Lipoprotein lipase gene mutations D9N and N291S in four pedigrees with familial combined hyperlipidaemia

Abstract. The role of the lipoprotein lipase (LPL) gene in familial combined hyperlipidaemia (FCH) is unclear at present. We screened a group of 28 probands with familial combined hyperlipidaemia and a group of 91 population controls for two LPL gene mutations. D9N and N291S. LPL-D9N was found in two probands and one normolipidaemic population control. LPL-N291S was found in four probands and four population controls. Subsequently, two pedigrees from probands with the D9N mutation and two pedigrees from probands with the N291S mutation were studied, representing a total of 24 subjects. Both LPL gene mutations were associated with a significant effect on plasma lipids and apolipoproteins. Presence of the D9N mutation (n = 7) was associated with hypertriglyceridaemia [2.69± 1.43 (SD) mmol L^-1] and reduced plasma high-density lipoprotein cholesterol (HDL-C) concentrations (0.92± 0.21 mmol L^-1) compared with 11 non-carriers (triglyceride 1.75± 0.64 mmol L^-1; HDL-C 1.23± 0.30 mmol L^-1, P = 0.03 and P = 0.025 respectively). LPL-D9N carriers had higher diastolic blood pressures than non-carriers. LPL-N291S carriers ( n = 6) showed significantly higher (26%) apo B plasma concentrations (174± 26 mg dL^-1) than non-carriers (138± 26 mg dL^-1; P = 0.023), with normal post-heparin plasma LPL activities. Linkage analysis revealed no significant relationship between the D9N or N291S LPL gene mutations and the FCH phenotype (hypertriglyceridaemia, hypercholesterolaemia or increased apo B concentrations). It is concluded that the LPL gene did not represent the major single gene causing familial combined hyperlipidaemia in the four pedigrees studied, but that the LPL-D9N and LPL-N291S mutations had significant additional effects on lipid and apolipoprotein phenotype.     

Intravenous fat-tolerance test in ischaemic heart disease and peripheral vascular disease.

1. The intravenous fat-tolerance test and serum lipid and lipoprotein measurements were carried out in ninety-three normal subjects, fifty-one patients with ischaemic heart disease and thirty patients with peripheral vascular disease. 2. The fractional turnover rate of exogenous triglyceride was significantly slower in patients with ischaemic heart disease and in patients with peripheral vascular disease than in normal men. The rate was also slower in normal men than normal women. 3. Serum triglyceride and cholesterol concentrations were higher in both vascular disease groups than in control subjects. 4. The proportion of both groups of patients who had a subnormal fractional turnover rate of exogenous triglyceride was 35%, and 32% of patients had hypertriglyceridaemia in the fasting state; 27% of patients were hypercholesterolaemic. 5. Although the intravenous fat-tolerance test did not provide significantly better discrimination between cardiovascular patients and control subjects than did measurement of serum triglyceride, the results suggest that hypertriglyceridaemia in such patients may be separable into a group in which impaired triglyceride clearance may be partly responsible, and a group in which overproduction of serum triglyceride may be the major mechanism of the hyperlipidaemia.     

Apolipoprotein A and B (Sf 100-400) metabolism during bezafibrate therapy in hypertriglyceridemic subjects.

