Point mutations in apolipoprotein A-I mimic the phenotype observed in patients with classical lecithin:cholesterol acyltransferase deficiency.
We have analyzed the effect of charged to neutral amino acid substitutions around the kinks flanking helices 4 and 6 of apoA-I and of the deletion of helix 6 on the in vivo activity of LCAT and the biogenesis of HDL. The LCAT activation capacity of apoA-I in vitro was nearly abolished by the helix 6 point (helix 6P-apoA-I[R160V/H162A]) and deletion {helix 6Delta-apoA-I[Delta(144-165)]} mutants, but was reduced to 50% in the helix 4 point mutant (helix 4P-apoA-I[D102A/D103A]). Following adenovirus-mediated gene transfer in apoA-I deficient mice, the level of plasma HDL cholesterol was greatly reduced in helix 6P and helix 6Delta mutants. Electron microscopy and two-dimensional gel electrophoresis showed that the helix 6P mutant formed predominantly high levels of apoA-I containing discoidal particles and had an increased prebeta1-HDL/alpha-HDL ratio. The helix 6Delta mutant formed few spherical particles and had an increased prebeta1-HDL/alpha-HDL ratio. Mice infected with adenovirus expressing the helix 4P mutant or wild-type apoA-I had normal HDL cholesterol and formed spherical alpha-HDL particles. Coinfection of mice with adenoviruses expressing human LCAT and the helix 6P mutant dramatically increased plasma HDL and apoA-I levels and converted the discoidal into spherical HDL, indicating that the LCAT activity was rate-limiting for the biogenesis of HDL. The LCAT treatment caused only a small increase in HDL cholesterol and apoA-I levels and in alpha-HDL particle numbers in the helix 6Delta mutant. The findings indicate a critical contribution of residue 160 of apoA-I to the in vivo activity of LCAT and the subsequent maturation of HDL and explain the low HDL levels in heterozygous subjects carrying this mutation.
Chroni
Angeliki
A
Molecular Genetics, Whitaker Cardiovascular Institute, Department of Medicine, Center for Advanced Biomedical Research, Boston University School of Medicine, Boston, Massachusetts 02118, USA.
Duka
Adelina
A
Kan
Horng-Yuan
HY
Liu
Tong
T
Zannis
Vassilis I
VI
D000818:Animals; D016632:Apolipoprotein A-I / Q000235:genetics* / Q000378:metabolism; D001483:Base Sequence; D053442:High-Density Lipoproteins, Pre-beta; D006801:Humans; D008075:Lipoproteins, HDL / Q000378:metabolism; D051379:Mice; D018716:Molecular Mimicry; D010641:Phenotype; D010713:Phosphatidylcholines / Q000172:deficiency* / Q000235:genetics / Q000378:metabolism; D017354:Point Mutation; D002785:Sterol O-Acyltransferase / Q000172:deficiency* / Q000235:genetics / Q000378:metabolism
D016632:Apolipoprotein A-I; D053442:High-Density Lipoproteins, Pre-beta; D008075:Lipoproteins, HDL; D010713:Phosphatidylcholines; D002785:Sterol O-Acyltransferase