The V73M mutation in the hepatic lipase gene is associated with elevated cholesterol levels in four Dutch pedigrees with familial combined hyperlipidemia

Familial combined hyperlipidemia (FCHL) is a heritable lipid disorder characterized by multiple lipoprotein phenotypes within a single family. Previously, we have shown an increased incidence of mutations in the LPL gene which was associated with elevated levels of very low density lipoprotein (VLDL) and decreased levels of high density lipoprotein among the families studied. Now, we report the results of our study on the hepatic lipase gene. We found the HL V73M variant to be present in four FCHL families. By means of a pedigree-based maximum log-likelihood method we analyzed the effect of this variant on the lipid levels in these families. Carriers of the HL V73M variant revealed significantly higher levels of total cholesterol (P < 0.01) and apoB (P <0.01). These findings show that the HL V73M mutant explains another part of the variability in the phenotype observed among FCHL family members, compared with mutations in the LPL gene. Family analysis shows that in these FCHL families, carriers of mutations in the LPL or HL genes have an increased risk for FCHL compared with their non-carrier relatives.

Familial chylomicronemia caused by a novel type of mutation in the APOE-CI-CIV-CII gene cluster encompassing both the APOCII gene and the first APOCIV gene mutation: APOCII-CIV(Nijmegen)

Apolipoprotein CII (ApoCII) deficiency is a relatively rare cause of the chylomicronemia syndrome, a disorder characterized by severe fasting hypertriglyceridemia and massive accumulation of chylomicrons in plasma. Here we present a case which is the first example of apoCII deficiency caused by a major rearrangement in the APOCII gene. Southern blot analysis revealed an approximately 7.5-kb deletion disrupting the APOCII gene including the promotor region and first exon. Interestingly, the deletion also encompasses the APOCIV gene, a recently discovered novel gene upstream of APOCII. This deletion is the first mutation to be reported in the APOCIV gene.

Severe hyperlipidemia in apolipoprotein E2 homozygotes due to a combined effect of hyperinsulinemia and an SstI polymorphism

More than 90% of patients with type III hyperlipoproteinemia are homozygous carriers of the apolipoprotein (apo) E*2 allele. The great majority of these apoE2(Arg158-->Cys) homozygotes in the general population, however, are normolipidemic. Apparently, expression of the hyperlipidemic state requires additional genetic and/or environmental factors, suggesting a multifactorial etiology. To elucidate these additional risk factors, we analyzed normolipidemic and hyperlipidemic apoE2 homozygotes. Hyperinsulinemia was observed in 27 of 49 apoE2 homozygotes and associated with elevated lipid levels: hyperinsulinemic apoE2 homozygotes had type III hyperlipoproteinemia 6 times more often than apoE2 homozygotes with normal insulin levels (odds ratio 6.2, P=0.02). We screened the normolipidemic and hyperlipidemic apoE2 homozygotes for common variants in candidate genes involved in lipolysis-the APOA1-C3-A4 gene cluster, lipoprotein lipase, and hepatic lipase-and analyzed for associations with the expression of hyperlipidemia. In the hyperinsulinemic group, the 7 carriers of the SstI polymorphism (S2) in the APOC3 gene displayed severely elevated VLDL cholesterol (P(insulin by SstI)<0.001) and VLDL triglyceride (P(insulin by SstI)<0.01) and low levels of HDL (P(insulin by SstI)<0.02). In the normoinsulinemic group, no such relation of the SstI polymorphism with hyperlipidemia was observed. These data provide the first evidence for a combined effect of hyperinsulinemia and the SstI polymorphism on the expression of hyperlipidemia in apoE2 homozygotes.

Increased risk for endogenous hypertriglyceridaemia is associated with an apolipoprotein C3 haplotype specified by the SstI polymorphism

BACKGROUND

Hypertriglyceridaemia is a common metabolic disorder frequently found in patients with coronary heart disease. Numerous studies have revealed an association between the SstI polymorphism in the APOC3 gene and increased plasma apoC3 and triglyceride levels. In addition, two different variants within the promoter region have been recently suggested to be the mutations of the APOC3 gene leading to hypertriglyceridaemia.

