Compound heterozygosity for an apolipoprotein A1 gene promoter mutation and a structural nonsense mutation with apolipoprotein A1 deficiency

Abstract

Apolipoprotein (apo) A1 plays a central role in the metabolism of HDL. We describe a novel genetic variant of the apoA1 gene identified in a patient with low concentrations of plasma HDL cholesterol. The proband, a 12-year- old Japanese boy, exhibited markedly low levels of both plasma apoA1 and HDL cholesterol. Genomic DNA sequencing of apoA1 genes of the patient showed a compound heterozygosity for an A to C substitution at 27 bp upstream of the transcription start site of 1 apoA1 allele, and a C to T substitution in another allele at residue 84 resulting in aberrant termination. The point mutation at nucleotide position -27 changed ATAAATA of the putative TATA box signal sequence to ATACATA. In addition to this mutation, the patient was heterozygous for a G to A substitution at position -75. Immunoblotting of an isoelectric focusing electrophoresis gel of the proband's plasma showed a trace amount of normal apoA1. No measurable plasma apoA1 and HDL cholesterol in a patient with homozygosity for nonsense mutation at residue 84 has been reported previously. To determine the effects of substitution either at position -27 or -75, plasmids containing the 5'-flanking region of the human apoA1 promoter fused to the CAT reporter gene were constructed and transfected in HepG2 cells. A construct with the A to C substitution at position -27 showed 41.8±4.2%, and G to A substitution at position -75 showed 72.8 ± 15.2% (means±SD, n=3) of CAT activities, compared with the wild-type promoter sequence. A construct with the double substitutions at positions -27 and -75 showed only 22.8±1.3% (mean±SD, n=3) activity relative to the wild type. Our patient is the first case with a TATA box mutation etiologically related to lipoprotein disorders.