The apolipoprotein E2 (Arg145Cys) mutation causes autosomal dominant type III hyperlipoproteinemia with incomplete penetrance

Mechanisms of Allosteric Inhibition of Insulin-Regulated Aminopeptidase

Inhibition of Insulin-Regulated Aminopeptidase is being actively explored for the treatment of several human diseases and several classes of inhibitors have been developed although no clinical applications have been reported yet. Here, we combine enzymological analysis with x-ray crystallography to investigate the mechanism employed by two of the most studied inhibitors of IRAP, an aryl sulfonamide and a 2-amino-4H-benzopyran named HFI-419. Although both compounds have been hypothesized to target the enzyme's active site by competitive mechanisms, we discovered that they instead target previously unidentified proximal allosteric sites and utilize non-competitive inhibition mechanisms. X-ray crystallographic analysis demonstrated that the aryl sulfonamide stabilizes the closed, more active, conformation of the enzyme whereas HFI-419 locks the enzyme in a semi-open, and likely less active, conformation. HFI-419 potency is substrate-dependent and fails to effectively block the degradation of the physiological substrate cyclic peptide oxytocin. Our findings demonstrate alternative mechanisms for inhibiting IRAP through allosteric sites and conformational restricting and suggest that the pharmacology of HFI-419 may be more complicated than initially considered. Such conformation-specific interactions between IRAP and small molecules can be exploited for the design of more effective second-generation allosteric inhibitors.

Association of African Ancestry-Specific APOE Missense Variant R145C With Risk of Alzheimer Disease

Importance

Numerous studies have established the association of the common APOE ε2 and APOE ε4 alleles with Alzheimer disease (AD) risk across ancestries. Studies of the interaction of these alleles with other amino acid changes on APOE in non-European ancestries are lacking and may improve ancestry-specific risk prediction.

Objective

To determine whether APOE amino acid changes specific to individuals of African ancestry modulate AD risk.

Design, Setting, and Participants

Case-control study including 31 929 participants and using a sequenced discovery sample (Alzheimer Disease Sequencing Project; stage 1) followed by 2 microarray imputed data sets derived from the Alzheimer Disease Genetic Consortium (stage 2, internal replication) and the Million Veteran Program (stage 3, external validation). This study combined case-control, family-based, population-based, and longitudinal AD cohorts, which recruited participants (1991-2022) in primarily US-based studies with 1 US/Nigerian study. Across all stages, individuals included in this study were of African ancestry.

Exposures

Two APOE missense variants (R145C and R150H) were assessed, stratified by APOE genotype.

Main Outcomes and Measures

The primary outcome was AD case-control status, and secondary outcomes included age at AD onset.

Results

Stage 1 included 2888 cases (median age, 77 [IQR, 71-83] years; 31.3% male) and 4957 controls (median age, 77 [IQR, 71-83] years; 28.0% male). In stage 2, across multiple cohorts, 1201 cases (median age, 75 [IQR, 69-81] years; 30.8% male) and 2744 controls (median age, 80 [IQR, 75-84] years; 31.4% male) were included. In stage 3, 733 cases (median age, 79.4 [IQR, 73.8-86.5] years; 97.0% male) and 19 406 controls (median age, 71.9 [IQR, 68.4-75.8] years; 94.5% male) were included. In ε3/ε4-stratified analyses of stage 1, R145C was present in 52 individuals with AD (4.8%) and 19 controls (1.5%); R145C was associated with an increased risk of AD (odds ratio [OR], 3.01; 95% CI, 1.87-4.85; P = 6.0 × 10-6) and was associated with a reported younger age at AD onset (β, -5.87 years; 95% CI, -8.35 to -3.4 years; P = 3.4 × 10-6). Association with increased AD risk was replicated in stage 2 (R145C was present in 23 individuals with AD [4.7%] and 21 controls [2.7%]; OR, 2.20; 95% CI, 1.04-4.65; P = .04) and was concordant in stage 3 (R145C was present in 11 individuals with AD [3.8%] and 149 controls [2.7%]; OR, 1.90; 95% CI, 0.99-3.64; P = .051). Association with earlier AD onset was replicated in stage 2 (β, -5.23 years; 95% CI, -9.58 to -0.87 years; P = .02) and stage 3 (β, -10.15 years; 95% CI, -15.66 to -4.64 years; P = 4.0 × 10-4). No significant associations were observed in other APOE strata for R145C or in any APOE strata for R150H.

