Overview

Clinical Phenotype: Alzheimer's Disease, Multiple Conditions
Position: (GRCh38/hg38):Chr19:44908730 G>A
Position: (GRCh37/hg19):Chr19:45411987 G>A
Transcript: NM_000041; ENSG00000130203
dbSNP ID: rs267606664
Coding/Non-Coding: Coding
DNA Change: Substitution
Expected RNA Consequence: Substitution
Expected Protein Consequence: Missense
Codon Change: GGC to GAC
Reference Isoform: APOE Isoform 1
Genomic Region: Exon 4

Findings

This variant was examined in a study of dementia, including Alzheimer’s disease (AD), and was proposed as being possibly protective (Rasmussen et al., 2020). In two cohorts totaling more than 100,000 individuals from Copenhagen, Denmark, the variant was found in 36 individuals, 19 who were at least 60 years old, none of whom had dementia (3.26 percent of non-carriers had dementia). In addition, carriers of the variant had high levels of ApoE in blood, a finding that suggested low AD risk based on an analyses of nine APOE variants, including G145D, showing that low ApoE in plasma was associated with increased AD risk, whereas high levels were associated with reduced risk, after adjustment for the common APOE2/E3/E4 alleles. The authors noted these features suggested a similarity between G145D and the protective allele R176C (APOE2). Interestingly, a subsequent study of the same Danish cohorts revealed yet another similarity with APOE2: an association with increased risk of age-related macular degeneration (aHR, 3.53; 95% CI, 1.14-10.96; Rasmussen et al., 2022).

The proposed protectiveness of the variant must be limited, however, given that at least one carrier, an individual from the Cretan Aging Cohort, was reported to have AD (Mathioudakis et al., 2022). Age at onset was 84 years. Of note, unlike most G145D carriers who carry the common APOE2 allele (see Non-Neurological Findings below), this subject was homozygous for APOE3.

Non-Neurological Findings

Several studies suggest this variant is a risk factor for hyperlipoproteinemia type III (HLPP3), also known as familial dysbetalipoproteinemia, which is characterized by elevated cholesterol and triglyceride levels in blood, and early onset atherosclerosis and heart disease. Its inheritance has been described as dominant with variable penetrance (Feussner et al., 1992; Miller et al., 1995; Koopal et al., 2017).

However, dissecting G145D’s contribution to HLPP3 has been complicated because it has nearly always been found together with the common allele R176C (APOE2) which, in homozygous form, is the most common cause of HLPP3. In the large Danish study, for example, 35 of 36 carriers had at least one APOE2 allele (Rasmussen et al., 2020). Eight percent were APOE2 homozygotes (versus one percent in the total population) and 75 percent had an APOE3/2 genotype (versus 12 percent in the total population). None of the G145D carriers had APOE3/4 or APOE4/4 genotypes, and 14 percent had an APOE2/4 genotype (also, see linkage disequilibrium data for APOE2 and G145D in Abou Kahlil et al., 2022: D’=1.0, r2=0.240). In addition, in the past, G145D was often studied as a result of identifying ApoE1, a protein defined by its migration upon isoelectric focusing, which carries both the G145D and APOE2 substitutions (see below, Weisgraber et al., 1984).

The ApoE1 species was first described in two members of a family with HLPP3 in Bethesda, Maryland (Gregg et al., 1983). It was presumed to be encoded by a novel genetic variant of APOE expressing in a codominant manner, but its sequence was unknown. In a subsequent study, the ApoE1 band was reported in a Finnish man suffering from hypertriglyceridemia (Weisgraber et al., 1984). Because sialylation can cause the common ApoE isoforms—ApoE2, 3, and 4—to migrate to the ApoE1 position upon isoelectric focusing, the authors used neuraminidase to remove sialic acid from ApoE isolated from the patient and then re-tested the protein’s migration. The migration was unchanged indicating a modification other than sialylation. Biochemical and sequence analysis of fragments of the ApoE1 variant revealed two substitutions, ApoE2 and G145D, which, together, were consistent with the protein’s migration pattern.

Subsequently, a study of a family of Turkish origin carrying G145D/APOE2 reported that the lipoprotein profiles and clinical phenotypes of the three heterozygotes and two homozygotes in this family were very similar to those of patients carrying only APOE2, suggesting G145D had little or no effect (Steinmetz et al., 1990). The heterozygotes all had normal blood lipid profiles, and only one of the homozygotes had HLPP3. APOE2 homozygosity is a major risk factor for HLPP3, but only a small fraction of APOE2 homozygotes develop the disease.