METHODS

In the present study, we have applied haplotype analysis to investigate whether these promoter polymorphisms are involved in the lipid disorders of patients with distinct types of hypertriglyceridaemia: combined hyperlipidaemia (CHL), familial dysbetalipoproteinaemia (FD) and endogenous hypertriglyceridaemia (HTG).

RESULTS

The -482 and -455 polymorphisms were significantly more frequent in FD patients (P = 0. 017) and endogenous HTG patients (P < 0.0001) than in CHL patients and a control group. The SstI polymorphism was only significantly more frequent in HTG patients (P < 0.0001). However, we did not find differences in frequencies for these polymorphisms in the APOC3 gene between CHL patients and a control group. Haplotype analysis indicates that the SstI polymorphism arose on the allele containing both promoter polymorphisms.

CONCLUSION

The haplotype containing the SstI polymorphism is found five times more frequently among HTG patients (OR 5.28, 95% CI 1.65-16.90), which strongly suggests it is associated with an increased risk for severe hypertriglyceridaemia.

Gender-related association between the -93T-->G/D9N haplotype of the lipoprotein lipase gene and elevated lipid levels in familial combined hyperlipidemia

Familial combined hyperlipidemia (FCHL) is a frequent cause of premature coronary artery disease. Affected family members are characterized by different combinations of elevated cholesterol and/or triglyceride levels. A reduction in lipoprotein lipase (LPL) activity has been observed in a subgroup of FCHL patients. Recently, we have demonstrated an increased frequency of mutations in the LPL gene in Dutch FCHL patients compared to normolipidemic controls. In the present study, we have applied a pedigree-based maximum likelihood method to study the effect of LPL mutations on the phenotypic expression of FCHL in families. In 40 FCHL probandi, three different previously reported mutations in the LPL gene were identified resulting in amino acid changes, D9N, N291S, and S447X. The D9N mutation in exon 2 appeared to be in strong linkage disequilibrium with a T-->G substitution at position -93 in the promoter region of the LPL gene. We present data that the -93T-->G/D9N haplotype is associated with significantly higher levels of LDL and VLDL cholesterol, and VLDL triglycerides. Interestingly, the effect was only observed in male carriers. In line with our previous observations, these results further sustain that the LPL gene is a susceptibility gene for dyslipidemia which explains part of the variability in the phenotype observed among FCHL family members.

Effect of apolipoprotein E and insulin resistance on VLDL particles in combined hyperlipidemic patients

Apolipoprotein (apo) E2 and high insulin levels are associated with the severity of hypertriglyceridemia in patients with combined hyperlipidemia. To study how these determinants affect very low-density lipoprotein (VLDL) in combined hyperlipidemic patients, we characterized VLDL particles in 106 unrelated patients with combined hyperlipidemia. The study was performed after 9 weeks of standardized dietary intake and after an overnight fast. Patients heterozygous for apoE2 had significantly higher mean levels of VLDL cholesterol by 0.71 mmol/l (95% CI, 0.30 to 1.12 mmol/l, P < 0.005) and VLDL triglycerides by 0.88 mmol/l, (95% CI, 0.30 to 1.47 mmol/l, P < 0.005) compared to patients without apoE2. The VLDL triglyceride content per particle and the calculated diameter of the VLDL particles were similar in both groups, which indicate a higher number of circulating VLDL particles in heterozygous apoE2 carriers. Patients with high fasting insulin levels (> or = 80 pmol/l) had a higher mean serum VLDL triglyceride level by 0.56 mmol/l (95% CI, 0.04 to 1.07 mmol/l, P < 0.05). The calculated VLDL diameter was larger by 3.7 nm (95% CI, 1.2 to 6.2 nm, P < 0.005) and the particles contained more triglycerides by 2.7 weight percent (95% CI, 0.3 to 5.1 weight percent, P < 0.05). These insulin-dependent changes in VLDL particles were only present in the absence of apoE2. In conclusion, patients heterozygous for apoE2 have higher numbers of circulating VLDL particles, whereas patients with high fasting insulin levels have larger, triglyceride enriched VLDL particles.