Conclusions and Relevance

In this exploratory analysis, the APOE ε3[R145C] missense variant was associated with an increased risk of AD among individuals of African ancestry with the ε3/ε4 genotype. With additional external validation, these findings may inform AD genetic risk assessment in individuals of African ancestry.

From degenerative disease to malignant tumors: Insight to the function of ApoE

Apolipoprotein E (ApoE) is a multifunctional protein involved in lipid transport and lipoprotein metabolism, mediating lipid distribution/redistribution in tissues and cells. It can also regulate inflammation and immune function, maintain cytoskeleton stability, and improve neural tissue Function. Due to genetic polymorphisms of ApoE (ε2, ε3, and ε4), its three common structural isoforms (ApoE2, ApoE3, ApoE4) are also associated with the risk of many diseases, especially degenerative diseases, such as vascular degenerative diseases including atherosclerosis (AS), coronary heart disease (CHD), and neurodegenerative disease like Alzheimer's disease (AD). The frequency of the ε4 allele and APOE variants were significantly higher than that of the ε2 and ε3 alleles in the patients with CHD or AD. In recent years, ApoE has frequently appeared in tumor research and become a tumor biomarker gradually. It has been found that ApoE is highly expressed in most solid tumor tissues, such as glioblastoma, gastric cancer, pancreatic ductal cell carcinoma, etc. Studies illustrated that ApoE could regulate the polarization changes of macrophages, participate in the construction of tumor immune microenvironment, regulate tumor inflammation and immune response and play a role in tumor progression, invasion, and metastasis. Of course, many functions of ApoE and its relationship with diseases are still under research. By reviewing the structure and function of ApoE from degeneration diseases to tumor neoplasms, we hope to better understand such a biomarker and further explore the value of ApoE in later studies.

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.

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.

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.

Variable phenotypic expression of nonsense mutation p.Thr5* in the APOE gene

Subjects with hypercholesterolemia who do not carry a mutation in the low density lipoprotein receptor gene, in the apolipoprotein B gene or in the proprotein convertase subtilisin/kexin type 9 gene, could possible carry a mutation in the apolipoprotein E (APOE) gene. DNA from 844 unrelated hypercholesterolemic subjects who did not carry a mutation in any of the three above mentioned genes, was subjected to DNA sequencing of the APOE gene. Two subjects were found to be heterozygous for mutation p.Thr5*. This mutation which generates a stop codon in the signal peptide, is assumed to prevent the synthesis of APOE. Family studies revealed that the mutation was carried on an APOE4 allele in both families. In one of the families only those who had an APOE2 allele as the second allele, had hypercholesterolemia. These were functionally hemizygous for APOE2 and presented with a Type III hyperlipoproteinemia phenotype. However, in the second family, hypercholesterolemia was observed in the index patient who had APOE3 as the second allele, but not in four heterozygous family members who also had APOE3 as the second allele. These findings underscore that the phenotypic expression of mutations in the APOE gene is variable and that the trait exhibits reduced penetrance.

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.

Apolipoprotein E mutations: a comparison between lipoprotein glomerulopathy and type III hyperlipoproteinemia

Apolipoprotein E (ApoE) serves as a ligand for the low-density lipoprotein (LDL) receptor and cell surface receptors of the LDL receptor gene family. More than 10 different causative apoE mutations associated with lipoprotein glomerulopathy (LPG) have been reported. ApoE polymorphisms including three common phenotypes (E2, E3, E4), and a variety of rare mutations can affect blood cholesterol and triglyceride levels. The N-terminal domain of apoE is folded into a four-helix bundle of amphipathic α-helices, and contains the receptor-binding domain in which most apoE mutations that cause LPG or dominant mode of type III hyperlipoproteinemia (HL) are located. No single apoE mutation has been reported that causes both LPG and the dominant mode of type III HL.