Subsequent studies of multiple families, however, suggested G145D has an effect on blood lipids independently of APOE2, but requires additional factors to express, similar to what has been reported for APOE2 homozygosity (Le et al., 2019). Analyses of three unrelated Canadian families (Miller et al., 1995), a French multigenerational pedigree (Richard et al., 1997), and a family from the British Isles (Wendham et al., 1993) indicated that G145D/APOE2 heterozygosity is associated with HLPP3.

Several studies have described multiple aspects of the lipid profiles of G145D carriers. For example, in the 36 carriers from the large Danish study, mean plasma levels of ApoE were high and cholesterol in low-density lipoprotein particles (LDL-C) levels were low compared with non-carriers, while levels of cholesterol in high-density lipoprotein particles (HDL-C), remnant cholesterol, and triglycerides, were similar to those of non-carriers (Rasmussen et al., 2020, Rasmussen et al., 2023). Two carriers who had combined hyperlipidemia had normal levels of very low-density cholesterol (VLDLc) and a normal ratio of VLDLc to VLDL triglycerides (Bea et al., 2023). One of these carriers fulfilled the set of criteria for HLPP3 (non-high density lipoprotein cholesterol (non-HDLc)/ApoB≥1.7 and triglycerides/ApoB≥1.35). G145D was also identified in two patients with elevated triglyceride levels diagnosed with familial combined hyperlipidemia, also known as hyperlipoproteinemia type IIb (HLPP2b) (Abou Khalil et al., 2022).

Additional studies have described patients with lipid abnormalities who carry the G145D variant with other variants that may affect lipid metabolism. These carriers include a German family with severe dysbetalipoproteinemia carrying the G145D/G49Vfs double mutation (Feussner et al., 1992), a 10-year-old French girl with hypertriglyceremia carrying the G145D/R269G double mutation (Richard et al., 1997), a French APOE2 homozygote and G145D heterozygote with mixed dyslipidemia (Wintjens et al., 2016), a Russian APOE2 homozygote and G145D heterozygote with hypertriglyceridemia and atherosclerosis at age 40 (Limonova et al., 2021), and a Canadian APOE2 homozygote with severe combined dyslipidemia carrying the G145D substitution in addition to an APOC2 mutation and 16 variants in multiple genes associated with elevated blood triglycerides (Le et al., 2019).

The frequency of G145D appears to vary between populations. Whereas two studies of German individuals suggested the mutation is rare even amongst HLPP3 patients (Feussner et al., 1998), a study of Spanish patients identified the mutation in three of 15 patients with HLPP3, suggesting it may be a relatively common cause of dyslipidemia in this population (Civeira et al. 1996). Evans and colleagues considered the relatively high incidence of the variant in some populations—they observed three German carriers in their lipid clinic—may be evidence that the variant is, in fact, not associated with hyperlipidemia (Evans et al., 2013). Of note, the gnomAD database reports a global frequency of 0.00015, with 25 of 29 carriers having European ancestry (v2.1.1, Apr 2022).

Biological Effect

G145 is near the ApoE receptor binding region, but its effects on receptor affinity remain uncertain. Weisgraber and colleagues found essentially no effect from G145D on the ability of synthetically lipidated mutant ApoE to compete with labeled low-density lipoprotein (LDL) for binding to the surface of fibroblasts (Weisgraber et al., 1984). G145D/ApoE2 had an affinity similar to that of ApoE2, approximately 4 percent that of ApoE3. Moreover, Miller and colleagues were unable to detect any effect from the mutation in LDL receptor-expressing macrophages exposed to patient LDLs (Miller et al., 1995).

However, in vitro binding assays using the LDL receptor ectodomain attached to plates exposed to VLDL particles isolated from a G145D carrier with an APOE2/E3 genotype, revealed increased affinity for low-density lipoprotein (LDL) receptors compared with APOE3/E3 particles (Bea et al., 2023). Moreover, G145D/ApoE2 mediated binding of β-very low-density lipoprotein (β-VLDL), a ligand of the LDL receptor-related protein (LRP), to fibroblasts deficient in LDL receptors about half as efficiently as ApoE3, but twice as efficiently as ApoE2 (Mann et al., 1995). Also of note, binding of G145D/ApoE2 to heparin beads in vitro was 38 percent of that observed for ApoE3, and lower than the 58 percent observed for ApoE2.