Apolipoprotein E1-Hammersmith (Lys146-->Asn;Arg147-->Trp), due to a dinucleotide substitution, is associated with early manifestation of dominant type III hyperlipoproteinaemia

Apolipoprotein E (apoE) is one of the major protein constituents of chylomicron and very low density lipoprotein (VLDL) remnants and plays a central role as a ligand in the receptor-mediated uptake of these particles by the liver. Here we describe a new variant of apoE, apoE1-Hammersmith, which is associated with dominantly expressed type III hyperlipidaemia. The propositus, aged 26, developed tubero-eruptive xanthomas at the age of 3, her daughter developed similar lesions at age 7 but her son, aged 3, shows no clinical abnormality so far. All three cases had an apoE3E1 phenotype and a broad beta band on lipoprotein electrophoresis. Cysteamine modification resulted in a shift of apoE1 to the apoE2 isoform position, indicating that the mutation leading to apoE1-Hammersmith occurred on an apoE3 background. ApoE genotyping confirmed these results. Sequence analysis of DNA of the propositus was performed for exons 3 and 4 and revealed a dinucleotide substitution causing two amino acid changes at adjacent positions (Lys146-->Asn) and (Arg147-->Trp).

The lipoprotein lipase (Asn291-->Ser) mutation is associated with elevated lipid levels in families with familial combined hyperlipidaemia

Familial combined hyperlipidaemia (FCHL) is one of the major genetic causes of coronary heart disease (CHD) and is characterised by elevated levels of plasma cholesterol and/or triglycerides in individuals within a single family. Decreased lipoprotein lipase (LPL) activity has been found in some cases of FCHL. A recent study revealed a common mutation in the LPL gene, LPL(Asn291-->Ser), with a frequency of 9.3% in Dutch FCHL patients (Reymer et al,. Circulation, 90 (1994) I-998). This mutation was found in 3 out of 17 FCHL families. Extensive family studies were subsequently performed to determine the effect of this mutation on the phenotypic expression of FCHL. Using a pedigree-based maximum likelihood estimate, we demonstrated that the LPL(Asn291-->Ser) mutation significantly affects the levels of plasma and very low density lipoprotein (VLDL) triglycerides (2.03 +/- 0.21 vs. 1.14 +/- 0.13 and 1.21 +/- 0.16 vs. 0.62 +/- 0.09 mmol/l, carriers and non-carriers, respectively) and VLDL- and high density lipoprotein (HDL) cholesterol (0.83 +/- 0.10 vs. 0.38 +/- 0.06 and 1.02 +/- 0.08 vs. 1.29 +/- 0.05 mmol l, carriers and non-carriers, respectively), but not those of plasma and low density lipoprotein (LDL) cholesterol. These findings indicate that the LPL(Asn291-->Ser) mutation is associated with elevated lipid levels, indicating it may be one of the genetic factors predisposing to FCHL in the families studied.

Effect of insulin resistance, apoE2 allele, and smoking on combined hyperlipidemia

Combined hyperlipidemia may result from the interaction of several metabolic and environmental factors. We explored to what extent fasting insulin concentration, apolipoprotein (apo) E2 frequency, and cigarette smoking explained the serum levels of triglyceride and high-density lipoprotein cholesterol (HDL-C) in patients with combined hyperlipidemia. Forty-nine untreated patients with combined hyperlipidemia were compared with 49 hypercholesterolemic patients who were matched for gender, age, and body mass index. All laboratory values were obtained after 9 weeks of standardized dietary intake and after an overnight fast. The patients with combined hyperlipidemia had a significantly higher (33 pmol/L, 50%) mean insulin concentration than matched hypercholesterolemic control subjects, indicating that the combined hyperlipidemic patients were more insulin resistant. However, the differences in the fasting insulin and triglyceride concentrations within the pairs were only slightly correlated (adjusted r = .29). The combined hyperlipidemic patients were also characterized by a higher frequency of apoE2 alleles (25% versus 6%) and smokers (41% versus 16%). In a matched multiple linear regression model, the differences in insulin concentration, apoE2 allele frequency, and smoking explained 12%, 8%, and 9%, respectively, of the mean paired difference in triglyceride concentration. The differences in insulin concentration or apoE2 allele frequency did not significantly explain the mean paired difference in HDL-C concentration, whereas smoking explained 17% of the difference. In conclusion, fasting insulin concentration, the presence of the apoE2 allele, and smoking may explain 30% of the hypertriglyceridemia and the low levels of HDL-C in nonobese patients with combined hyperlipidemia.(ABSTRACT TRUNCATED AT 250 WORDS)

