RESUMO
We have identified the genetic defect that leads to a deficiency of apoC-II in the proband from the Paris kindred. Analysis of the apoC-IIParis DNA by Southern blot hybridization revealed no major gene rearrangements, but sequencing of polymerase chain reaction-amplified apoC-IIParis DNA revealed an A to G transition that changed the initiation AUG (methionine) codon to GUG (valine). Potential initiation of translation at the closest inframe methionine codon eliminates the entire signal peptide and the first 8 amino-terminal residues of apoC-II which would prevent apoC-II secretion into plasma. In agreement with this, no apoC-II was detected in the patient's plasma by radioimmunoassay or by two-dimensional gel electrophoresis and immunoblotting. Direct sequencing of amplified patient DNA from 12 different polymerase chain reaction samples demonstrated the presence of the A to G substitution in all, indicating that the proband is a homozygote for the defect. We propose that in the apoC-IIParis gene, a mutation in the initiation methionine codon prevents the normal initiation of apolipoprotein synthesis and leads to a deficiency of apoC-II. This initiation methionine mutation represents a new type of molecular defect that can result in Type I hyperlipoproteinemia.
