2025 Volume 32 Issue 3 Pages 281-303
Apolipoprotein E (apoE) is a key apoprotein in lipid transport and is susceptible to genetic mutations. ApoE variants have been studied for four decades and more than a hundred of them have been reported. This paper presents an up-to-date review of the function and structure of apoE in lipid metabolism, the E2, E3, and E4 isoforms, the APOE gene, and various pathologies, such as familial type III hyperlipidemia and lipoprotein glomerulopathy, caused by apoE variants. Alzheimer’s disease was barely mentioned in this paper. But this review should help researchers obtain a comprehensive overview of human apoE in lipid metabolism.
Apolipoprotein E (apoE) is one of the key apolipoproteins in blood lipid transport. It has the following features: high hydrophobicity, the ability to bind to the low-density-lipoprotein (LDL) receptor and related receptors, production in various tissues throughout the body, acting as a central apoprotein of cerebrospinal fluid lipoproteins, having three major isoforms (apoE2, apoE3, and apoE4) that vary in their function and susceptibility to genetic mutations1). Disorders of apoE are associated with atherosclerosis, which causes dyslipidemia and myocardial infarction, and apoE-deficient mice are used as model animals for atherosclerosis2, 3). In fact, apoE is associated not only with dyslipidemia but also with lipoprotein glomerulopathy, a renal glomerular lesion, and late-onset Alzheimer’s disease (AD). In this review, we summarize the pathophysiology of apoE with a focus on lipid metabolism and review the impact of apoE variants in the pathogenesis of several different diseases except Alzheimer’s disease.
Proteins that exist in association with lipoproteins are called apolipoproteins (apoproteins). Many apoproteins, including apoE, have an amphipathic α-helix structure. Among apoproteins, apoprotein B100 (apoB100) and the amino terminal 48% of apo B100, which forms apoprotein B48 (apoB48), are very large-molecular-weight proteins. ApoB48 (molecular weight 240 kDa) is found on chylomicrons and chylomicron remnants, whereas apoB100 (molecular weight 500 kDa) is found on very low-density lipoprotein (VLDL), intermediate-density lipoprotein (IDL), and LDL. A single copy of both apoB forms is present on lipoprotein particles and they do not move between lipoprotein particles. In contrast, multiple copies of apoE and other small apoproteins with molecular weights of 6–50 kDa, such as apoprotein (apo)A-I, A-II, A-V, C-II, and C-III, are present on a single lipoprotein particle and can move between lipoproteins.
When LDL is taken up by cells, apoB100 binds to the LDL receptor as a ligand for uptake. ApoE is the only other ligand that can bind to the LDL receptor. While apoB100 can bind only to the LDL receptor, apoE is a ligand that can bind to many LDL receptor-related receptors and cell surface heparan sulfate proteoglycans (HSPGs), in addition to the LDL receptor4).
ApoE is a multifunctional protein synthesized and secreted by numerous tissues throughout the body, but hepatocytes contribute to approximately 75% of the apoE peripheral blood pool4). Because of the high lipophilicity of apoE, most of it is present on lipoproteins in the blood. In plasma, apoE associates primarily with chylomicrons, chylomicron remnants, VLDL, IDL, and high-density lipoprotein (HDL). One of the main functions of apoE is to mediate lipoprotein clearance through the uptake of remnants (chylomicron and VLDL) and HDL by hepatocytes (Fig.1). The LDL receptor-binding affinity of apoE was reported to be more than 20-fold greater than that of apoB100 5). ApoE binds a wide range of cellular receptors, including the LDL receptor (LDLR), the LDL receptor-related proteins (LRP) LRP1, LRP2, and LRP8, and the very low-density-lipoprotein receptor (VLDLR), which mediate the cellular uptake of the apoE-containing lipoprotein particles6). ApoE also exhibits heparin-binding activity and binds HSPGs on the surface of cells, a property that supports the capture and receptor-mediated uptake of apoE-containing lipoproteins by cells7). Through its interaction with these receptors and the HSPG-LRP pathway, apoE mediates remnant lipoprotein catabolism (Fig.1). ApoE is involved in the hepatic biosynthesis of VLDLs and their uptake by peripheral tissues, ensuring the delivery of triglycerides and energy storage in muscle, heart, and adipose tissues. ApoE also plays an important role in lipid transport in the central nervous system, regulating neuron survival and sprouting4).
ApoE binds to a wide range of cellular receptors, including the LDL receptor (LDLR), LDL receptor-related proteins, and very-low-density lipoprotein receptor (VLDLR), as well as heparan sulfate proteoglycans (HSPG), which mediate the cellular uptake of lipoprotein particles containing apoE. In vessels, meanwhile, apoE on VLDL and chylomicron particles competitively inhibits triglyceride degradation processing by lipoprotein lipase (LPL) promoted by apoC-II and apoA-V. GPIHBP1: glycosylphosphatidylinositol-anchored high-density lipoprotein binding protein 1, LRP1: LDL receptor-related protein 1.
Human apoE is a protein consisting of 299 amino acid residues with a molecular weight of 34 kDa8). ApoE has many amphipathic α-helices (Fig.2). These helices separate the hydrophobic and polar residues into two planes, allowing them to adsorb at polar–nonpolar interfaces such as lipoproteins. ApoE is divided into three structural domains: the N-terminal domain (residues 1–167) with four α-helices (helices 1–4) attached, the hinge domain (residues 168–205) with two helices (residues 168–180 and 190–199), and the C-terminal domain (residues 206–299) with three helices (residues 210–223, 236–266, and 271–276) that binds even more strongly to lipids (Fig.2). The first structural determination of the N-terminal domain of apoE3, reported in 1991, indicated that it is a unique four-α-helix bundle (Fig.2)9, 10). The structure of the N-terminal domain was subsequently more precisely determined by NMR for apoE3 and by X-ray crystallography for apoE3 and apoE4. As expected, the polar groups in helix 2 (residues 55–79) and the region in helix 4 (residues 131–164) were all shown to be exposed to solvent. Despite extensive analysis, it took 20 years until the whole structure of apoE, including the C-terminal domain, was determined because of its very high affinity for lipids, and the higher-order structure of the full-length protein was first reported in 2011 11). It was revealed that the C-terminal domain of apoE has three α-helices, which interact with the helices of the N-terminal domain by forming hydrogen bonds and salt bridges. The region of amino acid residues 134–150 of the N-terminal domain helix 4 includes many basic amino acids on its polar face and is important for binding to the LDL receptor (Fig.2). Meanwhile, heparin binding sites are located in both the N-terminal and the C-terminal domains; the heparin binding site in the N-terminal domain is located at residues 142–147, which overlaps with the LDL receptor binding region12). In fact, HSPG binding activity is significantly reduced by variants in the basic amino acids Arg142, Arg145, and Lys146, suggesting that these residues contribute to heparin binding. The interaction of apoE with HSPGs may be the first step in the clearance of lipoproteins containing apoE from plasma (Fig.1).
ApoE is divided into three structural domains: the N-terminal domain (residues 1–167) with four α-helices (helices 1–4) attached, the hinge domain (residues 168–205) with two helices (residues 168–180 and 190–199), and the C-terminal domain (residues 206–299) that binds even more strongly to lipids. The C-terminal domain of apoE interacts with the helix of the N-terminal domain by forming hydrogen bonds and salt bridges. The region of amino acid residues 134–150 of the N-terminal domain helix 4 contains many basic amino acids on its polar face and is important for binding to the LDL receptor.
There are three major isoforms at the APOE locus, ε2, ε3, and ε4, which are generated by the exchange of one amino acid residue at positions 112 and 158 resulting from a single base change (Fig.3). ε3 has Cys112 and Arg158, while ε4 (Cys112Arg, p.Cys130Arg) has arginine at both positions and ε2 (Arg158Cys, p.Arg176Cys) has cysteine at both positions (Fig.3)8).
There are three major isoforms at the APOE locus, namely, ε2, ε3, and ε4, which are generated by the exchange of one amino acid residue at positions 112 and 158, resulting from two single-nucleotide polymorphisms in exon 4.
The combination of these three variants results in three homozygous (ε2ε2, ε3ε3, ε4ε4) and three heterozygous (ε2ε3, ε3ε4, ε4ε2) APOE genotypes. ε3 is the most common APOE genotype in humans, being present in 65%–70% of the population, and is considered the wild type. Meanwhile, ε2 is present in 8%–10% of the population, and ε4 in 15%–20%. The common APOE genotypes can be easily determined by isotyping with the restriction enzyme Hha I13). The common isoforms ε2, ε3, and ε4 can be distinguished at the protein level by isoelectric electrophoresis into apoE2, apoE3, and apoE4 (Fig.3)14). The isoelectric point of many amino acids such as Cys is approximately neutral (pH 5–6), whereas the isoelectric point of Asp or Glu is acidic (pH 3), and that of His, Lys, or Arg is alkaline (pH 8–10). In addition, studies have reported a number of APOE variants (APOE1, E2, E3, E4, E5, E7) that can be detected by isoelectric electrophoresis due to changes in the isoelectric point caused by changes in amino acids. The Cys112Arg and Arg158Cys SNPs appear to be in almost perfect linkage disequilibrium (Fig.3), and the combination of arginine at position 112 and cysteine at position 158, which may define the apoE1, also called apoE3r isoform, is extremely rare15).
ApoE3 and apoE4 have normal binding activity to the LDL receptor, while apoE2 is defective in such binding (＜2% of the normal activity.) due to the presence of Arg158Cys near the LDL receptor binding site16). The presence of cysteine at position 158 in ε2 disrupts the normal salt bridging required for the interaction of arginine at position 150 in the receptor binding region with the LDL receptor17). However, apoE2 retains the ability to bind to HSPG and other related receptors. In the absence of lipid abnormalities, plasma apoE levels are generally reported to tend to be higher for apoE2 and lower for apoE4 18).
In contrast, Cys112Arg in apoE4 affects the organization and stability of both the N-terminal and the C-terminal domains and enhances the ability of apoE to bind to lipoproteins. As a result, apoE4 binds better and is more abundant on the surface of VLDL and chylomicron particles than apoE3 or apoE2 19).