The apolipoprotein C2-linked (Acl) gene: a new gene within the mouse apolipoprotein e-c1-c2 gene cluster

The apolipoprotein E, C1, and C2 genes are contained within a gene cluster in man. Previously, we have shown that this gene cluster has a similar structure in mouse. During the characterization of the mouse Apoc2 gene, evolutionarily conserved and transcribed sequences were found 5' of the Apoc2 gene. In this study, we have shown that these 5' sequences represent a novel gene within the gene cluster designated the apolipoprotein C2-linked gene (Acl). The Acl gene is located 2 kb 5' to the Apoc2 gene. The transcriptional orientation is identical to that of the other genes within the Apoe-c1-c2 gene cluster. We have sequenced the mouse Acl gene at the cDNA and the genomic levels. The gene is composed of three exons spanning a region of approximately 3.6 kb. The Acl gene is expressed in the liver as a transcript 473 bp in size and encodes a putative protein of 124 amino acid residues.

An acceptor splice site mutation in intron 16 of the low density lipoprotein receptor gene leads to an elongated, internalization defective receptor

In this report, we describe the characterization of a mutation in the low density lipoprotein (LDL) receptor gene of a true homozygous familial hypercholesterolemic (FH) patient. The combined use of denaturing gradient gel electrophoresis (DGGE) and DNA sequence analysis revealed a unique A to G transition in the penultimate 3'-nucleotide of intron 16 of the LDL receptor gene, which disrupts the acceptor splice site. cDNA sequence analysis indicated that a cryptic splice site was activated in intron 16, upstream from the original splice site, leading to the inclusion of 62 nucleotides and a reading frame-shift. The resulting new translation product contains a stretch of 154 amino acids at the carboxy-terminal that have no resemblance to the normal receptor protein. To elucidate the biological effects of the mutation, the structural and functional properties of the mutated LDL receptor protein were studied. Immunoprecipitation of the newly synthesized LDL receptors showed that an aberrant precursor form of the LDL receptor protein was synthesized, about 10 kDa larger than normal, which is not further processed to the mature form. Some 50% of the normal LDL binding activity was found on the cell surface of the patient's fibroblasts, whereas internalization and degradation of LDL were abolished.

The mouse apolipoprotein C1 gene: structure and expression

We have isolated and characterized cDNA and genomic clones containing the mouse apolipoprotein C1 (Apoc1) gene. The Apoc1 gene is part of the Apoe-c1-c2 gene cluster and is located 3.4 kb 3' of the Apoe gene. The mouse Apoc1 gene spans a region of approximately 3.3 kb and consists of four exons. The exon-intron structure is similar to those of human and baboon genes, although in mouse introns 2 and 3 are smaller. Significant sequence homology is found between man and mouse in the promoter and exonic regions (80 and 67%, respectively). Northern blotting and primer extension analysis of mouse RNA showed that a major transcript 409 bp in size is expressed primarily in fetal and adult liver. The mouse Apoc1 cDNA contains an open reading frame encoding a protein of 88 amino acids, including a signal peptide of 26 amino acid residues. Comparisons of the deduced amino acid sequence of mouse apoC1 with the human, baboon, rat, and dog sequences showed discrete regions with a high degree of conservation. The delineation of the sequence and structural organization of the mouse Apoc1 gene is an essential step in enhancing the use of mouse models to study the function of apoC1 in the lipoprotein metabolism.

