Heterozygosity for apolipoprotein E-4Philadelphia(Glu13—-Lys, Arg145—-Cys) is associated with incomplete dominance of type III hyperlipoproteinemia

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.

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.

Evaluation of the Non-HDL Cholesterol to Apolipoprotein B Ratio as a Screening Test for Dysbetalipoproteinemia

BACKGROUND

Familial dysbetalipoproteinemia is associated with the accumulation of remnant lipoproteins and premature cardiovascular disease. Identification of dysbetalipoproteinemia is important because family members may be affected. Diagnostic testing involves demonstration of β-lipoprotein in the VLDL fraction or characterization of apo E³. These investigations are complex and relatively expensive. The ratios of apo B to total cholesterol and triglycerides have been proposed as screening tests. However, the ratio of non-HDL cholesterol to apo B (NHDLC/apoB) could offer improved performance as the confounding effect of variations in HDL cholesterol is removed.

METHODS

We evaluated NHDLC/apoB as a screening test for dysbetalipoproteinemia, using β-quantification analysis as a reference standard. Data from 1637 patients referred over a 16-year period for β quantification were reviewed retrospectively. In 63 patients, diagnostic criteria for dysbetalipoproteinemia (VLDL cholesterol/triglyceride ratio ≥0.69 and presence of β-VLDL) were fulfilled, and 1574 patients had dysbetalipoproteinemia excluded.

RESULTS

Mean NHDLC/apoB in patients with dysbetalipoproteinemia was 7.3 mmol/g (SD, 1.5 mmol/g) and with dysbetalipoproteinemia excluded was 4.0 mmol/g (SD, 0.5 mmol/g). The optimum cutoff of >4.91 mmol/g achieved a diagnostic sensitivity of 96.8% (95% CI, 89.0-99.6) and specificity of 95.0% (95% CI, 93.8-96.0). NHDLC/apoB offered improved performance compared to total cholesterol/apoB [diagnostic sensitivity 92.1% (95% CI, 82.4-97.4) and specificity 94.5% (95% CI, 93.2-95.6) with a cutoff of >6.55 mmol/g]. NHDL/apoB reference ranges were not sex-dependent, although there was a significant difference between men and women for total cholesterol/apoB.

CONCLUSIONS

NHDLC/apoB offers a simple first-line test for dysbetalipoproteinemia in selecting patients with mixed hyperlipidemia for more complex investigations.

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.

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 key role of apolipoprotein E in atherosclerosis

Apolipoprotein E is a multifunctional protein that is synthesized by the liver and several peripheral tissues and cell types, including macrophages. The protein is involved in the efficient hepatic uptake of lipoprotein particles, stimulation of cholesterol efflux from macrophage foam cells in the atherosclerotic lesion, and the regulation of immune and inflammatory responses. Apolipoprotein E deficiency in mice leads to the development of atherosclerosis and re-expression of the protein reduces the extent of the disease. This review presents evidence for the potent anti-atherogenic action of apolipoprotein E and describes our current understanding of its multiple functions and regulation by factors implicated in the pathogenesis of cardiovascular disease.

Genetics of lipoprotein disorders

The study of lipoprotein metabolism has led to major breakthroughs in the fields of cellular physiology, molecular genetics, and protein chemistry. These advances in basic science are reflected in medicine in the form of improved diagnostic methods and better therapeutic tools. Perhaps the greatest benefit is the improved ability to identify at an early stage patients who are at high risk for atherosclerosis, providing clinicians the opportunity to proceed swiftly with intensive lipid-lowering therapy for the prevention of cardiovascular complications. Recent clinical trials have shown that such an approach is not only cost-effective but saves lives while improving the quality of life. They also emphasize the important role physicians can have in prevention. More than half of patients with premature CAD have a familial form of dyslipoproteinemia. This review of the genetics of atherogenic lipoprotein disorders underscores the importance of identifying major genetic defects. It also stresses the need to take into account multifactorial etiologies and clustering of risk factors, as well as gene-gene and gene-environment interactions in assessing the atherogenic potential of a lipid transport disorder. Table 2 summarizes the key points in the diagnosis, clinical implications, and treatment of the major inherited atherogenic dyslipidemias.

Increased serum remnant lipoproteins in patients with apolipoprotein E7 (apo E Suita)

Apolipoprotein (apo) E7 was originally identified by Yamamura et al. in subjects with atherosclerotic cardiovascular diseases (J. Clin. Invest. 1984;74:1229). However, the lipoprotein abnormalities associated with apo E7 phenotype have not been elucidated. In the current study, to clarify the physiological roles of apo E7, lipoprotein abnormalities were studied in 12 apo E7 heterozygotes. A total of seven subjects were hyperlipidemic and five subjects were normolipidemic. The apo E phenotype was apo E7/3 in 11 subjects and apo E7/4 in one subject. Polymerase chain reaction revealed that all of the subjects with apo E7 phenotype had the same mutation as that of apo E(Suita) as reported previously (J. Biochem. 1989;105:249). All the hyperlipidemic subjects were over 40 years of age and two of them also had and severe coronary heart disease. Ultracentrifugal analysis revealed that the cholesterol level both in very low density lipoprotein and in intermediate density lipoprotein (IDL) was substantially higher in hyperlipidemic apo E7 heterozygotes, compared with control subjects and that the IDL cholesterol was also increased even in normolipidemic apo E7 heterozygotes. Polyacrylamide gel electrophoresis of lipoproteins showed a midband, which implies the increase of remnant lipoproteins, in 11 subjects out of 12, irrespective of the presence or absence of hyperlipoproteinemia. In two cases, a broad beta pattern was observed similar to that seen in type III hyperlipoproteinemia. Dietary therapy was dramatically effective for the treatment of hyperlipidemia in patients with apo E7. These findings confirm that apo E is crucial for remnant lipoprotein metabolism and that apo E7 is related to the increase in serum remnant lipoproteins, which leads to hyperlipoproteinemia in association with obesity, aging and impaired glucose metabolism.