ApoE is an apoprotein with two sides: intravascular and extravascular. Outside of blood vessels, apoE acts as a ligand, binding to various receptors and causing intracellular uptake of lipoproteins, contributing to lipid lowering (Fig.1). Meanwhile, in vessels, apoE on VLDL and chylomicron particles competitively inhibits triglyceride degradation processing by lipoprotein lipase (LPL) promoted by apoC-II and apoA-V20). Therefore, apoE4, which is abundant on VLDL and chylomicrons, tends to inhibit triglyceride degradation by LPL more readily than apoE2 or apoE3 21).
The APOE gene is located on chromosome 19q.13.32 and forms a very close cluster with APOC1, APOC1 pseudogene, APOC4, and APOC2 (Fig.4). The human APOE gene is located in a region approximately 5000 bp in length and consists of four exons. As shown in Fig.4, the APOE start codon is located in exon 2, and exons 2, 3, and 4 are the regions of the APOE gene encoding the expressed amino acids22). ApoE is expressed as pre-apoE consisting of 317 amino acid residues; the signal peptide of 18 amino acid residues is cleaved from pre-apoE to produce mature apoE of 299 amino acid residues, which is then secreted (Fig.4).
The APOE gene is located on chromosome 19q.13.32 and clusters closely with APOC1 and other genes. The human APOE gene has four exons. The APOE start codon is located in exon 2, and exons 2, 3, and 4 are the parts of the APOE gene expressed at the protein level22). The signal peptide of 18 amino acid residues is cleaved to enable secretion as the 299-amino-acid mature apoE.
APOE gene variants have been reported for more than 40 years. Analysis of mutant proteins was also performed in parallel with this work, and thus for a long time APOE gene mutation sites were mostly reported based on the amino acid numbers (n=299) of mature apoE. However, recently, apoE mutational analyses have focused on the gene rather than the protein. In this context, the amino acid number from the universal start codon of the protein, methionine, is often used to represent the mutation site. In this case, the mutation site is always prefixed with “p.” to indicate that the numbering is from the start codon. Because many researchers have been unaware of this, some papers misidentify the number of amino acid residues in apoE, causing previously reported variants to be incorrectly reported as new ones. In the tables of this review, APOE variants are presented using the conventional mature amino acid representation, along with a protein-level “p.” for numbering from the start codon, a base-level “c.,” and a unique “rs” assigned to the SNP site, which is assigned to unique SNP sites. For example, ε4 is rs429358 and ε2 is rs7412. The mutation sites (mutation initiation sites) of Genome Reference Consortium Human (GRCh) Builds 37 and 38 are also listed to facilitate identification by next-generation sequencing (NGS) analysis (Supplementary Table 1).
| No. | Variant | Protein sequence change | Genomic change＊ | rs number | Structure | Characteristics | GRCh37 | GRCh38 | Ref. |
|---|---|---|---|---|---|---|---|---|---|
| 1 | APOE p.Thr5＊ | p.Trp5Ter | c.15G>A | rs777551553 | Signal | FDB | 45409896 | 44906639 | 39 |
| 2 | APOE p.Thr11Ala | p.Thr11Ala | c.31A>G | rs144354013 | Signal | CHL | 45409912 | 44906655 | 40 |
| 3 | APOE p.Thr11Ser | p.Thr11Ser | c.31A>T | rs144354013 | Signal | NA | 45409912 | 44906655 | 127 |
| 4 | APOE p.Phe12Tyr | p.Phe12Tyr | c.35T>A | rs747078681 | Signal | NA | 45409916 | 44906659 | 127 |
| 5 | APOE (c.44-1G>C) | NA | c.44-1G>C | NA | Intron 2 | HCh | 45411016 | 44907759 | 40 |
| 6 | APOE5 (Glu3Lys) | p.Glu21Lys | c.61G>A | rs121918392 | N-ter | CHL | 45411034 | 44907777 | 104,14,107 |
| 7 | APOE p.Ala23Val (Ala5Val) | p.Ala23Val | c.68C>T | rs776242156 | N-ter | NA | 45411041 | 44907784 | 127 |
| 8 | APOE4 Philadelphia (Glu13Lys, Arg145Cys) | p.Glu31Lys, p.Arg163Cys | c.91G>A, c.487C>T | rs201672011, rs769455 | N-ter | FDB | 45411064 | 44907807 | 63 |
| 9 | APOE p.Arg33Cys (Arg15Cys) | p.Arg33Cys | c.97C>T | rs752079771 | N-ter | NA | 45411070 | 44907813 | 127 |
| 10 | APOE (Trp20Ter) | p.Trp38Ter | c.114G>A | rs2122132512 | N-ter | FDB | 45411087 | 44907830 | 27 |
| 11 | APOE Kyoto (Arg25Cys) | p.Arg43Cys | c.127C>T | rs121918399 | N-ter | LPG | 45411100 | 44907843 | 87 |
| 12 | APOE4 Freiburg/Pittsburgh (Leu28Pro, Cys112Arg) | p.Leu46Pro, p.Cys130Arg | c.137T>C | rs769452 | Helix 1 | CHL, AD | 45411130 | 44907873 | 108,109 |
| 13 | APOE (Gly31fsTer29/Gly127Asp, Arg158Cys) | p.Gly49fs/ p.Gly145Asp, p.Arg176Cys | c.146Gdel/c.434G>A | rs2122132718 | Helix 1 | FDB | 45411117_9 | 44907860_2 | 37,38 |
| 14 | APOE p.Arg50Cys (Arg32Cys) | p.Arg50Cys | c.148C>T | rs11542029 | Helix 1 | NA | 45411121 | 44907864 | 126 |
| 15 | APOE (Arg32His)/Kyoto (Arg25Cys) | p.Arg50His/ p.Arg43Cys | c.149G>A/c.127C>T | rs762461580/ rs121918399 | Helix 1 | LPG | 45411122 | 44907865 | 88 |
| 16 | APOE p.Thr60Ala (Thr42Ala) | p.Thr60Ala | c.178A>G | rs28931576 | Helix 1 | NA | 45411151 | 44907894 | 126 |
| 17 | APOE p.Ser72Phe (Ser54Phe) | p.Ser72Phe | c.215C>T | rs1969840702 | Helix 2 | NA | 45411188 | 44907931 | 127 |
| 18 | APOE (c.237-33C>G, Arg158Cys) | NA | c.237-33C>G | NA | Intron 3 | FDB＊＊ | 45411757 | 44908500 | 44 |
| 19 | APOE (c.237-2A>G) | NA | c.237-2A>G | rs397514253 | Intron 3 | Deficiency, FDB | 45411788 | 44908531 | 32,33 |
| 20 | APOE p.Met82Ile (Met64Ile) | p.Met82Ile | c.246G>T | rs557845700 | Helix 2 | HTG | 45411799 | 44908542 | 44 |
| 21 | APOE deficiency (Glu79fsTer98) | p.Glu97fsTer98 | c.291del | rs527236160 | Helix 2 | Deficiency, FDB | 45411844 | 44908587 | 34 |
| 22 | APOE5 Frankfurt (Gln81Lys, Cys112Arg) | p.Gln99Lys, p.Cys130Arg | c.295C>A | rs1180612218 | Helix 2_3 | CHL | 45411848 | 44908591 | 112 |
| 23 | APOE5 (Pro84Arg, Cys112Arg) | p.Pro102Arg, p.Cys130Arg | c.305C>G | rs11083750 | Helix 2_3 | HCh | 45411858 | 44908601 | 113 |
| 24 | APOE p.Pro102Leu (Pro84Leu) | p.Pro102Leu | c.305C>T | rs11083750 | Helix 2_3 | HCh | 45411858 | 44908601 | 40 |
| 25 | APOE p.Pro102Gln (Pro84Gln) | p.Pro102Gln | c.305C>A | rs11083750 | Helix 2_3 | NA | 45411858 | 44908601 | 126 |
| 26 | APOE p.Arg108Trip (Arg90Trp) | p.Arg108Trp | c.322C>T | rs1050106163 | Helix 3 | NL | 45411875 | 44908618 | 44 |
| 27 | APOE3 Groningen (Glu96fsTer50) | p.Glu114fsTer50 | c.340insG:c.341insG | NA | Helix 3 | FDB | 45411893 | 44908636 | 41 |
| 28 | APOE p.Glu114Lys (Glu96Lys) | p.Glu114Lys | c.340G>A | rs1169728519 | Helix 3 | NA | 45411893 | 44908636 | 127 |
| 29 | APOE p.Ala117Thr (Ala99Thr) | p.Ala117Thr | c.349G>A | rs28931577 | Helix 3 | NA | 45411902 | 44908645 | 126 |
| 30 | APOE p.Arg121Trp (Arg103Trp) | p.Arg121Trp | c.361C>T | rs11542037 | Helix 3 | FDB | 45411914 | 44908657 | 44 |
| 31 | APOE4 (Cys112Arg) | p.Cys130Arg | c.388T>C | rs429358 | Helix 3 | Common | 45411941 | 44908684 | |
| 32 | APOE Tsukuba (Arg114Cys) | p.Arg132Cys | c.394C>T | rs11542041 | Helix 3 | LPG | 45411947 | 44908690 | 86 |
| 33 | APOE p.Arg132Ser (Arg114Ser) | p.Arg132Ser | c.394C>A | rs11542041 | Helix 3 | NA | 45411947 | 44908690 | 126 |
| 34 | APOE Arg114Pro | p.Arg132Pro | c.395G>C | rs1263042140 | Helix 3 | HCh | 45411948 | 44908691 | 43 |
| 35 | APOE p.Tyr136His (Tyr118His) | p.Tyr136His | c.406T>C | rs1969862344 | Helix 3 | NL | 45411959 | 44908702 | 44 |
| 36 | APOE p.Arg137Cys (Arg119Cys) | p.Arg137Cys | c.409C>T | rs573658040 | Helix 3 | FH | 45411962 | 44908705 | 40 |
| 37 | APOE p.Arg137His (Arg119His) | p.Arg137His | c.410G>A | rs11542035 | Helix 3 | FH | 45411963 | 44908706 | 40 |
| 38 | APOE p.Glu139Val (Glu121Val) | p.Glu139Val | c.416A>T | rs41382345 | Helix 3 | NA | 45411969 | 44908712 | 126 |
| 39 | APOE3 Leiden (121-127dupGluValGlnAlaM etLeuGly) | p.139_145dup | c.415_435dup | rs397514254 | Helix 3_4 | dFDB | 45411964 | 44908707 | 55 |
| 40 | APOE1 (Gly127Asp, Arg158Cys) | p.Gly145Asp, p.Arg176Cys | c.434G>A | rs267606664 | Helix 3_4 | FDB | 45411987 | 44908730 | 56,57 |
| 41 | APOE p.Glu150Gly (Glu132Gly) | p.Glu150Gly | c.449A>G | rs11542034 | Helix 4 | NA | 45412002 | 44908745 | 126 |
| 42 | APOE p.Arg152Gln (Arg134Gln) | p.Arg152Gln | c.455G>A | rs28931578 | Helix 4 | NA | 45412008 | 44908751 | 126 |
| 43 | APOE5 (Val135_Arg142dup) | p.Val153_Arg160dup | c.457_480dup24 | NA | Helix 4 | HTG | 45412010 | 44908753 | 114 |
| 44 | APOE 136fsTer96 | p.Arg154fsTer96 | c.460Cdel | rs121918393 | Helix 4 | FDB | 45412013 | 44908756 | 42 |
| 45 | APOE2 Heidelberg (Arg136Cys) | p.Arg154Cys | c.460C>T | rs121918393 | Helix 4 | dFDB | 45412013 | 44908756 | 47 |
