Incomplete dominance of type III hyperlipoproteinemia is associated with the rare apolipoprotein E2 (Arg136-->Ser) variant in multigenerational pedigree studies

Human apolipoprotein E glycosylation and sialylation: from structure to function

Human apolipoprotein E (ApoE) was first identified as a polymorphic gene in the 1970s; however, the genetic association of ApoE genotypes with late-onset sporadic Alzheimer's disease (sAD) was only discovered 20 years later. Since then, intensive research has been undertaken to understand the molecular effects of ApoE in the development of sAD. Despite three decades' worth of effort and over 10,000 papers published, the greatest mystery in the ApoE field remains: human ApoE isoforms differ by only one or two amino acid residues; what is responsible for their significantly distinct roles in the etiology of sAD, with ApoE4 conferring the greatest genetic risk for sAD whereas ApoE2 providing exceptional neuroprotection against sAD. Emerging research starts to point to a novel and compelling hypothesis that the sialoglycans posttranslationally appended to human ApoE may serve as a critical structural modifier that alters the biology of ApoE, leading to the opposing impacts of ApoE isoforms on sAD and likely in the peripheral systems as well. ApoE has been shown to be posttranslationally glycosylated in a species-, tissue-, and cell-specific manner. Human ApoE, particularly in brain tissue and cerebrospinal fluid (CSF), is highly glycosylated, and the glycan chains are exclusively attached via an O-linkage to serine or threonine residues. Moreover, studies have indicated that human ApoE glycans undergo sialic acid modification or sialylation, a structural alteration found to be more prominent in ApoE derived from the brain and CSF than plasma. However, whether the sialylation modification of human ApoE has a biological role is largely unexplored. Our group recently first reported that the three major isoforms of human ApoE in the brain undergo varying degrees of sialylation, with ApoE2 exhibiting the most abundant sialic acid modification, whereas ApoE4 is the least sialylated. Our findings further indicate that the sialic acid moiety on human ApoE glycans may serve as a critical modulator of the interaction of ApoE with amyloid β (Aβ) and downstream Aβ pathogenesis, a prominent pathologic feature in AD. In this review, we seek to provide a comprehensive summary of this exciting and rapidly evolving area of ApoE research, including the current state of knowledge and opportunities for future exploration.

Multifaceted roles of APOE in Alzheimer disease

For the past three decades, apolipoprotein E (APOE) has been known as the single greatest genetic modulator of sporadic Alzheimer disease (AD) risk, influencing both the average age of onset and the lifetime risk of developing AD. The APOEε4 allele significantly increases AD risk, whereas the ε2 allele is protective relative to the most common ε3 allele. However, large differences in effect size exist across ethnoracial groups that are likely to depend on both global genetic ancestry and local genetic ancestry, as well as gene-environment interactions. Although early studies linked APOE to amyloid-β - one of the two culprit aggregation-prone proteins that define AD - in the past decade, mounting work has associated APOE with other neurodegenerative proteinopathies and broader ageing-related brain changes, such as neuroinflammation, energy metabolism failure, loss of myelin integrity and increased blood-brain barrier permeability, with potential implications for longevity and resilience to pathological protein aggregates. Novel mouse models and other technological advances have also enabled a number of therapeutic approaches aimed at either attenuating the APOEε4-linked increased AD risk or enhancing the APOEε2-linked AD protection. This Review summarizes this progress and highlights areas for future research towards the development of APOE-directed therapeutics.

APOE and familial hypercholesterolemia

PURPOSE OF REVIEW

Autosomal dominant hypercholesterolemia is a common cause of cardiovascular disease. In addition to the classic genes that cause hypercholesterolemia, LDLR, APOB and PCSK9 , a new locus has emerged as a candidate to be the cause of this hyperlipidemia, the p.(Leu167del) mutation in the APOE gene.