Prevalence of the apolipoprotein E Arg145Cys dyslipidemia at-risk polymorphism in African-derived populations

Apolipoprotein E, a protein component of blood lipid particles, plays an important role in lipid transport. Different mutations in the apolipoprotein E gene have been associated with various clinical phenotypes. In an initiated study of Qataris, we observed that 17% of the African-derived genetic subgroup were heterozygotes for a rare Arg145Cys (R145C) variant that functions as a dominant trait with incomplete penetrance associated with type III hyperlipoproteinemia. On the basis of this observation, we hypothesized that the R145C polymorphism might be common in African-derived populations. The prevalence of the R145C variant was assessed worldwide in the "1000 Genomes Project" and in 1,012 whites and 1,226 African-Americans in New York, New York. The 1000 Genomes Project data demonstrated that the R145C polymorphism is rare in non-African-derived populations but present in 5% to 12% of Sub-Saharan African-derived populations. The R145C polymorphism was also rare in New York whites (1 of 1,012, 0.1%); however, strikingly, 53 of the 1,226 New York African-Americans (4.3%) were R145C heterozygotes. The lipid profiles of the Qatari and New York R145C heterozygotes were compared with those of controls. The Qatari R145C subjects had higher triglyceride levels than the Qatari controls (p <0.007) and the New York African-American R145C subjects had an average of 52% greater fasting triglyceride levels than the New York African-American controls (p <0.002). From these observations, likely millions of people worldwide derived from Sub-Saharan Africans are apolipoprotein E R145C. In conclusion, although larger epidemiologic studies are necessary to determine the long-term consequences of this polymorphism, the available evidence suggests it is a common cause of a mild triglyceride dyslipidemia.

Dysbetalipoproteinaemia: a mixed hyperlipidaemia of remnant lipoproteins due to mutations in apolipoprotein E

Atherosclerosis is strongly associated with dyslipoproteinaemia, and especially with increasing concentrations of low-density lipoprotein and decreasing concentrations of high-density lipoproteins. Its association with increasing concentrations of plasma triglyceride is less clear but, within the mixed hyperlipidaemias, dysbetalipoproteinaemia (Fredrickson type III hyperlipidaemia) has been identified as a very atherogenic entity associated with both premature ischaemic heart disease and peripheral arterial disease. Dysbetalipoproteinaemia is characterized by the accumulation of remnants of chylomicrons and of very low-density lipoproteins. The onset occurs after childhood and usually requires an additional metabolic stressor. In women, onset is typically delayed until menopause. Clinical manifestations may vary from no physical signs to severe cutaneous and tendinous xanthomata, atherosclerosis of coronary and peripheral arteries, and pancreatitis when severe hypertriglyceridaemia is present. Rarely, mutations in apolipoprotein E are associated with lipoprotein glomerulopathy, a condition characterized by progressive proteinuria and renal failure with varying degrees of plasma remnant accumulation. Interestingly, predisposing genetic causes paradoxically result in lower than average cholesterol concentration for most affected persons, but severe dyslipidaemia develops in a minority of patients. The disorder stems from dysfunctional apolipoprotein E in which mutations result in impaired binding to low-density lipoprotein (LDL) receptors and/or heparin sulphate proteoglycans. Apolipoprotein E deficiency may cause a similar phenotype. Making a diagnosis of dysbetalipoproteinaemia aids in assessing cardiovascular risk correctly and allows for genetic counseling. However, the diagnostic work-up may present some challenges. Diagnosis of dysbetalipoproteinaemia should be considered in mixed hyperlipidaemias for which the apolipoprotein B concentration is relatively low in relation to the total cholesterol concentration or when there is significant disparity between the calculated LDL and directly measured LDL cholesterol concentrations. Genetic tests are informative in predicting the risk of developing the disease phenotype and are diagnostic only in the context of hyperlipidaemia. Specialised lipoprotein studies in reference laboratory centres can also assist in diagnosis. Fibrates and statins, or even combination treatment, may be required to control the dyslipidaemia.

Different lipid profiles according to ethnicity in the Heart of Soweto study cohort of de novo presentations of heart disease

Background

Historically, sub-Saharan Africa has reported low levels of atherosclerotic cardiovascular disease (CVD). However as these populations undergo epidemiological transition, this may change.