| 46 | APOE2 Christchurch (Arg136Ser) | p.Arg154Ser | c.460C>A | rs121918393 | Helix 4 | dFDB, CHL | 45412013 | 44908756 | 26,48 |
| 47 | APOE3’ (Arg136His) | p.Arg154His | c.461G>A | rs200703101 | Helix 4 | FDB | 45412014 | 44908757 | 59 |
| 48 | APOE p.Leu155Phe (Leu137Phe) | p.Leu155Phe | c.463 C>T | rs1018669382 | Helix 4 | FH | 45412016 | 44908759 | 40 |
| 49 | APOE Tokyo/Maebashi (141-143del:143- 145del) | p.159_161del:p.161- 163del | c.475_483del9: c.480_488del9 | NA | Helix 4 | LPG | 45412033 | 44908776 | 73,74 |
| 50 | APOE3 (Arg142Cys) | p.Arg160Cys | c.478C>T | rs387906567 | Helix 4 | dFDB | 45412031 | 44908774 | 49,50 |
| 51 | APOE1 Nagoya (Arg142Ser, Arg158Cys) | p.Arg160Ser, p.Arg176Cys | c.478C>A | rs387906567 | Helix 4 | FDB | 45412031 | 44908774 | 60 |
| 52 | APOE2 (Arg142Leu) | p.Arg160Leu | c.479G>T | rs1452005331 | Helix 4 | dFDB | 45412032 | 44908775 | 51 |
| 53 | APOE 15 bp deletion (142-146del) | p.160_164del | c.477_491del15 | NA | Helix 4 | LPG | 45412033 | 44908776 | 75 |
| 54 | APOE p.Lys161Asn (Lys143Asn) | p.Lys161Asn | c.483G>T | rs867215645 | Helix 4 | NA | 45412036 | 44908779 | 127 |
| 55 | APOE2 (Arg145Cys) | p.Arg163Cys | c.487C>T | rs769455 | Helix 4 | FDB, FH | 45412040 | 44908783 | 64,65,57 |
| 56 | APOE Kochi (Arg145His) | p.Arg163His | c.488G>A | rs121918397 | Helix 4 | FDB | 45412041 | 44908784 | 61,62 |
| 57 | APOE Sendai (Arg145Pro) | p.Arg163Pro | c.488G>C | rs121918397 | Helix 4 | LPG | 45412041 | 44908784 | 72 |
| 58 | APOE2 (Lys146Gln) | p.Lys164Gln | c.490A>C | rs121918394 | Helix 4 | dFDB, AD | 45412043 | 44908786 | 45 |
| 59 | APOE1 Harrisburg (Lys146Glu) | p.Lys164Glu | c.490A>G | rs121918394 | Helix 4 | dFDB | 45412043 | 44908786 | 52,53 |
| 60 | APOE1 Hammersmith (Lys146Asn, Arg147Trp) | p.Lys164Asn, p.Arg165Trp | c.492G>C, c.493C>T | rs1446645173, rs1402219759 | Helix 4 | dFDB | 45412045_6 | 44908788_9 | 54 |
| 61 | APOE p.Arg165Trp (Arg147Trip) | p.Arg165Trp | c.493C>T | rs1402219759 | Helix 4 | FDB | 45412046 | 44908789 | 44 |
| 62 | APOE Chicago (Arg147Pro) | p.Arg165Pro | c.494G>C | rs1332591068 | Helix 4 | LPG | 45412047 | 44908790 | 76 |
| 63 | APOE p.Leu167del (Leu149del) | p.Leu167del | c.499_501delCTC: c.500_502delTCC | rs515726148 | Helix 4 | FH | 45412049 | 44908792 | 115,116,117 |
| 64 | APOE Okayama (Arg150Gly) | p.Arg168Gly | c.502C>G | rs867594573 | Helix 4 | LPG | 45412055 | 44908798 | 77 |
| 65 | APOE Modena (Arg150Cys) | p.Arg168Cys | c.502C>T | rs867594573 | Helix 4 | LPG | 45412055 | 44908798 | 78 |
| 66 | APOE Guangzhou (Arg150Pro) | p.Arg168Pro | c.503G>C | rs376170967 | Helix 4 | LPG | 45412056 | 44908799 | 79 |
| 67 | APOE (Arg150His) | p.Arg168His | c.503G>A | rs376170967 | Helix 4 | NL | 45412056 | 44908799 | 43 |
| 68 | APOE2 Kanto (Asp151dupAsp) | p.Asp169dupAsp | c.505_507insGAT:c.508_510insGAT | NA | Helix 4 | LPG | 45412058 | 44908801 | 70,80 |
| 69 | APOE Las Vegas (Ala152Asp) | p.Ala170Asp | c.509C>A | rs1969869057 | Helix 4 | LPG | 45412062 | 44908805 | 81 |
| 70 | APOE Chengdu (Leu155Pro) | p.Leu173Pro | c.518T>C | NA | Helix 4 | LPG | 45412071 | 44908814 | 82 |
| 71 | APOE1 (156_173del) | p.174_191del | c.520_573del | NA | Helix 4 | LPG | 45412073 | 44908816 | 83 |
| 72 | APOE2 (Arg158Cys) | p.Arg176Cys | c.526C>T | rs7412 | Helix 4 | Common | 45412079 | 44908822 | |
| 73 | APOE Osaka/Kurashiki (Arg158Pro) | p.Arg176Pro | c.527G>C | NA | Helix 4 | LPG | 45412080 | 44908823 | 84,85 |
| 74 | APOE p.Val179Ala (Val161Ala) | p.Val179Ala | c.536T>C | rs1421977676 | Helix 4 | CHL | 45412089 | 44908832 | 40 |
| 75 | APOE (Ala166fs) | p.Ala184fs | c.550Gdel | NA | Helix 4_ hinge | HTG | 45412103 | 44908846 | 43 |
| 76 | APOE p.Glu189Lys (Glu171Lys) | p.Glu189Lys | c.565G>A | rs11542032 | Hinge | NA | 45412118 | 44908861 | 126 |
| 77 | APOE p.Gly191Cys (Gly173Cys) | p.Gly191Cys | c.571G>T | rs1265472491 | Hinge | HCh | 45412124 | 44908867 | 44 |
| 78 | APOE1 Baden (Arg180Cys, Arg158Cys) | p.Arg198Cys, p.Arg176Cys | c.592C>T | rs1426426514 | Hinge | HTG | 45412145 | 44908888 | 119 |
| 79 | APOE2 Toranomon (Gln187Glu) | p.Gln205Glu | c.613C>G | rs2122138444 | Hinge | FDB | 45412166 | 44908909 | 66,67 |
| 80 | APOE p.Gln205Arg (Gln187Arg) | p.Gln205Arg | c.614A>G | rs11542030 | Hinge | NA | 45412167 | 44908910 | 126 |
| 81 | APOE p.Ala210Ser (Ala192Ser) | p.Ala210Ser | c.628G>T | NA | Hinge | NA | 45412181 | 44908924 | 127 |
| 82 | APOE p.Val213Glu (Val195Glu) | p.Val213Glu | c.638T>A | rs1227709957 | Hinge | FH | 45412191 | 44908934 | 40 |
| 83 | APOE Toyonaka (Ser197Cys, Arg158Cys) | p.Ser215Cys, p.Arg176Cys | c.644C>G | rs11542027 | Hinge | GP | 45412197 | 44908940 | 101 |
| 84 | APOE p.Ser215Phe (Ser197Phe) | p.Ser215Phe | c.644C>T | rs11542027 | Hinge | NA | 45412197 | 44908940 | 127 |
| 85 | APOE p.Gly218Cys (Gly200Cys) | p.Gly218Cys | c.652G>T | rs1488379910 | Hinge | FH | 45412205 | 44908948 | 40 |
| 86 | APOE p.Gly218Ser (Gly200Ser) | p.Gly218Ser | c.652G>A | rs1488379910 | Hinge | NA | 45412205 | 44908948 | 127 |
| 87 | APOE p.Pro220Leu (Pro202Leu) | p.Pro220Leu | c.659C>T | rs1265743589 | Hinge | CHL | 45412212 | 44908955 | 44 |
| 88 | APOE5 (Gln204Lys, Cys112Arg) | p.Gln222Lys, p.Cys130Arg | c.664C>A | rs1181840153 | Hinge | NL | 45412217 | 44908960 | 125 |
| 89 | APOE 10 bp deletion (Ala209GlyfsTer20) | p.Ala227fsTer20 | c.679_688del | NA | C-ter | Deficiency, FDB | 45412232 | 44908975 | 35 |
| 90 | APOE3 Washington (Trp210Ter) | p.Trp228Ter | c.683G>A | rs121918396 | C-ter | Deficiency, FDB | 45412236 | 44908979 | 36 |
| 91 | APOE5 (Glu212Lys) | p.Glu230Lys | c.688G>A | rs567353589 | C-ter | NL | 45412241 | 44908984 | 124,125 |
| 92 | APOE (Arg217Trp) | p.Arg235Trp | c.703C>T | rs530010303 | C-ter | FH | 45412256 | 44908999 | 29 |
| 93 | APOE2 Fukuoka (Arg224Gln) | p.Arg242Gln | c.725G>A | rs267606663 | C-ter | FH | 45412278 | 44909021 | 120 |
| 94 | APOE2 Dunedin (Arg228Cys) | p.Arg246Cys | c.736C>T | rs121918395 | C-ter | HTG | 45412289 | 44909032 | 121 |
| 95 | APOE Hong Kong (Asp230Tyr)/Kyoto (Arg25Cys) | p.Asp248Tyr/ p.Arg43Cys | c.742G>T | NA/ rs121918399 | C-ter | LPG | 45412298 | 44909038 | 89 |
| 96 | APOE p.Glu249Lys (Glu231Lys) | p.Glu249Lys | c.745G>A | rs762906934 | C-ter | CHL | 45412298 | 44909041 | 40 |
| 97 | APOE p.Glu252Lys (Glu234Lys) | p.Glu252Lys | c.754G>A | NA | C-ter | CHL | 45412307 | 44909050 | 40 |
| 98 | APOE2 (Val236Glu) | p.Val254Glu | c.761T>A | rs199768005 | C-ter | CHL | 45412314 | 44909057 | 122 |
| 99 | APOE7 Suita (Glu244Lys, Glu245Lys) | p.Glu262Lys, p.Glu263Lys | c.784G>A, c.787G>A | rs140808909, rs190853081 | C-ter | CHL | 45412337 | 44909080 | 123,14 |
| 100 | APOE3 (Arg251Gly, Cys112Arg) | p.Arg269Gly, p.Cys130Arg | c.805C>G | rs267606661 | C-ter | CHL | 45412358 | 44909101 | 122 |
| 101 | APOE1 (Leu252Glu, Arg158Cys) | p.Leu270Glu, p.Arg176Cys | c.808C>G, c.809T>A | rs992440908, NA | C-ter | HCh | 45412361_2 | 44909104_5 | 122 |
| 102 | APOE4’ (Arg274His, Cys112Arg) | p.Arg292His, p.Cys130Arg | c.875G>A | rs121918398 | C-ter | NL | 45412428 | 44909171 | 122 |
| 103 | APOE p.Val305Met (Val287Met) | p.Val305Met | c.913G>A | rs749102800 | C-ter | NA | 45412466 | 44909209 | 127 |
| 104 | APOE4 (Ser296Arg) | p.Ser314Arg | c.940A>C | rs28931579 | C-ter | NL | 45412493 | 44909236 | 122 |
＊: Main mutation sites are shown, ＊＊: There was linkage disequilibrium with apoE2, resulting in FDB in apoE2 homozygotes, GRCh37: Genome Reference Consortium Human Build 37, GRCh38: Genome Reference Consortium Human Build 38, Ref.: references, NA: not available, Signal: signal peptide, N-ter: N-terminal domain, Hinge: hinge domain, C-ter: C-terminal domain, Common: common isoforms, FDB: familial dysbetalipoproteinemia (familial type III HLP), dFDB: autosomal dominant familial dysbetalipoproteinemia (familial type III HLP), FH: familial hypercholesterolemia including autosomal dominant hypercholesterolemia (ADH) and severe familial type III HLP, HCh: hypercholesterolemia, HTG: hypertriglyceridemia, CHL: combined hyperlipidemia, AD: Alzheimer’s disease, NL: no lipid abnormalities