RECENT FINDINGS

Various studies have demonstrated the involvement of the p.(Leu167del) mutation in the APOE gene in hypercholesterolemia: Studies of family segregation, lipoprotein composition by ultracentrifugation and proteomic techniques, and functional studies of VLDL-carrying p.(Leu167del) internalization with cell cultures have demonstrated the role of this mutation in the cause of hypercholesterolemia. The phenotype of individuals carrying the p.(Leu167del) in APOE is indistinguishable from familial hypercholesterolemia individuals with mutations in the classic genes. However, a better response to lipid-lowering treatment has been demonstrated in these APOE mutation carrier individuals.

SUMMARY

Therefore, APOE gene should be considered a candidate locus along with LDLR, APOB , and PCSK9 to be investigated in the genetic diagnosis of familial hypercholesterolemia.

APOE3ch alters microglial response and suppresses Aβ-induced tau seeding and spread

A recent case report described an individual who was a homozygous carrier of the APOE3 Christchurch (APOE3ch) mutation and resistant to autosomal dominant Alzheimer's Disease (AD) caused by a PSEN1-E280A mutation. Whether APOE3ch contributed to the protective effect remains unclear. We generated a humanized APOE3ch knock-in mouse and crossed it to an amyloid-β (Aβ) plaque-depositing model. We injected AD-tau brain extract to investigate tau seeding and spreading in the presence or absence of amyloid. Similar to the case report, APOE3ch expression resulted in peripheral dyslipidemia and a marked reduction in plaque-associated tau pathology. Additionally, we observed decreased amyloid response and enhanced microglial response around plaques. We also demonstrate increased myeloid cell phagocytosis and degradation of tau aggregates linked to weaker APOE3ch binding to heparin sulfate proteoglycans. APOE3ch influences the microglial response to Aβ plaques, which suppresses Aβ-induced tau seeding and spreading. The results reveal new possibilities to target Aβ-induced tauopathy.

Establishing the relationship between Familial Dysbetalipoproteinemia and genetic variants in the APOE gene

Familial Dysbetalipoproteinemia (FD) is the second most common monogenic dyslipidemia and is associated with a very high cardiovascular risk due to cholesterol‐enriched remnant lipoproteins. FD is usually caused by a recessively inherited variant in the APOE gene (ε2ε2), but variants with dominant inheritance have also been described. The typical dysbetalipoproteinemia phenotype has a delayed onset and requires a metabolic hit. Therefore, the diagnosis of FD should be made by demonstrating both the genotype and dysbetalipoproteinemia phenotype. Next Generation Sequencing is becoming more widely available and can reveal variants in the APOE gene for which the relation with FD is unknown or uncertain. In this article, two approaches are presented to ascertain the relationship of a new variant in the APOE gene with FD. The comprehensive approach consists of determining the pathogenicity of the variant and its causal relationship with FD by confirming a dysbetalipoproteinemia phenotype, and performing in vitro functional tests and, optionally, in vivo postprandial clearance studies. When this is not feasible, a second, pragmatic approach within reach of clinical practice can be followed for individual patients to make decisions on treatment, follow‐up, and family counseling.

Diagnostic yield of sequencing familial hypercholesterolemia genes in individuals with primary hypercholesterolemia

INTRODUCTION AND OBJECTIVES

Our objective was to approximate the prevalence of mutations in candidate genes for familial hypercholesterolemia (FH) in a middle-aged Spanish population and to establish the predictive value of criteria for clinical suspicion in the detection of causative mutations.

METHODS

Unrelated individuals aged ≥ 18 years from the Aragon Workers' Health Study (AWHS) with high low-density lipoprotein cholesterol (LDL-C) and clinical suspicion of FH (participants with LDL-C concentrations above the 95th percentile, participants with premature cardiovascular disease and/or participants with high LDL-C [130 mg/dL] under statin therapy), assuming that any participant with FH exhibits at leats 1 trait, were selected and the LDLR, APOB, PCSK9, APOE, STAP1 and LDLRAP1 genes were sequenced by next generation sequencing technology.