This study describes the effects of bezafibrate, an analogue of clofibrate, on the plasma lipid and lipoprotein profiles of 11 hypertriglyceridemic subjects and on their metabolism of apolipoproteins A-I, A-II, and B. The major action of the drug was to lower plasma triglyceride (by 58%; P less than 0.01). This was accompanied by a reduction in the level of very low density lipoprotein apoprotein B (Svedberg units of flotation [Sf] 60-400), whose mean residence time in the plasma fell threefold (from 3.4 to 1.0 h). Synthesis of the B protein in this fraction was not significantly altered, so the drug acts to accelerate the transit of very low density lipoprotein particles down the delipidation cascade. The metabolism of very low density lipoprotein remnant apoprotein B (Sf 12-100) changed little in response to treatment, although we detected a 30% increment (P less than 0.05) in the plasma concentration of this fraction. The mean residence time of these remnant particles in the plasma did not correlate with that of Sf 100-400 very low density lipoprotein apoprotein B, nor was this parameter altered by the drug. The most consistent and significant perturbation seen in the Sf 0-12 fraction (low density lipoprotein) was a reduction in the fractional catabolism of its apoprotein B moiety (26%; P less than 0.05). In those subjects who were grossly hypertriglyceridemic and who responded well to treatment, the level of this protein rose substantially owing to a combined increase in its synthesis and a reduction in its catabolism. In the group as a whole, high density lipoprotein cholesterol rose 13% (P less than 0.02), and detailed examination showed that this was associated with a small but significant increment in the plasma concentration of the high density lipoprotein subfraction 2. High density lipoprotein subfraction 3 also rose on the average, but this was not a consistent feature in all patients. The plasma concentrations and turnovers of the A proteins (A-I and A-II) were not significantly altered by bezafibrate therapy.     

Serum lipids, lipoproteins and apolipoprotein E phenotypes in relatives of patients with type III hyperlipoproteinaemia

Abstract. Eighty-six relatives of nineteen probands with type III hyperlipoproteinaemia were studied to determine the occurrence of hyperlipidaemia and to investigate the relation between apo E phenotypes, the occurrence of hyperlipidaemia, and the composition of the very low density lipoprotein (VLDL) fraction. Thirty-nine relatives were hyperlipidaemic: four type IIa or IIb, nine type III and twenty-six type IV. The predisposition for hyperlipidaemia was independent of the apo E phenotype.
Hyperlipidaemic relatives with apo E phenotype E2/2 had a significantly ( P < 0·01) higher VLDL-cholesterol/VLDL-triglycerides ratio (1·26 ± 0·35 n = 9) than those heterozygous for apo E allele ɛ2 (0·66 ± 0·12, n = 23) or without apo E allele ɛ2 (0·69 ± 0·11, n = 7). Normolipidaemic homozygous apo E-2 relatives had also a significantly ( P < 0·05) higher ratio (0.97 ± 0·19, n = 6) than those heterozygous for (0·77 ± 0·19, n = 31) or without the apo E allele ɛ2 (0·74 ± 0·13, n = 10). Thus, both hyper- and normolipidaemic apo E2 homozygotes have higher concentrations of VLDL remnants than the subjects heterozygous or without allele ɛ2.     

Plasma triglyceride and low density lipoprotein metabolism

This study examines the relationship between plasma triglyceride and low density lipoprotein (LDL) levels by measuring the turnover of the native and 1,2 cyclohexanedione-treated lipoprotein in 25 healthy adults. Plasma triglyceride showed a strong positive correlation with circulating LDL apoprotein (apo LDL) mass. In order to achieve a satisfactory fit to the kinetic data it was necessary to postulate the existence of two plasma apo LDL pools (A and B). When subjects were grouped in quintiles on the basis of circulating apo LDL mass, pool A predominated in those in the lowest quintile. The fractional catabolic rate (FCR) of apo LDL from this pool was high (FCR = 0.57 +/- 0.06 pools day-1). As plasma triglyceride and apo LDL mass rose, apoprotein accumulated in the more slowly metabolized pool B as a result of an increase in the rate of input of apo LDL into the latter. The fractional clearance rate of protein from this pool remained unchanged at 0.26 +/- 0.04 pools day-1. Synthesis of apo LDL into pool B correlated with plasma triglyceride (r = 0.553, P less than 0.01), suggesting that the protein in this pool was derived from large, triglyceride-rich very low density lipoprotein.     