Familial type III HLP is also called familial dysbetalipoproteinemia (FDB). It is a metabolic disorder that results in increased cholesterol and triglyceride-rich lipoproteins termed β-VLDL, owing to their β-electrophoretic migration instead of the usual pre-β migration of typical VLDL. Patients with familial type III HLP are known to be susceptible to the early development of atherosclerosis. Clinical manifestations of familial type III HLP can include tuberous or planar xanthomas of the skin or tendon sheath xanthomas, as well as early-onset coronary artery disease. Familial type III HLP is caused by the accumulation of chylomicron remnants derived from intestinal lipoproteins and VLDL remnants derived from hepatic lipoproteins; these abnormal cholesterol, triglyceride, and apoE-enriched lipoproteins, collectively termed β-VLDL, are the diagnostic features of this disease. The definition and diagnostic details of familial type III HLP are discussed in other review articles23-25). Usually, in familial type III HLP, plasma cholesterol and triglycerides are almost equally elevated above 300 mg/dL. The primary molecular cause of familial type III HLP is the presence of the homozygous form of APOE2 (Arg158Cys), an LDL receptor binding-deficient isoform of apoE, which is associated with the recessive inheritance of the disease. In fact, more than 90% of affected individuals are APOE2 homozygotes, with the remainder having rare APOE variants, including those causing apoE deficiency or compound heterozygotes of APOE variants and APOE2 23, 25-28). However, less than 10% of APOE2 homozygotes develop type III HLP, and most APOE2/E2 carriers are either normolipidemic or hypocholesterolemic. Thus, additional genetic, hormonal, or environmental factors such as obesity, hypothyroidism, estrogen status, and diabetes are necessary for the development of familial type III HLP. Familial type III HLP is more common and tends to develop earlier in men than in women, in whom it rarely occurs until after menopause. As the diagnosis of type III HLP requires analysis of lipoproteins and apoE, measurement of serum cholesterol and triglyceride levels alone often leads to the diagnosis of familial hypercholesterolemia (FH), combined hyperlipidemia (CHL), or hypertriglyceridemia. Relatively few cases of familial type III HLP are correctly diagnosed as type III because of their good response to lipid-lowering drugs and the difficulty of achieving an accurate diagnosis28).
Severe type III HLP meets the criteria for FH. FH is caused primarily by variants in the LDLR gene. Other FH-causing genes include proprotein convertase subtilisin kexin 9 (PCSK9), low-density-lipoprotein receptor adapter protein 1 (LDLRAP1), APOB, and APOE. Genetic variants in LDLR, PCSK9, and APOB cause autosomal dominant hypercholesterolemia (ADH), while pathogenic variants in LDLRAP1 cause recessive FH. FH due to APOE deficiency is recessively inherited, as in APOE-deficient homozygotes, but APOE variants diagnosed as ADH have also been reported29).
To date, 13 different APOE deficiencies have been reported. Of these, the two early studies of complete APOE deficiency only involved analysis at the protein level and the site of mutation was not reported30, 31). Of the 11 cases of APOE deficiency with reported mutation sites, four were homozygotes and seven were compound heterozygotes or heterozygotes27, 29-40).
Two cases with no reported mutation site and homozygotes for APOE deficiency due to variant in the intron 3 acceptor site (c.237-2A>G)32, 33) and APOE deficiency (Glu79fsTer98)34) with almost no detectable apoE have been reported to cause severe type III HLP, including various xanthomas as well as palmar linear xanthomas (Table 1). Homozygotes for the APOE 10-bp deletion (Ala209GlyfsTer20), in which 19 amino acids from among the 209 amino acids at the C-terminus were nonfunctional, also exhibited severe familial type III HLP35). Meanwhile, homozygotes for APOE3 Washington (Trp210Ter) showed relatively mild type III HLP, possibly due to the structure of apoE retaining the N-terminal side (Table 1)36). Plasma lipid and lipoprotein cholesterol values of two heterozygous offspring were found to be within the normal range.
| Variant | No. | Protein sequence change | Genomic change＊ | rs number | Exon/intron | Protein structure | Homo/hetero | Characteristics | Ref. |
|---|---|---|---|---|---|---|---|---|---|
| 1 | APOE p.Thr5＊ | p.Thr5Ter | c.15G>A | rs777551553 | Exon 2 | Signal | Hetero | FDB | 39 |
| 2 | APOE (c.44-1G>C) | NA | c.44-1G>C | NA | Intron 2 | NA | Hetero | HCh | 40 |
| 3 | APOE (Trp20Ter) | p.Trp38Ter | c.114G>A | rs2122132512 | Exon 3 | N-ter | Hetero | FDB | 27 |
| 4 | APOE (Gly31fsTer29/Gly127Asp, Arg158Cys) | p.Gly49fs/ p.Gly145Asp, p.Arg176Cys | c.146Gdel/c.434G>A | rs2122132718 | Exon 3 | Helix 1 | Compound | FDB | 37,38 |
| 5 | APOE (c.237-2A>G) | NA | c.237-2A>G | rs397514253 | Intron 3 | NA | Homo | Deficiency, FDB | 32,33 |
| 6 | APOE deficiency (Glu79fsTer98) | p.Glu97fsTer98 | c.291Gdel | rs527236160 | Exon 4 | Helix 2 | Homo | Deficiency, FDB | 34 |
| 7 | apoE3 Groningen (Glu96fsTer50) | p.Glu114fsTer50 | c.340insG:c.341insG | NA | Exon 4 | Helix 3 | Hetero | FDB | 41 |
| 8 | APOE 136fsTer96 | p.Arg154fsTer96 | c.460Cdel | rs121918393 | Exon 4 | Helix 4 | Hetero | FDB | 42 |
| 9 | APOE (Ala166fs) | p.Ala184fs | c.550Gdel | NA | Exon 4 | Helix 4_ hinge | Hetero | HTG | 43 |
| 10 | APOE 10 bp deletion (Ala209GlyfsTer20) | p.Ala227fsTer20 | c.679_688del | NA | Exon 4 | C-ter | Homo | Deficiency, FDB | 35 |
| 11 | APOE Washington (Trp210Ter) | p.Trp228Ter | c.683G>A | rs121918396 | Exon 4 | C-ter | Homo | Deficiency, FDB | 36 |
|
APOE (c.237-33C>G, Arg158Cys) |
NA | c.237-33C>G | NA | Intron 3 | NA | Hetero | FDB＊＊ | 44 |
Ref.: references, NA: not available, Homo: homozygote, Hetero: heterozygote, Compound: compound heterozygote, Signal: signal peptide, N-ter: N-terminal domain, hinge: hinge domain, C-ter: C-terminal domain, hinge: hinge domain, FDB: familial dysbetalipoproteinemia (familial type III HLP), ＊: Main mutation sites are shown, ＊＊: There was linkage disequilibrium with apoE2, resulting in FDB in apoE2 homozygotes, HCh: hypercholesterolemia, HTG: hypertriglyceridemia, Deficiency: apoE deficiency
In heterozygotes with APOE deficiency, when the mutant apoE was absent or dysfunctional, the nature of the apoE of the remaining allele affected the manifestation of familial type III HLP, with some cases showing type III HLP and others showing no lipid abnormalities. A 60-year-old Caucasian male compound heterozygous for APOE1 (Gly127Asp, Arg158Cys) and a nucleotide deletion of guanosine at codon 31 (Gly31fsTer29) was reported to have a severe form of familial type III HLP (Table 1)37, 38). In family studies, normal to mild dyslipidemia was reported when the remaining allele was apoE3, while type III HLP was reported when it was APOE2. Heterozygous APOE (p.Thr5*) was found in two families in a screening of a hypercholesterolemic, hypertriglyceridemic population in Norway39). APOE (p.Thr5*) does not express apoE because the fifth position of the signal peptide is a stop codon. In one family, only two of the five carriers of the APOE2 allele showed type III HLP. Meanwhile, in the second family, the five carriers did not have the APOE2 allele and did not have type III HLP. Heterozygous APOE (c.44-1G>C) was recently discovered in the screening of hypercholesterolemic patients in a French cohort40). APOE (c.44-1G>C) involves a mutation of the intron 2 acceptor site and apoE is not expressed from this allele, but no details were available other than that the case involved hypercholesterolemia. Elsewhere, APOE (Trp20Ter) was detected by phenotypic and genotypic analyses of apoE in German type III HLP cases27). In a study of a single family, only one of the three APOE (Trp20Ter) carriers with the APOE2 allele had hypertriglycemia, while the two with the E3 allele were normolipidemic. APOE Groningen (Glu96fsTer50) was identified as a mutation in which a G was inserted in codon 95 or 96 (95AAG96GAG>95AAG96GGA-G) of the APOE3 allele41). APOE Groningen (Glu96fsTer50) was detected in the analysis of a type III HLP case. Five APOE Groningen (Glu96fsTer50) carriers were detected in a family study, of which only three cases with the E2 allele, including the proband, exhibited type III HLP. Heterozygosity for APOE (136fsTer96) was also found in an Indigenous Australian woman42). She had the E2 allele and was diagnosed with type III HLP, while a case with APOE (136fsTer96) heterozygosity along with the E4 allele was normolipidemic. In addition, a case heterozygous for APOE (Ala166fs) was discovered upon screening diabetic patients in the United Kingdom43). APOE (Ala166fs) showed E2 on isoelectric electrophoresis, while the other allele was E3 and was not associated with hyperlipidemia.
