND RESULTS

43 FCHL families (n=449) and 9 normolipidemic families (n=73) were recruited to assess the influence of apoE polymorphism on plasma lipids. The relative frequency of the epsilon4 allele in affected and unaffected FCHL relatives, spouses and normolipidemic members was 13.8%, 5.3%, 9.1% and 6.8%, respectively, with a significantly higher frequency in affected FCHL relatives, compared with unaffected FCHL relatives or normolipidemic members (p=0.0002 or p=0.029). In FCHL relatives, the apoE4 subset (E4/4 and E4/3) exhibited significantly higher levels of apoB, total cholesterol and low-density lipoprotein-cholesterol (LDL-C) than did the apoE3 (E3/3) subset, especially in women (all p<0.05), and there was significant elevation of LDL-C concentrations in men only (p<0.05). In men, the apoE2 (E3/2) subset indicated a decreased level of apoB and increased apoA1 compared with those in the apoE3 subset (p<0.05).

CONCLUSIONS

ApoE polymorphism appears to be associated with variance of the lipoprotein phenotype in Chinese families with FCHL.

The lipoprotein lipase gene in combined hyperlipidemia: evidence of a protective allele depletion

Background

Lipoprotein Lipase (LPL), a key enzyme in lipid metabolism, catalyzes the hydrolysis of triglycerides (TG) from TG-rich lipoproteins, and serves a bridging function that enhances the cellular uptake of lipoproteins. Abnormalities in LPL function are associated with pathophysiological conditions, including familial combined hyperlipidemia (FCH). Whereas two LPL susceptibility alleles were found to co-segregate in a few FCH kindred, a role for common, protective alleles remains unexplored. The LPL Ser447Stop (S447X) allele is associated with anti-atherogenic lipid profiles and a modest reduction in risk for coronary disease. We hypothesize that significant depletion of the 447X allele exists in combined hyperlipidemia cases versus controls. A case-control design was employed. The polymorphism was assessed by restriction assay in 212 cases and 161 controls. Genotypic, allelic, and phenotypic associations were examined.

Results

We found evidence of significant allelic (447X_control: 0.130 vs. 447X_case: 0.031, χ² = 29.085; 1df; p < 0.001) and genotypic association (SS: 0.745 vs. 0.939, and SX+XX: 0.255 vs. 0.061) in controls and cases, respectively (χ² = 26.09; 1df; p < 0.001). In cases, depletion of the 447X allele is associated with a significant elevation in very-low-density lipoprotein cholesterol (VLDL-C, p = 0.045). Consonant with previous studies of this polymorphism, regression models predict that carriers of the 447X allele displayed significantly lower TG, low-density lipoprotein cholesterol (LDL-C) and TG/high-density lipoprotein cholesterol (HDL-C) ratio.

Conclusion

These findings suggest a role for the S447X polymorphism in combined hyperlipidemia and demonstrate the importance of evaluating both susceptibility and protective genetic risk factors.

Impaired endothelium-dependent vascular reactivity in patients with familial combined hyperlipidaemia

BACKGROUND

Familial combined hyperlipidaemia (FCHL) is associated with a markedly increased risk of premature coronary artery disease. This study was designed to evaluate whether preclinical atherosclerotic functional abnormalities are detectable in the arteries of patients with FCHL.

METHODS

60 subjects were recruited for the study: 30 probands of families with FCHL (mean (standard deviation (SD)) age 48 (10) years, 77% men), defined by fasting total plasma cholesterol or triglyceride concentration >250 mg/dl (>6.5 mmol/l cholesterol, >2.8 mmol/l triglyceride) and by the occurrence of multiple lipoprotein phenotypes within a family, and 30 age-matched and sex-matched healthy controls. All subjects underwent high-resolution B-mode ultrasound examination and the brachial arterial reactivity, a marker of endothelial function, was measured by a semiautomated computerised program. Lipid profile, resting blood pressure, body mass index (BMI), smoking status, insulin and homocysteine levels were also determined.

