Interaction between the Asn291Ser variant of the LPL gene and insulin resistance on dyslipidaemia in high risk individuals for Type 2 diabetes mellitus.

AIMS: Lipoprotein lipase (LPL) is a major regulator of triglyceride clearance. A genetic variant of the LPL gene on chromosome 8p22, Asn291Ser, has previously been associated with dyslipidaemia and an increased frequency of cardiovascular disease as well as familial disorders of lipoprotein metabolism. The aim of this study was to test whether the phenotypic expression of the LPL Asn291Ser variant is dependent upon glucose tolerance and insulin resistance. Therefore, the Asn291Ser variant was examined in 192 patients with Type 2 diabetes, 278 subjects with normal glucose tolerance who are first degree relatives of patients with Type 2 diabetes and 226 healthy control spouses without family history of diabetes. METHODS: The subjects were genotyped with an allele-specific mini-sequencing method. Insulin resistance was estimated using the homeostasis model assessment (HOMA) index. RESULTS: The frequency of the Asn/Ser genotype was significantly increased in normoglycaemic subjects with hypertriglyceridaemia (> 1.7 mmol/1), and was associated with dyslipidaemia and increased systolic blood pressure. There was a significant interaction between Asn291Ser and insulin resistance in normoglycaemic subjects, indicating that dyslipidaemia is more severe in Asn/ Ser carriers with reduced insulin sensitivity. The frequency of the Asn/Ser genotype was not increased in diabetic subjects with hypertriglyceridaemia, but was associated with increased systolic blood pressure. CONCLUSIONS: The Asn/Ser genotype of the LPL gene is associated with dyslipidaemia in normoglycaemic subjects, and the dyslipidaemic phenotype is more severe in insulin-resistant subjects. This association is not seen in diabetic subjects.

Delayed clearance of postprandial large TG-rich particles in normolipidemic carriers of LPL Asn291Ser gene variant.

The carrier frequency of Asn291Ser polymorphism of the lipoprotein lipase (LPL) gene is 4;-6% in the Western population. Heterozygotes are prone to fasting hypertriglyceridemia and low high density lipoprotein (HDL) cholesterol concentrations especially when secondary factors are superimposed on the genetic defect. We studied the LPL Asn291Ser gene variant as a modulator of postprandial lipemia in heterozygote carriers. Ten normolipidemic carriers were compared to ten control subjects, who were selected to have similar age, sex, BMI, and apolipoprotein (apo)E-phenotype. The subjects were given a lipid-rich mixed meal and their insulin sensitivity was determined by euglycemic hyperinsulinemic clamp technique. The two groups had comparable fasting triglycerides and glucose utilization rate during insulin infusion, but fasting HDL cholesterol was lower in carriers (1.25 +/- 0.05 mmol/L) than in the control subjects (1. 53 +/- 0.06 mmol/L, P = 0.005). In the postprandial state the most pronounced differences were found in the very low density lipoprotein 1 (VLDL1) fraction, where the carriers displayed higher responses of apoB-48 area under the curve (AUC), apoB-100 AUC, triglyceride AUC, and retinyl ester AUC than the control subjects. The most marked differences in apoB-48 and apoB-100 concentrations were observed late in the postprandial period (9 and 12 h), demonstrating delayed clearance of triglyceride-rich particles of both hepatic and intestinal origin. Postprandially, the carriers exhibited enrichment of triglycerides in HDL fraction. Thus, in normolipidemic carriers the LPL Asn291Ser gene variant delays postprandial triglyceride, apoB-48, apoB-100, and retinyl ester metabolism in VLDL1 fraction and alters postprandial HDL composition compared to matched non-carriers.

Glucose intolerance in familial combined hyperlipidaemia. EUFAM study group.