G145 lies within a loop that connects N-terminal helices H3 and H4 (Wintjens et al., 2016). A structure-predicting algorithm, CUPSAT, predicted G145D to be stabilizing. However, the substitution modifies the protein’s net charge which could reduce its affinity for at least some cell surface receptors. Also consistent with a disruption in binding affinity, another structure-predicting algorithm, Chou-Fasman, predicted two disruptions of ApoE secondary structure, an alteration of a β-turn mediated by G145D, and disruption of an α-helix mediated by ApoE2 (Civeira et al. 1996).

Standard computational algorithms to predict G145D’s deleteriousness, including Polyphen2, SIFT, Mutation Taster, and Provean, yielded mixed results (Abou Khalil et al., 2022). Its PHRED-scaled CADD score (24.3), which integrates diverse information in silico, was above 20, suggesting a deleterious effect (CADD v.1.6, May 2022).

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References

Mutations Citations

1. APOE R176C (ApoE2)
2. APOE [R176C];[C130R] (ApoE2/4)
3. APOE R269G

Paper Citations

1. Rasmussen KL, Tybjaerg-Hansen A, Nordestgaard BG, Frikke-Schmidt R. APOE and dementia - resequencing and genotyping in 105,597 individuals. Alzheimers Dement. 2020 Dec;16(12):1624-1637. Epub 2020 Aug 18 PubMed.
2. Rasmussen KL, Tybjærg-Hansen A, Nordestgaard BG, Frikke-Schmidt R. Associations of Alzheimer Disease-Protective APOE Variants With Age-Related Macular Degeneration. JAMA Ophthalmol. 2023 Jan 1;141(1):13-21. PubMed.
3. Mathioudakis L, Dimovasili C, Bourbouli M, Latsoudis H, Kokosali E, Gouna G, Vogiatzi E, Basta M, Kapetanaki S, Panagiotakis S, Kanterakis A, Boumpas D, Lionis C, Plaitakis A, Simos P, Vgontzas A, Kafetzopoulos D, Zaganas I. Study of Alzheimer's disease- and frontotemporal dementia-associated genes in the Cretan Aging Cohort. Neurobiol Aging. 2023 Mar;123:111-128. Epub 2022 Jul 11 PubMed.
4. Feussner G, Funke H, Weng W, Assmann G, Lackner KJ, Ziegler R. Severe type III hyperlipoproteinemia associated with unusual apolipoprotein E1 phenotype and epsilon 1/'null' genotype. Eur J Clin Invest. 1992 Sep;22(9):599-608. PubMed.
5. Miller DB, Hegele RA, Wolfe BM, Huff MW. Identification, molecular characterization, and cellular studies of an apolipoprotein E mutant (E1) in three unrelated families with hyperlipidemia. J Clin Endocrinol Metab. 1995 Mar;80(3):807-13. PubMed.
6. Koopal C, Marais AD, Westerink J, Visseren FL. Autosomal dominant familial dysbetalipoproteinemia: A pathophysiological framework and practical approach to diagnosis and therapy. J Clin Lipidol. 2017 Jan - Feb;11(1):12-23.e1. Epub 2016 Oct 13 PubMed.
7. Abou Khalil Y, Marmontel O, Ferrières J, Paillard F, Yelnik C, Carreau V, Charrière S, Bruckert E, Gallo A, Giral P, Philippi A, Bluteau O, Boileau C, Abifadel M, Di-Filippo M, Carrié A, Rabès JP, Varret M. APOE Molecular Spectrum in a French Cohort with Primary Dyslipidemia. Int J Mol Sci. 2022 May 21;23(10) PubMed.
8. Weisgraber KH, Rall SC Jr, Innerarity TL, Mahley RW, Kuusi T, Ehnholm C. A novel electrophoretic variant of human apolipoprotein E. Identification and characterization of apolipoprotein E1. J Clin Invest. 1984 Apr;73(4):1024-33. PubMed.
9. Gregg RE, Ghiselli G, Brewer HB Jr. Apolipoprotein EBethesda: a new variant of apolipoprotein E associated with type III hyperlipoproteinemia. J Clin Endocrinol Metab. 1983 Nov;57(5):969-74. PubMed.