RESULTS

Of 5400 individuals from the AWHS, 4514 had complete data on lipid levels and lipid-lowering drugs, 255 participants (5.65%) met the criteria for suspicion of FH, 24 of them (9.41%) were diagnosed with hyperlipoproteinemia(a), and 16 (6.27% of those sequenced) were found to carry causative mutations in candidate genes: 12 participants carried 11 different pathogenic LDLR alleles and 4 participants carried 1 pathogenic mutation in PCSK9. LDL-C concentrations> 220 mg/dL and LDL-C> 130 mg/dL despite statin therapy showed the strongest association with the presence of mutations (P=.011).

CONCLUSIONS

Our results show that the prevalence of FH in Spain is 1:282 and suggest that the combination of high untreated LDL-C and high levels of LDL-C despite statin therapy are the best predictors of a positive FH genetic test.

APOE gene variants in primary dyslipidemia

Apolipoprotein E (apoE) is a major apolipoprotein involved in lipoprotein metabolism. It is a polymorphic protein and different isoforms are associated with variations in lipid and lipoprotein levels and thus cardiovascular risk. The isoform apoE4 is associated with an increase in LDL-cholesterol levels and thus a higher cardiovascular risk compared to apoE3. Whereas, apoE2 is associated with a mild decrease in LDL-cholesterol levels. In the presence of other risk factors, apoE2 homozygotes could develop type III hyperlipoproteinemia (familial dysbetalipoproteinemia or FD), an atherogenic disorder characterized by an accumulation of remnants of triglyceride-rich lipoproteins. Several rare APOE gene variants were reported in different types of dyslipidemias including FD, familial combined hyperlipidemia (FCH), lipoprotein glomerulopathy and bona fide autosomal dominant hypercholesterolemia (ADH). ADH is characterized by elevated LDL-cholesterol levels leading to coronary heart disease, and due to molecular alterations in three main genes: LDLR, APOB and PCSK9. The identification of the APOE-p.Leu167del variant as the causative molecular element in two different ADH families, paved the way to considering APOE as a candidate gene for ADH. Due to non mendelian interacting factors, common genetic and environmental factors and perhaps epigenetics, clinical presentation of lipid disorders associated with APOE variants often strongly overlap. More studies are needed to determine the spectrum of APOE implication in each of the diseases, notably ADH, in order to improve clinical and genetic diagnosis, prognosis and patient management. The purpose of this review is to comment on these APOE variants and on the molecular and clinical overlaps between dyslipidemias.

Dysbetalipoproteinemia and other lipid abnormalities related to apo E

Dysbetalipoproteinaemia (or type III hyperlipoproteinaemia) is a severe mixed hyperlipidaemia resulting from the accumulation of remnant chylomicron and VLDL particles in plasma, also called β-VLDL. It is caused by a defect in the recognition by hepatic LDL and lipoprotein receptor-related protein (LRP) of β-VLDL. Mutations in the APOE gene, especially in subjects homozygous for the ɛ2/ɛ2 allele, are responsible for this lack of receptor recognition. Dysbetalipoproteinaemia represents 2-5% of the mixed dyslipidaemias seen in Lipid Units, is highly atherogenic and predisposes to diffuse atheromatosis, either coronary, peripheral vascular, or carotid, so early diagnosis and treatment is necessary. The presence of hypertriglyceridaemia, with non-HDL cholesterol/apolipoprotein B ratios>1.43 (in mg/dL) followed by APOE genotyping is the method of choice in the diagnosis of dysbetalipoproteinaemia. It is a dyslipidaemia that responds well to hygienic-dietary treatment, although the combination of statin and fenofibrate is often necessary to achieve optimal control.

APOE and Alzheimer's disease: advances in genetics, pathophysiology, and therapeutic approaches

The APOE ε4 allele remains the strongest genetic risk factor for sporadic Alzheimer's disease and the APOE ε2 allele the strongest genetic protective factor after multiple large scale genome-wide association studies and genome-wide association meta-analyses. However, no therapies directed at APOE are currently available. Although initial studies causally linked APOE with amyloid-β peptide aggregation and clearance, over the past 5 years our understanding of APOE pathogenesis has expanded beyond amyloid-β peptide-centric mechanisms to tau neurofibrillary degeneration, microglia and astrocyte responses, and blood-brain barrier disruption. Because all these pathological processes can potentially contribute to cognitive impairment, it is important to use this new knowledge to develop therapies directed at APOE. Several therapeutic approaches have been successful in mouse models expressing human APOE alleles, including increasing or reducing APOE levels, enhancing its lipidation, blocking the interactions between APOE and amyloid-β peptide, and genetically switching APOE4 to APOE3 or APOE2 isoforms, but translation to human clinical trials has proven challenging.