Another example of an APOE variant, APOE (c.237-33C>G), recently reported in Spain, is a mutation adjacent to the splicing branchpoints of intron 3, which might impair splicing44). However, it was found to be in linkage disequilibrium with APOE2 (Arg158Cys) and only two of the four reported cases, where the other allele was also E2, had type III HLP, while the other two had CHL.
Approximately 10% of patients with familial type III HLP have other APOE variants or APOE deficiency. Some of these rare APOE variants that cause familial type III HLP were found to exhibit a dominant mode of inheritance45, 46). In these cases, a single defective APOE allele is usually sufficient to cause hyperlipidemia. A secondary factor promoting the accumulation of these residual dominant type III HLPs has been reported to exist in helix 4 of the N-terminal domain (Fig.2, Fig.5). These type III HLPs are highly penetrant dominant traits. Rare APOE variants associated with dominant type III HLPs include single-nucleotide substitutions at APOE2 Heidelberg (Arg136Cys)47), APOE2 Christchurch (Arg136Ser)26, 48), APOE2 (Arg142Cys)49, 50), APOE2 (Arg142Leu)51), APOE2 (Lys146Gln)45), and APOE1 Harrisburg (Lys146Glu)52, 53) (Table 2, Fig.5). In addition, APOE1 Hammersmith (Lys146Asn, Arg147Trp)54) and amino acid 121–127 tandem repeat insertions (APOE3 Leiden)55) have been reported as causative of dominantly inherited type III HLP (Table 2). APOE p.Arg121Trp (Arg103Trp) and APOE p.Arg165Trp (Arg147Trp) have also recently been reported to cause type III HLP, but only in one case each, and it was not known whether they were dominant44).
Variants that cause both familial type III HLP and LPG are concentrated in the LDL receptor binding region (amino acids 136–150). However, variants causing type III HLP do not cause lipoprotein glomerulopathy and present different pathologies from each other. ＊: non-dominant familial type III HLP.
| No. | Variant | Protein sequence change | Genomic change＊ | rs number | Protein structure | ApoE phenotypes | Characteristics |
gnomAD/ TOPMed＃ frequency |
Ref. |
|---|---|---|---|---|---|---|---|---|---|
| 1 | APOE p.Arg121Trp (Arg103Trp) | p.Arg121Trp | c.361C>T | rs11542037 | Helix 3 | NA | FDB | 0.000005 | 44 |
| 2 | APOE Leiden (121-127dup GluValGlnAlaMetLeuGly) | p.139_145dup | c.415_435dup | rs397514254 | Helix 3_4 | E3 | dFDB | NA | 55 |
| 3 | APOE1 (Gly127Asp, Arg158Cys) | p.Gly145Asp, p.Arg176Cys | c.434G>A | rs267606664 | Helix 3_4 | E1 | FDB | 0.00015300 | 56,57 |
| 4 | APOE Heidelberg (Arg136Cys) | p.Arg154Cys | c.460C>T | rs121918393 | Helix 4 | E2 | dFDB | 0.00008980 | 47 |
| 5 | APOE Christchurch (Arg136Ser) | p.Arg154Ser | c.460C>A | rs121918393 | Helix 4 | E3 | dFDB, CHL | 0.00001280 | 26,48 |
| 6 | APOE3’ (Arg136His) | p.Arg154His | c.461G>A | rs200703101 | Helix 4 | E4 | FDB | NA | 59 |
| 7 | APOE2 (Arg142Cys) | p.Arg160Cys | c.478C>T | rs387906567 | Helix 4 | E3 | dFDB | 0.000006 | 49,50 |
| 9 | APOE Nagoya (Arg142Ser, Arg158Cys) | p.Arg160Ser, p.Arg176Cys | c.478C>A | rs387906567 | Helix 4 | E1 | FDB | NA | 60 |
| 8 | APOE2 (Arg142Leu) | p.Arg160Leu | c.479G>T | rs1452005331 | Helix 4 | E2 | dFDB | NA | 51 |
| 10 | APOE2 (Arg145Cys) | p.Arg163Cys | c.487C>T | rs769455 | Helix 4 | E2 | FDB, FH | 0.00211000 | 64,65,57 |
| 10_1 | APOE Philadelphia (Glu13Lys, Arg145Cys) | p.Glu31Lys, p.Arg163Cys | c.91G>A, c.487C>T | rs201672011, rs769455 | Helix 4 | E4 | FDB | 0.00013200 | 63 |
| 11 | APOE Kochi (Arg145His) | p.Arg163His | c.488G>A | rs121918397 | Helix 4 | E3 | FDB | 0.00000648 | 61,62 |
| 12 | APOE2 (Lys146Gln) | p.Lys164Gln | c.490A>C | rs121918394 | Helix 4 | E2 | dFDB, AD | 0.00000651 | 45 |
| 13 | APOE Harrisburg (Lys146Glu) | p.Lys164Glu | c.490A>G | rs121918394 | Helix 4 | E1 | dFDB | 0.000007 | 52,53 |
| 14 | APOE1 Hammersmith (Lys146Asn, Arg147Trp) | p.Lys164Asn, p.Arg165Trp | c.492G>C, c.493C>T | rs1446645173, rs1402219759 | Helix 4 | E1 | dFDB | NA | 54 |
| 15 | APOE p.Arg165Trp (Arg147Trp) | p.Arg165Trp | c.493C>T | rs1402219759 | Helix 4 | NA | FDB | 0.000004# | 44 |
| 16 | APOE2 Toranomon (Gln187Glu) | p.Gln205Glu | c.613C>G | rs2122138444 | Hinge | E2 | FDB | NA | 66,67 |
＊: Main mutation sites are shown, gnomAD: Genome Aggregation Database, TOPMed: Trans-Omics for Precision Medicine, ＃: TOPMed frequency, Ref.: references, NA: not available, Hinge: hinge domain, FDB: familial dysbetalipoproteinemia (familial type III HLP), dFDB: autosomal dominant familial dysbetalipoproteinemia (familial type III HLP), CHL: combined hyperlipidemia, FH: familial hypercholesterolemia, AD: Alzheimer’s disease
The Gly127Asp variant is in linkage disequilibrium with APOE2 (Arg158Cys) and was detected as APOE1 (Gly127Asp, Arg158Cys)26, 38, 56, 57). This variant is not causative of a dominantly inherited familial type III HLP and is not directly related to receptor binding, but may be one of the factors that cause hyperlipidemia in association with the ε2 allele26, 58). Nine carriers (all heterozygotes) of APOE3’ (Arg136His) were reported to have levels of very low-density-lipoprotein (VLDL) cholesterol and triglycerides twice as high as those of nine noncarriers59). APOE3’ (Arg136His) might be associated with recessive expression of type III HLP. Meanwhile, APOE1 Nagoya (Arg142Ser, Arg158Cys) was reported from a case of familial hypercholesterolemia and type III HLP, occurring in an apoE2 homozygote60). Only one case of this kind was reported, and it was unclear whether it involved dominant type III HLP. Moreover, APOE3 Kochi (Arg145His) was reported to exhibit type III HLP, but this was not a highly penetrant dominant trait61, 62). Some APOE3 Kochi (Arg145His) carriers have also been reported to have normal lipid levels or hypertriglyceridemia. Furthermore, severe type III HLP has been reported in APOE4 Philadelphia (Glu13Lys, Arg145Cys) homozygotes63), while APOE (Arg145Cys) heterozygotes in Africa-derived populations were associated with higher triglyceride levels64). APOE (Arg145Cys) was inherited dominantly with incomplete penetrance and was thought to be dependent on environmental factors64, 65). APOE (Arg145Cys) homozygote was also found in a cohort of autosomal dominant hypercholesterolemia (ADH) patients in France57). Finally, APOE2 Toranomon (Gln187Glu) located in the hinge region was found in association with the recessive expression of type III HLP with coronary atherosclerosis66, 67).