10. Steinmetz A, Assefbarkhi N, Eltze C, Ehlenz K, Funke H, Pies A, Assmann G, Kaffarnik H. Normolipemic dysbetalipoproteinemia and hyperlipoproteinemia type III in subjects homozygous for a rare genetic apolipoprotein E variant (apoE1). J Lipid Res. 1990 Jun;31(6):1005-13. PubMed.
11. Le R, Abbas M, McIntyre AD, Hegele RA. Severe Combined Dyslipidemia With a Complex Genetic Basis. J Investig Med High Impact Case Rep. 2019 Jan-Dec;7:2324709619877050. PubMed.
12. Richard P, Beucler I, Pascual De Zulueta M, Biteau N, De Gennes JL, Iron A. Compound heterozygote for both rare apolipoprotein E1 (Gly127-->Asp, Arg158-->Cys) and E3(Cys112-->Arg, Arg251-->Gly) alleles in a multigeneration pedigree with hyperlipoproteinaemia. Clin Sci (Lond). 1997 Jul;93(1):89-95. PubMed.
13. Wenham PR, McDowell IF, Hodges VM, McEneny J, O'Kane MJ, Davies RJ, Nicholls DP, Trimble ER, Blundell G. Rare apolipoprotein E variant identified in a patient with type III hyperlipidaemia. Atherosclerosis. 1993 Mar;99(2):261-71. PubMed.
14. Rasmussen KL, Luo J, Nordestgaard BG, Tybjærg-Hansen A, Frikke-Schmidt R. APOE and vascular disease: Sequencing and genotyping in general population cohorts. Atherosclerosis. 2023 Nov;385:117218. Epub 2023 Aug 9 PubMed.
15. Bea AM, Larrea-Sebal A, Marco-Benedi V, Uribe KB, Galicia-Garcia U, Lamiquiz-Moneo I, Laclaustra M, Moreno-Franco B, Fernandez-Corredoira P, Olmos S, Civeira F, Martin C, Cenarro A. Contribution of APOE Genetic Variants to Dyslipidemia. Arterioscler Thromb Vasc Biol. 2023 Jun;43(6):1066-1077. Epub 2023 Apr 13 PubMed.
16. Wintjens R, Bozon D, Belabbas K, MBou F, Girardet JP, Tounian P, Jolly M, Boccara F, Cohen A, Karsenty A, Dubern B, Carel JC, Azar-Kolakez A, Feillet F, Labarthe F, Gorsky AM, Horovitz A, Tamarindi C, Kieffer P, Lienhardt A, Lascols O, Di Filippo M, Dufernez F. Global molecular analysis and APOE mutations in a cohort of autosomal dominant hypercholesterolemia patients in France. J Lipid Res. 2016 Mar;57(3):482-91. Epub 2016 Jan 22 PubMed.
17. Limonova AS, Ershova AI, Meshkov AN, Kiseleva AV, Divashuk MG, Kutsenko VA, Drapkina OM. Case Report: Hypertriglyceridemia and Premature Atherosclerosis in a Patient With Apolipoprotein E Gene ε2ε1 Genotype. Front Cardiovasc Med. 2020;7:585779. Epub 2021 Jan 18 PubMed.
18. Feussner G, Feussner V, Hoffmann MM, Lohrmann J, Wieland H, März W. Molecular basis of type III hyperlipoproteinemia in Germany. Hum Mutat. 1998;11(6):417-23. PubMed.
19. Civeira F, Pocoví M, Cenarro A, Casao E, Vilella E, Joven J, González J, Garcia-Otín AL, Ordovás JM. Apo E variants in patients with type III hyperlipoproteinemia. Atherosclerosis. 1996 Dec 20;127(2):273-82. PubMed.
20. Evans D, Beil FU, Aberle J. Resequencing the APOE gene reveals that rare mutations are not significant contributory factors in the development of type III hyperlipidemia. J Clin Lipidol. 2013 Nov-Dec;7(6):671-4. Epub 2013 May 25 PubMed.
21. Mann WA, Meyer N, Weber W, Meyer S, Greten H, Beisiegel U. Apolipoprotein E isoforms and rare mutations: parallel reduction in binding to cells and to heparin reflects severity of associated type III hyperlipoproteinemia. J Lipid Res. 1995 Mar;36(3):517-25. PubMed.

Other Citations

1. G49Vfs

Further Reading

No Available Further Reading