ClinVar

By extensive sequence analysis of the 2 alleles of the APOB gene in a man with moderate hypercholesterolemia (FHCL2; 144010), who was originally reported by Vega and Grundy (1986) and was found to be heterozygous for familial defective apolipoprotein by Innerarity et al. (1987), Soria et al. (1989) demonstrated a mutation in the codon for amino acid 3500 that results in the substitution of glutamine for arginine. This same mutant allele was found in 6 other, unrelated subjects and in 8 affected relatives in 2 of these families. A partial haplotype of this mutant apoB100 allele was constructed by sequence analysis and restriction enzyme digestion at positions where variations in the apoB100 are known to occur. This haplotype was found to be the same in 3 probands and 4 affected members of 1 family and lacks a polymorphic XbaI site whose presence has been correlated with high cholesterol levels. Thus, it appears that the mutation in the codon for amino acid 3500 (CGG-to-CAG), a CG mutation hotspot, defines a minor apoB100 allele associated with defective low density lipoproteins and hypercholesterolemia.

Ludwig and McCarthy (1990) used 10 markers for haplotyping at the APOB locus in cases of familial defective apolipoprotein B100: 8 diallelic markers within the structural gene and 2 hypervariable markers flanking the gene. In 14 unrelated subjects heterozygous for the mutation, 7 of 8 unequivocally deduced haplotypes were identical, and 1 revealed only a minor difference at one of the hypervariable loci. The genotypes of the other 6 affected subjects was consistent with the same haplotype. Familial defective apolipoprotein B100 (FDB) results from a G-to-A transition at nucleotide 10708 in exon 26 of the APOB gene. Ludwig and McCarthy (1990) interpreted the data as consistent with the existence of a common ancestral chromosome.

In a screening for the APOB3500 mutation by PCR amplification and hybridization with an allele-specific oligonucleotide, Loux et al. (1993) found only 1 case among 101 French subjects with familial hypercholesterolemia. The son of this individual, a 45-year-old man, was found also to have the mutation. Haplotype analysis revealed strict identity to that previously reported by Ludwig and McCarthy (1990), thus supporting a unique European ancestry. The family lived in the southwest of France and had no knowledge of Germanic origin.

Rauh et al. (1992) stated that the frequency of the arg3500-to-gln mutation has been found to be approximately 1/500 to 1/700 in several Caucasian populations in North America and Europe. On the other hand, Friedlander et al. (1993) found no instance of this mutation in a large screening program in Israel. They pointed out that the mutation has also not been found in Finland (Hamalainen et al., 1990) and is said to be absent in Japan. Tybjaerg-Hansen and Humphries (1992) gave a review suggesting that the risk of premature coronary artery disease in the carriers of the mutation is increased to levels as high as those seen in patients with familial hypercholesterolemia; at age 50, about 40% of males and 20% of females heterozygous for the mutation have developed coronary artery disease.

Marz et al. (1992) found only moderate hypercholesterolemia in a 54-year-old man who was homozygous for the arg3500-to-gln mutation and on a normal diet without lipid-lowering medication. There was no evidence of atherosclerosis and no history of cardiovascular complaints. The levels of apoE-containing lipoproteins were normal. Marz et al. (1992) suggested that the intact metabolism of apoE-containing particles decreases LDL production in this disorder, explaining the difference from familial hypercholesterolemia due to a receptor defect in which apoE levels are raised. Marz et al. (1993) investigated possible compensatory mechanisms that may have alleviated the consequences of the familial defective apoB100 (FDB). They showed that the receptor interaction of buoyant LDL is normal due to the presence of apoE in these particles. In addition, they provided evidence that the arg3500-to-gln substitution profoundly alters the conformation of the apoB receptor binding domain when apolipoprotein B resides on particles at the lower and upper limits of the LDL density range. They concluded that these mechanisms distinguish FDB from FH and account for the mild hypercholesterolemia in homozygous FDB. Among 43 patients with clinically and biochemically defined type III hyperlipoproteinemia (107741), Feussner and Schuster (1992) found no instance of the arg3500-to-gln mutation.

In the course of investigating 2 unrelated French patients heterozygous for mutations in the LDLR gene (606945) who had aggravated hypercholesterolemia, Benlian et al. (1996) found that each carried the identical arg3500-to-gln mutation in the APOB gene, i.e., were double heterozygotes. One of the patients was a 10-year-old boy when he was referred for hypercholesterolemia discovered at the time of a cardiac arrest. He had no planar xanthomata, although he exhibited bilateral xanthomas of the Achilles and metacarpal phalangeal tendons. Peripheral arterial disease was demonstrated by the presence of arterial murmurs and by arterial wall irregularity on ultrasound analysis. Stenoses of coronary arteries necessitated surgical angioplasty. The second patient was a 39-year-old man with myocardial infarction and acute ischemia of the legs. Both families came from the Perche region from which many French Canadians originated. The LDLR mutations trp66-to-gly (606945.0003) and glu207-to-lys (606945.0007) had previously been described in French Canadians. Rubinsztein et al. (1993) described an Afrikaner family with 6 FH/FDB double heterozygotes carrying another LDLR mutation, asp206-to-glu (606945.0006). (Benlian et al. (1996), in the title of their article, correctly referred to these patients as double heterozygotes; in the paper itself they incorrectly referred to them as FH/FDB compound heterozygotes. The latter term is used for heterozygosity for alleles at the same locus.)

