D.J. Galton

Apolipoprotein B gene variants are involved in the determination of serum cholesterol levels: a study in normo- and hypelipidaemic individuals☆

We have investigated the frequencies of 3 restriction fragment length polymorphisms (RFLPs) of the apolipoprotein B (apo B) gene in normo- and hyperlipidaemic individuals. In individuals with type III hyperlipidaemia, the allele frequency for the RFLP detected with XbaI was significantly different from the allele frequency in normolipidaemic individuals and in those with other types of hyperlipidaemia. No significant difference in allele frequency was found among these groups for the RFLPs detected with MspI or EcoRI. Within a sample of 62 normolipidaemic individuals, homozygotes for the X2 allele (cutting site) of the XbaI RFLP had a significantly higher serum cholesterol level than homozygotes for the XI allele, with individuals of the genotype X1X2 having an intermediate value (X2X2 mean 5.71 mmol/l, X1X1 mean 4.81 mmol/l, X1X2 mean 5.30 mmol/l). There were also significant differences in serum triglyceride levels in individuals with different XbaI genotypes. In these normolipidaemic individuals there was no correlation between the EcoRI and MspI RFLP genotypes and levels of any serum lipid variable. Information from the XbaI and EcoRI RFLPs was used in conjunction to define apo B haplotypes. These haplotypes are a more precise measure of the genotypic variation, and they explain a greater fraction of the serum cholesterol and triglyceride levels than the single-site polymorphisms considered separately. This study suggests that variations in the gene for apo B are associated with the determination of serum cholesterol and triglyceride levels both in patients with type III hyperlipidaemia and in the normal population.

DNA polymorphisms at the lipoprotein lipase gene: associations in normal and hypertriglyceridaemic subjects

Lipoprotein lipase is a rate determining enzyme for the removal of triglyceride-rich lipoproteins from the blood stream. We examined whether genetic variation at the lipoprotein lipase gene locus was related to the fasting plasma level of triglycerides in both a normal and hypertriglyceridaemic population. Two restriction fragment length polymorphisms revealed by the enzymes PvuII and HindIII generated alleles designated H1, 17.5 kb;H2, 8.7 kb;P1, 7.0 kb;P2, 4.4 and 2.5 kb, respectively. These were studied in 46 Caucasian hypertriglyceridaemic subjects in comparison with 86 normolipidaemic controls. The respective allelic frequencies were H1 0.211, H2 0.789 and H1 0.414, H2 0.586 (p less than 0.01). Similar differences in allelic frequencies were found in a smaller group of Japanese hypertriglyceridaemic subjects (n = 29) compared to Japanese controls (n = 41, p less than 0.01). Ninety-three healthy Caucasians were genotyped for both polymorphic sites to relate to levels of plasma triglyceride. We found that individuals with genotype P1P1 had fasting triglyceride levels of 0.96 +/- 0.31 mmol/l (n = 20) compared to genotype P2P2 with levels of 1.31 +/- 0.66 mmol/l (n = 30, p less than 0.02); heterozygous subjects (P1P2) had intermediate levels of plasma triglyceride (1.15 +/- 0.46 mmol/l, n = 43). The HindIII alleles were not significantly associated with variation in levels of plasma triglyceride, cholesterol, or HDL-cholesterol. We conclude that DNA variations at, or around, the lipoprotein lipase gene may constitute genetic determinants for both the population variation in plasma triglyceride levels as well as for the common metabolic disorder of primary hypertriglyceridaemia.

High-density and low-density lipoproteins and prevalence of vascular disease in diabetes mellitus.

The prevalence of vascular disease among 154 diabetic patients was analysed in relation to the serum concentrations of individual lipoproteins. Overal the presence of vascular disease (59 cases) was positively associated with serum cholesterol and low-density-lipoprotein cholesterol but negatively associated with high-density-lipoprotein (HDL) cholesterol. The negative relation between HDL and vascular disease was not observed in all subgroups of diabetics. We conclude that there may be no overriding association between HDL and vascular disease in diabetics as proposed for some non-diabetic populations.