LPG is a rare renal disease first reported in Japan in 1989 68). Since then, over 150 cases have been reported, primarily in East Asia, especially in Japan and China, but also from other regions of the world. LPG is characterized by dilated glomerular capillaries with lipoprotein clots showing lamellar formation, as seen on renal biopsy69).
LPG was initially identified as a glomerular disease associated with type III HLP68). However, its genetics and pathology differ from those of apoE2 homozygote glomerulopathy, which causes type III HLP. The renal involvement in apoE2 homozygote glomerulopathy is thought to follow the mechanism of atherosclerosis, with mesangial cells and macrophages infiltrating the mesangial space taking up lipoproteins and contributing to the development of glomerulosclerosis. In contrast to apoE2 homozygote glomerulopathy, LPG is characterized by dilation of glomerular capillaries with lipoprotein thrombi showing lamellar formation, directly inducing glomerular damage without inducing foamy changes68).
Clinically, LPG was found to be associated with several distinctive features, including proteinuria and dyslipidemia. LPG-induced kidney damage gradually progresses to end-stage renal failure. While proteinuria may start out mild, it frequently progresses to nephrotic syndrome69). LPG is more seen in men and is often familial in nature. The dyslipidemia seen in LPG is similar to type III HLP, with elevated levels of serum triglycerides and apoE. Unlike in type III HLP, however, patients with LPG do not develop palmar linear xanthomas or tendon xanthomas, and they have a low risk of complications related to atherosclerosis70). Prevention of hypertriglyceridemia may be important in LPG therapy, and the efficacy of fibrates, including fenofibrate, has been reported in several cases in Japan and China. Hu et al. compared patient and kidney survival over 3 years between fenofibrate-treated and control groups, confirming the value of fenofibrate in LPG treatment71).
APOE gene variants have been identified in nearly all cases of LPG, which is considered to be an inherited disease caused by the accumulation of abnormal lipoproteins containing mutant apoE in the glomeruli.
The first APOE variant associated with LPG was reported in Japan and is known as APOE Sendai (Arg145Pro)72). To date, 14 different variants in the APOE gene have been reported as causes of LPG (Fig.5, Table 3). Most were heterozygous apoE missense variants close to the LDL receptor binding site.
| No. | Variant | Protein sequence change | Genomic change＊ | rs number | Protein structure | ApoE phenotypes | Characteristics |
gnomAD/ TOPMed＃ frequency |
Ref. |
|---|---|---|---|---|---|---|---|---|---|
| 1 | APOE Kyoto (Arg25Cys) | p.Arg43Cys | c.127C>T | rs121918399 | N-ter | E2 | LPG | 0.00000799 | 87 |
| 1_1 | APOE Kyoto (Arg25Cys)/ (Arg32His) | p.Arg43Cys/ p.Arg50His | c.127C>T/c.149G>A | / rs762461580 | N-ter | NA | LPG | / 0.000015＃ | 88 |
| 1_2 | APOE Kyoto (Arg25Cys)/ Hong Kong (Asp230Tyr) | p.Arg43Cys/ p.Asp248Tyr | c.127C>T/c.742G>T | / NA | N-ter | NA | LPG | / NA | 89 |
| 2 | APOE Tsukuba (Arg114Cys) | p.Arg132Cys | c.394C>T | rs11542041 | Helix 3 | E2 | LPG | 0.00006390 | 86 |
| 3 | APOE Tokyo/Maebashi (141-143del:143-145del) | p.159_161del:p.161- 163del | c.475_483del9:c.480_488del9 | NA | Helix 4 | E1 | LPG | NA | 73,74 |
| 4 | APOE 15 bp deletion (142- 146del) | p.160_164del | c.477_491del15 | NA | Helix 4 | NA | LPG | NA | 75 |
| 5 | APOE Sendai (Arg145Pro) | p.Arg163Pro | c.488G>C | rs121918397 | Helix 4 | E2 | LPG | 0.000006 | 72 |
| 6 | APOE Chicago (Arg147Pro) | p.Arg165Pro | c.494G>C | rs1332591068 | Helix 4 | E2 | LPG | NA | 76 |
| 6_1 | APOE Chicago (Arg147Pro)/ E5(Glu3Lys) | p.Arg165Pro/ p.Glu21Lys | c.494G>C/c.61G>A | rs1332591068 | Helix 4 | E4 | LPG | / 0.000007 | 90 |
| 7 | APOE Okayama (Arg150Gly) | p.Arg168Gly | c.502C>G | rs867594573 | Helix 4 | E2 | LPG | NA | 77 |
| 8 | APOE Modena (Arg150Cys) | p.Arg168Cys | c.502C>T | rs867594573 | Helix 4 | E2 | LPG | NA | 78 |
| 9 | APOE Guangzhou (Arg150Pro) | p.Arg168Pro | c.503G>C | rs376170967 | Helix 4 | E2 | LPG | 0.000137 | 79 |
| 10 | APOE Kanto (Asp151dupAsp) | p.Asp169dupAsp | c.505_507insGAT:c.508_510insGAT | NA | Helix 4 | E2 | LPG | NA | 70,80 |
| 11 | APOE Las Vegas (Ala152Asp) | p.Ala170Asp | c.509C>A | rs1969869057 | Helix 4 | E2 | LPG | NA | 81 |
| 12 | APOE Chengdu (Leu155Pro) | p.Leu173Pro | c.518T>C | NA | Helix 4 | E3 | LPG | NA | 82 |
| 13 | APOE1 (156_173del) | p.174_191del | c.520_573del | NA | Helix 4 | E1 | LPG | NA | 83 |
| 14 | APOE Osaka/Kurashiki (Arg158Pro) | p.Arg176Pro | c.527G>C | NA | Helix 4 | E2 | LPG | NA | 84,85 |
| APOE Toyonaka (Ser197Cys, Arg158Cys) | p.Ser215Cys, p.Arg176Cys | c.644C>G | NA | Hinge | E2 | GP | NA | 101 |
＊: Main mutation sites are shown, gnomAD: Genome Aggregation Database, TOPMed: Trans-Omics for Precision Medicine, ＃: TOPMed frequency, Ref.: references, NA: not available, N-ter: N-terminal domain, Hinge: hinge domain, LPG: lipoprotein glomerulopathy, GP: glomerulopathy due to APOE Toyonaka and APOE2
Seven of these variants, APOE Tokyo/Maebashi (141–143del or 142–144del)73, 74), APOE 15bp deletion (142–146del)75), and single-nucleotide substitutions of Arg145, Arg147, and Arg150, APOE-Sendai (Arg145Pro)72), APOE Chicago (Arg147Pro)76), APOE Okayama (Arg150Gly)77), APOE Modena (Arg150Cys)78), and APOE Guangzhou (Arg150Pro)79), are in the LDL receptor binding site of helix 4 of the N-terminal domain (Fig.2, Fig.5, Table 3). The causative APOE variants in LPG were often found in the LDL receptor binding site of helix 4, similar to the causative APOE mutation sites in familial type III HLP, but the same variant did not appear to cause LPG and familial type III HLP.
Five variants in helix 4 that were not in the LDL receptor binding site were identified (Fig.5), but were reported to cause LPG, namely, APOE Kanto (Asp151dupAsp)70, 80), APOE Las Vegas (Ala152Asp)81), APOE Chengdu (Leu155Pro)82), APOE1 (156_173del)83), and APOE Osaka/Kurashiki (Arg158Pro)84, 85). APOE Tsukuba (Arg114Cys) was identified as a mutation near Cys112 in helix 3 86). No dyslipidemia was seen in these cases, but proteinuria led to renal biopsy, which showed glomerular capillary lumen dilatation and apoE deposition containing pale-stained clots with a layered structure. Another variant, APOE Kyoto (Arg25Cys), is located upstream of helix 1 of the N-terminal domain (Fig.5, Table 3)87). A survey of LPG-affected families also revealed that each variant exhibits low penetrance. APOE Kyoto was also reported as a compound heterozygote of p.Arg50His (Arg32His)88) and APOE Hong Kong (Asp230Tyr)89), while APOE Chicago was also reported as a compound heterozygote of APOE5 (Glu3Lys)90). It is likely that APOE Kyoto and APOE Chicago act in the pathogenesis of LPG, although this has not been investigated in detail.
As for incorrectly reported variants, APOE Ganzhou (Arg43Cys)91) is correctly described as APOE (p.Arg43Cys), which is the same variant as APOE Kyoto (Arg25Cys)87). Meanwhile, APOE (p.Arg150Cys)92) is correctly identified as APOE (Arg150Cys), the same variant as APOE Modena (Arg150Cys)78).
The majority of LPG cases have been reported in Japan and China. APOE Sendai (Arg145Pro) was observed mainly in central and northern Japan, especially in Yamagata and Miyagi, but not in China71, 93). Moreover, APOE Kyoto (Arg25Cys)87) has been reported as the most common variant in LPG worldwide, including southwestern China, Japan, France, and the United States70, 94). Because the Chinese have been more involved in international population movements in ancient times than the Japanese, the APOE Kyoto gene could have spread from China to other countries around the world71). Meanwhile, APOE Tokyo/Maebashi (141–143del) has been reported in cases from Japan and eastern China94). In animal models, APOE Sendai (Arg145Pro) and APOE Kyoto (Arg25Cys) variants have been expressed in APOE-deficient mice, with LPG-like renal pathology having been observed in each95, 96).
ApoE2 (Arg158Cys) homozygote glomerulopathy has been reported in 10 cases worldwide97, 98). The histological features of renal biopsies of ApoE2 homozygote glomerulopathy include a prominent foamy macrophage infiltrate that can be explained by the mechanism of atherosclerosis glomerulosclerosis99). In some cases of apoE2 homozygote glomerulopathy, non-lamellar lipoprotein thrombi were observed, which is distinct from lipoprotein glomerulopathy100). In the presence of diabetes, it is often difficult to distinguish between ApoE2 homozygote glomerulopathy and diabetic nephropathy without a diagnosis of type III HLP.