The clinical and laboratory investigation of dysbetalipoproteinemia

Familial dysbetalipoproteinemia (type III hyperlipoproteinemia) is a potentially underdiagnosed inherited dyslipidemia associated with greatly increased risk of coronary and peripheral vascular disease. The mixed hyperlipidemia observed in this disorder usually responds well to appropriate medical therapy and lifestyle modification. Although there are characteristic clinical features such as palmar and tuberous xanthomata, associated with dysbetalipoproteinemia, they are not always present, and their absence cannot be used to exclude the disorder. The routine lipid profile cannot distinguish dysbetalipoproteinemia from other causes of mixed hyperlipidemia and so additional investigations are required for confident diagnosis or exclusion. A range of investigations that have been proposed as potential diagnostic tests are discussed in this review, but the definitive biochemical test for dysbetalipoproteinemia is widely considered to be beta quantification. Beta quantification can determine the presence of "β-VLDL" in the supernatant following ultracentrifugation and whether the VLDL cholesterol to triglyceride ratio is elevated. Both features are considered hallmarks of the disease. However, beta quantification and other specialist tests are not widely available and are not high-throughput tests that can practically be applied to all patients with mixed hyperlipidemia. Using apolipoprotein B (as a ratio either to total or non-HDL cholesterol or as part of a multi-step algorithm) as an initial test to select patients for further investigation is a promising approach. Several studies have demonstrated a high degree of diagnostic sensitivity and specificity using these approaches and apolipoprotein B is a relatively low-cost test that is widely available on high-throughput platforms. Genetic testing is also important in the diagnosis, but it should be noted that most individuals with an E₂/₂ genotype do not suffer from remnant hyperlipidemia and around 10% of familial dysbetalipoproteinemia cases are caused by rarer, autosomal dominant mutations in APOE that will only be detected if the gene is fully sequenced. Wider implementation of diagnostic pathways utilizing apo B could lead to more rational use of specialist investigations and more consistent detection of patients with dysbetalipoproteinemia. Without the application of a consistent evidence-based approach to identifying dysbetalipoproteinemia, many cases are likely to remain undiagnosed.

APOE Variants E2, E3, and E4 Can Be Miscalled By Classical PCR-RFLP When The Christchurch Variant Is Also Present

BACKGROUND

The APOE Christchurch (APOECh) is a rare variant (c.543C>A) in codon 154. It was first described in an E2 patient with type III dyslipidemia, and thus initially called E2Ch. Its prevalence and the lipid profile of carriers remain unclear.

METHODS

E2, E3, and E4 screening for the APOE gene was performed by PCR-RFLP. The rare APOECh variant was firstly found after detecting an unexpected 109 base-pair band in the high-resolution agarose gel electrophoresis leading to a genotype misinterpretation: the presence of APOECh alters the restriction-bands pattern. To confirm the Ch variant, a second PCR-RFLP method was specifically designed to detect this variant and Sanger sequencing was also performed for all positive samples.

RESULTS

We identified 12 unrelated subjects for the APOECh among a cohort of 2,560 patients: nine E3/E3Ch, two E3Ch/E4, and one E2/E3Ch or E2Ch/E3. The frequency of the variant is 0.4% in our study population, which represents the highest percentage published so far. If there is a 109 bp band, it is easy to recognize the presence of the variant. However, in APOE routine genotyping, an E4Ch allele is indistinguishable from a standard E3. Therefore, E4Ch alleles might be underrepresented in the results.