Linkage and Association Between Distinct Variants of the APOA1/C3/A4/A5 Gene Cluster and Familial Combined Hyperlipidemia

Objective—Combined hyperlipidemia is a common disorder, characterized by a highly atherogenic lipoprotein profile and a substantially increased risk of coronary heart disease. The purpose of this study was to establish whether variations of apolipoprotein A5 (APOA5), a newly discovered gene of lipid metabolism located 30 kbp downstream of the APOA1/C3/A4 gene cluster, contributes to the transmission of familial combined hyperlipidemia (FCHL). Methods and Results—We performed linkage and association tests on 128 families. Two independent alleles, APOA5c.56G and APOC3c.386G, of the APOA1/C3/A4/A5 gene cluster were overtransmitted in FCHL (P =0.004 and 0.007, respectively). This was paired with reduced transmission of the common APOA1/C3/A4/A5 haplotype (frequency 0.4461) to affected subjects (P =0.012). The APOA5c.56G genotype accounted for 7.3% to 13.8% of the variance in plasma triglyceride levels in probands (P <0.004). The APOC3c.386G genotypes accounted for 4.4% to 5.1% of the variance in triglyceride levels in FCHL spouses (P <0.007), suggesting that this allele marks a FCHL quantitative trait as well as representing a susceptibility locus for the condition. Conclusions—A combined linkage and association analysis establishes that variation at the APOA1/C3/A4/A5 gene cluster contributes to FCHL transmission in a substantial proportion of northern European families.

Lipoprotein and hepatic lipase gene variants in coronary atherosclerosis

Lipoprotein lipase is the rate determining enzyme for the removal of triglyceride rich lipoproteins from the blood stream. We examined whether genetic variation at the lipoprotein lipase gene locus is related to the occurrence of premature coronary artery disease. Two restriction fragment length polymorphisms, revealed by the enzymes HindIII and PvuII, demonstrated alleles designated H1 (17.5 kb), H2 (8.7 kb), P1 (7.0 kb), P2 (4.4 kb and 2.5 kb) respectively. These were studied in 70 Caucasian subjects with severe coronary atherosclerosis in comparison with 122 Caucasian healthy controls. The allelic frequencies for cases and controls were respectively: H2 0.770, 0.579 (P less than 0.001); P2 0.575, 0.554 (P NS). The allelic frequencies of the HindIII and BglII polymorphic sites at the hepatic lipase gene locus were also studied in the same groups of subjects. These showed no differences between cases and controls. We conclude that DNA variation at or adjacent to the lipoprotein lipase gene may contain genetic determinants for the occurrence of premature coronary artery disease.

GENETIC POLYMORPHISMS OF APOLIPOPROTEIN C-III AND INSULIN IN SURVIVORS OF MYOCARDIAL INFARCTION

Genetic polymorphisms of apolipoprotein C-III (apo C-III) and insulin were studied in 48 Caucasian post-myocardial infarction patients. 10 patients (21%) had an uncommon allelic variant of the apo A-I/C-III gene cluster (on the long arm of chromosome 11); in 47 control subjects this variant was present in only 2 and in none of those who were normotriglyceridaemic. Distribution of genotypes between post-infarct and normotriglyceridaemic control groups was significantly different and the difference persisted when the normotriglyceridaemic subgroups of these patients were compared. In contrast, there was no difference in distribution of alleles at the highly polymorphic locus on the short arm of chromosome 11 adjacent to the insulin gene.

Type 1 (insulin-dependent) diabetes and a highly variable locus close to the insulin gene on chromosome 11

SummaryA polymorphic DNA sequence in the 5′-flanking region of the human insulin gene was studied in relation to Type 1 (insulin-dependent) diabetes. In 141 Caucasoid subjects analysed by Southern blot hybridisation techniques, two major DNA insertions were observed: a Class 1 allele or a Class 3 allele. The Class 2 allele was not observed in this group of subjects. Genotype frequencies in a control population (n = 88) were: homozygous 1/1, 42%; heterozygous 1/3, 50%; and homozygous 3/3, 8%. Corresponding genotype frequencies in 53 Type 1 diabetic patients were 79%, 21% and 0%, respectively (p<0.0005 from χ2 test). This confirms prevalence data reported by Bell et al. [16]. There appeared to be no coinheritance with HLA-DR3/DR4 related antigens, nor with autoimmune features. Analysis of 17 Type 1 diabetic pedigrees including 34 diabetic and 69 non-diabetic subjects did not demonstrate genetic linkage of these DNA inserts with diabetes, using an autosomal recessive, single locus model of inheritance.