BACKGROUND: Familial combined hyperlipidaemia (FCHL) is a common hereditary disorder. Hypertriglyceridaemia is associated with glucose intolerance and insulin resistance. METHODS: To study glucose tolerance in FCHL patients with different lipid phenotypes [hypercholesterolaemia (IIA), mixed hyperlipidaemia (IIB), hypertriglyceridaemia (IV)], we investigated 253 family members and 92 spouses arising from 33 well-defined Finnish FCHL pedigrees. RESULTS: In oral glucose tolerance tests the affected family members had higher values for glucose area under the curve than did non-affected family members [673+/-127 min mmolL(-1), 754+/-145 min mmol L(-1), 846+/-180 min mmol L(-1) and 838+/-183 min mmol L(-1) for phenotypes normal, IIA, IIB and IV respectively; P < 0.001 after adjustment for body mass index, waist circumference and age]. Impaired glucose tolerance and diabetes were more common among affected than non-affected family members (prevalences of normal glucose tolerance 94.0%, 80.0%, 54.3% and 58.5% for phenotypes normal, IIA, IIB and IV). CONCLUSION: Affected FCHL family members were more glucose intolerant than non-affected family members. In men, this disturbance was not related to lipid phenotype nor was it explained by obesity.

Phenotype expression in familial combined hyperlipidemia.

Familial combined hyperlipidaemia (FCHL) is one of the most common hereditary disorders predisposing to early coronary death. The affected family members have elevations of serum total cholesterol, triglycerides or both. Despite intensive research efforts the genetic and metabolic defects underlying this complex disorder are still unknown. To dissect the metabolism and genetics of FCHL the phenotype of an individual must be precisely defined. We assessed the influence of different diagnostic criteria on the phenotype definition and studied factors affecting the phenotype expression in 16 large Finnish families (n = 255) with FCHL. The fractile cut-points used to define abnormal lipid values had a profound influence on the diagnosis of FCHL. If the 90th percentile cut-point was used, approximately 45% of the family members were affected, in concord with the presumed dominant mode of transmission for FCHL. If the 95th percentile was used only 22% of study subjects were affected. To characterize the metabolic differences or similarities between the different lipid phenotypes, we determined very low density lipoprotein (VLDL), intermediate density lipoprotein (IDL), low density lipoprotein (LDL) and high density lipoprotein (HDL) particles separated by ultracentrifugation. In linkage analysis no single ultracentrifugation variable could discriminate reliably affected family members from non-affected family members. Our data emphasizes the need for re-evaluation of FCHL diagnostic criteria. Preferably, the diagnosis should be based on a single, reliable metabolic marker.

No evidence of linkage between familial combined hyperlipidemia and genes encoding lipolytic enzymes in Finnish families.

Familial combined hyperlipidemia (FCHL) is characterized by different lipid phenotypes (IIa, IIb, IV) and elevated apolipoprotein B (apo B) levels in affected family members. Despite intensive research, the genes involved in the expression of this complex disorder have not been identified, probably because of problems associated with phenotype definition, unknown mode of inheritance, and most probably genetic heterogeneity. To explore the genetics of FCHL in the genetically homogeneous Finnish population, we collected 14 well-documented Finnish pedigrees with premature coronary heart disease and FCHL-like dyslipidemia. The lipolytic enzymes lipoprotein lipase (LPL), hepatic lipase (HL), and hormone-sensitive lipase (HSL) were selected as initial candidate genes because of their central roles in apo B and triglyceride metabolism. On the basis of the pedigree structures, a dominant mode of inheritance was adopted for linkage analyses, and serum total cholesterol and/or triglyceride levels exceeding the 90th percentile level were set as diagnostic criteria (criterion 1). In pairwise linkage analyses with intragenic markers, no evidence for linkage was found. Instead, the significantly negative LOD scores suggested exclusion of all three loci for single major gene effect. LOD scores were -14.63, -5.03, and -5.70 for the three LPL polymorphisms (theta=0.00); -9.40, -6.30, and -4.74 for the three HL polymorphisms (theta=0.00); and -15.29 for the HSL polymorphism (theta=0.00). The results were very similar when apo B levels over the 90th percentile were used as criteria for affected status (criterion 2). Also, when linkage calculations were carried out using an intermediate or recessive mode of inheritance, the results of pairwise linkage analysis remained negative. Furthermore, when haplotypes were constructed from multiple polymorphisms of the LPL and HL genes, no segregation with the FCHL phenotype could be observed in the 14 Finnish families. Data obtained by the affected sib-pair method supported these findings, suggesting that the LPL, HL, or HSL genes do not represent major loci influencing the expression of the FCHL phenotype.