CONCLUSION

We recommend APOE exon 4 sequencing to unequivocally detect the common three variants E2, E3, and E4 and the rare variants as well, to find out the real role they play in atherosclerosis and to estimate its real frequency which is nowadays unclear, in part by the small number of cases identified.

The p.Leu167del Mutation in APOE Gene Causes Autosomal Dominant Hypercholesterolemia by Down-regulation of LDL Receptor Expression in Hepatocytes

CONTEXT

The p.Leu167del mutation in the APOE gene has been associated with hyperlipidemia.

OBJECTIVES

Our objective was to determine the frequency of p.Leu167del mutation in APOE gene in subjects with autosomal dominant hypercholesterolemia (ADH) in whom LDLR, APOB, and PCSK9 mutations had been excluded and to identify the mechanisms by which this mutant apo E causes hypercholesterolemia.

DESIGN

The APOE gene was analyzed in a case-control study.

SETTING

The study was conducted at a University Hospital Lipid Clinic.

PATIENTS OR OTHER PARTICIPANTS

Two groups (ADH, 288 patients; control, 220 normolipidemic subjects) were included.

INTERVENTION

We performed sequencing of APOE gene and proteomic and cellular experiments.

MAIN OUTCOME MEASURE

To determine the frequency of the p.Leu167del mutation and the mechanism by which it causes hypercholesterolemia.

RESULTS

In the ADH group, nine subjects (3.1%) were carriers of the APOE c.500_502delTCC, p.Leu167del mutation, cosegregating with hypercholesterolemia in studied families. Proteomic quantification of wild-type and mutant apo E in very low-density lipoprotein (VLDL) from carrier subjects revealed that apo E3 is almost a 5-fold increase compared to mutant apo E. Cultured cell studies revealed that VLDL from mutation carriers had a significantly higher uptake by HepG2 and THP-1 cells compared to VLDL from subjects with E3/E3 or E2/E2 genotypes. Transcriptional down-regulation of LDLR was also confirmed.

CONCLUSIONS

p.Leu167del mutation in APOE gene is the cause of hypercholesterolemia in the 3.1% of our ADH subjects without LDLR, APOB, and PCSK9 mutations. The mechanism by which this mutation is associated to ADH is that VLDL carrying the mutant apo E produces LDLR down-regulation, thereby raising plasma low-density lipoprotein cholesterol levels.

Hyperlipoproteinemia type 3: the forgotten phenotype

Hyperlipoproteinemia type 3 (HLP3) is caused by impaired removal of triglyceride-rich lipoproteins (TGRL) leading to accumulation of TGRL remnants with abnormal composition. High levels of these remnants, called β-VLDL, promote lipid deposition in tuberous xanthomas, atherosclerosis, premature coronary artery disease, and early myocardial infarction. Recent genetic and molecular studies suggest more genes than previously appreciated may contribute to the expression of HLP3, both through impaired hepatic TGRL processing or removal and increased TGRL production. HLP3 is often highly amenable to appropriate treatment. Nevertheless, most HLP3 probably goes undiagnosed, in part because of lack of awareness of the relatively high prevalence (about 0.2% in women and 0.4-0.5% in men older than 20 years) and largely because of infrequent use of definitive diagnostic methods.

Biophysical analysis of apolipoprotein E3 variants linked with development of type III hyperlipoproteinemia

Background

Apolipoprotein E (apoE) is a major protein of the lipoprotein transport system that plays important roles in lipid homeostasis and protection from atherosclerosis. ApoE is characterized by structural plasticity and thermodynamic instability and can undergo significant structural rearrangements as part of its biological function. Mutations in the 136–150 region of the N-terminal domain of apoE, reduce its low density lipoprotein (LDL) receptor binding capacity and have been linked with lipoprotein disorders, such as type III hyperlipoproteinemia (HLP) in humans. However, the LDL-receptor binding defects for these apoE variants do not correlate well with the severity of dyslipidemia, indicating that these variants may carry additional properties that contribute to their pathogenic potential.

