Discovery and consequences of apolipoprotein-epsilon(3Groningen): a G-insertion in codon 95/96 that is predicted to cause a premature stop codon.

Background: We found an unexplained, persistent discrepancy between the outcomes of two apolipoprotein-E (apo-E) genotyping methods for a patient with features of familial dysbetalipoproteinaemia (FD). Polymerase chain reaction - restriction fragment length polymorphism resulted in the apo-epsilon(2)/epsilon(2) genotype, whereas minisequencing indicated apo-epsilon(2)/epsilon(3). The discrepancy was predicted to derive from a novel mutation.

Methods: Sequencing of patient DNA, set-up of a mutation analysis method and establishment of mutation occurrence in 19 family members of the proband and investigation of its association with serum lipid indices.

Results: Sequencing demonstrated a G-insertion in codon 95 or 96 ((95)AAG-(96-)GAG-->(95)AAG-(96)GGA-G) of the apo-epsilon(3) allele. The mutation, designated apo-epsilon(3Groningen), was predicted to cause a frameshift, a premature stop codon at codon 146 (AAG-->TAA) and the expression of a truncated apo-E protein, if any. Four family members with the apo-epsilon(3Groningen) were identified. Two family members with apo-epsilon(3)/epsilon(3Groningen) had serum lipid indices within reference ranges but low-serum apo-E. Three subjects with apo-epsilon(2)/epsilon(3Groningen), proband included, had serum cholesterol, triglycerides and calculated low-density lipoprotein-cholesterol levels above the reference ranges. Their electrophoresis pattern showed the classical broad-beta band, indicative of FD.

Conclusion: Apo-epsilon(3Groningen) heterozygosity is unlikely to precipitate FD, unless provoked by compound apo-epsilon(2) heterozygosity or other FD precipitating factors.