Structure and expression of the mouse apolipoprotein C2 gene

Three cDNA clones containing the mouse apolipoprotein C2 (Apoc2) gene were isolated from a mouse liver cDNA library. The inserts from two cDNA clones were 500 bp in size while the insert from the third clone was unexpectedly large, 962 bp. All three clones contained a single open reading frame encoding apoC2. The exon-intron structure of the mouse Apoc2 gene was determined by sequence analysis. Northern blotting and primer extension analysis of mouse RNA showed that the major liver transcript is 500 bp in size and is encoded by four exons. Transcripts for Apoc2 were found in fetal liver, adult liver, intestine, and peritoneal macrophages. The largest cDNA clone, mAPOC2c4, contained an additional 440 bp at the 5' end that are evolutionary conserved between man and mouse. These additional sequences are encoded by two exons located 5' to the major liver start site. Although the larger transcript could not be detected by Northern blot analysis, products resulting from an upstream transcription initiation site were detected in the liver using RT-PCR analysis. The sizes of the RT-PCR products are consistent with alternative splicing.

The mouse low density lipoprotein receptor gene: cDNA sequence and exon-intron structure

The low density lipoprotein (LDL) receptor plays a central role in the cholesterol metabolism. The cDNA sequence of the mouse low density lipoprotein receptor (Ldlr) gene has been determined and shows 76% homology with the human gene. The exon-intron structure has been determined for the 129/J mouse strain. The gene is composed of 18 exons and spans a region of 28 kb. In addition, the promoter regions of the mouse and human genes are homologous. Northern blot analysis revealed an mRNA of approximately 5 kb. The cloning of the Ldlr gene will enhance the usefulness of the mouse for the study of cholesterol metabolism and, in particular, for carrying out gene targeting experiments.

Evolutionary conservation of the mouse apolipoprotein e-c1-c2 gene cluster: structure and genetic variability in inbred mice

The human apolipoprotein E (APOE), APOC1, pseudo APOC1 (APOC1'), and APOC2 genes are clustered within 48 kb on the long arm of chromosome 19. A mouse Apoe cDNA probe was used to isolate overlapping cosmid clones from a cosmid library of the C57BL/Rij inbred mouse strain. These clones were investigated for the presence of the Apoc1 and Apoc2 genes by heterologous hybridization. Our results show that the Apoe-c1-c2 gene cluster is conserved in the mouse. In line with evolutionary data, the mouse lacks the equivalent of APOC1'. These data were confirmed using a mouse Apoc2 cDNA clone, and surprisingly the cDNA clone isolated here was 965 bp in size, which is on average 450 bp longer than other APOC2 cDNAs described so far. Correspondingly, the Apoc2 gene occupies an unusually large genomic region, due to an extended 5' end. Interestingly, a variable number of tandem repeat (VNTR) in the third intron of the human APOC2 gene shows a high sequence homology and is located at the identical position in the mouse gene. Despite the high copy number of this VNTR (27 or 34 copies) only two variants were found among 11 different inbred strains. With the aid of six restriction fragment length variations in this gene cluster only two different haplotypes could be deduced, indicating that the Apoe-c1-c2 gene cluster is highly conserved in the inbred strains that were studied.

Nucleotide sequence comparison of five human pepsinogen A (PGA) genes: evolution of the PGA multigene family

To unravel the genetic basis for the pepsinogen A (PGA) protein polymorphism, we have isolated and characterized a number of PGA genes, distinguishable by polymorphic EcoRI fragments of 12.0, 15.0, and 16.6 kb. Using a HindIII or AvaII polymorphism, we can discriminate between different 15.0 (15.0 and 15.0*) and 12.0 (12.0s and 12.0l) genes, respectively. The coding sequences of a 15.0 and a 16.6 gene were determined, together with considerable stretches of the 5'- and 3'-flanking regions and introns. The genes were demonstrated to encode Pg5 and Pg4, respectively. Because substitutions in codons 43 and 207 appeared to be critical in the determination of the encoded proteins, we sequenced only these regions in the two 12.0 genes and the 15.0* gene. On the basis of these partial sequences, we assume that these genes encode Pg3. In the evolutionary model of the PGA gene cluster presented here, the 12.0 genes arose by an unequal, but homologous crossover. The results of sequence analysis of the second intron of the 12.0s, 12.0l, 15.0, and 16.6 genes suggest that the two 12.0 genes have arisen from two different crossover events.