Methodology/Principal Findings

In this study we examined whether three type III HLP predisposing apoE3 variants, namely R136S, R145C and K146E affect the biophysical properties of the protein. Circular dichroism (CD) spectroscopy revealed that these mutations do not significantly alter the secondary structure of the protein. Thermal and chemical unfolding analysis revealed small thermodynamic alterations in each variant compared to wild-type apoE3, as well as effects in the reversibility of the unfolding transition. All variants were able to remodel multillamelar 1,2-Dimyristoyl-sn-glycero-3-phosphocholine (DMPC) vesicles, but R136S and R145C had reduced kinetics. Dynamic light scattering analysis indicated that the variant R136S exists in a higher-order oligomerization state in solution. Finally, 1-anilinonaphthalene-8-sulfonic acid (ANS) binding suggested that the variant R145C exposes a larger amount of hydrophobic surface to the solvent.

Conclusions/Significance

Overall, our findings suggest that single amino acid changes in the functionally important region 136–150 of apoE3 can affect the molecule's stability and conformation in solution and may underlie functional consequences. However, the magnitude and the non-concerted nature of these changes, make it unlikely that they constitute a distinct unifying mechanism leading to type III HLP pathogenesis.

Tendon Xanthomas in Familial Hypercholesterolemia Are Associated With Cardiovascular Risk Independently of the Low-Density Lipoprotein Receptor Gene Mutation

OBJECTIVE

To investigate the significance of tendon xanthomas (TX) in heterozygous subjects with familial hypercholesterolemia (hFH).

METHODS AND RESULTS

951 men and women with genetic diagnosis of hFH were studied, of whom 278 (29.2%) presented TX. TX frequency increased with age from 6.9% in subjects 20 to 30 years to 38.3% at 51 to 60 years, with a decrease in those older than 60 years. Total and low-density lipoprotein (LDL) cholesterol were higher in TX+ than in TX- subjects (439.0+/-78.5 mg/dL and 363.1+/-76.5 mg/dL versus 400.6+/-73.4 and 323.3+/-71.0, respectively; P=0.001). High-density lipoprotein (HDL) cholesterol was lower in TX+ than in TX- subjects (50.4+/-15.0 mg/dL versus 53.1+/-14.8 mg/dL; P=0.005). Lp(a), apolipoprotein E genotype, and type of LDL receptor gene mutation showed no differences between groups. 102 TX+ reported premature cardiovascular disease (CVD) (36.7%) versus 93 TX- (13.8%) (P=0.001). The relative odds for premature CVD were higher in women (4.49 versus 2.26), and increased in hFH younger than 51 years to 3.60 (95% CI, 1.703 to 7.608) in men and to 17.1 (95% CI, 2.697 to 108.920) in women. In the multivariate analysis, age, male sex, LDL cholesterol, and hypertension showed significant positive association with TX, whereas body mass index showed negative association with TX.

CONCLUSIONS

TX are associated with cardiovascular risk factors and higher CVD, indicating that their detection indicates the need for more aggressive lipid-lowering intervention.

Apolipoprotein E genotype is not associated with cardiovascular disease in heterozygous subjects with familial hypercholesterolemia

BACKGROUND

Familial hypercholesterolemia (FH) is a genetic disorder characterized by high low-density lipoprotein cholesterol levels and premature cardiovascular disease (CVD). There are important differences in the presence of CVD among heterozygous subjects with FH. Some of this variability can be explained by genetic factors, and the apolipoprotein (apo) E genotype has been proposed as a useful marker.

METHODS

We analyzed the apo E genotype in 706 non-related subjects who were heterozygous for FH from Spain. CVD was present in 198 subjects (28%), 132 men (41%) and 66 women (17%).

RESULTS

Apo E allele frequencies for the epsilon 3, epsilon 4, and epsilon 2 alleles were 0.89, 0.09, and 0.02 respectively. Age, body mass index, smoking status, high blood pressure, diabetes mellitus, presence of tendon xanthomas, total cholesterol level, triglyceride levels, high-density lipoprotein cholesterol level, low-density lipoprotein cholesterol level, and Lp(a) did not differ among genotypes. The incidence of CVD and the age of onset of CVD did not differ among genotypes either. In the multivariant analysis, apo E genotype did not contribute significantly to CVD.