RESULTS

Compared with controls, patients with FCHL had significantly higher BMI, diastolic blood pressure and insulin levels. No difference was observed in baseline brachial diameter between the two groups (mean (SD) 3.45 (0.51) mm for FCHL v 3.60 (0.63) mm for controls; p = 0.17). In response to flow increase, the arteries of the controls dilated (mean (SD) 8.9% (4.9%), range 2.3-20.8%), whereas in the patients with FCHL, brachial arterial reactivity was significantly impaired (5.5% (2.5%), range 0-10.1%; p = 0.002). In multivariate linear regression analysis, apolipoprotein B and BMI were independent determinants of brachial artery response to reactive hyperaemia.

CONCLUSIONS

The findings of our study suggest that vascular reactivity is impaired in the arteries of patients with FCHL.

Genetic variation in the hepatic lipase gene is associated with combined hyperlipidemia, plasma lipid concentrations, and lipid-lowering drug response

BACKGROUND

Combined hyperlipidemia (CHL) is a very frequent dyslipidemia, being lipid-lowering drugs often necessary in its management. Some genetic loci have been associated with CHL, and modulation of lipid-lowering treatment by genetic polymorphisms has been reported. We have investigated whether common polymorphisms in the hepatic lipase gene (LIPC) influence the baseline lipid concentration and the response to atorvastatin or bezafibrate in patients with CHL.

METHODS

Two genetic polymorphisms in LIPC (-514C-->T and +651A-->G) were determined by polymerase chain reaction and restriction analysis in 118 subjects of the ATOMIX (Atorvastatin in Mixed dyslipidemia) study who were randomized to treatment with either atorvastatin or bezafibrate and in 114 normolipidemic controls.

RESULTS

The -514T allele frequency was higher in the ATOMIX group (0.297) than in the control group (0.193) (P = .01). The -514T allele carriers in the control group showed higher high-density lipoprotein cholesterol (HDL-C) concentrations than the -514C homozygotes, 50.8 +/- 1.86 versus 45.9 +/- 1.40 mg/dL (P = .02). The +651G carriers in the ATOMIX group showed lower total cholesterol and low-density lipoprotein cholesterol than the +651A homozygotes, 274 +/- 3.72 and 181 +/- 3.50 mg/dL versus 289 +/- 4.0 and 194 +/- 3.76 mg/dL, respectively (P < .01). Homozygotes for the -514C allele on bezafibrate treatment had greater decrease in triglycerides and greater increase in HDL-C than -514T allele carriers after 12 months of bezafibrate treatment, -39.4% and +35.8% versus -25.5% and +20.4%, respectively (P = .080 and P = .007, respectively).

CONCLUSIONS

A higher frequency of the -514T allele of LIPC suggests a role of this locus in the pathogenesis of CHL. The -514T allele is associated with higher HDL-C concentration in normolipidemic population. The -514C-->T polymorphism modulates the lipid-lowering response to bezafibrate, with a better effect in homozygous CC subjects.

Treatment of dyslipidemia in children and adolescents

The early lesions of atherosclerosis begin in childhood, and are related to antecedent cardiovascular disease risk factors. Environmental and genetic factors such as diet, obesity, exercise, and certain inherited dyslipidemias influence the progression of such lesions. The identification of youth at risk for atherosclerosis includes an integrated assessment of these predisposing factors. Treatment starts with a diet low in total and saturated fat and cholesterol, the use of water-soluble fiber and plant sterols, weight control, and exercise. Drug therapy, for example, with inhibitors of hydroxymethylglutaryl CoA reductase, bile acid sequestrants, and cholesterol absorption inhibitors, can be considered in those with a positive family history of premature coronary artery disease and a low-density lipoprotein cholesterol above 160 mg/dL, after dietary and hygienic measures. Candidates for drug therapy often include those with familial hypercholesterolemia, familial combined hyperlipidemia, the metabolic syndrome, polycystic ovarian syndrome, type I diabetes, and the nephrotic syndrome. We review the safety and efficacy of dietary and drug therapy, and propose an updated diagnostic and therapeutic algorithm that includes the metabolic syndrome. The early identification and treatment of youth with dyslipidemias is likely to retard the atherosclerotic process.