Methods

This was an observational cohort study performed at Chris Hani Baragwanath Hospital in Soweto, South Africa. As part of the Heart of Soweto study, a clinical registry captured detailed clinical data on all de novo cases of structural and functional heart disease presenting to the Cardiology unit during the period 2006 to 2008. We examined fasting lipid profiles in 2 182 patients (of 5 328 total cases) according to self-reported ethnicity. The study cohort comprised 1 823 patients of African descent (61% female, aged 56 ± 16 years), 142 white Europeans (36% female, aged 57 ± 13 years), 133 Indians (51% female, aged 59 ± 12 years) and 87 of mixed ancestry (40% female, aged 56 ± 12 years).

Results

Consistent with different patterns in heart disease aetiology, there were clear differences in total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C) and triglycerides across ethnicities (p < 0.001): patients of African descent had the lowest TC and LDL-C levels and Indians the highest. However, there were no significant differences in high-density lipoprotein cholesterol (HDL-C) levels between ethnicities (p = 0.20). Adjusting for age, gender and body mass index, patients of African descent were significantly less likely to record a TC of > 4.5 mmol/l (OR 0.33, 95% CI: 0.25–0.41) compared to all ethnic groups (all p < 0.001).

Conclusions

These data confirm important blood lipid differentials according to ethnicity in patients diagnosed with heart disease in Soweto, South Africa. Such disparities in CVD risk factors may justify the use of specialised prevention and management protocols.

Apolipoprotein E polymorphisms and spontaneous pregnancy loss in patients with endometriosis

Endometriosis affects >10% of women during their reproductive years, many of whom report high rates of spontaneous pregnancy loss (SPL). We examined whether gene polymorphisms in apolipoprotein E (APOE), which is involved in lipoprotein metabolism, are associated with endometriosis and/or endometriosis-associated infertility. We conducted a cross-sectional genetic association study of women surgically confirmed to have endometriosis (n = 345) and no surgical evidence of the disease (n = 266). Genotyping of APOE polymorphism (ε2, ε3, ε4) was conducted by polymerase chain reaction-restriction fragment length polymorphism followed by visualization of specific patterns by gel electrophoresis. Statistical significance of differences in genotype and allelic frequencies was assessed using Pearson's χ(2) test and Risk analysis. Overall, we found no association between APOE genotype and diagnosis of endometriosis. However, patients with endometriosis who reported at least one SPL were three times more likely to be ε2 carriers and 2-fold less likely to be ε4 carriers. Compared with ε3 carriers, patients with endometriosis who were ε2 carriers and had at least one live birth reported four times the rate of SPL, while ε4 carriers were <0.4-fold less likely to report an SPL. Our data suggest that there may be an association between APOE allelic frequency and SPL in patients with endometriosis, which appears to be independent of mechanisms associated with infertility, an intriguing observation that deserves further investigation.

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.

Tuberous Xanthomas Associated with Olanzapine Therapy and Hypertriglyceridemia in the Setting of a Rare Apolipoprotein E Mutation

OBJECTIVE

To describe a patient with tuberous xanthomas and high levels of cholesterol and triglycerides, who was found to have type III hyperlipoproteinemia (HLP) and a rare apolipoprotein E (apoE) mutation.

METHODS

We present a case report with extensive clinical, laboratory, and genetic documentation.

RESULTS

A 33-year-old African American man presented for evaluation of hypertriglyceridemia. His medical history was remarkable for schizophrenia necessitating ongoing olanzapine therapy for the past 6 years. A few months after olanzapine treatment was begun, he noted the development of nontender, firm, papular skin lesions on his elbows and knees. His family history was negative for lipid disorders or premature vascular disease. Physical examination revealed the presence of prominent tuberous xanthomas on both elbows and knees. Results of a lipid panel demonstrated a total cholesterol level of 374 mg/dL (9.7 mmol/L) and triglycerides of 828 mg/dL (9.3 mmol/L). A work-up for causes of secondary hyper-triglyceridemia was negative. Results of apoE genotyping by a commercial laboratory showed the E3/E3 genotype, based on gene sequencing at codons 112 and 158. Because the skin lesions were typical for type III HLP, his entire apoE gene was sequenced. This analysis revealed an apoE2/E2 (arginine 145 to cysteine) mutation, previously reported to be a rare cause of type III HLP in 5 patients of African descent. Triglyceride-lowering therapy with gem-fibrozil was initiated, in addition to lifestyle modification. At follow-up several months later, total cholesterol was 276 mg/dL (7.14 mmol/L) and triglycerides were 479 mg/dL (5.41 mmol/L).