Serum complement and familial combined hyperlipidemia.

Familial combined hyperlipidemia (FCHL) is one of the most common inherited lipid disorders. Resistance of adipocytes to the effects of acylation stimulating protein (ASP) may contribute to ineffective triglyceride synthesis and thereby prolonged postprandial lipemia and increased fatty acid flux to the liver seen in FCHL patients. Interestingly, ASP is identical to C3a-desArg, fragment of the third component of complement. We examined the relationships between serum levels of complement components C3 and C4 and markers of lipid and glucose metabolism in 11 large FCHL families (n = 53). Median serum C3 levels were 38% higher in affected compared to non-affected male FCHL family members (1.90 g/l vs. 1.38, P = 0.0027). The strongest correlations were observed between serum complement C3 and apolipoprotein B levels, reaching 0.77 in males. These relations were not confounded by obesity or impaired glucose tolerance. In conclusion, serum levels of the main complement components C3 and C4 correlated significantly with serum lipid levels. Further studies are needed to clarify the importance of disturbances in the complement system on the pathogenesis of FCHL and other lipid disorders.

Development and evaluation of an ELISA method for the determination of lipoprotein lipase mass concentration--comparison with a commercial, one-step enzyme immunoassay.

We developed a non-competitive, enzyme-linked, immunosorbent assay (ELISA) for the quantitation of lipoprotein lipase (LPL) in human postheparin plasma using affinity-purified antihuman milk lipoprotein lipase antibodies produced in chicken eggs and a monoclonal antibody directed against human lipoprotein lipase. We compared our ELISA method with a commercially available sandwich-enzyme immunoassay (Markit-F LPL EIA Kit, Dainippon Pharmaceutical Co, Ltd. Osaka, Japan). The reference values for lipoprotein lipase catalytic activity concentration and mass concentration in healthy Finns were determined. Lipoprotein lipase activity concentration (mean +/- SD) was 297 +/- 112 U/l in women, and mass concentration as measured by the ELISA method was 1058 +/- 367 micrograms/l. The corresponding values for men were 247 +/- 97 U/l and 815 +/- 207 micrograms/l, respectively. Across the whole concentration range of the ELISA method, the control samples' intra- and inter-assay coefficients of variation (CV) were 5.1% and 6.5%, respectively. The correlation between the ELISA and EIA methods was good, r = +0.81. The importance of the correct standardisation of immunoassays is discussed.

Prevalence of hyperapobetalipoproteinemia and factors affecting the phenotype expression in children and young adults. The Cardiovascular Risk in Young Finns Study.

Hyperapobetalipoproteinemia (hyperapoB) is one of the most common phenotypes in patients with premature coronary heart disease. In this study the factors that affect the expression of the hyperapoB phenotype were evaluated in young individuals. A cohort of 1125 children and young adults aged 9-24 years was classified into three groups by sex: (1) normal serum apolipoprotein B (apoB), (2) high apoB (> or = 90th percentile) and normal low density lipoprotein cholesterol (LDL-C < 90th), (3) high apoB and high LDL-C (> or = 90th percentile). In females, alcohol use (11, 33, 0%, in groups 1-3, P < 0.05) and oral contraceptive use (35, 83, 47%, P < 0.01) were significantly different between the groups and the highest frequencies were seen in the hyperapoB group (group 2). In both sexes smoking tended to be more common in the hyperapoB group (29, 43, 18%, P < 0.14). The two hyperapoB definition criteria (high apoB and low LDL-C/apoB ratio) were studied with multiple linear regression analyses. Oral contraceptive use correlated positively with apoB values (coefficient beta = 0.101, R2 = 2.1%, P < 0.01) and negatively with LDL-C/apoB ratio (beta = -0.134, R2 = 3.3%, P < 0.001). Alcohol use (beta = -0.072, R2 = 2.9%, P < 0.001) and smoking (beta = -0.050, R2 = 1.0%, P < 0.05) correlated negatively with LDL-C/apoB ratio. Prevalence of the hyperapoB phenotype was 4.4%. According to the results, the expression of the hyperapoB phenotype may be influenced by common lifestyle habits. This should be considered if high risk young individuals are identified through the expression of the hyperapoB phenotype.