Increased lipogenesis and fatty acid reesterification contribute to hepatic triacylglycerol stores in hyperlipidemic Txnip-/- mice

The effect of decreased fatty acid oxidation on liver lipid metabolism in HcB-19 mice, a mouse model of hyperlipidemia (Txnip(-/-)), was investigated using metabolic labeling. De novo cholesterol synthesis and de novo lipogenesis were quantified using 1-(13)C(1) acetic acid, and liver triacylglycerol (TAG) derived from dietary fatty acids was quantified using dietary glyceryl tri(hexandecanoate-d(31)). Tissue samples were analyzed for TAG, free cholesterol (FC), and cholesterol ester (CE) content. Txnip(-/-) mice had significantly elevated (P < 0.05) serum nonesterified fatty acids compared with wild-type (WT) littermates; their livers weighed more and contained more TAG and total cholesterol. Txnip(-/-) liver also contained measurable CE; CE was not detectable in WT mice. Liver CE content was elevated despite lower cholesterol fractional synthesis rates (16 vs. 31%/d in Txnip(-/-) and WT mice, respectively). FC absolute synthesis rate (ASR) in WT mice (0.28 +/- 0.0 micromol/d) was similar to the combined synthesis rates of FC (0.13 +/- 0.10 micromol/d) and CE (0.10 +/- 0.00 micromol/d) in Txnip(-/-) mice. Lipogenesis, as assessed by TAG-palmitate ASR, was significantly greater in Txnip(-/-) mice (1.47 +/- 0.08 vs. 0.49 +/- 0.06 micro mol/d) and liver fatty acid synthase activity was also higher (7.96 +/- 2.53 vs. 4.83 +/- 1.44 U/mg protein). Both elevated lipogenesis and increased fatty acid reesterification to glycerol and cholesterol contributed to fat in the livers of Txnip(-/-) mice. These data support elevated fatty acid synthesis as the primary contributor to liver TAG in Txnip(-/-) mice, although increased esterification of fatty acids also contributed to excess liver TAG. The absolute total cholesterol synthesis rate was not altered, but esterification of fatty acids to cholesterol provided an additional means to buffer physiologically the negative results of excess fatty acid availability.

Rat model of familial combined hyperlipidemia as a result of comparative mapping

Total genome scan was carried out in 266 F2intercrosses from the Prague hypertriglyceridemic (HTG) rat that shares several clinical characteristics with human metabolic syndrome. Two loci for plasma triglycerides (TG) were localized on chromosome 2 (Chr 2) (LOD 4.4, 3.2). The first locus overlapped with the rat syntenic region of the human locus for the metabolic syndrome and for small, dense LDL, while the second overlapped with the syntenic region of another locus for small, dense LDL in humans by the comparative mapping approach. Loci for TG on rat Chr 13 (LOD 3.3) and Chr 1 (LOD 2.7) overlapped with the syntenic region of loci for human familial combined hyperlipidemia (FCHL) in Finnish and Dutch populations, respectively. The concordances of loci for TG localized in this study with previously reported loci for FCHL and its related phenotypes are underlying the generalized importance of these loci in dyslipidemia. These data suggest the close relationship between dyslipidemia in HTG rats and human FCHL, establishing a novel animal model for exploration of pathophysiology and therapy based on genomic determinants.

A genome-wide search for genes involved in type 2 diabetes in a recently genetically isolated population from the Netherlands

Multiple genes, interacting with the environment, contribute to the susceptibility to type 2 diabetes. We performed a genome-wide search to localize type 2 diabetes susceptibility genes in a recently genetically isolated population in the Netherlands. We identified 79 nuclear families with type 2 diabetes who were related within 13 generations and performed a 770-marker genome-wide scan search for shared founder alleles. Twenty-six markers yielded a logarithm of odds (LOD) score >0.59 (nominal P < 0.05), of which 7 reached LOD scores >1.17 (nominal P < 0.01). The strongest evidence for a type 2 diabetes locus was at marker D18S63 on chromosome 18p (LOD 2.3, P = 0.0006). This region was investigated further using additional markers. For one of these markers (D18S1105), we found a significant association with type 2 diabetes (odds ratio 6.7 [95% CI 1.5-30.7], P = 0.005 for the 97-bp allele, assuming a dominant model), which increased when limiting the analysis to patients with high BMI (12.25 [2.1-71], P = 0.003). A locus on chromosome 18p in patients with high BMI was suggested earlier by Parker et al. Our study is the first to confirm this locus.