A new glomerular disease was identified in four non-consanguineous carriers possessing the novel variant APOE Toyonaka (Ser197Cys) (Fig.5, Table 3)99, 101-103). This variant was considered to be in linkage disequilibrium with apoE2 (Arg158Cys). It was reported that glomerular lesions only occurred in APOE Toyonaka (Ser197Cys, Arg158Cys) when the other allele was ApoE2. The first and fourth reported cases presented with proteinuria and hematuria, but not dyslipidemia, and renal biopsy revealed nonimmune membranous nephropathy-like features in the glomeruli101, 98). Tandem mass spectrometry of the glomeruli identified an accumulation of apoE, and high serum apoE levels were seen. The other two cases were accompanied by marked hyperlipidemia and histologically showed foam cells and non-lamellar lipoprotein thrombi frequently observed in apoE2 homozygote glomerulopathy99, 102).
Hypercholesterolemia (HCh) and hypertriglycemia (HTG) are conditions that can occur even in the absence of APOE variants. The APOE variants described below have been reported from cases of hyperlipidemia (Table 4), although some cases may have been caused by factors other than APOE variants. Note that HCh and HTG in the same cases or in the same families are defined as CHL in Table 4.
| No. | Variant | Protein sequence change | Genomic change＊ | rs number | Structure | ApoE phenotypes | Characteristics |
gnomAD/ TOPMed＃ frequency |
Ref. |
|---|---|---|---|---|---|---|---|---|---|
| 1 | APOE p.Thr11Ala | p.Thr11Ala | c.31A>G | rs144354013 | Signal | NA | CHL | 0.00017＃ | 40 |
| 2 | APOE5 (Glu3Lys) | p.Glu21Lys | c.61G>A | rs121918392 | N-ter | E5 | CHL | 0.000007 | 104,14,107 |
| 3 | APOE4 Freiburg/ Pittsburgh (Leu28Pro, Cys112Arg) | p.Leu46Pro, p.Cys130Arg | c.137T>C | rs769452 | Helix 1 | E4 | CHL, AD | 0.00252 | 108,109 |
| 4 | APOE p.Met82Ile (Met64Ile) | p.Met82Ile | c.246G>T | rs557845700 | Helix 2 | NA | HTG | 0.000014 | 44 |
| 5 | APOE5 Frankfurt (Gln81Lys, Cys112Arg) | p.Gln99Lys, p.Cys130Arg | c.295C>A | rs1180612218 | Helix 2_3 | E5 | CHL | 0.000004 | 112 |
| 6 |
APOE5 (Pro84Arg, Cys112Arg) |
p.Pro102Arg, p.Cys130Arg | c.305C>G | rs11083750 | Helix 2_3 | E5 | HCh | 0.0000284 | 113 |
| 7 |
APOE p.Pro102Leu (Pro84Leu) |
p.Pro102Leu | c.305C>T | rs11083750 | Helix 2_3 | E3 | FH | 0.000028 | 40 |
| 8 | APOE Arg114Pro | p.Arg132Pro | c.395G>C | rs1263042140 | Helix 3 | E2 | HCh | 0.000004＃ | 43 |
| 9 |
APOE p.Arg137Cys (Arg119Cys) |
p.Arg137Cys | c.409C>T | rs573658040 | Helix 3 | NA | FH | 0.000006 | 40 |
| 10 | APOE p.Arg137His (Arg119His) | p.Arg137His | c.410G>A | rs11542035 | Helix 3 | NA | FH | 0.000019 | 40 |
| 11 |
APOE5 (Val135_ Arg142dup) |
p.Val153_ Arg160dup | c.457_480dup24 | NA | Helix 4 | E5 | HTG | NA | 114 |
| 12 |
APOE p.Leu155Phe (Leu137Phe) |
p.Leu155Phe | c.463 C>T | rs1018669382 | Helix 4 | NA | FH | 0.000007 | 40 |
| 13 | APOE p.Leu167del (Leu149del) | p.Leu167del | c.499_501delCTC, c.500_502delTCC | rs515726148 | Helix 4 | E3 | FH | 0.00002 | 115,116,117 |
| 14 | APOE p.Val179Ala (Val161Ala) | p.Val179Ala | c.536T>C | rs1421977676 | Helix 4 | NA | CHL | 0.000008 | 40 |
| 15 |
APOE p.Gly191Cys (Gly173Cys) |
p.Gly191Cys | c.571G>T | rs1265472491 | Hinge | NA | HCh | 0.000007 | 44 |
| 16 | APOE1 Baden (Arg180Cys, Arg158Cys) | p.Arg198Cys, p.Arg176Cys | c.592C>T | rs1426426514 | Hinge | E1 | HTG | 0.000011 | 119 |
| 17 | APOE p.Val213Glu (Val195Glu) | p.Val213Glu | c.638T>A | rs1227709957 | Hinge | NA | FH | 0.00005 | 40 |
| 18 |
APOE p.Gly218Cys (Gly200Cys) |
p.Gly218Cys | c.652G>T | rs1488379910 | Hinge | NA | FH | 0.00007＃ | 40 |
| 19 |
APOE p.Pro220Leu (Pro202Leu) |
p.Pro220Leu | c.659C>T | rs1265743589 | Hinge | NA | CHL | 0.000008 | 44 |
| 20 | APOE p.Arg235Trp (Arg217Trp) | p.Arg235Trp | c.703C>T | rs530010303 | C-ter | NA | FH | 0.000023 | 29 |
| 21 | APOE2 Fukuoka (Arg224Gln) | p.Arg242Gln | c.725G>A | rs267606663 | C-ter | E2 | FH | 0.000004＃ | 120 |
| 22 | APOE2 Dunedin (Arg228Cys) | p.Arg246Cys | c.736C>T | rs121918395 | C-ter | E2 | HTG | 0.000057 | 121 |
| 23 | APOE p.Glu249Lys (Glu231Lys) | p.Glu249Lys | c.745G>A | rs762906934 | C-ter | NA | CHL | NA | 40 |
| 24 | APOE p.Glu252Lys (Glu234Lys) | p.Glu252Lys | c.754G>A | NA | C-ter | NA | CHL | NA | 40 |
| 25 |
APOE2 (Val236Glu) |
p.Val254Glu | c.761T>A | rs199768005 | C-ter | E2 | CHL | 0.000452 | 122 |
| 26 | APOE7 Suita (Glu244Lys, Glu245Lys) | p.Glu262Lys, p.Glu263Lys | c.784G>A, c.787G>A | rs140808909, rs190853081 | C-ter | E7 | CHL | 0.000199 | 123,14 |
| 27 |
APOE3 (Arg251Gly, Cys112Arg) |
p.Arg269Gly, p.Cys130Arg | c.805C>G | rs267606661 | C-ter | E3 | CHL | 0.00036 | 122 |
| 28 |
APOE1 (Leu252Glu, Arg158Cys) |
p.Leu270Glu, p.Arg176Cys | c.808C>G, c.809T>A | rs992440908, NA | C-ter | E1 | HCh | NA | 122 |
＊: Main mutation sites are shown, gnomAD: Genome Aggregation Database, TOPMed: Trans-Omics for Precision Medicine, ＃: TOPMed frequency, Ref.: references, NA: not available, Signal: signal peptide, N-ter: N-terminal domain, C-ter: C-terminal domain, Hinge: hinge domain, FH: familial hypercholesterolemia including autosomal dominant hypercholesterolemia (ADH) and severe familial type III HLP, HCh: hypercholesterolemia, HTG: hypertriglyceridemia, CHL: combined hyperlipidemia, AD: Alzheimer’s disease
APOE5 (Glu3Lys) is predominantly present in the Japanese population14, 104) and has been shown to be associated with increased binding to the LDL receptor compared with apoE3 105). Carriers of APOE5 (Glu3Lys), including a homozygote, have been shown to exhibit high lipoprotein levels without type III HLP106, 107).
APOE4 Freiburg/Pittsburgh (Leu28Pro, Cys112Arg) was found in the same year in a screening focused on hyperlipidemia and in a screening for AD108, 109). Four APOE4 Freiburg/Pittsburgh (Leu28Pro, Cys112Arg) homozygotes were reported to have various types of hyperlipoproteinemia (types IIa, IIb, IV, V), including two hypertriglyceridemic patients with coronary artery disease108). APOE4 Freiburg/Pittsburgh (Leu28Pro, Cys112Arg) heterozygotes tended to have higher triglyceride levels. The APOE4 isoform has consistently emerged as a factor conferring susceptibility to late-onset AD. Studies have reported that APOE4 Freiburg/Pittsburgh (Leu28Pro, Cys112Arg) was structurally unstable and was associated with a higher risk of developing AD than APOE4 (Cys112Arg)110, 111). In addition, a case of an APOE p.Met82Ile (Met64Ile) heterozygote showed hypertriglyceridemia44). Moreover, APOE5 Frankfurt (Gln81Lys, Cys112Arg) heterozygosity was identified in a 43-year-old man with moderate CHL112). Furthermore, APOE5 (Pro84Arg,Cys112Arg) heterozygosity was found in a 54-year-old woman of European descent with elevated LDL cholesterol levels113). A case of an APOE (Arg114Pro) heterozygote was also found upon the screening of diabetic patients in the UK who presented with hypercholesterolemia, but this patient’s serum lipid levels were normalized by 10 mg of simvastatin43). Finally, APOE5 (Val135_Arg142dup ValArgLeuAlaSerHisLeuArg) heterozygosity was detected in a patient with elevated triglyceride levels114).
As another example of an APOE variant, APOE p.Leu167del (Leu149del) was originally reported to be associated with sea blue histiocytosis and familial combined hyperlipidemia115-117). Subsequently, whole-exome sequencing and functional analysis of large French and Italian families containing many FH patients revealed that APOE p.Leu167del (Leu149del) is involved in ADH29, 57, 118). This variant was also erroneously reported as APOE p.Leu149del116, 117).
APOE p.Gly191Cys (Gly173Cys), APOE1 Barden (Arg180Cys), and APOE p.Pro220Leu (Pro202Leu) were shown to be located in the hinge region44, 119). Moreover, a case exhibiting APOE p.Gly191Cys (Gly173Cys) heterozygosity showed isolated hypercholesterolemia44). Furthermore, two APOE1 Barden (Arg180Cys) heterozygotes in a family study showed hypertriglyceridemia112), while a case of APOE p.Pro220Leu (Pro202Leu) with CHL was also reported44).