Substituting polyunsaturated for saturated fat as a single change in a Swedish diet: effects on serum lipoprotein metabolism and glucose tolerance in patients with hyperlipoproteinaemia

Abstract. A diet with a high content of polyunsaturated fatty acids (PUFA) with a ratio between polyunsaturated and saturated fatty acids (P/S) of 20 and a fat content of 44% was worked out. After an initial 2 weeks' period on a control diet (P/S ratio 0–2) the PUFA diet was fed under isoenergetic conditions at a metabolic ward for 2 weeks to thirty patients with hyperlipoproteinaemia type IIa (n= 7), type IIb (n= 5) and type IV (n=18). The two diets were based on ordinary foodstuff and differed only in regard of the quality of the fat, while the amount of fat as well as the content of other nutrients were kept constant. Compared with the control diet the serum cholesterol concentration decreased by 10%, 13% and 12% on the PUFA diet in patients with hyperlipoproteinaemia type IIa, IIb and IV respectively. In hyperlipoproteinaemia Ila the low density lipoprotein cholesterol decreased by 9% (n.s.) and the high density lipoprotein cholesterol by 16% (P < 005). In hyperlipoproteinaemia type lib the very low density lipoprotein cholesterol decreased by 18% (P < 005), the low density lipoprotein cholesterol by 13% (P < 005) and the high density lipoprotein cholesterol by 5% (n.s.). In type IV the very low density lipoprotein cholesterol decreased by 18% (P < 0–01), the low density lipoprotein cholesterol by 7% (P < 0–05)) while the high density lipoprotein cholesterol remained unchanged. The serum triglyceride concentration decreased by 10% (type IIa), 14% (type lib) and 13% (type IV) on the PUFA diet. The serum concentrations of apolipoprotein A-I and B were reduced by 6% (P < 0–05) and 11% (P < 005) respectively in patients with hyperlipoproteinaemia type IV while the serum apolipoprotein concentration did not change in the patients with hypercholesterolae-mia. Inverse relationships between very low density lipoprotein triglycerides and high density lipoprotein cholesterol were found before treatment (r= 0–49, P < 0–01) which was altered by the treatment (r= 0–28, P > 005). The very low density lipoprotein triglycerides were also found to be inversely related to low density lipoprotein cholesterol both on the control diet (r= -0–65, P < 0001) and the PUFA diet (r= -0–56, P < 001). The regression lines of the latter equations were parallel. The intravenous glucose tolerance was improved (P < 0 05) in patients with hyperlipoproteinaemia type IV on the PUFA diet. The fatty acid composition of the serum lipid esters was significantly changed during the treatment. The relative concentrations of oleic acid and saturated fatty acids decreased while the linoleic acid content increased. The effects of the PUFA diet were less pronounced than the effects of conventional lipid lowering diets where also the fat content has been reduced and where complex carbohydrates have been substituted for simple carbohydrates.     

Biliary lipids in familial combined hyperlipidaemia: effects of acipimox therapy

Biliary lipid composition and plasma lipoprotein levels were determined in nine gallstone-free male patients with familial combined hyperlipidaemia (FCHL). In the basal situation, stimulated fasting duodenal bile from the patients contained a higher relative concentration of cholesterol than bile obtained from age- and sex-matched normal controls (n = 22), 6.5 +/- 0.3 (SEM) vs. 4.7 +/- 0.2 mol % (P less than 0.01). This resulted in a higher cholesterol saturation of bile from FCHL patients, 85 +/- 6 vs. 70 +/- 2% (P less than 0.05). After 6 weeks of treatment with acipimox, 750 mg day-1, total plasma triglycerides were lowered from 7.5 +/- 1.5 to 4.6 +/- 0.7 mmol l-1 (P less than 0.05) and plasma cholesterol decreased from 8.0 +/- 0.1 to 7.1 +/- 0.3 mmol l-1 (P less than 0.05) in the FCHL patients. These changes were mainly due to a decrease in very low density lipoprotein concentrations while low density lipoprotein levels remained unaltered. The relative proportion of cholesterol in stimulated fasting duodenal bile was reduced from 6.5 +/- 0.3 to 4.3 +/- 0.5 mol % (P less than 0.01), resulting in 'normalization' of biliary cholesterol saturation, from 85 +/- 6 to 58 +/- 6% (P less than 0.005). No correlations between the changes in biliary lipid composition and those in plasma lipoprotein levels were observed. The results indicate that treatment with acipimox in patients with FCHL, a disorder commonly associated with supersaturated bile, does not increase biliary cholesterol, and presumably not the risk for gallstone formation.     

Plasma triglyceride secretion and metabolism in chronic renal failure.