Lipoprotein distribution in the metabolic syndrome, type 2 diabetes mellitus, and familial combined hyperlipidemia

Metabolic abnormalities associated with the metabolic syndrome are also present in patients with type 2 diabetes mellitus and in those with familial combined hyperlipidemia (FCHL). These abnormalities include central obesity, insulin resistance with hyperinsulinemia, hypertension, increased plasma triglycerides, and decreased high-density lipoprotein cholesterol levels. Other characteristics associated with FCHL include the presence of small, dense low-density lipoprotein cholesterol and increased apolipoprotein B. Patients with these abnormalities are at an increased risk for premature coronary artery disease. Treatment of the dyslipidemia associated with type 2 diabetes and FCHL with a combination of a statin and a thiazolidinedione or niacin offers the most comprehensive modality to correct the various lipid abnormalities.

Small, dense LDL and elevated apolipoprotein B are the common characteristics for the three major lipid phenotypes of familial combined hyperlipidemia

OBJECTIVE

Familial combined hyperlipidemia (FCHL) is associated with variable lipid and lipoprotein phenotypes arbitrarily defined as type IIa, IIb, and IV based on plasma total cholesterol and triglyceride levels. This study sought to characterize consistent lipoprotein and lipid abnormalities across the 3 lipoprotein phenotypes in 62 patients with documented FCHL (IIa [n=14], IIb [n=19], and IV [n=29]) and 44 healthy individuals.

METHODS AND RESULTS

The lipoprotein cholesterol distribution was determined over 38 fractions obtained by density gradient ultracentrifugation. As expected, FCHL patients with hypertriglyceridemia (IIb and IV) had higher cholesterol levels in VLDL than IIa, whereas IIa showed higher cholesterol in the big, buoyant LDL and in HDL. LDL cholesterol was higher in IIb than IV; most of the increase in LDL cholesterol was associated with big, buoyant LDL rather than small, dense LDL (sdLDL). The differences in lipoproteins between phenotypes were attributable to changes in VLDL and big, buoyant LDL levels. Comparison of the FCHL patients with healthy individuals showed a significant elevation in plasma apolipoprotein B levels and sdLDL in all 3 FCHL phenotypes.

CONCLUSIONS

Although triglyceride and cholesterol levels are variable by lipoprotein phenotype, sdLDL and elevated plasma apolipoprotein B levels are consistent characteristics of FCHL shared by the 3 different lipoprotein phenotypes.

Biochemical and genetic association of plasma apolipoprotein A-II levels with familial combined hyperlipidemia

Apolipoprotein A-II (apoA-II) is a major protein on high-density lipoprotein (HDL) particles, and in mice, its levels are associated with triglyceride and glucose metabolism. In particular, transgenic mice overexpressing apoA-II exhibit hypertriglyceridemia, increased body fat, and insulin resistance, whereas apoA-II-null mice have decreased triglycerides and increased insulin sensitivity. Given the phenotypic overlap between familial combined hyperlipidemia (FCH) and apoA-II transgenic mice, we investigated the relationship of apoA-II to this disorder. Despite having lower HDL-cholesterol (HDL-C), FCH subjects had higher apoA-II levels compared with unaffected relatives (P<0.00016). Triglyceride and HDL-C levels were significant predictors of apoA-II, demonstrating that apoA-II variation is associated with several FCH-related traits. After adjustment for multiple covariates, there was evidence for the heritability of apoA-II levels (h2=0.15; P<0.02) in this sample. A genome scan for apoA-II levels identified significant evidence (LOD=3.1) for linkage to a locus on chromosome 1q41, coincident with a suggestive linkage for triglycerides (LOD score=1.4). Thus, this locus may have pleiotropic effects on apoA-II and FCH traits. Our results demonstrate that apoA-II is biochemically and genetically associated with FCH and may serve as a useful marker for understanding the mechanism by which FCH develops.

Genetic analysis of a polymorphism in the human apoA-V gene: effect on plasma lipids