An abnormal triglyceride-rich lipoprotein containing excess sialylated apolipoprotein C-III.

An abnormal triglyceride-rich lipoprotein has been isolated from some patients with chronic renal failure or severe hypertriglyceridemia. The abnormal lipoprotein was characterized by an increased content of apolipoprotein (apo) C-III-2 (57.5% of total apo C-III peptides compared with 35.5% for controls, P less than 0.001) as characterized by isoelectric focusing and scanning densitometry. As determined by a substrate competition assay, the abnormal lipoprotein was a less efficient substrate for purified bovine milk lipoprotein lipase than control lipoproteins. Neuraminidase digestion of abnormal or control lipoprotein resulted in a reduction of the apo C-III-2 band with a corresponding increase in the region of apo C-III-0, which suggests that the increased content of apo C-III-2 in the abnormal is due to excessive sialylation of the C-III peptide. Limited incubation of the abnormal lipoproteins with neuraminidase caused a partial loss of sialic acid and resulted in a triglyceride-rich lipoprotein with a normal C-III-2:C-III-1 ratio. This preparation displayed normal substrate interaction with lipoprotein lipase. Three severely hypertriglyceridemic patients with the abnormal lipoprotein showed a marked reduction in serum triglyceride concentration, which is associated with a reversion to a normal C-peptide profile after dietary therapy. The results suggest that the extent of sialylation of the apo C-III peptide carried on triglyceride-rich lipoproteins may be critical for their interaction with lipoprotein lipase.

Insulin-independent diabetes: A defect in the activity of lipoprotein lipase in adipose tissue

SummaryThe activity of lipoprotein lipase (E.C.3.1.1.3.) has been measured in adipose tissue from insulin-independent diabetics with hypertriglyceridaemia, non-diabetics with hypertriglyceridaemia and control patients, all of whom were obese. Although all groups showed an increase of plasma insulin after oral glucose, both the diabetic and nondiabetic hypertriglyceridaemics had impaired activities of lipoprotein lipase in adipose tissue compared to the obese normals (p<0.02, p<0.03, respectively). A course of insulin therapy (20 u.o.d.) for one week increased the activity of lipoprotein lipase extracted from adipose tissue, lowered plasma triglycerides and improved triglyceride clearance from plasma in a group of diabetics with hypertriglyceridaemia (mean plasma triglyceride 8.7 mmol/l). Our results suggest that a feature in the development of insulin resistance in adult diabetics may be a failure of maintenance of key intracellular enzyme activities involved in lipid metabolism.

Confirmed Locus on Chromosome 11p and Candidate Loci on 6q and 8p for the Triglyceride and Cholesterol Traits of Combined Hyperlipidemia

Background—Combined hyperlipidemia is a common disorder characterized by a highly atherogenic lipoprotein profile and increased risk of coronary heart disease. The etiology of the lipid abnormalities (increased serum cholesterol and triglyceride or either lipid alone) is unknown. Methods and Results—We assembled 2 large cohorts of families with familial combined hyperlipidemia (FCHL) and performed disease and quantitative trait linkage analyses to evaluate the inheritance of the lipid abnormalities. Chromosomal regions 6q16.1-q16.3, 8p23.3-p22, and 11p14.1-q12.1 produced evidence for linkage to FCHL. Chromosomes 6 and 8 are newly identified candidate loci that may respectively contribute to the triglyceride (logarithm of odds [LOD], 1.43; P =0.005) and cholesterol (LOD, 2.2; P =0.0007) components of this condition. The data for chromosome 11 readily fulfil the guidelines required for a confirmed linkage. The causative alleles may contribute to the inheritance of the cholesterol (LOD, 2.04 at 35.2 cM; P =0.0011) component of FCHL as well as the triglyceride trait (LOD, 2.7 at 48.7 cM; P =0.0002). Conclusions—Genetic analyses identify 2 potentially new loci for FCHL and provide important positional information for cloning the genes within the chromosome 11p14.1-q12.1 interval that contributes to the lipid abnormalities of this highly atherogenic disorder.