CONCLUSIONS

Heterozygous men with FH have a very high risk of coronary disease in a Mediterranean country, and the apo E genotype in this large group of adults with FH is not associated either with CVD or lipid values, in contrast with the established effect in the general population.

Two Italian kindreds carrying the Arg136-->Ser mutation of the Apo E gene: development of premature and severe atherosclerosis in the presence of epsilon 2 as second allele

BACKGROUND AND AIMS

Type III hyperlipoproteinemia, or dysbetalipoproteinemia, is commonly associated with apolipoprotein E2 homozygosity (Cys112, Cys158). Apo E2-Christchurch (Arg136-->Ser), a rare mutation of the Apo E gene, located in the receptor-binding domain of the protein, has been found to be associated in the vast majority of cases of dysbetalipoproteinemia.

METHODS AND RESULTS

This is the first report of two Italian kindreds carrying the Arg136-->Ser mutation. One family is a four-generation kindred from Genoa (Liguria, Italy) with a high rate of mortality due to coronary artery disease: the proband was a 51-year-old woman with previous myocardial infarction and residual angina, severe carotid atherosclerosis, peripheral arterial vascular disease and arterial hypertension. The other family was identified in Palermo (Sicily, Italy): the proband was an overweight 62-year-old man with a mixed form of hyperlipidemia. The mutation, which was identified by means of Apo E genotyping followed by direct sequencing, co-segregated with the same haplotype in the two families.

CONCLUSIONS

The family histories and clinical examinations of these subjects clearly show that the Apo E Arg136-->Ser variant fully expresses a type III phenotype in association with a second allele coding for Apo E2, and only partially in association with a second allele coding for Apo E4.

Dietary fat clearance in type V hyperlipoproteinaemia secondary to a rare variant of human apolipoprotein E: the apolipoprotein E3 (Arg 136-->Ser)

This present case report describes two siblings with severe type V hyperlipoproteinaemia, diagnosed very early in life and due to the combination of the common apolipoprotein (Apo) E2 allele and rare mutant variant of ApoE, ApoE3 (Arg 136-->Ser). Phenotyping of ApoE falsely identified E2/E2 phenotype. The presence of mutated ApoE was suspected on an unusual restriction polymorphism of a Hha 1 restriction site and confirmed by sequence analysis of the cloned polymerase chain reaction fragment of exon 4 and familial segregation study. The severity of the hypertriacylglycerolaemia was modulated by the lipid content of the diet. A low-fat diet enriched in medium-chain triacylglycerol (TAG) decreased but did not normalize plasma TAG levels in both affected patients of the pedigree. A standardized lipid-enriched test meal showed a marked impairment of TAG-rich lipoprotein (TRL) clearance, especially the exogeneous TRL bearing ApoB-48 which still represented 79% of total TRL 7 h after the fat load. Finally, differences between the male and female siblings with the existence of a consanguine relationship in their parents suggested the involvement of other genetic factors in modulating the severity of phenotypic expression. This observation reinforces the usefulness of genotyping of ApoE for the characterization of genetic hypertriacylglycerolaemia and selection of the appropriate diet and treatment.

Comparison of the hypolipidemic effect of gemfibrozil versus simvastatin in patients with type III hyperlipoproteinemia

BACKGROUND

Type III hyperlipoproteinemia is characterized by the accumulation of chylomicron and very low density lipoprotein (VLDL) remnants. Individuals with this disorder have a high risk of premature atherosclerosis, and hypolipidemic drugs are useful in their management.

METHODS

We compared, in a double-blind, placebo-controlled, randomized crossed study, the effects of gemfibrozil (1200 mg/day) and simvastatin (20 mg/day) on lipids, apolipoprotein AI, apolipoprotein B, and apolipoprotein E and on lipids and apolipoprotein B content in VLDL, intermediate-density lipoprotein (IDL), low-density lipoprotein (LDL), and high-density lipoprotein (HDL) in 10 patients with type III hyperlipoproteinemia.