Recent discovery and characterization of APOAV suggests a role in metabolism of triglyceride (TG)-rich lipoproteins. Previously, variation at the APOAV locus was shown to modestly influence plasma TGs in normolipidemic samples. The aims of this study were to assess the effects of a polymorphism in APOAV (T-1131C) in terms of its frequency among three dyslipidemic populations and a control population, differences of allele frequency across available ethnic groups, and associations with specific lipoprotein TG and cholesterol compartments. We found a striking elevation in the frequency of the rare allele in a Chinese population (P = 0.0002) compared with Hispanic and European populations. The rare allele of the polymorphism was associated with elevated plasma TG (P = 0.012), VLDL cholesterol (P = 0.0007), and VLDL TG (P = 0.012), LDL TG (P = 0.003), and HDL TG (P = 0.016). Linear regression models predict that possession of the rare allele elevates plasma TG by 21 mg/dl (P = 0.009) and VLDL cholesterol by 8 mg/dl (P = 0.0001), and reduces HDL cholesterol by 2 mg/dl (P = 0.017). The association of the polymorphism with altered lipoprotein profiles was observed in combined hyperlipidemia, hypoalphalipoproteinemia, and hyperalphalipoproteinemia, and in controls. These findings indicate that APOAV is an important determinant of plasma TG and lipoprotein cholesterol, and is potentially a risk factor for cardiovascular disease.

Apolipoprotein and apolipoprotein receptor genes, blood lipids and disease

PURPOSE OF REVIEW

Apolipoproteins and their receptors are the main controllers of lipid metabolism and, as such, have a major impact not only on the risk of cardiovascular disease but also on the development and degeneration of the central nervous system. Variations in the genes coding for these apolipoproteins and their receptors and the interaction with the environment determine individual susceptibility to metabolic disturbances, the response to dietary or pharmacological intervention and, finally, to disease.

RECENT FINDINGS

This review will focus on recent findings, such as the latest concepts regarding apolipoprotein E in neurodevelopment, the newly identified apolipoprotein A-V and its influence in triglyceride metabolism, and the improved understanding of apolipoprotein A-I and HDL metabolism in the light of the discovery of the ABC family of transporters. Other key aspects of lipoprotein metabolism and cardiovascular disease risk such as apolipoprotein B-100, the LDL receptor, apolipoprotein C-III or apolipoprotein (a) will be updated.

SUMMARY

Variations in these genes will be analysed in relation to plasma lipid levels, their interactions with diet, treatment or other environmental stimuli, and their influence on the risk of cardiovascular disease and neurological disorders.

Clinical relevance of the biochemical, metabolic, and genetic factors that influence low-density lipoprotein heterogeneity

Traditional risk factors for coronary artery disease (CAD) predict about 50% of the risk of developing CAD. The Adult Treatment Panel (ATP) III has defined emerging risk factors for CAD, including small, dense low-density lipoprotein (LDL). Small, dense LDL is often accompanied by increased triglycerides (TGs) and low high-density lipoprotein (HDL). An increased number of small, dense LDL particles is often missed when the LDL cholesterol level is normal or borderline elevated. Small, dense LDL particles are present in families with premature CAD and hyperapobetalipoproteinemia, familial combined hyperlipidemia, LDL subclass pattern B, familial dyslipidemic hypertension, and syndrome X. The metabolic syndrome, as defined by ATP III, incorporates a number of the components of these syndromes, including insulin resistance and intra-abdominal fat. Subclinical inflammation and elevated procoagulants also appear to be part of this atherogenic syndrome. Overproduction of very low-density lipoproteins (VLDLs) by the liver and increased secretion of large, apolipoprotein (apo) B-100-containing VLDL is the primary metabolic characteristic of most of these patients. The TG in VLDL is hydrolyzed by lipoprotein lipase (LPL) which produces intermediate-density lipoprotein. The TG in intermediate-density lipoprotein is hydrolyzed further, resulting in the generation of LDL. The cholesterol esters in LDL are exchanged for TG in VLDL by the cholesterol ester tranfer proteins, followed by hydrolysis of TG in LDL by hepatic lipase which produces small, dense LDL. Cholesterol ester transfer protein mediates a similar lipid exchange between VLDL and HDL, producing a cholesterol ester-poor HDL. In adipocytes, reduced fatty acid trapping and retention by adipose tissue may result from a primary defect in the incorporation of free fatty acids into TGs. Alternatively, insulin resistance may promote reduced retention of free fatty acids by adipocytes. Both these abnormalities lead to increased levels of free fatty acids in plasma, increased flux of free fatty acids back to the liver, enhanced production of TGs, decreased proteolysis of apo B-100, and increased VLDL production. Decreased removal of postprandial TGs often accompanies these metabolic abnormalities. Genes regulating the expression of the major players in this metabolic cascade, such as LPL, cholesterol ester transfer protein, and hepatic lipase, can modulate the expression of small, dense LDL but these are not the major defects. New candidates for major gene effects have been identified on chromosome 1. Regardless of their fundamental causes, small, dense LDL (compared with normal LDL) particles have a prolonged residence time in plasma, are more susceptible to oxidation because of decreased interaction with the LDL receptor, and enter the arterial wall more easily, where they are retained more readily. Small, dense LDL promotes endothelial dysfunction and enhanced production of procoagulants by endothelial cells. Both in animal models of atherosclerosis and in most human epidemiologic studies and clinical trials, small, dense LDL (particularly when present in increased numbers) appears more atherogenic than normal LDL. Treatment of patients with small, dense LDL particles (particularly when accompanied by low HDL and hypertriglyceridemia) often requires the use of combined lipid-altering drugs to decrease the number of particles and to convert them to larger, more buoyant LDL. The next critical step in further reduction of CAD will be the correct diagnosis and treatment of patients with small, dense LDL and the dyslipidemia that accompanies it.

