Correspondence

Association of Mutations in the Apolipoprotein B Gene with Hypercholesterolemia and the Risk of Ischemic Heart Disease

N Engl J Med 1998; 339:1640-1642November 26, 1998

Article

To the Editor:

We disagree with several of the conclusions of Tybjærg-Hansen et al. (May 28 issue)1 concerning the clinical significance of mutations in the apolipoprotein B gene with respect to lipoprotein metabolism. First, the failure to identify persons with the Arg3500Trp mutation in a Scandinavian population cannot justify disregarding this mutation in others. Other studies have found 6 subjects with the Arg3500Trp mutation and 29 subjects with the Arg3500Gln mutation2,3 (and unpublished data). Second, Boren et al.4 have shown that the presence of an arginine at residue 3500 is crucial. Third, data on two families showed that the Arg3500Trp mutation cosegregated with elevated serum cholesterol levels.2 The elimination of the tryptophan mutation from screening programs would lead to the omission of an important cause of hypercholesterolemia in non-Scandinavian populations, especially Asian populations.3

Tybjærg-Hansen et al. did not use critical cosegregation studies within families to evaluate the contribution of the Arg3531Cys mutation to hyperlipidemia. In an analysis of 44 patients with this mutation, we found that the mutation cosegregated with serum cholesterol levels within families (P<0.001). In vitro studies have clearly established that this mutation reduces binding to the low-density lipoprotein (LDL) receptor to about 25 percent of the normal level. Also, mutant Cys3531 particles accumulate preferentially in vivo. The discordant observation of a lower incidence of coronary disease among Danish patients with the Arg3531Cys mutation could be explained by the fact that six of the seven subjects with the mutation were women whose median age was over 70.

Thus, the conclusion of Tybjærg-Hansen et al. that the Arg3500Gln mutation is the only mutation worth screening for is not well founded.

Dairena Gaffney, Ph.D.
Glasgow Royal Infirmary, Glasgow G31 2ER, United Kingdom

Clive Pullinger, Ph.D.
John Kane, M.D., Ph.D.
University of California, San Francisco, San Francisco, CA 94143-0130

4 References

1. 1

   Tybjaerg-Hansen A, Steffensen R, Meinertz H, Schnohr P, Nordestgaard BG. Association of mutations in the apolipoprotein B gene with hypercholesterolemia and the risk of ischemic heart disease. N Engl J Med 1998;338:1577-1584
   Full Text | Web of Science | Medline

2. 2

   Gaffney D, Reid JM, Cameron IM, et al. Independent mutations at codon 3500 of the apolipoprotein B gene are associated with hyperlipidemia. Arterioscler Thromb Vasc Biol 1995;15:1025-1029
   CrossRef | Web of Science | Medline

3. 3

   Choong ML, Koay ES, Khoo K-L, Khaw M-C, Sethi SK. Denaturing gradient-gel electrophoresis screening of familial defective apolipoprotein B-100 in a mixed Asian cohort: two cases of arginine 3500 → tryptophan mutation associated with a unique haplotype. Clin Chem 1997;43:916-923
   Web of Science | Medline

4. 4

   Boren J, Lee I, Zhu W, Arnold K, Taylor S, Innerarity TL. Identification of the low density lipoprotein receptor-binding site in apolipoprotein B100 and the modulation of its binding activity by the carboxyl terminus in familial defective apo-B100. J Clin Invest 1998;101:1084-1093
   CrossRef | Web of Science | Medline

To the Editor:

Tybjærg-Hansen and colleagues addressed the important question of the clinical relevance of apolipoprotein B-100 mutations — Arg3500Gln, Arg3500Trp, and Arg3531Cys — and found that only the Arg3500Gln mutation causes severe hypercholesterolemia and increases the risk of ischemic heart disease in persons with familial defective apolipoprotein B. However, they missed a crucial point.

First, the function of apolipoprotein B-100 can be replaced in part by apolipoprotein E, another apolipoprotein that can bind to the LDL receptor. Thus, an analysis of apolipoprotein B gene mutations tells only part of the story. Second, patients who are homozygous for the apolipoprotein B-100 Arg3500Gln mutation do surprisingly well and have only mild atherosclerosis, as compared with patients who are homozygous for familial hypercholesterolemia.1 Recently, we found that apolipoprotein E–mediated removal of LDL precursors was enhanced in a patient who was homozygous for familial defective apolipoprotein B, resulting in decreased production of LDL.2 Therefore, the effects of apolipoprotein E can account for the differences in the lipoprotein phenotype and clinical outcome in these patients. The apolipoprotein E phenotype (E2, E3, or E4) has a crucial effect on lipoprotein metabolism.3 Could Tybjærg-Hansen et al. indicate the apolipoprotein E phenotypes of their subjects? We speculate that the presence of the apolipoprotein E phenotype associated with reduced LDL-receptor–binding properties (i.e., E2/E2) would lead to more severe dyslipidemia and coronary artery disease in persons with familial defective apolipoprotein B. An answer to this question would strengthen the results of this otherwise very important study.

Juergen R. Schaefer, M.D.
Matthias Herzum, M.D.
Bernhard Maisch, M.D.
Philipps University Marburg, D-35033 Marburg, Germany

3 References

1. 1

   Schaefer JR. Präventive Kardiologie. Stuttgart, Germany: Schattauer Verlag, 1998:41-4.
   