RESULTS

Levels of total cholesterol, VLDL cholesterol, IDL cholesterol, and apolipoprotein B decreased with both drugs. Larger reductions in triglycerides (109 +/- 28.2 mg/dL, P =.005), VLDL cholesterol (24.7 +/- 10.9 mg/dL, P =.05), and VLDL triglycerides (86.3 +/- 20.2 mg/dL, P =.003) were obtained with gemfibrozil compared with simvastatin. LDL cholesterol reduction was more effective with simvastatin than with gemfibrozil (44.3 +/- 17.1 mg/dL, P =.03). HDL cholesterol after gemfibrozil was 5.71 +/- 2.37 mg/dL higher than after simvastatin.

CONCLUSIONS

In patients with type III hyperlipoproteinemia gemfibrozil is more effective in reducing total triglyceride and VLDL lipid levels than simvastatin, and simvastatin is better in reducing LDL cholesterol than gemfibrozil is. IDL and apolipoprotein E levels were reduced similarly with both drugs.

Effects of a frequent apolipoprotein E isoform, ApoE4Freiburg (Leu28-->Pro), on lipoproteins and the prevalence of coronary artery disease in whites

Different isoforms of apoE modulate the concentrations of plasma lipoproteins and the risk for atherosclerosis. A novel apoE isoform, apoE4Freiburg, was detected in plasma by isoelectric focusing because its isoelectric point is slightly more acidic than that of apoE4. ApoE4Freiburg results from a base exchange in the APOE4 gene that causes the replacement of a leucine by a proline at position 28. Analysis of the allelic frequencies in whites in southwestern Germany revealed that this isoform is frequent among control subjects (10:4264 alleles) and is even more frequent in patients with coronary artery disease (21:2874 alleles; P=0.004; adjusted odds ratio, 3.09; 95% confidence interval, 1.20 to 7.97). ApoE4Freiburg affects serum lipoproteins by lowering cholesterol, apoB, and apoA-I compared with apoE4 (P<0.05). Our 4 apoE4Freiburg homozygotes suffered from various phenotypes of hyperlipoproteinemia (types IIa, IIb, IV, and V). In vitro binding studies excluded a binding defect of apoE4Freiburg, and in vivo studies excluded an abnormal accumulation of chylomicron remnants. ApoE4Freiburg and apoE4 accumulated to a similar extent in triglyceride-rich lipoproteins. HDLs, however, contained about 40% less apoE4Freiburg than apoE4. In conclusion, our data indicate that apoE4Freiburg exerts its possible atherogenic properties by affecting the metabolism of triglyceride-rich lipoproteins and HDL.

Apo E variants in patients with type III hyperlipoproteinemia

Type III hyperlipoproteinemia (HLP III) is characterized by the reduced catabolism and accumulation of chylomicron and very low density lipoprotein (VLDL) remnants. Most HLP III patients are homozygous for the apolipoprotein E2 (Cys112, Cys158) allele; however, several other mutations at this gene locus have been associated with this HLP. In order to assess the presence of rare apo E variants in our population, we have examined apo E phenotypes by isoelectric focusing (IEF) and genotypes by restriction enzyme analysis of polymerase chain reaction (PCR) amplified DNA in 15 patients with HLP III. Lack of concordance between these two methods was observed in 11 subjects (73.3%). DNA sequencing analysis of the receptor binding domain of the apo E gene in the 11 HLP III patients with discrepancies demonstrated the presence of six carriers of the epsilon 3(Arg136-->Ser) allele and three carriers of the epsilon 2(Gly127-->Asp) allele. Five HLP III patients were apo E2/E2 using IEF, but only 2 of them were epsilon 2 homozygous using PCR. Two patients were E3/E3 homozygous with normal DNA sequence in the low density lipoprotein receptor binding domain of apo E. In conclusion, our results show that a number of different apo E genotypes are associated with HLP III in this population. More specifically, mutations at positions 127 and 136 might be frequent in Spain and occur in patients with HLP III.