Genetic influences contributing to LDL particle size in familial combined hyperlipidaemia

The nature of the genetic and environmental factors influencing low density lipoprotein (LDL) particle size in patients with familial combined hyperlipidaemia (FCHL) is under debate. We measured LDL peak particle size in 553 subjects belonging to 48 Finnish FCHL families. Individuals with high triglyceride (TG) concentrations (phenotype IV) or combined hyperlipidaemia (phenotype IIB) had significantly smaller LDL particles than those with hypercholesterolaemia (phenotype IIA) or unaffected subjects (P<0.001). In stepwise regression analyses, serum TGs (r(2)=43%, P<0.001) and high density lipoprotein cholesterol (HDL-C) (r(2)=4.5%, P<0.001) were the only significant predictors of LDL peak particle size. Familial correlations support the conclusion that LDL peak particle size is familial, and most probably influenced by genes in these families. Segregation analysis of LDL peak particle size, a quantitative trait, was performed to model this genetic influence. Our results suggest a polygenic background for LDL size with a recessive major gene that may contribute to large LDL peak particle size in women. Serum TG and HDL-C concentrations predict the majority of variations in LDL particle size.

Allelic polymorphism -491A/T in apo E gene modulates the lipid-lowering response in combined hyperlipidemia treatment

BACKGROUND

Combined hyperlipidemia (CHL) is one of the dyslipidemias more frequently found in clinical practice, and lipid-lowering drugs are often necessary in its management. Some genetic loci have been associated with CHL expression, and some studies have shown modulation of drugs efficiency in the treatment of dyslipidemias by genetic polymorphisms. We have investigated whether common polymorphisms and mutations in the apolipoprotein (apo) E, lipoprotein lipase (LPL), and apo CIII genes influence atorvastatin or bezafibrate responses in patients with CHL.

DESIGN

One hundred and sixteen subjects participating in the ATOMIX study (Atorvastatin in Mixed dyslipidemia) were randomized to treatment with either atorvastatin or bezafibrate. Apolipoprotein E genotype and common -491A/T and -219T/G polymorphisms in the apo E gene promoter region, Sst I polymorphism in the apo CIII gene (3238C/G), and D9N and N291S common mutations in the LPL gene were determined by polymerase chain reaction (PCR) and restriction enzyme digestion.

RESULTS

Statistical analysis showed the influence of the -491A/T polymorphism in atorvastatin and bezafibrate treatments. Subjects carrying the -491T allele showed an increased LDL-cholesterol-lowering effect with atorvastatin compared with -491T allele noncarriers (-35% vs. -27%, P = 0.037). Subjects carrying the -491T allele, when on bezafibrate treatment, showed a lower triglyceride reduction compared with -491T allele noncarriers (-23% vs. -39%, P = 0.05).

CONCLUSIONS

In our study, the -491A/T polymorphism in the apo E gene promoter region modulated the lipid-lowering efficiency of atorvastatin and bezafibrate in CHL patients. Such influence might explain some of the interindividual response variabilities observed for the two drugs, and could help in CHL management.

