apolipoprotein e mutants, hyperlipidemia and arteriosclerosis.pdf

APOLIPOPROTEIN E MUTANTS, HYPERLIPIDEMIA AND ARTERIOSCLEROSIS Gerd Utermann Institut fuer Humangenetik und Genetische Poliklinik der Philipps-Unversitaet Marburg, Federal Republic of Germany INTRODUCTION A genetic polymorphism of apolipoprotein E was first described in 1977 and it was shown at that time that one phenotype in this genetic system is associated with primary dysbetalipoproteinemia.1 It is now well established that genetic apo E polymorphism is under the control of 3 common alleles (I2, I3, I4) at the apo E structural gene locus, that specify for isoforms E2, E3, and E4 and have fre- quencies of 0.077 (I2), 0.773 (I3) and 0.150 (I4) in the German pop- ulation.2,3 Apo E2 and apo E4 differ from the parent apo E3 isoform by single cysteine for arginine interchanges at one of two positions in the primary structure: apo E2 (Arg158~Cys) and apo E4 (Cys112 ---?Arg).4,5 Examination of a DNA sequence that included the known apo E polymorphic sites showed that the mutant apo E phenotypes can be explained on the basis of a single base substitution in the first position of each of the respective codons.6 Recently additional rare apo E mutants have been identified that were designated E2* (Arg145~Cys), E2** (Lys146---?Gln), E3-Leiden and E2-Damascus7-9, G. Utermann, unpublished observations. The ami- noacid interchanges in position 145(Arg---?Cys), 146(Lys~Gln) and 158(Arg~Cys) all result in E2 mutants that are functionally defec- tive in that the, display reduced binding activity to LDL(B/E)- receptors.7, 10, 1 The most common E2 (Arg158~Cys) mutant is how- ever more severely defective «2% of E3 control) in binding to the receptor than the other two E2 variants (about 40-50% of control E3). Homozygotes and compound heterozygotes for the E2 mutants accumulate remnants of triglyceride-rich lipoproteins in plasma, apparently due to the failure to catabolize these lipoproteins efficiently through receptor mediated endocytosis in the liver.12,13 Despite the accumu-173 D. Kritchevsky et al. (eds.), Drugs Affecting Lipid Metabolism VIII © Plenum Press, New York 1985174 G.UTERMANN lation of cholesterol-rich remnants (beta-VLDL) in plasma, however, cholesterol levels are not increased in most E2 homo zygotes but, on the contrary, are subnormal and especially LDL-cholesterol is greatly reduced in E-2/2 subjects. 1, 14,15 Some E2 homozygotes do, however, develop severe type III hyperlipoproteinemia and there is evidence from the genetic and biochemical studies that additional indepen- dent factor(s) (genetic and/or environmental) operate in such pa-tients. 14, 16, 17 Heterozygotes for the E2 isoform (phenotypes E-3/2 and -4/2) that comprise about 16% of the population on the average have plasma cholesterol, triglyceride, apo B and apo E levels that are intermedi~te between E2 homo zygotes and individuals that lack an I2 allele15 , 1~, utermann, unpubli~hed results. The higher frequency of a beta-VLDL subfraction in fasting plasma from E2 heterozygotes that is indicative for the presence of remnants in the postabsorptive state suggests that heterozygotes tend to a milder form of dysbetalipoproteinemia.15 Since about half of the apo E in heterozygotes is a defective protein, they may be more prone to develop hyperlipidemia under certain circumstances. The observations that apo E genes contribute to the normal vari- ance of plasma lipid and apolipoprotein concentrations in the popu- lation and are associated with a specific form of hyperlipidemia sug- gests that they may also contribute to the genetic risk for arterio-sclerosis. RARE APO E MUTANTS The presumably rare apo E2* (Arg145~CyS) and apo E2** (LYS146 --7Gln) isoforms are identical in charge with the common E2 (Arg158 --7Cys) mutant. E3-Leiden has the same charge as the parent apo E3 isoform. Hence the common and rare isoforms cannot be distinguished by IEF or 2D-electrophoresis. All of the rare isoforms were distin- guished from their common counterparts only by sequence analysis and/or binding properties to LDL(B/E)-receptors. These proteins were investigated in detail because they were found in patients with classical features of type III disease and it therefore was antici- pated that the proteins might be abnormal. In population stUdies or in asymptomatic patients such mutants will however not be detected by present day screening methods. Therefore, the assumption that these mutants are rare is based on limited evidence and there are no data on their actual frequency in different populations. We have recently shown that apo E2 (Arg158~Cys) but not apo E2* (Arg145~Cys) and apo E2** (LYS146~Gln) exhibits anomalously slow mobility on SDS-polyacrylamide gels19 resulting in an apparent Mr of apo E2 (Arg158---?Cys) that is about 1500 Daltons higher than that of the other isoforms (Fig. 1A). The defective binding of apo E2 (Arg158~Cys) to LDL(B/E)-receptors and the anomalous behaviour in MUTANTS, HYPERLIPIDEMIA AND ARTE。