CONCLUSION

We speculate that olanzapine therapy, with its known metabolic side effects, exacerbated this patient's underlying lipoprotein metabolic abnormality. To our knowledge, this is the first report of an association between olanzapine therapy and tuberous xanthomas and the sixth report of this rare apoE2/E2 (arginine 145 to cysteine) mutation in the literature.

LDL receptor deficiency or apoE mutations prevent remnant clearance and induce hypertriglyceridemia in mice

We have used adenovirus-mediated gene transfer and bolus injection of purified apolipoprotein E (apoE) in mice to determine the contribution of LDL receptor family members in the clearance of apoE-containing lipoproteins in vivo and the factors that trigger hypertriglyceridemia. A low dose [5 x 10(8) plaque-forming units (pfu)] of an adenovirus expressing apoE4 did not normalize plasma cholesterol levels of apolipoprotein E-deficient (apoE(-/-)) x low density lipoprotein receptor-deficient (LDLr(-/-)) mice and induced hypertriglyceridemia. A similar phenotype of combined dyslipidemia was induced in apoE(-/-) or apoE(-/-) x LDLr(-/-) mice after infection with a low dose (4 x 10(8) pfu) of an adenovirus expressing the apoE4[R142V/R145V] mutant previously shown to be defective in receptor binding. In contrast, a low dose of 5 x 10(8) pfu of the apoE4-expressing adenovirus corrected hypercholesterolemia in apoE(-/-) mice and did not trigger hypertriglyceridemia. Bolus injection of purified apoE in apoE(-/-) x LDLr(-/-) mice did not clear plasma cholesterol levels and induced mild hypertriglyceridemia. In contrast, similar injection of apoE in apoE(-/-) mice cleared plasma cholesterol and caused transiently mild hypertriglyceridemia. These findings suggest that a) the LDL receptor alone can account for the clearance of apoE-containing lipoproteins in mice, and the contribution of other receptors is minimal, and b) defects in either the LDL receptor or in apoE that affect its interactions with the LDL receptor, increase the sensitivity to apoE-induced hypertriglyceridemia in mice.

The apoE isoform binding properties of the VLDL receptor reveal marked differences from LRP and the LDL receptor

Apolipoprotein E (apoE) associates with lipoproteins and mediates their interaction with members of the LDL receptor family. ApoE exists as three common isoforms that have important distinct functional and biological properties. Two apoE isoforms, apoE3 and apoE4, are recognized by the LDL receptor, whereas apoE2 binds poorly to this receptor and is associated with type III hyperlipidemia. In addition, the apoE4 isoform is associated with the common late-onset familial and sporadic forms of Alzheimer's disease. Although the interaction of apoE with the LDL receptor is well characterized, the specificity of other members of this receptor family for apoE is poorly understood. In the current investigation, we have characterized the binding of apoE to the VLDL receptor and the LDL receptor-related protein (LRP). Our results indicate that like the LDL receptor, LRP prefers lipid-bound forms of apoE, but in contrast to the LDL receptor, both LRP and the VLDL receptor recognize all apoE isoforms. Interestingly, the VLDL receptor does not require the association of apoE with lipid for optimal recognition and avidly binds lipid-free apoE. It is likely that this receptor-dependent specificity for various apoE isoforms and for lipid-free versus lipid-bound forms of apoE is physiologically significant and is connected to distinct functions for these receptors.

Apolipoprotein epsilon 2/3 genotype and type III hyperlipoproteinemia among Taiwanese

BACKGROUND

Type III hyperlipoproteinemia (HLP) is a genetic disorder of lipid metabolism in humans that predisposes affected subjects to the premature development of atherosclerosis. Most type III HLP occurs in homozygous carriers of apolipoprotein (apo) E2. The aims of this study were to determine the frequencies of different apo E genotypes in type III HLP in Taiwanese and to assess the possibility of apo E mutants in these patients.

MATERIALS AND METHODS

Four hundred and seven patients with hyperlipoproteinemia were recruited. Electrophoresis, apo E genotyping and sequencing were performed.