Locus for elevated apolipoprotein B levels on chromosome 1p31 in families with familial combined hyperlipidemia

Familial combined hyperlipidemia (FCH), a common cause of premature coronary artery disease, is genetically complex and poorly understood. Recently, a major locus on chromosome 1q21-23 exhibiting highly significant linkage was identified in Finnish FCH families by use of a parametric analysis. We now report highly significant evidence of linkage (maximum LOD score 3.8, recombination fraction 0) of an important FCH phenotype, elevated apolipoprotein B (apoB) levels, to a distinctly separate locus on chromosome 1p31 in Dutch pedigrees. ApoB is the major protein on very low density and low density lipoproteins, and elevated apoB levels have been used as a surrogate trait for FCH. Additional microsatellite markers in the 1p31 region were genotyped, and evidence of linkage improved (maximum LOD score 4.7) in a multipoint analysis of two markers in the peak region. The leptin receptor gene resides within this locus and is involved in obesity and insulin/glucose homeostasis. However, there was no evidence of an association between leptin receptor and apoB levels, raising the possibility that another gene on this chromosomal region contributes to elevated apoB levels in this Dutch population. This is one of the first loci identified for apoB levels in humans and is the second major locus implicated in the genetic etiology of FCH.

Lipoprotein and apolipoprotein abnormalities in familial combined hyperlipidemia: a 20-year prospective study

In order to characterize the lipoprotein abnormalities in familial combined hyperlipidemia (FCHL) and to describe factors associated with the stability of the FCHL phenotype during 20-year follow-up, 287 individuals from 48 families with FCHL originally identified in the early 1970s (baseline) were studied. Hyperlipidemia was defined as lipid-lowering medication use, or > or =age- and sex-specific 90th percentile for triglycerides or cholesterol. Triglyceride, cholesterol and medical history data were obtained at baseline and 20-year follow-up. Additional follow-up measures included HDL-C, LDL-C, LDL particle size, lipoprotein(a), apolipoprotein (apo) A-I, apoB, and apoE polymorphism. Longitudinally, two-thirds of relatives were consistently normolipidemic or hyperlipidemic, and one third were discordant for hyperlipidemic status at baseline and 20-year follow-up. Individuals with hyperlipidemia at baseline and/or follow-up had higher apoB levels than those with consistently normal lipids (P<0.05), whereas small LDL size was associated with concurrent hyperlipidemia. Among individuals who were normolipidemic at baseline, the following variables were independently associated with development of hyperlipidemia over 20 years: older age at baseline, male sex, greater increase in BMI during follow-up, and apoE alleles epsilon 2 or epsilon 4. In conclusion, apoB is associated with hyperlipidemia and apoE polymorphism is associated with later onset of hyperlipidemia in FCHL.

Genetic dissection of familial combined hyperlipidemia

Familial combined hyperlipidemia (FCHL) is the most common genetic hyperlipidemia in man. FCHL is characterized by familial clustering of hyperlipidemia and clinical manifestations of premature coronary heart disease, i.e., before the age of 60. Although FCHL was delineated about 25 years ago, at present the FCHL phenotype and its complex genetics are not fully understood. Initially, the familial aggregation of high plasma total cholesterol and triglyceride levels, with a bimodal distribution of triglycerides, was taken as evidence of a dominant mode of inheritance. However, it is now clear that the genetics of FCHL is more complex, and it has been suggested that FCHL is heterogeneous. Several approaches can be taken to identify genes contributing to the disease phenotype in complex genetic disorders either by studying the disease in the human situation or by using animal models. Recent reports have shown that a combination of genetic linkage studies, association studies, and differential gene expression studies provides a useful tool for the genetic dissection of complex diseases. Therefore, the genetic strategies that will be used to dissect the genetic background of FCHL are reviewed.

Insulin resistance in the St. Thomas' mixed hyperlipidaemic (SMHL) rabbit, a model for familial combined hyperlipidaemia

