2.2. Apolipoprotein B

Apo B exists in human plasma as two isoforms, apo B-48 and apo B-100. Apo B-100 is the major physiological ligand for the LDL receptor. It is the largest monomeric protein sequenced so far, containing 4536 amino acid residues (Chen et al. 1986, Law et al. 1986). Its gene has been mapped on the short arm of chromosome 2, with an approximate length of 43 kilobases and 29 exons (Ludwig et al. 1987). The LDL-binding domain of the molecule is proposed to be located between the residues 3129 and 3532 (Knott et al. 1986). Apo B-100 is synthesised in the liver and is required for the assembly of very low density lipoproteins (VLDL). It does not interchange between lipoprotein particles, as do the other lipoproteins, and it is found in IDL and LDL particles after the removal of the apolipoproteins A, E and C (Young 1990).

Apo B-48 is present in chylomicrons and chylomicron remnants and plays an essential role in the intestinal absorption of dietary fats (Kane 1983). Apo B-48 is synthesised in the small intestine. It comprises the N-terminal 48% of apo B-100 and is produced due to posttransscriptional apo B-100 mRNA editing at codon 2153, which creates a stop codon in the intestine instead of a glutamine in the liver (Chen et al. 1987).

Mutations occurring in the apo B gene can alter blood cholesterol levels. Most of the mutations lower blood cholesterol levels due to the production of truncated apo B. The mechanisms by which blood cholesterol is lowered are not yet fully understood. Two mutations in the apo B gene have been associated with elevated blood cholesterol. The apo B-3500 ArgÆGln substitution causes familial defective hypercholesterolemia (FDB) due to defective binding of LDL to its receptor (Vega & Grundy 1986, Soria et al. 1989). The prevalence of the mutation in the general population in Central Europe is 1/204-1/700 (Innerarity et al.1990, Tybjaerg-Hansen et al. 1990, Schuster et al. 1990). The highest prevalence has been reported from Switzerland (Miserez et al. 1994), and so far apo B -3500 has not been found in Finland (Hämäläinen et al. 1990). Another mutation in the LDL receptor binding area causing apo B-3531 ArgÆCys has been described to cause moderate hypercholesterolemia due to defective binding of LDL to its receptor (Pullinger et al. 1995).

Several restriction fragment length polymorphisms (RFLP) in the Apo B gene have been defined (Humphries & Talmud 1995). The most widely studied of these is the XbaI polymorphism in exon 26, which does not result in an amino acid substitution. In some populations the presence of the XbaI cutting site is associated with hypercholesterolemia in both normolipemic (Berg 1986, Talmud et al. 1987, Aalto-Setälä et al. 1988) and hypercholesterolemic (Leren et al. 1988, Aalto-Setälä et al. 1989) individuals. The absence of the XbaI cutting site was associated with higher triglyceride levels in one study (Deeb et al. 1986). Several studies have failed to reveal any association between the XbaI polymorphism and lipid values (Hegele et al. 1986, Aburatani et al. 1988, Rajput-Williams et al. 1988, Darnfors et al. 1989, Gajra et al. 1994,) and in one study the association of the presence of the XbaI cutting site with elevated cholesterol and triglyceride levels was only observed in patients with peripheral artery disease (Monsalve et al. 1988).

The EcoRI restriction fragment length polymorphism in exon 29 is associated with an amino acid change Gln Æ Lys4154 . Most studies have revealed no association between the EcoRI polymorphism and cholesterol or triglyceride levels (Ma et al. 1987, Dunning et al. 1988, Jenner et al. 1988, Aburatani et al. 1988, Peacock et al. 1992,). An association between elevated triglycerides and the absence of the EcoRI cutting site has been reported in coronary heart disease patients (Paulweber et al. 1990, Tybjaerg-Hansen et al. 1991) and in healthy males (Paulweber et al. 1990).

The MspI RFLP in exon 26 is associated with an amino acid change Arg Æ Gln3611. The MspI polymorphism is not associated with differences in serum lipid concentrations (Deeb et al. 1986, Hegele et al. 1986, Xu et al. 1989, Genest et al. 1990).

The apo B signal peptide contains a leucine-alanine-leucine insertion/deletion polymorphism affecting the amino acids 14-16 producing signal peptides with 24 or 27 amino acids (Boerwinkle & Chan 1989). The ins allele has been associated with elevated serum triglycerides (Tikkanen & Heliö 1992), low serum cholesterol and apo B (Hansen et al. 1993), and coronary artery disease (Peacock et al. 1992) in some populations, whereas in others the del allele has been connected with elevated total and LDL cholesterol but not with myocardial infarction (Bohn et al. 1994). No association between the polymorphism and lipids was detected in Asian patients, but the del allele was associated with coronary artery disease (Wu et al. 1994). A strong linkage disequilibrium between the XbaI and ins/del polymorphisms has been reported (Hansen et al. 1993).