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

Type III hyperlipoproteinemia (type III HLP) is an atherogenic disorder of lipoprotein metabolism characterized by the accumulation of cholesterol-enriched VLDL and is usually associated with homozygosity for a normal variant of apoE, apoE2. ApoE2(Arg145Cys) is a rare variant arising from a C-->T transition at nucleotide 4031 and has been linked to type III HLP. Ten subjects from a group of 42 unrelated individuals with proven type III HLP were found to be either heterozygous or homozygous for the apoE2(Arg145Cys) mutation by DNA sequencing. The apoE4-Philadelphia (Glu13Lys, Arg145Cys) variant was subsequently excluded. None of 4 homozygotes (3 blacks and 1 of mixed ancestry) developed ischemic heart disease, but they did present with xanthomata. In contrast, 6 heterozygous subjects presented mainly with ischemic heart disease but generally lacked physical signs. Cholesterol concentrations ranged from 6.2 mmol/L to 13.3 mmol/L and triglyceride levels from 3.2 to 13.2 mmol/L. The dyslipoproteinemia in homozygous and heterozygous subjects was indistinguishable. Family investigation identified an additional 10 heterozygous mutant-allele carriers, of whom 3 had type III HLP. This unique cohort of patients indicates that the apoE2(Arg145Cys) mutation is relatively common in several population groups in our region and may be particularly prevalent in blacks. There was no clear allele dosage effect present for the development of dyslipoproteinemia or atherosclerosis. The mode of inheritance is for the first time clearly established to be autosomal dominant with incomplete penetrance.

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.

Three-dimensional structure of the LDL receptor-binding domain of the human apolipoprotein E2 (Arg136-->Cys) variant

The familial lipoprotein disorder type III hyperlipoproteinemia (HLP) is usually inherited as a recessive trait. Indeed, more than 90% of affected individuals are homozygous for a receptor binding-defective isoform of apolipoprotein (apo) E, apo E2. However, some rare apo E variants have been described that dominantly (thus in a single dose) predispose to the disease. Amino acid substitutions, which are accompanied with the loss of positive charges within the proposed apo E binding-region to lipoprotein receptors, seem to be responsible in most of these cases for the dominance with respect to the expression of type III HLP. So far available data in the literature on the naturally occurring human apo E2 (Arg136-->Cys) variant are not conclusive about its recessive or dominant character. We recently identified a subject heterozygous for this mutation, presenting the typical clinical and biochemical characteristics of type III HLP. In the present study we performed further analysis of the mutation on apo E structure and function based on computer modeling. Our combined data point to a dominant influence of the apo E2 (Arg136-->Cys) variant with respect to the transmission of the disease.

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

In the process of screening apolipoprotein (apo) E genotypes in a population of subjects with lipid abnormalities, we have identified five subjects (one homozygote and four heterozygotes) with an abnormal 109 base pairs band following apo E restriction isotyping of amplified DNA with the restriction endonuclease CfoI. The polymerase chain reaction (PCR) products were cloned and their sequencing revealed a C-->A substitution at the first nucleotide of codon 136. This mutation resulted in an amino acid substitution Arg to Ser, previously described as apo E2 Christchurch. Family studies were carried out for four of the probands. In these kindreds, stepwise multiple regression analyses indicated that 78% of the cholesterol variability in men was predicted by body mass index, age and the rare apo E2 (Arg136-->Ser) variant. In women, age and the apo E2 (Arg136-->Ser variant predicted 54.9% of the variability in cholesterol levels. Linkage analysis suggested that the presence of the apo E2 (Arg136-->Ser) variant was linked with the occurrence of cholesterol enriched triglyceride rich lipoproteins and with an incomplete dominance of type III hyperlipoproteinemia. Our data indicates that this mutation may be a relatively common cause of dyslipidemia in the Spanish population.

Genetic heterogeneity of apolipoprotein E and its influence on plasma lipid and lipoprotein levels

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. Including the most common variant, apoE3, 30 apoE variants have been characterized. At present, 14 apoE variants have been found to be associated with familial dysbetalipoproteinemia, a genetic lipid disorder characterized by elevated plasma cholesterol and triglyceride levels and an increased risk for atherosclerosis. Seven apoE variants were found to be associated with other forms of hyperlipoproteinemia. This report presents an overview of all currently known apoE variants and their effects on lipoprotein metabolism.