2. 2

   Schaefer JR, Scharnagl H, Baumstark MW, et al. Homozygous familial defective apolipoprotein B-100: enhanced removal of apolipoprotein E-containing VLDLs and decreased production of LDLs. Arterioscler Thromb Vasc Biol 1997;17:348-353
   CrossRef | Web of Science | Medline

3. 3

   Utermann G, Kindermann I, Kaffarnik H, Steinmetz A. Apolipoprotein E phenotypes and hyperlipidemia. Hum Genet 1984;65:232-236
   CrossRef | Web of Science | Medline

Author/Editor Response

The authors reply:

To the Editor: We agree with Gaffney et al. that the Arg3500Trp mutation could be a quantitatively important cause of hypercholesterolemia in Asian populations. However, available data suggest that this may not be the case in white populations. Among 2190 patients with hyperlipidemia, this mutation was identified in only 2, 1 of whom was Indian, whereas the Arg3500Gln mutation was identified in 36 patients.1-4 In addition, we screened 948 patients with ischemic heart disease, 36 unrelated patients with familial hypercholesterolemia, and 2021 subjects from a general population with cholesterol and apolipoprotein B levels above the 70th percentile and found that none had the Arg3500Trp mutation.

In vitro, the Arg3531Cys mutation causes reduced binding of LDL to the LDL receptor, but this level of binding is more than twice that associated with the Arg3500Gln mutation.5 In vivo we found that the Arg3500Gln mutation increased cholesterol, LDL cholesterol, and apolipoprotein B levels, whereas the Arg3531Cys mutation did not. To us this finding seems the most likely explanation for the absence of ischemic heart disease among carriers of the Arg3531Cys mutation, rather than the preponderance of women among the carriers. This possibility is further supported by the fact that of the carriers who were identified in the general-population sample, six of seven carriers of the Arg3500Gln mutation reported a predisposition to ischemic heart disease or hypercholesterolemia, as compared with only one of seven carriers of the Arg3531Cys mutation. Finally, in all studies of patients with hyperlipidemia, the Arg3500Gln mutation was more frequent than the Arg3531Cys mutation.2-5 However, we cannot rule out the possibility that under certain circumstances the Arg3531Cys mutation may increase susceptibility to hypercholesterolemia, ischemic heart disease, or both.5

We agree with Schaefer et al. that the apolipoprotein E genotype may modulate the effect on the cholesterol level. The apolipoprotein E genotypes of the carriers of the Arg3500Gln mutation in our study were as follows: nine had E3/E3, three had E3/E4, and two had E3/E2. This distribution cannot account for the high cholesterol levels in the carriers. Likewise, the apolipoprotein E genotype cannot explain the low cholesterol levels in carriers of the Arg3531Cys mutation: six had the E3/E3 genotype, and two had the E3/E2 genotype.

Thus, we conclude that the Arg3500Gln mutation is so far the only mutation of the apolipoprotein B gene that is worth screening for in patients with hyperlipidemia or ischemic heart disease and in their relatives, because this mutation has a severe effect on cholesterol levels, is frequent in whites, and is associated with an increased odds ratio for ischemic heart disease of 7.0. It is possible, however, that the Arg3500Trp mutation may be frequent among Asians and cause hypercholesterolemia and ischemic heart disease, but this needs to be determined before testing for this mutation can be recommended.

Anne Tybjærg-Hansen, M.D., D.M.Sc.
Herlev University Hospital, DK-2730 Herlev, Denmark

Børge G. Nordestgaard, M.D., D.M.Sc.
Glostrup University Hospital, DK-2600 Glostrup, Denmark

5 References

1. 1

   Gaffney D, Reid JM, Cameron IM, et al. Independent mutations at codon 3500 of the apolipoprotein B gene are associated with hyperlipidemia. Arterioscler Thromb Vasc Biol 1995;15:1025-1029
   CrossRef | Web of Science | Medline

2. 2

   Wenham PR, Henderson BG, Penney MD, Ashby JP, Rae PW, Walker SW. Familial ligand-defective apolipoprotein B-100: detection, biochemical features and haplotype analysis of the R3531C mutation in the UK. Atherosclerosis 1997;129:185-192
   CrossRef | Web of Science | Medline

3. 3

   Talmud PJ, Tamplin OJ, Heath K, Gaffney D, Day IN, Humphries SE. Rapid testing for three mutations causing familial defective apolipoprotein B100 in 562 patients with familial hypercholesterolaemia. Atherosclerosis 1996;125:135-137
   CrossRef | Web of Science | Medline

4. 4

   Ludwig EH, Hopkins PN, Allen A, et al. Association of genetic variations in apolipoprotein B with hypercholesterolemia, coronary artery disease, and receptor binding of low density lipoproteins. J Lipid Res 1997;38:1361-1373
   Web of Science | Medline

5. 5

   Pullinger CR, Hennessy LK, Chatterton JE, et al. Familial ligand-defective apolipoprotein B: identification of a new mutation that decreases LDL receptor binding affinity. J Clin Invest 1995;95:1225-1234
   CrossRef | Web of Science | Medline

Citing Articles (3)

Citing Articles

1. 1

   Dairena Gaffney, Clive R. Pullinger, Denis St.J. O'Reilly, Michael S. Hoffs, Isobel Cameron, J.Keith Vass, Medha V. Kulkarni, John P. Kane, Verne N. Schumaker, Gerald F. Watts, Chris J. Packard. (2002) Influence of an asparagine to lysine mutation at amino acid 3516 of apolipoprotein B on low-density lipoprotein receptor binding. Clinica Chimica Acta 321:1-2, 113-121
   CrossRef

2. 2

   Børge G. Nordestgaard, Anne Tybjærg-Hansen Glostrup. (1999) Susceptibility mutations for ischemic heart disease. Current Atherosclerosis Reports 1:2, 108-114
   CrossRef

3. 3

   &NA;. (1999) Lipid metabolism. Current Opinion in Lipidology 10:4, 347-362
   CrossRef