The St. Thomas mixed hyperlipidaemic (SMHL) rabbit exhibits an inherited hyperlipidaemia similar to that seen in familial combined hyperlipidaemia (FCHL). In this study, we investigated whether the SMHL rabbit is insulin resistant, a condition often associated with FCHL. Six young and six mature combined hyperlipidaemic SMHL rabbits, age/sex matched New Zealand White (NZW) control rabbits and six young hypercholesterolaemic Watanabe heritable hyperlipidaemic (WHHL) control rabbits were fed a 0.08% (w/w) cholesterol-enriched diet for at least 1 month prior to the start of the experiment. We performed an oral glucose tolerance test after an overnight fast by dosing the rabbits with a solution of 1 g of glucose per kg body weight. Blood was withdrawn just before and 15, 30, 45, 60 and 120 min after administration of the oral glucose dose. Plasma glucose levels were similar in SMHL, WHHL and NZW rabbits throughout the oral glucose tolerance test. Fasting glucose levels were slightly increased in WHHL rabbits but not in young and adult SMHL rabbits as compared to NZW rabbits. The area under the curve (AUC) for the insulin response was significantly increased for both young (P<0.05) and mature (P<0.05) SMHL rabbits, and in WHHL rabbits, compared with NZW rabbits. The AUC for the ratio of glucose:insulin response to the glucose dose was decreased in young and mature SMHL rabbits (P<0.05 and P<0.01, respectively) and in young WHHL rabbits (P<0.05), compared with NZW rabbits. Only WHHL rabbits showed an increased AUC for the non-esterified fatty acid response compared to NZW rabbits. Log-transformed plasma triglycerides values were significantly correlated with the log-transformed AUC for the insulin response in young SMHL rabbits (r=0.81; P<0.05) and with the AUC for the insulin response in mature SMHL rabbits (r=0.84; P<0.05). WHHL rabbits showed no significant correlation. In conclusion, SMHL rabbits are insulin resistant, the severity of which appears to increase with age. Therefore, the SMHL rabbit offers a valuable animal model in which to study the relation between hypertriglyceridaemia and insulin resistance.

Relationship of insulin sensitivity and ApoB levels to intra-abdominal fat in subjects with familial combined hyperlipidemia

Familial combined hyperlipidemia (FCHL) is one of the most common familial dyslipidemias associated with premature heart disease. Subjects with FCHL typically have elevated apolipoprotein B (apoB) levels, variable elevations in cholesterol and/or triglycerides, and a predominance of small, dense, low density lipoprotein particles. It is thought that insulin resistance is important in the expression of the combined hyperlipidemia phenotype. To further characterize the relationship between insulin resistance and increased apoB levels, 11 subjects from well-characterized FCHL families and normal control subjects matched for weight and/or age underwent measurement of intra-abdominal fat (IAF) and subcutaneous fat (SQF) by CT scan, insulin sensitivity (Si) by the frequently sampled intravenous glucose tolerance test, and lipoprotein levels. Body mass index and IAF were higher and Si was lower (more insulin resistant) in the FCHL group than in the age-matched group, but the values were similar in the FCHL group and the age- and weight-matched control group. When the relationship between body fat distribution and Si was tested with multiple linear regression, only IAF was significantly correlated with Si after the addition of SQF and body mass index as independent variables. For any level of insulin sensitivity or IAF, however, apoB levels remained higher in the FCHL subjects than in the control groups. In conclusion, in FCHL, visceral obesity is an important determinant of insulin resistance. Visceral obesity and insulin resistance, however, do not fully account for the elevated levels of apoB in this disorder, and this study provides physiological support for separate, but additive, genetic determinants in the etiology of the lipid phenotype.

A genome scan for familial combined hyperlipidemia reveals evidence of linkage with a locus on chromosome 11

Familial combined hyperlipidemia (FCHL) is a common familial lipid disorder characterized by a variable pattern of elevated levels of plasma cholesterol and/or triglycerides. It is present in 10%-20% of patients with premature coronary heart disease. The genetic etiology of the disease, including the number of genes involved and the magnitude of their effects, is unknown. Using a subset of 35 Dutch families ascertained for FCHL, we screened the genome, with a panel of 399 genetic markers, for chromosomal regions linked to genes contributing to FCHL. The results were analyzed by use of parametric-linkage methods in a two-stage study design. Four loci, on chromosomes 2p, 11p, 16q, and 19q, exhibited suggestive evidence for linkage with FCHL (LOD scores of 1.3-2.6). Markers within each of these regions were then examined in the original sample and in additional Dutch families with FCHL. The locus on chromosome 2 failed to show evidence for linkage, and the loci on chromosome 16q and 19q yielded only equivocal or suggestive evidence for linkage. However, one locus, near marker D11S1324 on the short arm of human chromosome 11, continued to show evidence for linkage with FCHL, in the second stage of this design. This region does not contain any strong candidate genes. These results provide evidence for a candidate chromosomal region for FCHL and support the concept that FCHL is complex and heterogeneous.