RESULTS

Of the 407 hyperlipoproteinemia, 8 were identified as type III HLP. In contrast to reports of high apo epsilon 2/2 genotype prevalence, only two of the type III HLP subjects were of the apo epsilon 2/2 genotype (25%). Fifty percent of the type III HLP were of apo epsilon 2/3 genotype. This observation was further reflected in a lower frequency of the epsilon 2 allele (0.563) and higher epsilon 3 allele (0.375) frequency. No rare apo E variant was found by direct sequencing.

CONCLUSION

The most common genotype of type III HLP in Taiwan was apo epsilon 2/3 instead of apo epsilon 2/2.

Non-denaturing polyacrylamide gradient gel electrophoresis for the diagnosis of dysbetalipoproteinemia

Dysbetalipoproteinemia, an uncommon but highly atherogenic mixed hyperlipidemia due to the accumulation of remnants of triglyceride-rich lipoproteins, is characterized by cholesterol-enriched VLDL that migrates in the beta-position on agarose gels. The demonstration of a broad beta-band on agarose gel electrophoresis of plasma is an insensitive method and ultracentrifugation is an impractical method of diagnosing this condition. Non-denaturing polyacrylamide gradient gel electrophoresis (PGGE) was investigated as a screening method for the diagnosis of dysbetalipoproteinemia. A minigel procedure separating the Sudan Black prestained apolipoprotein B (apoB)-containing lipoproteins on a 2-8% polyacrylamide gel at 4 degrees C overnight was analyzed for ultracentrifugally and genetically proven dysbetalipoproteinemic subjects as well as matched controls for mixed hyperlipidemia. Visual inspection revealed that the presence of only small VLDL- and IDL-like particles in untreated patients was highly sensitive (72%) and specific (95%) for dysbetalipoproteinemia. Videodensitometric analysis of area under the curve for large and small VLDL, as well as IDL and LDL, permitted even better discrimination in subjects whose profiles included some staining in the LDL-like region. A ratio of area under the curve of more than 0.5 for IDL-LDL allowed for a specificity of 100% and a sensitivity of 89% for the diagnosis of dysbetalipoproteinemia. This modified PGGE system may be useful in screening for dysbetalipoproteinemia.

Diminished LDL receptor and high heparin binding of apolipoprotein E2 Sendai associated with lipoprotein glomerulopathy

Variants of apolipoprotein E (apoE) have been linked to lipoprotein glomerulopathy, a new glomerular disease characterized by the deposition of lipoproteins in mesangial capillaries. One third of affected patients are heterozygous for apoE2 Sendai (Arg(145) Pro). Variants of apoE can also produce type III hyperlipoproteinemia (HLP). Recessive type III HLP is caused by apoE2 (Arg(158) Cys), a mutant with diminished low-density lipoprotein (LDL) receptor binding but halfnormal heparin binding. Dominant type III HLP is caused by mutations that markedly alter heparin binding but modestly reduce receptor binding. This study examined whether apoE2 Sendai (Arg(145) Pro) was functionally different from type III HLP-producing apoE variants by expressing apoE3, apoE2 (Arg(158) Cys), apoE1 (Arg(146) Glu), a dominant apoE variant, and apoE2 Sendai (Arg(145) Pro) in the baculovirus system. LDL receptor binding was studied using recombinant apoE complexed to phospholipid vesicles and to very lowdensity lipoprotein from a patient with familiar apoE deficiency. Compared with apoE3, receptor-binding activities of apoE2 (Arg(158) Cys), apoE1 (Arg(146) Glu), and apoE2 Sendai (Arg(145) Pro) all were less than 5%. Heparin-binding activities were 53%, 23%, and 66%, respectively, of apoE3. The distribution of apoE2 Sendai among the major plasma lipoprotein fractions was similar to that of apoE3 and apoE2 (Arg(158) Cys). ApoE2 Sendai (Arg(145) Pro) represents the only known mutation within the heparin-binding domain of apoE (residues 142 through 147), revealing diminished receptor binding and almost normal heparin binding. These unique characteristics of apoE2 Sendai (Arg(145) Pro) may relate to the development of lipoprotein glomerulopathy.