In a patient homozygous for the R3500Q mutation, Schaefer et al. (1997) found LDL cholesterol and apoB concentrations approximately twice normal, whereas apoE plasma level was low. Using a stable-isotope labeling technique, they obtained data showing that the in vivo metabolism of apoB100-containing lipoproteins in FDB is different from that in familial hypercholesterolemia, in which LDL receptors are defective. Although the residence times of LDL apoB100 appeared to be increased to approximately the same degree, LDL apoB100 synthetic rate was increased in FH and decreased in FDB. The decreased production of LDL apoB100 in FDB may originate from enhanced removal of apoE-containing LDL precursors by LDL receptors, which may be upregulated in response to the decreased flux of LDL-derived cholesterol into hepatocytes.

Almost all individuals with familial defective apoB100 are of European descent, and in almost all cases the mutation is on a chromosome with a rare haplotype at the apoB locus, suggesting that all probands are descended from a common ancestor in whom the original mutation occurred. Distribution of the mutation is consistent with an origin in Europe 6,000 to 7,000 years ago. Myant et al. (1997) estimated the amount of recombination between the APOB gene and markers on chromosome 2 in 34 FDB (R3500Q) probands in whom the mutation is on the usual 194 haplotype. Significant linkage disequilibrium was found between the APOB gene and marker D2S220. They identified 3 YACs that contained the APOB gene and D2S220. The shortest restriction fragment common to the 3 YACs that contain both loci was 240 kb long. No shorter fragments with both loci were identified. On the assumption that 1000 kb corresponds to 1 cM, Myant et al. (1997) deduced that the recombination distance between D2S220 and the APOB gene is about 0.24 cM. Combining this value with the linkage disequilibrium observed between the 2 loci in the probands, they estimated that the ancestral mutation occurred about 270 generations ago. They postulated that the original mutation occurred in the common ancestor of living FDB (R3500Q) probands, who lived in Europe about 6,750 years ago.

Tybjaerg-Hansen et al. (1998) found that the R3500Q mutation in the APOB gene is present in approximately 1 in 1,000 persons in Denmark and causes severe hypercholesterolemia and increases the risk of ischemic heart disease. Heterozygous carriers of the arg3531-to-cys (107730.0017) mutation, which is present in the population in approximately the same frequency and also is associated with familial defective apolipoprotein B100, was not associated with higher-than-normal plasma cholesterol levels or an increased risk of ischemic heart disease.

Saint-Jore et al. (2000) estimated the respective contributions of the LDLR gene defect, APOB gene defect, and other gene defects in autosomal dominant type IIa hypercholesterolemia by studying 33 well-characterized French families in which this disorder had been diagnosed over at least 3 generations. Using the candidate gene approach, they found that defects in the LDLR gene accounted for the disorder in about 50% of the families. The estimated contribution of an APOB gene defect was only 15%. This low estimation of involvement of the APOB gene defect was strengthened by the existence of only 2 probands carrying the R3500Q mutation. Surprisingly, 35% of the families were estimated to be linked to neither LDLR nor APOB. The results suggested that genetic heterogeneity in type IIa hypercholesterolemia had been underestimated and that at least 3 major groups of defects were involved. The authors were unable to estimate the contribution of the FH3 gene (603776).

Boren et al. (2001) concluded that normal receptor binding of LDL involves an interaction between arginine-3500 and tryptophan-4369 in the carboxyl tail of apoB100. Trp4369 to tyr (W4369Y) LDL and arg3500 to gln (R3500Q) LDL isolated from transgenic mice had identically defective LDL binding and a higher affinity for a monoclonal antibody that has an epitope flanking residue 3500. Boren et al. (2001) concluded that arginine-3500 interacts with tryptophan-4369 and facilitates the conformation of apoB100 required for normal receptor binding of LDL. They developed a model that explained how the carboxyl terminus of apoB100 interacts with the backbone of apoB100 that enwraps the LDL particle. The model explained how all known ligand-defective mutations in apoB100, including a newly discovered R3480W mutation, cause defective receptor binding.

Horvath et al. (2001) studied 130 unrelated individuals with hypercholesterolemia in Bulgaria. Four of these individuals were found to be carriers of this mutation. Horvath et al. (2001) concluded that this mutation accounts for 0.99 to 8.17% (95% CI) of cases of hypercholesterolemia in Bulgaria and therefore represents the most common single mutation associated with this condition in Bulgaria.

Bednarska-Makaruk et al. (2001) found the arg3500-to-gln mutation in 2.5% (13/525) of unrelated patients with hypercholesterolemia in Poland. All the patients belonged to the type IIA hyperlipoproteinemia group. In 65 patients with the clinical characteristics of familial hypercholesterolemia, the frequency of the arg3500-to-gln mutation was 10.8% (7/65). The same haplotype at the APOB locus in the carriers of this mutation in Poland as in other populations from western Europe suggested its common origin.