Endogenous very low density lipoprotein triglyceride (VLDL-TG) production rate has been studied in 13 patients with chronic renal failure who were not on dialysis treatment. In these patients the VLDL-TG plasma concentration was significanlty raised when compared with the control group; the fractional turnover rate was reduced but the absolute turnover rate was increased. The results suggest that increased hepatic production of VLDL-TG is a contributory factor to the hypertriglyceridaemia of chronic renal failure.     

Suppression by diets rich in fish oil of very low density lipoprotein production in man.

The highly polyunsaturated fatty acids in fish oils lower the plasma triglyceride concentration. We have studied the effect of a diet rich in fish oil on the rate of production of the triglyceride-transporting very low density lipoprotein (VLDL). Seven subjects, five normal and two with hypertriglyceridemia received up to 30% of daily energy needs from a fish oil preparation that was rich in eicosapentaenoic acid and docosahexaenoic acid, omega-3 fatty acids with five and six double bonds, respectively. Compared with a diet similarly enriched with safflower oil (in which the predominant fatty acid is the omega-6 linoleic acid, with two double bonds), the fish oil diet lowered VLDL lipids and B apoprotein concentrations profoundly. High density lipoprotein lipids and A1 apoprotein were also lowered, but the effect on low density lipoprotein (LDL) concentration was inconsistent. The daily production or flux of VLDL apoprotein B, calculated from reinjected autologous 125I-labeled lipoprotein, was substantially less in six subjects studied after 3 wk of fish oil, compared with after safflower oil. This effect on flux was more consistent than that on the irreversible fractional removal rate, which was increased in the four normolipidemic but inconsistent in the hypertriglyceridemic subjects. This suggests that fish oil reduced primarily the production of VLDL. The daily production of VLDL triglyceride, calculated from the kinetics of the triglyceride specific radioactivity-time curves after [3H]glycerol was injected, also showed very substantial reductions in five subjects studied. The marked suppression in VLDL apoprotein B and VLDL triglyceride formation was found not to be due to diminished plasma total free fatty acid or plasma eicosapentaenoic flux, calculated during constant infusions of [14C]eicosapentaenoic acid and [3H]oleic acid in four subjects. In two subjects there was presumptive evidence for substantial independent influx of LDL during the fish oil diet, based on the precursor-product relationship between the intermediate density lipoprotein and LDL apoprotein B specific radioactivity-time curves.     

Measurement of serum apolipoprotein B by radioimmunoassay

Abstract. A sensitive and specific double antibody radioimmunoassay for the major apolipoprotein (apo B) of human serum very low density lipoprotein (VLDL) and low density lipoprotein (LDL) is described. Using anti-LDL and anti-apo B antibodies the immunoreactivity of LDL and apo B were compared. Human LDL and its isolated apo B were not immunologically identical when each antiserum was used with its homologous label; a population of antibodies was selected which reacted with antigenic sites unique to the antigen itself as well as to those which were common to the closely related protein. When the heterologous label was used with either antiserum, a population of antibodies directed against antigenic sites shared by the LDL and apo B molecules was selected.
Apo B in sera samples can be measured using either anti-LDL or anti-apo B antibodies provided that intact LDL was used for preparation of the iodinated tracer and standard. Serum apo B levels in healthy normolipi-daemic males and females were 0.93 ±0.25 g/l (range 0.58–1.39) and 0.90 ± 0.15 g/l (range 0.58–1.12), respectively. The total cholesterol and apo B, and phos-pholipid and apo B concentrations for both males and females were significantly correlated (P<0.05). In another normolipidaemic population ( n = 52), total serum apo B values correlated positively with LDL cholesterol ( r= 0.92, P< 0.001).
Apo B was measured in sera from patients with abetalipoproteinaemia, familial hypercholesterolaemia and Tangiers disease. Apo B was not detected in the serum of subjects with abetalipoproteinaemia, while the apo B level in the familial hypercholesterolaemic subjects was significantly elevated (range 3.26–4.94 g/l) compared to normals (P<0.001). Serum apo B (0.80 g/l) of the subject with Tangier disease was within the normal range.