The C-terminal domain of APOE has high affinity for lipids, and there are several amphipathic α-helices in this domain that are responsible for the binding of apoE to lipids. Within a Norwegian hyperlipidemic cohort, APOE p.Arg235Trp (Arg217Trp) was found in a 56-year-old man with total cholesterol, triglyceride, LDL, and HDL levels of 363, 239, 247, and 69.6 mg/dL, respectively, along with the apoE3/E4 isoforms29).
Elsewhere, APOE2 Fukuoka (Arg224Gln) heterozygosity was identified in a 54-year-old Japanese woman with xanthomas120). When untreated (at 48 years old), she had dyslipidemia with total cholesterol of 1149 mg/dL and triglycerides of 1162 mg/dL. Possibly because of the variant being located in the C-terminal domain, recombinant APOE2 Fukuoka (Arg224Gln) showed almost the same LDL receptor-binding activity and heparin-binding capacity as recombinant APOE3 120). APOE2 Dunedin (Arg228Cys)121), APOE2 (Val236Glu)122), APOE7 Suita (Glu244,245Lys)123, 14) (all heterozygotes), and APOE3 (Arg251 Gly, Cys112Arg) 122) in the C-terminal domain were identified as variants that cause hypertriglyceridemia or CHL. APOE1 (Leu252Glu, Arg158Cys) was found in the course of an apoE screening among hypercholesterolemic patients in Münster in 1985 122).
Finally, 10 novel APOE variants, namely, APOE p.Thr11Ala, APOE p.Pro102Leu (Pro84Leu), APOE p.Arg137Cys (Arg119Cys), APOE p.Arg137His (Arg119His), APOE p.Leu155Phe (Leu137Phe), APOE p.Val179Ala (Val161Ala), APOE p.Val213Glu (Val195Glu), APOE p.Gly218Cys (Gly200Cys), APOE p.Glu249Lys (Glu231Lys), and APOE p.Glu252Lys (Glu234Lys), were reported in a recent French cohort of ADH and CHL patients (Table 4)40).
This section addresses APOE variants without lipid abnormalities and APOE variants without any stated abnormalities of lipid metabolism. APOE p.Arg108Trip (Arg90Trp) and APOE p.Tyr136His (Tyr118His) heterozygotes were detected in a normolipidemic group (Table 5)44). Moreover, screening of diabetic patients in the United Kingdom found one African patient who was heterozygous for APOE (Arg150His) with the E2/E2 phenotype, but no serum lipid abnormalities43). Elsewhere, APOE5 (Glu212Lys) was identified in a Turkish family124). Examination of the proband’s kindred revealed six heterozygous and two homozygous variant carriers. Compared with the non-carriers, carriers of the variant had slightly higher triglyceride levels (110.7 versus 98.3 mg/L). In another study, APOE5 (Gln204Lys, Cys112Arg) and APOE5 (Glu212Lys) were found to be present at almost polymorphic levels (allele frequencies: 0.006 and 0.012–0.013, respectively) in the Ethiopian population and no subjects with these variants had abnormal lipid or apolipoprotein patterns125). Moreover, APOE4’ (Arg274His, Cys112Arg) and APOE4 (Ser296Arg) were identified from cases selected during IEF-phenotype screening of a randomly collected population of healthy 35-year-old Dutch males, and they were normolipidemic122).
| No. | Variant | Protein sequence change | Genomic change＊ | rs number | Protein structure | Characteristics | Ref. |
|---|---|---|---|---|---|---|---|
| 1 | APOE p.Thr11Ser | p.Thr11Ser | c.31A>T | rs144354013 | Signal | NA | 127 |
| 2 | APOE p.Phe12Tyr | p.Phe12Tyr | c.35T>A | rs747078681 | Signal | NA | 127 |
| 3 | APOE p.Ala23Val (Ala5Val) | p.Ala23Val | c.68C>T | rs776242156 | N-ter | NA | 127 |
| 4 | APOE p.Arg33Cys (Arg15Cys) | p.Arg33Cys | c.97C>T | rs752079771 | N-ter | NA | 127 |
| 5 | APOE p.Arg50Cys (Arg32Cys) | p.Arg50Cys | c.148C>T | rs11542029 | Helix 1 | NA | 126 |
| 6 | APOE p.Thr60Ala (Thr42Ala) | p.Thr60Ala | c.178A>G | rs28931576 | Helix 1 | NA | 126 |
| 7 | APOE p.Ser72Phe (Ser54Phe) | p.Ser72Phe | c.215C>T | rs1969840702 | Helix 2 | NA | 127 |
| 8 | APOE p.Pro102Gln (Pro84Gln) | p.Pro102Gln | c.305C>A | rs11083750 | Helix 2-3 | NA | 126 |
| 9 | APOE p.Arg108Trip (Arg90Trp) | p.Arg108Trp | c.322C>T | rs1050106163 | Helix 3 | NL | 44 |
| 10 | APOE p.Glu114Lys (Glu96Lys) | p.Glu114Lys | c.340G>A | rs1169728519 | Helix 3 | NA | 127 |
| 11 | APOE p.Ala117Thr (Ala99Thr) | p.Ala117Thr | c.349G>A | rs28931577 | Helix 3 | NA | 126 |
| 12 | APOE p.Arg132Ser (Arg114Ser) | p.Arg132Ser | c.394C>A | rs11542041 | Helix 3 | NA | 126 |
| 13 | APOE p.Tyr136His (Tyr118His) | p.Tyr136His | c.406T>C | rs1969862344 | Helix 3 | NL | 44 |
| 14 | APOE p.Glu139Val (Glu121Val) | p.Glu139Val | c.416A>T | rs41382345 | Helix 3 | NA | 126 |
| 15 | APOE p.Glu150Gly (Glu132Gly) | p.Glu150Gly | c.449A>G | rs11542034 | Helix 4 | NA | 126 |
| 16 | APOE p.Arg152Gln (Arg134Gln) | p.Arg152Gln | c.455G>A | rs28931578 | Helix 4 | NA | 126 |
| 17 | APOE p.Lys161Asn (Lys143Asn) | p.Lys161Asn | c.483G>T | rs867215645 | Helix 4 | NA | 127 |
| 18 | APOE (Arg150His) | p.Arg168His | c.503G>A | rs376170967 | Helix 4 | NL | 43 |
| 19 | APOE p.Glu189Lys (Glu171Lys) | p.Glu189Lys | c.565G>A | rs11542032 | Hinge | NA | 126 |
| 20 | APOE p.Gln205Arg (Gln187Arg) | p.Gln205Arg | c.614A>G | rs11542030 | Hinge | NA | 126 |
| 21 | APOE p.Ala210Ser (Ala192Ser) | p.Ala210Ser | c.628G>T | NA | Hinge | NA | 127 |
| 22 | APOE p.Ser215Phe (Ser197Phe) | p.Ser215Phe | c.644C>T | rs11542027 | Hinge | NA | 127 |
| 23 | APOE p.Gly218Ser (Gly200Ser) | p.Gly218Ser | c.652G>A | rs1488379910 | Hinge | NA | 127 |
| 24 | APOE5 (Gln204Lys, Cys112Arg) | p.Gln222Lys, p.Cys130Arg | c.664C>A | rs1181840153 | C-ter | NL | 125 |
| 25 | APOE5 (Glu212Lys) | p.Glu230Lys | c.688G>A | rs567353589 | C-ter | NL | 124,125 |
| 26 | APOE4’ (Arg274His, Cys112Arg) | p.Arg292His, p.Cys130Arg | c.875G>A | rs121918398 | C-ter | NL | 122 |
| 27 | APOE p.Val305Met (Val287Met) | p.Val305Met | c.913G>A | rs749102800 | C-ter | NA | 127 |
| 28 | APOE4 (Ser296Arg) | p.Ser314Arg | c.940A>C | rs28931579 | C-ter | NL | 122 |
＊: Main mutation sites are shown, Ref.: references, Signal: signal peptide, N-ter: N-terminal domain, Hinge: hinge domain, C-ter: C-terminal domain, NA: not available, NL: no lipid abnormalities
The following 10 variants, APOE p.Arg50Cys (Arg32Cys), APOE p.Thr60Ala (Thr42Ala), APOE p.Pro102Gln (Pro84Gln), APOE p.Ala117Thr (Ala99Thr), APOE p.Arg132Ser (Arg114Ser), APOE p.Glu139Val (Glu121Val), APOE p.Glu150Gly (Glu132Gly), APOE p.Arg152Gln (Arg134Gln), APOE p.Glu189Lys (Glu171Lys), and APOE p.Gln205Arg (Gln187Arg), have only been analyzed in silico for APOE SNPs and their clinical data were unknown (Table 5)126). Genetic screening of the APOE gene for an association with dementia conducted in Denmark reported 20 rare nonsynonymous APOE variants in 105,597 individuals127). Nine of these variants had already been reported, but the following 11 variants had not (Table 5): APOE p.Thr11Ser, APOE p.Phe12Tyr, APOE p.Ala23Val (Ala5Val), APOE p.Arg33Cys (Arg15Cys), APOE p.Ser72Phe (Ser54Phe), APOE p.Glu114Lys (Glu96Lys), APOE p.Lys161Asn (Lys143Asn), APOE p.Ala210Ser (Ala192Ser), APOE p.Ser215Phe (Ser197Phe), APOE p.Gly218Ser (Gly200Ser), and APOE p.Val305Met (Val287Met) 127).
In this review, the importance of apoE in lipoprotein metabolism was demonstrated and the relationship between APOE variants and the pathology was explained as clearly as possible based on our current understanding. Abnormalities caused by APOE variants were classified and APOE variants were also described to the extent possible in order of domain and amino acid number (Supplemental Table 1). However, the present review also has some limitations. Here, we have reviewed the variants in and around the exons of the APOE gene that affect apoE as a protein, as reported in the literature to date. As such, the APOE variants in this review do not include synonymous substitutions, mutations in regulatory regions, intronic mutations other than in splicing sites, or linkage disequilibrium with other gene regions. We also did not review the very large number of papers analyzing the relationship between Alzheimer’s disease and APOE variants. Nonetheless, we consider this work to be valuable given that ApoE is not only a very important apoprotein, but also has genetic variants that readily cause pathological conditions.
The authors have no conflicts of interest to declare.
