Genetic abnormalities in apolipoprotein B

Hypobetalipoproteinemia associated with apo B-48.4, a truncated protein only 14 amino acids longer than apo B-48

Familial hypobetalipoproteinemia is an autosomal codominant trait that can be caused by mutations in the apo B gene. Here we report a novel apo B gene mutation causing hypobetalipoproteinemia, that is associated with the synthesis of a truncated apo B protein in a young healthy male subject and his mother. The mutation is an A deletion at position 6627 of the apo B cDNA leading to a truncated protein of 2166 amino acids (apo B-48.4). This truncated apo B was detected mainly in VLDL, LDL and in trace amounts in HDL, but not in the lipoprotein deficient plasma fraction. Affected family members present with elevated levels of HDL-cholesterol, mainly due to an increase in HDL2 particles. Postprandial triglycerides and retinyl esters in the d < 1.006 g/ml lipoprotein in the proband showed a normal response to an oral fat load compared to a group of eight matched healthy controls. In summary this novel mutation is associated with hypobetalipoproteinemia with a normal fat absorption as expected for a protein with a length similar to that of apo B-48.

Diabetes mellitus in a new kindred with familial hypobetalipoproteinemia and an apolipoprotein B truncation (apoB-55)

Familial hypobetalipoproteinemia is an autosomal co-dominant disorder, which in a minority of cases is due to a truncation producing mutation in the apoB gene. We have identified an apoB mutation in a 40-year old hypobetalipoproteinemic man with Type II diabetes mellitus. Immunoblotting of plasma revealed a major band for apoB-100 and a minor band with estimated size between apoB-52 and apoB-55. The proband's 75-year old father with Type II diabetes and a non-diabetic daughter also possessed the truncated protein. Direct sequencing of the amplified fragment of genomic DNA revealed a C-->T transition at nt 7692 in exon 26 of the apoB gene. This substitution yielded a premature stop codon at residue 2495 and abolished a BsaI restriction endonuclease site. The identical mutation has been described previously; however, the genotypes and ancestors of the kindred were different, suggesting that the mutation may have occurred independently. The majority of apoB-55 was eluted as particles smaller than LDL-sized apoB-100, and floated mostly between the LDL and HDL density range. It is worth noting that despite the presence of Type II diabetes, both the proband and his father have very low plasma lipid levels and neither have any clinically manifest macrovascular complications.

Decreased production and increased catabolism of apolipoprotein B-100 in apolipoprotein B-67/B-100 heterozygotes

Apolipoprotein (apo) B-67 is a truncated form of apoB-100 due to deletion of an adenine at cDNA 9327. Heterozygotes have one allele making apoB-100; therefore, plasma apoB levels would be predicted to be at least 50% of normal. However, apoB-67 heterozygotes have total plasma apoB levels that are 24% of normal. To determine the mechanisms responsible for the lower-than-expected levels of apoB, in vivo kinetics of apoB-100 were performed in three apoB-67/apoB-100 heterozygotes and compared with those of six control subjects by using a primed-constant infusion of [5,5,5-2H3]leucine in the fed state. Kinetic parameters were calculated by multicompartmental modeling of the data. The mean total apoB plasma concentration of the apoB-67 subjects was 21.8+/-6.1 mg/dL, or 24% of that of control subjects (89.6+/-24.1 mg/dL, P=.002). ApoB-67 subjects had lower mean VLDL apoB-100 production rates (3.6+/-1.2 versus 13.9+/-3.5 mg x kg(-1) x d(-1), P=.002) and lower mean transport rates of apoB-100 into LDL (3.5+/-1.4 versus 12.6+/-4.1 mg x kg(-1) x d(-1), P=.008) compared with control subjects. The transport rate into IDL was not significantly different (1.2+/-0.5 versus 6.2+/-4.0 mg x kg(-1) x d(-1), P=.07). The fractional catabolic rate of VLDL apoB-100 was significantly higher in apoB-67 subjects than in control subjects (18.1+/-8.6 versus 7.6+/-1.6 mg x kg(-1) x d(-1), P=.017). ApoB-100 IDL and LDL fractional catabolic rates were not significantly different. VLDL apoB-100 pool size in apoB-67 subjects was 11% of that of control subjects (15.8+/-7.7 versus 141.6+/-33.7 mg, P=.0004) due to a 74% lower production rate (26% of control values) and a 2.4-fold higher fractional catabolic rate. LDL apoB-100 pool size in apoB-67 subjects was 22% of that of control subjects (665.3+/-192.4 versus 2968.3+/-765.2 mg, P=.002) due primarily to a lower production rate (27% of control values). Thus, both decreased production of VLDL and LDL apoB-100 and increased catabolism of VLDL apoB-100 are responsible for the low levels of apoB-100 in apoB-67 subjects.

Angioid streaks associated with abetalipoproteinemia

Angioid streaks were observed in two patients with abetalipoproteinemia. The progression of the angioid streaks was minimal over the years that these patients received vitamin A and E supplementation, though in one patient the development of subretinal neovascular membranes within the angioid streaks was the cause of rapid central visual loss. The simultaneous appearance of two rare entities in unrelated individuals strengthens the relationship between these two disorders that has been suggested by previous case studies. The authors propose a common metabolic pathway involving trace element deficiencies that may account for this relationship as well as the association of angioid streaks with other rare disorders such as Paget's disease, hypoparathyroidism, lead poisoning, hyperphosphatemia, and a number of hemoglobinopathies. Their study of these two patients underscores the need for further investigations as to the role of copper, zinc and omega-3 fatty acids in the pathogenesis of retinopathy in abetalipoproteinemia.